Crystallographic Investigations into the Polar Polymorphism of BaTeW2O9: Phase Transformation, Controlled Crystallization, and Linear and Nonlinear Optical Properties

Controlling polymorphism crystallization can be used to tune the intrinsic properties of a material without introducing any other atoms and is, therefore, of important technological significance. Although diversity in polymorphisms has been extensively studied, an intensive understanding of the relationships between their structure and properties is limited. Herein, a systematic investigation on the phase transformation, synthesis, structure, controlled growth, as well as the functional properties was performed for the polar metastable polymorph β-BaTeW2O9, which was grown via the top-seeded solution growth method using TeO2–WO3 as a flux. β-BaTeW2O9 crystallizes in an acentric space group P21 (No. 4, a = 5.499 (6) Å, b = 7.469 (9) Å, c = 8.936 (10) Å, and Z = 2), with a two-dimensional layered structure consisting of WO6 octahedra linked to TeO4 polyhedra. A notable phase transformation occurred from β-BaTeW2O9 to α-BaTeW2O9 at ∼608 °C and was demonstrated by the differential scanning calorimetry analysis combined with the in situ powder X-ray pattern measurements. This phase transformation was irreversible. This compound exhibited strong second harmonic generation response of 1.5 × KTiOPO4 (KTP) with a type-I phase-matching characteristic. Moreover, it also showed an excellent thermal stability and possessed a greatly broad transparent range from 0.325 to 5.70 μm with a large bandgap of 3.50 eV. In addition, electronic structure calculations revealed that WO6 and TeO4 are critical for producing a substantial nonlinear optical (NLO) response. These outstanding attributes indicate that β-BaTeW2O9 is a promising NLO material.