Phase transition and crystal structures of BaZn2Si2O7

The phase transition and crystal structures of the low (LT) and high (HT) t emperature phases of BaZn2Si2O7 were studied by neutron and X-ray diffracti on techniques. BaZn2Si2O7 undergoes a first-order phase transition at about 250 degrees C. The LT phase of BaZn2Si2O7 is iso-structural with BaCo2Si2O 7, crystallizing in the monoclinic space group C2/c with the lattice consta nts, a = 7.2782(43 Angstrom, b = 12.8009(7) Angstrom, c = 13.6869(7) Angstr om and beta = 90.093(6)degrees at room temperature. The structure of the HT phase, which was established by analyzing the X-lay powder-diffraction dat a using the direct method, crystallizes in the orthorhombic space group Ccm 2(1), with the lattice constants, a = 7.6199(4) Angstrom, b = 13.0265(6) An gstrom and c = 6.7374(2)Angstrom at 280 degrees C. The underlying framework s for both structures ale similar and can be regarded as an extensive three -dimensional tetrahedral system [Zn2Si2O7], in which both silicon and zinc atoms are coordinated tetrahedrally by oxygen atoms. The barium atoms are l ocated within the channels of the framework, but their coordination polyhed ra are different for the LT and HT structures. The lower coordination numbe r of Ba atoms and the breaking of the Ba-O bonds in the HT phase may accoun t for the endothermal behavior of the phase transition. The coefficient of volume expansion of the HT phase is smaller than that of the LT phase, and over the 280 degrees C-900 degrees C temperature range the cell constant al ong the a-axis decreases with increasing temperature. (C) 1999 Elsevier Sci ence Ltd. All rights reserved.