Jean-Paul Auffrédic

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.

Thermal decomposition of neutral and basic lanthanum nitrates studied with temperature-dependent powder diffraction and thermogravimetric analysis

Abstract The thermal behaviours of La(NO3)3 · 6H2O and La(OH)2NO3 · H2O have been reconsidered using temperature-dependent X-ray powder diffraction and thermogravimetric analysis. For the neutral lanthanum nitrate, the decomposition schemes are atmosphere dependent. For a water-vapour pressure greater than 4.7 Torr, five crystalline phases and one amorphous form have been displayed during the thermal decomposition to the anhydrous phase, while for PH2O lower than 4.3 Torr, only three crystalline phases and one amorphous form have been identified. Depending on the PH2O range, two polymorphic varieties of the tetrahydrated phase have also been obtained. Above 350 °C, the neutral and basic lanthanum nitrates have the same behaviour, and yield, in inert atmosphere, LaONO3, La3O4NO3 and La2O3 successively. In air LaONO3 is strongly reactive with regard to CO2 to give the monoclinic form of La2O2CO3. The crystal structure of the tetragonal oxide nitrate ( a = 4.127(4) A , c = 9.239(1), S.G. P4/mmm) is described, as well as the microstructural properties of La2O3 dominated by structural imperfections.

Etude thermodynamique de la décomposition thermique des hydroxynitrates de zinc

Abstract The reaction scheme of thermal decomposition for four zinc hydroxynitrates was investigated by means of differential scanning calorimetry, thermogravimetry, mass spectrometry, and radiocrystallography. The thermal transformation of Zn(OH)(NO3) · H2O and of Zn3(OH)4(NO3)2 involves the formation of gaseous water and nitric acid from an actual chemical reaction. This reaction is not observed for Zn5(OH)8(NO3)2 · 2H2O and Zn5(OH)8(NO3)2. These results show that the formation of gaseous nitric acid molecules inside the solids is specific to hydroxynitrates of divalent metals M, whose lamellar crystalline structure is characterized by a stacking of hexagonal close-packed layers of formula MX2+m, where m = 0 or 1 and X = OH−, H2O, or NO−3.

A study of the thermal decomposition of ammine zinc hydroxide nitrates

Abstract A new diammine zinc hydroxide nitrate, Zn5(OH)8(NO3)2 · 2NH3 and a new aquoammine zinc hydroxide nitrate, Zn5(OH)8(NO3)2 · 1.3NH3 · 0.7H2O, isostructural with Zn5(OH)8(NO3)2 · 2H2O, have been synthesized. It is demonstrated that the thermal decomposition process of these three compounds into ZnO, investigated by means of thermogravimetry, thermodiffractometry and mass spectrometry, depends upon the substitution rate of H2O by NH3 molecules. The diammine compound decomposes in two stages and the two others in three stages. A new ammine zinc hydroxide nitrate with the proposed chemical formula Zn(OH)(NO3) · NH3 is formed in the first stage of the decomposition of the ammines. It has been characterized by its X-ray powder diffraction pattern. The formation of Zn3(OH)4(NO3)2 during the decompositions requires that the compounds are hydrated. However, the anhydrous solid Zn5(OH)8(NO3)2 is obtained only from the dihydrated hydroxysalt.

Temperature-resolved X-ray powder diffractometry of a new cadmium hydroxide nitrate

A new cadmium hydroxide nitrate, Cd3(OH)5NO3, was prepared by an interdiffusion method and its X-ray powder diffraction pattern was indexed. The symmetry is orthorhombic and the cell parameters are a = 3.4209(3)A, b = 10.0292(6)A, and c = 11.0295(6)A. The thermal decomposition of this solid was analyzed in a vacuum and in a nitrogen atmosphere by the TG, MS, and TRXD methods. Temperature-resolved diffractometry was carried out by means of a curved position-sensitive detector combined to a conventional X-ray powder diffractometer. A new modification of Cd(OH)NO3 was displayed during the first stage of the decomposition and a detailed decomposition scheme is proposed. This study shows that the rate of the structural transformation can be quite different from that of the departure of the gaseous molecules. From TRXD, the evolution of microstructural properties of Cd(OH)NO3 and CdO as a function of temperature is also described.

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.

Thermal decomposition of cerous ammonium nitrate tetrahydrate studied with temperature-dependent X-ray powder diffraction and thermal analysis

Abstract The thermal decomposition process of Ce(NH 4 ) 2 (NO 3 ) 5 ·4H 2 O has been determined by means of temperature-dependent X-ray diffraction and TG-DSC. The transformations are particle-size dependent and have been studied in nitrogen and vacuum. The occurrence of Ce(NH 4 ) 2 (NO 3 ) 5 ·3H 2 O, Ce(NH 4 ) 2 (NO 3 ) 5 ·2H 2 O, Ce(NH 4 ) 2 (NO 3 ) 5 , Ce 2 (NH 4 ) 3 (NO 3 ) 9 or a mixture of these last two phases, as well as the formation of amorphous NH 4 NO 3 have been demonstrated. The diffraction powder patterns have been indexed, a linear variation of the chemical-formula-unit equivalent volumes V eq with the number of water molecules has been derived from the unit-cell volumes.

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.

The thermal behaviour of ceric ammonium nitrate studied by temperature-dependent X-ray powder diffraction

Abstract The thermal decomposition process of Ce(IV)(NH 4 ) 2 (NO 3 ) 6 has been determined by means of temperature-dependent X-ray diffraction and TG-DSC. The first stage is characterised by the simultaneous formation of CeO 2 and the cerous compound Ce(III) 2 (NH 4 ) 3 (NO 3 ) 9 , isostructural with the compounds Ce(III) 2 M(I) 3 (NO 3 ) 9 (M  K, Rb), which decomposes into CeO 2 in the second stage. The process of the reduction of Ce(IV), induced by thermal decomposition, is discussed

Structure and thermal behaviour of lanthanum aluminium nitrates

The structure of the new phase LaAl(NO 3 ) 6 .12H 2 O has been determined from single-crystal diffraction data. The symmetry is trigonal [a=10.948(1) A, c=16.802(2) A, S.G. R 3] and the structure consists of discrete lanthanum and aluminium polyhedra. La atoms are surrounded by six bidentate nitrate groups in the form of an icosahedron and Al atoms are octahedrally coordinated to six water molecules. The remaining H 2 O molecules are located in the holes of the structure, whose cohesion is ensured by hydrogen bonds. The thermal decomposition gives first the cubic phase LaAl(NO 3 ) 6 .6H 2 O [a=12.301(1) A, S.G. P2,3], whose structure has been solved from single-crystal diffraction data. The structure is also built from La(NO 3 ) 3- 6 anions and Al(H 2 O) 3+ 6 cations. The final decomposition product is LaAlO 3 , which crystallises from an amorphous state at about 450°C. A study of the microstructure of LaAlO 3 from diffraction line broadening shows the presence of a significant amount of microstrains at 900°C.