Structural, elastic, electronic and vibrational properties of XAl2O4 (X = Ca, Sr and Cd) semiconductors with orthorhombic structure

Abstract

Structural, elastic, electronic and vibrational properties of XAl2O4 (X = Ca, Sr and Cd) compounds with orthorhombic structure are studied by first principles method within generalized gradient approximation. The calculated negative formation enthalpy for each compounds indicates the thermodynamical stability of the studied phase. Band structure calculations reveal that CaAl2O4, SrAl2O4 and CdAl2O4 compounds have a direct band gap of 4.86, 4.54 and 2.46 eV, respectively. Besides, from the analysis of the band gap values, one can notice that the replacement of Ca atoms by Sr and Cd atoms in these compounds reduces the band gap energy values. It is also observed that the CaAl2O4, SrAl2O4 and CdAl2O4 compounds are less compressible along b-axis and their compressibility decreases in the sequence SrAl2O4 > CdAl2O4 > CaAl2O4. Similarly, it is also noticeable that the CaAl2O4 compound have more resisting power against the monoclinic shear distortion along {100} plane and along {110} direction compared to CdAl2O4 and SrAl2O4 compounds. Moreover, Cauchy pressures confirm that the CaAl2O4 and SrAl2O4 compounds are ductile while the CdAl2O4 compound is brittle in nature. This fit very well with the forecast from B/G relation. The calculated elastic constants and the phonon dispersion relations of the studied compounds show that these compounds are both mechanically and dynamically stable. Moreover the temperature dependence of the specific heat and entropy have been discussed in detail and calculated Debye temperature is in good agreement with the related study in literature. (C) 2019 Elsevier B.V. All rights reserved.