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Displacement Ellipsoids (displacement + ellipsoid)

Distribution by Scientific Domains

Chemistry	100%

Selected Abstracts

Dynamic proton disorder and the II,I structural phase transition in (NH4)3H(SO4)2

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2009
Yoo Jung Sohn
X-ray powder diffraction, differential scanning calorimetry (DSC)/thermogravimetry (TG) and single-crystal neutron diffraction methods were used to investigate triammonium hydrogen disulfate (NH4)3H(SO4)2 (TAHS) in the temperature range between 293 and 493,K. The temperature-dependent X-ray powder diffraction measurements show a clear hysteresis of the I II phase transition of TAHS with transition temperatures of Tup = 412.9,(1),K on heating and of Tdown = 402.6,(1),K on cooling. From the existence of hysteresis and from the jump-like changes of the lattice parameters, the I II phase transition of TAHS is considered to be first order. With DSC/TG measurements we confirmed that there is only one phase transition between 293 and 493,K. Through careful investigation on single crystals of TAHS using neutron diffraction, the correct space group (C2/c) of room-temperature TAHS-II phase was confirmed. Crystal structure analysis by single-crystal neutron diffraction showed a strongly elongated displacement ellipsoid of the proton which lies in the middle of the (SO4)H(SO4) dimer with local symmetry. The protons of the NH4 groups also show strongly enlarged anisotropic mean-square displacements. These findings are interpreted in terms of a characteristic proton disorder in the TAHS-II phase. [source]

Octahedral tilting in Pb-based relaxor ferroelectrics at high pressure

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2010
Bernd J. Maier
We have employed a combination of powder neutron diffraction and single-crystal synchrotron X-ray diffraction to characterize the pressure-induced phase transitions that occur in the perovskite-type relaxor ferroelectric PbSc0.5Ta0.5O3 (PST) and Pb0.78Ba0.22Sc0.5Ta0.5O3 (PST-Ba). At ambient pressure the symmetry of the average structure for both compounds is as a result of partial ordering of the Sc and Ta cations on the octahedral sites. At pressures above the phase transition both the neutron and X-ray diffraction patterns exhibit an increase in the intensities of h,k,l = all odd reflections and no appearance of additional Bragg reflections. Synchrotron single-crystal X-ray diffraction data show that the intensity of hhh peaks, h,=,2n,+,1, does not change with pressure. This indicates that the structural distortion arising from the phase transition has a glide-plane pseudo-symmetry along the ,111, cubic directions. Rietveld refinement to the neutron powder data shows that the high-pressure phase has either or symmetry, depending on whether the presence of 1:1 octahedral cation ordering is neglected or taken into account, and comprises octahedral tilts of the type a,a,a, that continuously evolve with pressure. The cubic-to-rhombohedral transition is also marked by a large increase in the anisotropy of the displacement ellipsoids of the Pb cations, indicating larger displacements of Pb cations along the rhombohedral threefold axis rather than within the perpendicular plane. For PST the anisotropy of the Pb displacement parameters decreases at approximately 3,GPa above the phase-transition pressure. For both PST and PST-Ba the average magnitudes of Pb-cation displacements expressed in terms of isotropic displacement ellipsoids gradually decrease over the entire pressure range from ambient to 7.35,GPa. [source]

Le phosphate de cobalt et de lithium à valence mixte Li4+xCo2,x(P2O7)2 (x = 0,03): étude structurale et analyse de distribution de charge

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 1 2010
Salah Kouass
The title compound, namely lithium cobalt(II/III) bis(diphosphate), Li4.03Co1.97(P2O7)2, is a new mixed-valent lithium/cobalt(II/III) phosphate. Three metal sites out of seven are occupied simultaneously by Li+ and CoII/III ions. This disorder was established both from an analysis of the atomic displacement ellipsoids and Li/Co,O bond distances, and by means of a charge-distribution (CHARDI) model, which provides satisfactory agreement on the computed charges (Q) for all the cations. [source]