Engineering Transport Properties in Interconnected Enargite-Stannite Type Cu2+x Mn1-x GeS4 Nanocomposites.

Understanding the mechanisms that connect heat and electron transport with crystal structures and defect chemistry is fundamental to develop materials with thermoelectric properties. In this work, we synthesized a series of self-doped compounds Cu2+x Mn1-x GeS4 through Cu for Mn substitution. Using a combination of powder X-ray diffraction, high resolution transmission electron microscopy and precession-assisted electron diffraction tomography, we evidence that the materials are composed of interconnected enargite- and stannite-type structures, via the formation of nanodomains with a high density of coherent interfaces. By combining experiments with ab initio electron and phonon calculations, we discuss the structure-thermoelectric properties relationships and clarify the interesting crystal chemistry in this system. We demonstrate that excess Cu+ substituted for Mn2+ dopes holes into the top of the valence band, leading to a remarkable enhancement of the power factor and figure of merit ZT.

The crucial role of selenium for sulphur substitution in the structural transitions and thermoelectric properties of Cu5FeS4 bornite.

Bornite Cu5FeS4-xSex (0 ≤ x ≤ 0.6) compounds have been synthesized, using mechanical alloying, combined with spark plasma sintering (SPS). High temperature in situ neutron powder diffraction data collected on pristine Cu5FeS4 from room temperature up to 673 K show that SPS enables the stabilization of the intermediate cubic (IC) semi-ordered form (Fm3[combining macron]m, aIC ∼ 10.98 Å) at the expense of the ordered orthorhombic form (Pbca, aO ∼ 10.95 Å, bO ∼ 21.86 Å, cO ∼ 10.95 Å) in the 300-475 K temperature range, whereas above 475 K the IC form coexists with the high temperature cubic (C) form (Fm3[combining macron]m, aC ∼ 5.50 Å). The ability of Se for S substitution to induce disorder and consequently to enhance the IC phase formation is also emphasized. This disordering effect is explained by the high quenching efficiency of the SPS method compared to conventional heating. The existence of topotactic phase transformations, as well as Se for S substitution is shown to have a significant effect on the transport properties. As expected, electrical transport properties indicate a change towards a more metallic behaviour with increasing Se content. The electrical resistivity reduces from ∼21.4 mΩ cm for the pristine Cu5FeS4 to ∼3.95 mΩ cm for Cu5FeS3.4Se0.6 at room temperature. A maximum power factor of 4.9 × 10-4 W m-1 K-2 is attained at 540 K for x = 0.4 composition. The influence of selenium substitution on the carrier effective mass and mobility is discussed based on single parabolic band approximation. Furthermore, a detailed investigation of the thermal conductivity by this isovalent anion substitution reveals a significant reduction of the lattice thermal conductivity due to the alloying effect. Finally, the important role of structural transitions in the thermoelectric properties is addressed. A maximum ZT of 0.5 is attained at 540 K for Cu5FeS3.8Se0.2 composition.

Rich crystal chemistry and magnetism of "114" stoichiometric LnBaFe4O7.0 ferrites.

Stoichiometric LnBaFe4O7.0 oxides with Ln = Dy to Lu have been synthesized and protected in order to prevent oxidation at room temperature. The structural study of these compounds, using laboratory and synchrotron X-ray as well as neutron powder diffraction, shows the extraordinary flexibility of the tetrahedral [Fe4] sublattice of these compounds, which exhibit various distortions. At room temperature they all are tetragonal (I4), and at higher temperature (T > 580 K) they exhibit a cubic symmetry (F43m). Moreover, the low-temperature structures of these oxides are dependent on the nature of the Ln(3+) cation. At 110 K, compounds with Ln = Dy and Ho adopt the same monoclinic (P12(1)1) structure as YBaFe4O7.0, whereas YbBaFe4O7.0 possesses a new centered monoclinic cell (I121), and members with Ln = Er and Lu keep the tetragonal (I4) symmetry. Neutron diffraction patterns evidence long-range magnetic ordering only for the most distorted structures (Ln = Dy and Ho), showing that the geometric frustration generated by the tetrahedral [Fe4]∞ sublattice can be lifted only with the most severe distortions. The other oxides (Ln = Er, Yb, and Lu) with weakly distorted [Fe4]∞ sublattices do not exhibit magnetic ordering down to 4 K, demonstrating the importance of magnetic frustration. The behavior of these "114" iron oxides is compared to the cobalt family, showing in both cases a striking underbonding of barium.

Impact of Mn3+ upon structure and magnetism of the perovskite derivative Pb(2-x)Ba(x)FeMnO5 (x ∼ 0.7).

On the basis of the Mn(3+) for Fe(3+) substitution in Pb(2-x)Ba(x)Fe2O5, a novel oxide Pb1.3Ba0.7MnFeO5 has been synthesized at normal pressure. Though it belongs to the same structural family, the mixed "MnFe" oxide exhibits a very different structural distortion of its framework compared to the pure "Fe2" oxide, due to the Jahn-Teller effect of Mn(3+). Combined neutron diffraction, high resolution electron microscopy/high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) investigations allow the origin of this difference to be determined. Here we show that the MO6 octahedra of the double perovskite layers in the "MnFe" structure exhibit a strong tetragonal pyramidal distortion "5 + 1", whereas the "Fe2" structure shows a tetrahedral distortion "4 + 2" of the FeO6 octahedra. Similarly, the MO5 polyhedra of the "MnFe" structure tend toward a tetragonal pyramid, whereas the FeO5 polyhedra of the "Fe2" structure are closer to a trigonal bipyramid. Differently from the oxide Pb(2-x)Ba(x)Fe2O5, which is antiferromagnetic, the oxide Pb1.3Ba0.7MnFeO5 exhibits a spin glass behavior with Tg ∼ 50 K in agreement with the disordered distribution of the Mn(3+) and Fe(3+) species.

Structure and physical properties of new iron hydrogenophosphates: K2Fe(HP2O7)(H2PO4)2, LiH3Fe2(P2O7)2, and FeH2P2O7.

The exploration of the systems Fe-H-P-O, Li-Fe-H-P-O, and K-Fe-H-P-O using soft chemistry methods has allowed three new hydrogenophosphates to be synthesized, whose structures have been determined by ab initio calculations. The structures of two of them--FeH(2)P(2)O(7) and LiH(3)Fe(P(2)O(7))(2)--exhibit close relationships: their 3D framework consists of [FeO(4)](infinity) and [LiFe(2)O(12)](infinity) chains of edge-sharing octahedra, respectively, interconnected through diphosphate groups. These two diphosphates exhibit a paramagnetic to antiferromagnetic transition at low temperatures. The former phase exhibits intrachain ferromagnetic interactions (theta(p) > 0) in competition with the antiferromagnetic interchain ordering, whereas for the second one, the ferromagnetic interactions have disappeared due to the presence of Li in the chains. Differently, the third phosphate, K(2)Fe(HP(2)O(7))(H(2)PO(4))(2), exhibits a chain structure, involving isolated FeO(6) octahedra, and consequently is paramagnetic in the whole temperature range (4-300 K). Among these three phosphates, only the latter exhibits ionic conductivity, which may originate from the proton mobility.

Electron transport and thermoelectric properties of layered perovskite LaBaCo(2)O(5.5).

We have investigated systematically the physical transport properties of layered 112-type cobaltite by means of electrical resistivity, magnetoresistance and thermopower measurements. In order to understand the complex transport mechanism of LaBaCo(2)O(5.5), the data have been analysed using different theoretical models. The compound shows an electronic transition between two semiconducting states around 326 K, which coincides with the ferromagnetic transition. Interestingly, the system also depicts a significant magnetoresistance (MR) effect near the ferro/antiferromagnetic phase boundary and the highest value of MR is close to 5% at 245 K under ± 7 T. The temperature dependence of thermopower, S(T), exhibits p-type conductivity in the 60 K≤T≤320 K range and reaches a maximum value of around 303 µV K(-1) (at 120 K). In the low temperature antiferromagnetic region the unusual S(T) behaviour, generally observed for the cobaltite series LnBaCo(2)O(5.5) (Ln = rare earth), is explained by the electron magnon scattering mechanism.

Zero magnetization in a disordered (La(1-x/2)Bi(x/2))(Fe(0.5)Cr(0.5))O(3) uncompensated weak ferromagnet.

The orthorhombic perovskite, (La(1-x/2)Bi(x/2))(Fe(0.5)Cr(0.5))O(3) was investigated for 0≤x≤1. Its space group, Pnma, compatible with the disordering of iron and chromium in the B sites, confirms previous observations. More importantly this compound is found to be an uncompensated weak ferromagnet, with a very peculiar zero magnetization behaviour, generally observed for ordered magnetic cations in the B sites. It exhibits a magnetic transition at high temperature (T(C)) above 450 K, while the zero magnetization occurs between 100 and 160 K depending on the x-value. The AC magnetic susceptibility study shows that this compound does not exhibit a spin glass or cluster glass behaviour, in contrast to what was suggested for the x = 0 compound. This zero magnetization phenomenon can be interpreted by the fact that this perovskite is an uncompensated weak ferromagnet, which consists of canted weak ferromagnetic domains and clusters of pure chromium and pure iron composition, antiferromagnetically coupled through Cr-O-Fe interactions.

New layered hydrogenophosphate, protonic conductor: Mn(H2PO4)2.

A new hydrogenophosphate Mn(H 2PO 4) 2 has been synthesized from an aqueous solution. Its ab initio structure resolution shows that the original layered structure of this phase consists of PO 2(OH) 2 tetrahedra and MnO 5OH octahedra, sharing corners to form [MnP 2O 8H 4] infinity layers, whose cohesion is ensured through hydrogen bonds. The excitation and emission spectra of this phase are characteristic of Mn (2+) species. This phosphate is shown to be a good protonic conductor with a conductivity of 10 (-4.4) S/cm at 90 degrees C (363 K).

The crucial role of mixed valence in the magnetoresistance properties of manganites and cobaltites.

The mixed valence Mn3+/Mn4+ and Co3+/Co4+ in manganites and cobaltites with the perovskite structure is absolutely necessary for the appearance of magnetotransport properties. It is shown that in these systems the Jahn Teller effect of the transition element, the charge and orbital ordering and the oxygen stoichiometry play a key role in the appearance of large and even colossal magnetoresistance. It has been discovered that these oxides exhibit a new phenomenon, the crystallographic and electronic phase separation. It is this phenomenon that is at the origin of the competition between ferromagnetism and antiferromagnetism or spin glass behaviour and which leads to the negative magnetoresistance (MR). The doping of these materials at different sites appears then to be a means of inducing large MR effects.

A novel layered titanoniobate LiTiNbO5: topotactic synthesis and electrochemistry versus lithium.

A new layered titanoniobate, LiTiNbO5, an n = 2 member of the A(x)M(2n)O(4n+2) family, has been synthesized using a molten salt reaction between HTiNbO5 and an eutectic "LiOH/LiNO3". This compound crystallizes in the P2(1)/m space group with a = 6.41 A, b = 3.77 A, c = 8.08 A, and beta = 92 degrees . It exhibits |TiNbO5|(infinity) layers similar to HTiNbO5, but differs from the latter by a "parallel configuration" of its |TiNbO6|(infinity) ribbons between the two successive layers. The topotactic character of the reaction suggests that exfoliation plays a prominent role in the synthesis of this new form. This new phase intercalates reversibly 0.8 lithium through a first-order transformation leading to a capacity of 94 mAh/g at a potential of 1.67 V vs Li/Li+.

Re-entrant metallicity and magnetoresistance induced by Ce for Sr substitution in SrCoO(3-δ).

Cerium for strontium substitution allows an oxygen deficient perovskite Sr(1-x)Ce(x)CoO(3-δ) to be stabilized with a cerium solubility limited to x≤0.15 (Trofimenko et al 1997 Solid State Ion. 100 183). For these samples, the magnetic properties depend clearly upon the oxygen content: Sr(0.9)Ce(0.1)CoO(2.74) and Sr(0.9)Ce(0.1)CoO(2.83) are weak and strong ferromagnets (T(C) = 160 K), respectively, the maximum ac-magnetic susceptibility of the latter being larger by two orders of magnitude than that of the former. In contrast to other Sr(1-x)L(x)CoO(3-δ) series (L =  lanthanide, Y(3) or Th(4+)), the electrical resistivity (ρ) behaviour does not simply reflect the magnetic behaviour. For x = 0.10 the re-entrant ρ feature becomes more pronounced whereas the ferromagnetic fraction and cobalt oxidation state increase. This unexpected behaviour could be related to the Ce(3+)/Ce(4+) mixed valency, the 4f localized moment of the Ce(3+) cations interacting with the conduction electrons through a Kondo-like mechanism. It is also found that the increase of oxygen vacancies favours the appearance of magnetoresistance at low T, reaching -50% at 5 K in 7 T for the Sr(0.95)Ce(0.05)CoO(2.61) sample prepared in a sealed tube.

New aluminate with a tetrahedral structure closely related to the c84 fullerene.

A new aluminate Sr33Bi(24+delta)Al48O(141+3delta/2), having an F3m cubic structure (a = 25.090 angstroms, Z = 4) and forming a close packed face centered cubic array of "Al84" fullerene geometry, has been discovered. This original structure consists of corner-sharing AlO4 tetrahedra forming "Al84O106" cubic units whose assemblage delimits five types of cages, three of them being empty, one being occupied by strontium, and the fifth one forming the huge spheric fullerene-type cavity. In the latter, strontium, oxygen, and bismuth ions form concentric spheres, with an onion-skin-like configuration. The latter ions are disposed into a compact "Bi16O(52-n[]n)" anion whose the exceptional geometry is characterized by a strong stereoactivity of the 6s2 lone pair of Bi3+.

Thermoelectric power of HoBaCo2O5.5: possible evidence of the spin blockade in cobaltites.

Thermoelectric power measurements have been performed for an ordered oxygen-deficient perovskite, HoBaCo2O5.5, in which the alternative layers of CoO6 octahedra and of [CoO(5)](2) bipyramids are occupied by Co3+ species. The T-dependent Seebeck coefficient S shows a clear change of the conduction regime at the metal-insulator (MI) transition (T(MI) approximately 285 K). The sign change of S from S<0 to S>0 can be explained assuming that a spin state transition occurs at T(MI). In the metallic state, Co2+ e(g) electrons are moving in a broad band on the background of high or intermediate spin Co3+ species. In contrast, the insulating behavior may result from the Co3+ spin state transition to a low-spin Co3+ occurring in the octahedra. In this phase the transport would occur by hopping of the low-spin Co(4+)t(2g) holes, whereas the high-spin Co2+ electrons become immobilized due to a spin blockade.

Ultrasharp magnetization steps in perovskite manganites.

We report a detailed study of steplike metamagnetic transitions in polycrystalline Pr0.5Ca0.5Mn0.95Co0.05O3. The steps have a sudden onset below a critical temperature, are extremely sharp (width <2x10(-4) T), and occur at critical fields which are linearly dependent on the absolute value of the cooling field in which the sample is prepared. Similar transitions are also observed at low temperature in non-Co doped manganites, including single crystal samples. These data show that the steps are an intrinsic property, qualitatively different from either previously observed higher temperature metamagnetic transitions in the manganites or metamagnetic transitions observed in other materials.

A five-dimensional structural investigation of the misfit layer compound

The structure of the misfit layer compound [Bi0.87SrO2]2[CoO2]1.82, bismuth strontium cobaltite, was determined by single-crystal X-ray diffraction using the five-dimensional superspace-group formalism. This composite crystal, of monoclinic symmetry, is composed of two subsystems exhibiting incommensurate periodicities along b, the binary axis direction. The first composite part [Bi0.87SrO2] displays an intrinsic modulation of planar monoclinic type characterized by the wavevector q* = 0.293a* + 0.915c*. The second composite part [CoO2] shows two different centered lattice variants. The structure of the misfit layer crystal can be described as an alternation along c of distorted rock-salt-type slabs, formed from [BiO] and [SrO] layers (first subsystem), and of [CoO2] layers (second subsystem) displaying a distorted CdI2-type structure. Two main structural results are obtained. First, as a consequence of the intrinsic modulation, disordered zones, characterized by Bi vacancies, are regularly distributed in the [BiO] layers. Second, strong chemical bonds are implied between the strontium atoms of the first subsystem and the oxygen atoms of the second one.

Room-temperature and low-temperature structure of Nd1-xCaxMnO3 (0.3

The structure of the insulating manganites Nd(1-x)Ca(x)MnO(3) (0.3

Cluster configurations in modulated EuVxMo8+/-yO14 crystals.

Three orthorhombic crystals of chemical formula Eu(x)V(y)Mo(8+/-z)O(14) were investigated by X-ray diffraction (Mo Kalpha radiation, lambda = 0.71073 Å). They have nearly the same lattice parameters (a approximately 11.3, b approximately 10.0, c approximately 9.2 Å), display one-dimensional incommensurate modulations of wavevector q* = gammac* and are characterized by the same superspace group Cmca(00gamma)s00. The crystals differ both in their compositions (namely Eu(0.976(6))V(1.13(5))Mo(7.10(5))O(14), Eu(0.986(4))V(1.10(3))Mo(7.30(1))O(14) and EuMo(7.96(1))O(14)) and in their gamma components [0.195 (2), 0.245 (2) and 0.286 (3), respectively]. The average structures of these crystals appear closely related to the structures of LaMo(7.7)O(14) (not modulated) and LaMo(8)O(14) (modulated); however, two main differences are outlined: first, the modulation direction is c in the Eu-containing crystals but b in the modulated La-containing crystal [q* = (1/3)b*], second, the Eu-containing crystals have centrosymmetric structures while the La-containing crystals have polar structures (space group C2ca). The Mo (or Mo and V) atoms are stacked to form (001) layers of metallic clusters. The density modulation of these structures implies the existence of the new types of clusters Mo(9), Mo(10), Mo(6)V(4), Mo(7)V(3) and Mo(8)V(2) besides the clusters M(8) (Mo(8), Mo(6)V(2) and Mo(7)V) and M(7) (Mo(7) and Mo(6)V) which are already known. Mo(8) units with cis and trans configurations and Mo(6)V(2) units with a trans configuration appear as the main cluster types in these crystals. The nature of the metallic clusters changes along c, but inside one (001) layer it is likely that only one cluster type with a given configuration is present. The main structural result is the formation, in some unit cells, of strong intercluster Mo-Mo, Mo-V or V-V bonds with distances close to 2.6 Å within a layer as well as between two neighbouring layers.

Commensurate Modulated Displacement and Disorder in the Mixed Valent Vanadium Monophosphate Sr(2)V(2)O(PO(4))(3).

A new mixed-valent monophosphate Sr(2)V(2)O(PO(4))(3) has been synthesized. Its 4-fold superstructure has been refined using the 4D formalism for modulated structures and using X-ray single-crystal diffraction data. The average structure can be described as a stacking of [V(2)P(3)O(13)](infinity) layers and [Sr(2)](infinity) layers. In these layers, the VO(6) octahedra, occupied by V(III), the VO(5) trigonal bipyramids, occupied by V(IV), and the PO(4) tetrahedra form chains running along b. The 4D description allows us to give a better description of the measured intensities in reciprocal space (main and first order satellite reflections) and particularly to explain why the second order satellite reflections are not observed using a classical X-ray source. The corresponding structural mechanism consists of an alternation of disordered ribbons of VO(5) bipyramids and PO(4) tetrahedra in two enantiomorphic configurations, separated by displacively modulated ribbons of VO(6) octahedra and PO(4) tetrahedra. Crystal data: monoclinic, superstructure space group P2(1)/c, Z = 8, a = 17.389(1) Å, b = 5.094(1) Å, c = 30.032(4) Å, beta = 132.17(1) degrees; superspace group P2(1)/m(0,0,(1)/(4))0s, Z = 2, c' = c/4.