Investigation of the Thermal Properties and Crystal Growth of the Nonlinear Optical Crystals AgGaS2 and AgGaGeS4

The thermal properties of orthorhombic AgGaGeS4 and chalcopyrite AgGaS2 crystals, including thermal expansion, specific heat, and thermal conductivity, have been investigated. For the AgGaS2 crystal, we have accurately determined the thermal expansion coefficients alpha(a) and alpha(c) with a thermal dilatometer in the temperature range of 298-850 K. It is found that alpha(c) decreases with increasing temperature, which confirms the negative thermal expansion of a AgGaS2 crystal along the c axis, and we have given a reasonable explanation of the negative thermal expansion mechanism. Further, the least-squares method has been applied to get linear curve fittings for alpha(a) and alpha(c). In addition, we also have deduced the Gruncision parameters, specific heat capacity, and thermal conductivity of AgGaS2 and all of them exhibit anisotropic behavior. For AgGaGeS4, both high-temperature X-ray powder diffraction measurements and a thermal dilatometer were adopted to study the thermal expansion behavior of a AgGaGeS4 crystal, and we have compared the results of these two different test methods. Furthermore, the isobaric specific heat and harmonic specific heat of AgGaS2 and AgGaGeS4 were obtained by experiment and calculation, and the anharmonic part of the specific heat was obtained by fitting. In addition, lattice thermal conductivities of AgGaS2 in the temperature range of 298-850 K have been calculated, which are consistent with our actual measurement results at 298-473 K, while the values of thermal conductivity along [100], [010], and [001] of AgGaGeS4 at room temperature are found to be 0.629, 0.495, and 0.336 W/(m K), respectively. Finally, using an AgGaS2 single crystal seed with the c axis and AgGaGeS4 single crystal seeds with the a, b, and c axes, with an improved growth process according to the obtained thermodynamic parameters, we have grown high-quality AgGaS2 and AgGaGeS4 single crystals with a diameter of 40 mm and a length of 60-90 mm.