Pressure dependence of spin-orbit levels of Co2+ in KZnF3

Low temperature emission measurements were performed at atmospheric and high pressures in KZnF3:Co2+. Comparing the zero-phonon line positions and using the phonon dispersion relation of the host material, the fine structure in the emission spectrum was identified. Pressure coefficients were obtained for several among the emission lines from which the values for zero-phonon lines were deduced. Analysis shows that the spin-orbit parameter changes with pressure are negligible in this material. (C) 1999 American Institute of Physics. [S0021-9606(99)70608-2].11083995399

NMR and Raman Scattering Studies of Temperature- and Pressure-Driven Phase Transitions in CH3NH2NH2PbCl3 Perovskite

Three-dimensional methylhydrazinium (CH3NH2NH2+, MHy+) lead halides, related to the famous methylammonium (CH3NH3+, MA+) and formamidinium (CH(NH2)2+, FA) perovskites, are attractive optoelectronic materials crystallizing in polar structures. In this work, temperature-dependent 1H and 207Pb magic-angle spinning (MAS) NMR, Raman as well as high-pressure Raman studies of CH3NH2NH2PbCl3 (MHyPbCl3) are reported. Raman spectroscopy reveals many similarities between phonon properties of MHy lead halides and the MA and FA analogues. In particular, these families of compounds show an increase in wavenumber of cage modes when large I- ions are replaced by smaller Br- and then Cl- ones. They also show strong sensitivity of the CH3 torsional mode on size of the cavity occupied by MHy+ cation that decreases with decreasing size of the halide anion. The cage modes of MHyPbCl3 are, however, observed at significantly lower wavenumbers than similar modes of MAPbCl3 and FAPbCl3, indicating higher softness of MHyPbCl3. Temperature-dependent Raman and NMR studies demonstrate that the MHy+ cations in MHyPbCl3 are significantly less affected by the temperature-induced phase transition than the Pb-Cl framework. This suggests a displacive type of the phase transition dominated by tilting and deformation of the PbCl6 octahedra. Analysis of the 207Pb MAS NMR spectra reveals the presence of two differently distorted PbCl6 octahedra and diminishing (increasing) distortion of the less (more) distorted octahedra in the high-temperature phase. Pressure-dependent Raman studies reveal the presence of a single first-order pressure-induced phase transition between 0.72 and 1.27 GPa. Analysis of the spectra indicates that the driving forces for the pressure-induced phase transition in MHyPbCl3 are tilting and distortion of the PbCl6 octahedra accompanied by reorientation of MHy+ cations. Raman spectra do not show evidence of any additional phase transition or amorphization up to 6.95 GPa

Smooth interface effects on the Raman scattering in zinc-blende AlN/GaN superlattices

Raman spectra of (AlN)(8-delta)/(AlxGa1-xN)(delta)/(GaN)(8-delta)/(AlxGa1-xN)(delta) superlattices with interface thickness varying between delta = 0 and delta = 3 are calculated. The influence of the nonabrupt interface related broadening is described in the complete range of scattering, with special attention to the modes giving stronger contribution to the Raman intensity. It is shown that the dispersion of folded acoustic phonons does not change appreciably with the interface smoothing. For delta = 0 the Raman spectra display new peaks due to the enhancement of some confined optic modes.6119130601306

Phonon Raman spectra of colloidal CdTe nanocrystals: effect of size, non-stoichiometry and ligand exchange

Resonant Raman study reveals the noticeable effect of the ligand exchange on the nanocrystal (NC) surface onto the phonon spectra of colloidal CdTe NC of different size and composition. The oleic acid ligand exchange for pyridine ones was found to change noticeably the position and width of the longitudinal optical (LO) phonon mode, as well as its intensity ratio to overtones. The broad shoulder above the LO peak frequency was enhanced and sharpened after pyridine treatment, as well as with decreasing NC size. The low-frequency mode around 100 cm-1 which is commonly related with the disorder-activated acoustical phonons appears in smaller NCs but is not enhanced after pyridine treatment. Surprisingly, the feature at low-frequency shoulder of the LO peak, commonly assigned to the surface optical phonon mode, was not sensitive to ligand exchange and concomitant close packing of the NCs. An increased structural disorder on the NC surface, strain and modified electron-phonon coupling is discussed as the possible reason of the observed changes in the phonon spectrum of ligand-exchanged CdTe NCs

STRUCTURAL PHASE-TRANSITIONS IN RBLISO4

Unpolarized Raman spectra of RbLiSO4 were obtained for pressures in the range 1 atm-9.5 GPa. Several modifications cations observed in the spectra were interpreted in terms of two-phase transitions. The critical pressures for these transitions were determined as 2.5 and 5.5 GPa on increasing the pressure. The first transition is irreversible and can also occur for pressures <2.5 GPa, although through a process varying slowly. The higher pressure transition is reversible but with a marked hysteresis.24313313

SHARP LINE EMISSION OF KZNF3-CR3+

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Pressure Raman scattering of CdTe quantum dots

A series of zinc-blende CdTe quantum dots in glass was investigated using Raman scattering and luminescence. The dot size was controlled by varying the annealing conditions. Luminescence was employed as a probe for quality of the sample and its adequacy for resonance. Resonant Raman spectra at the lowest confined energy levels of the quantum dots were obtained in function of the pressure in the range 1 bar to 4.6 GPa. The upper limit was established by a phase transition probably to NaCl structure, simultaneous with disappearance of Raman signal. In the latter phase, first-order Raman scattering is forbidden. The Gruneisen parameter for the LO phonon of CdTe quantum dots is given.55116743674

Vibrational spectra of alpha-Ge(HPO4)(2)center dot H2O compound

We have measured Raman and infrared spectra of alpha-Ge(HPO4)(2)center dot H2O compound at room temperature. The analysis of vibrational modes indicated the presence of two non-equivalent HPO42- units in agreement with P-31 nuclear magnetic resonance measurements. A tentative assignment of all the observed modes is proposed based on the previous works reported for other hydrogenphosphate-based compounds. (c) 2005 Elsevier B.V. All rights reserved.40220921