Optical anisotropy in Mo-Te-O frameworks constructed from distorted MoO₆ and TeO₃ units

Two new Mo-Te-O compounds, (NH₄)₂MgMo₄Te₂O₁₈ and β-MoTe₂O₇, have been synthesized and structurally characterized as frameworks constructed from distorted MoO₆ octahedra and TeO₃ trigonal pyramids with stereochemically active lone pairs. The two phases feature distinct Mo-O-Te connectivity and dimensionality, leading to different degrees of optical anisotropy. Polarized optical microscopy measurements reveal that both compounds exhibit moderate birefringence, which is further rationalized by density functional theory calculations. The incorporation of Mg²⁺ in (NH₄)₂MgMo₄Te₂O₁₈ results in a more structurally elaborate framework, and this compound shows a black crystal surface with a white interior, indicative of surface-related electronic effects. Combined experimental and theoretical results highlight the role of cooperative MoO₆-TeO₃ distortions and framework connectivity in governing optical anisotropy in Mo-Te-O oxides.