Recent progress in nonlinear optical molybdenum/tungsten tellurites: Structures, crystal growth and characterizations

The pursuit of second-order nonlinear optical (NLO) materials for mid-infrared (mid-IR, 3–5 μm) applications has been a significant research frontier due to their potential in optoelectronic technologies. In this review, we focus on noncentrosymmetric (NCS) molybdenum/tungsten tellurites incorporating d⁰ transition metal cations (Mo⁶⁺/W⁶⁺) and Te⁴⁺ cation with stereo-chemically active lone electron pair, which are prone to the second-order Jahn-Teller (SOJT) distortions. These compounds exhibit exceptional NLO effects, high optical transmittance in the 3–5 μm range, moderate birefringence, and ease of crystal growth in ambient conditions. Furthermore, they possess wider band gaps compared to conventional mid-IR NLO materials like metal phosphides and chalcogenides, enhancing their laser damage resistances. We categorize the notable NCS molybdenum/tungsten tellurites into nine series based on their cationic types, concentrating on ternary and quaternary systems. The review outlines their synthesis methods, crystal structures, growth techniques, and physical properties, with an emphasis on the relationship between SOJT-active units, NCS structures, and NLO performances. This work provides a clear perspective on the understanding of these materials and aims to accelerate the exploration of high-performance mid-IR NLO crystals to meet the increasing technological demands.