Academic interests
- Inorganic chemistry and crystallography.
- Metal oxides with fractional valences.
- Less common combinations of properties: towards halfmetallic ferromagnet.
- Relationship between structure and phase transitions into valence-controlled states: temperatures of valence mixing, of superconductivity.
- Less common compositions: carbides-oxides as pseudonitrides, nitrides-fluorides as pseudooxides.
- Oxide materials with less-common properties: high conductivity, photocatalysis.
Academic interests, methods
- Syntheses of new compounds at less-common temperature- and redox conditions.
- Precise valence control in solid oxides of transition metals.
- Structure determination from synchrotron x-ray and neutron diffraction data.
- Mössbauer spectroscopy of valence-, spin-, and coordination states (in collaboration).
- Magnetic, electric and thermal properties of inorganic solids (in collaboration).
Teaching
Background
- Positions related to University of Oslo: Professor, lecturer, researcher.
- Earlier positions: lecturer, University of Chemical Technology, Prague.
Awards
- Textbook of the year, 1987 (University of Chemical Technology, Prague)
- First place in national Chemistry Olympiad in Prague, 1966
Scientific service
- IUPAC, titular member of Division II, Inorganic Chemistry
- IUPAC, member of the Interdivisional Committee on Terminology, Nomenclature and Symbols
- International Centre for Diffraction Data, a member
- Norwegian Chemical Society, member of nomenclature committee
Collaboration
An extensive collaboration in Finland and USA
Projects
IUPAC project Toward a Comprehensive Definition of Oxidation State
IUPAC project Toward a Comprehensive Definition of Valence
IUPAC project Heuristic quantities and rules in inorganic chemistry
IUPAC project Revision and Update of the Red Book – Nomenclature of Inorganic Chemistry
Tags:
Inorganic chemistry,
Crystallography,
Mixed valence
Publications
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Karen, Pavel; Armelao, Lidia; Butler, Ian S.; Tomišić, Vladislav & Yamashita, Makoto
(2025).
Toward a definition of valence as a quantity (IUPAC Technical Report).
Pure and Applied Chemistry.
ISSN 0033-4545.
97(2),
p. 149–187.
doi:
10.1515/pac-2023-0402.
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Valence has a rich history in chemistry, as a bonding concept, in terms of quantitative context, and as a
true quantity. In the latter, a survey preceding this project revealed differing perceptions of valence values
and helped formulate candidate definitions. This IUPAC task group evaluated nine quantities behind eight
alternative definitions on 39 chemical entities of 48 bonding formulas, each giving a set of meaningful values with
mutual relationships. Given the reflection principle of IUPAC normative work, 15 comparative examples with
high variation of these alternative valences for an element were selected, and chemistry articles in English
searched for valence-termed quantities of the compared compounds to imply the definition behind the stated
valence value, the frequency of such use, and the chemistry field. Summarized preferences for the alternative
definitions show two main areas of use. Organic and physical chemists count valence as a number of two-electron
bonds at the atom. Inorganic chemists working with semi-metallic and metallic elements use n-valent as an
adjective for oxidation state. The diverse yet infrequent use cannot be covered by a single definition of the valence
quantity. Clarity in articles that use valence as a quantity is essential and achievable by stating the intended
context.
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Karen, Pavel
(2024).
Synthesis ranges for YBaFe<inf>4</inf>O<inf>7</inf> and its topotactic oxidation in ambient air to YBaFe<sup>III</sup> <inf>4</inf>O<inf>8.5</inf>.
Journal of Solid State Chemistry.
ISSN 0022-4596.
332.
doi:
10.1016/j.jssc.2024.124555.
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Conditions for YBaFe4O7 synthesis from oxide/carbonate amorphous precursors are investigated versus temperature and partial pressure of oxygen at high-temperature equilibrium with the sample in flowing atmosphere of humidified Ar and H2 gas mixtures. Upon topotactic oxidation in ambient air of the cooled-down product, YBaFeIII4O8.5 forms in a couple of minutes. Neutron powder diffraction suggests that added oxygens enter octahedral holes in the cubic closest packing of O and Ba atoms of YBaFe4O7. Up to two oxygen atoms add to some Fe coordination tetrahedra in the kagome-like layer of YBaFe4O7 in a disordered distribution described as partially occupied crystal-structure sites. The oxidized YBaFeIII4O8.5 is metastable and decomposes at high temperatures into FeIII oxides of barium or yttrium. The synthesized YBaFe4O7 is a powerful room-temperature O2 getter that can be reverted by annealing in the above gas mixture.
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Karen, Pavel
(2023).
Crystal structure and oxygen nonstoichiometry of ErBa2Fe3O8.
Journal of Solid State Chemistry.
ISSN 0022-4596.
322.
doi:
10.1016/j.jssc.2023.123953.
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Ten samples across the range of oxygen nonstoichometry of the ErBa2FeIII3O8 triple perovskite were obtained by high-temperature equilibration and quenching from flowing gas of controlled oxygen partial pressure. The crystal structure accommodating additional oxygen upon oxidation, and oxygen vacancies in the central coordination octahedron upon reduction, was studied by synchrotron X-ray powder diffraction. Close to the homogeneity limit upon reduction, the tetragonal P4/mmm structure distorts orthorhombic upon ordering of vacancies. The structural compromises across the nonstoichiometry range are discussed in terms of bond-valence sums at the atoms.
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Karen, Pavel
(2023).
Heuristic algorithms for understanding chemistry via simple quantities.
Pure and Applied Chemistry.
ISSN 0033-4545.
95(6),
p. 683–700.
doi:
10.1515/pac-2022-1118.
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Abstract Periodicity of elements is the basis of teaching and understanding inorganic chemistry. This review exemplifies simple rules and counting procedures as heuristic algorithms yielding often-dimensionless quantities that, as such or together with auxiliary parameters, allow us to predict not only the stoichiometry and bonding of compounds, but also some of their properties or reactions.
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Karen, Pavel
(2023).
LaBa2Fe3O8 cubic perovskite; oxygen-nonstoichiometry defects and conductivity.
Journal of Solid State Chemistry.
ISSN 0022-4596.
324.
doi:
10.1016/j.jssc.2023.124137.
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Point defects behind nonstoichiometry of the highly oxygen-vacant LaBa2FeIII3O8 single-perovskite structure are studied by quenching a set of nine samples close to FeIII from equilibria at two partial pressures of oxygen, pO2, at four temperatures T. This supports a simplified defect model that fits well the high-temperature electrical conductivity around and below the FeIII point, while deviating somewhat in the oxidized range. The conductivity fitted by sum of its electronic and ionic components versus T and pO2 in the surrounding gas reveals a good oxygen-ionic conductor. Oxygen diffusion behind conductivity transients upon a pO2 change is discussed.
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Karen, Pavel
(2022).
Thermal evolution of the EuBaCo2O5.5 crystal structure.
Journal of Solid State Chemistry.
ISSN 0022-4596.
311.
doi:
10.1016/j.jssc.2022.123102.
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Crystal structure of the EuBaCo2O5.502(1) perovskite obtained by equilibration and slow cooling in closed ampoule is studied between 100 and 400 K by high-resolution synchrotron powder X-ray diffraction, and its thermally induced phase transitions by differential scanning calorimetry (DSC). The low-temperature Pmma perovskite supercell 2ap 2bp 2cp has Eu and Ba alternating along c, Co3+ coordination square pyramids and octahedra alternating along b and orbital-order alternating along a. The last order is lost upon warming through a transition topping at 365 K (ΔS ≈ 2.25R per mole EuBaCo2O5.5 by DSC) that forms an ap 2bp 2cp supercell as refined at 380 and 400 K. The DSC detects also the twin magnetic phase transition at 248 K and 269 K as weak endothermic effects upon heating (ΔS ≈ 0.41 J/molK and 0.19 J/molK, resp.). Intermittent negative thermal expansion of some cobalt coordination polyhedra is discussed in the context of possible Co3+ spin states and of the DSC entropy change at the structure transition.
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Karen, Pavel
(2022).
Defect chemistry and oxygen nonstoichiometry of CeBaFe2O5.5.
Journal of Solid State Chemistry.
ISSN 0022-4596.
307.
doi:
10.1016/j.jssc.2021.122743.
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While lanthanoids Ln = Gd to Pr form Ba–Ln ordered double-cell iron perovskites with a wide equilibrium nonstoichiometry from LnBaFe2O5 to over O5.5, the Ce variant of this study no longer forms the CeBaFe2O5 composition. Point-defect equilibria behind oxygen nonstoichiometry of the CeIIIBaFeIII2O5.5 that forms are studied by quenching from flowing gas of controlled oxygen partial pressures pO2 and temperatures. Their thermodynamic parameters are evaluated on the defect model of pure undoped CeIIIBaFeIII2O5.5 with an electronic-defect pair e′ and h•, anion-Frenkel pair Oi′′ and vO••, and redox behavior at equilibrium with oxygen in the reaction atmospheres to fit the experimentally determined nonstoichiometry.
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Karen, Pavel
(2022).
Why is Mn charge ordered and AFM coupled in YBaMn2O5.
Journal of Solid State Chemistry.
ISSN 0022-4596.
315.
doi:
10.1016/j.jssc.2022.123469.
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Synthesis conditions to obtain single-phase YBaMn2O5 are reported and its crystal structure refined from neutron powder diffraction data collected between 25 and 600 °C in the paramagnetic state. The structure parameters are used to reason why the phase maintains ordered ionic charges of Mn instead of mixing them at an elevated temperature, why this Mn2+ and Mn3+ ordering is of the checkerboard type in all three dimensions, and why these ions would couple antiferromagnetic upon cooling.
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Karen, Pavel
(2022).
Structural changes and valence-mixing transition in EuBaCo2O5.
Journal of Solid State Chemistry.
ISSN 0022-4596.
314.
doi:
10.1016/j.jssc.2022.123367.
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High-intensity high-resolution synchrotron powder diffraction (SXPD) and differential scanning calorimetry (DSC) are used to investigate the valence-mixing transition in EuBaCo2O4.999(3) sample of composition homogenized by annealing with zirconium-metal oxygen getter in a sealed ampoule. The crystal structures are refined between 100 and 400 K. Fine scans by SXPD as well as DSC reveal two continuous transitions with no hysteresis: At 233 K, disappearance of the long-range charge-ordered Co2+/Co3+ superstructure from SXPD patterns, at 360 K, transition into paramagnetic state. The smooth evolution of lattice parameters as well as the DSC results suggest no spin-state change.
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Karen, Pavel
(2021).
Oxygen nonstoichiometry in LnBaFe2O5+w across Ln = Nd, Sm, Gd.
Journal of Solid State Chemistry.
ISSN 0022-4596.
301.
doi:
10.1016/j.jssc.2021.122297.
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In double-cell perovskites LnBaFe2O5+w, only lanthanoids Ln = Pr to Gd support wide equilibrium nonstoichiometry w from 0 to >0.5. This study investigates point-defect equilibria behind the variation in w along Ln = Nd, Sm, Gd by quenching the oxides from flowing gas of controlled oxygen partial pressures and temperatures. The experimental data of w as a function of pO2 and T are least-squares fit via standard enthalpies and entropies of the defect-equilibrium reactions of formation for electronic-defect pair e′ and h• and for anion-Frenkel pair Oi′′ and vO••, and of the Fe2+ oxidation. NdBaFe2O5+w is closest to behaving as a pure undoped oxide at 1000 °C, where it has a single plateau in the w versus pO2 plot at the w = 0.5 level. A second plateau, at w = 0, emerges at lower temperatures. The smaller Sm and Gd increase the size difference versus Ba, and their defect equilibria show two plateaus even at 1000 °C. The temperature-dependent defect equilibria across this series are therefore approximated by a formal Ln3+ for Ba2+ donor-doping model, and the results are presented in Brouwer diagrams of defect concentrations. The decrease of the anion-Frenkel defect-equilibrium constant KaF due to decreasing ionic size from Nd3+ towards Gd3+ is magnified upon decrease in temperature that feeds the entropy of the anion-Frenkel disorder; at 400 °C, Ln = Nd has KaF = 10−5.4, Sm 10−15.0 and Gd 10−24.3.
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Karen, Pavel
(2021).
Synthesis and equilibrium oxygen nonstoichiometry of PrBaFe2O5+w.
Journal of Solid State Chemistry.
ISSN 0022-4596.
299,
p. 1–8.
doi:
10.1016/j.jssc.2021.122147.
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A double-cell perovskite PrBaFe2O5+w is synthesized and its equilibrium nonstoichiometry investigated after quenching from flowing gas of varied oxygen partial pressures and temperatures. Whereas w = 0 is reached at 856 °C in wet H2 of pH2O = 0.02 bar yielding pO2 = 4×10−21 bar, w = 0.75 obtains at 400 °C in O2. Below ~1000 °C, compositions with w ≥ 0.5 order into a Cmmm superstructure that fully converts back upon heating to 1020 °C. PrBaFe2O5 of all equivalent Fe2.5+ sites orders into Fe2+ and Fe3+ upon cooling through a first-order transition at 204 K, with a 2 K hysteresis, as investigated by powder X-ray diffraction and differential scanning calorimetry. The valence-mixed and charge-ordered structures are refined from synchrotron X-ray powder diffraction patterns. The nonstoichiometry w is expressed as a function of T and pO2 of the flowing gas in a formal donor-dopant defect model via least-squares fit of thermodynamic parameters for point-defect equilibria in the solid.
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Linden, J; Lindroos, F. & Karen, Pavel
(2017).
Orbital occupancy evolution across spin- and charge-ordering transitions in YBaFe2O5.
Journal of Solid State Chemistry.
ISSN 0022-4596.
252,
p. 119–128.
doi:
10.1016/j.jssc.2017.04.036.
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Thermal evolution of the Fe2+–Fe3+ valence mixing in YBaFe2O5 is investigated using Mössbauer spectroscopy. In this high-spin double-cell perovskite, the d6 and d5 Fe states differ by the single minority-spin electron which then controls all the spin- and charge-ordering transitions. Orbital occupancies can be extracted from the spectra in terms of the dxz, dz2 and either dx2−y2 (Main Article) or dxy (Supplement) populations of this electron upon conserving its angular momentum. At low temperatures, the minority-spin electrons fill up the ordered dxz orbitals of Fe2+, in agreement with the considerable orthorhombic distortion of the structure. Heating through the Verwey transition supplies 93% of the mixing entropy, at which point the predominantly mixing electron occupies mainly the dx2−y2/dxy orbitals weakly bonding the two Fe atoms that face each other across the bases of their coordination pyramids. This might stabilize a weak coulombic checkerboard order suggested by McQueeney et alii in Phys. Rev. B 87(2013)045127. When the remaining 7% of entropy is supplied at a subsequent transition, the mixing electron couples the two Fe atoms predominantly via their dz2 orbitals. The valence mixing concerns more than 95% of the Fe atoms present in the crystalline solid; the rest is semi-quantitatively interpreted as domain walls and antiphase boundaries formed upon cooling through the Néel and Verwey-transition temperatures, respectively.
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Karen, Pavel; McArdle, Patrick & Takats, Josef
(2016).
Comprehensive definition of oxidation state
(IUPAC Recommendations 2016).
Pure and Applied Chemistry.
ISSN 0033-4545.
88,
p. 831–839.
doi:
10.1515/pac-2015-1204.
Full text in Research Archive
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Oxidation state (OS) is defined using ionic approximation of bonds. Two principal algorithms are outlined for OS determination in a chemical compound described by a Lewis formula or bond graph. Typical origins of ambiguous OS values are pointed out, and the relationship between OS and the dn electron configuration of transition metals is commented on.
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Karen, Pavel; McArdle, Patrick & Takats, Josef
(2014).
Toward a comprehensive definition of oxidation state (IUPAC Technical Report).
Pure and Applied Chemistry.
ISSN 0033-4545.
86(6),
p. 1017–1081.
doi:
10.1515/pac-2013-0505.
Full text in Research Archive
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Olsen, Jørn Eirik; Karen, Pavel; Sørby, Magnus Helgerud & Hauback, Bjørn
(2014).
Effect of chloride substitution on the order-disorder transition in NaBH4 and (NaBD4)-B-11.
Journal of Alloys and Compounds.
ISSN 0925-8388.
587,
p. 374–379.
doi:
10.1016/j.jallcom.2013.10.171.
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Seibel, HA; Karen, Pavel; Wagner, TR & Woodward, PM
(2009).
Synthesis and characterization of color variants of nitrogen- and fluorine-substituted TiO2.
Journal of Materials Chemistry.
ISSN 0959-9428.
19(4),
p. 471–477.
doi:
10.1039/b815758h.
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Andersson, K. Kristoffer & Karen, Pavel
(2009).
Mössbauer Spectroscopy in the Nordic countries; University of Oslo.
Mössbauer Effect Reference and Data Journal.
ISSN 0163-9587.
32(1),
p. 34–36.
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Professor of Physical Biochemistry K. Kristoffer Andersson at University of Oslo (Department of Molecular Biosciences) has used Mössbauer spectroscopy to study heme and di-iron proteins since 1982. Together with Prof. Eckard Münck (University of Minnesota) he performed detailed magnetic studies on (multi-)heme proteins from ammonia oxidizing bacteria. In these systems, both the 24 heme hydroxylamine-oxidoreductase (HAO) and tetra-heme cytochrome c554 revealed a novel type of weak heme-heme interaction where the EPR resonances were found frequency dependent. This was determined by the use of parallel mode EPR for integer spin EPR in combination with Mössbauer spectroscopy. The ferric cytochrome c554 has a very unusual low temperature EPR spectrum at neutral pH with a very broad resonance and zero crossing at g = 3.3 at (X-band EPR 9.2 GHz). The whole spectrum is shifted in S-band EPR (3.3 GHz) with the zero crossing at at g = 3.0. The novel P460 chromophore in HAO and the P460-cytochrome both in their reduced forms has an unusually large quadrupole doublet (Eq = 4.2 mm/sec seen by Mössbauer spectroscopy). The ferric P460 has an integer spin EPR signal in HAO due to spin coupling to one other heme. Also for this state the EPR spectra changed with EPR frequency (published in J. Am. Chem. Soc., J. Biol. Chem.). These novel findings of weak magnetic couplings and novel heme groups have later been found in other proteins.
Other Mössbauer studies include examination of di-iron oxygen proteins such as the hydroxylase components of soluble methane monooxygenase (with E. Münck, published in J. Am. Chem. Soc.) and of the RiboNucleotide Reductase (RNR) p53R2 protein (unpublished with Åsmund K. Røhr, V. Schünemann and A.X. Trautwein) in different redox states. These clusters, Cu-proteins and radicals were further examined with e.g. protein crystallography, Raman, magnetic circular dichroism (with Edward I. Solomon, Stanford University) and EPR up to 285 GHz (with Anne-Laure Barra, CNRS Grenoble).
Recently we the Andersson Group studied mono-heme cytochrome c552 from ammonia oxidizing bacteria with Mössbauer spectroscopy (with Volker Schünemann and Alfred X. Trautwein in Germany) in combination with EPR and paramagnetic NMR (with Kara L. Bren). We could find a novel correlation between the unusual highly anisotropic low spin (HALS) ferric cytochrome c552 EPR signals with large gmax values and their paramagnetic shifted heme methyl groups (published in J. Am. Chem. Soc.). Furthermore we have examined with Mössbauer, EPR, Raman and single protein crystal light absorption spectroscopy a Fe(IV)OH form of hydrogenperoxide oxidized myoglobin compound II (published in J. Biol. Chem). We are together with Volker Schünemann preparing experiments using the recently developed 57Fe dependent technique Nuclear Resonance Scattering and Raman Microscopy in order to further characterize the myoglobin compound II as well as our novel peroxy-myoglobin.
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Karen, Pavel; Gustafsson, K & Linden, J
(2007).
EuBaFe2O5: Extent of charge ordering by Mossbauer spectroscopy and high-intensity high-resolution powder diffraction.
Journal of Solid State Chemistry.
ISSN 0022-4596.
180.
doi:
10.1016/j.jssc.2006.09.021.
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Linden, J; Karen, Pavel; Nakamura, J; Karppinen, M & Yamauchi, H
(2006).
Transport and magnetotransport properties across the two-step Verwey transition in BaGdFe2O5+w.
Physical Review B. Condensed Matter and Materials Physics.
ISSN 1098-0121.
73.
doi:
10.1103/PhysRevB.73.064415.
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Magnetoresistance, electrical transport, and magnetic properties associated with a two-step Verwey-type transition in BaGdFe 2 O 5 + w (− 0.015 < w < 0.181 ) are studied as a function of temperature and oxygen nonstoichiometry (w). A disproportionation of Fe 2.5 + into Fe 2.5 − ϵ and Fe 2.5 + ϵ upon cooling through the transition temperature Tp and charge ordering into Fe 2 + and Fe 3 + at TV are manifested in electrical-conductivity and Seebeck-coefficient data. Above TV, electrical conductivity shows an activated hopping behavior with activation energy of ∼ 0.13 eV . Seebeck measurements identify holes as charge carriers below TV. Above TV, both holes and valence-mixing electrons need to be considered, although the Seebeck coefficient remains positive up to room temperature. This suggests that the activation energy for electrons is higher than that for holes, and the actual value is close to that obtained from conductivity data. Increasing w increases electrical conductivity and decreases Seebeck coefficient in the charge-ordered state. In the valence-mixed state, increasing w increases Seebeck coefficient, but conductivity increases only up to w≈ 0.1 , from which the decay of the valence mixing takes over and conductivity begins to approach values extrapolated from the charge-ordered state. Magnetoresistance peaks with negative ratio up to ∼ 2 % are observed, corresponding to a small magnetic-susceptibility change at TV. © 2006 American Physical Society
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Karen, Pavel; Moodenbaugh, AR; Goldberger, J; Santhosh, PN & Woodward, PM
(2006).
Electronic, magnetic and structural properties of A(2)VMoO(6) perovskites (A = Ca, Sr).
Journal of Solid State Chemistry.
ISSN 0022-4596.
179.
doi:
10.1016/j.jssc.2006.04.013.
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Moritomo, Y; Hanawa, M; Xu, S; Ishikawa, H; Ohishi, Y & Kato, K
[Show all 10 contributors for this article]
(2004).
Pressure effects on the charge-ordering transition of BaYCo2O5.0.
Physical Review B. Condensed Matter and Materials Physics.
ISSN 1098-0121.
69,
p. art. no.–134118.
doi:
10.1103/PhysRevB.69.134118.
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Pressure effects on the charge-ordering (CO) transition have been investigated for an oxygen-deficient double-perovskite cobalt oxide, BaYCo 2 O 5.0 , through resistivity measurements using a piston-cylinder-type clamp cell. The charge-ordering temperature TCO rises (+19 K / G P a ) as pressure increases, making a sharp contrast with the pressure suppression of TCO seen for the isostructural BaSmFe 2 O 5.0 . We have further determined the atomic coordinates under elevated pressures, and ascribed this unexpected behavior of BaYCo 2 O 5.0 to the pressure enhancement of the orthorhombic distortion s (≡ a − b ) . © 2004 American Physical Society
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Lindén, Johan; Karen, Pavel; Yamauchi, Hisao & Karppinen, Maarit
(2004).
Valence mixing, separation and ordering in double-cell perovskite GdBaFe2O5+w.
Journal of Magnetism and Magnetic Materials.
ISSN 0304-8853.
272-276,
p. E267–E268.
doi:
10.1016/j.jmmm.2003.11.181.
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Nakamura, J; Karppinen, M; Karen, Pavel; Linden, J & Yamauchi, H
(2004).
Isovalent-substitution effect on the Verwey-type transition in the A-site-ordered double perovskite (Ba,Sr)RFe2O5.
Physical Review B. Condensed Matter and Materials Physics.
ISSN 1098-0121.
70.
doi:
10.1103/PhysRevB.70.144104.
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Two types of chemical-pressure effects on the Verwey-type transition in Ba Sm Fe 2 O 5 are studied, viz., ionic-size contractions at the Ba and Sm sites. Whereas the replacement of Sm by the smaller Eu and Gd atoms increases the transition temperature TV from 232 K to 264 K , the Sr-for-Ba substitution decreases it from 232 K to 180 K for x=0.15 in (Ba 1 − x Sr x ) Sm Fe 2 O 5 . Of various structural parameters determined by synchrotron x-ray diffraction above TV, the magnitude of the orthorhombic distortion (=b− a ) is found to correlate with TV in a manner that unifies both substitution schemes. It is suggested that when an increase in b− a above TV is achieved by means of the structural substitution, it makes it easier for the dxz Fe 2 + orbital ordering to occur below TV. © 2004 American Physical Society
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Woodward, Patrick M.; Suard, Emmanuelle & Karen, Pavel
(2003).
Structural tuning of charge, orbital, and spin ordering in double-cell perovskite series between NdBaFe2O5 and HoBaFe2O5.
Journal of the American Chemical Society.
ISSN 0002-7863.
125,
p. 8889–8899.
doi:
10.1021/ja034813.
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Karen, Pavel; Woodward, Patrick M.; Santhosh, P.N.; Vogt, Tom; Stephens, P.W. & Pagola, S.
(2002).
Verwey transition under oxygen loading in RBaFe2O5+w (R = Nd and Sm).
Journal of Solid State Chemistry.
ISSN 0022-4596.
167,
p. 480–493.
doi:
10.1006/jssc.2002.9665.
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Double-cell perovskites RBaFe2O5+w, (R = Nd and Sm) are synthesized in the -0.03 < w < 0.47 portion of the
nonstoichiometry range. The ideal composition with w = 0 has di- and trivalent iron in equal proportions and exhibits a
charge-ordering Verwey transition upon cooling, preceded by a weaker premonitory transition. Both transitions are detectable
by differential scanning calorimetry. The changes in entropy, volume, orthorhombic distortion and electrical conductivity at the
main transition are larger for the variant with the smaller Sm atom than for R = Nd. The discontinuity of the SmBaFe2O5+w,,
transition also persists into much higher levels of the oxygen nonstoichiometry w than for the R = Nd variant. Whereas
3D-charge-ordered SmBaFe2O5.00 is isostructural with TbBaFe2O5.00 adopting space group Pmma, NdBaFe2O5.00
undergoes a more profound structural change upon entering the charge-ordered state, the structure of which is refined in the
P21ma symmetry from high-resolution X-ray synchrotron diffraction data.
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Karen, Pavel; Woodward, Patrick M.; Lindén, Johan; Vogt, Tom; Studer, Andrew & Fischer, Peter
(2001).
Verwey transition in mixed-valence TbBaFe2O5: Two attempts to order charges.
Physical Review B. Condensed Matter and Materials Physics.
ISSN 1098-0121.
64(21),
p. 214405/1–/14.
doi:
10.1103/PhysRevB.64.214405.
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Electronic, magnetic, and structural phase transitions in nearly stoichiometric TbBaFe 2 O 5 + w ( 0.00 < w < 0.05 ) have been investigated. At high temperatures this compound is a paramagnetic, mixed-valence (Fe 2 . 5 + ) conductor with identical square-pyramidal coordinations at all iron atoms. Upon cooling below TN=450 K , an antiferromagnetic (AFM) spin order appears, accompanied by a magnetostrictive orthorhombic distortion. At lower temperatures the increasing distortion sets the frame for a first attempt to order charges. Mössbauer spectroscopy shows that one squeezed and one expanded square pyramid appear with different orientations of their magnetic and electric field tensors, each centered by its own mixed-valence iron state, one Fe 2 . 5 + ε , the other Fe 2.5 − ε . The lattice retains its distortion, but a small, structurally homogeneous, and continuous increase in volume is experienced. At somewhat lower temperature (TV) a discontinuous increase of the orthorhombic distortion occurs, marking the second attempt to order charges, now with the classical symptoms of the Verwey transition: a large change in volume, entropy, and electrical conductivity. Below TV, a normal Fe 3 + high-spin state in a symmetrical square-pyramidal coordination appears, whereas Fe 2 + is distorted. The long-range order of this arrangement is solved from high-resolution powder neutron diffraction data. Rietveld refinements show that the charge-ordered spins have AFM interactions in all three directions (G type) whereas in the mixed-valence state a ferromagnetic (FM) interaction appears between the iron atoms facing each other across the Tb layer. This FM interaction is suggested to be essential for the appearance of the mixed-valence state via the double-exchange sharing of the Fe 2 + -originated electron. This also allows for the total ordered spin moment being unchanged at the Verwey transition, following one single Brillouin curve. Analogous cases are pointed out where the Verwey transition proceeds in a similar manner, also at the molecular level. © 2001 American Physical Society
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Karen, Pavel & Kjekshus, Arne
(2000).
Phase Diagrams and Thermodynamic Properties.
In Gschneidner, Karl A. Jr.; Eyring, LeRoy & Maple, Brian M. (Ed.),
Handbook on the Physics and Chemistry of the Rare Earths. Volume 30, High-Temperature Superconductors - I.
Elsevier Science B.V., The Netherlands.
ISSN 9780444505286.
p. 229–373.
doi:
10.1016/S0168-1273(00)30007-1.
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Phase diagrams and thermodynamic data for selected cuprates of alkaline-earth (A) and rare-earth (R) elements except Sc are presented in this chapter, covering binary to quintenary oxides, and including relevant solid-solution series with other elements. Attempts have been made to list all known occurring solid phases up to date. A short information on crystal structure is listed for each phase giving it its identity, as well as the concise chemical information which defines its character. The chemistry of phases is illustrated in mostly subsolidus isothermal phase diagrams, drawn after original sources,and referring to diverse chemical conditions defined by partial pressures of oxygen in particular. Reactivity towards CO2 and formation of oxide carbonates are treated systematically as well, having been one of the major experimental obstacles in pursuit of data relevant to pure oxides. Nevertheless, in some cases, the Ba-rich regions are still unclear as to the identity of the occurring phases. For phase diagrams of the individual elements, the reader is directed to the monograph by Young (1991) as well as the books edited by Massalski (1990) and by Villars et al.(1995) for selected and evaluated binary and ternary systems of the elements, respectively. Although not cited in the following text because only original works have been consulted for this matter, an excellent monograph edited by Whitler and Roth (1991) on phase diagrams related to high-Tc superconductors should be mentioned. When available, chemical information on the thermodynamics of solid phases is represented also in a numerical form. Treatment of liquid phases as well as most of the solid miscibilities is beyond the scope of this review, but references are given to those original communications where complete phase diagrams are assessed and such data provided. In this chapter, essential thermodynamic data for evaluations of phase equilibria appear in the form of entropies and enthalpies at standard and phase-transition temperatures together with heat-capacity functions. Other possibilities are temperature polynomials for standard Gibbs energies of formation, or for partial pressures of gaseous components. Efforts have been made to compile such suitable data for chemical phases relevant to superconducting oxides containing rare earths. When possible, for elements and simple oxides in particular, data selected in monographs by JANAF (1971), Hultgren et al. (1973), Pankratz (1982), Gschneidner et al. (1973), Dinsdale (1991) and Barin et al. (1993) have been chosen. However, the majority of data for the more complex oxide phases in this rapidly expanding field stems from original sources.
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Vogt, Tom; Woodward, Patrick M.; Karen, Pavel; Hunter, B.A.; Henning, P. & Moodenbaugh, A.R.
(2000).
Low to High Spin-State Transition Induced by Charge Ordering in Antiferrimagnetic YBaCo2O5.
Physical Review Letters.
ISSN 0031-9007.
84(13),
p. 2969–2972.
doi:
10.1103/PhysRevLett.84.2969.
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The oxygen-deficient double perovskite YBaCo 2 O 5 , containing corner-linked CoO 5 square pyramids as principal building units, undergoes a paramagnetic to antiferromagnetic spin ordering at 330 K. This is accompanied by a tetragonal to orthorhombic distortion. Below 220 K orbital ordering and long-range Co 2 + / Co 3 + charge ordering occur as well as a change in the Co 2 + spin state from low to high spin. This transition is shown to be very sensitive to the oxygen content of the sample. To our knowledge this is the first observation of a spin-state transition induced by long-range orbital and charge ordering. © 2000 American Physical Society
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Karen, Pavel; Kjekshus, Arne; Huang, Q.; Karen, V. L.; Mighell, A. D. & Santoro, A.
(2000).
Neutron powder diffraction study of the crystal structure of the oxycarbonate phase YBa2Cu2.85(CO3)0.15O6.73.
Physica. C, Superconductivity.
ISSN 0921-4534.
336,
p. 279–286.
doi:
10.1016/S0921-4534(00)00275-6.
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Karen, Pavel & Woodward, Patrick M.
(1999).
Synthesis and Structural Investigations of the Double Perovskites REBaFe2O5+w (RE = Nd, Sm).
Journal of Materials Chemistry.
ISSN 0959-9428.
9,
p. 789–797.
doi:
10.1039/A809302D.
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Karen, Pavel; Kjekshus, Arne; Huang, Q. & Karen, V. L.
(1999).
The Crystal Structure of Magnesium Dicarbide.
Journal of Alloys and Compounds.
ISSN 0925-8388.
282,
p. 72–75.
doi:
10.1016/S0925-8388(98)00828-7.
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The crystal structure of MgC<SUB>2</SUB> is solved from powder X-ray and neutron diffraction data. Magnesium dicarbide is synthesized from Mg powder and ethyne, with yields up to 40wt.% (balance: unreacted Mg and MgO). The X-ray pattern is indexed on a tetragonal unit cell, with space group P4<SUB>2</SUB>/mnm suggested from systematic extinctions. The unit cell refined from the powder neutron data has a=3.9342(7) and c=5.021(1)Å and Z=2. The crystal structure contains C<SUB>2</SUB> groups, with a triple-bond length of 1.215(6)Å, aligned into ... MgCCMgCCMg... chains having Mg-C bond lengths of 2.174(4)Å and a weak interaction [2.510(1)Å] between Mg and the triple bond of the crossing chains above and below.
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Lindén, Johan; Karen, Pavel; Kjekshus, Arne; Miettinen, J. & Karppinen, Maarit
(1999).
Partial Oxygen Ordering in Cubic Perovskite REBa2Fe3O8+w (RE= Gd, Eu, Sm, Nd).
Journal of Solid State Chemistry.
ISSN 0022-4596.
144,
p. 398–404.
doi:
10.1006/jssc.1999.8178.
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Lindén, Johan; Karen, Pavel; Kjekshus, Arne; Miettinen, J.; Pietari, T. & Karppinen, Maarit
(1999).
Valence State Mixing and Separation in SmBaFe2O5+w.
Physical Review B. Condensed Matter and Materials Physics.
ISSN 1098-0121.
60(22),
p. 15251–15260.
doi:
10.1103/PhysRevB.60.15251.
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A mixed-valence state, formally denoted as Fe 2 . 5 + , is observed in the 300 K Mössbauer spectra of the most reduced samples of SmBaFe 2 O 5 + w . Upon cooling below the Verwey-type transition temperature (TV≈ 200 K ) , the component assigned to Fe 2 . 5 + separates into a high-spin Fe 3 + state and an Fe 2 + state with an unusually low internal field. The separation of the mixed-valence state at TV is also confirmed by magnetic susceptibility measurements and differential scanning calorimetry. A model is proposed which accounts for the variation of the amount of the mixed-valence state with the oxygen content parameter w. © 1999 American Physical Society
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Linden, Johan; Kjekshus, Arne; Karen, Pavel; Miettinen, J. & Karppinen, Maarit
(1998).
A 57Fe Mössbauer study of REBa2Fe3O8+w triple perovskites with varied oxygen content (RE = Dy, Er and Y).
Journal of Solid State Chemistry.
ISSN 0022-4596.
139,
p. 168–175.
doi:
10.1006/jssc.1998.7828.
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Lindén, Johan; Lippmaa, Mikk; Kjekshus, Arne; Karen, Pavel & Karppinen, Maarit
(1998).
A 57Fe Mössbauer study of the cubic perovskite-type phase LaBa2Fe3O8+w (−0.20 < w < 0.83).
Journal of Solid State Chemistry.
ISSN 0022-4596.
p. 87–97.
doi:
10.1006/jssc.1998.7757.
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Karen, Pavel; Harlow, R.L.; Li, Z.; Meenan, P & Parziale, P.A.
(1998).
The crystal structure of 2-amino-4,6-dihydroxypyrimidine determined from powder X-ray synchrotron diffraction.
Acta Chemica Scandinavica.
ISSN 0904-213X.
52,
p. 1051–1055.
doi:
10.3891/acta.chem.scand.52-1051.
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Atanassova, Y.K.; Hadijev, V.G.; Karen, Pavel & Kjekshus, Arne
(1994).
Raman scattering from YBa2Fe3O8+d.
Physical review B (PRB).
ISSN 2469-9950.
50,
p. 586–589.
doi:
10.1103/PhysRevB.50.586.
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The results of polarized micro-Raman scattering experiments on YBa 2 Fe 3 O 8 + δ with δ=0.1 are reported. We identified seven out of the total ten by symmetry Raman-allowed phonons at 134 cm − 1 ( A 1 g ), 148 cm − 1 ( E g ), 219 cm − 1 ( A 1 g ), 300 cm − 1 ( B 1 g ), 478 cm − 1 ( A 1 g ), 493 cm − 1 ( E g ), and 691 cm − 1 ( A 1 g ). The Raman-active phonons are assigned to definite atomic vibrations in close comparison with the structurally similar YBa 2 Cu 3 O 6 , usually simple considerations based on charge and bond length. © 1994 American Physical Society
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Natali-Sora, I.; Huang, Q.; Lynn, J.W.; Rosov, N.; Karen, Pavel & Kjekshus, Arne
[Show all 9 contributors for this article]
(1994).
Neutron-powder-diffraction study of the nuclear and magnetic structures of the substitution compound (Y1−xCax)Ba2Fe3O8+d (x = 0.05, 0.10 and 0.20).
Physical review B (PRB).
ISSN 2469-9950.
49,
p. 3465–3472.
doi:
10.1103/PhysRevB.49.3465.
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The nuclear and magnetic structures of ( Y 1 − x Ca x ) Ba 2 Fe 3 O 8 + δ with x=0.05, 0.10, and 0.20 have been investigated by neutron-powder diffraction at room temperature. The compound crystallizes with the symmetry of space group P4/mmm and has a structure similar to that of the superconductor YBa 2 Cu 3 O 7 (1:2:3). There are two types of Fe atoms in the unit cell, one having fivefold pyramidal coordination and the other octahedral coordination. The Ca and Y atoms are randomly distributed over the 1/2,1/2,1/2 position, which are exclusively occupied by Y in the 1:2:3 superconductor. Since the oxygen sites on the basal plane of the structure are fully occupied to achieve octahedral coordination for the iron atoms on this plane, the oxygen content is eight atoms per formula unit, the barium atoms are twelvefold coordinated, and the coordination polyhedron is a cuboctahedron. Extra oxygen atoms corresponding to δ=0.08 and 0.05 are located in the Y/Ca layer. The presence of these atoms produces disorder in the structure resulting in a shift of the oxygen atoms located near the plane of the pyramidal iron atoms. The charge compensation required by the substitution of Y3+ by Ca 2 + is achieved in these materials by elimination of the extra oxygen, rather than oxidation of the iron atoms. The magnetic structure is based on a unit cell related to that of the nuclear structure by the transformation of axes (1,-1,0/1,1,0/0,0,2). The magnetic origin of some of the observed diffraction peaks was established by polarized neutron diffraction measurements. As in YBa 2 Fe 3 O 8 , the iron moments are coupled antiferromagnetically within each ( FeO 2 ) layer as well as along the c axis of the structure, and they lie in the planes perpendicular to c. The magnetic moments of the two iron atoms are practically identical and have values at room temperature of 〈μ〉=3.50(3) μ B , 3.52(2) μ B , and 3.62(2) μ B for the three compositions x=0.05, 0.10, and 0.20, respectively. © 1994 American Physical Society
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Karen, Pavel
(2005).
Verwey transition in a perovskite-type structure.
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Above Verwey transition at 120 K, magnetite (Fe3O4) is a spin-polarized conductor, feasible for applications in spin computing. Owing to continuing controversy about charge order in the complex low-temperature phase, Verwey transition in magnetite remains a mystery. Double-cell perovskites of the YBaFe2O5 type contain iron in the average oxidation state +2.5 and exhibit all symptoms of the Verwey transition. The charge-ordered superstructures are simple and the refined iron–oxygen bond distances suggest a 70 to 80% of long-range charge separation into di- and trivalent iron, likely to mean a 100% charge separation on the local scale. The charge order violates Coulombic minimization of point charges, implying orbital ordering. The associated orthorhombic distortion identifies the doubly-occupied dxz orbital of Fe2+. Magnetic interactions define the orientation of the orthorhombic distortion, but the charge order occurs independently of any specific spin coupling. Magnetostriction opens for an incipient partial charge ordering that occurs before the Verwey transition itself distorts the structure under continued cooling. Replacing Y with Sm and Nd shows that when the size of this seemingly uninvolved atom becomes unfavorable (a large projected ∆V at the transition), orbital ordering continues at a cost of symmetry lowering. Oxygen nonstoichiometry decreases Verwey-transition entropies ΔS and transition temperatures TV in a manner that corresponds to an ideal solution of YBaFe2O6 in the YBaFe2O5 solvent. Verwey transition is a klassengleich switch from a valence-mixed halfmetal into a valence-ordered semiconductor, achieved via orbital ordering of one single, singly or doubly occupied d orbital of t2g symmetry.
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Karen, Pavel; Woodward, Patrick M. & Lindén, Johan
(2002).
Charge ordering that only succeeds after nature's second attempt.
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Lindén, Johan; Nakamura, Jin; Karen, Pavel; Kjekshus, Arne; Karppinen, Maarit & Yamauchi, Hisao
(2000).
Observation of mixed-valence state in the BaSm(Cu1-xFex)2O5+.delta.double-perovskite phase.
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Karen, Pavel
(1999).
Ordering, Mixed-Valence and Metallic Character Along the Redox Range in LnBaM2O5+w (M = Fe and Co).
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A full replacement of the perovskite coordination-octahedra With ordered square pyramids leads to doubling of the unit-cell according to formula AA'M2O5. Suitably sized trivalent lanthanides (Ln) as A atoms and Ba as the A' atom are able to stabilize this arrangement for transition metals M like Mn, Fe and Co. The implied non-integer average valence M2.5+ and an entire vacant oxygen site are the main ingredients for a multitude of structural, electronic, magnetic and electric properties changing along a wide solid-state redox-stability range. LnBaM2O5+w phases with selected combinations of Ln = Nd, Sm, Gd, Y and M = Fe, Co are studied on quenched samples with w varied between 0.00 and 0.80. Structural consequences are investigated by powder neutron and synchrotron X-ray diffraction and involve distortions of the tetragonal (S.G. P4/mmm) prototype and/or further ordering schemes for significant compositions in terms of w. Valence and spin states of M are studied by 57Fe Mössbauer spectroscopy (for M = Co, 1% of it is
replaced by 57Fe). A mixed-valence AF state Fe2.5+ is observed at room temperature for the prototype composition and fades away with increasing w. Upon cooling below the transition temperature of around 200 K, the Fe2.5+ state separates into a high-spin Fe3+ state and Fe2+ with unusually low internal magnetic field. The transition is associated with a small change in magnetic susceptibility and a weak thermal effect, implying a charge ordering into an electron localized state. Samples with the most reduced M = Fe and the most oxidized M = Co have a silver luster appearance at room temperature, and optical reflectivity spectra as well as electrical conductivity measurements illustrate changes in the occurrence of the metallic character as a function of w and temperature. The Fe2.5+ valence state mixing coincides with the metallic state and is therefore closely reminiscent of the Verwey transition in magnetite, Fe3O4, one of the earliest examples of charge ordering in a transition metal oxide.
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Karen, Pavel
(2004).
Tuning the Verwey transition as an orbital ordering, ILL Annual Report 2003, p.24-5.
Institute Laue--Langevin.
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Nov. 11, 2010 9:00 AM
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