Lattice dynamics, Raman spectroscopy, and inelastic neutron scattering of orthoenstatite Mg{sub 2}Si{sub 2}O{sub 6}
- Mineral Physics Group, Department of Geological Sciences, Box 351310, University of Washington, Seattle, Washington 98195-1310 (United States)
- Intense Pulsed Neutron Source, Chemistry and Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439-4814 (United States)
- Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Bombay 400085 (India)
Enstatite Mg{sub 2}Si{sub 2}O{sub 6} is an important rock-forming silicate of the pyroxene group. It exists in several polymorphs, the structures of which are characterized by double [MgO{sub 6}] octahedral bands and single silicate chains. This paper reports lattice-dynamical rigid-ion model calculations and polarized Raman and inelastic-neutron-scattering measurements of orthoenstatite Mg{sub 2}Si{sub 2}O{sub 6}, which is orthorhombic (Pbca) with 80 atoms in the unit cell. The calculated elastic constants, phonon frequencies, density of states, and specific heat are in good agreement with the experimental data. The optical-phonon branches along the {Sigma} and {Delta} directions are relatively flat without significant dispersion, but have moderate dispersion along the {Lambda} direction, reflecting the strong structural anisotropy in orthoenstatite. Orthoenstatite undergoes a displacive-reconstructive phase transformation at {approximately}1360thinspK to protoenstatite Mg{sub 2}Si{sub 2}O{sub 6}, which is also orthorhombic (Pbcn) with 40 atoms in the unit cell and the a dimension half that of orthoenstatite. However, the computed phonon-dispersion relations in orthoenstatite do not exhibit any lattice instability in the entire Brillouin zone. The calculations predict that at the {Gamma} point the lowest optic A{sub g} mode involving translations of Mg{sup 2+} ions and translations and rotations of the tetrahedral silicate groups softens from 104thinspcm{sup {minus}1} in protoenstatite to 82thinspcm{sup {minus}1} in orthoenstatite, which is consistent with the single-crystal polarized Raman-scattering measurements. The phonon spectra obtained from inelastic-neutron-scattering measurements have been interpreted on the basis of model calculations. The broad peak in the 20{endash}80-meV range in orthoenstatite is mainly due to Mg translations and the librations of the nearly rigid tetrahedral [SiO{sub 4}] groups, whereas the internal Si-O bond stretching vibrations of the [SiO{sub 4}] groups contribute mainly above 80 meV. The bridging oxygens in the silicate chains are vibrationally distinct from the nonbridging oxygens, leading to significant differences in the vibrational spectra of orthoenstatite and protoenstatite with tetrahedral silicate chains from those in forsterite Mg{sub 2}SiO{sub 4} with isolated silicate tetrahedra. The band gaps found in the phonon density of states of forsterite are filled by the vibrations of the bridging oxygens in the silicate chains in the phonon densities of states of orthoenstatite and protoenstatite. {copyright} {ital 1998} {ital The American Physical Society}
- OSTI ID:
- 638788
- Journal Information:
- Physical Review, B: Condensed Matter, Vol. 58, Issue 2; Other Information: PBD: Jul 1998
- Country of Publication:
- United States
- Language:
- English
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