Intense interests in mid-infrared (MIR) nonlinear optical (NLO) crystals have erupted in recent years due to the development of optoelectronic applications ranging from remote monitoring to molecular spectroscopy. Here, two polar crystals Ca 3 (TeO 3 ) 2 (MO 4 ) (M = Mo, W) were grown from TeO 2 -MO 3 flux by high-temperature solution methods. Ca 3 (TeO 3 ) 2 (MoO 4 ) and Ca 3 (TeO 3 ) 2 (WO 4 ) are isostructural, which feature novel structures consisting of asymmetric MO 4 tetrahedra and TeO 3 trigonal pyramids. Optical characterizations show that both crystals display ultrawide transparency ranges (279 nm to 5.78 μm and 290 nm to 5.62 μm), especially high optical transmittance over 80% in the important atmospheric transparent window of 3−5 μm, and superhigh laser damage thresholds (1.63 GW/cm 2 and 1.50 GW/cm 2 ), 54.3 and 50 times larger than that of state-of-the-art MIR NLO AgGaS 2 , respectively. Notably, they exhibit the widest band gaps and the loftiest laser-induced threshold damages among the reported tellurates so far. Moreover, Ca 3 (TeO 3 ) 2 (MO 4 ) exhibit type I phase matching at two working wavelengths owing to their large birefringence and strong secondharmonic generation responses from the distorted anions, as further elucidated by the first-principles calculations. The above characteristics indicate that Ca 3 (TeO 3 ) 2 (MO 4 ) crystals are high-performance MIR NLO materials, especially applying in high-power MIR laser operations.

Transition metal ions with d 0 electronic states (Ti 4 + , Zr 4 + , Nb 5 + and Ta 5 + ) are widely investigated as functional materials. This work first illustrates that Sc 3 + ion, long-time ignored, displays a second-order Jahn-Teller (SOJT) effect similar to asymmetric oxide-coordinated transition metal ions, thus providing a new ground to seek for asymmetric functional materials with enhanced performances. In Ba 3 Sc 2 (BO 3 ) 4 , BO 3 groups are parallelly arranged, satisfying the ideal arrangement to produce large birefringence. Importantly, distorted octahedral ScO 6 with Sc 3 + ion in its d 0 electronic state enlarges birefringence unexpectedly up to 0.149 @ 550 nm, which is larger than previously reported borates containing solely BO 3 , even to B 3 O 6 units. Subsequently, the SOJT influence of distorted ScO 6 octahedra on birefringence is verified by a comparison between experimental data and theoretical calculations. In addition, Ba 3 Sc 2 (BO 3 ) 4 also displays a high transmittance in the range of 230 nm-3.5 μm with a UV cut-off wavelength at 198 nm and a large laser induced damage threshold (2.7 GW/cm 2 ), comparable to α-BaB 2 O 4 . Above characteristics imply that the title compound may be a promising birefringent material.