Pressure induced phase transition and pressure dependence of crystal structure in low {(alpha)} and Ca/A1-doped cristobalite.
The phase stability and atomic-level compression mechanisms for both SiO{sub 2} cristobalite, and for cristobalite partially stabilized by Ca/Al doping (Ca{sub x/2} Si{sub 2-x}Al{sub x}O{sub 4}), have been investigated. A phase transition to a lower symmetry phase, observed with in situ high-pressure energy-dispersive x-ray diffraction, occurs at about 1.2 GPa. Structure models of the low-pressure phase were obtained by Rietveld analysis of neutron powder-diffraction data from powdered samples contained in a gas pressure apparatus. These data were collected at pressures up to 0.6 GPa and at 298 and 60 K. The results suggest collapse of the corner-connected framework from rotations of the rigid SiO{sub 4} tetrahedra at high pressures and low temperatures as the dominant mechanism for the densification of both materials. Compared to pure SiO{sub 2} cristobalite at the same pressure and temperature, the Ca/Al-doped material has a larger unit-cell volume. It also has a larger Si-O-Si bending angle and a more expanded framework as evidenced by the smaller rotations of the rigid SiO{sub 4} tetrahedra. The rate of change of these parameters as a function of pressure and temperature is the same for both pure and Ca/Al-doped cristobalite. These observations are consistent with Ca occupying positions within the cavities formed by the (Si, Al)-O framework and bracing it against collapse.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- DOE Contract Number:
- DE-AC02-06CH11357
- OSTI ID:
- 937695
- Report Number(s):
- ANL/MSD/JA-12116; JAPIAU; TRN: US200905%%486
- Journal Information:
- J. Appl. Phys., Vol. 75, Issue 3 ; 1994; ISSN 0021-8979
- Country of Publication:
- United States
- Language:
- ENGLISH
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