This study investigates the mechanical, thermal, and thermodynamic properties of zinc-blende (zb) CdS and CdSe using Density Functional Theory (DFT) within the LDA, PBE, and PBE+U approximations. All three functionals confirm the mechanical stability of both compounds, with PBE+U providing results that best align with available theoretical and experimental data. Based on PBE+U calculations, CdS exhibits higher stiffness (B = 71.75 GPa, E = 36.71 GPa, G = 12.99 GPa) and faster sound velocity ( \(\nu\) = 1828 \(\hbox {ms}^{-1}\) ) than CdSe (B = 53.85 GPa, E = 38.88 GPa, G = 14.13 GPa, \(\nu\) = 1746 \(\hbox {ms}^{-1}\) ). Temperature-dependent analyses using the quasi-harmonic approximation reveal anomalous thermal contraction at low temperatures, transitioning to normal expansion beyond the zero thermal expansion points (113.92 K for CdS and 61.50 K for CdSe). The electron chemical potential shows a non-monotonic temperature dependence with transition temperatures of 1483 K for CdS and 853 K for CdSe. Heat capacities approach the Dulong-Petit limit ( \(\approx\) 49 J \(\hbox {mol}^{-1}\) \(\hbox {K}^{-1}\) ) at high temperatures, with CdSe reaching this limit earlier due to its softer lattice. CdSe also displays higher entropy, consistent with its heavier atomic mass and enhanced anharmonicity. Overall, CdS is mechanically stiffer and thermally more stable, while CdSe exhibits greater vibrational disorder. Overall, CdS is mechanically stiffer and thermally more stable, while CdSe shows greater anharmonicity and entropy.
