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

A new polar polymorph of BaTeMo2O9, α-BaTeMo2O9, has been synthesized by spontaneous crystallization with molten flux based on the TeO2–MoO3 solvent. The two polymorphous phases of BaTeMo2O9 are referred to as β- and α-BaTeMo2O9 according to their crystallization temperature from high to low. The structure of α-BaTeMo2O9 contains two-dimensional (Te2Mo4O18)4– anionic layers interleaved with Ba2+ cations. Each anionic layer is composed of Mo4O20 tetramers connected to TeOx (x = 3, 4) polyhedra. The Mo6+ and Te4+ 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-BaTeMo2O9 is phase-matchable. Polarization measurements indicate that α-BaTeMo2O9 is not ferroelectric, i.e., the polarization is not “switchable”. In addition, the powder X-ray patterns of the BaTeMo2O9 samples obtained from solid-state reactions revealed that BaTeMo2O9 undergoes an irreversible phase transition from β-BaTeMo2O9 to α-BaTeMo2O9 at ∼570 °C. BaMoO4 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-BaTeMo2O9, orthorhombic, space group Pca21 (No. 29), a = 14.8683(2) Å, b = 5.6636(1) Å, c = 17.6849(3) Å, V = 1489.21(4) Å3, and Z = 8.