Single-crystal Ca3Ge3O9 calcium germanate: structure, thermal and optical properties from experimental and first-principles analyses

Abstract

Single-crystal calcium germanate (Ca₃Ge₃O₉) was successfully synthesized by spontaneous crystallization. A combination of experimental and first-principles analyses was used to characterize its structure, thermal stability, and optical properties. Ca₃Ge₃O₉ crystallizes in the triclinic system with space group P, forming a stable three-dimensional framework built from oxygen-sharing GeO₄ tetrahedra and CaO₆ octahedra. Thermogravimetric analysis (TGA) indicates a decomposition temperature of 958.9 °C, demonstrating excellent high-temperature stability. Optical measurements reveal transmittance exceeding 85% for wavelengths above 770 nm. Based on density functional theory (DFT), calculations using the Perdew–Burke–Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) show that Ca₃Ge₃O₉ is an indirect bandgap semiconductor with a bandgap of 3.604 eV. In addition, Ca₃Ge₃O₉ exhibits a high absorption coefficient in the ultraviolet (UV) region, reaching approximately 10⁵ cm⁻¹. This strong absorption capability indicates that Ca₃Ge₃O₉ is a promising material for ultraviolet light detectors.