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Low temperature heat capacity of TTF (TCNQ)
Vol. 17, No. 1
ABSTRACTS OF PAPERS TO APPEAR IN J. PHYS. CHEM. SOLIDS
and 2.1 eV. This asymmetric H centre is formed from a primary symmetric H centre, designated the HN2 centre. The HN2 centre is <110)-oriented. The transformation of the symmetric HN2 centre into the asymmetric HN1 one is associated with important changes in the values of the isotropic hyperfine interaction and of the g1 shift. Received 28 May 1974 Revised 13 January 1975
15.
PARAMAGNETIC RESONANCE UNEWIDTH OF EuO NEAR THE CURIE TEMPERATURE M.S. Seehra and D.W. Sturm,Physics Department, West Virginia University, Morgantown, West Virginia 26506, U.S.A.
Measurements of the EPR linewidth i~Hof EuO at 9 GHz are reported in the temperature range 66—300°K) with particular attention to the region near T~(69.60 K). Comparison with the earlier data of Eastman at 25 GHz in the critical region shows considerable suppression of ~H at 25 GHz. The ternperature-dependent behavior of L~H in EuO at 9 GHz is similar to the observations in CrBr3 and it is in qualitative agreement with the zero-field predictions of Huber and Maleev. The quantitative discrepancies are believed to be due to the effect of the resonance magnetic field on EPR spin dynamics near T, and an inadequate decoupling of the four-spin correlation functions used in the theories. Received 8 January 1975 16.
LOW TEMPERATURE HEAT CAPACITY OF TTF (TCNQ) R. Viswanathan, Brookhaven National Laboratory, Upton, New York 11973, U.S.A. and D.C. Johnston, Institute for Pure and Applied Physical Sciences, University of California, San Diego, La Jolla, California 92037, U.S.A.
Low temperature heat capacity measurements of two TTF (TCNQ) polycrystalline samples are reported. The existence of a linear term and an anomolously
xi
high Debye temperature for one of them at low ternperatures are attributed to impurity effects. The literature data on other (TCNQ) salts are discussed. Received 18 February 1975
17.
TEMPERATURE AND PRESSURE DEPENDENCES OF THE PROPERTIES AND PHASE TRANSITION IN PARATELLURITE (TeO2): ULTRASONIC, DIELECTRIC AND RAMAN AND BRILLOUIN SCATTERING RESULTS P.S. Peercy, I.J. Fritz and G.A. Samara, Sandi Laboratories, Albuquerque, New Mexico 87115, U.S.A. The effects of temperature and pressure on the
ultrasonic propagation properties, dielectric constants and the Raman and Brfflouin spectra in paratellurite (Te02) were investigated with emphasis on the behavior in the vicinity of the newly-discovered, pressure-induced phase transition. The transition is found to be second-order and purely strain-induced, driven by a soft shear acoustic mode propagating along a <110) and polarized along a <1 TO) crystal direction. Such pure-strain transitions were previously discussed by Anderson and Blount and the transition in paratellurite is the first observation of this kind of transition. No evidence was found for any coupling of the soft mode to any other acoustic or optic mode, although small anomalies associated with lattice strains accompanying the transition were observed in some of the elastic and dielectric constants. Analysis of the effective elastic constant C’ governing the soft mode velocity indicates that, within experimental uncertainty, the transition can be described by mean-field theory. Although thesignificantly apparent attenuation of the soft mode increased near the transition, it is concluded that this effect is probably due to the fact that the phase and group velocities are not parallel rather than to intrinsic dissipative processes in the crystal. With the exception of C~,the remaining elastic constants and Raman-active phonon frequencies displayed normal increases with pressure. No soft Raman-active modes were observed in either phase. The static dielectric constants e~are large, due to the large electronic polarizability of TeO2, and the anisotropy in e results almost entirely from the
ABSTRACTS OF PAPERS TO APPEAR IN J. PHYS. CHEM. SOLIDS
and 2.1 eV. This asymmetric H centre is formed from a primary symmetric H centre, designated the HN2 centre. The HN2 centre is <110)-oriented. The transformation of the symmetric HN2 centre into the asymmetric HN1 one is associated with important changes in the values of the isotropic hyperfine interaction and of the g1 shift. Received 28 May 1974 Revised 13 January 1975
15.
PARAMAGNETIC RESONANCE UNEWIDTH OF EuO NEAR THE CURIE TEMPERATURE M.S. Seehra and D.W. Sturm,Physics Department, West Virginia University, Morgantown, West Virginia 26506, U.S.A.
Measurements of the EPR linewidth i~Hof EuO at 9 GHz are reported in the temperature range 66—300°K) with particular attention to the region near T~(69.60 K). Comparison with the earlier data of Eastman at 25 GHz in the critical region shows considerable suppression of ~H at 25 GHz. The ternperature-dependent behavior of L~H in EuO at 9 GHz is similar to the observations in CrBr3 and it is in qualitative agreement with the zero-field predictions of Huber and Maleev. The quantitative discrepancies are believed to be due to the effect of the resonance magnetic field on EPR spin dynamics near T, and an inadequate decoupling of the four-spin correlation functions used in the theories. Received 8 January 1975 16.
LOW TEMPERATURE HEAT CAPACITY OF TTF (TCNQ) R. Viswanathan, Brookhaven National Laboratory, Upton, New York 11973, U.S.A. and D.C. Johnston, Institute for Pure and Applied Physical Sciences, University of California, San Diego, La Jolla, California 92037, U.S.A.
Low temperature heat capacity measurements of two TTF (TCNQ) polycrystalline samples are reported. The existence of a linear term and an anomolously
xi
high Debye temperature for one of them at low ternperatures are attributed to impurity effects. The literature data on other (TCNQ) salts are discussed. Received 18 February 1975
17.
TEMPERATURE AND PRESSURE DEPENDENCES OF THE PROPERTIES AND PHASE TRANSITION IN PARATELLURITE (TeO2): ULTRASONIC, DIELECTRIC AND RAMAN AND BRILLOUIN SCATTERING RESULTS P.S. Peercy, I.J. Fritz and G.A. Samara, Sandi Laboratories, Albuquerque, New Mexico 87115, U.S.A. The effects of temperature and pressure on the
ultrasonic propagation properties, dielectric constants and the Raman and Brfflouin spectra in paratellurite (Te02) were investigated with emphasis on the behavior in the vicinity of the newly-discovered, pressure-induced phase transition. The transition is found to be second-order and purely strain-induced, driven by a soft shear acoustic mode propagating along a <110) and polarized along a <1 TO) crystal direction. Such pure-strain transitions were previously discussed by Anderson and Blount and the transition in paratellurite is the first observation of this kind of transition. No evidence was found for any coupling of the soft mode to any other acoustic or optic mode, although small anomalies associated with lattice strains accompanying the transition were observed in some of the elastic and dielectric constants. Analysis of the effective elastic constant C’ governing the soft mode velocity indicates that, within experimental uncertainty, the transition can be described by mean-field theory. Although thesignificantly apparent attenuation of the soft mode increased near the transition, it is concluded that this effect is probably due to the fact that the phase and group velocities are not parallel rather than to intrinsic dissipative processes in the crystal. With the exception of C~,the remaining elastic constants and Raman-active phonon frequencies displayed normal increases with pressure. No soft Raman-active modes were observed in either phase. The static dielectric constants e~are large, due to the large electronic polarizability of TeO2, and the anisotropy in e results almost entirely from the