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Optical properties
Influence of atmospheric humidity on the symmetry and phase transitions of layered potassium oxyfluorides K$_2$NbOF$_5$ $\cdot$ H$_2$O
S. V. Melnikovaa,
N. M. Laptashb a L. V. Kirensky Institute of Physics, Siberian Branch of the Russian Academy of Sciences, Krasnoyarsk
b Institute of Chemistry, Far East Branch of the Russian Academy of Sciences, Vladivostok
Abstract:
Crystals of K
$_2$NbOF
$_5$ $\cdot$ H
$_2$O have been grown, polarization optical investigations have been performed, and the birefringence and rotation angle of the optical indicatrix have been measured in the temperature range 100–400 K. It has been found that, depending on the degree of atmospheric humidity, the layered K
$_2$NbOF
$_5$ $\cdot$ H
$_2$O crystal at room temperature can be in three states, namely,
$A$,
$B$, and
$C$, which differ in symmetry and properties of the crystal. The K
$_2$NbOF
$_5$ $\cdot$ H
$_2$O crystal in the A state exists at a relative humidity RH = 90–100% and undergoes a first-order improper ferroelastic phase transition
$P\bar 1$ $\leftrightarrow$ $C2/m$, which is accompanied by strong anomalies of the optical characteristics, twinning, and shear strain
$x_6$ at temperatures
$T_2^\downarrow$ = 308 K and
$T_2^\uparrow$ = 313 K. The most stable state of the K
$_2$NbOF
$_5$ $\cdot$ H
$_2$O crystal is the
$B$ state (RH = 20–90%), which retains the monoclinic symmetry
$C2/m$ in the temperature range 100–370 K. In a dry atmosphere (RH = 0–20%) or at
$T_1\approx$ 370 K, the crystal becomes anhydrous (K
$_2$NbOF
$_5$) with the symmetry
$P4/nmm$ (the
$C$ state). The difference between the crystals in the states
$A$ and
$B$ is explained by the presence or absence of water molecules in interlayer spaces.
Received: 22.11.2012
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