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[摘要]：A new polar polymorph of BaTeMo(2)O(9), alpha-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 beta- and alpha-BaTeMo(2)O(9) according to their crystallization temperature from high to low. The structure of alpha-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 alpha-BaTeMo(2)O(9) is phase-matchable. Polarization measurements indicate that alpha-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 beta-BaTeMo(2)O(9) to alpha-BaTeMo(2)O(9) at similar to 570 degrees 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: alpha-BaTeMo(2)O(9), orthorhombic, space group Pca2(1) (No. 29), a = 14.8683(2) angstrom, b = 5.6636(1) angstrom, c = 17.6849(3) angstrom, V = 1489.21(4) angstrom(3), and Z = 8. |
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