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The transformations ia-x the iron oxide-hydroxide system have been interpreted in a rational crystallochemical manner,

The peak effect (PE) has been observed in a twinned crystal of YBa 2 Cu 3 O 7−δ for H c in the low field range, close to the zero field superconducting transition temperature (T c (0)) . A sharp depinning transition succeeds the peak temperature T p of the PE. The PE phenomenon broadens and its internal structure smoothens out as the field is increased or decreased beyond the interval between 250 Oe and 1000 Oe. Moreover, the PE could not be observed above 10 kOe and below 20 Oe. The locus of the T p (H) values shows a reentrant characteristic with a nose like feature located at T p (H)/T c (0)≈0.99 and H≈100 Oe (where the FLL constant a 0 ≈penetration depth λ). The upper part of the PE curve (0.5 kOe

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Xray diffraction studies of microstructural transformations in singlecrystal silicon during current filamentation

The second magnetization peak and the peak effect anomaly coexisting in a given isothermal magnetization hysteresis loop show striking similarities in Ca 3 Rh 4 Sn 13 , a low-T c superconductor and YBa 2 Cu 3 O 7Ϫ␦ , a high-T c superconductor. The observed variation of the hysteresis width with field could imply a modulation in the degree of the plastic deformation of the elastic vortex solid. The characteristics of the high-T c cuprates, such as large Ginzburg number, short coherence length, decoupling of the Josephson coupled pancake vortices, etc., are unlikely to be the cause of the observed behavior.

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T HE anhydrous carbonates of metals may be divided into two groups: CaCO 8 exists in both groups as calcite and aragonite. Divalent metals having smaller ionic radii than that of calcium belong to the calcite series. The others, with greater ionic radii, are members of the aragonite series. The structures of calcite and aragonite were analysed by Bragg (1914Bragg ( , 1924. In calcite, the Ca ions are arranged in a cubic close-packed (ABCABCA...) way in layers perpendicular to the threefold axis of the rhombohedral cell. Aragonite, on the other hand, is orthorhombic but also contains CO 3 groups; here the Ca ions are arranged in an approximately hexagonal close-packing, the Ca layers being perpendicular to the pseudo-hexagonal axis, which is the c-axis of the orthorhombic cell. In the structures of both calcite and aragonite, each carbonate group occupies a position between six Ca ions. In calcite, COs is placed so that each oxygen touches two calcium ions and is half-way between two layers of calcium ions. In aragonite each oxygen of a carbonate group touches three calcium ions (one in the lower and two in the upper layers) and is placed at a heigh~ of two-thirds from the lower layer of calcium. Though all the Ca ions are hexagonally close-packed in aragonite, the arrangement of the CO 3 groups lowers the symmetry to orthorhombic. All the CO s are normal to the c-axis of aragonite and are similarly oriented with respect to' the Ca ions, but they are arranged alternately up and down along a row parallel to (110) of the orthorhombic cell ( fig. 1). From the similarity of their structures,