Jean-Michel Savariault

Structure refinements by the Rietveld method of partially substituted hydroxyapatite: Ca9Na0.5(PO4)4.5(CO3)1.5(OH)2

Abstract The structural location and effect due to partially substitution of Na + and CO 3 2− to Ca ++ and PO 4 3− , respectively, in hydroxyapatite, Ca 10 (PO 4 ) 6 OH 2 (HAP), synthesised in aqueous solution have been investigated by X-ray powder pattern fitting methods. The compound crystallises in the hexagonal system (space group P63/m ) with the following cell: a = b =9.3892(4) A, c =6.9019(3) A. Using the Rietveld method, the refinement of occupancy factors shows that the sodium is located preferentially in the calcium site II of the apatite structure. Vacancies in both calcium sites are found and the OH site is fully occupied. No carbonate ion is found in the apatite channel, leading to a B type carbonate apatite. The carbonate ions are located on two faces of the PO 4 tetrahedron. A n extra shift of the OH − ion outside of the metal II triangle is attributed to the replacement of calcium by sodium and a vacancy. The corresponding new substitution mechanism is compared to already known fundamental mechanisms.

Extended CNDO/2 studies of the isoelectronic series AM(CO)3 (AM η6-C6H6Cr,η5-C5H5Mn,η4-C4H4Fe,η3-C3H5Co,η2-C2H4Ni) and of (η6-C6H6)2Cr

Abstract The electronic structure of isoelectronic AM(C0)3 (AM  η6-C6H6Cr,η5-C5-H5Mn,η4-C4H44Fe,η3-C3H5Co,η2-C2H4Ni) and of (η6-C6H6)2Cr is investigated within an extended CNDO/2 formalism using both experimental and standard geometries. The computed trends for bond strengths and stretching frequencies (as measured by Wiberg indices or bicentric energy terms) and for reactivity (charge distributions) correlate with the experimental ones. For chromium complexes, the extended CNDO/2 results seem more reasonable, for example the charge and the orbital populations on chromium, than are the ab initio single > results. A general agreement is observed between CNDO/2 and SCCC electronic structures.

Silver insertion mode in β-AgxV2O5 tunnel structure

The β-Ag x V 2 O 5 phase of silver vanadate has a large homogeneity range, 0.29≤x≤0.41; the (V 2 O 5 ) n network of its crystal structure is typical of the β phase of the vanadium oxide bronzes with its oxygenated tunnels running along [010]. Ag atoms are inserted in a novel manner, being distributed onto two distinct crystallographic sites, which explains the overstepping of the theoretical boundary limit of the β phases, x=0.33

Intercalation chemistry in layered transition metal oxide structures: Pyridine vanadium pentoxide

Abstract The reaction of pyridine, pyrazine, phenazine and 4-4′ dipyridine in m-xylene solutions on V2O5 yields new intercalation compounds. Only the pyridine compound corresponds to a definite formula: V2O5(C5H5N)1.1. The EXAFS study at the vanadium K-edge is consistent with a pyridine molecule bonded to every second vanadium atom of the layer via coordination bond of length VN = 2.34 A. From X-ray powder diffraction study, the orthorhombic crystal cell (a = 11.543(8) A, b = 7.111(4) A, c = 21.56(1) A) is built up from two V2O5 layers separated by 6.2 A, leading to a value expected for pyridine intercalation. Temperature dependent susceptibility measurements show that 10% of the total vanadium is in oxidation state IV. An interpretation of the presence of the V4+ is established on the basis of the mechanism involved in oxidation reactions catalyzed by V2O5.

Synthesis and structural investigation of a new potassium vanadium oxide bronze: ϱ-K0.50V2O5

Abstract ϱ-K 0.50 V 2 O 5 , synthesized at 350°C by interaction of K 2 C 2 O 4 on V 2 O 5 , crystallizes in the orthorhombic system, space group Cmcm , with the cell parameters a = 3.674(1)A˚, b = 11.607(3)A˚, c = 18.670(8)A˚, V = 756.20(4)A˚ 3 , and Z = 8. Despite the poor quality of the single crystal obtained, structural determination gave an R factor of 0.067 for 498 reflections. The original layer structure of ϱ-K 0.50 V 2 O 5 exhibits [V 2 O 5 ] n sheets built up by four VO 6 octahedra sharing edges and making quadruple endless strings associated by edges along [100]. These sheets, depicted as double layers D4, and their mirror image D4M alternate along [001]. The intercalated potassium atoms hold together the [V 2 O 5 ] n layers. They partially occupy two sites, 80% in K1 at the center of an oxygenated rectangular prism (CN8) and 20% in K2 surrounded by six oxygen atoms forming a trigonal prism (CN6). K1O 8 and K2O 6 polyhedra share a rectangular face, with the short distance between the potassium sites preventing simultaneous occupancy. The K2 site occupancy is related to an electronic localization within the [V 2 O 5 ] n layers. Finally, a comparison is made with other prototype structures containing D4 layers like δ- and ν-K x V 2 O 5 phases.

Pyrochlore-type tin niobate

A single crystal of Sn(1.59)Nb(1.84)O(6.35) was grown at 1273 K from a mixture of sodium niobate and tin(II) chloride. The structure is of pyrochlore type A(2)B(2)O(7). The tin is partially oxidized to tin(IV) and competes with niobium for the occupation of site B. The stereoactivity of the Sn(2+) lone pair induces displacement of tin towards the O atoms of the tunnel.

Une extension de la méthode CNDO/2 à l'étude des complexes d'éléments de transition@@@An extension of the CNDO/2formalism for the study of transition metal complexes: II. Cr(CO)6 et Fe(CO)5@@@II. Cr(CO)6 and Fe(CO)5

The previously proposed extension of the CNDO/2 formalism is used for investigating the electronic structure of Cr(CO)6 and Fe(CO)5. Thus, the ordering and the eigen values of molecular energy levels agree well with results provided byab initio calculations and photoelectron spectroscopy. The formal charge on metal is found to be in any case positive, as in Ni(CO)4 and Ni(PF3)4. Moreover, the validity of our parametrization is supported by the pretty agreement which exists between energy levels distribution and electronic structure provided by our technique and Veillardsab initio results for Ni(CN)42−. The parameters for the (Cr, Fe, Ni) set are now available and will allow to study large series of complexes in order to interpret their physical and chemical behaviour.

New phosphosilicates with Nasicon structure type

New series of compounds, Bi{sub 1/3}Pb{sub 1/6}Zr{sub 2}(PO{sub 4}){sub 8/3}(SiO{sub 4}){sub 1/3} and Ln{sub 1/6}Pb{sub 1/3}Zr{sub 2}(PO{sub 4}){sub 17/6}(SiO{sub 4}){sub 1/6} with Ln = La, Nd, Sm, Eu, Gd, Tb, Er, Yb, have been synthesized via sol-gel routes and characterized by powder X-ray diffraction pattern techniques. Chemical analyses as well as the cell parameter evolution versus the ionic radii of rare earths are reported. The structure of the lanthanum-containing phase, which was refined using Rietveld methods, was found to crystallize in the trigonal system, space group R{bar 3}.

Two original infinite chains in the new caesium tetramolybdate compound Cs2Mo4O13

Although dicaesium tetramolybdate, Cs 2 Mo 4 O 13 , is a member of the molybdate family, its structure deviates from those of other tetramolybdates. It crystallizes in the monoclinic system with an a parameter six times greater than those of the other tetramolybdates. The structure comprises two different infinite chains of [Mo 4 O 13 ] n 2- , which propagate along the [001] direction. One contains blocks made up of two MoO 6 octahedra sharing a face, which is unusual in molybdate compounds. The asymmetric unit of the other chain consists of a set of two MoO 6 octahedra, one MoO 5 square pyramid and one MoO 4 tetrahedron sharing edges and corners.

Heat capacity and thermal expansion study of relaxor-ferroelectric Ba0.92Ca0.08Ti0.76Zr0.24O3

Heat capacity and thermal expansion of the Ba0.92Ca0.08Ti0.76Zr0.24O3 compound is measured using the methods of adiabatic calorimetry and optic-mechanical dilatometry in the temperature range 100–370 K. Three blurred anomalies on the Cp(T) and α(T) dependencies are observed in the temperature range 200–360 K. The results of the studies are discussed together with data on the structure and dielectric properties in the framework of spherical random bond–random field model.