Cristobalite, cubic-SiO2
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It can be seen from the picture that the bonds between oxygen atoms and two neighbouring silicon atoms line in straight lines. This arrangement is not common in similar silicate materials, and calculations show that it is energetically unfavourable. These materials prefer a bond angle of around 145° rather than 180°. To achieve this the oxygen atoms must be displaced in there perpendicular direction, giving rise to disorder in the crystal structure. At low temperature the cubic structure transforms into a lower-symmetry tetragonal structure which has the normal Si–O–Si bond angles. But at high temperature, the disorder is dynamic.
We have looked at the phase transition in cristobalite and the structure of its high-temperature phase using a variety of methods. Using molecular dynamics simulations and neutron total scattering methods coupled with the Reverse Monte Carlo method we have
Methods
Crystal structures were analysed by neutron powder diffraction. Cristobalite was the first material I studied using the neutron total scattering method and to which we applied the Reverse Monte Carlo method.
Collaborators
My first collaborator in the study of cristobalite was Ian Swainson, one of my first PhD students in Cambridge. Neutron total scattering and RMC work was carried out in collaboration with Matt Tucker (now Oak Ridge National Laboratory, USA) and David Keen (ISIS neutron tactility, UK).
References
Landau free energy and order parameter behaviour of the α/β phase transition in cristobalite. WW Schmahl, IP Swainson, MT Dove and A Graeme-Barber. Zeitschrift für Kristallographie 201, 125–145, 1992 (https://doi.org/10.1524/zkri.1992.201.1-2.125)
Low-frequency floppy modes in β-cristobalite. IP Swainson and MT Dove. Physical Review Letters 71, 193–196, 1993 (https://doi.org/10.1103/PhysRevLett.71.193)
Direct measurement of the Si–O bond length and orientational disorder in β cristobalite. MT Dove, DA Keen, AC Hannon and IP Swainson. Physics and Chemistry of Minerals 24, 311–317, 1997 (https://doi.org/10.1007/s002690050043)
Rigid Unit Modes and dynamic disorder: SiO2 cristobalite and quartz. M Gambhir, MT Dove and V Heine. Physics and Chemistry of Minerals 26, 484–495, 1999 (https://doi.org/10.1007/s002690050211)
Crystal structure of the high-pressure monoclinic phase-II of cristobalite, SiO2. MT Dove, MS Craig, DA Keen, WG Marshall, SAT Redfern, KO Trachenko, MG Tucker. Mineralogical Magazine 64, 569–576, 2000 (https://doi.org/10.1180/002646100549436)
Dynamic structural disorder in cristobalite: Neutron total scattering measurement and Reverse Monte Carlo modelling. MG Tucker, MD Squires, MT Dove and DA Keen. Journal of Physics: Condensed Matter 13, 403–423, 2001 (https://doi.org/10.1088/0953-8984/13/3/304)
Infrared and Raman spectroscopy studies of the α–β phase transition in cristobalite. IP Swainson, MT Dove, and DC Palmer. Physics and Chemistry of Minerals, 30, 353–365, 2003 (https://doi.org/10.1007/s00269-003-0320-8)