Narrow Results

Bonding nature as well as structural, optoelectronic and thermal properties of the cubic XMg₂O₄(X = Si, Ge) spinel compounds have been calculated using a full-potential augmented plane-wave plus local orbitals (FP-APW+lo) method within the density functional theory. The exchange-correlation potential was treated with the PBE-GGA approximation to calculate the total energy. Moreover, the modified Becke–Johnson potential (TB-mBJ) was also applied to improve the electronic band structure calculations. The computed ground-state parameters (a, B, B′ and u) are in excellent agreements with the available theoretical data. Calculations of the electronic band structure and bonding properties show that these compounds have a direct energy band gap (Γ-Γ) with a dominated ionic character and the TB-mBJ approximation yields larger fundamental band gaps compared to those obtained using the PBE-GGA. Optical properties such as the complex dielectric function ε(ω), reflectivity R(ω) and energy loss function L(ω), for incident photon energy up to 40 eV, have been predicted. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the effects of pressure P and temperature T on the thermal expansion coefficient, Debye temperature and heat capacity for the considered compounds are investigated for the first time.

The structural, electronic, elastic and phonon properties of the cubic spinels AB₂O₄ (A = Ge, Si; B = Mg, Zn, Cd) compounds at zero pressure are investigated via density functional theory (DFT) using the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional. It has been shown that the predicted values of the structural parameters ($a_0$ and $u$), bulk modulus $(B)$, elastic constants $(C_{ij})$, shear modulus $G$ and $B/G$ ratio are in good agreement with the previously reported results. The phonon dispersion curves of the AB₂O₄ (A = Ge, Si; B = Mg, Zn, Cd) are calculated for the first time using the direct method. The estimated phonon spectra indicate that GeMg₂O₄, GeZn₂O₄, GeCd₂O₄, SiMg₂O₄ and SiZn₂O₄ are dynamically stable in the cubic spinel structure.