Polymorphism of BaTeO ₃ under High Pressure: Single-Crystal Structure Analysis and Characterization of HP-BaTeO ₃

A new high-pressure polymorph of barium tellurate, HP-BaTeO₃, was synthesized using multianvil high-pressure/high-temperature techniques (4 GPa, 900 °C). The compound crystallizes in the monoclinic space group <i>P</i>2₁/<i>c</i> and consists of stacked trigonal pyramidal [TeO₃]^2- units interconnected by secondary bonds. Structural analysis identifies significant differences between HP-BaTeO₃ and its ambient-pressure polymorph, BaTeO₃(I), including a doubling of the <i>c</i>-axis and additional secondary bonding within the <i>bc</i> plane. The optical properties of HP-BaTeO₃ were investigated using ultraviolet-visible spectroscopy, revealing a widened bandgap compared to BaTeO₃(I), attributed to changes in orbital overlap and lone pair orientation. Thermal analysis and high-temperature powder X-ray diffraction confirmed the metastable nature of HP-BaTeO₃, with a phase transition to BaTeO₃(I) occurring at approximately 550 °C. This study highlights the structural and electronic modifications induced by high-pressure synthesis and provides insights into the relationship between the two polymorphs.