High pressure Brillouin scattering and energy dispersive x-ray
diffraction study of (NH4)2ZnCl4
J. L. Burris, L. Tereshchenko, P. Kabos, H. D. Hochheimer
Department of Physics, Colorado State University, Fort Collins, CO 80523, USA
Y. Luspin Centre de Recherche sur les Matèriaux à Haute Tempèrature (CRMHT), CNRS,
45071 Orleans Cedex 2, France
C. R. Evenson and P. K. Dorhout Department of Chemistry, Colorado State University,
Fort Collins, CO 80523, USA
We have performed a high pressure Brillouin scattering and energy dispersive X-ray diffraction study on (NH4)2ZnCl4 in thetemperature range from room temperature to 150oC. At ambient pressure three phase transitions occur in this temperature range. Of special interest are the phase transitions at 91oC and 133oC because an incommensurate phase has been detected in this temperature range. We compare the results of our high pressure Brillouin scattering study with the ones obtained from other studies of the elastic constants. Our results for the anomaly in C33 observed at the commensurate-incommensurate transition at 133oC at ambient pressure contradict the results of the other Brillouin scattering studies but agree well with the results of an ultrasonic study. This result seems to indicate that critical fluctuations of the order parameter at the phase transition are reduced by pressure. Values of the calculated adiabatic bulk modulus, B-S, and the experimentally determined isothermal bulk modulus, B-T, for various materials of the A2BX4 family, are compiled and discussed. (C) 2000 Elsevier Science Ltd. All rights reserved. top of page
High pressure pptical and x-ray diffraction studies of two
polymorphs of K(RE)P2Se6 (RE = Pr and Tb )
I. Orgzall Hochdrucklabor bei der Universität Potsdam, Telegrafenberg A43, D-14473
Potsdam, Germany
B. Lorenz Texas Center for Superconductivity, University of Houston, Houston, TX
77204-5932,USA
P. K. Dorhout, P.M. Van Calcar Department of Chemistry, Colorado State University, Fort
Collins, CO 80523, USA
K. Brister CHESS, Wilson Laboratory, Cornell University, Ithaca, NY 14853, USA
T. Sander, H. D. Hochheimer Department of Physics, Colorado State University, Fort
Collins, CO 80523, USA
The results of a high pressure absorption, X-ray diffraction and Raman scattering study indicate a pressure induced phase transition in KTbP2Se6 at about 9.2GPa which is accompanied by a discontinuous jump of the absorption edge of about 0.5eV into the red. The phase transition is also reflected in the Raman spectrum at this pressure. The result of the energy dispersive X-ray diffraction study shows that the high pressure phase above 9.2GPa is also orthorhombic but the b axis is doubled with respect to the low pressure phase. We propose that the high pressure phase is due to charge transfer from the selenium to the phosphorous accompanied by the formation of a Se-Se bond. We discuss also a model which involves a 4f-5d charge transfer in Tb, leading to the intervalence charge transfer from Tb3+P4+ -- Tb4+P3+. Studies on KPrP2Se6 did not show a phase transition in this pressure range. top of page
Study of semiconductor lasers under simultaneous uniaxial stress and
hydrostatic pressure
H. D. Hochheimer, F. Widulle, J. Th. Held, G. Strehl,
Department of Physics, Colorado State University, Fort Collins, CO 80524, USA
R. T. Kotitschke, A. R. Adams
Department of Physics, University of Surrey, Guildford GU2 5XH, United Kingdom
We present the design of a device for the simultaneous application of uniaxial stress and hydrostatic pressure. This new apparatus will allow for the first time measurements at constant strain. Results of the simultaneous application of uniaxial stress and hydrostatic pressure to a semiconductor laser are presented and discussed.
Optical Investigation of template synthesized conducting polypyrrole
under pressure
J. Mikat, I. Orgzall , University of Potsdam, Institute of Physics, P.O. Box 60 15 53,
D-14415 Potsdam, German and Institute for Thin Film Technology and Microsensorics (IDM e.
V.)Telegrafenberg A43, D-14473 Potsdam, Germany
S. Sapp , C. R. Martin , Department of Chemistry, Colorado State University, Fort
Collins, CO 80523, USA
H. D. Hochheimer, Department of Physics, Colorado State University, Fort Collins, CO
80523, USA
The changes in the characteristics of vibrational and optical properties of the conducting form of polypyrrole under pressure is studied.
Several techniques used in combination with high pressure to study phase transitions are described. The usefulness of these techniques is then demonstrated by presenting results of a number of studies of materials which undergo structural phase transitions or isostructural transitions, where the valence state of the atom or the nature of the spin-density wave state changes with pressure.