Sr-rich Compositions Research Articles

Antiferroelectric to Antiferroelectric-Relaxor Phase Transition in Calcium Strontium Sulfoaluminate.

Structural phase transitions of calcium strontium sulfoaluminate series, (Ca1-xSrx)8[AlO2]12(SO4)2 ((CS)AS-x) with x = 0.80-1.00, are systematically investigated by powder X-ray diffraction, dielectric measurements, and pyroelectric measurements, to clarify a phase diagram of (CS)AS-x (x = 0.80-1.00). A pure strontium sulfoaluminate, (CS)AS-1.00, is found to undergo three phase transitions, which take place successively on cooling from a prototypical cubic phase with the symmetry of Im3̅m. Though the room-temperature phase of (CS)AS-1.00 was previously reported to be of polar Pcc2, the pyroelectric measurements clarified a nonpolar character of the crystal symmetry. The dielectric measurements suggest a possibility of an antiferroelectric ground state of (CS)AS-x in the Sr-rich compositions. As x decreases, the ground state changes to a short-range-ordered state, implying a unique phase transition from the antiferroelectric state to the antiferroelectric-relaxor state. The present study provides an intriguing playground for designing new ferro/antiferroelectric materials.

The solid solution–aqueous solution system (Sr,Ba,Ra)SO4 + H2O: A combined experimental and theoretical study of phase equilibria at Sr-rich compositions

Solid solution formation between RaSO4 and other isostructural sulfates has been known for a long time as a process potentially controlling Ra concentrations in aquifers. Here, we measured the Ra uptake in the Sr-rich corner of the ternary (Sr,Ba,Ra)SO4 solid solution - aqueous solution (SS-AS) system by equilibrating SrSO4, BaSO4 and mixed (Sr,Ba)SO4 solids with Ra-bearing aqueous solutions for up to 1302 days at 90 °C at a solid/liquid ratio of 5 g/L. The recrystallization experiments were combined with electron microscopy studies of the solids. The evolution of the solid and aqueous phases was interpreted based on thermodynamic modelling applying a recently revised thermodynamic dataset for the (Sr,Ba,Ra)SO4 + H2O system.The recrystallization process involved several metastable phases, starting from the least soluble, Ba- and Ra-rich precipitates and ending with Sr-rich solids, whose compositions approached the predicted equilibrium states. The composition and the time evolution of metastable phases were correlated with changes in the computed supersaturation functions. Particularly, the formation of Ra- Ba- and Sr-rich rims on primary barite grains in the experiment with the mechanical mixture of 99% of celestine and 1% of barite has been explained by combining calculated supersaturation conditions with considerations of structural misfit.A key result was the observed final Raaq concentration which is about one order of magnitude lower compared to the initial concentration, implying an efficient uptake into the newly formed solid phases. These concentrations appeared to be just slightly lower than those predicted by the thermodynamic calculations, implying that the complete SS-AS equilibrium was close to be reached in Sr-rich systems in the recrystallization experiments lasting for about 3.5 years.

X-ray diffuse scattering observations for Sr x Ba1−x Nb2O6 single crystals with x=0.35 and 0.81

ABSTRACT In an attempt to contribute to understanding of structure–property relationship in ferroelectric relaxor SrBaNbO, we studied X-ray diffuse scattering for compositions with x = 0.35 and x = 0.81 at room temperature. Within signal-rich reciprocal-space images, features related to (i) local deformation due to chemical inhomogeneity, (ii) incommensurate modulation and (iii) fluctuation of polarization have been identified. There are substantial differences between the two compositions, some of which can be straightforwardly attributed to different states of ferroelectric ordering in these materials. In the Sr-rich composition, we have observed additional features in the form of -oriented streaks on planes. They indicate that transverse correlation of uniaxial polarization is anisotropic. At the same time, the fact that streaks appear also on the superlattice planes raises the question whether the modulation and polarization order parameter are interrelated.

Thermodynamics of the solid solution - Aqueous solution system (Ba,Sr,Ra)SO4 + H2O: I. The effect of strontium content on radium uptake by barite

Thermodynamic properties of mixing in the ternary (Ba,Sr,Ra)SO4 solid solution are determined using first principles based total energy calculations and Monte Carlo simulations. Two levels of theory, which correspond to the regular mixing and the generalized Ising model, are considered. The results show that the regular mixing parameters increase along the row of Ba-Ra, Ba-Sr and Sr-Ra binary systems proportionally to the squared difference of molar volumes of the end-members. The magnitudes of pairwise interactions similarly increase along the same row, manifesting a tendency to short-range ordering (SRO). In the (Ba,Sr)SO4 system the SRO effect is approximately equivalent to a 40% decrease in the value of the regular mixing parameter. The ternary solid solution is well described as a regular mixture with the binary parameters WBaRa = 2.47 ± 0.22, WBaSr = 4.95 ± 0.75 and WSrRa = 17.50 ± 1.40 kJ/mol. These values imply that admixing RaSO4 to the (Ba,Sr)SO4 solid solution stabilizes Ba-rich and destabilizes Sr-rich compositions. Consequently, an addition of a small amount of RaSO4 to a Sr-rich solid solution leads to a nucleation of a Ba- and Ra rich phase. This phenomenon, predicted in our thermodynamic modelling study, is directly confirmed by our experiments on recrystallizing a powder of celestite with traces of Ba in the presence of an aqueous Ra-bearing solution. At a measurably high content of Ra in the system Ra-uptake by celestite occurs via a formation of a Ra-rich phase. The aqueous concentration of Ra in such systems would be governed mainly by the common anion effect caused by the relatively high solubility of Sr-rich sulphates. At lower Ra contents the retention of Ra would be enhanced both by the common anion and the dilution effects. Our simulations with the GEM-Selektor code predict that the optimum condition for Ra uptake is achieved when the barite solid solution contains 10 ± 5 mol % of SrSO4.

Late Eocene–Early Oligocene two-mica granites in NW Turkey (the Uludağ Massif): Water-fluxed melting products of a mafic metagreywacke

This paper deals with the petrogenesis of two-mica granites within the amphibolite-facies metamorphic rocks in the Uludağ Massif (NW Turkey). The two-mica granites are represented by three different types (i) a ~17km long and ~1.5km-wide strongly foliated and lineated metagranite, (ii) a relatively younger circular granite, ~11km by ~6.5km, free of any penetrative fabric, and (iii) up to 5m thick dikes of unfoliated granite-porphyries. Mineral constituents include quartz, plagioclase, microcline, biotite, muscovite and minor apatite, zircon, ilmenite and monazite. U–Pb zircon dating indicates that the metagranite was emplaced at 35±1Ma (2σ, latest Eocene) and the unfoliated granite at 28±1Ma (2σ) (Early-Late Oligocene boundary). Overall, the two-mica granites are characterized by (i) a restricted bulk composition (SiO2 ~70–76wt.%), (ii) near-linear to linear inter-element variations, (iii) high concentrations of Al2O3, Sr, Ba, LREEs, and low abundances of Rb, Y, Sc and HREEs, comparable with adakitic rocks, (iv) variably pronounced concave-upward shape of HREEs and absent to feeble Eu anomaly, (v) relatively high Na2O, CaO and Sr, and lower K2O and Rb contents at identical silica contents relative to the classical S-type granites. 87Sr/86Sr(i) and εNd(i) values range from 0.7052 to 0.7080, and 0.7 to −3.6, respectively. The 206Pb/204Pb(i) values range from 18.23 to 18.69, 207Pb/204Pb(i) from 15.63 to 15.70 and 208Pb/204Pb(i) from 38.48 to 38.77. 87Sr/86Sr(i) and εNd(i), and εNd(i) and 207Pb/204Pb(i) values are negatively correlated. Isotopic variation and negative correlations can best be explained by a heterogeneous source, including an older crustal component and a more juvenile basic component. Magma temperatures are estimated as 754–790°C based on zircon and monazite saturation. All these features such as (i) relatively Na-, Ca- and Sr-rich compositions, (ii) isotopically heterogeneous source with inferred residual phases such as garnet and hornblende, (iii) occurrence in a ductile strike-slip zone, and (iv) relatively low magma temperatures can be accounted for by water-fluxed melting of a metagreywacke source with substantial mafic component at middle to lower crustal depths.

Oxygen interstitials and vacancies in LaSrGa3O7-based melilites

The Sr-rich composition of the layered tetrahedral meililite, La0.8Sr1.2Ga3O6.9, was synthesized and a structural investigation on La0.8Sr1.2Ga3O6.9 using neutron powder diffraction revealed a site preference of oxygen vacancies on the bridging oxygen sites of the 4-linked GaO4 tetrahedra. Impedance measurement revealed limited ionic conduction in the oxygen-deficient La0.8Sr1.2Ga3O6.9, presumably associated with oxygen vacancies, which is ~2 orders of magnitude higher than the parent material LaSrGa3O7 but ~3–4 orders of magnitude lower than the interstitial oxide ionic conductivity in La-rich composition, La1.54Sr0.46Ga3O7.27. Low temperature neutron powder diffraction characterization was performed for the oxygen-excess, La-rich composition, La1.54Sr0.46Ga3O7.27, which confirmed the position near the center of the pentagonal tunnels for the oxygen interstitials identified previously using the room temperature data. Solid state 71Ga NMR data collected on these LaSrGa3O7-based materials with stoichometric, excess, and deficient oxygen contents was found not able to distinguish these three compositions. A metastability temperature gap within 850–1280°C was identified for the oxygen interstitial-conducting La1.54Sr0.46Ga3O7.27. The structures of these oxygen excess and deficient gallate melilites further demonstrate the structural flexibility of the LaSrGa3O7-based layer tetrahedral network.

Crystallization dynamics and interface stability of strontium titanate thin films on silicon.

Different physical vapor deposition methods have been used to fabricate strontium titanate thin films. Within the binary phase diagram of SrO and TiO2 the stoichiometry ranges from Ti rich to Sr rich, respectively. The crystallization of these amorphous SrTiO3 layers is investigated by in situ grazing-incidence X-ray diffraction using synchrotron radiation. The crystallization dynamics and evolution of the lattice constants as well as crystallite sizes of the SrTiO3 layers were determined for temperatures up to 1223 K under atmospheric conditions applying different heating rates. At approximately 473 K, crystallization of perovskite-type SrTiO3 is initiated for Sr-rich electron beam evaporated layers, whereas Sr-depleted sputter-deposited thin films crystallize at 739 K. During annealing, a significant diffusion of Si from the substrate into the SrTiO3 layers occurs in the case of Sr-rich composition. This leads to the formation of secondary silicate phases which are observed by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy.

Strain relaxation mechanisms of elastic softening and twin wall freezing associated with structural phase transitions in (Ca,Sr)TiO3 perovskites

Resonant ultrasound spectroscopy has been used to measure the bulk modulus (K), shear modulus (G) and acoustic dissipation of polycrystalline perovskite samples across the CaTiO3–SrTiO3 solid solution in the temperature range ∼10–1350 K. A remarkable pattern of up to ∼25% softening of G as a function of both temperature and composition is due to coupling of shear strain with order parameters for the Pm4/mcm, I4/mcm ↔ Pnma and I4/mcm ↔ Pbcm transitions. Anomalies in K associated with the phase transitions are small, consistent with only weak coupling of octahedral tilting order parameter(s) with volume strain. A change from tricritical character for the Pm 4/mcm transition towards second order character at Sr-rich compositions appears to be due to changing properties of the soft optic mode rather than to changes in magnitude of strain/order parameter coupling coefficients. Precursor softening of G ahead of the Pm 4/mcm transition, due to fluctuations or clustering, occurs over a temperature interval of up to ∼200 K, and also changes character at the most Sr-rich compositions. The tetragonal structure with Sr-rich compositions is characterized by additional softening with falling temperature which is most likely related to the proximity of a ferroelectric instability. The I4/mcm ↔ Pnma transition is accompanied by stiffening, which is attributed to the effects of strong coupling between order parameters for M-point and R-point tilting. The pattern of attenuation at RUS frequencies in the tetragonal phase can be understood in terms of the mobility of twin walls which become pinned below ∼500 K, and the loss mechanism most likely involves local bowing of the walls by lateral motion of ledges rather than the advance and retraction of needle tips. Twin wall mobility is suppressed in the orthorhombic structure.

Structure evolution and microwave dielectric response of (Ca0.5+xSr0.5−x)[(Al0.5Nb0.5)0.5Ti0.5]O3 solid solutions

The phase assemblage, crystal structure evolution and microwave dielectric response of (Ca0.5+xSr0.5−x)[(Al0.5Nb0.5)0.5Ti0.5]O3 ceramics (abbreviated as CSANT hereafter) are investigated. Single perovskite solid solution is formed in the CSANT ceramics in Sr-rich composition range of x < −0.05, however, Ca4Ti3O10-type layered perovskite phase begins to segregate after x = −0.05. The CSANT perovskites crystallized in Fm3m cubic symmetry in the composition range of x ≤ −0.2, however, as the Ca2+ content in A-site increased, the oxygen octahedral began to be anti-phase tilted at x = −0.1 and the crystal structure transited to P21/n pseudo-orthorhombic space group thereafter. The microwave dielectric response of the CSANT ceramics is elaborately discussed in terms of their crystallographic structure and chemical composition. When sintered at 1500 °C for 4 h, a dielectric constant ɛr of 52.5, a Qf product of 28000 GHz and a τf of +25.4 ppm/°C microwave dielectric ceramic can be obtained in the CSANT ceramics at x = 0.3.

Correlation Between Tolerance Factor and Temperature Coefficient of Dielectric Constant of (Ba,Sr)(Zn1/3Ta2/3)O3 Dielectric Resonator

The effect of substitution of Sr at the A site of Ba(Zn1/3Ta2/3)O3 was investigated with respect to tolerance factor (t) which determines the degree of packing of A and B site ions of perovskite unit cell, with an intension to understand the correlation between the temperature dependent dielectric properties with structure. The investigation reveals that the Ba rich compositions with highly symmetric cubic phase exhibits negative variation of dielectric constant with temperature and the Sr rich compositions with distorted structure always show positive variation of dielectric constant with temperature. The profile of τε variation of all the compositions was scaled with respect of reduced temperature which exhibits an exact signature of τε variation with the compositions at room temperature. The observed mechanism was explained using Clausius Mossoti equation.

Correlation Between Tolerance Factor and Temperature Coefficient of Dielectric Constant of (Ba,Sr)(Zn1/3Ta2/3)O3Dielectric Resonator

The effect of substitution of Sr at the A site of Ba(Zn1/3Ta2/3)O3 was investigated with respect to tolerance factor (t) which determines the degree of packing of A and B site ions of perovskite unit cell, with an intension to understand the correlation between the temperature dependent dielectric properties with structure. The investigation reveals that the Ba rich compositions with highly symmetric cubic phase exhibits negative variation of dielectric constant with temperature and the Sr rich compositions with distorted structure always show positive variation of dielectric constant with temperature. The profile of τε variation of all the compositions was scaled with respect of reduced temperature which exhibits an exact signature of τε variation with the compositions at room temperature. The observed mechanism was explained using Clausius Mossoti equation.

Preparation, characterization and dielectric properties of Ba3−xSrxLiM3Ti5O21[M=Nb and Ta, x = 0 to 3] ceramics

The ceramic compositions Ba3−xSrxLiM3Ti5O21[M=Nb and Ta, x = 0 to 3] were prepared through conventional solid state ceramic route. A detailed study has been carried out to correlate the structure of Ba3−xSrxLiM3Ti5O21[M=Nb and Ta, x = 0 to 3] with respect to their dielectric properties. The structure and microstructure of ceramic samples were studied using powder X-ray diffractometer and Scanning Electron Microscopic techniques. The dielectric properties of the sintered ceramic compacts have been studied. The Ba-rich compositions were identified as promising candidates for high frequency applications whereas the Sr-rich compositions were excellent ionic conductors and can be commercially exploited for applications in solid-state batteries.

Enhanced Tc of Bi(Pb)2223 sintered bulks up to 115 K

Enhancement of Tc was attempted for Bi(Pb)2223 sintered bulks by optimizations of starting cation composition and post-annealing conditions. Tc was enhanced from 110.4 K to 111.4 K after post-annealing at ~700°C in 1%O2. Additional high temperature annealing at ~830°C in air largely improved crystallinity of Bi(Pb)2223 and high Tc up to 114.3 K was achieved. Increases in the c-axis length indicated that the enhancement of Tc is possibly due to changes in cation composition towards the stoichiometric one. An as-synthesized Bi(Pb)2223 bulks starting from Sr-rich compositions showed slightly high Tc of 110.8 K, suggesting suppression of partial substitution of Ca and/or Bi for the Sr-site. In addition, Bi-rich starting composition resulted in decreases of secondary phases without suppression of Tc. High temperature post annealing in air performed for these Sr-rich or Bi-rich bulks also improved Tc's up to 114.4 and 114.8 K, respectively.

Luminescence Efficiency Enhancement by Impurity Doping in SrIn2O4 : Pr3 +

A red-emitting phosphor, is a candidate of Cd-free phosphor for vacuum fluorescent tubes. Luminescence of is quenched at a Sr-rich composition by formation of defects that work as nonradiative centers. At the stoichiometric composition , the luminescence efficiency is increased by addition of , while decreased by addition. Measurement of the unit cell volume showed that and occupy the site, while presumably occupies the site. This site preference shifts the atomic ratio of the site to the Sr-poor side by doping and to the Sr-rich side by doping, resulting in an increase or a decrease in efficiency at the stoichiometric composition. Under electron excitation and at the dopant concentration of per . , doping provides the maximum efficiency about twice higher than the maximum efficiency of an undoped sample. The optimized Sr fraction for doping, , has led to efficiency about 2.7 times as high as efficiency of the undoped .

Structure and dielectric properties of Ba1−xSrxZnTi7O16 hollandite ceramics

Hollandite-type Ba1−xSrxZnTi7O16 (x=0, 0.2, 0.4, 0.6, 0.8, and 1) ceramics have been synthesized by the conventional solid-state ceramic route. The phase purity and microstructure of these compositions have been characterized using x-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD analysis shows that an increase in strontium concentration in the A-site causes pairing of vacant tunnel sites, and hence the structure becomes unstable due to the collapse of the tunnel walls. The dielectric properties such as dielectric constant (er), loss tangent (tan δ), and temperature variation of dielectric constant (τer) have been measured up to the 13 MHz region. The present study shows that zinc hollandites have relatively high dielectric constant and low loss tangent. The temperature variation of dielectric constant studies reveal that Ba-rich compositions have high positive τer and Sr-rich compositions have high negative τer in the 0–100°C region. Proper tailor making of these compositions has been attempted to arrive at near-zero temperature variation of the dielectric constant.

Some Characteristics in Particle Growth in Gel and Oscillatory Zoning

An experiment for the particle growth in gelatin was performed. Particle is a solid solution of (Ba,Sr)SO4. First of all, we performed a counter-diffusion type experiment where BaCl2 and SrCl2 diffuse in gelatin and Na2SO4 diffuses toward the opposite direction. This is a condition proposed by Putnis et al. (1992). Oscillatory zoning found out by Putnis et al. (1992) was reproduced. Then, chemical composition changed periodically in a particle, thickness of each circular stripe being several microns in the order of magnitude. In order to investigate the particle growth process in a gel, we performed experiments for particle growth with several kinds of processes with reactions and diffusions. In these experiments, some kinds of chemical species were dissolved in gelatin, and other chemical species were diffused in gelatin. Findings of this work indicated that core portions of particles of the reaction-diffusion process were mainly composed of BaSO4 which determines fundamental shape of particles. After formation of this core, hemi-cylindrical portions with Sr-rich composition were deposited on the surface of the core. Although the characteristics were always kept for all of the experimental conditions of this work, the rate of deposition of hemi-cylindrical portion was dependent on the way of diffusion which was controlled by an experimental setup and gelation time. Fundamental information for particle growth in gel was obtained in this work, and potential activity for particle formation with reaction-diffusion mechanism was pointed out.

Hard-mode infrared spectroscopy of perovskites across the CaTiO3-SrTiO3solid solution

Powder infrared spectra of perovskites across the CaTiO 3 -SrTiO 3 solid solution have been collected at room temperature for the range 20-700 cm -1 . Frequency, intensity, and line broadening parameters vary with a pattern that appears to follow the development of the orthorhombic shear strain for the Pnma structure. The data are at least consistent with the Pm3m ↔ I4/mcm transition at Sr-rich compositions being continuous in character and the tetragonal ↔ orthorhombic transition at ~62% SrTiO 3 content being discontinuous. In spite of the fact that crystals with intermediate composition have recently been shown to have Pnma rather than Cmcm symmetry, autocorrelation analysis of their IR spectra shows that they have features which distinguish them from crystals with Pnma symmetry for CaTiO 3 rich compositions. In particular, crystals of intermediate composition have the highest values of the line broadening parameter, △corr, for the solid solution, suggesting that they are characterized by relatively high degrees of local structural heterogeneity.

Synthesis and crystal structure of the oxygen defect perovskites containing copper and tantalum

Perovskite-related solid solutions, Sr 2− x La x Cu 1− y Ta 1+ y O z , were synthesized by solid state reaction at 1050–1080°C in air. The crystalline phases were identified by powder X-ray diffraction. Three types of crystal structure were observed including simple cubic perovskite-type, tetragonal perovskite-type and tetragonally distorted (NH 4) 3FeF 6-type structure in which Cu 2+ and Ta 5+ were ordered alternatively. The average valency state of copper in these solid solutions was nearly equal to +2. The tetragonal phases were observed in a compositional range, z≥5.85, regardless of cation composition. The tetragonality ( c/ a) for both the perovskite-type and (NH 4) 3FeF 6-type phases increased with increasing oxygen content. Ordering between Cu 2+ and Ta 5+ was not observed in the Sr-rich composition. These facts suggest that oxygen content affects the appearance of the Jahn–Teller effect of Cu 2+ leading to the tetragonal distortion and that the cation on the A-site affects the ordering of the cation on the B-site.

Effects of oxidants on the deposition and dielectric properties of the SrTiO3 thin films prepared by liquid source metal-organic chemical vapor deposition (MOCVD)

Abstract SrTiO3 thin films are deposited by a liquid source metal-organic chemical vapor deposition (MOCVD). The effects of oxidants on the deposition characteristics and dielectric properties of the films are mainly tested. O2, N2O and O2 + N2O gases are used as the oxidants and the films with Ti-rich and Sr-rich compositions are obtained when O2 and N2O is used, respectively. Deposition of thin initial layer under O2 atmosphere is very effective to obtain large dielectric constant of the SrTiO3 thin film when the main layer is deposited under O2 + N2O atmosphere. The dielectric constants of 40 nm thick SrTiO3 films with thin O2, N2O initial layers and without the initial layers are 235, 145 and 210, respectively.

Improved Electrical Properties of SrTiO3, Thin Films by Electron Cyclotron Resonance Plasma Chemical Vapor Deposition

ABSTRACTSrTiO3 thin films with various compositions have been prepared on Pt coated sapphire substrates by electron cyclotron resonance plasma chemical vapor deposition at 450 °C and 600 °C. The stoichiometric (Sr/(Sr+Ti)=0.50) films have maximum dielectric constants (εr) of 190 for the 600 °C deposition and 170 for the 450 °C deposition. The dielectric constant decreases abruptly with increasing or decreasing the Sr/(Sr+Ti) value from the stoichiometry, especially for the 450 °C deposition. The Ti-rich films include titanium suboxide (TiOx). In the Sr-rich films, excess SrO is thought to be inserted into the SrO plane and TiO2 plane in SrTiO3. In addition, SrCO3 is included in Sr-rich films. The concentration of these impurities is greater for the 450 °C deposition than for the 600 °C deposition. The leakage current density increases with increasing the Sr/(Sr+Ti) value, and the leakage current density for the 450 °C deposition is larger than for the 600 °C deposition in the Sr-rich composition region. These impurity phases may be related to the higher leakage current density. These results suggest that precise composition control in stoichiometry is necessary to deposit SrTiO3 films with superior electrical properties.