Perovskite CaTiO3 Research Articles

Preparation and optical characterization of Eu3+-doped CaTiO3 perovskite powders

CaTiO3 perovskite powders doped with 1.0, 1.5, 2.0, 2.5, 3.0, and 5.0mol% Eu3+ were prepared by sol–gel technique followed by annealing at high temperatures. The powders were characterized by X-ray diffraction, scanning electron microscopy, Raman scattering, absorption, and photoluminescence spectroscopy. The obtained powders possessed orthorhombic crystal structure. Raman spectra of the CaTiO3:Eu3+ powders exhibited seven new peaks at 798, 1048, 1188, 1371, 1441, 1601, and 1644cm−1 which were assigned to the localized vibrational modes related to the complexes containing Eu3+. It was found that the band edge of the material shifted to the higher-energy side with increasing Eu3+-impurity content. The photoluminescence of Eu3+ ions results from the radiative intra-configurational f–f transitions that happen between the 5DJ (J=0, 1–3) exited states and the 7FJ (J=0,1–4) ground states; the photoluminescence excitation of Eu3+ ions takes place from the 7F0 ground state to the 5DJ (J=1–4), 5L6, and 5G2,6 exited states.

Nature of bonding in CaTiO3 and SrTiO3: A Compton scattering study

Compton profile measurements for CaTiO3 and SrTiO3 perovskite compounds have been undertaken using 20 Ci 137Cs Compton spectrometer. To understand the nature of bonding in both the compounds, we have analyzed the Compton line shape in terms of equal-valence-electron-density profiles. Electron momentum densities, energy bands and density of states computed using linear combination of the atomic orbitals method (within the framework of density functional theory) are analyzed using the experimental Compton line shapes. It is seen that the recently developed second order generalized gradient approximation gives a better description of momentum densities than the local density approximation. Moreover, bonding in both the ceramic compounds has also been discussed in terms of Mulliken's population analysis.

Rietveld refinement and impedance spectroscopy of calcium titanate

Single phase perovskite CaTiO3 has been synthesized by conventional solid state reaction technique. The ceramic was characterized by XRD at room temperature and its Rietveld refinement inferred orthorhombic crystal structure with the space group Pbnm. The field dependence of dielectric relaxation and conductivity was measured over a wide frequency range from room temperature to 673 K. Analysis of Nyquist plots of CaTiO3 revealed the contribution of many electrically active regions corresponding to bulk mechanism, distribution of grain boundaries and electrode processes. The dc conductivity depicted a semiconductor to metal type transition. Frequency dependence of dielectric constant (ε′) and tangent loss (tan δ) show a dispersive behavior at low frequencies and is explained on basis of Maxwell-Wagner model and Koop's theory. Both conductivity and electric modulus formalisms have been employed to study the relaxation dynamics of charge carriers. The variation of ac conductivity with frequency at different temperatures obeys the universal Jonscher's power law (σacα ωs). The values of exponent ‘s’ lie in the range 0.13 ≤ s ≤ 0.33, which in light of CBH model suggest a large polaron hopping type of conduction mechanism.

Iron and chromium doped perovskite (CaMO3 M = Ti, Zr) ceramic pigments, effect of mineralizer

Solid solutions Ca(DxM1−x)O3 (M=Ti, Zr and D=Fe,Cr), have been studied as ceramic pigment in conventional ceramic glazes using 0.5mol/mol of NH4Cl as flux agent by solid state reaction and by ammonia coprecipitation route. Ca(CrxTi1−x)O3 compositions obtained without addition of NH4Cl as mineralizer, produce pink color in glazes at low x but CaCrO4 crystallizes when x increases, producing undesired green colors. The crystallization of chromates can be avoided using NH4Cl as mineralizer, giving a complete solid solution that produce pink color in glazes at low x and dark blue shades at high x. Coprecipitated sample produce blue colors at low x and at low temperature than ceramic sample (1000°C instead 1200°C for CE sample). Cr4+ ion acts as red chromophore, but at higher x values (blue samples) Cr3+ ion entrance affects the color. Ca(FexTi1−x)O3 system crystallizes perovskite CaTiO3 and pseudobrookite Fe2TiO5 together with rutile as residual crystalline phase, glazed samples change from a yellow to a pink color associated to the increase of pseudobrookite with firing temperature. Ca(FexTi1−x)O3 and Ca(CrxZr1−x)O3 systems crystallize perovskite CaZrO3 and zirconia (ZrO2) in both monoclinic and cubic polymorphs, but iron or chromium oxides are not detected in the powders. Coprecipitated sample stabilises cubic form. The solid solution is not reached completely in these samples and is not stable in glazes.

Stick-like titania precursor route to MTiO3 (M = Sr, Ba, and Ca) polyhedra

In this paper, MTiO3 (M = Sr, Ba, and Ca) perovskite polyhedra have been successfully synthesized through a facile hydrothermal route, in which appropriate amounts of metal salts and sodium hydroxide are mixed with the stick-like titania powders to fabricate the products. Cubic SrTiO3, dodecahedral BaTiO3, and cross-shaped CaTiO3 perovskites are finally obtained. The experimental conditions have also been investigated in detail and a feasible growth mechanism is proposed to illustrate the formation of MTiO3 perovskites. Furthermore, we demonstrate the possibility of the method, in which titania nanorods act as the precursor, to produce titanate compounds with a unique morphology. This work not only enriches the synthesis methods of MTiO3 perovskites, but also facilitates the study of potential applications of titanate perovskite compounds.

High-pressure crystal structure of elastically isotropic CaTiO3 perovskite under hydrostatic and non-hydrostatic conditions

The structural evolution of orthorhombic CaTiO3 perovskite has been studied using high-pressure single-crystal x-ray diffraction under hydrostatic conditions up to 8.1 GPa and under a non-hydrostatic stress field formed in a diamond anvil cell (DAC) up to 4.7 GPa. Under hydrostatic conditions, the TiO6 octahedra become more tilted and distorted with increasing pressure, similar to other 2:4 perovskites. Under non-hydrostatic conditions, the experiments do not show any apparent difference in the internal structural variation from hydrostatic conditions and no additional tilts and distortions in the TiO6 octahedra are observed, even though the lattice itself becomes distorted due to the non-hydrostatic stress. The similarity between the hydrostatic and non-hydrostatic cases can be ascribed to the fact that CaTiO3 perovskite is nearly elastically isotropic and, as a consequence, its deviatoric unit-cell volume strain produced by the non-hydrostatic stress is very small; in other words, the additional octahedral tilts relevant to the extra unit-cell volume associated with the deviatoric unit-cell volume strain may be totally neglected. This study further addresses the role that three factors—the elastic properties, the crystal orientation and the pressure medium—have on the structural evolution of an orthorhombic perovskite loaded in a DAC under non-hydrostatic conditions. The influence of these factors can be clearly visualized by plotting the three-dimensional distribution of the deviatoric unit-cell volume strain in relation to the cylindrical axis of the DAC and indicates that, if the elasticity of a perovskite is nearly isotropic as it is for CaTiO3, the other two factors become relatively insignificant.

First-principles calculations of lattice dynamics in CdTiO3and CaTiO3: Phase stability and ferroelectricity

First-principles calculations of various phases of CdTiO${}_{3}$ carried out with the aim of obtaining insights into the mechanism of the ferroelectric phase transition and the structure of the low-temperature ferroelectric phase are reported. The results indicate that the preferred symmetry of the low-temperature phase is Pna2${}_{1}$, rather than $P$2${}_{1}ma$, corresponding to a small relative shift of the Ti and O ions in the paraelectric Pnma phase with the polarization axis parallel to the long axis. Calculated phonon dispersion curves show a distinct soft mode at the \ensuremath{\Gamma} point of the Pnma phase, which vanishes in the Pna2${}_{1}$ phase, confirming that the transition to the ferroelectric phase is of the soft-mode displacive type. Calculations of perovskite CaTiO${}_{3}$, which also has an orthorhombic Pnma structure at room temperature but, unlike CdTiO${}_{3}$, does not exhibit a ferroelectric phase transition down to 4.2 K, were also carried out to help characterize the factors controlling ferroelectric phase transitions in perovskite titanates.

Temperature dependence of luminescence properties of praseodymium-doped perovskite CaTiO3:Pr3+

A polycrystalline perovskite CaTiO3:Pr3+ was synthesized and the temperature dependence of luminescence properties was investigated in the temperature range of 6–440K. CaTiO3:Pr3+ exhibits red emission ascribed to the f–f transition from the excited state 1D2 to the ground state 3H4 of Pr3+ upon the band gap photo-excitation between valence and conduction band, and upon the IVCT (inter valence charge transfer between Pr3+ and Ti4+) photo-excitation. Below 330K, the emission intensity upon the band gap excitation increased with temperature, while the temperature dependence of emission intensity upon the IVCT excitation is relatively small, indicating that the electron transfer process between Pr3+ and the host accompanied by the band gap excitation is thermally activated. Above 330K, the thermal quenching of emission was observed upon both of the band gap and the IVCT excitation and the energy barrier for thermal quenching was estimated to be ∼5500cm−1.

Multifunctionality of Perovskites BaTiO3 and CaTiO3 in a Composite with Hydroxyapatite as Orthopedic Implant Materials

In the present article, the multifunctional behavior of perovskites BaTiO3 and CaTiO3 has been studied for their potential orthopedic applications. The biocompatibility and electrical conductivity properties of HA-40 wt%BaTiO3 (25.9 vol% BaTiO3) and HA-40 wt% CaTiO3 (35.3 wt% CaTiO3) composites were investigated to verify its suitability as hard tissue replacement materials. L929 mouse fibroblasts and Human osteoblasts (SaOS2) cells were used to study the cell adhesion and proliferation behavior on the composite surfaces. It has been observed that the developed composites are biocompatible and the conductivity and dielectric behavior of the composite is comparable to dry human bone.

Formation Mechanism of CaTiO3 Hollow Crystals with Different Microstructures

The crystal growth of CaTiO(3) hollow crystals with different microstructures has been investigated. In a water-free poly(ethylene glycol) 200 (PEG-200) solution, CaTiO(3) nanocubes formed first. The nanocubes underwent an oriented self-assembly into spherical particles, enhanced by the surface-adsorbed polymer molecules. Since the growth of nanocubes and their aggregation took place simultaneously, the nanocubes in the outer shells were larger than those in the cores. Disappearance of the small nanocubes in the cores of the spheres during an Ostwald ripening process led to spherical hollow crystals. Addition of a small amount of water (1.25 vol %) in the polymer solution enhanced surface recrystallization of the aggregated spheres, forming a cubic morphology. The orthorhombic distortion of the perovskite CaTiO(3) structure did not have a significant effect on the nanocube aggregation, resulting in a domain structure in the shells. Single-crystalline hollow cubes were produced with a slightly higher water content, e.g., 5 vol %. This process of (1) aggregation of nanocubes and (2) surface crystallization followed by (3) surface-to-core extension of recrystallization gives a good example of the reversed crystal growth route in ceramic materials. The proposed formation mechanism of the hollow CaTiO(3) crystals would enable us to control the microstructures of these materials and to explain the formation of many other hollow crystals.

Doping and Atomic Site Substitution in the Dielectric Ceramics Ti(Al)O2 Rutile and (1−x)Ca2/5Sm2/5TiO3–xLi1/2Sm1/2TiO3 Perovskite

Temperature‐stable, high‐permittivity (high‐dielectric constant) ceramics are used as resonators for microwave telecommunications. Research and development in this area is aimed at increasing the dielectric constant (in order to reduce component size) while maintaining very high quality factor (low dielectric loss). This present work is a synchrotron X‐ray powder diffraction study of high quality factor, high permittivity ceramics of the Ti(Al)O2 and (1−x)Ca2/5Sm2/5TiO3–xLi1/2Sm1/2TiO3 families. The synchrotron powder diffraction was used to investigate doping of Al onto the Ti site in Ti(Al)O2. It was found that the lattice parameters in Ti(Al)O2 are not affected by the Al doping. Although this null result does not give any evidence for substitution of Al on the Ti site, it does not exclude the possibility that a small proportion of the Al ions do substitute for Ti in Ti(Al)O2. In (1−x)Ca2/5Sm2/5TiO3–xLi1/2Sm1/2TiO3 the variation of lattice parameters and crystal structure versus x, and the possibility of substitution of Li onto the Ti site were investigated. Single phase samples with the orthorhombic CaTiO3 perovskite structure were obtained for 0.20<x<0.50. It was found that the unit cell volume increased in an inverse linear relationship with the (decreasing) vacancy concentration. Rietveld analysis of the powder diffraction data shows that there is no substitution of Li on the Ti site in (1−x)Ca2/5Sm2/5TiO3–xLi1/2Sm1/2TiO3.

Mixed-valence manganites

Mixed-valence manganese oxides (R1-χAχ)MnO3 (R=rare-earth cation, A=alkali or alkaline earth cation), with a structure similar to that of perovskite CaTiO3, exhibit a rich variety of crystallographic, electronic and magnetic phases. Historically they led to the formulation of new physical concepts such as double exchange and the Jahn-Teller polaron. More recent work on thin films has revealed new phenomena, including colossal magnetoresistance near the Curie temperature, dense granular magnetoresistance and optically-induced magnetic phase transitions. This review gives an account of the literature on mixed-valence manganites, placing new results in the context of established knowledge of these materials, and other magnetic semiconductors. Issues addressed include the nature of the electronic ground states, the metal-insulator transition as a function of temperature, pressure and applied magnetic field, the electronic transport mechanisms, dielectric and magnetic polaron formation, magnetic localization, ...

Preparation of Ca2Ti2O6 pyrochlore films by MOCVD

Ca-Ti-O films were prepared on quartz substrates by MOCVD using the precursors of Ti(O-i-Pr)2(dpm)2 and Ca(dpm)2. The effects of deposition conditions on the crystal structure and microstructure of Ca-Ti-O films were investigated. The deposition temperatures (Tdep) were changed from 787 to 1187 K and the total pressures in the reaction chamber (Ptot) was kept at 0.8 kPa. The molar ratio of Ca to Ti precursors (RCa/Ti) was changed from 0.34 to 0.95. At temperatures higher than 1073 K, CaTiO3 perovskite films in a single phase were obtained. Mixtures of Ca2Ti2O6 in a pyrochlore structure and CaTiO3 in a perovskite structure were obtained at 787 to 1073 K and the content of Ca2Ti2O6 phase increased with decreasing RCa/Ti. Ca2Ti2O6 pyrochlore films in a single phase were obtained at Tdep = 1073K and RCa/Ti = 0.38. Ca2Ti2O6 is a face-centered cubic structure with a=0.990 nm. Pyrochlore showed a granular microstructure about 500 nm in grain size. The transmittance of the Ca2Ti2O6 films in a single phase was 50% to 70% in the wavelength range of 500 to 2500 nm. The band-gap of pyrochlore was about 3.3 eV that calculated by the transmittance. The permittivity of Ca2Ti2O6 films was about 100 from room temperature to 700K.

Structural phase transition and octahedral tilting in the calcium titanate perovskite CaTiO 3

Crystal structure of the calcium titanate perovskite CaTiO 3 was refined by the Rietveld analysis of neutron diffraction data collected over the temperature range of 296 to 1720 K. The orthorhombic Pbnm-tetragonal I4/ mcm phase transformation reversibly occurred at 1512 ± 13 K and the tetragonal I4/ mcm-cubic Pm3̄ m transition was reversibly observed at 1635 ± 2 K. The structural phase transitions are characterized by the tilt of the TiO 6 octahedron. The tilt systems of the Pbnm, I4/ mcm, and Pm3̄ m CaTiO 3 are ϕ x − ϕ y − ϕ z + ( ϕ x − = ϕ y −), ϕ x 0 ϕ y 0 ϕ z −, and ϕ x 0 ϕ y 0 ϕ z 0, respectively. Here the ϕ x −, ϕ z +, and ϕ x 0 stand for the out-of-phase tilt angle of TiO 6 octahedron along x axis, the in-phase octahedral tilt angle along z axis, and no octahedral tilting along x axis, respectively. All the tilt angles, ϕ x −, ϕ z +, and ϕ z − decreased with increasing temperature where the critical exponents were about 0.25. The ϕ x − and ϕ y − of the orthorhombic structure decreased discontinuously to ϕ z − of the tetragonal structure or 0° through the Pbnm- I4/ mcm transition. The tilt angle, the unit-cell parameters and unit-cell volume discontinuously changed at the Pbnm- I4/ mcm transition temperature, indicating the first-order nature. The increase of the cell volume in the Pbnm- I4/ mcm transition was 0.088 vol.%. In contrast, the unit-cell volume continuously increased and the tilt angle ϕ z − continuously decreased with increasing temperature and became 0° at the I4/ mcm- Pm3̄ m transition temperature, indicating a continuous nature of the transformation. The Pbnm- I4/ mcm and I4/ mcm- Pm3̄ m phase transitions are induced by the tilting of TiO 6 octahedra, indicating the displacive nature of the transitions. The larger thermal motion of O2 atom in I4/ mcm CaTiO 3 indicates the larger positional disorder of O2 atom, which is consistent with the tilt system ϕ x 0 ϕ y 0 ϕ z −.

Cytotoxicity of Piezoelectric Materials in Colony Formation Assay

Biocompatible piezoelectric materials are becoming increasingly important for actuators and sensors in medical devices. In this paper, we highlighted on some perovskite oxides MgSiO3, CaSiO3, CaTiO3 and CaZrO3 which are biocompatible piezoelectric materials discovered by first-principles calculation in our previous studies. In order to verify their biocompatibility, the cytotoxicity of their isomeric oxides with the same components was examined as comparing with typical perovskite oxides Pb(Zr, Ti)O3 and BaTiO3. The fibroblast (L929) cells were cultured during 7 days and the effect of materials was evaluated by the half maximal inhibitory concentration (IC50). As a result, the colony formation assay indicated that BaTiO3, CaTiO3, MgSiO3 and CaZrO3 has the higher IC50 values. On the other hand, CaSiO3 and Pb(Zr,Ti)O3 shows the strong toxicity. The obtained IC50 values had a good correlation with the proliferation ratio in our previous studies.

Peierls dislocation modelling in perovskite (CaTiO3): comparison with tausonite (SrTiO3) and MgSiO3 perovskite

We present here a numerical modelling study of dislocations in perovskite CaTiO3. The dislocation core structures and properties are calculated through the Peierls–Nabarro model using the generalized stacking fault (GSF) results as a starting model. The GSF are determined from first-principles calculations using the VASP code. The dislocation properties such as collinear, planar core spreading and Peierls stresses are determined for the following slip systems: [100](010), [100](001), [010](100), [010](001), [001](100), [001](010), $$ [001](\bar{1}10), $$ $$ [\bar{1}10](001) $$ and $$ [110](\bar{1}10). $$ All dislocations exhibit lattice friction, but glide appears to be easier for $$ [\bar{1}10](001), $$ [100](010) and [010](100). $$ [110](\bar{1}10), $$ $$ [\bar{1}10](001), $$ $$ [001](\bar{1}10), $$ [001](010) and [001](100) exhibit collinear dissociation. Comparing Peierls stresses among tausonite (SrTiO3), perovskite (CaTiO3) and MgSiO3 perovskite demonstrates the strong influence of orthorhombic distortions on lattice friction. However, and despite some quantitative differences, CaTiO3 appears to be a satisfactory analogue material for MgSiO3 perovskite as far as dislocation glide is concerned.

Nanostructure of CaMgSi2O6 (Diopside) and CaTiO3 (Perovskite) mechanically activated in carbon dioxide

The micro- and nanostructure of materials resulting from mechanochemical interactions of natural diopside (CaMgSi2O6) and synthetic perovskite (CaTiO3) with CO2 have been studied by transmission electron microscopy (TEM) and high-resolution TEM. The results indicate that CO2 absorption is accompanied by CO2 “dissolution” in the form of CO32− ions in the structurally disordered silicate or titanate matrix. The diopside activation product is a quasi-homogeneous amorphous carbonate-silicate phase. The mechanically activated perovskite is a nanocomposite consisting of CaTiO3 nanocrystals embedded in a carbonated amorphous titanate matrix.

The Kinetics of Sorption of CO2 by Perovskite CaTiO3 and the Degree of Perovskite Decomposition with Nitric Acid after Its Mechanical Activation

The kinetics of mechanosorption of CO2 by perovskite CaTiO3 was analyzed. Data that characterized an increase in the reactivity of perovskite with respect to nitric acid caused by its mechanical activation in the atmosphere of carbon dioxide and in air were obtained.

Mechanosorption of Carbon Dioxide by CaTiO3 Perovskite: Structure-Chemical Changes

Processes that occur during the mechanical activation of CaTiO3 perovskite in a centrifugal-planetary mill in the atmosphere of CO2 and in air were studied. Mechanical activation in CO2 was accompanied by substantial chemisorption of carbon dioxide with perovskite with the formation of carbonate groups not only on the surface but also in the volume of structurally disordered calcium titanate particles. The content of CO2 in perovskite samples reached 12.8% after 50 min of mechanical activation. Carbon dioxide atmosphere contributed to a more intense accumulation of structure imperfections in perovskite under mechanical actions, whereas the dispersion of the compound was more effective in air.

Perovskite hollow cubes: morphological control, three-dimensional twinning and intensely enhanced photoluminescence

Hollow sub-micrometer sized cubes of perovskite CaTiO3 with twinned and textured orthorhombic (pseudocubic) nanodomains in three dimensions are prepared via a facile solvothermal route from calcium nitrate and titanium n-butoxide in poly(ethylene glycol) solvent.