New transparent conductors with the M7O12 ordered oxygen-deficient fluorite structure: from In4Sn3O12 to In5.5Sb1.5O12

A new transparent conductor, containing pentavalent antimony, In_4+xSn_3−2xSb_xO₁₂, has been synthesized for 0?x?1.5. The latter exhibits an ordered oxygen-deficient fluorite structure with an ordered distribution of Sb⁵⁺ and In³⁺/Sn⁴⁺ species in the octahedral and seven-fold coordinated sites, respectively. More importantly, it is shown that the electronic conductivity of this transparent conducting oxide (TCO) at room temperature, is one order of magnitude larger for x=1 (In₅SnSbO₁₂) than for x=0 (In₄Sn₃O₁₂) and it turns to a semi-metallic behavior in contrast to In₄Sn₃O₁₂ which is a semi-conductor. The potential of this new material, as TCO, is also shown by its reflectance spectra, similar to In₄Sn₃O₁₂, involving only a small increase of the optical bandgap, by 0.15 eV.     

Probing the local structure of crystalline ZITO: In2−2xSnxZnxO3 (x≤0.4)

The local structure of In₂O₃ cosubstituted with Zn and Sn (In_2−2xSn_xZn_xO₃, x≤0.4 or ZITO) was determined by extended X-ray absorption fine structure (EXAFS) for x=0.1, 0.2, 0.3 and 0.4. The host bixbyite In₂O₃ structure is maintained up to the enhanced substitution limit (x=0.4). The EXAFS spectra are consistent with random substitution of In by the smaller Zn and Sn cations, a result that is consistent with the “good-to-excellent” conductivities reported for ZITO.     

Thermoelectric properties of p-type pseudo-binary (Ag0.365Sb0.558Te)x-(Bi0.5Sb1.5Te3)1−x (x=0-1.0) alloys prepared by spark plasma sintering

In this paper, pseudo-binary (Ag_0.365Sb_0.558Te)_x-(Bi_0.5Sb_1.5Te₃)_1−x (x=0-1.0) alloys were prepared using spark plasma sintering technique, and the composition-dependent thermoelectric properties were evaluated. Electrical conductivities range from 7.9×10⁴ to 15.6×10⁴ Ω⁻¹ m⁻¹ at temperatures of 507 and 318 K, respectively, being about 3.0 and 8.5 times those of Bi_0.5Sb_1.5Te₃ alloy at the corresponding temperatures. The optimal dimensionless figure of merit (ZT) of the sample with molar fraction x=0.025 reaches 1.1 at 478 K, whereas that of the ternary Bi_0.5Sb_1.5Te₃ alloy is 0.58 near room temperature. The results also reveal that a direct introduction of Ag_0.365Sb_0.558Te in the Bi-Sb-Te system is much more effective to the property improvement than naturally precipitated Ag_0.365Sb_0.558Te in the Ag-doped Ag-Bi-Sb-Te system.     

Snx[BPO4]1−x composites as negative electrodes for lithium ion cells: Comparison with amorphous SnB0.6P0.4O2.9 and effect of composition

A comparative study of two Sn-based composite materials as negative electrode for Li-ion accumulators is presented. The former SnB_0.6P_0.4O_2.9 obtained by in-situ dispersion of SnO in an oxide matrix is shown to be an amorphous tin composite oxide (ATCO). The latter Sn_0.72[BPO₄]_0.28 obtained by ex-situ dispersion of Sn in a borophosphate matrix consists of Sn particles embedded in a crystalline BPO₄ matrix. The electrochemical responses of ATCO and Sn_0.72[BPO₄]_0.28 composite in galvanostatic mode show reversible capacities of about 450 and 530 mAh g⁻¹, respectively, with different irreversible capacities (60% and 29%). Analysis of these composite materials by ¹¹⁹Sn Mössbauer spectroscopy in transmission (TMS) and emission (CEMS) modes confirms that ATCO is an amorphous Sn^II composite oxide and shows that in the case of Sn_0.72[BPO₄]_0.28, the surface of the tin clusters is mainly formed by Sn^II in an amorphous interface whereas the bulk of the clusters is mainly formed by Sn⁰. The determination of the recoilless free fractions f (Lamb-Mössbauer factors) leads to the effective fraction of both Sn⁰ and Sn^II species in such composites. The influence of chemical composition and especially of the surface-to-bulk tin species ratio on the electrochemical behaviour has been analysed for several Sn_x[BPO₄]_1−x composite materials (0.17<x<0.91). The cell using the compound Sn_0.72[BPO₄]_0.28 as active material exhibits interesting electrochemical performances (reversible capacity of 500 mAh g⁻¹ at C/5 rate).     

Chemical shifts of atomic core levels and structure of K1−xTi1−xSbxOPO4, x=0-0.23, solid solutions

Antimony-doped K_1−xTi_1−xSb_xOPO₄, x=0.23, crystals have been prepared by spontaneous nucleation from the flux in the quaternary system K₂O-TiO₂-P₂O₅-Sb₂O₅. Crystal structure observation with TEM method reveals the presence of superstructure ordering. Core level electronic parameters have been studied by X-ray photoelectron spectroscopy. Strong effect of Sb doping has been detected for inner shells of Ti⁴⁺ ions. Prominent decreasing of the binding energy difference Δ(O 1s-Ti 2p_3/2) correlates with the shortening of mean oxide bond length L(Ti−O) at x=0.23 that suggests increased ionicity of Ti−O bonds in K_1−xTi_1−xSb_xOPO₄ solid solutions.     

Phase equilibria in systems Ce-M-Sb (M=Si, Ge, Sn) and superstructure Ce12Ge9−xSb23+x (x=3.8±0.1)

Phase relations in the ternary systems Ce-M-Sb (M=Si, Ge, Sn) in composition regions CeSb₂-Sb-M were studied by optical and electron microscopy, X-ray diffraction, and electron probe microanalysis on arc-melted alloys and specimens annealed in the temperature region from 850 to 200 °C. The results, in combination with an assessment of all literature data available, were used to construct solidus surfaces and a series of isothermal sections. No ternary compounds were found to form in the Ce-Si-Sb system whilst Ce₁₂Ge_9−xSb_23+x (3.3<x<4.2) and CeSn_xSb₂ (0.1<x<0.8) participate in phase equilibria in the composition region investigated. Crystallographic parameters for the ternary compound Ce₁₂Ge_9−xSb_23+x (x=3.8±0.1) were determined from X-ray single crystal and powder diffraction. For the binary system Ge-Sb a eutectic was defined L⇔(Ge)+(Sb) at 591.6 °C and 22.5 at%. Ge EPMA revealed a maximal solubility of 6.3 at% Ge in (Sb) at the eutectic temperature.     

Crystal chemistry of Co-doped Zn7Sb2O12

Zn₇Sb₂O₁₂ forms a full range of Co-containing α solid solutions, Zn_7−xCo_xSb₂O₁₂, with an inverse-spinel structure at high temperature. At low temperatures for x<2, the solid solutions transform into the low temperature β-polymorph. For x=0, the β→α transition occurs at 1225±25 °C; the transition temperature decreases with increasing x. At high x and low temperatures, α solid solutions are formed but are non-stoichiometric; the (Zn+Co):Sb ratio is >7:2 and the compensation for the deficiency in Sb is attributed to the partial oxidation of Co²⁺ to Co³⁺. From Rietveld refinements using ND data, Co occupies both octahedral and tetrahedral sites at intermediate values of x, but an octahedral preference attributed to crystal field stabilisation, causes the lattice parameter plot to deviate negatively from the Vegard's law. Sub-solidus compatibility relations in the ternary system ZnO-Sb₂O₅-CoO have been determined at 1100 °C for the compositions containing ?50% Sb₂O₅.     

Synthesis and electrical conductivity of perovskite-type PrCo1−xMgxO3

Oxides in the system PrCo_1−xMg_xO₃ (x=0.0, 0.05, 0.10, 0.15, 0.20, 0.25) were synthesized by citrate technique and characterized by powder X-ray diffraction and scanning electron microscope. All compounds have a cubic perovskite structure (space group ). The maximum ratio of doped Mg in the system PrCo_1−xMg_xO₃ is x=0.2. Further doping leads to the segregation of Pr₆O₁₁ in PrCo_1−xMg_xO₃. The substitution of Mg for Co improves the performance of PrCoO₃ as compared to the electrical conductivity measured by a four-probe electrical conductivity analyzer in the temperature range from 298 to 1073 K. The substitution of Mg for Co on the B site may be compensated by the formations of Co⁴⁺ and oxygen vacancies. The electrical conductivity of PrCo_1−xMg_xO₃ oxides increases with increasing x in the range of 0.0-0.2. The increase in conductivity becomes considerable at the temperatures ?673 K especially for x?0.1; it reaches a maximum at x=0.2 and 1073 K. From x>0.2 the conductivity of PrCo_1−xMg_xO₃ starts getting lower. This is probably a result of the segregation of Pr₆O₁₁ in PrCo_1−xMg_xO₃ , which blocks oxygen transport, and association of oxygen vacancies. A change in activation energy for all PrCo_1−xMg_xO₃ compounds (x=0-0.25) was observed, with a higher activation energy above 573 K and a lower activation energy below 573 K. The reasons for such a change are probably due to the change of dominant charge carriers from Co⁴⁺ to Vö in PrCo_1−xMg_xO₃ oxides and a phase transition mainly starting at 573 K.     

Preparation, crystal structure, and photoluminescence of Ca2SnO4:Eu, Y

Eu³⁺-doped Ca₂SnO₄ (solid solutions of Ca_2−xEu_2xSn_1−xO₄, 0?x?0.3) and Eu³⁺ and Y³⁺-codoped Ca₂SnO₄ (Ca_1.8Y_0.2Eu_0.2Sn_0.8O₄) were prepared by solid-state reaction at 1400 °C in air. Rietveld analysis of the X-ray powder diffraction patterns revealed that Eu³⁺ replaced Ca²⁺ and Sn⁴⁺ in Eu³⁺-doped Ca₂SnO₄, and that Eu³⁺ replaced Ca²⁺ and Y³⁺ replaced Sn⁴⁺ in Ca_1.8Y_0.2Eu_0.2Sn_0.8O₄. Red luminescence at 616 nm due to the electric dipole transition ⁵D_o→⁷F₂ was observed in the photoluminescence (PL) spectra of Ca_2−xEu_2xSn_1−xO₄ and Ca_1.8Y_0.2Eu_0.2Sn_0.8O₄ at room temperature. The maximum PL intensity in the solid solutions of Ca_2−xEu_2xSn_1−xO₄ was obtained for x=0.1. The PL intensity of Ca_1.8Y_0.2Eu_0.2Sn_0.8O₄ was 1.26 times greater than that of Ca_2−xEu_2xSn_1−xO₄ with x=0.1.     

Local environment in Ba2In2−xWxO5+3x/2 oxide ion conductors

The actual oxygen environment of the tungsten dopant in the Ba₂In_2−xW_xO_5+3x/2 solid solution was revealed by combining X-ray absorption spectroscopy at the tungsten L_I and L_III edges and at the indium L_I edge. Whatever the substitution ratio, the tungsten atoms exhibit a regular octahedral environment. When the substitution ratio increases, the oxygen vacancies are progressively filled until their total occupancy for x=2/3. For x?0.3, the perovskite structure is stabilised; the tungsten atoms are randomly distributed in the structure. Although X-ray diffraction revealed a cubic symmetry for these compositions, a local distortion of the indium environment is observed when a tungsten atom is in its surrounding.     

Three-layer Aurivillius phases containing magnetic transition metal cations: Bi2−xSr2+x(Nb,Ta)2+xM1−xO12, M=Ru, Ir, Mn, x≈0.5

We report the synthesis of Aurivillius-type phases incorporating magnetic M⁴⁺ cations (M=Mn, Ru, Ir), based on the substitution of M⁴⁺ for Ti⁴⁺ in Bi₂Sr₂(Nb,Ta)₂TiO₁₂. The key to incorporating these magnetic transition metal cations appears to be the partial substitution of Sr²⁺ for Bi³⁺ in the α-PbO-type layer of the Aurivillius phase, leading to a concomitant decrease in the M⁴⁺ content; i.e., the composition of the prepared compounds was Bi_2−xSr_2+x(Nb,Ta)_2+xM_1−xO₁₂, x≈0.5. These compounds only exist over a narrow range of x, between an apparent minimum (x≈0.4) Sr²⁺ content in the α-PbO-type [Bi₂O₂] layer required for Aurivillius phases to form with magnetic M⁴⁺ cations, and an apparent maximum (x≈0.6) Sr²⁺ substitution in this [Bi₂O₂] layer. Rietveld-refinement of synchrotron X-ray powder diffraction data making use of anomalous dispersion at the Nb and Ru K edges show that the overwhelming majority of the incorporated M cations occupy the central of the three MO₆ octahedral layers in the perovskite-type block. Magnetic susceptibility measurements are presented and discussed in the context of the potential for multiferroic (magnetoelectric) properties in these materials.     

Electron-poor SrAuxIn4−x (0.5?x?1.2) and SrAuxSn4−x (1.3?x?2.2) phases with the BaAl4-type structure

Solid solutions SrAu_xIn_4−x (0.5?x?1.2) and SrAu_xSn_4−x (1.3?x?2.2) have been prepared at 700 °C and their structures characterized by powder and single-crystal X-ray diffraction. They adopt the tetragonal BaAl₄-type structure (space group I4/mmm, Z=2; SrAu_1.1(1)In_2.9(1), a=4.5841(2) Å, c=12.3725(5) Å; SrAu_1.4(1)Sn_2.6(1), a=4.6447(7) Å, c=11.403(2) Å), with Au atoms preferentially substituting into the apical over basal sites within the anionic network. The phase width inherent in these solid solutions implies that the BaAl₄-type structure can be stabilized over a range of valence electron counts (vec), 13.0-11.6 for SrAu_xIn_4−x and 14.1-11.4 for SrAu_xSn_4−x. They represent new examples of electron-poor BaAl₄-type compounds, which generally have a vec of 14. Band structure calculations confirm that substitution of Au, with its smaller size and fewer number of valence electrons, for In or Sn atoms enables the BaAl₄-type structure to be stabilized in the parent binaries SrIn₄ and SrSn₄, which adopt different structure types.     

Thermochemistry of Li1+xMn2−xO4 (0?x?1/3) spinel

Lithium substituted Li_1+xMn_2−xO₄ spinel samples in the entire solid solution range (0?x?1/3) were synthesized by solid-state reaction. The samples with x<0.25 are stoichiometric and those with x?0.25 are oxygen deficient. High-temperature oxide melt solution calorimetry in molten 3Na₂O·4MoO₃ at 974 K was performed to determine their enthalpies of formation from constituent binary oxides at 298 K. The cubic lattice parameter was determined from least-squares fitting of powder XRD data. The variations of the enthalpy of formation from oxides and the lattice parameter with x follow similar trends. The enthalpy of formation from oxides becomes more exothermic with x for stoichiometric compounds (x<0.25) and deviates endothermically from this trend for oxygen-deficient samples (x?0.25). This energetic trend is related to two competing substitution mechanisms of lithium for manganese (oxidation of Mn³⁺ to Mn⁴⁺ versus formation of oxygen vacancies). For stoichiometric spinels, the oxidation of Mn³⁺ to Mn⁴⁺ is dominant, whereas for oxygen-deficient compounds both mechanisms are operative. The endothermic deviation is ascribed to the large endothermic enthalpy of reduction.     

Structural and magnetic properties of Zn1−xSbxCr2−x/3Se4 (x=0.11, 0.16 and 0.20) single crystals

Single crystals of Zn_1−xSb_xCr_2−x/3Se₄ based on the ZnCr₂Se₄ spinel, which is known to exhibit interesting magnetic and electronic transport properties, have been prepared by solid state reaction from the appropriate selenides. Three compounds of different Sb content (x=0.11, 0.16, and 0.20) were studied by X-ray diffraction, X-ray photoelectron scattering technique and macroscopic magnetic measurements with the aim to determine (i) stability of the cubic symmetry and (ii) influence of the Sb admixture on the magnetic properties. The results show that the Sb³⁺ and Zn²⁺ ions share the tetrahedral sites in the spinel structure, while the Cr³⁺ions carrying magnetic moments, are located in the octahedral sites. The X-ray photoelectron spectroscopy (XPS) data indicate that in this series of compounds the chromium ions have a 3d³ electronic configuration. The three samples studied order antiferromagnetically at low temperatures, with the magnetic characteristics being hardly altered with respect to those reported for the parent ZnCr₂Se₄ compound.     

Evolution of structure and magnetic properties in electron-doped double perovskites, Sr2−xLaxMnWO6 (0?x?1)

Powder neutron and X-ray diffraction studies show that the double perovskites in the region 0?x?1 exhibit two crystallographic modifications at room temperature: monoclinic P2₁/n and tetragonal I4/m, with a boundary at 0.75<x<0.9. Magnetic susceptibility measurements indicate that for x=0 and 0.5 Sr_2−xLa_xMnWO₆ orders antiferromagnetically (AFM) at 15 and 25 K, respectively, for 0.75?x<1.0, a contribution of weak ferromagnetism (FM), probably due to canted-AFM order, increases with increasing x. The end point compound SrLaMnWO₆ shows the strongest FM cluster effect; however, no clear evidence of magnetic order is discernable down to 4.2 K. X-ray absorption spectroscopy (XAS) confirms Mn²⁺ and mixed-valent W^6+/5+ formal oxidation states in Sr_2−xLa_xMnWO₆.     

O MAS NMR study of the oxygen local environments in the anionic conductors Y2(B1−xB′x)2O7(B, B′=Sn, Ti, Zr)

The local environments for oxygen in yttrium-containing pyrochlores and fluorites, Y₂(B_1−xB′_x)₂O₇ (B=Ti, B′=Sn, Zr) are investigated by using solid state ¹⁷O MAS NMR spectroscopy. The quadrupolar coupling constants of the nucleus, ¹⁷O are sufficiently small for these ionic oxides, that high-resolution spectra are obtained from the MAS spectra. Different oxygen NMR resonances are observed due to local environments with differing numbers of metal cations (Y³⁺, Sn⁴⁺, Ti⁴⁺ and Zr⁴⁺), allowing the numbers of different local environments to be quantified and cation mixing to be investigated. Evidence for pyrochlore-like local ordering is detected for Y₂Zr₂O₇, which nominally adopts the fluorite structure.     

Synthesis, optical properties and electronic structures of polyoxometalates K3P(Mo1−xWx)12O40 (0?x?1)

Various compositions of solid solutions K₃P(Mo_1−xW_x)₁₂O₄₀ (0?x?1) were prepared using two solid state synthetic routes. The crystallite size was determined by linewidth refinements of X-ray diffraction patterns using the Warren-Averbach method, and the grain size distribution by laser scattering experiments. Optical properties were determined by diffuse reflectance measurements in the UV-visible range. The optical gap E_g was found to increase exponentially from ∼2.5 to ∼3.30 eV with increasing x, and is systematically shifted to a higher energy when the grain size decreases. The relation between E_g and x was analyzed by calculating the HOMO-LUMO gaps of the [P(Mo_1−xW_x)₁₂O₄₀]³⁻ anions on the basis of tight-binding electronic structure calculations.     

Solution precursor synthesis and magnetic properties of Eu1−xCaxTiO3

Europium titanate, EuTiO₃, is a paraelectric/antiferromagnetic cubic perovskite with T_N=5.5 K. It is predicted that compressive strain could induce simultaneous ferroelectricity and ferromagnetism in this material, leading to multiferroic behavior. As an alternative to epitaxial strain, we explored lattice contraction via chemical substitution of Eu²⁺ with the smaller Ca²⁺ cation as a mechanism to tune the magnetic properties of EuTiO₃. A modified sol-gel process was used to form homogeneously mixed precursors containing Eu³⁺, Ca²⁺, and Ti⁴⁺, and reductive annealing was used to transform these precursors into crystalline powders of Eu_1−xCa_xTiO₃ with x=0.00, 0.05, 0.10, 0.15, 0.25, 0.35, 0.50, 0.55, 0.60, 0.65, 0.80, and 1.00. Powder XRD data indicated that a continuous Eu_1−xCa_xTiO₃ solid solution was readily accessible, and the lattice constants agreed well with those predicted by Vegard's law. SEM imaging and EDS element mapping indicated a homogeneous distribution of Eu, Ca, and Ti throughout the polycrystalline sample, and the actual Eu:Ca ratio agreed well with the nominal stoichiometry. Measurements of magnetic susceptibility vs. temperature indicated antiferromagnetic ordering in samples with x≤0.60, with T_N decreasing from 5.4 K in EuTiO₃ to 2.6 K in Eu_0.40Ca_0.60TiO₃. No antiferromagnetic ordering above 1.8 K was detected in samples with x>0.60.     

Superstructure of hollandite KxMg(8+x)/3Sb(16−x)/3O16 (x≈1.76)

Single crystals of K_xMg_(8+x)/3Sb_(16−x)/3O₁₆ (x≈1.76) with a hollandite superstructure were grown. Ordering schemes for guest ions (K) and the host structure were confirmed by the structure refinement using X-ray diffraction intensities. The space group is I4/m and cell parameters are a=10.3256(6), c=9.2526(17)Å with Z=3. Superlattice formation is primarily attributed to the Mg/Sb occupational modulation in the host structure. Mg/Sb ratios at two nonequivalent metal sites are 0.8977/0.1023 and 0.1612/0.8388. Two types of the cavity are seen in the tunnel, where parts of K ions deviate from the cavity center along the tunnel direction. Probability densities for K ions in the two cavities are different from each other, which seems to have arisen from the Mg/Sb modulation.     

Synthesis of layered cathode material Li[CoxMn1−x]O2 from layered double hydroxides precursors

Cathode materials Li[Co_xMn_1−x]O₂ for lithium secondary batteries have been prepared by a new route—precursor method of layered double hydroxides (LDHs). In situ high-temperature X-ray diffraction (HT-XRD) and thermogravimetric analysis coupled with mass spectrometry (TG-MS) were used to monitor the structural transformation during the reaction of CoMn LDHs and LiOH·H₂O: firstly the layered structure of LDHs transformed to an intermediate phase with spinel structure; then the distortion of the structure occurred with the intercalation of Li⁺ into the lattice, resulting in the formation of layered Li[Co_xMn_1−x]O₂ with α-NaFeO₂ structure. Extended X-ray absorption fine structure (EXAFS) data showed that the Co-O bonding length and the coordination number of Co were close to those of Mn in Li[Co_xMn_1−x]O₂, which indicates that the local environments of the transitional metals are rather similar. X-ray photoelectron spectroscopy (XPS) was used to measure the oxidation state of Co and Mn. The influences of Co/Mn ratio on both the structure and electrochemical property of Li[Co_xMn_1−x]O₂ have been investigated by XRD and electrochemical tests. It has been found that the products synthesized by the precursor method demonstrated a rather stable cycling behavior, with a reversible capacity of 122.5 mAh g⁻¹ for the layered material Li[Co_0.80Mn_0.20]O₂.