R.E. Gladyshevskii

Standardization of crystal structure data as an aid to the classification of crystal structure types

Abstract Owing to the different ways in which crystal structures may be described, isotypic compounds are often not identified as such. To remedy this situation, crystal structure data can be standardized by means of the structure tidy program. In the standardized data of isotypic structures, occupied sites have the same Wyckoff representation. This makes it possible to use the Wyckoff sequence (the letters of occupied Wyckoff sites) to classify crystal structure types. This classification is much finer than the previously used classification based on the Pearson code and is of great help if one wants to know whether a particular atom arrangement is already known. The standardization has enabled us not only to demonstrate new cases of isotypism, but also to discover structural relationships between different structure types with the same space group, for example substitution, vacancy or filled-in variants.

Er2RhSi3 and R2CoGa3 (R Y, Tb, Dy, Ho, Er, Tm, Yb) with Lu2CoGa3 type structure: new members of the A1B2 structure family

Abstract The structure of Er2RhSi3 was redetermined by X-ray single-crystal diffraction ( λ(Mo Kα)=0.71073 A , μ=42.392 mm −1 , F(000)=892, T=293 K , R=0.037, wR=0.030 for 244 contributing unique reflections). It is shown that this silicide has a hexagonal structure of the Lu2CoGa3 type, hP24, (194) P63/mmc-khfb, a=8.1130(7), c=7.7556(9) A , V=442.09(9) A 3 , Z=4, M r =521.68, D x =7.838 mg mm −3 . The structure of isotypic Er2 Co1.4Ga2.6 (hP24, (194) P63/mmc-khfb, a=8.607(3), c=6.898(3) A , V=442.5(4) A 3 , Z=4, M r =598.30, D x =8.980 mg mm −3 ) was refined by X-ray powder diffraction ( λ(Fe Kα)=1.93735 A , F(000)=1017.6, T=293 K , R=0.092 for 61 reflections). The cell parameters of R2CO1+xGa3−x phases (R  Y, Tb, Dy, Ho, Er, Tm, Yb; x= ±0.4) with the same structure type were obtained from X-ray powder diagrams. The Lu2CoGa3 structure is a distorted substitution variant of the A1B2 type, where the trigonal Lu6 prisms centred by cobalt atoms share triangular faces in infinite columns. The cobalt atoms are displaced away from the prism centres like the mercury atoms in the orthorhombic KHg2 structure type. The distortions in Er2RhSi3 are of a lesser magnitude than those observed in Lu2CoGa3. A progressive substitution of gallium by cobalt along the RGa2-RCoGa cross-sections of the R-Co-Ga systems (R  Y, Tb, Dy, Ho, Er, Tm, Yb, Lu) leads to a step by step deformation of the trigonal prisms. The main features of other deformation and substitution derivatives of the A1B2 type are discussed.

The crystal structure of orthorhombic Gd3Ni5Al19, a new representative of the structure series R2+mT4+mAl15+4m

The structure of Gd3Ni5Al19 has been determined from single-crystal diffraction data [λ(MoKα) = 0.71073 A, μ = 16.614 mm−1, F(000) = 2316, T = 293 K, wR = 0.023 for 997 contributing unique reflections]. This ternary aluminide crystallizes with a new, orthorhombic structure type, Pearson code oS108, (63) Cmcm, Wyckoff sequence f12c3, a = 4.0893(7), b = 15.993(2), c = 27.092(4) A, V = 1771.8(5) A3, Z = 4, Mr = 1277.95, Dx = 4.790 mg mm−3. The unit cell parameters of GdNiAl4 with orthorhombic YNiAl4 type, Pearson code oS24, (63) Cmcm, a = 4.0863(7), b = 15.520(4), c = 6.612(1)A, were refined from single-crystal diffraction data [λ(MoKα) = 0.71073 A, T = 293K]. The Gd3Ni5Al19 structure can be considered as an intergrowth of two kinds of slab, one cut from the orthorhombic YNiAl4 type and the second one corresponding to the translation unit of a hypothetical monoclinic R2T4Al15 structure. The general formula of this inhomogeneous linear structure series can be expressed as R2+mT4+mAl15+4m, where m is the number of consecutive intergrown YNiAl4-type slabs. The Gd3Ni5Al19 type is the simplest member of this structure series with m = 1. The following member with m = 2 is the monoclinic Y4Ni6Al23 type.

New Bi-based high-Tc superconducting phases obtained by low-temperature fluorination

Abstract A new superconducting phase, Bi 2 Sr 2 Ca 2 Cu 3 O 8 F 4 , was obtained by fluorination of standard Bi(2223) at moderate temperatures (250–300°C), using as fluorine source ammonium hydrogen difluoride, NH 4 HF 2 . The presence of fluorine in this new phase was confirmed by energy-dispersive X-ray analysis. X-ray and neutron diffraction experiments furthermore showed that fluorine atoms replace oxygen atoms in the BiO layers in the ratio 2:1. The additional anions form a square-mesh layer between neighboring BiF layers. The incorporation of fluorine increases the crystallographic c -axis parameter by ∼1.8 A ( a =5.409 A, b =5.407 A, c =38.792 A). The CuO 2 layers remain undistorted but the distance from the Cu atom to the apical oxygen atom of the square pyramids is decreased to 2.27 A, with respect to 2.42 A in the original phase. The superconducting transition temperature, determined from magnetic susceptibility measurements, was found to be 75 K. When applied to Bi(2212), the same fluorination process produced the new phase Bi 2 Sr 2 CaCu 2 O 6 F 4 with similar structural features.

Magnetic and thermal properties of the 116 K superconductor Tl-1223

Abstract The ( Tl ,Pb,Bi) 1 ( Sr ,Ba) 2 Ca 2 Cu 3 O 9 (Tl-1223) superconducting phase is characterized by means of X-ray diffraction, SEM and optical micrographs, AC susceptibility, resistivity, magnetization M ( T , H ) both in the reversible and in the irreversible regime up to 5 T, and specific heat near T c up to 14 T. Reversible magnetization data are analyzed using the London model in the low field limit, and the lower critical field is determined from the first flux penetration. Values are given for the upper critical field, μ 0 H c2c (0) = 75 T, the thermodynamic critical field, μ 0 H c (0) = 0.81 T, the lower critical field, μ 0 H c1c (0) = 0.037 T, the penetration depth, λ ab (0) = 1370 A , the Ginzburg-Landau parameter, κ ( T c ) = 45, κ (0) = 65, and for the mean-field specific heat jump, ΔC / T c = 18 mJ/(K 2 mol). The variation of the entropy with the field is measured near T c and agrees with magnetization data, thus demonstrating equilibrium conditions. Large magnetic hysteresis is observed only at temperatures below 20 K. The irreversibility line in the high temperature regime follows the functional dependence H irr ( T )∞(1− T / T c ) 2 . A systematic departure from this behaviour is observed below 70 K, possibly as a result of a phase transformation in the vortex system.

Preparation by in-situ reaction and physical characterization of Ag(Au) and Ag(Pd) sheathed (Tl, Pb, Bi) (Sr, Ba) 2Ca2Cu3O9−δ tapes

Superconducting tapes with high-purity substituted Tl(1223) powders (> 95 wt.%, Tc ∼120 K) were produced by a new method based on an in-situ reaction under high pressure of helium (25 bar He/0.5 bar O2). A Ag(20 wt.% Au) alloy showed to be a good sheath material for an in-tape reaction up to 990°C, even when involving partial melting of the oxide which favors the grain growth. Due to an interaction between Pd and the oxide, a Ag(6 wt.% Pd) alloy is much less suitable for tape preparation. Reproducible transport critical current densities of 9 kA/cm2 (77 K, 0 T) were obtained for Ag(Au) sheathed Tl(1223) tapes with in-tape-reacted powder, the highest value being 10 kA/cm2. The magnetic-field dependence of jc of such tapes was improved with respect to Ag sheathed tapes with pre-reacted Tl(1223) powders. Locally textured samples with up to 50 μm large plate-like grains were obtained after partial melting in Ag(Au) sheathed tapes.

AC LOSSES AND CRITICAL CURRENTS IN AG/(TL,PB,BI)-1223 TAPE

Abstract The results of AC loss measurements of silver-sheathed (Tl,Pb,Bi)-1223 tapes prepared by the powder-in-tube method are presented. Losses arising from an external AC magnetic field are compared with those generated by AC transport currents (self-field losses). Experimental results, obtained by both methods, show that energy losses are purely hysteretic in nature in the range of applied frequencies (30–200 Hz). Critical current densities are derived from the magnetic-loss data and compared with those measured by the four-point transport method. Further, the dependence of the transport critical currents on the external DC magnetic field, for different geometrical configurations of the tapes with respect to the field, i.e. field parallel or perpendicular to the plane of the sample, are presented. The experimental results are compared with theoretical predictions based on the critical-state model.

Y2Co3Al9 with Y2Co3Ga9 type structure: an intergrowth of CsCl- and Th3Pd5-type slabs

Abstract The structure of Y2Co3Al9 was determined by the single-crystal diffraction method (ψ( Mo K α ) = 0.71073 A , μ = 19.931 mm−1, F(000) = 1104, T = 293 K, ωR = 0.037 for 555 unique reflections). This new aluminide has an orthorhombic structure of Y2Co3Ga9 type, oS56, (63) Cmcm − hg2feca, a = 12.740(2)A, b = 7.4635(9) A , c = 9.321(1) A , V = 886.3(2) A 3 , Z = 4, Mr = 597.44, Dx = 4.478 mg mm−3. The Y2Co3Al9 structure can be interpreted as being built up of two kinds of slabs cut from the simple structures CsCl (composition CoAl) and Th3Pd5 (composition Y2Al5): Co3Al3+Y2Al8=Y2Co3Al9. The cobalt atoms are surrounded by eight aluminium atoms at contact distances, forming a distorted cube, and four yttrium atoms at relatively longer distances, the 12 atoms together forming an icosahedron. The yttrium atoms centre capped hexagonal prisms, the coordination numbers for the aluminium atoms are 10 or 11.

Structural characterization and superconducting properties of (Tl0.5Pb0.5)(Sr2−xBax)Ca2Cu3O9−δ

Abstract Samples of nominal composition (Tl0.5Pb0.5)(Sr2−xBax)Ca2Cu3O9−δ, 0 ≤ x ≤ 0.6, and (Tl0.6Pb0.2Bi0.2)(Sr1.8Ba0.2)-Ca2Cu3O9−δ were characterized by X-ray powder diffraction, scanning electron microscopy and AC susceptibility measurements. In the Bi-free samples, the highest weight fraction of the 1223 phase, 91.3%, was observed for x = 0.4. Above this Ba content, BaPbO3 perovskite appeared as main impurity phase. In the Bi-containing sample the weight fraction was even higher, 92.6%. The Ba content of the 1223 phase, as determined from X-ray diffraction data, was in agreement with the nominal composition of the samples. The unit cell parameters increased when the Sr atoms were progressively replaced by the larger Ba atoms. The refinement of the crystal structure of the 1223 phase revealed a significant off-centering of the Tl site. The distance from the splitted site to the ideal position on the four-fold axis was found to be ∼ 0.26 A for the Ba-containing samples, but only 0.07 A for the Ba-free sample. The distance from the Cu atom to the apical atom of the square pyramid formed by the oxygen atoms increased from 2.29 to 2.50 A as the Ba content was raised. The critical temperature remained between 116 and 120.5 K and was not significantly affected by changes in composition or structure.

Unsubstituted Tl-1223: a possible candidate for high current applications of superconductivity

Abstract The Tl–Ba–Ca–Cu–O system has been investigated. We report on the superconducting properties and the pathway for the formation of the Tl-1223 phase and on equilibrium phase relations. The formation of Tl-1223 implies the double TlO layer phases and occurs at high temperature (890–910°C) by a progressive de-intercalation of TlO layers from Tl-2223. As a consequence of this non-equilibrium process, transient intergrowth phases may be formed and have been unambiguously identified. This path of formation results in a platelet-like morphology for Tl-1223 grains which, in the view of the fabrication of tapes, is much more interesting than the globular shape observed for (Tl,Pb)(Sr,Ba) 2 Ca 2 Cu 3 O z . Moreover, the superconducting properties ( T c and irreversibility line) of the unsubstituted Tl-1223 phase compare well with those of (Tl,Pb)(Sr,Ba) 2 Ca 2 Cu 3 O z . With regard to phase equilibria, we show the existence of a three phase field Tl 2 Ca 3 O 6 –(Tl-2212)–(Ca,Tl) 1− x CuO y which prevents the formation of Tl-2223 for Tl contents higher than two.