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Schlagwörter:
high temperature diffraction; BaGa2O4; monobarium gallate; phase transitions; framework structure
Zusammenfassung:
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 = 770degreesC. The crystal structure of the previously unknown paraelectric high temperature modification beta-BaGa2O4 has been solved from a single crystal data set collected at 800degreesC. The compound is isotypic with beta- BaAl2O4 and closely related with high-kalsilite, adopting space group P6(3)22 (a = 5.3925(13), c = 8.9739(24) Angstrom, V = 226.0(1) Angstrom(3), Z = 2, D-calc = 5.01 g cm(-3), R(\F\) = 0.025 for 125 independent reflections with I > 2sigma (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 Angstrom 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 145.0degrees. 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 alpha-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 alpha-beta 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. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.
