Raman Spectra Of Mixed Crystals Research Articles

Effect of isotope disorder on the Raman spectra of cubic boron arsenide

Boron arsenide (c-BAs) is at the forefront of research on ultrahigh thermal conductivity materials. We present a Raman scattering study of isotopically tailored cubic boron arsenide single crystals for 11 isotopic compositions spanning the range from nearly pure c-$^{10}$BAs to nearly pure c-$^{11}$BAs. Our results provide insights on the effects of strong mass disorder on optical phonons and the appearance of two-mode behavior in the Raman spectra of mixed crystals. Strong isotope disorder also relaxes the one-phonon Raman selection rules, resulting in disorder-activated Raman scattering by acoustic phonons.

Clustering/anticlustering effects on the GeSi Raman spectra at moderate (Ge,Si) contents: Percolation scheme vs. ab initio calculations

We test a presumed ability behind the phenomenological percolation scheme used for the basic description of the multi-mode Raman spectra of mixed crystals at one dimension along the linear chain approximation, to determine, via the Raman intensities, the nature of the atom substitution, as to whether this is random or due to local clustering/anticlustering. For doing so, we focus on the model percolation-type GeySi1−y system characterized by six oscillators {1×(Ge−Ge),3×(Ge−Si),2×(Si−Si)} and place the study around the critical compositions y ∼ (0.16, 0.71, and 0.84) corresponding to nearly matching of intensities between the like Raman modes from a given multiplet (Ge−Si triplet or Si−Si doublet). The interplay between the GeySi1−y Raman intensities predicted by the percolation scheme depending on a suitable order parameter κ of local clustering/anticlustering is found to be consistent with ab initio calculations of the GeySi1−y Raman spectra done with the Ab Initio Modeling PROgram code using large (64-, 216-, and 512-atoms) disordered cubic supercells matching the required (y,κ) values. The actual “percolation vs. ab initio” comparative insight at moderate/dilute-(Ge,Si) limits, with an emphasis on the κ-induced intra-bond transfer of oscillator strength, extends a pioneering one earlier achieved at an intermediate composition (y ∼ 0.50) by using small (32-atom) supercells [O. Pagès et al., J. Appl. Phys. 114, 033513 (2013)], mainly concerned with the inter-bond transfer of oscillator strength, providing altogether a complete picture.

Effects of the phase transition in Hg2(Br,I)2 crystals

The effects of the phase transition in Hg2(Br,I)2 crystals have been investigated over a wide range of temperatures by the Raman scattering spectroscopy and X-ray diffraction analysis. The overtones (at the X point of the Brillouin zone boundary) and the fundamental tones (at the center of Brillouin zone) of soft modes are found in the Raman spectra of these crystals and studied in detail. The density of one-phonon states of the soft TA branch manifests itself in the Raman spectra of mixed crystals. The potentialities of the soft-mode spectroscopy are realized in full measure. Analysis of the ratio between intensities of overtones and fundamental tones of the soft modes has demonstrated the applicability of the Landau phenomenological theory of phase transitions. The orthorhombic splitting of the reflections corresponding to the basal plane is revealed in the X-ray diffraction patterns and thoroughly explored. The temperature dependences of the isotropic and shear spontaneous strains are obtained. It is shown that the shear spontaneous strain plays a decisive role. The critical indices are determined and the model of the improper ferroelastic phase transition D 4 17 → D 2 17 in the vicinity of the tricritical point is corroborated.

Temperature‐dependent Raman spectra of mixed crystals of NaNO3–KNO3: evidence for limited solid solutions

The temperature dependence of the Raman spectra of NaNO3–KNO3 was employed to follow structural phase transitions of mixed crystals with different compositions. Solid solutions in the NaNO3 terminal phase behaved like NaNO3 I, solid solutions in the KNO3 terminal phase behaved like KNO3 I and solid solutions in the intermediate phase appeared to be a mixture of NaNO3 I and KNO3 I on the scale of a few tens to hundreds of nanometers. The three different behaviours of the solid solutions below the solidus suggest that NaNO3–KNO3 might well be regarded as a system with limited solid solutions instead of a continuous series of solid solutions. The limited solid solution model seems to be more consistent with the phase diagrams obtained by differential scanning calorimetry. Copyright © 1999 John Wiley & Sons, Ltd.

Temperature-dependent Raman spectra of mixed crystals of NaNO3-KNO3: evidence for limited solid solutions

The temperature dependence of the Raman spectra of NaNO3–KNO3 was employed to follow structural phase transitions of mixed crystals with different compositions. Solid solutions in the NaNO3 terminal phase behaved like NaNO3 I, solid solutions in the KNO3 terminal phase behaved like KNO3 I and solid solutions in the intermediate phase appeared to be a mixture of NaNO3 I and KNO3 I on the scale of a few tens to hundreds of nanometers. The three different behaviours of the solid solutions below the solidus suggest that NaNO3–KNO3 might well be regarded as a system with limited solid solutions instead of a continuous series of solid solutions. The limited solid solution model seems to be more consistent with the phase diagrams obtained by differential scanning calorimetry. Copyright © 1999 John Wiley & Sons, Ltd.

Laser Raman spectra of mixed crystals of [(NH4)1−x Kx]2 SO4

The Raman spectra of mixed crystals of [(NH4)1−x K x ]2 SO4 in the region 50–3400 cm−1 at 293 K and below 223 K have been reported. At room temperature 293 K, as the concentration of K+ ion increases in the crystal up to 50%, the frequencies of the totally symmetric vibrations of SO 4 2− and NH 4 + ions increase and thereafter the frequency of SO 4 2− vibration decreases and attains the value in K2SO4. This change in frequency up to 50% of potassium concentration is due to the breaking of hydrogen bonds of the type N-H...O. The behaviour of Raman intensities of A g (v 1) mode of SO 4 2− for various concentrations (x=0, 0·03, 0·11, 0·5, 0·85) suggest that the phase transition changes from first order type to one of second order. The phase transition in mixed crystals of [(NH4)1−x K x ]2 SO4 can be a cooperative phenomenon arising from a coupling between (NH4)+ ions through hydrogen bonds with the distorted SO 4 2− ions in the low temperature phase.

The Raman spectra of mixed crystals of metal carbonyls in the 2000 cm − region

The Raman spectra of mixed crystals of M x M′ 1- x (CO) 6, 0 < x < 1 M′,M = Cr, Mo, W show an averaging of spectral features in the v(CO) region for features associated with the molecular e g modes but a superposition for the a 1g feature. This is correlated with the absence of factor group splitting in the latter case. The implication and generalisation of these observations are briefly discussed.

Spectres Raman de Basses Frequences de Cristaux de Derives Benzeniques Paradisubstitues: p‐Br2C6H4, p‐Br2C6D4, p‐BrClC6H4 et p‐ClC6H4. Étude du désordre dans le dérive p‐BrClC6H4

AbstractAn assignment of the Raman active lattice modes in terms of factor group symmetry species and rotational vibrations is given for paradibromobenzene‐H4 and ‐D4, parabromochlorobenzene and parachloroiodobenzene. The analysis of the Raman spectra of mixed crystals of paradibromobenzene and paradichlorobenzene at 90K with 1 cm−1 resolution and the temperature dependence of the linewidths of the three librations are evidence for the static nature of the disorder in the unsymmetrical derivatives.

Phonons in disordered molecular solids: Raman spectra of heavily doped mixed crystals of benzene and perdeuterobenzene

Raman spectra of mixed crystals of benezene-h6 and benzene-d6 have been studied throughout the whole concentration range (0%–100%). They are discussed in the virtual crystal limit, suitable for small trap depths. It is shown that within this limit: (1) All the phonons are delocalized in mixed crystals; (2) k-designation is still valid, and (3) no appreciable broadening is expected in mixed crystals. In particular, because the trap depths are the same for all three rotations in benzene mixed crystals, it is further inferred that: (1) There is a one-to-one correspondence between the mixed crystal absorption and the pure crystal absortion; (2) spectral line positions depend on the concentration linearly; and (3) the normal coordinates in the mass (or moment of inertia)-weighted coordinates remain unchanged from benzene-h6 to mixed crystals to benzene-d6. All these conclusions are supported by the experimental data. More general cases, where trap depths are different for different degrees of freedom, are also discussed. Implications of the phonon delocalization in isotopic mixed crystals to the interpretation of phonon sidebands of electronic spectra of isotopic mixed crystals are also pointed out.

Interpretation of the Infrared and Raman Spectra of Mixed Crystals

Interpretation of the Infrared and Raman Spectra of Mixed Crystals

Raman spectra of mixed crystals

Raman spectra of mixed crystals