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キーワード:
High temperature diffraction; BaGa2O4; Monobarium gallate; Phase transition; Framework structure
要旨:
Differential thermal analysis and high temperature single crystal diffraction on monobarium gallate indicate that there is a reversible paraelectric–ferroelectric phase transition at about Tc=770 °C. The crystal structure of the previously unknown paraelectric high temperature modification β-BaGa2O4 has been solved from a single crystal data set collected at 800 °C. The compound is isotypic with β-BaAl2O4 and closely related with high-kalsilite, adopting space group P6322 (a=5.3925(13), c=8.9739(24) Å, V=226.0(1) Å3, Z=2, Dcalc=5.01 g cm−3, R(|F|)=0.025 for 125 independent reflections with I>2σ(I) and 13 parameters). Main building element are GaO4-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 Ba2+. The bridging oxygen atoms between the layers are statistically distributed over three positions displaced 0.56 Å 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-BaGa2O4 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 Tc.
