NH4FEPO4F - STRUCTURAL STUDY AND MAGNETIC-PROPERTIES

Hitherto unknown NH4FePO4F has been isolated by low-temperature hydrothermal synthesis. It is orthorhombic and acentric (space group Pna2(1) (No. 33); a = 12.993(3) angstrom, b = 6.468(1) angstrom, c = 10.640(3) angstrom, V = 894.2(6) angstrom3, Z = 8). This compound belongs to the KTiOPO4 (KTP) structure type, with a strict O-F ordering, the Ti-O short bond in KTP being replaced by a Fe-F bonding. The two iron sites, clearly evidenced by Mossbauer spectroscopy, correspond to two different local situations with 2 F- in cis position for Fe(1) and in trans position for Fe(2). With such a configuration, the main magnetic coupling probably occurs within the chains of octahedra via Fe-F-Fe superexchange interactions only, like in AFeF5 and A2FeF5 compounds in which the Neel temperature is close to 10 K. However, the high magnetic transition temperature of NH4FePO4F(45(1)K) shows that the influence of super-superexchange coupling is rather important in this compound. (C) 1994 Academic Press, Inc.