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

Abstract

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.

Novel Polarized Crystal BaTeW2O9: Crystal Growth, Characterization, and Wide-Band Polarized Prism

“…In recent years, a sequence of oxide materials containing octahedrally coordinated d 0 transition metals (Ti 4+ , W 6+ , etc.) and cations with stereochemically active lone pairs have been synthesized and grown due to the inspiration of second-order Jahn–Teller distortions. − To date, most of these crystals such as β-BaTeMo 2 O 9 , , β-BaTeW 2 O 9 , Na 2 TeW 2 O 9 , , and Cs 2 TeMo 3 O 12 , have been paid attention for the second-order nonlinear optical, piezoelectric, and electro-optical properties due to their noncentrosymmetric structures. In 2015, the mid-IR prism made of biaxial crystals α-BaTeMo 2 O 9 (α-BTM) was reported by our group .…”

Investigations on the Synthesis, Crystal Structure, Linear- and Nonlinear-Optical Properties of the Zinc Germanate Rb2ZnGe2O6

“…To exploit new second-order NLO compounds covering the mid-IR band, various effective strategies have been developed and implemented by researchers. For instance, one of the commonly used strategies is to obtain noncentrosymmetric compounds by introducing d 0 transition-metal ions, for instance, Nb 5+ , W 6+ , and Mo 6+ , and heavy cations with lone-pair stereochemical activity including Te 4+ , Pb 2+ , and Bi 3+ , , both of which tend to induce asymmetric coordination environments, such as BaTeMo 2 O 9 , BaTeW 2 O 9 , and Cs 2 TeMo 3 O 12 . − However, these crystals can only be grown by using a flux method because of their incongruent melting nature. In addition to oxide-based compounds, other researchers have focused on metal halides and chalcogenides because of their rich structural and compositional diversities, such as BaGa 4 S 7 , BaGa 4 Se 7 , Li 2 Ga 2 GeS 6 , Na 2 SbF 5 , CsHgBr 3 , and HgBr 2 .…”

Rb21.89W32.66O108: An Excellent Mid- and Far-IR Nonlinear-Optical Material with a Wide Band Gap

“…So, the RWO crystal can only be grown by a high-temperature solution method . In addition, it can be seen from the test results that RWO has a very good thermal stability performance, obviously better than many previously reported tungstate/molybdate crystals, such as CdTeMoO 6 (congruently melting at 766 °C), β-BaTeW 2 O 9 (incongruently melting at 784 °C), Na 2 TeW 2 O 9 (incongruently melting at 562 °C), etc. This also shows that RWO has greater advantages in production and application.…”