The crystal structure of SnYb3Rh4Sn12, a new ternary superconducting stannide

The crystal structure of superconducting SnYb₃Rh₄Sn₁₂ has been determined from single-crystal X-ray diffraction data. This compound is cubic, space group Pm3n, $\text{a}{}_{\mathrm{o}}\text{= 9.676}\text{A}\text{?}$ (1) and has two formulae per unit cell. The structure was solved from Patterson and subsequent Fourier synthesis. The least squares refinement was based on 375 independent reflections. The final R and wR factors were 0.015 and 0.014, respectively. The two Sn(1) atoms occupy the 2a (000) positions, the six Yb atoms the 6d ($\text{1}\text{4}$$\text{1}\text{2}$ 0) positions, the eight Rh atoms the 8e ($\text{1}\text{4}$$\text{1}\text{4}$$\text{1}\text{4}$) positions and the twenty-four Sn(2) atoms the 24k (Oyz) positions (y ～ 0.31, z ～ 0.15). The Sn(2) atoms form a tridimensional array of corner-sharing trigonal prisms whose centers are occupied by the rhodium atoms. The Sn(1) and the Yb atoms occupy the icosahedral and cuboctahedral holes of this array, respectively. They form a sublattice which has the arrangement found in the structure of the A15 compounds. The structure of SnYb₃Rh₄Sn₁₂ can be described as containing two interpenetrated structures, namely Yb₃Sn and RhSn₃, or as having an A15 arrangement of clusters of atoms such as (SnSn₁₂) and (YbSn₁₂). These clusters are bound together by face-sharing among them; and by the rhodium atoms. An analogy is drawn between SnYb₃Rh₄Sn₁₂ and the perovskite-like ternary oxides A′A″₃B₄O₁₂.     

High pressure synthesis of the mixed valent and nonsuperconducting ternary YbRhxSny compound

The ternary compound YbRh_1.4Sn_4.6 with the phase I structure (simple cubic) when subjected to a pressure of 40 kbar at 800°C is found to transform to phase III structure (f.c.c.) with the composition YbRh_1.1Sn₃. The latter compound has a lattice parameter of $\text{a = 13.735}\text{A}\text{?}$ which suggests that the Yb is in an intermediate valence state. The temperature dependence of magnetic susceptibility suggests that the Yb is in a homogeneously mixed valence state in the pressure synthesized product. In the phase I structure YbRh_1.1Sn_4.6 is superconducting at 8.6°K, but in the phase III structure the compound YbRh_1.1Sn₃ is not superconducting down to 0.9°K. It is suggested that superconductivity and mixed valence are incompatible.     

Structural aspects of the metal-insulator transition in V4O7

V₄O₇ has a transition with decreasing temperature at 250 K and the structure has been refined at 298 and 200 K. The triclinic structure (A$\text{1}$) consists of rutile-like layers of VO₆ octahedra extending indefinitely in the a-b plane and four octahedra thick along the c-axis. The average VO distances for the four independent V atoms are 1.967, 1.980, 1.969, and 1.984 Å at 298K and 1.948, 1.992. 1.961, and 2.009 Å at 200K. At 200K there is a clear separation into strings of V³⁺ or V⁴⁺ ions running parallel to the pseudorutile c-axis. In addition, all of the 3+ and half of the 4+ sites are paired to form short VV bonds. The remaining V⁴⁺ atom is displaced toward one oxygen so as to balance its electrostatic charge. The distortion at the metal-insulator transitions in V₄O₇, Ti₄O₇, VO₂ + Cr, and NbO₂ are compared.     

Relationship between the crystal structure and the reentrant superconducting properties of (Sn1-xErx)Er4Rh6Sn18

Single crystals of (Sn_1-xEr_x)Er₄Rh₆Sn₁₈ are superconducting (T_c = 1.3K) for x 0, are reetrant superconducting (T_c = 1.24K and T_M = 0.34K) for x ～.30 and undergo a singel magnetic transition (T_M=0.68K) for x～.75. Since the occupancy of the [Sn(1)_1-xEr(1)_x]Er(2)₄ sublattice is responsible for the variation of the low-temperature properties, one can make predictions as to new reentrant superconductors in the MRh_xSn_y series (M=RE). This appears to be the first system of reentrant superconductors where stoichiometry within a sublattice controls both magnetic ordering and superconductivity.     

Oxygen vacancy ordering in the BaBiO3−y system

Compounds with the general formula BaBiO_3?y have been investigated. By determining the mass loss as a function of heat treatment temperature, two new phases, BaBiO_2.97 and BaBiO_2.77, have been shown to exist. They have been characterized by X-ray and electron diffraction and by electron microscopy. BaBiO_2.97 is monoclinic as is BaBiO₃; however, satellites and/or diffuse streaks due to an ordering of the oxygen vacancies appear on the diffraction photographs. Powder films of BaBiO_2.77 indicate a cubic symmetry, whereas single crystal photographs reveal the same satellites and streaks as in the case of BaBiO_2.97. The crystals of BaBiO_2.77 are thus disordered and microtwinned.     

Structural determinations of single-crystal K β-alumina and cobalt-doped K β-alumina

Single-crystal structural refinements of K β-alumina and Co²⁺-doped K β-alumina show that the distribution of potassium ions in the diffusing planes is very similar to that in the isomorphous compound Na β-alumina but quite different from Ag β-alumina. The Co²⁺ ions selectively fill the underbonded, tetrahedrally coordinated, Al(2) sites in the middle of the spinel block portion of the structure. This tends to support the aluminum vacancy mechanism of charge compensation. The positional parameters of the spinel block are well-determined and appear insensitive to the diffusing plane ions and arrangement.     

Thin polycrystalline films of W9Nb8O47−x As an electrochromic material

Using r.f. sputtering and ion beam deposition techniques, thin polycrystalline films of W₉Nb₈O_47?x have been prepared. X-ray diffraction measurements show these films have the same lattice spacings as bulk crystals which exhibit an elaborate tunnel structure. These films are electrochromic and have been evaluated for coloration and bleaching speed and for corrosion resistance.