LOW-TEMPERATURE SYNTHESES OF NBFEO4 AND TAFEO4 - INFLUENCE OF RECRYSTALLIZATION ON THE MAGNETIC-PROPERTIES

Hydroxides of iron and niobium, or iron and tantalum, precipitate by adding NH4OH to the metal oxalate solutions and give NbFeO4 and TaFeO4 by decomposition. Owing to the thermal treatment, different samples are obtained. NbFeO4 of α-PbO2-type structure crystallizes at 700°C with large diffraction lines while a very well-crystallized disordered phase is obtained by heating the precipitate at 1100°C for half an hour and quenching it down to room temperature. The wolframite ordered form is obtained at 1000°C. A pure rutile TaFeO4 is obtained at 1100°C by avoiding annealings between 700 and 900°C. The low temperature phase of α-PbO2 structure crystallizes in very small quantities at 1000°C. The ordered NbFeO4 wolframite sample is antiferromagnetic below 50 K unlike the complete disordered ones. Intermediate behaviors are observed owing to the thermal treatment, particularly a ferromagnetism within iron clusters. The Curie constants extracted from the Curie-Weiss behavior are very below the theoretical Curie constants for disordered samples and for TaFeO4 indicating strong interactions between irons. © 1990.