Pressure and Temperature Behavior of Framework Silicates and Nitrides
The response of silicates and nitrides to changes of pressure or of temperature depends on the flexibility of the bond angles of the crystal structures. Where bond angles can vary between neighboring tetrahedra linked at one corner, relative tilting or rotations occur which modify the three dimensional framework and this dominates the response of the crystal structure to temperature or pressure changes. Direct evidence for this behavior is available from measurements by time-of-flight neutron diffraction with multi-component profile refinement procedures, where atomic position changes have been determined to pressures of ~ 3 × 109 Pa. The importance of the framework structural type has been illustrated by studies of the flexible α-quartz structures of SiO2 and GeO2 which have volume compressibility coefficients Kv ~ 26 × 10−12Pa−1, the partially flexible Structures Si2N2O and Ge2N2O with K ~ 9 × 10−12Pa−1, and the rigid structures α and β Si3N4 with Kv ~ 3 × 10−12Pa−1.
The response of silicates to high pressures is different for the three-dimensional frameworks (Kv ~ 22 × 10−12Pa−1), for chain-like pyroxenes (Kv ~ 9 × 10−12Pa−1) and for the discrete silicate structures (Kv ~ 7 × 10−12Pa−1). This suggests that their behavior corresponds to flexible, partially flexible and rigid framework structure respectively. The silicate chains of all the pyroxenes respond to pressure in the same way with the same compressibility coefficient along the chain direction.
There is a less systematic variation of the thermal expansion coefficients with structural type since the thermal vibrations can modify the behavior of the framework structure. Information about the response of silicate or nitride frameworks to stresses is more easily and directly obtainable by high pressure studies which can provide essential preliminary data towards understanding thermal expansion characteristics.
KeywordsThermal Vibration Thermal Expansion Behavior Silicate Chain Volume Thermal Expansion Coefficient Compressibility Behavior
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