Sr2V3O9 and Ba2V3O9: quasi one-dimensional spin-systems with an anomalous low temperature susceptibility

The magnetic behaviour of the low-dimensional Vanadium-oxides Sr2V3O9 and Ba2V3O9 was investigated by means of magnetic susceptibility and specific heat measurements. In both compounds, the results can be very well described by an S=1/2 Heisenberg antiferromagnetic chain with an intrachain exchange of J = 82 K and J = 94 K in Sr2V3O9 and Ba2V3O9, respectively. In Sr2V3O9, antiferromagnetic ordering at T_N = 5.3 K indicate a weak interchain exchange of the order of J_perp ~ 2 K. In contrast, no evidence for magnetic order was found in Ba2V3O9 down to 0.5 K, pointing to an even smaller interchain coupling. In both compounds, we observe a pronounced Curie-like increase of the susceptibility below 30 K, which we tentatively attribute to a staggered field effect induced by the applied magnetic field. Results of LDA calculations support the quasi one-dimensional character and indicate that in Sr2V3O9, the magnetic chain is perpendicular to the structural one with the magnetic exchange being transferred through VO4 tetrahedra.Comment: Submitted to Phy. Rev.

High-pressure dependent ferroelectric phase transition in lead titanate

International audienceTi K edge x-ray absorption near-edge structure (XANES) measurements across the pressure induced ferroelectric-paraelectric phase transition in PbTiO3, a prototypical ferroelectric perovskite, are reported. A quantitative analysis of the XANES allows us to obtain the relative local displacement, directly related to the local electric polarization, of Ti and Pb atoms under pressure. In particular, we found that above the critical pressure, in the so-called cubic-paraelectric phase, the Ti atoms remain locally displaced from their cubic site, indicating that this phase transition has, in a general way, an essential order-disorder component. However, the magnitude of these local displacements is strongly affected by the applied pressure as opposed to what happens as a function of temperature. These conclusions have important consequences in the long standing controversial debate on the interpretation of the ferroelectric phase transition of PbTiO3 and related family compounds

High-pressure dependent ferroelectric phase transition in lead titanate

International audienceTi K edge x-ray absorption near-edge structure (XANES) measurements across the pressure induced ferroelectric-paraelectric phase transition in PbTiO3, a prototypical ferroelectric perovskite, are reported. A quantitative analysis of the XANES allows us to obtain the relative local displacement, directly related to the local electric polarization, of Ti and Pb atoms under pressure. In particular, we found that above the critical pressure, in the so-called cubic-paraelectric phase, the Ti atoms remain locally displaced from their cubic site, indicating that this phase transition has, in a general way, an essential order-disorder component. However, the magnitude of these local displacements is strongly affected by the applied pressure as opposed to what happens as a function of temperature. These conclusions have important consequences in the long standing controversial debate on the interpretation of the ferroelectric phase transition of PbTiO3 and related family compounds

High pressure macromolecular crystallography: The 140-MPa crystal structure at 2.3 A resolution of urate oxidase, a 135-kDa tetrameric assembly

We report the three-dimensional structure determined by high-pressure macromolecular crystallography (HPMX) of a 135-kDa homo-tetrameric enzyme, urate oxidase from Aspergillus flavus complexed with its potent inhibitor 8-azaxanthin. Urate oxidase crystals are quite sensitive to pressure, as three-dimensional order is lost at about 180 MPa. A highly complete 2.3 A resolution data set was collected at 140 MPa, close to the critical pressure. Crystal structures at atmospheric pressure and at high pressure were refined in the orthorhombic space group I222 with final crystallographic R factors 14.1% and 16.1%, respectively. The effect of pressure on temperature factors, ordered water molecules, hydrogen bond lengths, contacts, buried surface areas as well as cavity volume was investigated. Results suggest that the onset of disruption of the tetrameric assembly by pressure has been captured in the crystalline stat

High-Pressure Macromolecular Crystallography (HPMX): Status and prospects.

Recent technical developments, achievements and prospects of high-pressure (HP) macromolecular crystallography (MX) are reviewed. Technical difficulties associated with this technique have been essentially solved by combining synchrotron radiation of ultra-short wavelength, large-aperture diamond anvil cells and new sample-mounting techniques. The quality of diffraction data collected at HP can now meet standards of conventional MX. The exploitation of the potential of the combination of X-ray diffraction and high-pressure perturbation is progressing well. The ability of pressure to shift the population distribution of conformers in solution, which is exploited in particular by NMR, can also be used in the crystalline state with specific advantages. HPMX has indeed bright prospects, in particular to elucidate the structure of higher-energy conformers that are often of high biological significance. Furthermore, HPMX may be of interest for conventional crystallographic studies, as pressure is a fairly general tool to improve order in pre-existing crystals with minimal perturbation of the native structur

High pressure macromolecular crystallography: The 140-MPa crystal structure at 2.3 A resolution of urate oxidase, a 135-kDa tetrameric assembly

We report the three-dimensional structure determined by high-pressure macromolecular crystallography (HPMX) of a 135-kDa homo-tetrameric enzyme, urate oxidase from Aspergillus flavus complexed with its potent inhibitor 8-azaxanthin. Urate oxidase crystals are quite sensitive to pressure, as three-dimensional order is lost at about 180 MPa. A highly complete 2.3 A resolution data set was collected at 140 MPa, close to the critical pressure. Crystal structures at atmospheric pressure and at high pressure were refined in the orthorhombic space group I222 with final crystallographic R factors 14.1% and 16.1%, respectively. The effect of pressure on temperature factors, ordered water molecules, hydrogen bond lengths, contacts, buried surface areas as well as cavity volume was investigated. Results suggest that the onset of disruption of the tetrameric assembly by pressure has been captured in the crystalline stat