Role of chemical pressure on optical and electronic structure of Ho2GexTi2−xO7

Abstract

Chemical pressure plays a crucial role in determining the electronic properties of the quantum materials. Investigation of electronic structure of Ho2GexTi2−xO7 (x = 2, 1.9, 1.75, 1.5 1, 0.5, 0.25, 0.1 and 0) series has been performed. Pyrochlore and Pyrogermanate, Re2B2O7 (Re = Ho3+, B = Ti4+ and Ge4+; rare earth titanates and germanates), substituted with increasing amount of Ge4+ at the Ti4+ site and vice versa develops structural distortions. Distinct shrinkage effect has been established in the Ho2Ti2O7 matrix upon Ge+4 substitutions at B site, resulting in the modification of band gap value. Band gap of 5.24 eV drastically drops to 3.92 eV with immediate Ti4+ substitution in Ho2Ge2O7. Electronic states of Ho3+ (4f forbidden transitions) had also been identified. We observe favored sub level transition (Specific Stark component) corresponding to5F5 to 5I8 electronic transition for Ho3+ at λexc. = 450 nm. The upper valence band consisted of O 2p state hybridized with Ho 5p and Ti and Ge 4p states and conduction band primarily formed by Ho 5d state hybridized with Ti 3d and Ge 4d states as obtained from density of states (DOS) calculations. Strong hybridization between Ho 5p1/2 and Ti 3p orbital upon Ti4+ inclusion in Ho2Ge2O7 has been observed through both theoretical studies using LDA—1/2 and UV–Vis, photoluminescence, ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy. The evolution of total DOS of all studied composition shows that valence band edge is more sensitive than conduction band to composition. These results provide chemical pressure as an excellent tool to tailor the band gap and fine tune the intermediate electronic states in Ho2GexTi2−xO7.