High‐pressure metastable phase transitions in β‐Ge3N4 studied by Raman spectroscopy (original) (raw)

Pressure-induced transformations in α- and β-Ge3N4: in situ studies by synchrotron X-ray diffraction

Journal of Solid State Chemistry, 2004

Metastable high-pressure transformations in germanium nitride (a-and b-Ge 3 N 4 polymorphs) have been studied by energy-and angle-dispersive synchrotron X-ray diffraction at high pressures in a diamond anvil cell. Between P=22 and 25 GPa, the phenacitestructured b-Ge 3 N 4 phase (P6 3 /m) undergoes a 7% reduction in unit-cell volume. The densification is primarily concerned with the a-axis parameter, in a plane normal to the hexagonal c-axis. Based on results of previous LDA calculations and Raman spectroscopic studies, we propose that the structural collapse is due to transformation into a new metastable polymorph (d-Ge 3 N 4 ) that has a unit-cell symmetry based upon P3, that is related to the low-pressure b-Ge 3 N 4 phase by concerted displacements of N atoms away from special symmetry sites in the plane normal to the c-axis. No such transformation occurs for a-Ge 3 N 4 , due to the different stacking of linked GeN 4 layers. All three polymorphs (a-, band d-Ge 3 N 4 ) are based on tetrahedrally coordinated Ge atoms, unlike the spinel-structured g-Ge 3 N 4 phase, that contains octahedrally coordinated Ge 4+ . Experimentally determined bulk modulus values for a-Ge 3 N 4 (K 0 =165(10) GPa, K 0 0 =3.7(4)) and b-Ge 3 N 4 (K 0 =185(7) GPa, K 0 0 =4.4(5)) are in excellent agreement with theoretical predictions. The bulk modulus for the new d-Ge 3 N 4 polymorph is only determined above the b2d transition pressure (P=24 GPa); K=161(20) GPa, assuming K 0 =4. Above 45 GPa, both aand d-Ge 3 N 4 polymorphs become amorphous, as determined by X-ray diffraction and Raman scattering. r

Phonon softening and high-pressure low-symmetry phases of cesium iodide

Physical Review Letters, 1992

The relative stability of various high-pressure phases of CsI is studied from first principles and analyzed using the Landau theory of phase transitions. We demonstrate that the cubic-to-orthorhombic transition recently observed to occur slightly below 20 Gpa is driven by the softening of an acoustic phonon at the M point of the Brillouin zone. The coupling between this mode and anisotropic strain makes the transition slightly first order (with a volume variation of the order of 0. 1%), and stabilizes the experimentally observed orthorhombic phase with respect to other competing symmetry-allowed structures.

High pressure phase transition in metallic LaB 6: Raman and X-ray diffraction studies

Solid State Communications, 2004

High pressure Raman and angle dispersive X-ray diffraction (ADXRD) measurements on the metallic hexaboride LaB 6 have been carried out upto the pressures of about 20 GPa. The subtle phase transition around 10 GPa indicated in Raman measurements is confirmed by ADXRD experiments to be a structural change from cubic to orthorhombic phase. Ab-initio electronic band structure calculations using full potential linear augmented plane wave method carried out as a function of pressure show that this transition is driven by the interception of Fermi level by electronic band minimum around the transition pressure.

Phonon modes in spontaneously orderedGaInP2studied by micro-Raman scattering measurements

Physical Review B, 1997

We have performed micro-Raman-scattering experiments on ordered GaInP 2 alloy samples in three different geometries where the phonon wave vector is either parallel or perpendicular to the ordering axis of the crystal. By comparing results from the ͑111͒ backscattering and right-angle scattering measurements with the C 3v symmetry of the crystal, we found that the recently discovered peaks at 205 and 354 cm Ϫ1 in the Raman spectra of ordered alloys are due to longitudinal-phonon modes with A 1 symmetry in these geometries. In the ͑110͒ backscattering geometry, selection-rule forbidden longitudinal-phonon modes appear in the Raman spectra measured in parallel polarizations. Possible mechanisms for this selection-rule violation are discussed. ͓S0163-1829͑97͒04128-3͔

Raman excitations and orientational ordering in deuterium at high pressure

Physical Review B, 1996

High-resolution Raman spectra of ortho/para mixed crystals of deuterium to 4 K and pressures to 40 GPa reveal a number of phenomena. We find for para-containing samples a large enhancement of highfrequency vibron sidebands, a total of 11 vibron peaks, and significant changes in the low-frequency rotational excitations upon passage into the low-temperature phase. Analysis of the spectra reveals that the phase is orientational ordered and may form a superstructure derived from the high-temperature hcp lattice. The results also suggest that orientational frustration accompanies the transition in ortho/para mixed crystals.

(Ga,In)P: A standard alloy in the classification of phonon mode behavior

Physical Review B, 2006

Contrary to a broadly accepted assumption we show that random (Ga,In)P is not an exception with respect to the crude classification of the phonon mode behavior of random mixed crystals in terms of 1-bond→1-mode systems or 2-bond→1-mode systems, as established from the simple criterion derived by Elliott et al. [R.J. Elliott et al., Rev. Mod. Phys. 46, 465 (1974)]. Consistent understanding of the puzzling Raman/infrared behavior of (Ga,In)P, that has been a subject of controversy, is achieved via a basic version of our 1-bond→2mode model originally developed for (Zn,Be)-chalcogenides, that exhibit a large contrast in the bond properties, and recently extended under a simplified form to the usual (Ga,In)As alloy. The Raman/infrared features from (Ga,In)P are accordingly reassigned , with considerable change with respect to the previous approaches. In particular the In impurity mode, previously assigned within (∼390 cm −1) the optical band of the host GaP compound (368-403 cm −1), is reassigned below it (∼350 cm −1). Accordingly the GaP and In-P transverse optical branches do not overlap, which reconciles (Ga,In)P with the Elliott's criterion. Besides, we show that the idea of two bond lengths per species in alloys, supported by our 1-bond→2-phonon picture, opens an attractive area for the discussion of spontaneous ordering in GaInP2, and in mixed crystals in general. Essentially this is because it allows to play with the related competition effects regarding the minimization of the local strain energy due to the bond length mismatch between the parent compounds. In particular the unsuspected issue of intrinsic limit to spontaneous ordering comes out (η ∼0.5 in GaInP2). The whole discussion is supported by detailed re-examination of the (Ga,In)P Raman/infrared data in the literature, full contour modeling of the transverse and longitudinal optical Raman lineshapes via our phenomenological 1-bond→2-mode model, and first-principles bond length calculations concerned with the minority bond species close to the impurity limits (Ga ∼ 0, 1) and to the GaP (Ga ∼0.19) and In-P (Ga ∼0.81) bond percolation thresholds. In the latter case we discriminate between connected and isolated bonds, not in the usual terms of next-nearest neighbors.

Compositional behavior of Raman-active phonons inPb(Zr1−xTix)O3ceramics

Physical Review B, 2015

A systematic study of the Raman spectra of Pb(Zr 1−x Ti x)O 3 (PZT) ceramics has been performed in a broad temperature interval (10-600 K) and a broad Ti/Zr concentration range around the morphotropic phase boundary (x = 0.25-0.70). The number of the spectral components was estimated by a standard fitting procedure with damped harmonic oscillators as well as by counting the number of peaks and shoulders with the help of a purposely designed mathematical analysis based on frequency derivatives of the Raman spectra. This last method proves to be very useful to study Raman spectra of disordered materials. For the case of PZT, the comparison with the Raman modes of PbTiO 3 allows us to assign the phonon bands on both sides of the morphotropic phase boundary, and the crossover from the tetragonal to rhombohedral phase spectra is clearly visible. However, there are no indications of a systematic splitting of the E-symmetry modes into monoclinic A-A doublets in the morphotropic samples. Detailed adjusting of the response function to the spectrum requires to assume additional Raman-active modes, but this holds for a much broader concentration range than that of the anticipated monoclinic phase. In particular, the lowest frequency transverse optic mode of E-symmetry (soft mode of the ferroelectric phase transition) is split into two components, a THz frequency anharmonic (central modelike) component and a resonant component (at ω ∼ 80 cm −1), both related to the same normal coordinate. The additional Raman band appearing in this frequency range (ω ∼ 65 cm −1) at low temperatures is rather associated with the antiphase tilt vibrations of the oxygen octahedra.

Raman signatures of pressure induced electronic topological and structural transitions in Bi2Te3

Solid State Communications, 2012

We report Raman signatures of electronic topological transition (ETT) at 3.6 GPa and rhombohedral (α-Bi 2 Te 3 ) to monoclinic (β-Bi 2 Te 3 ) structural transition at ~ 8 GPa. At the onset of ETT, a new Raman mode appears near 107 cm -1 which is dispersionless with pressure. The structural transition at ~ 8 GPa is marked by a change in pressure derivative of A 1g and E g mode frequencies as well as by appearance of new modes near 115 cm -1 and 135 cm -1 . The mode Grüneisen parameters are determined in both the α and β-phases. a) Electronic mail-asood@physics.iisc.ernet.in

Pressure-tuning FT-Raman spectroscopic study of the T2g phonon mode of a diamond-anvil cell

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 1997

The stress behaviour of a type-IIA diamond in a commercial diamond-anvil cell has been examined by measuring the position of the TZg phonon mode of the top diamond in the cell (originally located at-1332 cm-') during a pressure-tuning FT-Raman microspectroscopic study at various pressures throughout the 0.001-62.2 kbar range. In general, the components of the scattered Raman signal from different depths appear as discrete band envelopes rather than a continuous gradient throughout the depth of the diamond. The changes taking place in the band structure with the variation in depth indicate the occurrence not of phase changes but of discontinuities in the pressure gradient throughout the diamond. For measurements made at the bottom edge of the diamond (i.e. at the sample-diamond interface), there is a linear relationship between the position (v, wavenumbers) of the T,, phonon mode and the pressure (P, kbar) such that dv/dP = 0.16 cm-' kbar-'.