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Phase transition and crystal structures of BaZn2Si2O7
Abstract
The phase transition and crystal structures of the low (LT) and high (HT) temperature phases of BaZn2Si2O7 were studied by
neutron and X-ray diffraction techniques. BaZn2Si2O7 undergoes a first-order phase transition at about 2508C. The LT phase of
BaZn2Si2O7 is iso-structural with BaCo2Si2O7, crystallizing in the monoclinic space group C2/c with the lattice constants, a à 7.2782(4) A° , b à 12.8009(7) A° , c à 13.6869(7) A° and b à 90.093(6)8 at room temperature. The structure of the HT phase,
which was established by analyzing the X-ray powder-diffraction data using the direct method, crystallizes in the orthorhombic
space group Ccm21, with the lattice constants, a à 7.6199(4) A° , b à 13.0265(6) A° and c à 6.7374(2) A° at 2808C. The
underlying frameworks for both structures are 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 located
within the channels of the framework, but their coordination polyhedra are different for the LT and HT structures. The lower
coordination number of Ba atoms and the breaking of the Ba–O bonds in the HT phase may account 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 2808C–9008C temperature range the cell constant 无忧论文 【http://www.uklunwen.com】along the a-axis decreases with increasing temperature. q1999
Elsevier Science Ltd. All rights reserved.
Keywords: D. Crystal structure; D. Phase transitions; C. Neutron scattering; A. Inorganic compounds |
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