Dielectrics

The role of structural and nonstructural water in oxyfluoride K$_2$WO$_2$F$_4$ $\cdot$ H$_2$O

S. V. Melnikova, A. D. Vasil'ev, A. G. Kocharova

L. V. Kirensky Institute of Physics, Siberian Branch of the Russian Academy of Sciences, Krasnoyarsk

Abstract: X-ray structural and polarization optical investigations have been performed, and birefringence and rotation angles of the optical indicatrix $\varphi_b$ and $\varphi_c$ of the K$_2$WO$_2$F$_4$ $\cdot$ H$_2$O crystal have been measured in the temperature range of 100–600 K. The structure and symmetry of compounds at room temperature have been refined. It has been established that the layered crystal K$_2$WO$_2$F$_4$ $\cdot$ H$_2$O can exist in two states (A and B) depending on the atmospheric humidity and undergoes the sequence of reversible and irreversible phase transformations G$_3$ $\leftrightarrow$ G$_2$ $\to$ G$_1$ $\to$ G$_0$. The sequences of changes in the phase symmetry $P\bar1\leftrightarrow C2/m\to P4/nmm$ for samples A and $m\leftrightarrow C2/m\to P4/nmm$ for samples B have been found. The second-order proper ferroelastic phase transition $(P\bar1\leftrightarrow C2/m)$ at $T_{03}$ = 270–290 K $(G_3\leftrightarrow G_2)$ is accompanied by twinning and appearance of the shift deformation $x_6$. The crystal system of the substance for the B crystals remains invariable after the second-order phase transition $G_3\leftrightarrow G_2$. The irreversible first-order phase transition $G_2\to G_1$ occurs in a temperature range $T_{02}\approx$ 350–380 K; it is accompanied by the loss of the crystallization water, which then is reduced easily from the atmosphere for a day. The substance decomposes at $T_{01}\approx$ 510 K $(G_1\to G_0)$. The distinction between the A and B crystals has been explained by the presence or absence of free water in interlayer spacings.

Received: 24.05.2011

 English version: Physics of the Solid State, 2011, 53:12, 2435–2440

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