Research of thermodynamic properties of mid-infrared single crystal ZnGeP2

Chalcopyrite ZnGeP2 single crystal with sufficient size and quality was grown by modified vertical Bridgman method. The thermal expansion coefficients along a-axis and c-axis, αa and αc, were evaluated from the results of thermal dilatometer tests carried out in the range of 300-600 K. It is found that both αa and αc achieve positive values and αa is always larger than αc in the whole range indicating that the thermal expansion of the crystal exhibits anisotropic behavior. We selected the sample of c-axis and measured its thermal conductivity and molar specific heat. On this basis, the Grüneisen parameters of c-axis at different temperatures were calculated. Using these Grüneisen parameters, the lattice thermal conductivities have been deduced by one correction formula and we found that they are very close to the experimental results.