High-Pressure Structural Stability and Optical Properties of Scheelite-type ZrGeO4 and HfGeO4 X-ray Phosphor Hosts

Abstract

Ab initio calculations were performed on the scheelite-type MGeO4 (M = Hf and Zr) compounds, which find a wide range of applications such as in X-ray imaging. We have studied the high-pressure structural stability, elastic constants, electronic structure, and optical properties of these compounds through density functional theory calculations. Two different density functional approaches, plane wave pseudopotential method (PW-PP) and full potential linearized augmented plane wave method (FP-LAPW), were used for the present study. The ground-state structural and vibrational properties are calculated and found to be in good agreement with experimental data. The compressibility of Zr and Hf germanates is found to be anisotropic as the a-axis is less compressible over the c-axis due to the presence of Ge–O bonds along the a-axis, which is further confirmed from the ordering of the elastic constants that follows C11 > C33. The electronic structure of the compounds has been calculated through recently developed Tran Blaha-modified Becke Johnson potential. The calculated electronic structure shows that the compounds are insulators with a gap of 5.39 eV for ZrGeO4 and 6.25 eV for HfGeO4, respectively. Optical anisotropy of these compounds is revealed from the computed optical properties such as complex dielectric function, refractive index, and absorption coefficient. In addition, it is observed that Ti-doped ZrGeO4 and HfGeO4 turn out to be good phosphors as the pristine compounds have an energy gap greater than the visible range; upon Ti doping, the band gap reduces, and, as a result, emission spectra occur in the visible region and are well explained in the present study.