Zr1−xLnxW2O8−x/2 (Ln=Eu, Er, Yb): Solid solutions of negative thermal expansion-synthesis, characterization and limited solid solubility

Zr_1−xLn_xW₂O_8−x/2 solid solutions (Ln=Eu, Er, Yb) of different substitution fractions x have been synthesized. Their X-ray diffraction (XRD) patterns have been indexed and lattice parameters calculated based on the α-ZrW₂O₈ structure. The coefficients of thermal expansion (CTEs) of these solid solutions were estimated to be −10.3×10⁻⁶ K⁻¹ in temperature range of 30-100 °C. The solubility of lanthanide ions in these solid solutions decreases linearly with the increase in the radius of substituted lanthanide ions. Based on the concentration dependence of phase transition temperatures, a novel method for determination of solubility of the lanthanide ions in Zr_1−xLn_xW₂O_8−x/2 solid solutions has been developed. This method seems to be more sensitive as compared with that based on XRD technique.     

Solid solubilities of (La Nd,)2(Zr,Ti)2O7 phases deduced by neutron diffraction

Time-of-flight powder neutron diffraction has been performed on oxides with composition (La_1−xNd_x)₂Zr₂O₇ and Nd₂(Zr_1−xTi_x)₂O₇, where x=0, 0.2, 0.4,…1.0, in order to determine the solid solution behaviour across each series. Between La₂Zr₂O₇ and Nd₂Zr₂O₇, a cubic pyrochlore phase is observed (, Z=8). A linear decrease in the lattice parameter from 10.8047 to 10.6758 Å indicates complete miscibility of the two end-members. For the same series, the 48f oxygen x-parameter increases from 0.3313 to 0.3348, suggesting increased distortion of the 6 coordinate B sites and reduced distortion of the 8 coordinate A sites. There is limited solubility of Nd₂Ti₂O₇ in Nd₂Zr₂O₇. Exsolution of a monoclinic phase (P2₁, Z=8) rich in Nd₂Ti₂O₇ is observed at approximately x=0.56. The compositional range over which a solid solution exists is more extensive than that which has been previously reported. The solubility of Nd₂Zr₂O₇ in Nd₂Ti₂O₇ is very low.     

Structural Characterization of the Complex Perovskites Ba1−xLaxTi1−xCrxO3

The series Ba_1−xLa_xTi_1−xCr_xO₃ (0≤x≤1) was synthesized at 1400°C for about 60 h. Their structure was carefully analyzed by the use of powder X-ray diffraction and Rietveld analysis software GSAS (General Structure Analysis System). Four solid solutions are found in this series: tetragonal solid solution Ba_1−xLa_xTi_1−xCr_xO₃ (0≤x≤0.029), cubic solid solution Ba_1−xLa_xTi_1−xCr_xO₃ (0.0365≤x≤0.600), rhombohedral solid solution Ba_1−xLa_xTi_1−xCr_xO₃ (0.700≤x≤0.873), and orthorhombic solid solution Ba_1−xLa_xTi_1−xCr_xO₃ (0.956≤x≤1). There are corresponding two-phase regions between the adjacent two solid solutions. The detailed lattice parameters are presented. The relationship between the lattice parameters and the composition of the solid solutions is developed.     

Effects of firing schedule on solubility limits and transport properties of ZrO2-TiO2-Y2O3 fluorites

The low Y/high Zr edge of the cubic defect fluorite solid solution in the system ZrO₂-TiO₂-Y₂O₃ in air is reassessed, as it is these compositions which have been suggested to offer the highest levels of mixed conductivity. Vegard's law is obeyed for values of x which lie within the cubic defect fluorite phase in Zr_1−x−yY_yTi_xO_2−δ for values of y=0.2 and 0.25. Measured lattice parameters show good agreement with those calculated from the Kim relation. Deviation from Vegard's law places the limit of the solid solution at x=0.18 and 0.20 for values of y=0.2 and 0.25, respectively, at 1500 °C. Discrepancies in current literature data can be shown to be due to differences in firing schedule such as slight temperature fluctuations and/or different cooling rates. A high level of care of sintering temperature and cooling profile is essential to form the most promising single-phase materials which contain maximum Ti-contents with low Y-contents. Contraction of the phase limit as a result of poor synthesis control leads to erroneously high values of bulk ionic conductivity while values of electronic conductivity are shown to be less affected.     

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.     

Phase Diagram and Phase Transitions in the Relaxor Ferroelectric Pb(Fe2/3W1/3)O3-PbTiO3 System

A complete solid solution between relaxor ferroelectric Pb(Fe_2/3W_1/3)O₃ (PFW) and ferroelectric PbTiO₃ (PT), (1−x)PFW-xPT, was synthesized by a B-site precursor method and characterized by X-ray diffraction, differential scanning calorimetry, and dielectric measurements. A phase diagram between PFW and PT has been established. The diffuse phase transition temperature (T_max≈180 K) of PFW was found to continuously increase with the increasing amount of Ti⁴⁺ ions on the B-site. At the same time, the relaxor ferroelectric behavior of PFW is gradually transformed toward a normal ferroelectric state, as evidenced by sharp and nondispersive peaks of dielectric permittivity around T_C for x≥0.25. At room temperature, a transition from a cubic to a tetragonal phase takes place with x increased up to 0.25. A morphotropic phase boundary is located within the composition interval 0.25≤x≤0.35, which separates a pseudocubic (rhombohedral) phase from a tetragonal phase.     

O NMR studies of local structure and phase evolution for materials in the Y2Ti2O7-ZrTiO4 binary system

¹⁷O MAS NMR and XRD studies of precursor-derived Y_1.6Zr_0.4Ti₂O_7.2 and Y_1.2Zr_0.8Ti₂O_7.4 have been performed to investigate the development of local and long-range order in these materials as they evolve from a metastable amorphous state upon heating. Zirconium titanate (ZrTiO₄) was also investigated to help interpret the ¹⁷O NMR spectra of the ternary compositions. Consistent with earlier studies, crystallization was observed at 800 °C to form a fluorite structure and a small amount of rutile; weak broad reflections were also observed which were ascribed to the presence of small pyrochlore-like ordered domains or particles within the fluorite phase. As the temperature was increased further, the sizes of these domains grew along with the concentration of rutile. At the highest temperature studied (1300 °C), the reflections of the thermodynamic phases, pyrochlore and zirconium titanate (ZrTiO₄), dominated the XRD pattern. The ¹⁷O NMR spectra revealed a series of different peaks that were assigned to different 3- and 4-coordinate O local environments. The data were consistent with the formation of a metastable phase Y_2−xZr_xTi_2−yZr_yO_7+x with pyrochlore-like ordering but with Zr substitution on both cation sites of the pyrochlore structure. At low temperatures, doping on the A (Y³⁺) sites predominates (i.e., x>y), consistent with the fact that the pyrochlore develops out of a more disordered fluorite-like, phase. As the temperature is raised, the Zr doping on the A site decreases and the metastable phase at this temperature can now be written as Y_2−x′Zr_x′Ti_2−y′Zr_y′O_7+x′ (i.e., x′<y′); TiO₂ is also observed, consistent with this suggestion. At high temperatures, doping on the B site decreases and the resonances due to the stoichiometric pyrochlore yttrium titanate (Y₂Ti₂O₇) dominate the NMR spectra. Weaker ¹⁷O NMR resonances due zirconium titanate (ZrTiO₄) are also observed.     

Phase transition of negative thermal expansion Zr1-xHfxW2O8 solid solutions

A powder X-ray diffraction experiment was performed on cubic Zr_1-xHf_xW₂O₈ (x=0.25, 0.50 and 0.75) solid solutions from 90 to 560 K. The lattice parameters of Zr_1-xHf_xW₂O₈ at 121 K decreased linearly with increasing Hf contents, due to smaller ionic radius of hafnium than that of zirconium. Transition temperatures due to α-β structural phase transition increased with increasing Hf contents, reflecting the decrease of lattice free volume related to the orientation of unshared vertex of WO₄. Anomaly in the heat capacity of Zr_0.5Hf_0.5W₂O₈ was observed around 450 K which was 9 K lower than that by X-ray diffraction method. Transition entropy of Zr_0.5Hf_0.5W₂O₈ was 2.1 J mol^-1 K^-1, consistent with those of ZrW₂O₈ and HfW₂O₈. This consistent entropy supports that Zr_1-xHf_xW₂O₈ (x=0-1.0) has the same order-disorder phase transition mechanism. This revised version was published online in August 2006 with corrections to the Cover Date.     

X-ray powder diffraction study of monoclinic V-ZrO2 solid solutions obtained from gels

Rietveld refinement of six monoclinic V_xZr_1−xO₂ solid solutions, with x=0, 0.01, 0.02, 0.05, 0.075 and 0.1, prepared by heating dried gel precursors at 1300°C in air atmosphere, has been characterized using X-ray powder diffractometer data. The present results confirm that crystal structure of these solid solutions contain V⁴⁺(Zr⁴⁺) cations surrounded by seven oxygens, four at a distance between 2.13 and 2.28 Å (referred as to O(2) in the tetrahedrally coordinated oxygens) and other three at a distance between 2.03 and 2.20 Å (denoted as O(1) in the triangularly coordinated oxygens). The trends in the lattice parameter variation of V_xZr_1−xO₂ solid solutions specimens with the nominal vanadium amount are in accordance with previous results obtained by experiments measured using an internal standard.     

Synthesis and optical absorption property of the Zn2TixSn1−xO4 (0?x?1) solid solutions

Zn₂Ti_xSn_1−xO₄ (0?x?1) solid solutions with an inverse spinel structure (Fd3m) were synthesized by solid-state reactions at 1300°C of the stoichiometric mixtures of ZnO, TiO₂ and SnO₂. X-ray diffraction, thermogravimetric and differential thermal analyses, scanning electron microscopy, transmission electron microscopy and BET specific surface area measurements were used to gain insights into the solid-state reactions and phase transformation of the system. Optical absorption property of the Zn₂Ti_xSn_1−xO₄ (0?x?1) solid solutions was studied with the ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS). The Zn₂Ti_xSn_1−xO₄ (0?x?1) solid solutions showed optical absorptions of the semiconductors in the near ultraviolet region; the adsorption band shifts with the composition of the solid solution.     

Lanthanum pyrochlores and the effect of yttrium addition in the systems La2−xYxZr2O7 and La2−xYxHf2O7

The crystal structures of the compounds La_2−xY_xZr₂O₇ and La_2−xY_xHf₂O₇ with x=0.0, 0.4, 0.8, 1.2, 1.6, and 2.0 have been studied using neutron powder diffraction and electron microscopy to determine the stability fields of the pyrochlore and fluorite solid solutions. The limits of pyrochlore stability in these solid solutions are found to be close to La_0.8Y_1.2Zr₂O₇ and La_0.4Y_1.6Hf₂O₇, respectively. In both systems the unit cell parameter is found to vary linearly with Y content across those compositions where the pyrochlore phase is stable, as does the x-coordinate of the oxygen atoms on the 48f (x,,) sites. In both systems, linear extrapolations of the pyrochlore data suggest that the disordering is accompanied by a small decrease in the lattice parameter of approximately 0.4%. After the pyrochlore solid solution limit is reached, a sharp change is observed from x∼0.41 to 0.375 as the disordered defect fluorite structure is favoured. Electron diffraction patterns illustrate that some short-range order remains in the disordered defect fluorite phases.     

The Crystal Structures, Microstructure and Ionic Conductivity of Ba2In2O5 and Ba(InxZr1−x)O3−x/2

This paper describes the results of electron microscopy, high-temperature powder neutron diffraction, and impedance spectroscopy studies of brownmillerite-structured Ba₂In₂O₅ and perovskite structured Ba(In_xZr_1−x)O_3−x/2. The ambient temperature structure of Ba₂In₂O₅ is found to adopt Icmm symmetry, with disorder of the tetrahedrally coordinated (In³⁺) ions of the type observed previously in Sr₂Fe₂O₅. Ba₂In₂O₅ undergoes a ∼6-fold increase in its ionic conductivity over the narrow temperature range from ∼1140 K to ∼1230 K, in broad agreement with previous studies. This transition corresponds to a change from the brownmillerite structure to a cubic perovskite arrangement with disordered anions. Electron microscopy investigations showed the presence of extended defects in all the crystals analyzed. Ba(In_xZr_1−x)O_3−x/2 samples with x=0.1 to 0.9 adopt the cubic perovskite structure, with the lattice parameter increasing with x.     

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.     

Phase relation studies in the CeO2-Gd2O3-ZrO2 system

The phase relations in the CeO₂-Gd₂O₃-ZrO₂ system have been established after slowly cooling the samples from 1400 °C. Ceria has been used as a surrogate material in place of plutonia. About 80 compositions in Zr_1−xGd_xO_2−x/2, Ce_1−xGd_xO_2−x/2, Ce_1−xZr_xO_2.00, (Zr_0.5Ce_0.5)_1−xGd_xO_2−x/2, (Ce_0.5Gd_0.5)_1−xZr_xO_1.75+x/4, (Zr_0.5Gd_0.5)_1−xCe_xO_1.75+x/4, and (Ce_0.8Zr_0.2)_xGd_1−xO_1.5+x/2 were prepared by a three steps heating protocol. Based on the refinement of the XRD data, several phase regions namely; cubic fluorite type solid solution, C-type solid solution, and various biphasic regions could be delineated. This system showed the existence of a very wide cubic phase field. About 17.5 mol% GdO_1.5 was found to fully stabilize the cubic zirconia. On the other hand ceria did not stabilize the cubic zirconia. The anion-excess gadolinia, i.e., Gd_1−xCe_xO_1.5+x was found to retain the C-type lattite unlike pure gadolinia. The ternary phase relations were mainly characterized by the presence of wide homogeneity ranges of fluorite type or C-type phases.     

Structural investigation of KxBa1−xGa2−xGe2+xO8 solid solutions using the X-ray Rietveld method

The K_xBa_1−xGa_2−xGe_2+xO₈ (x=0.6−1.0) solid solutions undergo a structural phase transition that has a significant effect on their sintering behavior and their microwave dielectric properties. The crystal structures of both phases within the solid-solution region were determined by the Rietveld method using powder X-ray diffraction data. We found that the low-temperature-stable phase is isostructural with the pseudo-orthorhombic KGaGe₃O₈ (space group P2₁/a), while the high-temperature-stable phase has a typical monoclinic feldspar structure (space group C2/m). Due to the topological differences between the two structures, the T-O bonds within the tetrahedra must be partially recombined to make a new framework, which causes an endothermic effect during the P2₁/a to C2/m phase transition. The correlation between the crystal structures, the microwave dielectric properties and the phase-transition behaviors were discussed in terms of the crystallographic features, the lattice parameters, and the strain-induced anisotropic peak-broadening.     

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).     

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.     

Synthesis, structure and electrical properties of Cu3.21Ti1.16Nb2.63O12 and the CuOx-TiO2-Nb2O5 pseudoternary phase diagram

Subsolidus phase relations in the CuO_x-TiO₂-Nb₂O₅ system were determined at 935 °C. The phase diagram contains one new phase, Cu_3.21Ti_1.16Nb_2.63O₁₂ (CTNO) and one rutile-structured solid solution series, Ti_1−3xCu_xNb_2xO₂: 0<x<0.2335 (35). The crystal structure of CTNO is similar to that of CaCu₃Ti₄O₁₂ (CCTO) with square planar Cu²⁺ but with A site vacancies and a disordered mixture of Cu⁺, Ti⁴⁺ and Nb⁵⁺ on the octahedral sites. It is a modest semiconductor with relative permittivity ∼63 and displays non-Arrhenius conductivity behavior that is essentially temperature-independent at the lowest temperatures.     

Synthesis and CO2 capture evaluation of Li2−xKxZrO3 solid solutions and crystal structure of a new lithium-potassium zirconate phase

Solid solutions of lithium and potassium metazirconates Li_2−xK_xZrO₃ (where, 0?x?2) were prepared by coprecipitation. Samples were characterized by powder X-ray diffraction, scanning electron microscopy, and thermogravimetric analyses. Results showed that the solubility limits of potassium into Li₂ZrO₃ is x=0.2. Furthermore, at higher potassium concentrations, a new phase was synthesized, Li_2.27K_1.19Zr_2.16O_6.05. For structural studies of this new phase, XRD data were analyzed by Rietveld refinements. Additionally, at high potassium concentrations different phases of ZrO₂ were found, as potassium tends to sublimate. On the other hand, lithium-potassium metazirconate solid solutions, Li_2−xK_xZrO₃, were tested as CO₂ captors. Thermal analyses into a CO₂ flux showed that Li_2−xK_xZr₂O₃ solid solutions present a better CO₂ absorption than Li₂ZrO₃ pure. The differences observed in the CO₂ sorption processes were explained with thermodynamic data.     

Rietveld Refinement of Tetragonal V-ZrO2 Solid Solutions Obtained from Gels by X-ray Powder Diffraction

The crystal structure of three tetragonal V_xZr_1−xO₂ solid solutions, with x=0.025, 0.05, and 0.075, prepared by heating dried gel precursors at 450°C in air atmosphere, have been determined by Rietveld refinement on the basis of powder X-ray powder diffractometer data. They contain V⁴⁺ cations surrounded by eight oxygens, four at a distance between 2.079 and 2.093 Å and another four at longer distances between 2.369 and 2.348 Å. The estimation of the crystal average oxygen position from the X-ray lattice parameter of V_xZr_1−xO₂ conform with the relationship proposed by Howard et al. (J. Am. Ceram. Soc. 81, 241 (1998)).