M. Tournoux

Localisation du doublet solitaire dans les composés oxygènés cristallisés du thallium I

Resume Nos etudes sur les composes oxygenes du thallium I nous ont conduit a proposer un modele permettant le calcul, sur une base electrostatique, de la position du doublet solitaire (appele paire non liee par divers auteurs). Le principe de ce calcul consiste a evaluer le champ au niveau du thallium I et, connaissant sa polarisabilite, a determiner la position du doublet solitaire dont le deplacement traduit la polarisation du thallium I. La methode exposee est applicable a tout ion comportant un doublet solitaire et dont la polarisabilite est connue (ex. Pb II ). Les approximations liees a la nature du modele propose et les limites de validite sont discutees.

Chimie douce with solid precursors, past and present

This contribution deals with the evolution of chimie douce with Solid Precursors during the last twenty years through its two main components (i) redox intercalation deintercalation reactions (ii) acid-base processes based on exchanges, protonation of sites, and structural reconstruction from building blocks. In both cases a critical discussion of the present situation is made whereas possible evolutions for the next future are discussed.

A new bismuth vanadium antimony oxide related to aurivillius phases (Bi4V1.5Sb0.5O10.7)

Abstract A new mixed valence Aurivillius phase Bi 4 V 1.5 Sb 0.5 O 10.7 has been prepared and characterized. The structure is orthorhombic with a = 5.486(2) A , b = 5.513(2) A , c = 15.559(3) A , Z = 2 and was refined from single-crystal data in space group Fmmm. The structure is derived from the Aurivillius basic form in which Bi 2 O 2 layers are interleaved with oxygen lacunary perovskite like layers. A XPS study has shown the presence of Sb V and vanadium in both V IV and V V valence states.

The potassium niobyl cyclotetrasilicate K2(NbO)2Si4O12

Abstract The single-phase compound K2(NbO)2Si4O12 was prepared by solid-state reaction. It crystallizes in the tetragonal system, space group P4bm with a = 8.7404(8)A and c = 8.136(1)A, Z = 2. The structure was determined from 484 reflections collected with a Nonius CAD4 diffractometer with MoKα radiation. The final R index and weighted Rw index are 0.021 and 0.022, respectively. This framework structure is built up from chains of corner-shared NbO6 octahedra running parallel to the four-fold axes and linked together by four-membered Si4O12 single rings. This structure is very similar to that of K4(ScOH)2 Si4O12.

Synthesis, thermal stability, and crystal structure of antimony(V) phosphate SbOPO4

Abstract The compound SbOPO 4 was prepared two different ways: thermal decomposition of the phosphatoantimonic acid HSbP 2 O 8 · x H 2 O or synthesis at 1173 K from a stoichiometric mixture of Sb 2 O 5 · x H 2 O and (NH 4 )H 2 PO 4 . At 1218 K SbOPO 4 undergoes a deoxidation process leading to the Sb(III) phosphate SbPO 4 . SbOPO 4 crystallizes in the monoclinic system, space group C2 c with a = 6.791(1)A, b = 8.033(1) A, c = 7.046(1) A, β = 115.90(1)°, Z = 4. The structure was determined from 481 reflexions collected on a NONIUS CAD4 automatic diffractometer with Mo K α radiation. The final R index and weighted R w index are 0.028 and 0.035, respectively. The structure is built up from chains of corner-shared SbO 6 octahedra, parallel to the c axis, crosslinked by corner-shared tetrahedra. Each phosphate group is bonded to four SbO 6 octahedra of which two belong to the same chain. This structure is closely related to that of β-VOSO 4 and β-VOPO 4 .

Sur une nouvelle famille structurale MII2MITa5O15 structure cristalline de Ca2TlTa5O15

Abstract Ca 2 TlTa 5 O 15 crystallizes in the space group Pmm 2 with a = 10.495(15), b = 7.607(13), c = 3.868(6), Z = 1. The structure was refined by full-matrix least-squares techniques using automatic diffractometer data to R = 0.06. The structure built up from corner-sharing octahedra is closely related to that of tungsten bronzes.

The synthesis and crystal structures of novel antimony compounds: A4Sb4O8(X4O12) (A:K, Rb, Cs, Tl; X =Si, Ge)

Abstract A series of A 4 Sb 4 O 8 ( X 4 O 12 ) compounds ( A =K, Rb, Cs, Tl; X =Si, Ge) have been prepared either by solid-state reaction or via a sol-gel route. Single crystals of the cesium compounds have been isolated and their structure determined by X-ray diffraction. Cs 4 Sb 4 O 8 (Si 4 O 12 ) crystallizes in the tetragonal system, space group P 4 2 / mmc with a = 7.172(1)A, c = 19.645(2)A, Z = 2. The structure was determined from 557 reflections collected on a Nonius CAD4 diffractometer using the Mo Kα radiation. The final R index and weighted R w index are 0.026 and 0.024, respectively. The structure is built up from planes of cross-linked chains of corner-sharing SbO 6 octahedra, linked together via corner-sharing by four-membered Si 4 O 12 single rings. This three-dimensional arrangement delimits intercrossing tunnels wherein Cs cations are situated. Cs 4 Sb 4 O 8 (Ge 4 O 12 ) crystallizes in the tetragonal system, space group I 4 1 22 with a = 10.241(1) A, c = 40.073(6)A, Z = 8. Its structure was determined from 1303 reflections and refined to R = 0.034 and R w = 0.035. It appears as a slightly distorted form of the silicate analogue.

Cobalt Lithium Orthoborate, LiCoBO3

The title compound has been synthesized by a solid-state reaction. Its structure is isotypic with those of LiMgBO3, LiMnBO3 and LiZnBO3. CoII occupies statistically two close positions within the CoO5 trigonal bipyramids. These polyhedra share edges to form chains running along the [\overline{1}01] axis and are linked together, via corner sharing, by BO3 planar groups. The Li atoms occupy statistically the center of two face-sharing tetrahedra. Such pairs of tetrahedra share edges to form chains running along the c axis.

K2(NbO)2Si4O12: A new material for non-linear optics

Abstract The potassium niobyl cyclotetrasilicate K2(NbO)2Si4O12 crystallizes in the non-centrosymmetrical polar space group P4bm with a = 8.7404(8) A, c = 8.136(1) A, Z = 2. The acentric character of the structure is mainly due to a sequence of short (niobyl) and long Nb-O bonds along the 4-fold axes. Optical studies on small crystals have shown that the transparency range extends from 0.3 to 4.5 μm and that they are positively uniaxial. Second harmonic generation tests, as a function of temperature, have been performed on a polycrystalline sample with use of the 1.06 μm line of a YAG-Nd laser. They show that phase matching conditions are still obtained at 1080[ddot]C (limit of the equipment). K2(NbO)2Si4O12 melts congruently at 1180[ddot]C and appears as rather inert and chemically stable.

HTB-like six membered rings of octahedra in some new oxides : structural aspects and related properties

The HTB-like six-membered ring octahedra is the common building unit of all compounds presented in this review. In three of them the entire (001) HTB plane is observed, associated to either XO{sub 4}, X{sub 2}O{sub 7}, or X{sub 3}O{sub 9} groups (X = P, Si, Ge). This association can result in a 2D network as in K{sub 3}Sb{sub 3}P{sub 2}O{sub 14}, xH{sub 2}O, as well as in a condensation of similar layers leading to the 3D framework observed in Cs{sub 3}Sb{sub 3}Ge{sub 2}O{sub 13}. The 3D framework of Cs{sub 8}Nb{sub 10}O{sub 23}{open square}(Si{sub 3}O{sub 9}){sub 2} appears as the first term of an intergrowth between the pyrochlore and benitoite structural types. These compounds are good ion-exchangers or promising precursors for such materials. Another family of compounds containing HTB-like six-membered rings corresponds to the first term of an intergrowth between HTB and perovskite types. Their physical properties are discussed in relation with the nature of the cations located either in the framework octahedra or in the perovskite and HTB sites.