Splitting of optical mode in (NH_4)_2 PtBr_6 studied by Raman scattering

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Hyperfine interactions in amorphous Tm-Cu Alloys

Hyperfine interactions in sputtered amorphous Tm_xCu_1?x, with x=0.70, 0.50, and 0.33, have been measured over a temperature range from 1.7 K to room temperature using the Mössbauer effect in¹⁶⁹Tm. At low temperatures each spectrum shows a well resolved six-line hyperfine pattern indicative of magnetic order and a superimposed quadrupole doublet. The relative spectral weight of the magnetic component increases with increasing Tm concentration and decreases with increasing temperature. The magnetic component persists at unexpectedly high, temperatures with no decrease in overall splitting and no evidence of electronic relaxation. For all three concentrations, the magnetic splitting is the same as that of Tm metal at low temperatures, nearly the free-ion limit. The electric quadrupole shift of the magnetic component is the same for the three samples, but is significantly less than that of Tm metal (hexagonal) and much larger than that of crystalline TmCu (cubic). The non-magnetic doublets have quadrupole splittings that are very nearly the same for the three concentrations and show considerable asymmetry, a typical effect of electronic relaxation, which persists at temperatures above where the magnetic component disappears.     

Mössbauer,magnetic susceptibility,EPR, and EXAFS investigations of the vibrationally-induced low-spin/high-spin transition in a biomimetic Fe(III) complex

Magnetic susceptibility measurements from 2 to 520 K, Mössbauer measurements from 1.2 to 450 K, and EPR measurements at 10 K have been performed on the monomeric Fe^III complex (1,4,7-tris(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triaza-cyclononan)Fe. The complex exhibits a low-spin/high-spin transition at temperatures above 250 K. This behavior is quantitatively explained on the basis of a crystal-field model, which explicitly includes the vibrational properties of iron ligands. The EPR spectrum at 10 K yields a pure Fe^III low-spin signal withg values 2.58(5), 2.12(5), 1.45(5). The values are consistently described by a crystal-field model, which explicitly includes spin-orbit coupling within the t_2g subspace. The temperature dependence of the quadrupole splitting indicates a phase transition at approximately 100 K. The existence of the phase transition is corroborated by the temperature dependence of the effective thickness. The observation of only one quadrupole doublet up to 450 K indicates that the relaxation time between the high-spin and the low-spin configurations is shorter than the quadrupole precession time. X-ray structure analysis on single crystals at RT and temperature-dependent EXAFS investigation of powder material between 30 and 200 K indicate that the observed phase transition induces only changes of bond angles, while the low-spin/high-spin transition most likely induces changes of metal-ligand bond distances.     

Mössbauer study on M1 and M2 sites of Fe2+ in natural hypersthene under dynamic crystal field potential approach

⁵⁷Fe Mössbauer spectra of hypersthene, a natural silicate mineral belonging to the orthopyroxene group, have been taken over the temperature range 77–292 K. At temperatures above 77 K, they show asymmetric quadrupole peaks. This asymmetry arises from the overlapping of two quadrupole doublets from Fe²⁺(3d⁶,⁵D) ions in two different sites (M1 and M2). The quadrupole splitting, isomer shift and their temperature dependence are appreciably different for Fe²⁺ ions in the two sites. The Fe²⁺ quadrupole splitting in the M1 site decreases linearly with temperature, which can be explained quite satisfactorily by using a very simple model of the orbit-lattice interaction.     

First Mössbauer observation of the glass transition in an amorphous metal

We report the first Mössbauer spectra of a metallic glass obtained above the glass transition temperature (T _G ). The temperature at which both the Lamb-Mössbauer factor and the average quadrupole splitting decrease sharply is in agreement withT _G determined by thermal and mechanical measurements, but the width of the quadrupole splitting distribution starts to increase at a much lower temperature. These results are compared with data from similar measurements on more conventional glasses.     

Li and K NMR relaxation study of a LiKSO4 single crystal

The ⁷Li and ³⁹K NMR relaxations in a LiKSO₄ single crystal grown by the slow evaporation method were investigated by employing a pulse NMR spectrometer. From the experimental data, the quadrupole coupling constant and asymmetry parameter were determined at the temperatures of 180 and 300 K. The relaxation processes of ⁷Li and ³⁹K were studied for the LiKSO₄ crystal, and the relaxation times for the ⁷Li and ³⁹K nuclei exhibit remarkable changes near T_c2 (=190 K). The activation energies for ⁷Li and ³⁹K were determined in phases I and III. The large change in the activation energy at 190 K indicates that the Li and K ions are significantly affected during this transition. The correlation time of the ⁷Li calculated from the spin-lattice relaxation time and quadrupole parameters was larger than that of the ³⁹K calculated using the same method. The reason for this is that the Li ion undergoes molecular motion as in the LiO₄ groups.     

Mössbauer effect investigations of FeZn alloy and analysis using charge shift model

Mössbauer studies of metallurgically prepared FeZn alloy reveal that it undergoes an irreversible phase change on heating. The new phase starts precipitating at temperatures below 403 K and causes an apparent decrease in quadrupole splitting. The results are analysed in the framework of charge shift model and it is suggested that the vibrations of iron ion in zinc are likely to be much less anisotropic than those of zinc ions in pure zinc.     

Niobium hydride studied by the electric quadrupole interaction of100Rh

NbH_x (x=0.83, 0.72) was studied between 90K and 450K by observing the quadrupole interaction of¹⁰⁰Rh with the TDPAC method. The transition from a static interaction at low temperatures to the motional narrowing limit in the non-cubic beta phase and the cubic alpha’ phase is well observed.     

Mössbauer studies of FexNbS2 (x = 0.25, 0.33 and 0.5)

Mössbauer studies of Fe_xNbS₂ (x = 0.25, 0.33 and 0.5) have been carried out for temperatures from 4.2K to about 715K. The iron exists in high spin divalent state for all the compositions. The temperature dependence of quadrupole splitting and center shift shows a reversible phase transition at about 600K and a possible disordering of Fe vacancies beyond this temperature for Fe_0.25NbS₂ and a reversible phase transition at 490K for Fe_0.5NbS₂. The hyperfine magnetic fields have been evaluated from the magnetically ordered spectra. The observed temperature dependent line intensities of the quadrupole doublet are attributable to the temperature dependence of the difference in the meansquare amplitude of vibrations parallel and perpendicular to the EFG axis.     

Quadrupole splitting observed in some cubic Tm compounds by169Tm Mössbauer spectroscopy

The cubic CsCl structure compounds TmCu, TmAg and TmRh show a quadrupole splitting above their ordering temperatures. Quadrupole pair and magnetic parameters are determined for TmCu in the ordered state.     

Direct observation of a (3 x 3) phase in alpha-Pb/Ge(111) at 10 K

We have investigated the recently reported structural phase transition at low temperature (LT) for alpha-Pb/Ge(111) [from a (3 x 3) symmetry to a disordered phase] using scanning tunneling microscopy (STM). By tracking exactly the same surface regions with atomic resolution while varying the sample temperature from 40 to 140 K, we have observed that substitutional point defects are not mobile, in clear contrast to previous assumptions. Moreover, STM data measured at the lowest temperatures ever reported for this system (10 K) show that while filled-state images display the apparent signature of a glassy phase with no long-range order, in empty-state images honeycomb patterns with (3 x 3) periodicity, and not distinguishable from data measured at much higher temperatures, are clearly resolved. These new observations cast serious doubts on the nature and/or on the existence of a disordered phase at LT.     

NMR evidence for the coexistence of order-disorder and displacive components in barium titanate

A quadrupole coupling induced 47Ti and 49Ti satellite background which transforms into well-defined satellite lines below T(c) in the ferroelectric phase has been observed in the cubic phase of an ultrapure BaTiO3 single crystal. The results demonstrate the coexistence of a displacive and order-disorder component in the phase transition mechanism and tetragonal breaking of the cubic symmetry due to biased Ti motion between off-center sites in the paraelectric phase above T(c).     

Magnetic properties of the pseudo-binary Fe2(Nb1-xMnx)

The magnetic behavior of the pseudo-binary system Fe₂(Nb_1-xMn_x) is investigated by means of the experimental techniques of X-ray diffraction (XRD), Mössbauer Spectroscopy (MS) and magnetization studies. The XRD results indicate that, up to x=0.3, all samples are single phase with hcp structure. This corresponds to the solubility limit of manganese in this phase. Above x=0.3, all prepared samples present the coexistence of three phases, two with hcp structure and one fcc. The magnetization measurements at low temperatures indicate that the transition temperature increases with the addition of Mn atoms in the Fe₂Nb host (T_N=10 K) up to 58 K for x=0.1. The Mössbauer spectra were fitted with a quadrupole splitting distribution, which indicates that the average quadrupolar splitting increases slightly with the increase of the manganese concentration.     

In-beam Mössbauer spectroscopy:57Fe in Si and Ge

Fe has been studied in the semiconductors Si and Ge with the Coulomb excitation recoil implantation technique in a wide temperature range. In the case of Fe inSilicon it was found that one third of the implanted atoms land on interstitial sites. The long range diffusion of these atoms could be observed microscopically at temperatures around 600 K. The isomer shift of interstitial Fe in Si was determined. The remaining atoms exhibit a strong quadrupole splitting on disturbed sites. This component seems to relax into a state with higher symmetry above 700 K. InGermanium a similar situation is found. Whereas iron on disturbed sites dominates the spectra, the direct implantation into interstitial sites is also observed below 200 K. At higher temperatures the substitutional position is preferred. The isomer shifts for interstitial and substitutional Fe in Si and in Ge are in good agreement with calculated electron densities.     

Langevin description of Markov master equations II: Noise correlations

We present experimental results of dielectric and birefringence measurements on 3.5% and 8% Li∶KTaO₃ as a function of temperature. These results indicate that the 8% Li∶KTaO₃ sample undergoes a ferroelectric phase transition at 97 K. The 3.5% sample seems to undergo a distortive transition at ～75 K. Raman experiments show that the splitting of the TO₁ mode of the 8% Li sample extrapolates to zero at ～97 K supporting the conclusion that this sample undergoes a phase transition at this temperature. Raman spectra of both samples at temperatures above the phase transition or freezing temperatures exhibit in addition to second order Raman features first order Raman lines. These lines have the properties of fluctuation induced first order Raman scattering. Measurement of the integrated intensity of these lines is used to obtain the temperature dependence of the fluctuation intensity. The results are compared to the Halperin and Varma theory. The data obtained for the 3.5% Li sample partialy fits the theory. However the results for the 8% Li sample do not fit the theory. The misfit is attributed to the overlap of defect zones.     

Effect of Sn substitution on the elastic anomalies and phase transition behaviors of antiferroelectric PbHfO3 single crystals studied by Brillouin scattering

The phase transition sequence of a PbHf_0.7Sn_0.3O₃ single crystal was investigated by using Brillouin spectroscopy. The longitudinal acoustic mode showed three distinct changes at ∼370 K, ∼455 K and ∼495 K which corresponded to the antiferroelectric 1 → antiferroelectric 2 → intermediate → paraelectric phase transitions upon heating. The paraelectric phase was characterized by softening acoustic mode, increasing acoustic damping and the appearance of quasielastic central peaks which were observed upon approaching the phase transition temperature. The relaxation times derived from the acoustic mode anomalies and the central peak were consistent with each other and showed a slowing-down behavior. These precursor phenomena were attributed to the polar clusters having broken inversion symmetry. The substitution of Hf with Sn induced a very soft intermediate phase between the paraelectric and the antiferroelectric phases, where the longitudinal acoustic mode exhibited the lowest mode frequency along with substantially high acoustic damping. The transverse acoustic mode, which was observed in the two antiferroelectric phases, did not appear in the intermediate phase. These results indicated that the average symmetry of the intermediate phase may be cubic consisting of fine ferroelastic domains.     

Polar behaviour induced by lithium in potassium tantalate ceramics

Polar behaviour in K(1-x)Li(x)TaO(3) ceramics with x = 0:02, 0.05 and 0.10, processed by the conventional solid state method, is studied by Raman spectroscopy and thermally stimulated depolarization current (TSDC) techniques between 10 and 290 K. The TO1 mode of KTaO(3) is revealed to harden in the whole temperature range and to split in the low-temperature range by Li doping. One splitting is observed for x = 0:02 and two consequent splittings are detected for x = 0:05 and 0.10. The temperatures, where TO1 mode splitting occurs, are found to correspond to those of the peaks of TSDC, and hence to the onset of the electric polarization. Such behaviour provides evidence for the order-disorder ferroelectric phase transition induced in KTaO(3) by lithium doping, which emerges from deformations of the cubic phase developed on cooling either in one (for x = 0:02) or two steps (for x = 0:05 and 0.10).     

Temperature dependence of the Mössbauer effect on the semiconductors Pb0.78Sn0.22Te and Pb0.80Sn0.20Te:In

Crystals of the semiconductors Pb_0.78Sn_0.22Te and Pb_0.80Sn_0.20Te:In were grown by Bridgman method, and investigated by Mössbauer spectroscopy and X-ray diffraction. Mössbauer spectra were taken at temperatures between 80 and 300 K. The absence of the quadrupole splitting shows a cubic symmetry of the environment for the tin atom, which is in accordance with X-ray data. The temperature dependence of the spectral area is discussed and the Debye temperature is estimated.     

Superconducting phase diagram of Li metal in nearly hydrostatic pressures up to 67 GPa

The dependence of the superconducting transition temperature T(c) on nearly hydrostatic pressure has been determined to 67 GPa in an ac susceptibility measurement for a Li sample embedded in helium pressure medium. With increasing pressure, superconductivity appears at 5.47 K for 20.3 GPa, T(c) rising rapidly to approximately 14 K at 30 GPa. The T(c)(P) dependence to 67 GPa differs significantly from that observed in previous studies where no pressure medium was used. Evidence is given that superconductivity in Li competes with symmetry breaking structural phase transitions which occur near 20, 30, and 62 GPa. In the pressure range 20-30 GPa, T(c) is found to decrease rapidly in a dc magnetic field, the first evidence that Li is a type I superconductor.     

Field-induced splitting of the OH-Stretch absorption line in KTaO3 and KTaO3 :Li

Abstract

We have studied the electric field-induced splitting of a single OH stretch mode in KTaO₃ and KTaO₃ :Li. For the three prominent directions [100], [110] and [111] we observed polarized splitting patterns with up to six components. The line positions exhibit no mirror symmetry with respect to the zero-field frequency. This behaviour can be interpreted by taking into account a field-dependent shift of the Ta⁵⁺ ion and a change of the covalent character of the bond. Li-doped KTaO₃ samples show for [Li] < 2mol% a line broadening, while for [Li] < 3.5 mol% a field dependent phase transition has been observed with evidence for a 90 degree domain structure.