G.V. Chandrashekhar

Anomalous conductivity effects in (Na,K) mixed crystals of the β-Al2O3 type

Abstract Anomalous behavior has been observed in the ionic conductivity of (Na,K) mixed crystals of the alkali gallates and aluminate of the β-Al2O3 type fast ion conductors. The conductivity goes through a minimum at some intermediate composition as a consequence of a maximum in the activation energy, and is most pronounced in the (Na,K)-β-gallate, followed by (Na,K)-β-Al2O3 and (Na,K)-β“-gallate. The effect is similar to the well-known “mixed alkali effect” in glasses. A second anomaly consisting of a pronounced increase in conductivity over about a 70° temperature range, without any permanent change in activation energy, was observed for some compositions of (Na,K)-β“-gallate.

Absence of excited triplets in the EPR of the high Tc superconductors and the antiferromagnetic insulator CuO

Abstract We have been unable to observe electron paramagnetic resonance signals at temperatures of up to ∼ 570K from either the S = 1 2 Cu 2+ ions or the S = 1 excited states of the Cu2+ - Cu2+ pairs in the new Bi2Sr2CaCu2Ox (2212) high Tc ∼80K superconductors, in the YBa2Cu3Ox (123) high Tc∼90K superconductors, or in the antiferromagnetic insulator CuO with TN∼ 230K. No present theory of the finite spin susceptibility in the paramagnetic state is compatible with the absence of EPR. Short-range resonating-valence-bond (RVB) theory gives zero spin susceptibility at low temperatures, but the absence of the S = 1 excited states of pairs appears to eliminate that theory as well.

The mixed alkali effect in (K, Na)–β‐gallate fast ion conductor

K–β‐gallate is a potassium/gallium isomorph of the two‐dimensional fast ion conductor β‐alumina. It exhibits a strong ’’mixed alkali conductivity effect’’ when part of the potassium is replaced by sodium. At ?20 at.u2009% Na the conductivity at room temperature goes through a minimum about 1/5000th of that expected from extrapolation between the conductivities of the single alkali end members. This conductivity minimum is due to a corresponding maximum in the activation energy in the exponent of the conductivity relation, which overrides the opposing effect of a maximum in the pre‐exponent. The effect is attributed to local ordering of the alkali ions—three sodiums are bound in a triplet of interstitial sites around a vacancy, and the potassiums are distributed between normal lattice sites and pairs bound to interstitial oxygens. Raman spectroscopy reveals only one prominent low energy mode that can be related to the alkali ions. This vibrational frequency cannot be related in any obvious way to the mixed alk...

Phonons in the high temperature Bi2Can-1Sr2CunO4+2n superconductors

Abstract We consider phonons in two members of the Bi-high temperature superconductor family, Bi 2 Sr 2 CuO 6 and Bi 2 CaSr 2 Cu 2 O 8 . For these compounds, we sort the zone center phonon modes according to the irreducible representations of the appropriate point group. When there are two (or more) Cuue5f8O planes in the unit cell, there is a B 1g type of mode that only involves the motion (along the c-axis) of the oxygen atoms in these planes. The Raman spectrum of Bi 2 CaSr 2 Cu 2 O 8 is measured by back scattering from ab-plate-like crystals and some mode assignments are given. The unique B 1g phonon is assigned to the 464 cm −1 mode. This frequency is higher than that found for the B 1g mode in YBa 2 Cu 3 O 7 , but comparable to that in Sr 3 Ti 2 O 7 .

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 measurements of the superconductor (La2-xSrx)CuO4

Abstract We report polarized Raman measurements of single crystal (La2-xSrx)CuO4 with x=0, 0.08, and 0.12, which encompasses insulating and superconducting crystals. Group theory predicts four Raman allowed modes of symmetry 2A1g + 2Eg. For all of the crystals we observe two sharp A1g modes at approximately 226 and 430 cm−1, but could not detect either Eg mode. Tabularly summarizing the published Raman data on this material, a great deal of disagreement can be seen among the measurements. However, our results are in good agreement with some of the data. We discuss the absence of frequency dependence on x in our data.

Infrared spectra of the superionic conductors Na, K, Rb, Ag and Ti β-aluminates

Abstract A detailed analysis of the infrared reflectivity spectra of the superionic conductors X β-aluminates (X = Na, K, Ag, Rb and Tl) is presented. A Kramers-Kronig analysis and the oscillator fits to the data have yielded the frequencies of the transverse optic and associated longitudinal optic modes. A model which ignores the coupling between the spinel blocks accounts for the number of observed infrared modes as well as the Raman modes. It is shown that even for the modes characteristic of the motions of the disordered X-ions, the wave vector selection rule is still valid. The results are discussed with the available theoretical calculations.

Electron paramegnetic detection of ionic motional effects in the super ionic conductor β-sodium gallate

Abstract The EPR spectra of Cu2+, Mn2+ and Eu2+, incorporated into the conducting mirror planes of β-sodium gallate, have been studied. For all three impurities, the EPR spectra at or near room temperature show pronounced relaxation effects caused by the motion of the ions. From the relaxation effects the correlation time of the motion is found to be of the order of 10-11 sec. This time corresponds to a residence time for the paramagnetic ion at a lattice site in the conducting plane. From this time and the thermal activation energy an attempt frequency of 1012 Hz is determined.

A new gallate fast-ion conducting isomorph of β-Al2O3

Abstract A new gallium isomorph of the two-dimensional fast ion conductor, β-Al 2 O 3 , Zn-doped K-β-gallate, has been grown by K 2 O vaporization from K 2 O-Ga 2 O 3 melts. A crystal of composition (K 2 O) (0.812ZnO) (6.126Ga 2 O 3 ) with ≅68% excess K + over the stoichiometric composition is typical from a melt with 4% added ZnO. This crystal has all of the charge of the excess alkali compensated by the negative charge on a zinc center in the spinel block and, hence, has no interstitial oxygens in the conduction plane, as is the case with β-Al 2 O 3 and Na-β-gallate. The log σT vs 1/T plot shows a distinct change of slope at 300°C. No corresponding specific heat anomaly could be observed. There is a change in slope in the temperature dependence of two low-energy Raman modes near the same temperature, however. When the K + is replaced by Na + by ion exchange, the resulting log σT vs 1/T plot is linear over the entire measured temperature range, and the Raman spectrum reduces to a single, temperature-independent, low energy mode.

Raman results in the Na β- and β″-gallate superionic conductors

Abstract Raman results of two superionic conducting materials, Na β-and β″-gallate, are reported. The spectra divide clearly into a high energy part, above 160 cm -1 , and a low energy part that varies as Na is replaced with K or Rb. The low energy line, associated with the so called attempt frequency, occurs at 57 and 69 cm -1 for Na β- and β″-gallate respectively. Rather surprisingly, the lines are much narrower than observed in the aluminate analogues. We measure 5.6 and 3.0 cm -1 respectively. The high energy lines are very similar for the β- and β″-gallate crystals.