Structural, electronic, optical and thermodynamic properties of NaxRb1−xH and NaxK1−xH alloys

A theoretical study of the structural, electronic, optical and thermodynamic properties of NaxRb1−xH and NaxK1−xH ternary alloys in NaCl phase has been carried out using the first-principles method. We modeled the alloys at some selected compositions with ordered structures described in terms of periodically repeated supercells. The dependences on the composition of the lattice constant, band gap, dielectric constant, refractive index, Debye temperature, mixing entropy and heat capacities were analyzed for x=0, 0.25, 0.50, 0.75 and 1. The lattice constants of NaxRb1−xH and NaxK1−xH exhibit a marginal deviation from Vegard's law. A strong deviation of the bulk modulus from linear concentration dependence was observed for both alloys. We found that the composition dependence of the energy band gap is highly non linear and the large bowing coefficient for NaxRb1−xH is sensitive to the composition. Using the approach of Zunger and co-workers, the microscopic origins of the gap bowing were detailed and explained. The thermodynamic stability of these alloys was investigated by calculating the phase diagram. The thermal effect on some macroscopic properties was investigated using the quasi-harmonic Debye model. There is a good agreement between our results and the available experimental data for the binary compounds, which is a support for those of the ternary alloys that we report for the first time.

► Fundamental properties of NaxRb1−xH and NaxK1−xH are investigated.
► Energy band gaps using EV-GGA are larger than those within PBE-GGA.
► Microscopic origins of the gap bowing were examined.
► Heat capacities and mixing entropy are obtained.