Lattice dynamics and phase transitions

Calorimetric and dielectric studies of the (NH$_4$)$_2$MoO$_2$F$_4$ oxyfluoride

V. D. Fokina^ab, E. V. Bogdanov^a, E. I. Pogoreltsev^b, V. S. Bondarev^ab, I. N. Flerov^ab, N. M. Laptash<sup>c

a</sup> L. V. Kirensky Institute of Physics, Siberian Branch of the Russian Academy of Sciences, Krasnoyarsk
^b Siberian Federal University, Krasnoyarsk
^c Institute of Chemistry, Far East Branch of the Russian Academy of Sciences, Vladivostok

Abstract: The temperature dependences of the heat capacity, dielectric properties, and response to an external pressure and an electric field for the (NH$_4$)$_2$MoO$_2$F$_4$ oxyfluoride (space group Cmcm, Z = 4) have been studied. A comparative analysis of the data on the entropy of phase transitions, $p$–$T$ phase diagrams, permittivity, and anomalous heat capacity in combination with the results of previous studies of the related compounds (NH$_4$)$_2$WO$_2$F$_4$ and (ND$_4$)$_2$WO$_2$F$_4$ has made it possible to establish that both [MO$_2$F$_4$]$^{2-}$ anions and ammonium groups play a substantial role in the mechanism and nature of the structural transformations.

Received: 02.04.2009
Accepted: 15.05.2009

 English version: Physics of the Solid State, 2010, 52:1, 158–166

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