First-principles and experimental characterization of the electronic properties of CaGaSiN and CaAlSiN: the impact of chemical disorder.

We report a detailed investigation of the electronic, mechanical and optical properties of the recently discovered nitridogallosilicate CaGaSiN which has potential as a LED-phosphor host material. We focus on chemical disorder effects, originating from the Ga/Si site, and compared them to those of isostructural CaAlSiN. We calculate the elastic moduli and the Debye temperature in terms of quasi harmonical approximation. Spectral properties like the joint density of states (JDOS) are evaluated and the absorption, reflectance and energy loss function are obtained from the dielectric function. The optical band gap of CaGaSiN from experiment is compared to the electronic band gap in terms of electronic DOS and band structure calculations. All properties are evaluated for different ordering models of Ga/Si while the experimentally observed substitutional disorder is accounted for by utilizing the Coherent Potential Approximation (CPA). We conclude a shrinking of the band gap for both CaGaSiN and CaAlSiN due to atomic disorder, which is unfavorable for potential phosphor applications. This study contributes to materials design considerations, and provides a close look on the electronic impact of substitutional disorder. Moreover, we open the scope for future investigations on solid solutions and phosphor host materials with low doping concentrations.