Polymorphism of BaTeMo₂O₉: A New Polar Polymorph and the Phase Transformation

A new polar polymorph of BaTeMo 2 O 9, α-BaTeMo 2 O 9, has been synthesized by spontaneous crystallization with molten flux based on the TeO 2 –MoO 3 solvent. The two polymorphous phases of BaTeMo 2 O 9 are referred to as β- and α-BaTeMo 2 O 9 according to their crystallization temperature from high to low. The structure of α-BaTeMo 2 O 9 contains two-dimensional (Te 2 Mo 4 O 18 ) 4– anionic layers interleaved with Ba 2+ cations. Each anionic layer is composed of Mo 4 O 20 tetramers connected to TeO x ( x = 3, 4) polyhedra. The Mo 6+ and Te 4+ are in asymmetric coordination environments attributable to second-order Jahn–Teller (SOJT) effects. Second-harmonic generation (SHG) measurements using 1064 nm radiation show that α-BaTeMo 2 O 9 is phase-matchable. Polarization measurements indicate that α-BaTeMo 2 O 9 is not ferroelectric, i.e., the polarization is not “switchable”. In addition, the powder X-ray patterns of the BaTeMo 2 O 9 samples obtained from solid-state reactions revealed that BaTeMo 2 O 9 undergoes an irreversible phase transition from β-BaTeMo 2 O 9 to α-BaTeMo 2 O 9 at ∼570 °C. BaMoO 4 is found to lower the energy barrier and stimulate the transformation. Furthermore, structural distortions of both polymorphs were calculated to gain a better understanding of the structure–property relationships. Crystal data: α-BaTeMo 2 O 9, orthorhombic, space group Pca 2 1 (No. 29), a = 14.8683(2) Å, b = 5.6636(1) Å, c = 17.6849(3) Å, V = 1489.21(4) Å 3, and Z = 8.