H. D. Hochheimer

Effect of pressure and temperature on the Raman spectra of solid N2O

High pressure Raman studies up to 18 GPa have been made on solid CO2 at room temperature and low temperatures. Changes in the spectral features of external modes are used to draw conclusions on the structural behavior under compression. Anharmonic contributions to the intermolecular potential and their pressure dependence are determined from the pressure and temperature shift of librational frequencies. Anharmonic contributions to the intramolecular potential are determined by pressure tuning of the Fermi resonance.

High pressure Raman study of solid CS2

Solid molecular CS2 is investigated in a diamond anvil cell in the temperature and pressure range of 6–300 K and 0–20 GPa, respectively, by means of Raman spectroscopy. The pressure dependence of external and internal vibrational frequencies is determined, and used to draw conclusions about the p–T phase diagram. A very strong influence of sample preparation conditions on the crystal quality, especially at low temperatures, is noticed. It turned out that the librational Raman spectrum provides a sensitive test with respect to the crystalline or amorphous structure of the solids under investigation. In addition the origin of the ν2 fundamental and phonon sidebands, some‐times observed in spectra of amorphous CS2, will be discussed. Finally the influence of temperature and pressure is shown in the so‐called ‘‘black’’ CS2, a polymerized modification of CS2, which is formed at pressures above 10 GPa.

High pressure X-ray investigation of AlH3 and Al at room temperature

Abstract Aluminium hydride (AlH 3 ) was studied at pressures up to 35 GPa at room temperature using energy dispersive X-ray techniques. No phase transition was found. The constants obtained by fitting the Birch equation of state to the pressure-volume data are B 0 = 47.9 ± 1.0 GPa and B 0 = 3.3 ± 0.2. The volume change in AlH 3 was compared with the sum of the volume changes of the pure aluminium f.c.c. phase and the universal volume increment for the hydrogen atoms known from measurements of other transition metal hydrides. In contrast with previous findings in vanadium hydride a clear pressure dependent compressibility of the hydrogen volume was found and an approach to the partial volume of hydrogen characteristic of transition metal hydrides was estimated for a pressure of about 46 ± 2 GPa. From these results a continuous approach of hydrogen to metallic behaviour can be expected with increasing pressure. The pressure dependence of the hydrogen volume can be fitted by a Birch equation of state with constants B 0 = 30.9 ± 2.0 GPa and B 0 = 3.2 ± 0.4. No phase transition has been found in pure aluminium up to 27 GPa. A fit of the Birch equation of state to our pressure-volume data yields values of B 0 = 76.6 ± 1.0 GPa and B 0 = 3.2 ± 0.2.

Pressure induced valence change in ytterbium monoxide

Abstract We have investigated the pressure-volume relationship in YbO to 35 GPa using a diamond anvil cell and the energy dispersive X-ray diffraction technique. The anomalous compressibility exhibited by YbO above 8 GPa strongly suggests a valence change in Yb from divalent towards the trivalent state at higher pressures. The valence change appears to be continuous. The change in color from black to yellow in reflected light supports the valence change. The initial bulk modulus of YbO is B0 = 130 ± 10 GPa.

Temperature dependence of the elastic constants in CsCN measured by Brillouin scattering

Abstract The temperature dependence of the elastic constants of CsCN has been studied by Brillouin scattering in the high-temperature disordered cubic phase from 193 to 300 K. The elastic constants c11 and c44 decrease whereas c12 increases when approaching the phase transition from above. This anomalous behaviour is similar to that found in NaCN, KCN and RbCN. The softening of the symmetry-adapted elastic constants cEg= (c11−c12)/2 and c44 can be described by a linear coupling of the strain field and the order parameter.

High‐pressure Raman and x‐ray studies of the alkali cyanides up to 27 GPa

We have investigated the phase diagrams of the alkali cyanides NaCN, KCN, RbCn, and CsCN in the pressure range up to 27 GPa, using the Raman and energy‐dispersive x‐ray techniques. We completed the information concerning the concept of a general phase diagram of the alkali cyanides with our measurements. In the case of KCN and RbCN we found at room temperature the expected high pressure phase. This phase IV, with a monoclinic structure (space group Cm) can be followed up to about 27 GPa. A different behavior was observed for NaCN. This crystal has in the pressure range between 10–15 GPa a coexistence region between the antiferro‐ordered orthorhombic phase B and the ferro‐ordered monoclinic phase. However, this monoclinic phase IVa in NaCN has a different structure with space group Pm (C1s). Finally the combination of the x‐ray and Raman data allowed the determination of the structure of CsCN at low temperature and at high pressures. The structure is rhombohedral with space group R3m (phase VII).

Pressure-dependent crystal field splitting of Pr3+ in LaCl3: The observation of a crossing of crystal field levels

Abstract The dependence of the ground state crystal field splitting of Pr 3+ in LaCl 3 on hydrostatic pressure up to 14.0 GPa has been studied by fluorescence techniques and interpreted by the angular overlap model. For the first time a crossing of crystal field levels has been observed directly.

Raman spectroscopic study of tris-sarcosine calcium chloride in the antiferroelectric phase

Abstract The polarized Raman spectra of (CH 3 NHCH 2 COOH) 3 CaCl 2 (TSCC) have been obtained applying hydrostatic pressure in the paraelectric, ferroelectric and in the pressure-induced antiferroelectric phase. The phase transition between the paraelectric or the ferroelectric and the antiferroelectric phase appears to be of first order. No cell doubling could be observed in the antiferroelectric phase. The space group P2 1 a (C 5 2h ) for TSCC in this phase is compatible with our experimental results. The pressure-dependence of the Raman-active soft mode is discussed qualitatively.

Pressure Dependence of the Raman Spectra of TSCC

We have studied the polarized Raman spectra of tris-sarcosine calcium chloride under hydrostatic pressure (p<500 MPa) at low temperatures (100 K<T<180 K). The pressure and temperature-dependence of the soft mode in the ferroelectric phase near Tc has been determined, the spectra have been fitted to a damped harmonic-oscillator model and to a correlation function derived from a coupled pseudospin-phonon model assuming an order-disorder type transition.

Exafs high pressure study on RbCN: Strong coupling of Rb+ displacement and CN− rotational motion in the B1−and B2−phases

Abstract Room temperature EXAFS measurements at the Rbue5f8 K -edge of RbCN at 1 bar ( B 1-phase, NaCl-type structure) and 6.35 kbar ( B 2-phase, CsCl-type structure) show small mean square displacements σ 2 for the C- or N-atom of the CN − -ion relative to a neighbouring Rb + -ion. Large σ 2 are found for the Rb + −Rb + pairs which are in agreement with neutron diffraction data. These results give direct evidence that the displacement of the Rb + -ions from the ideal lattice positions is strongly coupled to the rotational motion of the CN − -ions in both phases.