АNISOTROPY OF ELASTIC AND ACOUSTIC PROPERTIES OF AgGaTe2 CRYSTALS
DOI:
https://doi.org/10.32782/pet-2022-1-9Keywords:
semiconductor, chalcopyrite, elastic properties, anisotropyAbstract
Within the framework of the density functional theory using the CASTEP program, which is based on the pseudopotential method with the basis of Bloch-type plane waves, first-principle calculations of the elastic properties of the AgGaTe2 crystal were carried out. The full matrix of elastic stiffness coefficients Сij was obtained using the generalized gradient approximation as an exchange-correlation functional. It is shown that the calculated elastic stiffness coefficients satisfy the Born criterion of material stability for tetragonal symmetry. The coefficients of elastic stiffness were used to estimate the linear compressibility ka and kc along the axes a and c, respectively. The calculated values of compressibility are as follows: ka = 8.9 × 10–3 GPa–1 and kс = 10.3 × 10–3 GPa–1, for a and c directions, respectively. Hence, the volumetric compressibility for the AgGaTe2 crystal is k = 28.1 × 10–3 GPa–1. It is shown that the studied crystal has significant anisotropy of elastic properties. The analysis of anisotropy was carried out for the purpose of its quantitative and qualitative assessment. The universal anisotropy index AU was calculated, which characterizes the anisotropy of both the bulk modulus of the crystal and the anisotropy of the shear modulus. For the AgGaTe2 crystal, the universal anisotropy index is equal to 1.015, which is significantly different from zero and indicates significant anisotropy. Usually, the main contribution to the anisotropy of elastic properties is given by the shear modulus G. Spatial 3D distributions of elastic moduli such as Young's modulus E, bulk modulus B, and shear modulus G of the crystal were constructed and analyzed. It is shown that the bulk modulus for the crystal is characterized by a spherical surface, which corresponds to the almost isotropic case. The largest anisotropy was obtained for the distribution of the shear modulus G. The distributions of the propagation velocities of acoustic waves in the crystal were calculated for pistons (100) and (001) and their anisotropy was analyzed. The theoretical Debye temperature of the investigated crystal was obtained.
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