Raman scattering study of the high-temperature vibrational properties and stability of CaGeO3 perovskite

Durben, Dan J.; Wolf, George H.; McMillan, Paul F.

doi:10.1007/bf00202573

Cited by 28 publications

(21 citation statements)

References 42 publications

(47 reference statements)

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“…Recently, Durben et al (1991) have performed Raman scattering measurements on one of our CaGeO3 specimens and found no evidence for symmetry-breaking structural transformations or soft-mode behavior between room temperature and 650 ~ C. These authors suggest that the small increase in thermal expansitivity observed in our study near 520 K may arise from a relaxation of stress associated with twin boundaries.…”

Section: Discussioncontrasting

confidence: 45%

“…We have two comments on the data and alternative explanation of Durben et al (1991). Firstly, although the behavior of the Raman Ag modes does not indicate any phase transition near 520 K, the Bg mode data are more ambiguous.…”

Section: Discussionmentioning

confidence: 91%

“…Although it is an improvement over previous studies, the data are not sufficient to conclude whether there is a phase change. Note that there is no point symmetry change across the first-order, Pbnm to Cmcm transition and, as pointed out by Durben et al (1991), the total number of Ag and Bg modes are the same for the two phases. Such a phase transition does Lockwood and Torrie (1974) have studied KMnF3 perovskite using Raman spectroscopy.…”

Section: Discussionmentioning

confidence: 92%

“…This ambiguity may be resolved in the future when theoretical and experimental Raman studies become available on perovskites with known Pbnm and Cmcm transitions. Secondly, we do not believe that the anomalous thermal expansion observed near 520 K can be due to stress relaxation, as suggested by Durben et al (1991). Our x-ray experiments were done with loose powder; the crystals were under free boundary conditions.…”

Section: Discussionmentioning

confidence: 97%

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Phase transition in CaGeO3 perovskite: Evidence from X-ray powder diffraction, thermal expansion and heat capacity

et al. 1991

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Abstract. High-temperature x-ray powder diffraction study by the full pattern Rietveld method of orthorhombic CaGeO3 (Pbnm at ambient condition) perovskite confirms the previously observed phase transition at Tc = 520 K. The measured volumetric thermal expansion coefficients are 3.1 x 10 -s (K -1) below Tc and 3.5x 10 .5 (K -1) above Tc. The space group at T>Tc has been tentatively identified as Cmcm. Such a transition involves the disappearance of one of the two octahedral rotations in the (001) plane, and the doubling of the unit cell volume, with c axis unchanged. Although this transition should be of first order from symmetry considerations, the distortion of the Pbnm phase decreases continuously as the temperate approaches Tc and there is no observable volume discontinuity at Tc. The measured heat capacity places an upper limit on the enthalpy of transition of 50 J/mol, which is quite reasonable in terms of the crystallographic nature of this phase transition.

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Section: Discussioncontrasting

confidence: 45%

Section: Discussionmentioning

confidence: 91%

Section: Discussionmentioning

confidence: 92%

Section: Discussionmentioning

confidence: 97%

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Phase transition in CaGeO3 perovskite: Evidence from X-ray powder diffraction, thermal expansion and heat capacity

et al. 1991

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“…For example, in the X(ZX)X geometry, only the Bag modes are allowed; however, the strong Ag modes were also observed. This leakage is due to the presence of off-axis scattering geometries that invariably arise when high numerical aperture focusing and collection optics are used as previously reported by other workers (Durben et al 1991). In addition, strain induced depolarisation and imperfect alignment of the single crystal also result in the leakage of unallowed modes.…”

Section: ) Mg L 4 Si 2 0 6 and Calculated Long Wavelength Phonon Frementioning

confidence: 81%

Lattice dynamics and Raman spectroscopy of protoenstatite Mg2Si2O6

et al. 1994

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Abstract. Enstatites (MgzSi206) are important rock forming silicates of the pyroxene group whose structures are characterised by double MgO6 octahedral bands and single silicate chains. Orthoenstatite transforms to protoenstatite above 1273 K with a doubling of the a axis and a rearrangement of the silicate chains with respect to the Mg 2+ ions. Lattice dynamical calculations based on a rigid-ion model in the quasi-harmonic approximation provide theoretical estimates of elastic constants, long wavelength phonon modes, phonon dispersion relations, total and partial density of states and inelastic neutron scattering cross-sections of protoenstatite. The computed elastic constants are in good agreement with experimental data. The computed density of states of a chain silicate such as protoenstatite is distinct from that of olivines (forsterite, MgzSiO4 and fayalite, FezSiO4) with isolated silicate tetrahedra. The band gaps in the density of states in forsterite are largely due to the separation in the frequency ranges of the external and internal vibrations of the isolated silicate group, whereas in protoenstatite these gaps are filled by the vibrations of the bridging oxygens of the silicate chain. The computed density of states is used to calculate the specific heat, the mean square atomic displacements and temperature factors. Validity of these calculations are supported by Raman scattering measurements. Polarised and unpolarised Raman spectra are obtained from small single crystals of protoenstatite (Li,Sc)o.6Mgl.4Si206 stable at room temperature using the 488nm or 514.5 nm lines of an Ar + ion laser and a micro-Raman spectrometer with backscattering geometry. The Raman spectra were analysed and interpreted based on the lattice dynamical model. The experimental Raman frequencies and mode assignments (based on polarised single crystal spectra) are in good agreement with those obtained from lattice dynamical calculations.

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Lattice dynamic investigation of the phonon wavenumbers of CaGeO3 perovskite

Gupta

Ashdhir

1999

J. Raman Spectrosc.

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A short-range force constant model was applied to investigate the phonons in CaGeO 3 perovskite in the orthorhombic phase. The calculations with six stretching and bending force constants provide good agreement for the Raman wavenumbers. The correct assignments of the observed Raman wavenumbers were made based on the present calculations. The infrared wavenumbers have also been assigned for the first time.

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Raman scattering study of the high-temperature vibrational properties and stability of CaGeO3 perovskite

Cited by 28 publications

References 42 publications

Phase transition in CaGeO3 perovskite: Evidence from X-ray powder diffraction, thermal expansion and heat capacity

Phase transition in CaGeO3 perovskite: Evidence from X-ray powder diffraction, thermal expansion and heat capacity

Lattice dynamics and Raman spectroscopy of protoenstatite Mg2Si2O6

Lattice dynamic investigation of the phonon wavenumbers of CaGeO3 perovskite

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