INSTABILITIES IN THE ELPASOLITE STRUCTURES: $Cs_{2}LiCo(CN)_{6}$ AND $Cs2LiIr(CN)_{6}$
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Date
1978
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Ohio State University
Abstract
The structural phase transformations in $Cs_{2}LiCo(CN)_{6}$ and $Cs_{2}LiIr(CN)_{6}$ have been probed using Raman scattering and optical microscopy. The $Ir^{+3}$ salt undergoes two phase transformations $(T^{1}_{c}= 418^{\circ}K, T^{2}_{c}= 335^{\circ}K)$ while the $Co^{+3}$ salt is observed optically to have one phase change $(T^{1}_{c}= 183^{\circ}K)$. The $418^{\circ}\;K$ phase change in the $Ir^{+3}$ salt is 1st order involving rotation of the $Ir(CN)_{6}^{-3}$ octahedra. The $335^{\circ}\;K$ transition is 2nd order with the structural distortion proceeding along a soft $Cs^{+}$ translational mode. The phase transformation in $Cs_{2}LiCo(CN)_{6}$ involves a simultaneous distortion among the same two phonons. The structural instabilities for these two salts are the same as those observed for other $R_{2}MM ^{\prime} L_{6}$ salts indicating that materials with the elpasolite structure may share a common lattice instability. The transition temperatures for $Cs_{2}LiM(CN)_{6}(M= Cr^{+3}\rightarrow Co^{+3})$ salts correlate with the size of the $Cs^{+}$ site, as is consistent with the earlier suggestion that the major driving force for the instabilities in these materials results from $Cs^{+}$ atoms occupying too large a site in the cubic lattice. As the $Cs^{+}$ site size decreases, the high temperature cubic cell is stabilized thereby lowering the critical temperatures for the phase transformations. The correlation of $Cs^{+}$ site size and the critical temperature is not maintained in comparison of the $Cr^{+}$ and $Ir^{+3}$ salts. The unusual behavior of the $Ir^{+3}$ material has been attributed to a cooperative interaction of the external phonon instabilities with the internal forces of the $Ir(CN)_{6}^{-3}$ complexion.
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Author Institution: Department of Chemistry, The University of Texas