The high temperature crystal structure of caesium enneaborate, gamma-Cs_2O.9B_2O_3, has been refined using the Rietveld technique, from neutron powder diffraction data obtained with the high resolution powder diffractometer (HRPD) at the ISIS spoliation pulsed neutron source. gamma-Cs_2O.9B_2O_3 is tetragonal, space group P4_1 22, with unit cell dimensions a_0 = 8.77646 +- 0.00016 and c_0 =15.94929 +- 0.00030 A. This polymorph was formed by devitrification at high temperature, before undergoing a displacive transition to the orthorhombic polymorph, beta-Cs_2O.9B_2O_3, detailed in earlier neutron studies, upon cooling below 316 K. The topology of the borate network remains unchanged, comprising two independent, interpenetrating borate networks of boroxol and triborate groups in the ratio 2:1. An analysis is presented of the detailed geometry of the borate units, and the probable existence of a third, low temperature, polymorph is discussed.
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