Structure and cation distribution in perovskites with small cations at the A site: the case of ScCoO3.

We synthesize ScCoO3 perovskite and its solid solutions, ScCo1-x Fe x O3 and ScCo1-x Cr x O3, under high pressure (6 GPa) and high temperature (1570 K) conditions. We find noticeable shifts from the stoichiometric compositions, expressed as (Sc1-xMx )MO3 with x = 0.05-0.11 and M = Co, (Co, Fe) and (Co, Cr). The crystal structure of (Sc0.95Co0.05)CoO3 is refined using synchrotron x-ray powder diffraction data: space group Pnma (No. 62), Z = 4 and lattice parameters a = 5.26766(1) Å, b = 7.14027(2) Å and c = 4.92231(1) Å. (Sc0.95Co0.05)CoO3 crystallizes in the GdFeO3-type structure similar to other members of the perovskite cobaltite family, ACoO3 (A3+ = Y and Pr-Lu). There is evidence that (Sc0.95Co0.05)CoO3 has non-magnetic low-spin Co3+ ions at the B site and paramagnetic high-spin Co3+ ions at the A site. In the iron-doped samples (Sc1-xMx )MO3 with M = (Co, Fe), Fe3+ ions have a strong preference to occupy the A site of such perovskites at small doping levels.

Bi3Cr2.91O11: a ferromagnetic insulator from Cr(4+)/Cr(5+) mixing.

The search for materials with ferromagnetic and semiconducting/insulating properties has intensified recently because of their potential use in spintronics. However, the number of materials is rather limited because of conflicting requirements needed for the appearance of ferromagnetic and insulating properties. Here we show that Bi3Cr2.91O11 belongs to the scarce family of ferromagnetic insulators. Bi3Cr2.91O11 was synthesized at high pressure of 6 GPa and high temperature of 1570 K. Its crystal structure and properties were studied using single crystals. It crystallizes in the KSbO3-type structure with space group Pn3 and the lattice parameter a = 9.2181(2) Å. Bi3Cr2.91O11 has almost a 1:1 mixture of Cr(4+) and Cr(5+) ions distributed in one octahedral crystallographic site. Bi3Cr2.91O11 is a rare example of oxides having chromium ions in unusual oxidation states. The presence of Cr(4+) and Cr(5+) results in ferromagnetic properties with ferromagnetic Curie temperature TC = 220 K.

A bona fide two-dimensional percolation model: an insight into the optimum photoactivator concentration in La2/3-x Eu x Ta2O7 nanosheets.

La-Eu solid solution nanosheets La2/3-x Eu x Ta2O7 have been synthesized, and their photoluminescence properties have been investigated. La2/3-x Eu x Ta2O7 nanosheets were prepared from layered perovskite compounds Li2La2/3-x Eu x Ta2O7 as the precursors by soft chemical exfoliation reactions. Both the precursors and the exfoliated nanosheets exhibit a decrease in intralayer lattice parameters as the Eu contents increase. However, there is a discontinuity in this trend between the nominal Eu content ranges x≤ 0.3 and x ≥ 0.4. This discontinuity is attributed to the difference in degree of TaO6 octahedra tilting for the La- and Eu-rich phases. La2/3-x Eu x Ta2O7 nanosheets exhibit red emission, characteristic of the f-f transitions in Eu3+ photoactivators. The photoluminescence emission can be obtained from both host and direct photoactivator excitation. However, photoluminescence emission through host excitation is much more dominant than that through direct photoactivator excitation, and this behavior is consistent with that of all the other rare-earth photoactivated nanosheets reported previously. The absolute photoluminescence quantum efficiency of the La2/3-x Eu x Ta2O7 nanosheets increases as the experimentally determined Eu contents increase up to x=0.45 and decrease above it. This result is in good agreement with the optimum photoactivator concentration expected from the percolation theory. These solid solution La2/3-x Eu x Ta2O7 nanosheets are excellent models for validating the theory of optimum photoactivator concentration in the truly two-dimensional photoactivator matrix.

Synthesis, crystal and electronic structures of new narrow-band-gap semiconducting antimonide oxides RE(3)SbO(3) and RE(8)Sb(3-delta)O(8), with RE = La, Sm, Gd, and Ho.

In the search for high-temperature thermoelectric materials, two families of novel, narrow-band-gap semiconducting antimonide oxides with the compositions RE(3)SbO(3) and RE(8)Sb(3-delta)O(8) (RE = La, Sm, Gd, Ho) have been discovered. Their synthesis was motivated by attempts to open a band gap in the semimetallic RESb binaries through a chemical fusion of RESb and corresponding insulating RE(2)O(3). Temperatures of 1350 degrees C or higher are required to obtain these phases. Both RE(3)SbO(3) and RE(8)Sb(3-delta)O(8) adopt new monoclinic structures with the C2/m space group and feature similar REO frameworks composed of "RE(4)O" tetrahedral units. In both structures, the Sb atoms occupy the empty channels within the REO sublattice. High-purity bulk Sm and Ho samples were prepared and subjected to electrical resistivity measurements. Both the RE(3)SbO(3) and RE(8)Sb(3-delta)O(8) (RE = Sm, Ho) phases exhibit a semiconductor-type electrical behavior. While a small band gap in RE(3)SbO(3) results from the separation of the valence and conduction bands, a band gap in RE(8)Sb(3-delta)O(8) appears to result from the Anderson localization of electrons. The relationship among the composition, crystal structures, and electrical resistivity is analyzed using electronic structure calculations.

Crystal and magnetic structures and properties of BiMnO(3+delta).

Crystal and magnetic structures of BiMnO(3+delta) (delta = 0.03, 0.08, and 0.14) have been determined by the Rietveld method from neutron diffraction data at 8-10 and 290 K. BiMnO(3.03) (= Bi(0.99)Mn(0.99)O(3)) crystallizes in a monoclinic system (the refinement was performed in space group C2/c; Z = 8; a = 9.5313(3) A, b = 5.57791(17) A, c = 9.7375(4) A, beta = 108.951(2) degrees at 290 K). BiMnO(3.08) (= Bi(0.974)Mn(0.974)O(3)) crystallizes in space group P2(1)/c (Z = 8; a = 9.5565(4) A, b = 5.51823(16) A, c = 9.7051(4) A, beta = 109.442(3) degrees at 290 K). It was found that Mn vacancies are localized mainly in one Mn site (among three sites) in Bi(0.974)Mn(0.974)O(3). Vacancy-ordering and charge-ordering scenarios are suggested as possible reasons for the crystal symmetry change compared with Bi(0.99)Mn(0.99)O(3). BiMnO(3.03) and BiMnO(3.08) are ferromagnetic below T(C) = 82 and 68 K, respectively, with magnetic moments along the monoclinic b axes. Refined magnetic moments at 10 K are 2.88(2)micro(B) in BiMnO(3.03) and 2.33(2)micro(B) in BiMnO(3.08). BiMnO(3.14) (= Bi(0.955)Mn(0.955)O(3)) crystallizes in an orthorhombic system (space group Pnma; Z = 4; a = 5.5136(4) A, b = 7.8069(8) A, and c = 5.5454(5) A at 290 K), and its structure is similar to that of LaMnO(3.11)-LaMnO(3.15). No magnetic reflections were found in BiMnO(3.14) down to 8 K, in agreement with its spin-glass magnetic state. Magnetic and chemical properties of BiMnO(3+delta) (0.02 < or = delta < or = 0.14) have also been investigated and compared with those of LaMnO(3+delta). Systematic changes of magnetic parameters in BiMnO(3+delta) were found to depend on delta.

Spinel-to-CaFe2O4-type structural transformation in LiMn2O4 under high pressure.

A new form of LiMn2O4 is reported. The structure is the CaFe2O4-type and 6% denser than the spinel. The structure transformation was achieved by heating at 6 GPa. Analysis of the neutron diffraction pattern confirmed an average of the structure; the unit cell was orthorhombic at a = 8.8336(5) angstroms, b = 2.83387(18) angstroms, and c = 10.6535(7) angstroms (Pnma). Electron diffraction patterns indicated an order of superstructure 3a x b x c, which might be initiated by Li vacancies. The exact composition is estimated at Li(0.92)Mn2O4 from the structure analysis and quantity of intercalated Li. The polycrystalline CaFe2O4-type compound showed semiconducting-like characters over the studied range above 5 K. The activation energy was reduced to approximately 0.27 eV from approximately 0.40 eV at the spinel form, suggesting a possible enhancement of hopping mobility. Magnetic and specific-heat data indicated a magnetically glassy transition at approximately 10 K. As the CaFe2O4-type transition was observed for the mineral MgAl2O4, hence the new form of the lithium manganese oxide would provide valuable opportunities to study not only the magnetism of strongly correlated electrons but also the thermodynamics of the phase transition in the mantle.