Research - Crystallography Laboratory at Virginia Tech

Nancy Ross and Ross Angel have been funded by NSF grants EAR-0105864,EAR-0408460 and EAR-0738692 to determine the role that bond compression plays in the compression of structures that can be described as polyhedral frameworks. It has often been assumed that such structures (feldspars, zeolites, perovskites) compress solely by the tilting of rigid atomic groups (or polyhedra), without compressing the metal-oxygen bonds of the framework. With the support of these two grants we have been able to develop the experimental techniques to measure extremely small bond compressions in structures at high pressures, and to show that in some structures bond compression in the framework, although small, determines the high pressure behavior of the mineral. These small changes in structure under pressure will have a large effect on cation partitioning between phases within the Earth, will influence the retention and diffusion rates of non-bonded species such as the rare gases that are important for isotope geochemistry, and will determine the elastic properties of these minerals.

Feldpsars and coesite
Both coesite and feldspars display complex variation of their volume under compression, but this can be explained in terms of the response of frameworks being that of essentially rigid tetrahedra. The compression of some Si-O bonds in coesite serves only to modify the rigid-unit behaviour of the framework. Details are given in the following publications (reprints available on request from the authors):
Angel RJ (2004) Equations of state of plagioclase feldspars. Contributions to Mineralogy and Petrology 146:506-512.
Brown JM, Abramson EH, Angel RJ (2006) Triclinic Elastic Constants for Low Albite. Physics and Chemistry of Minerals 33:256-265.
Benusa M, Angel RJ, Ross NL (2005) Compression of albite, NaAlSi3O8. American Mineralogist 90:1115-1120.
Angel RJ, Ross NL, Zhao J (2005) The compression of framework minerals: beyond rigid polyhedra. European Journal of Mineralogy 17:193-199
Angel RJ, Mosenfelder JL, Shaw CSJ (2001) Anomalous compression and equation of state of coesite. Physics of the Earth and Planetary Interiors 124:71-79.

Perovskites Through a series of high-pressure structural studies of various perovskites we have determined that bond compression in the octahedra determines the evolution of the perovskite at high pressures. If the octahedra are less compressible than the A-cation site, then the structure becomes more distorted at high pressure. Conversely, if the octahedra are more compressible than the A cation site, then pressure reduces the distortion of the perovskite, and can even lead to phase transitions to higher-symmetry structures. From these results we have developed a model that will predict the behaviour of perovskites at high pressure which is described in these papers:
Angel RJ, Zhao J, Ross NL (2005) General rules for predicting phase transitions in perovskites due to octahedral tilting. Physical Review Letters 95:025503.
Zhao J, Ross NL, Angel RJ (2006) Estimation of polyhedral compressibility and tilting in GdFeO3-type perovskites through compression of unit-cell axes. Acta Crystallographica B 62:431-439.
Zhao J, Ross NL, Angel RJ (2004) New view of the high-pressure behaviour of GdFeO3-type perovskites. Acta Crystallographica B 60:263-271.

Our latest results on silicate perovskites relevent to the Earth's lower mantle are in:
Vanpeteghem CB, Zhao J, Angel RJ, Ross NL, Bolfan-Casanova N (2006) Crystal structure and equation of state of MgSiO3 perovskite. Geophysical Research Letters 33:L03306, doi:10.1029/2005GL024955.
Vanpeteghem CB, Angel RJ, Ross NL, Jacobsen SD, Litasov KD, Ohtani E (2006) Al, Fe substitution in MgSiO3 perovskite structure: a single X-ray diffraction study. Physics of Earth and Planetary Interiors 155:96-103

For further information about our current experiments on perovskites, please contact Dr. Jing Zhao.

Phase Transitions

Ross Angel has been studying phase transitions at high pressures for many years. While the physics of structural phase transitions, even in complex systems, are now well-understood at high and low temperatures, our exploration of the same transitions at high pressure is still in its infancy. Of particular interest are systems that behave in unusual ways, for example; Clinopyroxenes undergo a sequence of transitions with space group changes C2/c -> P21/c -> C2/c on increasing pressure. Some of these transitions can be seen in the optical microscope.

But we are interested in the factors that determine the relative stabilities of the three phases, and also whether it is possible to drive a direct phase transition from C2/c to C2/c, which would be of fundamental importance as phase changes without symmetry change are extremely rare.

Lead phosphate is an improper ferroelastic, which undergoes a transition from monoclinic to trigonal symmetry on increasing temperature or pressure. Whereas the high-symmetry structure is dynamically disordered at high temperatures our recent high-pressure neutron powder diffraction study that it is statically disordered at high pressures. There are two papers describing these results:
Angel RJ, Bismayer U, Marshall WG (2004) Local and long range order in ferroelastic lead phosphate at high pressure. Acta Crystallographica B 60:1-9. This paper includes some diffuse scattering measurements, the results of which can also be viewed in more detail here.

Angel RJ, Bismayer U, Marshall WG (2001) Renormalization of the phase transition in lead phosphate,Pb3(PO4)2, by high pressure: structure. Journal of Physics-Condensed Matter 13:5353-5364 Download paper

The same general behaviour appears to occur in anorthite feldspar (Angel, 1988: American Mineralogist 73:1114-1119). By contrast, the high-pressure, high-symmetry phase of titanite is statically ordered compared to its disordered high-temperature phase (Angel et al. 1999: Phase Transitions, 68:533-543).

Molecules at High Pressure

Most of the high-pressure crystallographic studies have been performed on inorganic materials and especially minerals. Ross Angel, Carla Slebodnick and Maciej Bujak, in collaboration with members of the Chemistry Department, are now starting projects in which we use pressure to probe the balance between the strong intra-molecular forces (bonds) and the weaker inter-molecular forces within molecular solids. As we apply pressure to molecular solids the molecules will move together and the inter-molecular repulsive forces will increase. As a result we can suppress or halt dynamic behaviour of molecules, study phase changes and induce new properties in substances which can later be mimicked at atmospheric pressure by rational design and synthesis. With Prof. Brian Hanson we are probing metal-carbonyl compounds to determine whether the widely-held belief that their conformations are a product of steric interactions - if they are then we expect to induce phase transitions from one conformation to another by the application of pressure.