High temperature single crystal diffraction study on monobarium gallate-the crystal structure of β-BaGa_2O_4

摘要

Differential thermal analysis and high temperature single crystal diffraction on monobarium gallate indicate that there is a reversible paraelectric-ferroelectric phase transition at about T_c = 770 ℃. The crystal structure of the previously unknown paraelectric high temperature modification β-BaGa_2O_4 has been solved from a single crystal data set collected at 800 ℃. The compound is isotypic with β-BaAl_2O_4 and closely related with high-kalsilite, adopting space group P6_322 (a = 5.3925(13), c = 8.9739(24) A, V = 226.01(1) A~3, Z = 2, D_(calc) = 5.01 g cm~(-3), R(|F|) = 0.025 for 125 independent reflections with I > 2σ(I) and 13 parameters). Main building element are GaO_4-tetrahedra which are corner linked to form sheets normal to the c axis. These layers are in turn connected to build up a three-dimensional framework enclosing cavities which are occupied by Ba~(2+). The bridging oxygen atoms between the layers are statistically distributed over three positions displaced 0.56 A from the ideal position on the three-fold axis. The off-centering reduces the Ga-O-Ga bond angle from an energetically unfavorable 180 to 145.0°. Within a single layer of the high temperature phase the tetrahedra are arranged in six-membered rings with an UDUDUD conformation. A different framework topology is found in the low temperature modification α-BaGa_2O_4 which is based on two different ring configurations, UUUDDD, and UDUDUD. Due to the differences between the sequences of upwards and downwards pointing tetrahedra in both phases the α-β transformation is a first order transition showing a pronounced thermal hystereses between heating and cooling. The transition is also reflected in the evolution of the lattice parameters where significant discontinuities are observed at T_c.