Mechanism and nature of phase transitions in the (NH4)(3)MoO3F3 oxyfluoride

作者：Flerov I N^*; Fokina V D; Bovina A F; Bogdanov E V; Molokeev M S; Kocharova A G; Pogorel'tsev E I; Laptash N M

来源：Physics of the Solid State, 2008, 50(3): 515-524.

DOI：10.1134/S1063783408030219

The temperature dependences of the heat capacity, the unit cell parameter, and the permittivity for the (NH4)(3)MoO3F3 cryolite (space group Fm (3) over barm) are investigated. It is revealed that the compound undergoes ferroelectric and ferroelastic structural phase transitions at temperatures of 297 and 205 K, respectively. The mechanism of structural distortions is discussed in terms of the entropy parameters, pressure-temperature phase diagrams, and electron density maps for critical atoms. An analysis is made of the influence of the cation size and shape on the phase transitions in oxyfluorides of the general formula A(2)A'MO3F3 (A,A' = NH4, K; M = Mo, W).

• 出版日期2008-3