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• Andersen, Eline Haug; Sannes, Johnny Andreas; Frøen, Emil Herman; Nilsen, Ola & Valldor, Björn Martin (2025). Diamagnetic sulfide Ba3Sc2S6 – With infinite [ScS6/2] chains as structural element. Journal of Solid State Chemistry. ISSN 0022-4596. 342. doi: 10.1016/j.jssc.2024.125105. Full text in Research Archive Show summary

  The sulfide Ba3Sc2S6 was synthesized by the conventional high-temperature solid-state technique in an inert atmosphere, utilizing sealed silica ampules with low internal pressure of Ar gas. The crystal structure was determined by single crystal X-ray diffraction and can be described by a trigonal crystal lattice with rhombohedral symmetry, R 3‾ c (No. 167), with powder diffraction unit cell parameters a = 12.0402(1) Å and c = 13.3893(2) Å. The compound crystallizes with the K4CdCl6-type structure, which has been used to describe homologous sulfides. This crystal structure shares features with the 2H-perovskites but contains infinite 1D chains, Niggli notation [ScS6/2]∞1, consisting of alternating, distorted octahedra and trigonal prisms connected by face-sharing. Ba3Sc2S6 is diamagnetic, as determined from magnetic susceptibility measurements. UV–Vis spectroscopy data suggest a direct bandgap of about 2.88 eV. Further, DFT calculations reveal a band gap of about 1.8 eV and 2.9 eV using the PBE and the HSE06 functional, respectively, of which the latter agrees perfectly with UV–Vis observations.

• Frøen, Emil Herman; Nowak, Domenic; Adler, Peter & Valldor, Björn Martin (2024). Synthesis and Properties of BaMTeS (M = Fe, Mn, Zn) and the Disordered Structural Analog BaGe<inf>0.5</inf>TeS. Inorganic Chemistry. ISSN 0020-1669. doi: 10.1021/acs.inorgchem.4c00146. Full text in Research Archive Show summary

  A series of tertiary sulfide-tellurides, BaMxTeS (M = Fe, Mn, Zn, Ge), has been synthesized by solid-state synthesis. The compounds assume an orthorhombic crystal structure, described by the Cmcm (No. 63) space group, and are structural analogs of the BaMSO (M = Co, Zn) phases. The properties of all four analogs are investigated by DFT analysis. As only the BaFeTeS analog was prepared as a relatively pure phase, this homologue was subject to further experimental investigations, including heat capacity, magnetometry, and Mössbauer spectroscopy. BaFeTeS exhibits no obvious phase transition between 2 and 300 K, has no paramagnetic behavior, and lacks long-range magnetic ordering. However, the Mössbauer spectra, as well as electrical resistance data, indicate a hidden transition near 200 K that is tentatively explained by a dynamic charge-density-wave mechanism, based on a resonating valence bond (RVB) model.

• Frøen, Emil Herman & Valldor, Björn Martin (2024). Synthesis and Properties of La<inf>14</inf>TME<inf>6</inf>CuS<inf>24</inf>O<inf>4</inf>, Exhibiting Multivalent Spin Chains (TME = Cr, Fe). ACS Omega. doi: 10.1021/acsomega.4c00564. Full text in Research Archive Show summary

  Two homologues in a series of quinary oxysulfides, La14TME6CuS24O4 (TME = Cr or Fe), have been synthesized by solid-state synthesis in sealed ampules, and their structures are homologue to assume a novel crystal structure. X-ray diffraction analyses of single crystal and powder samples give a monoclinic lattice, described in the C2/m (No. 12) space group, with lattice parameters a = 15.3853(5) Å, b = 13.9729(5) Å, c = 10.5074(4) Å, and β = 116.227(3)° for the Cr analogue and a = 15.4303(2) Å, b = 14.0033(2) Å, c = 10.4909(2) Å, and β = 116.261(2)° for Fe. The crystal structure contains one-dimensional (1D) chains consisting of interconnected transition metal element (TME) trimers, which are further arranged into two-dimensional (2D) layers. These spin-chain planes are interspaced with 1D chains of lanthanum–oxygen coordinations and an apparent disordered occupation of copper sites. Alternating current (AC) and direct current (DC) magnetic susceptibility measurements show that the Cr and Fe analogues exhibit what is best described as spin-domain formation. Density functional theory (DFT) calculations suggest the formal oxidation state of the species is best represented in the form La14TME52+TME3+Cu+S24O4.

• Frøen, Emil Herman; Adler, Peter & Valldor, Björn Martin (2024). Synthesis and properties of La<inf>18</inf>Fe<inf>5</inf>Cu<inf>4</inf>S<inf>26</inf>O<inf>8</inf>, a compound containing large magnetic clusters. Journal of Materials Chemistry C. ISSN 2050-7526. doi: 10.1039/d4tc03067b. Full text in Research Archive Show summary

  A new quinary oxysulfide, La18Fe5Cu4S26O8, was synthesized by solid-state reaction, and is found to crystallize in a novel type structure. X-Ray diffraction on powder and single crystal samples reveal an orthorhombic lattice, which can be described with the Cmcm (no. 63) space group, with lattice constants a = 9.7999(2) Å, b = 26.1186(3) Å and c = 9.7767(1) Å. The matrix of the crystal structure is built up of layers of [La9O4]-units connected by sulfur ions and interspaced with transition metal arrangements. The structure exhibits a guest-in-host arrangement where the guest constitutes 1D chains of transition metal ions with substantial static disorder, which can be simulated using several split positions with partial occupancies. AC and DC magnetic susceptibility (χ) measurements reveal the formation of magnetic spin domains below ∼15 K; AC data indicate that the domain wall motions are thermally activated because Arrhenius-like frequency dependencies on both the real and imaginary parts of χ(T) are observed. However, the spin domains should have relatively large sizes because there is a significant entropy release during the domain formation, as indicated by specific heat measurements. Temperature-dependent 57Fe-Mössbauer spectroscopy data reveal that the compound lacks magnetic order down to 20 K, but exhibits signatures of spin-freezing at 6 K. Hence, it is suggested that the title compound exhibits large magnetic clusters with significant spin dynamics at high temperatures, which freeze into a disordered state at low temperatures.

• Sannes, Johnny Andreas; Chatzitakis, Athanasios; Andersen, Niels Højmark; Nilsen, Ola; Frøen, Emil Herman & Valldor, Björn Martin (2024). Characterization and Solid-State UV-Vis Investigations of Photoelectrocatalytically Active La5Cl7[TeO3]4, a Mixed Anion Compound with Alternating 2D Layers of Oxygen and Chlorine. Inorganic Chemistry. ISSN 0020-1669. 63(40), p. 18632–18641. doi: 10.1021/acs.inorgchem.4c02392. Full text in Research Archive Show summary

  An oxide chloride, La5Cl7[TeO3]4, was synthesized using the conventional high-temperature solid-state synthesis technique in an inert atmosphere. This compound possesses a novel crystal structure that can be described with the triclinic space group P1̅ (No. 2) and unit cell parameters: a = 7.2634(3) Å, b = 8.1241(3) Å, c = 9.1993(3) Å, α = 79.373(1)°, β = 83.599(1)°, and γ = 82.511(1)°. The preference of Te(IV) to coordinate to oxygen and direct its lone pair toward the lower charged chlorine results in 2D layers of both oxygen and chlorine, alternating along the crystallographic b-direction. Homoleptic coordination, solely to oxygen, and heteroleptic coordination to oxygen and chlorine are observed for lanthanum, forming layers connected through edge-sharing polyhedra. In the crystal structure, two distinct tellurium positions are observed, with three close Te–O distances, emphasizing an active lone pair. The compound has been investigated by solid-state UV–vis measurements, and a band gap of 3.44 eV has been determined by DFT calculations. Detailed photoelectrochemical measurements clearly indicate that the title compound is photoelectrocatalytically active, showing an n-type behavior. Raman spectroscopy confirms that complex tellurite ions are present in the crystal structure; several observed bands can be assigned to Te–O stretching, reflecting the relatively low crystallographic symmetry of the title compound.

• Frøen, Emil Herman; Adler, Peter & Valldor, Björn Martin (2023). Synthesis and Properties of Ba<inf>6</inf>Fe<inf>2</inf>Te<inf>3</inf>S<inf>7</inf>, with an Fe Dimer in a Magnetic Singlet State. Inorganic Chemistry. ISSN 0020-1669. 62(31), p. 12548–12556. doi: 10.1021/acs.inorgchem.3c01775. Full text in Research Archive Show summary

  A new quaternary sulfide telluride, Ba6Fe2Te3S7, was synthesized by a solid-state reaction, and its crystal structure is novel. X-ray diffraction data on powder and single crystals reveal an orthorhombic lattice with a = 9.7543(3) Å, b = 18.2766(6) Å, and c = 12.0549(4) Å, and the noncentrosymmetric space group Cmc21 (No. 36). The properties of the compound were studied by magnetic susceptibility investigations, specific heat measurements, Mössbauer spectroscopy, and density functional theory calculations. Assuming Ba2+ and, as verified by the Mössbauer spectra, Fe3+, the charge balance requires the presence of a polytelluride, suggested to be a straight-chain [Te34–] polyanion. Further, the crystal structure contains [Fe2S7]8– dimers of two vertex-sharing tetrahedra, with a nearly linear Fe–S–Fe atom arrangement. The dimer exhibits antiferromagnetic coupling, with a coupling constant J = −10.5 meV (H = −2JS1S2) and S = 5/2, resulting in a spin singlet ground state. The interdimer magnetic interaction is so weak that the magnetic dimers can be treated as individuals.

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• Frøen, Emil Herman; Adler, Peter & Valldor, Björn Martin (2023). The Absence of Expected Paramagnetic Behaviour in Ba6Fe2Te3S7. Full text in Research Archive
• Frøen, Emil Herman; Norby, Truls Eivind; Bjørheim, Tor Svendsen & Løvvik, Ole Martin (2019). Coexistent Oxide p-n Junctions: DFT Analysis of Interfaces Between Nickel Oxide and Zinc Oxide. Universitetet i Oslo. Full text in Research Archive Show summary

  NiO and ZnO are a pair of p- and n-type wide band gap metal oxide semiconductors that forms a partially soluble system with each other. Due to these properties, they are considered a potential semiconductor pair for the construction of a p-n heterojunction that remains thermodynamically stable under high-temperature applications. To this end, an in-depth understanding of the properties of the available interface arrangements, as well as of the effects of the formation of the mixed phases on the properties of the semiconductors is necessary. This study seeks to investigate these aspects by a theoretical approach, utilizing DFT to model the properties of the possible interfaces, as well as the behavior of the mixed phase systems at different concentrations of solute ions. The bulk properties of nickel substituted ZnO (Ni:ZnO) and zinc substituted NiO (Zn:NiO) were investigated utilizing a hybrid functional approach. Both systems were found to exhibit an increase in the VBM energy level relative to the pure materials, with maxima of 0.39 eV and 0.26 eV, respectively. The VBM change with solute ion concentrations has been found to reach saturation at 3.70% and 4.69%, again respectively. The VBM shifts of Ni:ZnO and Zn:NiO are found to be caused by the formation of a new valence band due to localized impurity states, and the formation of cubic ZnO bonding states, respectively. For both mixed phase materials, a reduction of the band gap width is observed: The Ni:ZnO band gap is reduced by 0.3 eV, while the Zn:NiO band gap changes linearly with concentration throughout the experimental concentration range, with a band gap reduction of 0.8 eV at 31.25% zinc concentration. The structural, energetic and electronic properties of the NiO-ZnO interfaces formed from pairs of low-Miller index surfaces were investigated utilizing a GGA+U functional approach. The polar-polar interfaces were found to exhibit the most favorable interface formation energies, for either strain distribution considered. The natural band offsets were found to all form a type II heterojunction with valence band offset values ranging from 0.73 to 1.84 eV. The majority of the considered interface arrangements exhibit considerable interface states: Conduction band interface states are the most prevalent, formed from nickel 3d-orbitals. The results indicate that the prevalence of these interface states, and the general NiO-ZnO heterojunction properties, may be manipulated to a considerable extent by choosing a suitable interface, and the growth conditions under which it is formed.

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