The fluorite-pyrochlore transformation of Ho2−yNdyZr2O7

Twelve members of the Ho_2−yNd_yZr₂O₇ series, prepared using conventional solid state methods, have been characterised by neutron powder diffraction. Ho₂Zr₂O₇ has a defect fluorite structure whereas Nd₂Zr₂O₇ is found to adopt the ordered pyrochlore structure with the composition induced fluorite-pyrochlore transformation occurring near y=1. Rietveld analysis on the neutron data for all the compositions reveals an increase in lattice parameter as a function of y across the entire series, with a small discontinuity associated with the transformation. The neutron profile results suggest that domains of pyrochlore-type initially begin to form before crystallising into a separate phase, and therefore that anion and cation ordering processes are distinct. There is a strong correlation between the extent of disorder in the anion sublattice and the x-parameter of 48f oxygen. These results point the way to a better understanding of the stability observed in pyrochlore structures.     

Mechanochemical-thermal preparation and structural studies of mullite-type Bi2(GaxAl1−x)4O9 solid solutions

A series of Bi₂(Ga_xAl_1−x)₄O₉ solid solutions (0≤x≤1), prepared by mechanochemical processing of Bi₂O₃/Ga₂O₃/Al₂O₃ mixtures and subsequent annealing, was investigated by XRD, EDX, and ²⁷Al MAS NMR. The structure of the Bi₂(Ga_xAl_1−x)₄O₉ solid solutions is found to be orthorhombic, space group Pbam (No. 55). The lattice parameters of the Bi₂(Ga_xAl_1−x)₄O₉ series increase linearly with increasing gallium content. Rietveld refinement of the XRD data as well as the analysis of the ²⁷Al MAS NMR spectra show a preference of gallium cations for the tetrahedral sites in Bi₂(Ga_xAl_1−x)₄O₉. As a consequence, this leads to a far from random distribution of Al and Ga cations across the whole series of solid solutions.     

Synthesis and structural study of stoichiometric Bi2Ti2O7 pyrochlore

Bi₂Ti₂O₇ has been synthesized using a co-precipitation route from H₂O₂/NH_3(aq) solutions of titanium with aqueous bismuth nitrate. The stoichiometric material crystallizes into a pale yellow cubic pyrochlore phase. A powder X-ray diffraction study showed this crystallization to be very temperature sensitive, the pure phase can only be obtained within a few degrees of 470°C. Time-of-flight powder neutron diffraction studies of Bi₂Ti₂O₇ (Space group , a=10.37949(4) Å at ambient temperature, Z=8, R_p=3.95%, R_wp=4.75%) revealed positional disorder in the bismuth site and in the O′ oxide site both at ambient temperature and at 2 K.     

Magnetic properties related to structure and complete composition analyses of nanocrystalline La1−xMn1−yO3 powders

Structural and magnetic properties were studied on La_1−xMnO_3±δ nanocrystalline powders exhibiting different La/Mn ratios. These compounds were prepared using a gel combustion method based on a cation solution soaking by acrylamide polymerization. Structural properties were studied both by transmission electron microscopy and X-ray diffraction (XRD). Complete chemical composition analyses were performed by induced coupled plasma spectroscopy and by iodometric titration. Proportions of parasitic phases in samples, as La₂O₃ or Mn₃O₄, and actual compositions of La_1−xMnO_3±δ phases were then determined from refinements of XRD data and sample chemical compositions. As a result, perovskite structure is not any more stable for La/Mn<0.9 as it decomposes into a mixing of La_0.9MnO₃ and of Mn₃O₄ phases, in agreement with results on thermodynamic equilibrium in the La-Mn-O phase diagram. For La/Mn>0.9, a high oxygen excess is observed and leads to consider the creation of vacancies on both lanthanum and manganese sites, whose concentrations are evaluated. Magnetic properties agree well with the proposed structures and sample compositions since for La/Mn<0.9, for which a La_0.9MnO₃ phase is always found, the Curie temperature remains constant and equal to 295 K (the highest temperature never observed before in such series of compositions), while for La/Mn>0.9, there is a formation of Mn vacancies giving rise to a lowering of Curie temperature resulting of a frustration of ferromagnetic interactions.     

The local crystal chemistry and dielectric properties of the cubic pyrochlore phase in the Bi2O3MONb2O5 (M=Ni and Mg) systems

The composition, dielectric properties and inherent displacive disorder of a Bi-based, misplaced-displacive cubic pyrochlore phase found in two ternary Bi₂O₃M²⁺ONb₂O₅ (M=Ni and Mg) systems has been investigated. The dielectric permittivities (up to 1 MHz) of (Bi_0.825Ni_0.125□_0.05)₂(Ni_0.25Nb_0.75)₂O₇ and (Bi_0.835Mg_0.085□_0.08)₂(Mg_0.235Nb_0.765)₂O₇ at room temperature are found to be 116 and 151, respectively, while the dielectric loss tangents are 0.00065 and 0.00042, respectively, at 100 kHz. A highly structured characteristic diffuse intensity distribution apparent in electron diffraction is reported in both cases and partially interpreted in terms of large amplitude, β-cristobalite-type tetrahedral rotations of the O’A₂ tetrahedral framework sub-structure of the ideal pyrochlore structure type. Bond valence sum calculations are used to investigate the local crystal chemistry responsible for this displacive disorder.     

Synthesis and characterization of the new pyrochlore Bi1.5Sb1.5−xNbxMnO7 solid solution

The solid solution with formula Bi_1.5Sb_1.5−xNb_xMnO₇ has been synthesized using the ceramic method at 1000 °C. This solid solution was crystallized in the pyrohlore structure in cubic lattice with Fd-3 m space group. The cell parameter increases linearly with increase in the niobium concentration. The magnetic susceptibility measurements achieved between 4 and 300 K show a paramagnetic behaviour with an effective moment 5.84 μ_β for Bi_1.5Sb_1.5MnO₇ compound (x = 0) and 5.61 μ_B for Bi_1.5Nb_1.5MnO₇ compound (x = 1.5) indicating “ + 2” oxidation state for manganese ion. The electric conductivity measured using the complex impedance spectroscopy method put in evidence an increase of the electric conductivity with the temperature, and the antimony compound is slightly more conductor than the niobium compound; results indicate electronic conductivity with semi-conducting behaviour.     

Luminescent and structural properties of the series Ba6−xEuxTi2+xTa8−xO30 and Ba4−yKyEu2Ti4−yTa6+yO30

The series Ba_6−xEu_xTi_2+xTa_8−xO₃₀ and Ba_4−yK_yEu₂Ti_4−yTa_6+yO₃₀ have been synthesized at 1400°C in air. They exhibit efficient excitation at about 400 nm and typical emission of Eu³⁺ at about 580-620 nm, form solid solutions within 0.0?x?2.0 and 0?y?4 respectively, and crystallized in P4/mbm at room temperature with Eu atoms occupied at centrosymmetric site (0, 0, 0). Their conductivity is very low (2.8×10⁻⁶ Ω⁻¹ cm⁻¹ at 740°C for Ba₆Ti₂Ta₈O₃₀).     

Synthesis and structural characterization of Al4Si2C5-homeotypic aluminum silicon oxycarbide, (Al6−xSix)(OyC5−y) (x∼0.8 and y∼1.6)

We have prepared a new layered oxycarbide, [Al_5.25(5)Si_0.75(5)][O_1.60(7)C_3.40(7)], by isothermal heating of (Al_4.4Si_0.6)(O_1.0C_3.0) at 2273 K near the carbon-carbon monoxide buffer. The crystal structure was characterized using X-ray powder diffraction, transmission electron microscopy and energy dispersive X-ray spectroscopy (EDX). The title compound is trigonal with space group R3?m (centrosymmetric), Z=3, and hexagonal cell dimensions a=0.32464(2) nm, c=4.00527(14) nm and V=0.36556(3) nm³. The atom ratios Al:Si were determined by EDX, and the initial structural model was derived by the direct methods. The final structural model showed the positional disordering of one of the three types of Al/Si sites. The reliability indices were R_wp=4.45% (S=1.30), R_p=3.48%, R_B=2.27% and R_F=1.25%. The crystal is composed of three types of domains with nearly the same fraction, one of which has the crystal structure of space group R3¯m. The crystal structure of the remaining two domains, which are related by pseudo-symmetry inversion, is noncentrosymmetric with space group R3m.     

Synthesis, Crystal Structure and Physical Properties of LixMg0.857−xCu2.143O3−y

Solid solutions of Li-doped Mg_0.857Cu_2.143O₃ (Li_xMg_0.857−xCu_2.143O_3−y) were prepared at 950°C for 12 h in air by the solid-state method using Li₂CO₃, MgO and CuO powders. The solid solutions were obtained as the single α phase with the güggenite structure in 0≦x≦0.06 region. With the increasing of the Li content x, the lattice parameters a, b and unit cell volume V decreased, while c increased. On the basis of the charge neutrality, hole carrier estimated by the oxygen content increased with the Li substitution. The Seebeck coefficient at room temperature of x = 0.03 sample was +400 μV/K. The electrical resistivity ρ at room temperature drastically decreased with the increasing x. Temperature dependences of ρ for x = 0.01, 0.03 and 0.06 samples were semi-conductive behavior from room temperature to about 12 K. Interaction between Cu²⁺ and Cu²⁺ through O²⁻ seems to be somewhat large antiferromagnetic one. Sperconducting transition was not detected in the temperature range.     

Homoatomic Clustering in the Intermediate Valence Compounds Yb1−yAl3−xSix and Yb1−yAl3−xGex

Two new phases, Yb_1−xAl_3−xSi_x and Yb_1−yAl_3−xGe_x, were found by systematic investigations of the according ternary systems. The crystal structures of Yb_1−yAl_2.8Si_0.2 and Yb_1−yAl_2.8Ge_0.2 (defect HT-PuAl₃ type) were studied by X-ray powder methods (CuKα₁ radiation, λ=1.54056 Å, hexagonal system, space group P6₃/mmc (No. 194), a=6.009(1) and 6.015(1) Å, c=14.199(2) and 14.241(5) Å, V=444.0(2) and 446.2(3) ų, 93 and 92 reflections, and 8200 and 8000 profile points for silicide and germanide, respectively). Full profile refinements with 11 and 13 structural parameters resulted in R_I=0.049 and 0.054, and R_p=0.088 and 0.104, respectively. The ternary structures are distorted closest packings in comparison with the binary YbAl₃ compound with AuCu₃-type structure. They are characterized by the formation of Al₃-, Si₃-, and Ge₃-homoatomic clusters and aluminum networks. Magnetization measurements show that both the silicide and germanide are valence fluctuation compounds with enhanced electronic density of states at the Fermi level similar to the binary YbAl₃. The characteristic maximum of the magnetic susceptibility increases from ≈120 K for YbAl₃ to ≈140 K for Yb_1−yAl_2.8Si_0.2or Yb_1−yAl_2.8Ge_0.2 and further to ≈150 K for Yb_1−yAl_2.75Si_0.25. The S-shape of the electrical resistivity curves is also characteristic of valence fluctuations.     

Structural and magnetic properties of Nd18Li8Co4−xFexO39−y and Nd18Li8Co4−xTixO39−y

Nd₁₈Li₈Co₃FeO_39−y, Nd₁₈Li₈CoFe₃O_39−y and Nd₁₈Li₈Co₃TiO_39−y have been synthesised and characterised by neutron powder diffraction, magnetometry and Mössbauer spectroscopy. Their cubic structure (Pm3?n, a∼11.9 Å) is based on intersecting <1 1 1> chains comprised of alternating octahedral and trigonal-prismatic coordination sites. These chains lie within hexagonal-prismatic cavities formed by a Nd-O framework. Each compound has an incomplete oxide sublattice (y∼1), with vacancies located around the octahedral sites that lie at the points of chain intersection. These sites are fully occupied by a disordered arrangement of transition-metal cations but only 75% of the remaining octahedral sites are occupied. The trigonal-prismatic sites are fully occupied by lithium except in the case of Nd₁₈Li₈CoFe₃O_39−y where some iron is present. Antiferromagnetic interactions are present on the Nd sublattice in each composition, but a spin glass forms below 5 K when a high concentration of spins is also present on the octahedral sites.     

Synthesis and structural characterization of Al7C3N3-homeotypic aluminum silicon oxycarbonitride, (Al7−xSix)(OyCzN6−y−z) (x∼1.2, y∼1.0 and z∼3.5)

A new aluminum silicon oxycarbonitride, (Al_5.8Si_1.2)(O_1.0C_3.5N_1.5), has been synthesized and characterized by X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). The title compound is hexagonal with space group P6₃/mmc and unit-cell dimensions a=0.322508(4) nm, c=3.17193(4) nm and V=0.285717(6) nm³. The atom ratios of Al:Si and those of O:C:N were, respectively, determined by EDX and EELS. The initial structural model was successfully derived from the XRPD data by the direct methods and further refined by the Rietveld method. The crystal is most probably composed of four types of domains with nearly the same fraction, each of which is isotypic to Al₇C₃N₃ with space group P6₃mc. The existence of another new oxycarbonitride (Al_6.6Si_1.4)(O_0.7C_4.3N_2.0), which must be homeotypic to Al₈C₃N₄, has been also demonstrated by XRPD and TEM.     

Synthesis and crystal structure of two new cerium rhodium oxides: Ce2/3−xRh2O4 (x∼0.12) with Ce mixed valency and CeRh2O5

The new compounds Ce_2/3−xRh₂O₄ (x∼0.11-0.14) and CeRh₂O₅ have been prepared. Their structures were determined from single crystal X-ray diffraction data. Electrical and magnetic properties were also evaluated. Based on the structural analysis and physical properties, oxidation states for CeRh₂O₅ can be assigned as Ce⁴⁺Rh³⁺₂O₅. A small variation in x was detected for Ce_2/3−xRh₂O₄ indicating a formula ranging from Ce^3.64+_0.55Rh³⁺₂O₄ to Ce^3.81+_0.525Rh³⁺₂O₄.     

Preparation and structure of NaSr0.5Al2B2O7 and NaCa0.5Al2B2O7

Two compounds NaSr_0.5Al₂B₂O₇ and NaCa_0.5Al₂B₂O₇, have been found to crystallize into a new structure type by Rietveld refinement from X-ray powder diffraction data. Their structure belongs to hexagonal space group P6₃/m, with lattice parameters of , for NaSr_0.5Al₂B₂O₇ and , for NaCa_0.5Al₂B₂O₇, respectively. The structure is built up by [Al₂B₂O₇]²⁻ double layer and Na⁺/Ca²⁺ or Na⁺/Sr²⁺ ions alternatively stacking along the c-axis. The sites in the inter-double layer are fully occupied jointly by Na and Ca or Sr, but the intra-double layer sites are only half occupied solely by Na. A mechanism of the transition of the structure from CaAl₂B₂O₇ to present structure type by replacing only 1% Ca by Na (2%) as observed by Chang and Keszler (Mater. Res. Bull. 33 (1998) 299) is also proposed.     

Synthesis, Electrical Properties, and Powder Neutron Crystal Structure Refinement of Pb1−xBixPt2O4 Compounds (0≤x≤0.3)

Substitution of Pb for Bi in the recently characterized mixed-valence lead-platinum oxide PbPt₂O₄ was attempted and a Pb_1−xBi_xPt₂O₄ solid solution was obtained for 0≤x≤0.3. Powder X-ray diffraction study showed that all substituted compounds crystallize with similar triclinic unit cell and PbPt₂O₄ lattice parameters. The structural model of Pb_0.7Bi_0.3Pt₂O₄ was refined from powder X-ray diffraction data using the Rietveld method and the results indicate the same crystal structure than PbPt₂O₄ with one mixed Pb/Bi atomic site. Neutron diffraction realized on the two limit compositions of the solid solution (x=0 and 0.3) allowed to confirm the PbPt₂O₄ and Pb_0.7Bi_0.3Pt₂O₄ stoichiometries. Mean oxidation degree of Pt atoms in the [PtO₄] infinite chains decreases from +3 for PbPt₂O₄ to +2.7 for Pb_0.7Bi_0.3Pt₂O₄. Conductivity measurements show a metallic behavior for all the compositions except the limit composition x=0.3 for which a semiconducting behavior appears.     

Solvothermal Synthesis of Superfine Li1−xMn2O4−σ Powders

Superfine Li_1−xMn₂O_4−σ powders were successfully synthesized by the alcohol-thermal method using 0.01 mol of MnO₂, 0.01mol of LiOH·H₂O, and 0.06mol of NaOH as starting materials at 160-200°C. The products are characterized by XRD, TEM, ED, BET, and ICP. Results show that the Li_0.74Mn₂O_3.74 powder prepared at 200°C has an average size of 180 nm with BET surface areas of 16.44 m²/g. A possible formation mechanism is proposed. It was concluded that the alcohol acts not only as the solvent but also as the reducing agent in the synthesis of Li_1−xMn₂O_4−σ powders. The effects of reaction temperature and the contents of NaOH and LiOH on the formation of single phase Li_1−xMn₂O_4−σ were investigated.     

Chemical Synthesis and Properties of Spinel Li1−xCo2O4−δ

Spinel Li_1−xCo₂O_4−δ samples with 0.44≤(1−x)≤1 have been synthesized by chemically extracting lithium with the oxidizer NO₂BF₄ in acetonitrile medium from the LT-LiCoO₂ synthesized at 400°C. Rietveld analysis of the X-ray diffraction data reveals that the Li_1−xCo₂O_4−δ samples adopt the normal cubic spinel structure with a cation distribution of (Li_1−x)_8a[Co₂]_16dO_4−δ. Redox iodometric titration data indicate that the LT-LiCoO₂ tends to lose oxygen on extracting lithium and the spinel Li_1−xCo₂O_4−δ samples are oxygen-deficient. Both infrared spectroscopic and magnetic susceptibility data suggest that the LiCo₂O_4−δ spinel is metallic with itinerant electrons. The tendency to lose oxygen on extracting lithium from the LT-LiCoO₂ and the observed metallic behavior of the spinel LiCo₂O_4−δ are explained on the basis of a qualitative band diagram.     

Thermoelectric Properties (Resistivity and Thermopower) in (Bi1.5Pb0.5Ca2−xMxCo2O8−δ (M=Sc, Y, or La)

In order to understand the origin of good thermoelectric (TE) properties in the transition metal oxides with the lattice structure isomorphous to the 232-structure, the bond nature between Co and O ions in Bi_1.5Pb_0.5Ca_2−xM_xCo₂O_8−δ-system has been tried to vary by replacing M with Sc³⁺, Y³⁺ or La³⁺ and by changing x from 0 to 0.3. The resistivity is minimum at x = 0.1 in Sc- and Y-systems, but very high in La-system. The large thermopower is obtained in every compound. The experimental TE properties have been discussed mainly within the framework of the charge-transfer scheme in which the ionic radii of Sc³⁺ and Y³⁺ smaller than Ca²⁺ reduce the energy between O 2p levels and Co e_g parentages but the large ionic radius of La³⁺ expands it. The oxygen solubility in the compounds and the lattice distortion peculiar to the 232-structure are also likely to contribute somewhat to the experimental results.     

Structural and magnetic characterisation of Aurivillius material Bi2Sr2Nb2.5Fe0.5O12

The n=3 Aurivillius material Bi₂Sr₂Nb_2.5Fe_0.5O₁₂ is investigated and combined structural refinements using neutron powder diffraction (NPD) and X-ray powder diffraction data (XRPD) data reveal that the material adopts a disordered, tetragonal (I4/mmm) structure at temperatures down to 2 K. Significant ordering of Fe³⁺ and Nb⁵⁺ over the two B sites is observed and possible driving forces for this ordering are discussed. Some disorder of Sr²⁺ and Bi³⁺ over the M and A sites is found and is consistent with relieving strain due to size mismatch. Highly anisotropic thermal parameters for some oxygen sites suggest that the local structure may be slightly distorted with some rotation of the octahedra. Magnetic measurements show that the material behaves as a Curie-Weiss paramagnet in the temperature range studied with no evidence of any long-range magnetic interactions. Solid solutions including Bi_3−xSr_xNb₂FeO₁₂, Bi₂Sr_2−xLa_xNb₂FeO₁₂ and Bi₂Sr₂Nb_3−xFe_xO₁₂ were investigated but single-phase materials were only successfully synthesised for a narrow composition range in the Bi₂Sr₂Nb_3−xFe_xO₁₂ system.