E. Gutiérrez Puebla

Alternation of TaO and TeO layers in a three-dimensional array: Crystal structure of Ta2Te2O9

Abstract Single crystals of monoclinic Ta2Te2O9 were grown by slow cooling of Ta2O5TeO2 melts. Ta2Te2O9 crystallizes in space group P2 1 c (No. 14), Z = 4, with a = 7.100(1), b = 7.486(2), c = 14.625(5) A and β = 102.98(2)°. The structure has been solved by X-ray single-crystal techniques up to a final R value of 0.061 using 1184 observed reflections. Ta2Te2O9 is three-dimensional, with infinite puckered layers of composition (Te4O12)n parallel to the ab plane, alternating along the c axis with infinite sheets constituted by nearly regular TaO6 octahedra sharing corners. The two kinds of tellurium atoms are four- and five-fold oxygen coordinated, the nonbonded pair being directed towards the vacant positions of a trigonal bipyramid and a strongly deformed octahedron, respectively. The structure of Ta2Te2O9 is compared with those known in the systems M2O5TeO2 (M = V, Nb).

Crystal growth of Ba2Cu3RO7 − x (R = Y, Nd, Gd, Yb)

Abstract Two methods of growing Ba 2 Cu 3 RO 7 − δ ( R = Y , Nd, Gd, Yb) crystals were followed. In the first method a molten and water quenched mass of molar ratios Ba : Cu : R = 2 : 3 : 1 is introduced into the furnace at 1040°C, soaked at this temperature for 5 h and cooled at 25°C h -1 . In the second, a mixture of CuO and BaO 2 (72 : 28) is employed as a flux and the crystals are grown by cooling from 1010°C at 5°C h -1 or 15°C h -1 . CuBaR 2 O 5 crystals are also grown. The first method leads to euhedral crystals, better formed than those grown using fluxes. The shape and growth mode of the crystals grown by the second method are accounted for by the presence of helicoidal dislocations of the same sign or by means of a Frank-Read mechanism. X-ray diffraction data for BaCuGd 2 O 5 and Ba 2 Cu 3 GdO 7 − x are included.

Crystal growth of superconducting LiTi2O4

Crystals of LiTi2O4, a member (s = 1) of the hypothetical LisTi(III)4-3sTi(IV)2s-1O4 (0.50≤s≤1.33) spi nel series, were grown by two methods: electrolysis (30 mA, 100 mV) of a bath containing a suspension of TiO2 (anatase) in molten LiBO2, and chemical reduction by addition of a metal, Zr, more electropositive than Ti, on the indicated suspension. LiTi2O4 crystals were characterized by energy dispersive spectroscopy (EDS), X-ray powder diffraction and scanning electron microscopy. Electrolysis led to crystal aggregates of about 1 mm3, grown on the cathode and to skeletal crystals, 0.15 mm3, of cubic morphology related to the development of growth spirals in the three directions. The reduction using Zr plates gave rise to rows of twinned crystals as well as to isolated crystals showing planar faces unequally developed. The crystal structure of LiTi2O4 was anisotropically refined by single-crystal X-ray diffraction in space group Fd3m(No. 227), with a = 8.372(1) A, V = 586.8(1) A3, Li and Ti at tetrahedral and octahedral sites, respectively, an oxygen parameter u = 0.2628(3) and, as a consequence, practically equal Li-O and Ti-O distances. As is shown by AC susceptibility measurements, electrotically grown crystals as well as those grown on Zr plates are very similar in their superconducting transitions, with Tc (onset) of 11.5 K, transition widths of 1.5 K, and diamagnetic shieldings of 65–70%. Magnetization measurements up to 2 T at 6 K on electrolytically grown crystals show a lower critical field Hc1 of 250 Oe and a critical current of 106 A cm-2.

Crystal structure and magnetic properties of CoR(BO2)5 (RY, Gd) and NiR(BO2)5 (RNd, Gd)

Abstract Single crystals of the title borates were grown from mixtures of R 2 O 3 , NiO or Co 3 O 4 and a large excess of B 2 O 3 . Single-crystal X-ray diffraction showed them to be monoclinic, centrosymmetric space group P 2 1 / n (No. 14), Z =4, with cell parameters (in angstroms) a =8.494(2) and 8.560(4), b =7.574(2) and 7.578(2), c =9.364(6) and 9.370(2), and β=94.03(3)° and 93.35(3)° for Co borates and RY and Gd respectively; the parameters were a =8.617(2) and 8.514(6), b =7.565(3) and 7.554(3), c =9.472(2) and 9.4329(6), and β=93.46(1)° and 93.81(2)° for Ni borate and RNd and Gd respectively. The crystals studied by X-ray diffraction were analyzed using energy-dispersive X-ray spectroscopy. Magnetic measurements were taken from 300 to 4 K for the four borates.

Dimorphism and magnetic properties in iron rare earth germanates

Abstract Polycrystalline samples of iron and lanthanide FeRGe 2 O 7 germanates, R=Dy, Ho, Er, Yb have been prepared. They present the new II-FeRGe 2 O 7 structure type, S.G. P2 1 /m (No. 11) Z =4. Results of the refinements from neutron diffraction data for Dy and Yb compounds as well as X-ray data (Ho, Er) are given. Lattice dimensions are a /A=9.6391(4), 9.635(1), 9.646(2) and 9.6554 (5); b /A=8.4743(3), 8.475(1), 8.511(1) and 8.5125(4); c /A=6.7113(3), 6.6701(9), 6.655(1) and 6.6804(4); and β /°=100.538(7), 100.612(7), 100.83(1) and 100.733(3), for R=Dy, Ho, Er and Yb, respectively. Magnetic susceptibility measurements between 350 and 1.7 K reveal the existence of two separated anomalies for each of the four compounds, which appear at T 1 and T 2 ( T 2 T 1 ): 41 and 24 K, 38 and 17 K, 40 and 8 K, 38 and 4 K, respectively. T 1 coincides with the setting up of magnetic ordering in Fe 3+ and R 3+ sublattices, as determined by neutron diffraction in Tb and Dy compounds, and correspond to the Neel temperatures. χ ( T ) maxima at T 2 do not correspond to any phase transition but they are caused by the exchange interaction of R 3+ magnetic moments with the ordered iron subsystem. The crystal structure and magnetic properties of these FeRGe 2 O 7 compounds are compared with those of I-FeRGe 2 O 7 (R=Pr, Nd and Gd) and other II- (R=Y, Tb) compounds.

Enhancement of critical current intensities in single crystals of the LiTi2O4 spinel

Abstract Single crystals of LiTi 2 O 4 grown by two methods and anisotropically refined by single crystal X-ray diffraction show critical temperatures of 11.5 K, transition widths of 1.5 K and diamagnetic shieldings of 65–70%. Magnetization measurements up to 2 T at 6 K show lower critical fields H Cl of 250 Oe and critical currents of 10 6 Acm −2 , four orders of magnitude higher than those previously reported for polycrystalline samples.

Syntesis and Crystal Structure of Ta 2 Te 2 O 9

During the last years some systematic X-ray diffraction studies on the ternary oxides of Te(IV) and M(V) (M = V, Nb, Ta) have been performed (1–5) but only the crystal structures of V 2 Te 2 O 9 and Nb 2 Te 3 O 11 (4,5) have been described. This work aims to report the preparation and crystal structures of Ta 2 Te 2 O 9, and to compare it to the known structures in the M 2 O 5 — TeO 2 (M = V, Nb) systems.