Structural and Optical Properties of Bi₄Si_x/2Ge_x/2V_2-xO_11-x/2 Compounds: Effect of Double Substitution and Phase Transitions

The solid-state reaction method was employed to successfully synthesize a range of compounds, Bi₄Si_x/2Ge_x/2V_2-xO_11-x/2, where 0.1 ≤ x ≤ 0.6. The main aim of this investigation is to examine how the structural and optical properties are influenced by the double substitution on vanadium site. The analyses using room temperature X-ray diffraction (XRD) revealed existence of α-monoclinic and β-orthorhombic phases in the composition range of 0.1 ≤ x ≤ 0.3 and 0.4 ≤ x ≤ 0.5, respectively. The α to β structural transition, as a function of the substitution rate, was further confirmed through Raman and infrared spectroscopies at room temperature, which both verified the presence of these two distinct phases depending on the composition. The X-ray diffraction (XRD) analysis, versus temperature, highlighted the α → β and β → γ phase transitions with increasing the temperature. These phase transitions were confirmed by DTA and Raman investigations as a function of temperature. Diffuse reflectance spectroscopy showed that the band gap varies around 2.1eV and that the compound with x = 0.1 has the lowest value of 2.08 eV.