Anisotropic thermal anharmonicity of CdSiP_2 and ZnGeP_2: Ab initio calculations

摘要

The anisotropic thermal anharmonicity of CdSiP_2 and ZnGeP_2 has been studied by calculating the a- and c-axial Grueneisen parameters separately to cast light on the mechanism of anisotropic thermal expansivity of ABC_2 chalcopyrite compounds. Both the Debye model and lattice dynamics theory were implemented to calculate the axial Griineisen parameters. The variation of shear modulus, calculated from the Debye model, demonstrated normal behavior for ZnGeP_2 but abnormal behavior for CdSiP_2, and was thus assumed to be the most important parameter that determines anisotropic thermal anharmonicity. Using phonon frequency-based lattice dynamics, the axial mode Griineisen parameters were calculated for not only the Γ-point but also for other K-ppoints in the first Brillouin zone. The lowest B_1 and B_2 modes of both compounds were found to be new soft modes that were not observed in previous studies of volume-dependent mode Griineisen parameters. The larger magnitude of these soft mode Griineisen parameters of CdSiP_2 was responsible for its greater abnormal axial thermal anharmonicity in the low-temperature range. The Grueneisen parameters became positive at 110K for CdSiP_2 and 80 K for ZnGeP_2.