An ab-initio investigation of mechanical and thermodynamic properties of Ag_2MgSn(S/Se)_4 in kesterite and stannite phases

摘要

In this work, mechanical and thermodynamic properties and phonon calculations of quaternary chalcogenide material Ag_2MgSn(S/Se)_4 in kesterite and stannite phases are calculated using density functional theory with generalized gradient approximation (GGA) functional. Elastic constants are calculated for all the four crystal structures, and it is found that they are mechanically stable in accordance with Born-Huang stability criterion. Also, it is observed that the strength of bonds along the [100] and [010] crystalline directions is as strong as the bonds in the [001] crystalline direction for all structures. The sum of elastic constants C_(11) and C_(12) is greater than C_(33) showing that elastic modulus calculated in the (001) plane is more compared to its values along the c-axis. With Pugh's theory analysis, it is found that the B/G ratio is greater than 1.75 for all four structures, and they are ductile in nature. Using the Debye model, vibrational heat capacity at constant volume and coefficient of thermal expansion are studied in the temperature range of 0-1200 K for all four structures. Also, the vibrational modes and phonon dispersion curve along the high symmetry point (Z-Γ-X-P-Γ-N) are analysed to investigate the stability of the structures. These findings should be useful in the fabrication of thin-film solar cells using these materials as absorber layer.