J.A. Campá

Sr9Ni6.64O21: A New Member (n= 2) of the Perovskite-RelatedA3n+3An′B3+nO9+6nFamily

Crystals of a new phase Sr{sub 9}Ni{sub 6.64}O{sub 21} were grown. This compound in the n = 2 member of the A{sub 3n+3}A{sub n}{prime}B{sub n+3}O{sub 6n+9} series. The composition and the crystal structure have been established form X-ray single crystal diffraction data. The structure contains face-shared chains of NiO{sub 6} polyhedra parallel to the c axis and is related with hexagonal 2H polytype perovskite. Sr{sub 9}Ni{sub 6.64}O{sub 21} crystallizes in the space group R3c (No. 167) with a = 9.467(2) {angstrom}, c = 35.87(5) {angstrom}, V = 2784.(4) {angstrom}{sup 3}, and Z = 6. A comparison is made between the structure of the title compound and the other members of the series.

Single-crystal growth of superconducting Bi2Sr2CaCu2O8 using rotary crucibles

Crystals grown using static and rotary crucibles from mixtures of analytical grade Bi2O3, Sr(OH)2·8H2O, CaCO3 and CuO had nominal compositions Bi2SrmCanCupOq (1.5 < m ≤ 2.0, 0.2 < n ≤ 2.0, 1.8 < p < 3.0), were characterized by X-ray powder diffraction and scanning electron microscopy, and contained intergrown crystals of Bi2Sr2CaCu2O8 (2212) and Bi2Sr2CuO6 (2201) in various relative amounts, together with crystals of CuO, (Sr, Ca)2CuO3 and Sr(Cu, Bi)O2. The intergrowths of 2212 and 2201 with highest 2212 contents were obtained from melts at 980°C of mixtures with m = p = 2 and n = 1, using rotary crucibles and a cooling rate of 0.5°C h-1. After being recrystallized, crystals of 2212 appeared free of even traces of 2201, were orthorhombic, had cell dimensions of a = 5.404(2) A, b = 5.418(4) A and c = 30.80(3) A, and showed a superconducting transition at 90 K as indicated by magnetization measurements. The crystal structure of one orthorhombic Sr(Cu, Bi)O2 crystal of unit-cell constants a = rm3.4865(5) A, b = rm16.176(2) A and c = rm3.8818(4) A was refined in the space group Cmcm arriving at the composition Sr(Cuin0.85Bi0.15)O 2.225, R = 0.037, and the main distances and angles which are given.

Crystal structure of BaGd2NiO5 prepared by reaction of Ni metal with BaCO3 and Gd2O3

Abstract Black prismatic crystals of BaGd2NiO5 were grown by reaction of an excess of Ni metal with a mixture of BaCO3 and Gd2O3, Ba:Gd=2:1, after heating in air at 1100°C for three days and slowly cooling. BaGd2NiO5 is orthorhombic, a=3.7872(4), b=5.8388(9), c=11.498(1) A , V=254.25(10) A 3 , Z=2, D c =7.71 g cm −3 , F (000)=504, μ( MoK α)=371.6 cm −1 , S.G. Immm (No. 71), T=295 K, and R=0.019 for 223 observed reflections. The crystal structure can be conceived as the result of packing the metallic coordination polyhedra: BaO10 bicapped quadrangular prisms, GdO7 capped trigonal prisms, and chain-forming NiO6 flattened octahedra.

Splitting of the bulk resistive transition in high-Tc superconductors : evidence for unconventional pairing

Abstract We report on measurements of the resistive transition in several samples of polycrystalline YBa 2 Cu 3 O 7− y and Bi 2 Sr 2 Ca 2 Cu 3 O 10+ x , and single crystal Bi 2 Sr 2 CaCu 2 O 8+ x . The results are analyzed in terms of the temperature derivative of the resistivity, d ϱ /d T . In the ceramic samples, we systematically find that the transition proceeds as a two-stage process. Initially, superconductivity establishes in some disconnected regions of the homogeneous phase. Long range ordering is obtained at lower temperatures as a consequence of a paracoherent-coherent transition involving the granular array. A new, reproducible and general feature is also revealed in our results. The principal maximum of d ϱ /d T , which denotes the intragranular transition, shows a two-peak splitting. This characteristic, which is observed in all investigated samples strongly suggests the occurrence of a double bulk superconducting transition, which is in itself a signature of unconventional order parameter symmetry.

The novel R2Ba1.25NiO5.25 (R = Tm, Yb, Lu) structure type

Abstract Crystals of isostructural R2Ba1.25NiO5.25 (R = Tm, Yb, Lu) have been grown and characterized. The crystal structure for R = Yb has been established from X-ray single crystal diffraction data. R2Ba1.25NiO5.25 are tetragonal, S.G. I4 m (No. 87), with unit-cell parameters a = b = 13.582(3), 13.553(4), and 13.549(3) A , c = 5.656(2), 5.638(2), and 5.633(2) A , V = 1043.4(8), 1035.6(9), and 1034.1(8) A 3 for R = Tm, Yb, and Lu, respectively; and Z = 8. Interatomic distances and principal angles are given for the Yb compound. The crystal structure of Yb2Ba1.25NiO5.25 is formed by square NiO5 pyramids, trigonal YbO7 prisms capped on one rectangular face, tetragonal bicapped Ba(1)O10 prisms, and irregular Ba(2)O10 polyhedra. A comparison is made between this structure type and those of Yb2BaNiO5 and Y2BaNiO5: the RO7 polyhedra look similar for the three types, and Yb2Ba1.25NiO5.25 contains NiO5 groups similar to those of Yb2BaNiO5 as well as Ba(2)O10 units which are shaped like those present at Y2BaNiO5.

Sr{sub 9}Ni{sub 6.64}O{sub 21}: A new member (n = 2) of the perovskite-related A{sub 3n+3}A{sub n}{prime}B{sub 3+n}O{sub 9+6n} family

Crystals of a new phase Sr{sub 9}Ni{sub 6.64}O{sub 21} were grown. This compound in the n = 2 member of the A{sub 3n+3}A{sub n}{prime}B{sub n+3}O{sub 6n+9} series. The composition and the crystal structure have been established form X-ray single crystal diffraction data. The structure contains face-shared chains of NiO{sub 6} polyhedra parallel to the c axis and is related with hexagonal 2H polytype perovskite. Sr{sub 9}Ni{sub 6.64}O{sub 21} crystallizes in the space group R3c (No. 167) with a = 9.467(2) {angstrom}, c = 35.87(5) {angstrom}, V = 2784.(4) {angstrom}{sup 3}, and Z = 6. A comparison is made between the structure of the title compound and the other members of the series.

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.

New rare-earth (Y, Yb) bismuth(III) germanates. An initial study of a promising series

Colored crystals of the middle term (a = 1) of the Bi2−aRaGeO5 series (R = Y (yellow), Yb (brown)) have been grown. The structure of these materials has been established by single-crystal X-ray diffraction. They are isostructural and crystallize in the orthorhombic system, space group Pbca (No. 61), Z = 8, with unit-cell parameters a = 5.341(8) and 5.315(2), b = 15.232(2) and 15.225(2), c = 11.084(3) and 11.025(2) A, for R = Y and Yb respectively. The structure is formed by [GeRO5]∞ layers containing both (RO7)6 units and GeO4 tetrahedra. Between these layers zig-zag chains of ψ-BiO5 octahedra run in the a direction. The stereoelectronic effect of the Bi3+ lone pair leads to this apparent zig-zag, and therefore to the kind of packing observed in this new type of structure. The downward deviation in the molar magnetic susceptibility of BiYbGeO5 from the Curie–Weiss behavior, observed at low temperatures, is attributed to the effect of the crystal field splitting of the 2F7/2 ground term of Yb3+. Energies of the maximum phonon, 799 and 797 cm−1 for Y and Yb materials, respectively, have been determined from room temperature Raman scattering spectra of polycrystalline samples.

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.

Thermal fluctuation effects on the magnetization above and below the superconducting transition in Bi2Sr2CaCu2O8 and YBa2Cu3O7−δ crystals in the weak magnetic field limit

We present detailed experimental data of the magnetization, Mab(T,H), of Bi2Sr2CaCu2O8 and YBa2Cu3O7−δ crystals on both sides of the superconducting transition, for magnetic fields, H, applied perpendicularly to the ab (CuO2) planes. The data are analyzed in terms of thermal fluctuations in the weak H limit: In the reversible mixed state below the transition, by taking into account the vortex fluctuations, as first proposed by Bulaevskii, Ledvij and Kogan, which are much more important in the Bi-based compounds. Above the transition, by taking into account the order parameter amplitude fluctuations (OPF), through a generalization to multilayered superconductors of the Schmidt-like approach.