The crystal and local atomic structure of monoclinic ReO2 (alpha-ReO2) under hydrostatic pressure up to 1.2 GPa was investigated for the first time using both X-ray absorption spectroscopy and high-resolution synchrotron X-ray powder diffraction and a home-built B4C anvil pressure cell developed for this purpose. Extended X-ray absorption fine-structure (EXAFS) data analysis at pressures from ambient up to 1.2 GPa indicates that there are two distinct Re-Re distances and a distorted ReO6 octahedron in the alpha-ReO2 structure. X-ray diffraction analysis at ambient pressure revealed an unambiguous solution for the crystal structure of the alpha-phase, demonstrating a modulation of the Re-Re distances. The relatively small portion of the diffraction pattern accessed in the pressure-dependent measurements does not allow for a detailed study of the crystal structure of alpha-ReO2 under pressure. Nonetheless, a shift and reduction in the (011) Bragg peak intensity between 0.4 and 1.2 GPa is observed, with correlation to a decrease in Re-Re distance modulation, as confirmed by EXAFS analysis in the same pressure range. This behavior reveals that alpha-ReO2 is a possible inner pressure gauge for future experiments up to 1.2 GPa.

ReO 2 presents two crystalline variants, with monoclinic and orthorhombic structures. The former is metastable and irreversibly transforms to an orthorhombic structure above 460 o C. The structure of the latter was determined from studies on monocrystalline samples, whereas for the monoclinic variant there are no single crystals available so far. It was found only one monoclinic variant and the structure associated with this variant is based on studies on polycrystals. We analyzed a monoclinic oxide powder sample by X-ray diffraction and refined its pattern by means of the Rietveld Method. We obtained that the monoclinic variant belongs to space group P 21/c, with lattice parameters a = 5.615(3), b = 4.782(2), c = 5.574(2)Å, β = 120.13 (1) o .

We have studied the properties of Ce doping in Ca-site of Ca 2 MnReO 6 double perovskite up to 10%. These compounds have presented a monoclinic cell with P2 1 /n space group. A small increase of lattice parameters in doped sample were observed through Rietveld refinement of X-ray diffraction pattern. The spectra at the Mn K-edge and Re L 3 -edge showed a mixed valence for Mn and Re in both samples. We do not observed any variation on Mn K-edge valence in XANES measurements under pressure. However a change was observed for Re L 3 -edge as a function of applied pressure. The Ce-doped sample showed a magnetic geometric frustration and a small decrease of the temperature frustration as compared with Ca 2 MnReO 6 sample. The decrease of temperature frustration can be related to the MnO 6 and ReO 6 octahedral distortion.

X-ray powder diffraction was used to study the phase composition of human renal calculi. The stones were collected from 56 donors in Vitória, Espírito Santo state, southeastern Brazil. An XRD phase quantification revealed that 61% of the studied renal stones were composed exclusively of calcium oxalate ͓34% formed only by calcium oxalate monohydrate ͑COM͒ and 27% presents both monohydrate and dihydratate calcium oxalate͔. The 39% multi-composed calculi have various other phases such as uric acid and calcium phosphate. Rietveld refinement of XRD data of one apparent monophasic ͑COM͒ renal calculus revealed the presence of a small amount of hydroxyapatite. The presence of this second phase and the morphology of the stone ͑ellipsoidal͒ indicated that this calculus can be classified as non-papillary type and its nucleation process developed in closed kidney cavities. In order to show some advantages of the X-ray powder diffraction technique, a study of the phase transformation of monohydrate calcium oxalate into calcium carbonate ͑CaCO 3 ͒ was carried out by annealing of a monophasic COM calculi at 200, 300, and 400°C for 48 h in a N 2 gas atmosphere. The results of the XRD for the heat treated samples is in good agreement with the thermogravimetric analysis found in the literature and shows that X-ray powder diffraction can be used as a suitable technique to study the composition and phase diagram of renal calculi.

Mercury superconductor samples doped with rhenium, Hg0.82Re0.18Ba2Ca2Cu3O8+d, were produced in a very narrow range of oxygen content (d = 0.05, 0.10 and 0.15). The lattice parameters indicated a slight change of the unitary cell as a function of the oxygen content. Furthermore, the reduction of the volume cell was associated with the slight increment of TC. The measurements of χac versus temperature, under external hydrostatic pressure, have shown sensitive results as a function of distinct oxygen content. In our opinion, this kind of measurement may be used as a tool to determine the oxygen content (underdoped, optimal or overdoped) of samples. For dTC/dP > 1.9 K GPa−1 the sample presents an underdoped oxygen content. One can understand that dTC/dP = (1.9 ± 0.2) K GPa−1 is associated with an intrinsic term value for the optimal oxygen doped sample, Hg0.82Re0.18Ba2Ca2Cu3O8.10.