Lattice dynamics and phase transitions

Investigation of thermal expansion, phase diagrams, and barocaloric effect in the (NH$_4$)$_2$WO$_2$F$_4$ and (NH$_4$)$_2$MoO$_2$F$_4$ oxyfluorides

M. V. Gorev^ab, E. V. Bogdanov^a, I. N. Flerov^ab, A. G. Kocharova^a, 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 thermal expansion along the principal crystallographic axes of the (NH$_4$)$_2$WO$_2$F$_4$ and (NH$_4$)$_2$MoO$_2$F$_4$ oxyfluorides has been studied. The anomalous behavior of $\alpha_i(T)$ due to the phase transitions has been revealed at $T_1$ = 271.4 K and $T_2\approx$ 180 K for the molybdate and at $T_1$ = 201.5 K and $T_2\approx$ 161 K for the tungstate. The quantities $dT/dp$ and $dT/d\sigma_i$, which characterize the dependence of the phase transition temperatures on the hydrostatic and uniaxial pressures, have been determined from analyzing the results of studies of the thermal expansion and heat capacity with the use of the Pippard relations. The $T$–$p$ and $T$–$\sigma_i$ phase diagrams reflect different characters of the influence of the pressure on the stability of the initial and distorted phases of the oxyfluorides. The magnitudes of the extensive and intensive barocaloric effects determined in the vicinity of the structural phase transitions are as follows: $\Delta S_{\mathrm{BCE}}$ varies from approximately -10 to -17 J/mol K and $\Delta T_{\mathrm{AD}}\approx$ 8–17 K for the molybdate and $\Delta S_{\mathrm{BCE}}$ varies from approximately -10 to -17 J/mol K and $\Delta T_{\mathrm{AD}}\approx$ 8–13 K for the tungstate.

Received: 28.04.2009
Accepted: 09.06.2009

 English version: Physics of the Solid State, 2010, 52:1, 167–175

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