Jean Pierre Chaminade

Crystal structure of the new borate Li3Gd(BO3)2. Comparison with the homologous Na3Ln(BO3)2 (Ln: La, Nd) compounds

Abstract The structure of Li3Gd(BO3)2 has been solved by X-ray diffraction study on single crystal. This novel borate crystallizes in the monoclinic system with the P21/c space group (Z=4). The cell parameters are respectively equal to a=8.724(2), b=6.425(2), c=10.095(2)xa0A and β=116.85(2)°. Refinements of 110 parameters using 2924 independent reflections having I>2σ(I) converged to R1=0.028 (wR2=0.058). The structure of Li3Gd(BO3)2 is made up of double layers of eightfold coordinated Gd atoms parallel to the (bc) plane. They are linked by respectively three- and four-coordinated boron and lithium atoms. The structure is compared to that of the homologous sodium compounds, Na3Ln(BO3)2 (Ln: La, Nd), in which LnO8 polyhedra also form a bi-dimensional array.

Etude des propriétés cristallographiques, diélectriques et d'optique non linéaire de quelques nouvelles phases de composition BaxLi5−2xNb5(1−y)Ta5yO15 et de structure bronzes oxygénés de tungstène quadratiques

Resume Deux series de phases de structure “bronzes oxygenes de tungstene quadratiques” et de composition BaxLi5−2xT5O15(T = Nb, Ta) ont ete mises en evidence au sein des systemes BaNb2O6ue5f8LiNbO3 et BaTa2O6ue5f8LiTaO3. Toutes ces phases possedent des transitions ferroelectriques-paraelectriques. La temperature de Curie TC augmente avec le taux en lithium. La valeur de TC correspondant a la phase Ba2.03Li0.94Nb5O15 est la valeur la plus elevee obtenue jusqua present pour le type structural envisage. Ces phases se comportent comme des doubleurs de frequence. Le niobate Ba2.14Li0.71 Nb5O15 presente un rendement harmonique tres eleve, de lordre de 250 fois celui du K.D.P., 2.5 fois superieur donc a celui du “banana” Ba2NaNb5O15. Les etudes radiocristallographique et dielectrique du systeme Ba2.14Li0.71Nb5O15ue5f8Ba2.14Li0.71Ta5O15 ont permis la mise en evidence de trois domaines respectivement antiferroelectrique, ferroelectrique et paraelectrique. La temperature de Curie ferroelectrique et le rendement harmonique en optique non lineaire diminuent lorsque le taux en tantale augmente.

BaZr(BO3)2: a non-centrosymmetric dolomite-type superstructure

Abstract The investigation of the ternary phase diagram for BaOue5f8ZrO 2 ue5f8B 2 O 3 has allowed single crystals to be grown by the flux method with formula BaZr(BO 3 ) 2 . As this composition has been already announced in the literature, but without any structural data, the structure of BaZr(BO 3 ) 2 has been solved using X-ray data collected on a Kappa CCD diffractometer. This borate crystallizes in the trigonal symmetry with non-centrosymmetric space group R 3 c (no. 161): a = b =5.167(1)xa0A and c =33.913(7)xa0A; Z =6. The structure derived from the dolomite-type with a doubling of the c parameter. Along the c -axis, alternated layers of Ba atoms and Zr atoms are separated by planes of BO 3 groups. BaO 6 and ZrO 6 octahedra so formed are linked by their corners and each Ba(Zr) octahedron is linked to six Zr(Ba) octahedra and to six borate groups. The small deviation from centrosymmetry is attested by considering refinement agreement for the (weak) reflections ( hxa0kxa0l ) with odd value of l , and positive response of the powder second harmonic generation (SHG) test. A new J.C.P.D.S.-I.C.D.D. card is proposed for this compound.

Un exemple de chaînes antiferromagnetiques alternees de spin : CuWO4

Abstract The ordering of Cu and W atoms in the distorted wolframite structure of CuWO 4 gives rise to zigzag chains of edge sharing CuO 6 octahedra linked by similar chains of WO 6 octahedra. The existence of two kinds of connections between the CuO 6 octahedra leads to alternating antiferromagnetic interactions along the chains. The magnetic behavior is explained using a Heisenberg type model of Duffy and Barr with the largest exchange integral : J = − 67 K and an alternation parameter : a = 0.8. k B EPR measurements have shown that threedimensional ordering occurs at T N ⋍ 24 K .

Crystal growth and characterization of Ce1-xPrxO2 (x≃0.05) single crystals

Abstract The growth of Ce1−xPrxO2 single crystals (x≃0.05) has been investigated in connection with a systematic research work o n rare earth based red pigments. Ce1−xPrxO2 crystals have been grown by spontaneous nucleation during slow cooling of a high temperature solution using PbF2-Pb3P2O8-NaF or Na2B4O7-NaF as solvents. X-ray diffraction, chemical analysis and optical study were performed on the obtained single crystals. Stabilization of Pr4+ in cerium oxide accounted for the observed red color.

Lithium intercalation in LiW3O9F: An electrochemical and NMR study

Abstract Lithium has been intercalated in LiW 3 O 9 F. This phase exhibits a peculiar packing of hexagonal tungsten bronze layers. Up to two lithium atoms can be intercalated reversibly. If the Li 3 W 3 O 9 F composition is overpassed the reaction becomes irreversible and an amorphization occurs. The variation of the diffusion coefficient versus the amount of lithium has been determined from the electrochemical relaxation curves. An NMR study has shown that Li + ions are not mobile in LiW 3 O 9 F and have a high mobility in intercalated materials at RT.

Lithium Intercalation in Perovskite and Hexagonal Tungsten Bronze Derivatives

Lithium has been intercalated chemically and electrochemically in LnNb 3 O 9 (Ln = La, Nd) perovskite-type phases and LiW 3 O 9 F which can be considered as a hexagonal tungsten bronze derivative. The crystallographic formula of the LnNb 3 O 9 starting material is □3/2 (Ln□ 1/2 )Nb 3 O 9 . In both systems, solid solutions are observed in the fir part of the intercalation reaction. While almost all perovskite cavities are filled in the neodymium phases, the higher ionic character of the La-O bonds prevents practically the Li intercalation in the □ sites. In the LiW 3 O 9 F phase two lithium atoms can be electrochemically intercalated via a single phase mechanism. The reaction is completely reversible. For higher amounts of intercalation (x > 3), irreversible structural modifications occur. In both systems the unit cell parameter variation is negligible during intercalation. This behavior results from the blocking up of the framework by Ln 3+ ions in LnNb 3 O 9 phases or by the stacking of the three octahedra triangular arrangement in the hexagonal tungsten bronze structure.