Anisotropy of the thermal expansion of CuIn$_{5}$Se$_{8}$ single crystals in two structural modifications

Thermal expansion in the temperature range 80–700 K is studied for two (trigonal and hexagonal) structural modifications of CuIn$_{5}$Se$_{8}$ single crystals grown by planar crystallization of the melt. From the data, the thermal-expansion coefficients are calculated for both modifications. It is established that, in the temperature range under study, the thermal expansion of both modifications is anisotropic. For the trigonal modification, the thermal-expansion coefficient in the direction of the $\mathbf{c}$ axis $(\alpha_{c})$ is larger than that in the direction of the orthogonal $\mathbf{a}$ axis $(\alpha_{a})$. For the hexagonal modification of the CuIn$_{5}$Se$_{8}$ crystal, the thermal-expansion coefficient in the direction of the $\mathbf{c}$ axis exhibits anomalous behavior: as the temperature is increased, the coefficient $\alpha_{c}$ increases, after which it decreases to negative values, reaches a minimum, and then increases further. Such behavior of the coefficient $\alpha_{c}$ is associated with the phase transformation of the hexagonal modification of the CuIn$_{5}$Se$_{8}$ compound into the trigonal modification.