Showing papers by "Juergen Eckert published in 1980"

Neutron scattering study of the pressure dependenceof short-range order in CsD/sub 2/PO/sub 4/

Abstract: We present the results of a neutron scattering study of the pressure dependence of the order-disorder phase transition in CsD/sub 2/PO/sub 4/. In an earlier study, it was shown that the dominant interaction propagates along one-dimensional chains of disordered hydrogen bonds linking phosphate groups along the b direction. In the absence of applied pressure, the interchain ordering is ferroelectric; at high pressure, it is antiferroelectric. This is shown to arise from extremely small interactions along the a direction between correlated b-c planes; with increasing pressure, these effective interactions change character so as to favor an antiferroelectric stacking of ferroelectric planes, resulting in a doubling of the unit cell along a. Interchain and intrachain couplings are inferred from a comparison of the diffuse scattering data with the predictions of a model based on weak coupling between one-dimensional chains. These results are compared with those obtained in a recent study of the dielectric behavior.

Phase transition and rotational excitations in Ni(NH3)6I2

Abstract: Neutron scattering techniques were used to investigate both the crystal structures and the rotational dynamics of the ammonia groups in Ni(NH3)6I2 above and below the phase transition near 20 K. The phase transition involves orientational ordering of the ammonia groups and is accompanied by a distortion of the crystal lattice from cubic (space group Fm3m) to trigonal symmetry along with a doubling of the primitive cell. A deuterated sample was employed for the structural studies. Its transition temperature was found to be 26.5±0.3 K. Extensive studies of the rotational dynamics of the NH3 groups above the phase transition showed that their motion is best described as one‐dimensional rotational diffusion about their trigonal axis. Diffusion constants derived from this analysis as a function of temperature exhibit increasing orientational correlations as the transition temperature is approached. At low temperatures the NH3 groups perform one‐dimensional tunneling motions. The resulting splitting of the librational ground state was determined to be 0.0634±0.0014 meV in good agreement with the value derived from earlier specific heat measurements. A further excitation at 9 meV is interpreted as a transition to the first excited librational state.

Search for charge density waves in potassium

Abstract: Motivated by the recent calculation of Guiliani and Overhauser in which a criterion for the preferred orientation of the wave vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}}$ of a charge density wave (CDW) in the alkali metals was derived, we have carried out a neutron-diffraction search for CDW's in potassium metal. Although the search was not exhaustive of all the possibilities, we have been unable to detect any satellite peaks at (or near) the predicted positions in reciprocal space. However, we have found small diffraction peaks along certain symmetry axes at other positions in reciprocal space. They are strongly temperature dependent, resembling the Debye-Waller effect in potassium metal, but they do not satisfy the symmetry requirements of a CDW modulation of the body-centered-cubic crystal structure.

Instrument development for materials science research at WNR

Abstract: The neutron scattering program at the Los Alamos spallation neutron source is based on the operational WNR facility which provides up to 11 ..mu..A of 800 MeV protons to a target in pulse widths up to 8 ..mu..s at 120 Hz. The immediate goals of the program are: to gain experience with neutron instrumentation at spallation neutron sources; and to explore the scientific potential for condensed matter research at these sources. The proton storage ring (PSR) funded for construction will provide 100 ..mu..A in 0.27 ..mu..s pulses at 12 Hz, therefore greatly improving intensity, time-of-flight (TOF) resolution, and repetition rate. The initial emphasis, given limited manpower and resources, has been placed on developing a set of prototype instruments which are relatively easy to implement and which take advantage of the unique characteristics of the present WNR when compared with reactor neutron sources.