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The thermopower, α, of many members of the Bi2Ca1−x Y x Sr2Cu2O8+δ, Bi2−y Pb y CaSr2Cu2O8 +δ, Tl y Ca1−x Y x Ba2Cu2O6+δ, TlCa1−x Ln x Sr2Cu2O6+δ and TlSr2−x La x CuO5+δ systems is positive down to T c, but is negat ive in some of them. The slope of the α-T plots above T c is generally negative independent of the sign of α. The value of α extrapolated to 0 K, α(0), from the high-tem perature regime is finite and generally, positive. Compared to the behaviour of YBa2Cu3O7, the data on the Bi and Tl cuprate systems examined here seem to suggest that electrons may be involved in the conduction process as well, at least in systems where both the α and the α-T slope are negative. Of the different systems examined here, Tl1−x Pb x CaSr2Cu2O6+δ contai ning Pb in the 4+state, deserves special mention since it shows negative α over the entire temperature range in addition to a negative slope suggesting that this is likely to be a genuine high-T c electron-superconductor. The thermopower of cuprates has been discussed in some detail in terms of entropic and quasiparticle contributions.
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Amine-templated rare earth sulfates of the compositions: [Ln2(H2O)2(SO4)5][C2N2H10]2, I, with Ln = La, Pr or Nd, [Nd2(SO4)4(H2O)2][C4N2H12], II, [Ln2(SO4)4][C2N2H10], III, with Ln = La or Nd, [La2(SO4)4][C3N2H12], IV and [Ln2(SO4)4(H2O)4][C6N2H14]2[C2N2H8][SO4][H2O]3, with V, Ln = La, Pr or Nd, have been synthesized under hydrothermal conditions. Both I and II have three-dimensional architectures with I possessing eight-membered apertures surrounding 16-membered apertures, and II having 12 membered apertures. Both III and V have interesting layered structures with LnO3 layers possessing (6, 3) net topology and the amine located in between the layers. V has a layered structure wherein the SO4 tetrahedra and the LnO9 polyhedra join together to form (4, 4) net sheets, with two different amines as well as the sulfate ions residing in the interlamellar space.
Reaction of Bi2O3 with MgO, NiO, Co3O4 and Al2O3 gives rise to the corresponding ternary bismuth oxides, Bi18Mg8O36, Bi18Ni8O36, Bi20Co6O39 and Bi24Al2O39. These oxides have the general formula Bi26−xMxO40−y and exhibit BCC structures related to α - Bi2O3. In the first three solids, the metal ions, M, replace bismuth randomly at the octahedral 24r sites (space group 123); in the last case, aluminium ions occupy the tetrahedral 2a sites, the phase being isostructural with Bi24Ge2O40. Starting from Bi2O3 and NiO, orthorhombic Bi2Ni2O5 has also been obtained.
A few oxides such as YMnO3, TbMnO3, YMn2O5 and BiFeO3 constituted the small family of well-characterized multiferroics until recently, but this area of research has been enlarged significantly due to the advent of a novel class of oxides exhibiting interesting multiferroic and magnetoelectric properties arising from magnetically induced ferroelectricity. Interestingly, these materials are simple transition metal oxides, most of them possessing the perovskite structure. In this review article, we present the significant features of multiferroic and magnetoelectric ferrites and chromites which owe their ferroelectricity to magnetic interactions. Some of the important systems discussed are BiFeO3 whose properties are affected by magnetic and electric fields, rare-earth orthoferrites LnFeO3 (Ln = Dy, Gd and Sm) and rare-earth orthochromites LnCrO3, where exchange-striction plays a significant role. Perovskite oxides of the type Y(A1−xBx)O3 (A, B = Fe, Cr, Mn) exhibit multiferroic properties, although the existence of these properties in YFeO3 and YCrO3 is in doubt. Such oxides with a non-magnetic rare-earth cation at the A site and two transition metal ions in the B-site permit tuning the transition temperatures by varying the B site ions and their relative proportions or the Ln ion. Multiferroic properties of simple ferrites such as Al(Ga)FeO3 where cation disorder appears to play a role are also discussed. Problems and challenges in this area of research are indicated.
By carrying out the reaction of Ga2O3 powder with carbon nanotubes around 1100 °C, nanowires, nanobelts and nanosheets of Ga2O3 have been obtained, the diameter and proportion of the nanowires depending on the flow rate of argon through the reaction zone. Reaction of Ga2O3 powder with activated carbon mainly gives rise to nanosheets and nanorods. The procedures employed in this study are attractive since they give high yields of nanowires and nanobelts. The nanowires are single crystalline with the growth direction perpendicular to the ( 1 ̄ 02) planes. The Ga2O3 nanowires exhibit good photoluminescence characteristics.
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Several $(La1-yLny)2-xSrx(Bax)CuO_4$ systems show supercond. in the 15-40 K region. Marginal metalicity of the non-superconducting phases and the role of the nominal $Cu^3^+/Cu^2^+$ ratio are discussed. The possible importance of hole-hole coupling (forming resonating O-O bonds) is indicated.
