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.     

Mild hydrothermal synthesis and magnetic properties of the manganates Pr1−xCaxMnO3

The calcium-doped manganates, Pr_1−xCa_xMnO₃ (x=0.39, 0.46, 0.70, 0.76), were synthesized as cube-shaped crystalline phases under mild hydrothermal conditions for the first time. The crystals could be grown in one step from solutions of metal salts and potassium hydroxide at temperatures ∼240 °C, and found to adopt perovskite-like structure (space group Pbnm). Samples were characterized by powder X-ray diffraction, scanning electron microscopy, inductively coupled plasma analysis and variable temperature dc/ac magnetic susceptibility. The studies indicate that formation of the materials is dependent on the alkalinity and composition of the initial reaction mixtures. The magnetic properties show spin-glass-like behavior due to competing ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions in Pr_1−xCa_xMnO₃ with x=0.39, 0.46.     

Composition-induced structural phase transitions in the (Ba1−xLax)2In2O5+x (0?x?0.6) system

Composition-induced structural phase changes across the high temperature, fast oxide ion conducting (Ba_1−xLa_x)₂In₂O_5+x, 0?x?0.6, system have been carefully analysed using hard mode infrared (IR) powder absorption spectroscopy, X-ray powder diffraction and electron diffraction. An orthorhombic brownmillerite to three-dimensionally disordered cubic perovskite phase transition in this system is signalled by a drastic change in slope of both wavenumber and average line widths of IR spectra as a function of composition. Some evidence is found for the existence of an intermediate tetragonal phase (previously reported to exist from electron diffraction data) around x∼0.2. The new spectroscopic data have been used to compare microscopic and macroscopic strain parameters arising from variation in composition. The strain and spectroscopic data are consistent with first-order character for the tetragonal→orthorhombic transition, while the cubic→tetragonal transition could be continuous. Differences between the variation with composition of spectral parameters and of macroscopic strain parameters are consistent with a substantial order/disorder component for the transitions. There is also evidence for precursor effects within the cubic structure before symmetry is broken.     

Structural and magnetic properties of Pr18Li8Fe5−xMxO39 (M=Ru, Mn, Co)

A polycrystalline sample of Pr₁₈Li₈Fe₄RuO₃₉ has been synthesized by a solid state method and characterized by neutron powder diffraction, magnetometry and Mössbauer spectroscopy; samples of Pr₁₈Li₈Fe_5−xMn_xO₃₉ and Pr₁₈Li₈Fe_5−xCo_xO₃₉ (x=1, 2) have been studied by magnetometry. All these compounds adopt a cubic structure (space group , a₀∼11.97 Å) based on intersecting 〈111〉 chains made up of alternating octahedral and trigonal-prismatic coordination sites. These chains occupy channels within a Pr-O framework. The trigonal-prismatic site in Pr₁₈Li₈Fe₄RuO₃₉ is occupied by Li⁺ and high-spin Fe³⁺. The remaining transition-metal cations occupy the two crystallographically-distinct octahedral sites in a disordered manner. All five compositions adopt a spin-glass-like state at 7 K (Pr₁₈Li₈Fe₄RuO₃₉) or below.     

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.     

La doping of the Sr2CoWO6 double perovskite: A structural and magnetic study

La-doped Sr₂CoWO₆ double perovskites have been prepared in air in polycrystalline form by solid-state reaction. These materials have been studied by X-ray powder diffraction (XRPD), neutron powder diffraction (NPD) and magnetic susceptibility. The structural refinement was performed from combined XRPD and NPD data (D2B instrument, λ=1.594 Å). At room temperature, the replacement of Sr²⁺ by La³⁺ induces a change of the tetragonal structure, space group I4/m of the undoped Sr₂CoWO₆ into the distorted monoclinic crystal structure, space group P2₁/n, Z=2. The structure of La-doped phases contains alternating CoO₆ and (Co/W)O₆ octahedra, almost fully ordered. On the other hand, the replacement of Sr²⁺ by La³⁺ induces a partial replacement of W⁶⁺ by Co²⁺ into the B sites, i.e. Sr_2−xLa_xCoW_1−yCo_yO₆ (y=x/4) with segregation of SrWO₄. Magnetic and neutron diffraction measurements indicate an antiferromagnetic ordering below T_N=24 K independently of the La-substitution.     

Synthesis and structural study of a new NASICON-type solid solution: Li1−xLax/3Zr2(PO4)3

A new complete solid solution of NASICON-type compounds between LiZr₂(PO₄)₃ and La_1/3Zr₂(PO₄)₃ was evidenced with the general formula Li_1−xLa_x/3Zr₂(PO₄)₃ (0?x?1). These phases were synthesized by a complex polymerizable method and structurally characterized from Rietveld treatment of their X-ray and neutron powder diffraction data. This solid solution results from the substitution mechanism Li⁺→1/3La³⁺+2/3□ leading to an increase of the vacancies number correlated to an increase of the La content. According to this substitution mechanism, the general formula can then be written Li_1−xLa_x/3□_2x/3Zr₂(PO₄)₃ (0?x?1) in order to underline the correlation between the La content and the vacancies rate. For all the compounds, the structure is clearly related to that of the NASICON family with three crystallographic domains evidenced. For 0?x?0.5, all the members adopt at high temperature the typical NASICON-type structure (s.g. R3¯c), while at lower temperature, their structure distorts to a triclinic form (s.g. C 1¯), as observed for LiZr₂(PO₄)₃ prepared above 1100 °C. Moreover, in this domain, the reversible transition is clearly soft and the transition temperature strongly depends of the x value. For 0.6?x?0.9, the compounds crystallize in a rhombohedral cell (s.g. R3¯), while for x=1, the phase La_1/3Zr₂(PO₄)₃ is obtained (s.g. P3¯, Z=6, a=8.7378(2) Å, c=23.2156(7) Å).This paper is devoted to the structure analysis of the series Li_1−xLa_x/3Zr₂(PO₄)₃ (0?x?1), from X-ray and neutron powder thermo diffraction and transmission electron microscopy (TEM) studies.     

Solid solubility and phase transitions in the system LaNb1−xTaxo4

The solid solubility between LaNbO₄ and LaTaO₄ was investigated by X-ray diffraction, and a two-phase region was observed in the composition region LaNb_1−xTa_xO₄ where 0.4?x?0.8. Single-phase LaNb_1−xTa_xO₄ (0?x?0.4) with the monoclinic Fergusonite structure at ambient temperature, was observed to transform to a tetragonal Scheelite structure by in-situ high-temperature X-ray diffraction, and the phase transition temperature was shown to increase with increasing Ta-content. This ferroelastic to paraelastic second-order phase transition was described by Landau theory using spontaneous strain as an order parameter. The thermal expansion of LaNb_1−xTa_xO₄ (0?x0.4) was shown to be significantly higher below the phase transition than above. Single-phase LaNb_1−xTa_xO₄ (0.8?x?1) with another monoclinic crystal structure at ambient temperature was shown to transform to an orthorhombic crystal structure by X-ray diffraction and differential scanning calorimetry. The phase transition temperature was observed to decrease with decreasing Ta-content. Finally, orthorhombic LaTaO₄ could also be transformed to monoclinic LaTaO₄ at ambient temperature by applying a uniaxial pressure of 150-170 MPa, reflecting the lower molar volume of monoclinic LaTaO₄.     

Thermoelectric properties of polycrystalline La1−xSrxCoO3

Thermoelectric properties of polycrystalline La_1−xSr_xCoO₃, where Sr²⁺ is substituted in La³⁺ site in perovskite-type LaCoO₃, have been investigated. Sr-doping increases the electrical conductivity (σ) of La_1−xSr_xCoO₃, and also decreases the Seebeck coefficient (S) for 0.01?x?0.40. A Hall coefficient measurement reveals that the increase in electrical conductivity arises from increases in both carrier concentration and the Hall mobility. The decrease in the Seebeck coefficient is caused by a decrease in carrier effective mass as well as increase in carrier concentration. The highest power factor (σS²) is 3.7×10⁻⁴ W m⁻¹ K⁻² at 250 K for x=0.10. The thermal conductivity (κ) is about 2 W m⁻¹ K⁻¹ at 300 K for 0?x?0.04, and increases for x?0.05 because of an increase in heat transport by conductive carrier. The thermoelectric properties of La_1−xSr_xCoO₃ are improved by Sr-doping, and the figure of merit (Z=σS² κ⁻¹) reaches 1.6×10⁻⁴ K⁻¹ for x=0.06 at 300 K (ZT=0.048). For heavily Sr-doped samples, the thermoelectric properties diminish mainly because of the decrease in the Seebeck coefficient and the increase in thermal conductivity.     

X-ray and neutron powder diffraction studies of Ba(NdxY2−x)CuO5

Ba(R,R′)₂CuO₅ (R,R′=lanthanides and Y) plays an important role as a flux-pinning agent in enhancing the superconducting properties of the Ba₂(R,R′)Cu₃O_6+x (R,R′=lanthanides and Y) coated conductors. Using X-ray diffraction and neutron diffraction, we found that the Ba(Nd_xY_2−x)CuO₅ solid solution adopts two structure types. In the Nd-rich region (1.8?x?2.0), the materials are of brown color (commonly referred to as the ‘brown phase’), and the structure is tetragonal with space group I4/mbm (no. 127). In the Y-rich region (0.0?x?1.4), the materials are green (commonly referred to as the ‘green phase’) and the structure is orthorhombic with space group Pnma (no. 62). A two-phase region (1.4<x<1.8) exists between the orthorhombic and tetragonal phases. The crystal chemistry and crystallography of the orthorhombic ‘green phase’ series, Ba(Nd_xY_2−x)CuO₅ (isostructural to BaY₂CuO₅), are discussed in this paper.     

Structure and conductivity of strontium-doped cerium orthovanadates Ce1−xSrxVO4 (0≤x≤0.175)

A-site substituted cerium orthovanadates, Ce_1−xSr_xVO₄, were synthesised by solid-state reactions. It was found that the solid solution limit in Ce_1−xSr_xVO₄ is at x=0.175. The crystal structure was analysed by X-ray diffraction and it exhibits a tetragonal zircon structure of space group I4₁/amd (1 4 1) with a=7.3670 (3) and c=6.4894 (1) Å for Ce_0.825Sr_0.175VO₄. The UV-vis absorption spectra indicated that the compounds have band gaps at room temperature in the range 4.5-4.6 eV. Conductivity measurements were performed for the first time up to the strontium solid solution limit in air and in dry 5% H₂/Ar with conductivity values at 600 °C ranging from 0.3 to 30 mS cm⁻¹ in air to 30-45 mS cm⁻¹ in reduced atmosphere. Sample Ce_0.825Sr_0.175VO₄ is redox stable at a temperature below 600 °C although the conductivity is not high enough to be used as an electrode for solid oxide fuel cells.     

Doping studies of the magnetic cobaltocuprate CoSr2Y2−xCexCu2O9±δ

A structural, magnetic and electronic study of the cobaltocuprate CoSr₂Y_2−xCe_xCu₂O_9±δ (x=0.5-0.8) has been performed. All materials crystallise in the orthorhombic Cmcm symmetry space group in which chains of corner linked CoO₄ tetrahedra run parallel to the 1 1 0 direction. An antiferromagnetic transition is observed for x=0.5-0.8; T_M increases with x. A change in the dimensionality of the magnetic order occurs at x=0.8 as the interchain distance increases to a critical value. There is charge transfer between the cuprate planes and cobaltate layer as Ce doping increases, so that Co³⁺ is partially oxidised to Co⁴⁺ with a concomitant reduction in the valence of Cu. Superconductivity is not observed in any of the samples and a crossover from Mott to Efros and Shklovskii variable range hopping behaviour is evidenced as x increases from 0.5 to 0.8.     

Structural and Magnetic Chemistry of La2Sr2BMnO8 (B=Mg, Zn)

Polycrystalline samples of the layered perovskites La₂Sr₂MgMnO₈ and La₂Sr₂ZnMnO₈ have been studied by X-ray and neutron powder diffraction, electron diffraction and magnetometry. X-ray and neutron powder diffraction indicate that the average structure is that of K₂NiF₄, with disordering of Mn and (Zn, Mg) cations over the octahedral sites. Electron diffraction data indicate that cation ordering is present over these sites in the xy planes, with the xy ordered planes being stacked in a disordered manner along z. No long-range magnetic ordering is observed in the temperature range 5≤T (K)≤300.     

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.     

Electrical properties and sulfur tolerance of La0.75Sr0.25Cr1−xMnxO3 under anodic conditions

Complex metal oxides with composition of La_0.75Sr_0.25Cr_1−xMn_xO₃(x=0.4,0.5,0.6) (LSCM) have been synthesized and examined as anode materials for solid oxide fuel cells (SOFCs). LSCM compositions show excellent tolerance to both reduction and oxidation but the crystal structure transforms from hexagonal in air to orthorhombic in H₂. The volume change associated with this phase transformation is only about 1%, thus having little effect on other properties. The total electrical conductivity increases with the content of Mn, whereas the resistance to sulfur poisoning increases with the content of Cr. Fuel cells using LSCM as the anode show very good performance when pure hydrogen is used as the fuel. However, they do not appear to be stable in fuels containing 10% of H₂S.     

Phase relations in NaxCrxTi8−xO16 at 1350 °C and crystal structure of hollandite-like Na2Cr2Ti6O16

Phase relations of rutile, freudenbergite, and hollandite structures were examined in the pseudobinary system NaCrO₂-TiO₂ (i.e., Na_xCr_xTi_8−xO₁₆) at 1350 °C. The hollandite structure was obtained in the composition range 1.7?x?2.0. The symmetry of the samples at room temperature was tetragonal for x=1.7 and 1.75, and monoclinic for x=1.8 and above. Single crystals of monoclinic hollandite Na₂Cr₂Ti₆O₁₆ were grown and the structure refinement has been carried out using an X-ray diffraction technique. The space group was I2/m and cell parameters were a=10.2385(11), b=2.9559(9), c=9.9097(11)Å, and β=90.545(9)° with Z=1. The Na ion distribution in the tunnel was markedly deformed from that in the tetragonal form. It was suggested that Cr/Ti ratios were different between the two framework metal sites.     

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.     

Structural and magnetic study of the cation-ordered perovskites Ba2−xSrxErMoO6

A series of perovskite phases have been prepared from the appropriate carbonates and oxides by heating under reducing conditions at temperatures up to 1300 °C. Complete ordering between ErO₆ and MoO₆ octahedra and a disordered distribution of Sr²⁺ and Ba²⁺ occur in all compounds. Neutron powder diffraction experiments show that the substitution of Sr²⁺ into Ba₂ErMoO₆ introduces a progressive reduction in symmetry from Fm3¯m (x=0) to I4/m (x=0.5, 0.8) to P2₁/n (x=1.25, 1.75, 2.0). Magnetic susceptibility measurements indicate that all of these compounds show Curie-Weiss paramagnetism and that for x<1.25 this behaviour persists down to 2 K. The monoclinically distorted compounds show magnetic transitions at low temperature and neutron diffraction has confirmed the presence of long-range antiferromagnetic order below 2.5 and 4 K in Ba_0.25Sr_1.75ErMoO₆ and Sr₂ErMoO₆, respectively. Ba_0.75Sr_1.25ErMoO₆, Ba_0.25Sr_1.75ErMoO₆ and Sr₂ErMoO₆ do not undergo structural distortion on cooling from room temperature.     

A novel route to synthesize cubic ZrW2−xMoxO8 (x=0-1.3) solid solutions and their negative thermal expansion properties

Cubic ZrW_2−xMo_xO₈ (c-ZrW_2−xMo_xO₈) (x=0-1.3) solid solutions were prepared by a novel polymorphous precursor transition route. X-ray diffraction (XRD) analysis reveals that the solid solutions are single phase with α- and β-ZrW₂O₈ structure for 0?x?0.8 and 0.9?x?1.3, respectively. The optimum synthesis conditions of ZrWMoO₈ are obtained from differential scanning calorimetry-thermal gravimetric analysis (DSC-TGA), XRD and mass loss-temperature/time curves. Following the above experience, the stoichiometric solid solutions of c-ZrW_2−xMo_xO₈ (x=0-1) are obtained within 1 wt% of mass loss. The relationships of lattice parameters (a), phase transition temperatures (T_c) and instantaneous coefficients of thermal expansion (α_i) against the content x of Mo are discussed based on the variation of order degree parameters of ZrW_2−xMo_xO₈.     

Synthesis and properties of the Co7Se8−xSx and Ni7Se8−xSx solid solutions

We report the synthesis and elementary properties of the Co₇Se_8−xS_x (x=0-8) and Ni₇Se_8−xS_x (x=0-7) solid solutions. Both systems form a NiAs-type structure with metal vacancies. In general, the lattice parameters decrease with increasing x, but in the Ni₇Se_8−xS_x system c increases on going from x=5 to 7. Magnetic susceptibility measurements show that all samples exhibit temperature-independent paramagnetism from 25-250 K. Samples within the Co₇Se_8−xS_x system, as well as Ni₇Se₈ and Ni₇SeS₇, were found to be poor metals with resistivities of ∼0.20 and ∼0.06 mΩ cm at 300 K, respectively. The Sommerfeld constant (γ) was determined from specific heat measurements to be ∼13 mJ/mol_CoK² and ∼7 mJ/mol_NiK² for Co₇Se_8−xS_x and Ni₇Se_8−xS_x, respectively.