M. Louër

Time-resolved study by X-ray powder diffraction with position-sensitive detector: rate of the β-Cs2Cdl4 transformation and the effect of preferred orientation

A time-resolved X-ray powder diffraction of the transformation, at room temperature, of the metastable β-Cs2Cdl4 phase into α-Cs2Cdl4 is described. It is based on data collected by means of a conventional X-ray diffraction system combined to a curved position-sensitive detector. The transformation is governed by a nucleation and a three-dimensional growth process, and its rate depends on the particle size and crystal imperfections. When particle sizes are less than 40 μm, the rate of the transformation strongly decreases in the later stage of growth, due to the stresses produced by structural modifications. Particular attention is paid to the fact that the quantitative interpretation of experimental data in kinetic terms, can be influenced by the existence of a preferred orientation effect of the crystallites in the sample.

Crystal structure determination of Zr(OH)2(NO3)2 · 4.7H2O from X-ray powder diffraction data

Abstract The crystal structure of Zr(OH)2(NO3)2 · 4.7H2O has been solved ab initio from X-ray powder data obtained from a conventional diffractometer. The unit cell was obtained by means of the successive dichotomy indexing method. This is triclinic, space group P 1 with the following lattice parameters: a = 9.541(1) A, b = 9.590(1) A, c = 6.753(1) A, α = 98.64(1)°, β = 92.89(1)° and γ = 118.60(1)°, Z = 2. A total of 136 integrated intensities, unambiguously indexed, were used to generate a Patterson function from which the approximate heavy atom coordinates were derived. The remaining atoms were located by an interpretation of successive three-dimensional Fourier maps. The structure was refined by means of the Rietveld method (RF = 0.039, RB = 0.063, Rp = 0.084 and Rwp = 0.108). In accordance with the chemical analysis, the structure model shows that the most probable number of water molecules is 4.7. The structure of Zr(OH)2(NO3)2 · 4.7H2O consists of infinite and isolated chains of trigonal dodecahedra sharing edges. The zirconium atom is eightfold coordinated by four OH groups, two water molecules, and one bidentate nitrate group. The chains are held together by hydrogen bonds through additional water molecules and nitrate groups located between the chains.

Structure and thermal expansion of KGe2(PO4)3

KGe{sub 2}(Po{sub 4}){sub 3} has been prepared in both crystalline and vitreous forms. The NZP-type structure of the crystalline variety and its thermal expansion have been investigated by X-ray powder diffraction. The crystal structure has been refined by the Rietveld method in the space group R{bar 3}. The thermal expansion coefficients have been determined between room temperature and 600 C and compared to dilatometry measurements. The parameters a and c have a positive and isotropic behavior and the average macroscopic thermal expansion coefficient, , is 10 {times} 10{sup {minus}6} C{sup {minus}1} at 600 C. A comparison with the thermal expansion of the related Li and Na compounds is reported.

Structure solution from powder data of the phosphate hydrate tinticite

The crystal structure of the mineral tinticite has been solved by direct methods from integrated intensities of X-ray powder diffraction data and subsequently refined with the Rietveld technique. The sample used for the structure solution comes from the Gava-Bruguers area (20 km SW of Barcelona), which contains a large variety of phosphates, some of which were exploited in gallery mines during the ancient neolithic. Tinticite crystallizes in the triclinic space group P 1 with unit cell parameters a = 7.965(2) A, b = 9.999(2) A, c = 7.644(2) A, α = 103.94(2)°, β = 115.91(2)°, ω = 67.86(2)° and cell content Fe 3+ 5.34 (PO 4 ) 3.62 (VO 4 ) 0.38 (OH) 4 ·6.7 H 2 O; ρ exp = 2.94 g/cm 3 ; ρ cale = 2.88 g/cm 3 . The Rietveld refinement of the data set converged to R wp = 13.1 % and χ 2 = 3.3. Due to the complexity of the disorder in this structure, the refined structure model could only account for part of it. The octahedrally coordinated Fe 3+ ions form dreier single chains of general formula ∞ 1 [Fe 3 O 14 ] at y = 0 and trimers of type cis-[Fe 3 O 14 placed at y = 1/2. While the dreier single chains are linked to each other by fully occupied PO 4 groups yielding in this way predominantly ordered layers, the trimers arc partially disordered and connected to each other and to the ordered layers both by PO 4 groups and through H-bonds. The higher stability of the ordered layers is consistent with the observed platy nature of the microcrystals of tinticite.

Temperature-dependent X-ray diffraction and crystal structure of CeRb2(NO3)5 · 4H2O

Abstract The structure of CeIIIRb2(NO3)5 · 4H2O was determined from single crystal diffraction. The symmetry is monoclinic (space group Cc): a = 11.050(1) A , b = 8.977(1) A , c = 17.859(2) A , β = 100.877(9) °, Z = 4. The structure consists of icosahedra [Ce(NO3)5(H2O)2]2− linked by hydrogen bonds. The Rb atoms and two water molecules are located between these polyhedra and ranged in an alternating sequence along [010]. From temperature-dependent X-ray diffraction and thermogravimetry measurements, the occurrence of CeRb2(NO3)5 · 3H2O, CeRb2(NO3)5 · 2H2O, α- or β-CeRb2(NO3)5 in crystalline or amorphous forms, and the mixture of Ce2Rb3(NO3)9 and CeRb(NO3)4, have been shown during the thermal decomposition. The transformations are atmosphere and particle-size dependent, and the dehydration mechanisms are discussed.

Structure cristalline et polymorphisme du nitrate de cadmium anhydre

Cd(NO3)2 undergoes a phase transition at 160°C. The high temperature form is cubic and isomorphic with M(NO3)2 (M = Ba, Ca, Sr, Pb). The crystal structure of the low temperature phase has been solved by X-ray diffraction at 20°C, using 774 independent reflections collected with a 4-circle diffractometer. The dimensions of the orthorhombic unit cell are: a ⋍ c = 7.5073 (14) A, b = 15.3692 (35) A, Z = 8, space group Pca21. The structure has been refined to the final weighted R = 0.044. The cadmium atoms are nearly in a face-centered arrangement. Each cadmium is octahedrally surrounded by six oxygen, the CdO distances varying from 2.34 to 2.46 A. Each nitrate group belongs through its three oxygens to three different octahedra. The structural change cubic Cd(NO3)2 → orthorhombic Cd(NO3)2 is characterized by a small rotation of NO3 groups in their plane; the face-centered array of cadmium atoms is only slightly modified. The coordination of cadmium atoms changes from 12 to 6, and the approximate doubling of parameter (b) as well as the difference of symmetry can be explained by two different directions of rotation of the NO3 groups situated in the same plane.

Crystal structure and temperature-resolved powder diffractometry of Cd5(OH)8(NO3)2 · 2H2O

A new cadmium hydroxide nitrate Cd{sub 5}(OH){sub 8}(NO{sub 3}){sub 2} {center dot} 2H{sub 2}O was prepared by an interdiffusion method. It is monoclinic with the cell parameters a=18.931(3) {angstrom}, b=6.858(2) {angstrom}, c=5.931(1) {angstrom}, {beta}=94.85(2){degree}; the space group is C2/m with Z=2. The crystal structure has been solved from single-crystal data by means of Patterson and Fourier synthesis (R=0.043,2218hkl). The structure, related to the brucite type, is built up from OH{sup -} ion layers parallel to (100) with 3/4 of the octahedral holes filled with cadmium atoms. The remaining metal atoms are located above and below the empty octahedral sites; they are sixfold coordinated by three OH{sup -}, one water molecule, and one bidentate nitrate group. The thermal decomposition of this compound was investigated by means of TRXD and TG methods. It proceeds in four or three stages, depending upon the environmental atmosphere. Under vacuum, the hemihydrate Cd{sub 5}(OH){sub 8}(NO{sub 3}){sub 2} {center dot} 0.5H{sub 2}O is obtained during the first stage, whereas a new anhydrous hydroxide nitrate is displayed in the second one.

Structure cristalline et moleculaire du tricarbonyl (1,2,3,4,5,6-η-3-ethylendo-7-phenyl-1,3,5 cycloheptatriene) chrome.

Abstract The title compound crystallizes in the space group P2 1 /n with four molecules in a unit cell of dimensions a = 11.795 (3), b = 16.260 (4), c = 8.273 (2) A, β = 94.66 (2)°. Data were collected on a NONIUS CAD-4 automatic diffractometer. The structure was solved by direct methods and refined to R and R″ values of 0.067 and 0.079 for 2069 independent reflections. It consists of discrete molecules of formula C 18 H 16 CrO 3 . It is compared with (1,2,3,4,5,6-η-exo-7 phenyl-1,3,5 cycloheptatriene) tricarbonylchromium structure and with 1,2,3,4,5,6-η benzene chromium tricarbonyl.

A powder X-ray diffraction study of lead chloride oxalate Pb 2 Cl 2 (C 2 O 4 ): ab initio structure determination and thermal behavior

Mixed lead chloride oxalate, Pb 2 Cl 2 (C 2 O 4 ), has been obtained in a polycrystalline form in the course of a study on precursors of nanocrystalline PZT-type oxides. Its crystal structure has been solved ab initio from powder diffraction data collected using a monochromatic radiation from a conventional X-ray source. The symmetry is monoclinic, space group C 2/ m , the cell dimensions are a =5.9411(3) A, b =5.8714(4) A, c =9.4212(4) A, β=95.232(4)° and Z=2. The structure consists of a stacking of complex double sheets, built from lead polyhedra, parallel to (001) and connected together through oxalate groups. The lead atom is nine-fold coordinated by four O atoms from one bidentate and two monodentate oxalate groups and five Cl atoms. The polyhedron can be described as a highly distorted square antiprism mono-capped by a Cl atom. The thermal behavior of lead chloride oxalate, in vacuum and in air, is carefully described from temperature-dependent powder diffraction and thermogravimetric measurements. It is shown that reaction pathways are complicated by the identification of various oxide chloride phases.

Synthèse et structures cristallines de Cs3MI4NO3 (M = Zn, Co, Cd): Un exemple de substitution sélective

Abstract The compounds Cs 3 MX 5 ( M is a bivalent metal, and X an halogen) consist of Cs + , I − , and distorted ( M I 4 ) 2− ions. The separate X − ion suggests a possible substitution by another monovalent anion. The new compounds Cs 3 M I 4 NO 3 ( M = Zn, Co, Cd) have been synthesized and characterized by X-ray diffraction. They are orthorhombic Pnma , a = 10.114(4), b = 11.601(5), c = 14.290(9) A for Cs 3 ZnI 4 NO 3 ; a = 10.078(8), b = 11.621(4), c = 14.262(6) A for Cs 3 CoI 4 NO 3 ; a = 10.177(4), b = 11.784(5), c = 14.336(7) A for Cs 3 CdI 4 NO 3 ; Z = 4. The crystal structures are described. The NO 3 groups surrounded by six Cs + cations occupy the same sites as the separate I − ion in the Cs 3 M I 5 compounds.