M.W. Shafer

Raman measurements of the superionic conductor AgI

Abstract Using the Raman technique, we have observed large abrupt reversible changes in the phonon spectrum at the transition temperature, T c = 147°C, of the superionic conductor AgI. The defect nature of the high temperature phase completely breaks down the selection rules and allows the Raman measurements to give a measure of the frequency dependent conductivity, σ ( ω ), assuming a frequency independent matrix element. Similarities as well as differences in σ ( ω ) above and below T c are shown.

New fluoride ion conductor, KxBi1−xF3−2x (.02≤ x ≤.12), with the tysonite structure

Abstract A study of the phase relationships in the KF-BiF 3 system shows the existence of a new compound occurring in the composition range from about 3 to 10 mole percent KF. X-ray analysis of this compound showed it to be isomorphous with the trivalent rare earth fluorides having the tysonite structure. This compound has been characterized by X-rays, lattice parameter and density measurements, DTA and chemical analysis. Ionic conductivity measurements on both single crystals and pressed pellets in the temperature range from 25 to ∼ 450°C show it to be an excellent fluoride ion conductor.

Raman results in the superionic conductor RbAg4I5

Abstract Raman measurements between 77°K and 296°K are reported for the superionic conductor RbAg4I5. Careful attention is paid to the temperature region of the two phase transitions at 121°K and 208°K. We can detect no shifts in any of the numerous phonon modes except one at 22.9 cm-1, which abruptly and reversibly appears in the lowest temperature phase. Raman results for the isomorphic material KAg4I5 are the same with the same mode appearing in the low temperature phase. Thus, the results in these systems are markedly different from those in AgI, where there are very large changes at the superionic conducting transition temperature.

Fluoride ion conductivity - composition relationships in the fluorite phase region of the KF-BiF3 system

Abstract We have studied the KBiF 4 -BiF 3 system in terms of its compositional relationships and ionic conductivity. The extent of the fluorite phase has been determined and lattice parameters and densities have been measured on compositions having the fluorite phase. Ionic conductivities on single crystals have been measured from 25 to 450°C. A maximum in the conductivity as a function of bismuth concentration and a composition independent conductivity at approximately 375°C have been observed.

Effect of charge density wave fluctuations on the frequencies of optic phonons in 2H-TaSe2 and -NbSe2

The temperature dependence of the frequencies of the Raman active optic phonons between 200 and 250 cm−1 in 2H-TaSe2 and -NbSe2 has been measured for 2 K<T<300 K. The observed shifts are considerably larger than would be expected on the basis of measurements made on the isostructural semiconducting layer compound 2H-MoSe2. The anomalous temperature dependence in the charge density wave (CDW) state. The temperature dependence of the phonon frequencies for temperatures above the CDW phase transition suggests the presence of localized CDW distortions in the normal state.

Raman spectroscopy of intercalated layered structure compounds

Abstract The Raman spectra of intercalated 1TTaS2 and 2HTaSe2 have been measured for the first time. Normal symmetry allowed optic phonons and charge density wave induced modes are observed in both systems. The spectra of the ethylenediamine intercalated compounds are qualitatively similar to those of the pure materials with small quantitative shifts in frequency and changes in the relative intensities of the CDW induced modes. The qualitative similarities in the spectra of the pure and the intercalated compounds reflect the two dimensional character of these materials and show that the dynamic properties of the charge density wave states are largely determined by intralayer effects.

Symmetry of the top of the valence band in MoS2

Abstract It is shown that the previously reported EPR parameters for various acceptors in MoS 2 are characteristic of p z symmetry for the top of the valence band and not of d z 2 symmetry as was previously believed.

Pressure dependence of the Raman modes in Sr2TiO4

Abstract Sr 2 TiO 4 is an insulator with the same crystal structure as the high temperature superconductor La 2−x Sr x CuO 4 . In order to try to better understand these materials, we have studied the effect of hydrostatic pressure on the four allowed Raman modes of Sr 2 TiO 4 up to 7 GPa at room temperature. All modes shift to higher frequencies with increasing pressure. Based on the pressure dependence and bond lengths, we tentatively assign the two high frequency modes to those involving mostly the oxygen motions and the two low frequency ones to those involving primarily the strontium motions. Comparison of mode frequencies in other materials with the same structure is also discussed.

Testing the assumptions of the theoretical calculations in simple superionic conductors: The phonon response in KBiF4

Abstract Theoretical calculations of the phonon infrared and Raman response in simple superionic conductors such as AgI, CuI, and CaF 2 types are based on two fundamental assumptions. First, most of the response can be understood in terms of a breakdown of the selection rules due to disorder (lack of translational symmetry) and second, harmonic lattice dynamics can be used with a good degree of accuracy. This is tested here experimentally in the superionic conductor K 1−x Bi 1+x F 4+2x which has the CaF 2 structure (x=0.0 is analogous to 2CaF 2 ) and the assumptions are found to be appropriate. The calculations are also compared to recent experimental results of the density of states of PbF 2 . Again agreement is good.

Thermoreflectance spectroscopy of systems showing charge density wave transitions

Abstract The thermoreflectance spectra of 1T-TaS2, -TaSe2 and alloys of these compounds have been measured and show substantial changes at the different charge density wave transitions. The changes include the appearance of new structure and the modification of the shape and sign of the modulated spectra. These changes are due to the new gaps introduced into the band structure by the presence of a charge density wave.