A new stable polymorph of Li2TeMo3O12 with wide mid-infrared transparency and a large Raman response

Abstract

Polymorphism is crucial for unraveling crystallization mechanisms, predicting crystal structures and discovering novel materials. A new high-symmetry polymorph of Li₂TeMo₃O₁₂, β-Li₂TeMo₃O₁₂, was identified successfully by a spontaneous crystallization technique, and it exhibits improved thermal stability and crystallizes in the trigonal space group R (No. 148) in contrast to the metastable α-Li₂TeMo₃O₁₂. Thermal analysis and variable-temperature X-ray diffraction powder patterns demonstrated an irreversible phase transition of Li₂TeMo₃O₁₂ from α-Li₂TeMo₃O₁₂ to β-Li₂TeMo₃O₁₂. High-quality and centimetre-sized single crystals of β-Li₂TeMo₃O₁₂ were extracted via a facile flux approach. Moreover, optical characterization showed that it has a large bandgap of 3.05 eV accompanied by high transparency from 0.34 to 5.4 μm, encompassing a 3–5 μm atmospheric window. Remarkably, the title compound features the intriguing merits of an intrinsically strong Raman shift located at 955 cm⁻¹ and a narrow line width of 12.54 cm⁻¹, which enable it to act as a stimulated Raman laser, making it a promising mid-infrared Raman crystal. Additionally, theoretical investigations are used to evaluate the fundamental relationships between the compound's structure and optical properties.