Knowledge Bank

The structural phase transformations in $Cs2LiCo(CN)6$ and $Cs2LiIr(CN)6$ have been probed using Raman scattering and optical microscopy. The $Ir+3$ salt undergoes two phase transformations $(Tc1=418\circ K,Tc2=335\circ K)$ while the $Co+3$ salt is observed optically to have one phase change $(Tc1=183\circ K)$. 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 $Cs2LiCo(CN)6$ involves a simultaneous distortion among the same two phonons. The structural instabilities for these two salts are the same as those observed for other $R2MM\prime L6$ salts indicating that materials with the elpasolite structure may share a common lattice instability. The transition temperatures for $Cs2LiM(CN)6(M=Cr+3\to Co+3)$ 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.