Lattice dynamics, Raman spectroscopy, and inelastic neutron scattering of orthoenstatiteMg2Si2O

Abstract

Enstatite Mg 2 Si 2 O 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 6 ͔ octahedral bands and single silicate chains. This paper reports lattice-dynamical rigid-ion model calculations and polarized Raman and inelasticneutron-scattering measurements of orthoenstatite Mg 2 Si 2 O 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 ⌺ and ⌬ directions are relatively flat without significant dispersion, but have moderate dispersion along the ⌳ direction, reflecting the strong structural anisotropy in orthoenstatite. Orthoenstatite undergoes a displacive-reconstructive phase transformation at ϳ1360 K to protoenstatite Mg 2 Si 2 O 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 ⌫ point the lowest optic A g mode involving translations of Mg 2ϩ ions and translations and rotations of the tetrahedral silicate groups softens from 104 cm Ϫ1 in protoenstatite to 82 cm Ϫ1 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 20-80-meV range in orthoenstatite is mainly due to Mg translations and the librations of the nearly rigid tetrahedral ͓SiO 4 ͔ groups, whereas the internal Si-O bond stretching vibrations of the ͓SiO 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 2 SiO 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.

Inelastic Neutron Scattering, Lattice Dynamics, Computer Simulation and Thermodynamic Properties

“…The simulated density of states of enstatite is found to be in overall agreement with results obtained from lattice dynamics calculations and available inelastic neutron scattering data [53]. Uranium, thorium, and lithium oxides are superionic conductors whose solid-state diffusion coefficients are comparable to that of liquids.…”

“…Due to the structural complexity involved, an approach based on interatomic potentials has been used to derive phonon density of states and various microscopic and macroscopic properties for several minerals including forsterite [48,49], fayalite [50], enstatite [51][52][53], MgSiO 3 perovskite [54,55], almandine, pyrope, grossular, spessartine, [19,20], zircon [16][17][18], sillimanite, andalusite, kyanite [56], and so on. …”

Shock-induced transformation of olivine to a new metastable (Mg,Fe)2SiO4 polymorph in Martian meteorites

“…The spinelloids are based on cubic packing of the O 2-sublattice, rather than the hcp symmetry observed here, and they contain SiOSi linkages between polymerised SiO 4 units that would give rise to strong Raman peaks both at 600-700 cm -1 and 950-1100 cm -1 [29,30]. Instead, the Raman spectra of the shocked olivines remain characteristic of an orthosilicate structure containing isolated SiO 4 4-units (Fig.…”

Redefinition of the mode Grüneisen parameter for polyatomic substances and thermodynamic implications

“…The dependence of optic frequencies on q j (dispersion) is known for only a few silicates, and the existing data obtained through inelastic neutron scattering are not complete (e.g., ref. 12). Hence, the Grüneisen parameters of the acoustic modes have been postulated to represent the thermodynamic average (e.g., ref.…”