Sr1-xCax TiO3 Research Articles

Epitaxial growth of SrCaTiO3 films on GaN by molecular beam epitaxy with a TiO2 buffer layer

We demonstrate the epitaxial growth of (111)-oriented Sr1 − xCaxTiO3 (SCTO) thin films on (0002) GaN using a thin (100) TiO2 buffer layer by RF-plasma-assisted oxide molecular beam epitaxy. We explore the growth window of SCTO for both x = 0 (i.e., SrTiO3) and x = 0.24 with various TiO2 buffer layer thicknesses and find that a substrate temperature of 650 °C and TiO2 thickness of 1 nm produce the highest quality films, as determined by reflection high energy electron diffraction, x-ray diffraction, and atomic force microscopy. SCTO films deposited without a buffer layer are polycrystalline, confirming the necessity of the TiO2 buffer layer to facilitate (111)-oriented epitaxial growth of SCTO on GaN. Transmission electron microscopy performed on the samples shows that the SCTO films grown with a TiO2 buffer layer are highly textured, and selected-area electron diffraction patterns indicate that some domains in the oxide exhibit cation ordering along the (111) direction. We also show that the growth mode changes from 2D to 3D as the thickness of SCTO increases past 10 nm, which leads to film relaxation and a rougher surface morphology.

Electro-optic effect in SrTiO3 and Sr1–x Ca x TiO3 (x = 0.014)

The results of the investigation of the quadratic electro-optic effect in Sr1–xCaxTiO3 with x = 0.014 (SCT) and in nominally pure SrTiO3 (STO) at room temperature in applied direct-current (dc) and alternating-current (ac) electric fields have been presented. It has been shown that the quadratic (in polarization) electro-optic coefficients of STO and SCT crystals coincide within the accuracy of the determination (±5%). It has been found that, in nominally pure STO measured in a dc electric field, there is a relaxation of the electro-optic effect with a relaxation time τ ≈ 30 s due to the formation of a space charge in the sample. No similar effect in SCT has been observed. A possible mechanism for the formation of a space charge in STO and SCT has been discussed.

Low temperature x-ray diffraction study of the phase transitions in Sr1-xCaxTiO3 (x=0.02,0.04): Evidence for ferrielectric ordering

X-ray diffraction studies on Sr1-x Cax TiO3 in the temperature range 12 to 300 K reveal cubic to tetragonal (I4/mcm) antiferrodistortive and tetragonal to orthorhombic (Ic2m) polar phase transitions at ∼170 and 225, 32, 34 K, respectively, for x=0.02 and 0.04. It is shown that the polarization in the orthorhombic phase is along the ⟨110⟩ pseudocubic direction, in agreement with the experimentally reported easy polarization direction by Bednorz and Muller. However, in disagreement with them the moments are found to be ferrielectrically coupled in the neighboring (001) planes that is responsible for the anomalously low values of the spontaneous polarization in the polar phase.

The various phases of the system Sr1-xCaxTiO3—A Raman scattering study

This paper deals with the structural Raman-active and the disorder-induced TO and LO Raman modes in Sr1−xCaxTiO3 (SCT) ceramics. Spectra were recorded at ambient pressure in a wide range of temperatures from 8 K up to ≈500 K. The chemical composition (x) was chosen so as to investigate the phase boundaries which have been previously determined in the x-T phase diagram. An eigenvector analysis is given for all the known phases. The Raman data confirm the occurrence of a double-tilt system of Imma space group lying between the tetragonal I4/mcm and the orthorhombic Pbnm structures for x ≥ 0.06. Higher-order transformation of the antiferroelectric phase (0.20 ≤ x ≤ 0.40) at low temperature is also suggested. The main interesting disorder-induced effects originate from the temperature behaviour of the hard polar modes TO2 and TO4, which appear in the Raman spectra of the paraelectric domain. This behaviour allows one to discuss the precursor range of ferroelectricity in the SCT system in terms of ferroelectric- or antiferroelectric-type dipolar interactions. Copyright © 2005 John Wiley & Sons, Ltd.

ARaman scattering study of the phase transitions in SrTiO3and in the mixed system (Sr1-xCax)TiO3 atambient pressure from T = 300 K down to 8 K

The room-temperature Raman spectra of Sr1-xCaxTiO3 (SCT) for x = 0.06 (SCT06), 0.12 (SCT12)and 0.30 (SCT30) are shown to contain first-order Raman lines corresponding to Eg and B1g modes, which appear below105 K in pure SrTiO3. The Eg line shows slightasymmetry, indicating the orthorhombic structure of SCT for x⩾0.06. The low-temperature Raman spectra of SCT06 andSCT12 contain Raman-active modes of symmetry B2g andEg, predicted by group theory but not observed in SrTiO3 orSCT0.7 (x = 0.007) by previous workers. The polar hard modesTO2 (175 cm-1) and TO4 (550 cm-1) arepresent even at room temperature for SCT06 and SCT12 due toCa2+-centred ferroelectric microregions. The intensityof these modes starts increasing below 55 and 80 K for SCT06and SCT12 respectively, which are nearly 2 Tc, where Tc is thetemperature of the smeared dielectric peak. The presence ofsuch an extended precursor region is akin to relaxor behaviour.For SCT30, two new lines at 79 and 128 cm-1 appear belowthe paraelectric to antiferroelectric phase transition temperature of 230 K. It is shown that Raman lines characteristicof both ferroelectric and antiferroelectric phases are presentin SCT12 below 100 K, confirming the frustration model for thehighly smeared dielectric response for this composition.

Evolution of Crystallographic Phases in (Sr1−xCax)TiO3 with Composition (x)

Results of X-ray powder diffraction, neutron powder diffraction, electron microdiffraction, and convergent beam electron diffraction (CBED) studies are presented to show that the space group of (Sr1−xCax)TiO3 (SCT) in the composition range 0.35<x≤0.55 cannot be Cmcm. The correct space groups are Pbcm for 0.35<x≤0.40 and Pbnm for 0.40<x≤0.55. The analysis of powder XRD data reveals that the structure of SCT is noncubic for x≥0.06. It is shown that four different types of orthorhombic phases appear in the SCT system with increasing Ca2+ content.

Local configuration instability as an origin of relaxor-type properties of ferroelectric solid solutions SBN, SCT and KLTN

A model of phase transitions (PT) in concentrated and diluted ferroelectric oxide solid solutions with iso-charged doping is considered on examples of SBN (SrxBa1−xNb2O6), x = 0.61 for congruent composition, of SCT (Sr1−xCaxTiO3) for x <<1, and of KLTN (K1−xLixTa1-yNbyO3) with weak (Nb5+) and strong (Li+) off-center impurities for x <<1, y<<1 cases. This nano-cluster model with taking into account in-cluster local configuration instability (LCI) can explain uncommon relaxor-type properties of these systems as well as the origin of active polar clusters. It is shown that cooperative mechanism of cluster polarization formation due to co-operative LCI can be important for SBN and for SCT. The model also explains strong suppressing of ferroelectric PT by Cr-impurities in SBN. While the SBN and the SCT are characterised by LCI which is induced by the chemical fluctuation field, KLTN consists in Nb-rich nano-clusters obey Li-impurities as cluster cores. Multi-mode soft dynamics with local in-cluster piezoeffect is responsible for the origin of long-range electric dipole-dipole interaction as well as for co-existence of ferroelectric and glass-like states with reentrant glass phenomenon. The former is a key effect in the phenomenon of small concentration impurity influence on the PT in KLTN. This model explains also an appearance of a new PT in KLTN with strong hysteresis and cusp-type susceptibility.

Relaxor properties of dilute and concentrated polar solid solutions

The close analogy between dielectric dispersion and polydispersivity of both dilute quantum paraelectrics like Sr1−xCaxTiO3 (SCT) and concentrated “solid solutions” like PbMg1/3Nb2/3O3 (PMN) suggests the appearance of polar nanoclusters to be the phe-nomenological link between both kinds of systems. Their size distribution gives rise to polydispersivity and dynamic heterogeneity. Whereas quenched random-fields are believed to be at the origin of these clusters in concentrated relaxors, the Poisson statistical distribution of impurities and their large polar correlation radius seems to give rise to the inherent nanocluster structure in dilute relaxors like SCT and K1−xLixTaO3 (KLT). The present status of the nanocluster hypothesis is presented both from the experimental and the theoretical point of view.

Novel features of Sr1-xCaxTiO3 phase diagram: evidence for competing antiferroelectric and ferroelectric interactions

Structural and dielectric evidences are advanced to show that the Sr1-xCaxTiO3 (SCT) system undergoes an antiferroelectric (AFE) phase transition in the composition range 0.18</=x</=0.40. Stabilization of the AFE phase, instead of the ferroelectric (FE) phase known for 0<x<0.12, is shown to be intimately linked with the nonpolar antiferrodistortive phase transitions driven by the R and M point instabilities preceding the AFE transition. It is proposed that the disorder and frustration introduced by CaTiO3 substitution in the incipient FE matrix of SrTiO3 leads to the smearing of the varepsilon(')(T) response in SCT for 0.002<x<0.12 akin to dipole glasses and relaxor ferroelectrics.

Nonlinear susceptibility of SrTiO3:Ca

The nonlinear dielectric response of Sr1−xCaxTiO3(SCT) is described in terms of various quantum model approaches. Field and temperature dependencies and the corresponding scaling properties are approached both by simulations within the framework of a heterogeneous transverse Ising model (TIM) and by analytical solutions within the framework of TIM and a coupled anharmonic quantum oscillator model. Alternatively, we propose an empirical approach, which models both phonon- and random field-induced clustering processes.

Novel structural features and phase transition behaviour of (Sr1-xCax)TiO3: II. X-ray diffraction studies

Results of a careful XRD study of the room temperature structure of (Sr1-xCax)TiO3 (SCT) with are presented to show that the structure of SCT is CaTiO3- (CT-) like for . High temperature XRD studies on the phase transition behaviour of SCT with show the existence of an intermediate phase in between the room temperature orthorhombic phase and the high temperature cubic phase. This is analogous to the phase transition behaviour of pure CT and is attributed to the softening of the R25 and M3 point phonon modes.

Domain state properties of weakly doped SrTiO3:Ca

Abstract Linear birefringence, Raman scattering, dielectric, and second-harmonic generation (SHG) studies on Sr1-xCax TiO3, x=0.007, reveal a smeared transition into a ferroelectric-improper ferroelastic phase at Tc=18 K. Polar disorder persists on a nanometric scale as a consequence of the percolative nature of the transition under the constraint of quenched random fields. The ferroelectric nanoregions exhibit superparaelectric response to an external ac field and interact piezoelectrically with acoustic quantum fluctuations of the host lattice, SrTiO3. Their morphology is probed by interface SHG, the angular dependence of which is incompatible with the global orthorhomic symmetry of the ferroelectric phase.