Investigation of the structural phase transitions in the ammonium trifluorides of zinc, manganese and cobalt by means of X-ray and neutron diffraction

Abstract

The compounds NH₄MF₃ (M=Zn, Co, Mn) exhibit structural phase transitions at T_c=115.1, 124.5 and 181K respectively. The crystal structures have been determined at 4.2 and 300K by neutron diffraction using powdered samples. At 300K the compounds have the ideal cubic perovskite structure, while below T_c they are tetragonal (c/a>1). In the Co and Mn compounds an antiferromagnetic spin structure of type G has been observed at 4.2K. No evidence could be found for the weak ferromagnetic moment in the Co compound as has been detected by susceptibility and magnetisation measurements. The change in cell dimensions with temperature has been followed using X-rays. The discontinuity of the cell dimensions at T_c indicates a first-order character of the structural transitions, in agreement with the hysteresis in the specific heat measurements. It is argued that the experimentally observed reorientation of the NH₄⁺ ions at the transition temperature is a consequence of the structural transition, which itself is driven by the instability of the cubic phase as expressed by the Goldschmidt tolerance factor.