Rare 3 × 3 Layering in High-Pressure Pseudo-Perovskite Sr₄Te₄O₁₅

A novel strontium tellurate, Sr₄Te₄O₁₅, has been synthesized under high-pressure/high-temperature conditions using multianvil techniques (9 GPa, 700°C). This unique composition constitutes both Te⁴⁺ and Te⁶⁺ sites and crystallizes in the orthorhombic space group Pnma with unit cell parameters a = 1332.4(1), b =1236.4(1), and c = 748.02(9) pm. The crystal structure of Sr₄Te₄O₁₅ consists of heavily corrugated oxotellurate sheets made up of corner-sharing [TeO₆]^6- octahedra and [TeO₄]^4- bisphenoids separated by layers of Sr²⁺. Some of its structural features are comparable to those of 2D perovskites, and the deviation of the compound from this class is discussed in terms of both structure and composition. The stability of this compound was investigated through high-temperature powder X-ray diffraction and thermal analysis revealing its transformation to SrTeO₄ at elevated temperatures. Comprehensive characterization, including UV-Vis, IR, and Raman spectroscopy, as well as density functional theory calculations, was performed to investigate its electronic structure, vibrational properties, and electron localization of the Te⁴⁺ lone electron pairs. The direct and indirect bandgaps were determined to be 3.17 and 2.97 eV, respectively. The study provides insights into the structural and electronic properties of Sr₄Te₄O₁₅, contributing to the understanding of rare corrugated 2D pseudo-perovskite systems.