Order-to-Disorder Transition and Hydrogen Bonding in the Jahn–Teller Active NH4CrF3 Fluoroperovskite

• Published in: Inorganic chemistry Vol. 63; no. 23; pp. 10594 - 10602
• Main Authors: Fjellvåg, Øystein S., Gonano, Bruno, Bernal, Fabian L. M., Amedi, Salah B., Lyu, Jike, Pomjakushin, Vladimir, Medarde, Marisa, Chernyshov, Dmitry, Marshall, Kenneth, Valldor, Martin, Fjellvåg, Helmer, Hauback, Bjørn C.
• Format: Journal Article
• Language: English
• Published: American Chemical Society 10.06.2024; ACS Publications
• ISSN: 0020-1669; 1520-510X; 1520-510X
• DOI: 10.1021/acs.inorgchem.4c00931

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.