FLUFFY – Fluoridbaserte Na-ion batterikatoder for stasjonær energilagring

• Fjellvåg, Øystein Slagtern; Saha, Dipankar; Chernyshov, Dmitry; Bernal, Fabian Leonardo Martinez; Fjellvåg, Helmer & Hauback, Bjørn Christian [Vis alle 7 forfattere av denne artikkelen] (2025). On the low temperature crystal structure of KCrF3. Journal of Solid State Chemistry. ISSN 0022-4596. 352. doi: 10.1016/j.jssc.2025.125558. Fulltekst i vitenarkiv Vis sammendrag

  We report on the low-temperature phase transition and crystal structure of KCrF3, revisiting the previously reported reentrant tetragonal transition at 5 K. Using high-resolution X-ray powder diffraction across a temperature range of 5–300 K in two independent experiments, we study the structural developments and apply symmetry-mode analysis to describe the transition from tetragonal to monoclinic. A tilt of the CrF6-octahedra drives the symmetry reduction. Evaluation of the bond lengths and angles shows no sign of a reentrant or incipient transition. Our findings confirm that the low-temperature phase of KCrF3 is monoclinic.

• Fjellvåg, Øystein Slagtern; Park, Heesoo; Veillon, Fabien; Lyu, Jike; Medarde, Marisa & Amedi, Salah Bra [Vis alle 10 forfattere av denne artikkelen] (2025). ACrF3 Jahn–Teller-Distorted Fluoroperovskites: Expanding to RbCrF3 and CsCrF3. Inorganic Chemistry. ISSN 0020-1669. 64(36), s. 18316–18322. doi: 10.1021/acs.inorgchem.5c02668. Fulltekst i vitenarkiv Vis sammendrag

  The Jahn–Teller effect significantly affects the CrF6 octahedra in Cr(II) fluoroperovskites. Here, we report the synthesis, crystal structures, and magnetic properties of RbCrF3 and CsCrF3, thereby completing a comprehensive investigation of the ACrF3 fluoroperovskites. Powder samples are prepared using a wet-chemical method, which allows stabilization of Cr(II). RbCrF3 predominantly adopts the P4/mbm space group; however, the absence of the expected (211) peak suggests a limited correlation length, and we demonstrate that this is a result of stacking faults in the rotation of the distorted CrF6 octahedra along the c-axis. We estimate around 5–10% of I4/mcm-type stacking in P4/mbm for RbCrF3. In contrast, CsCrF3 showcases a two-phase coexistence of I4/mcm and P4/mbm polymorphs and, as such, parallels to KCuF3. We discuss and highlight the interplay between the impact of A-site cations and the Jahn–Teller effect acting on CrF6 octahedra. Antiferromagnetic interactions dominate the paramagnetic regime, and RbCrF3 and CsCrF3 both have a Weiss temperature of −12 K, associated with Néel temperatures of 50 and 47 K, respectively. The effective paramagnetic moments of 4.65 and 4.68 μB for RbCrF3 and CsCrF3 are in good agreement with S = 2 spin configuration.

• Gonano, Bruno; Fjellvåg, Øystein Slagtern; Steciuk, Gwladys; Marshall, Kenneth; Fjellvåg, Helmer & Valldor, Björn Martin (2025). PVDF fluorination of La₄Co₃O₁₀: Synthesis, structure and physical properties of La₄Co₃O₁₀F₂. Scripta Materialia. ISSN 1359-6462. 255. doi: 10.1016/j.scriptamat.2024.116379. Fulltekst i vitenarkiv Vis sammendrag

  Topochemical fluorination of oxides recently gained interest as it opens the way to substantial structural modifications and drastic changes in physical properties. We present a study of La4Co3O10F2, synthesized via low-temperature topochemical methods using polyvinylidene difluoride (PVDF). Using synchrotron X-ray powder diffraction, we find the compound to crystallize in a monoclinic cell (A2/a, with a = 5.32969(9) Å, b = 5.37555(1) Å, c = 30.5958(6) Å and β = 90.994(1) °), where the fluoride anions occupy the rock-salt layer's interstitial sites, inducing a tilting of the CoO6 octahedra. The physical properties of this novel compound are compared with those of the parent compound La4Co3O10, evidencing changes in resistivity and magnetism, highlighting the possibility of tuning magnetic interactions and electronic correlations through PVDF fluorination.

• Fjellvåg, Øystein Slagtern; Gonano, Bruno; Bernal, Fabian Leonardo Martinez; Amedi, Salah Bra; Lyu, Jike & Pomjakushin, Vladimir [Vis alle 12 forfattere av denne artikkelen] (2024). Order-to-Disorder Transition and Hydrogen Bonding in the Jahn-Teller Active NH4CrF3 Fluoroperovskite. Inorganic Chemistry. ISSN 0020-1669. 63(23), s. 10594–10602. doi: 10.1021/acs.inorgchem.4c00931. Fulltekst i vitenarkiv Vis sammendrag

  Large quantities of high-purity NH4CrF3 have been synthesized using a wet-chemical method, and its structural chemistry and magnetic properties are investigated in detail for the first time. NH4CrF3 is a tetragonal fluoroperovskite that displays an ordering of the ammonium (NH4+) groups at room temperature and C-type orbital ordering. The ammonium groups order and display distinct signs of hydrogen bonds to nearby fluoride anions by buckling the Cr–F–Cr angle away from 180°. The ammonium ordering remains up to 405 K, much higher than in other ammonium fluoroperovskites, indicating a correlation between the flexibility of the Jahn–Teller ion, the hydrogen bond formation, and the ammonium ordering. At 405 K, an order-to-disorder transition occurs, where the ammonium groups disorder, corresponding to a transition to higher symmetry. This is accompanied by a contraction of the unit cell from breaking hydrogen bonds, similar to the phenomenon observed in water ice melting. The compound orders antiferromagnetically with a Neél temperature of 60 K, an effective paramagnetic moment of 4.3 μB, and a Weiss temperature of −33 K. An A-type antiferromagnetic structure is identified by neutron diffraction, with an ordered moment of 3.72(2) μB.

Se alle arbeider i NVA

• Fjellvåg, Øystein Slagtern (2025). Cr(II) fluoroperovskites – Jahn-Teller active materials. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern (2025). Investigations of Cr(II) fluoroperovskites and other systems at SNBL . Fulltekst i vitenarkiv

Se alle arbeider i NVA