The paper presents the first observation of photoluminescence spectra of V3+ impurity in SrTiO3. The broad band of the observed emission is located in the near IR region and at low temperatures consists of a pronounced zero-phonon line (ZPL) with a maximum at 1157.1 nm (8642 сm-1) at 77 K and developed vibronic sidebands extending to 1450 nm. The observed ZPL is associated with an intracenter 1T2g → 3T1g or a closely located 1Eg → 3T1g emission transition in V3+ (3d2) ions replacing Ti4+ ions. It was found that the temperature shift of zero-phonon line is unusually large and its frequency decreases upon lowering the temperature. The local configuration instability of V3+ ions in the 3T1g ground state caused by soft TO1 temperature-dependent phonon mode present in SrTiO3 is considered as a possible source of the observed unusual temperature shift.

— The found effect of lowering the symmetry of the crystal structure of thin (less than 1 mm) plates and bars of strontium titanate (SrTiO 3 ) single crystals is discussed. This symmetry lowering manifests itself in the electron spin resonance (ESR) spectra of impurity centers (Fe 3+ and Mn 4+ ions, used as paramagnetic probes). It is shown that symmetry lowering is observed at temperatures T > 105 K, which generally correspond to the cubic phase of SrTiO 3 ; it leads to the formation of a tetragonal nonpolar structure, differing from the antiferrodistortive (AFD) phase D_4h^18 , which is characteristic of strontium titanate at T < 105 K and was not observed previously in SrTiO 3 . The factors determining the distortion value are found to be the geometry and ratio of sample sizes, surface finishing quality, and crystallographic orientation of plates.

The paper presents the first observation of photoluminescence spectra of V3+ impurity in SrTiO3. The broad band of the observed emission is located in the near IR region and at low temperatures consists of a pronounced zero-phonon line (ZPL) with a maximum at 1157.1 nm (8642 cm–1) at 77 K and developed vibronic sidebands extending to 1450nm. The observed ZPL is associated with an intracenter 1T2g → 3T1g or a closely located 1Eg → 3T1g emission transition in V3+ (3d2) ions replacing Ti4+ ions. It was found that the temperature shift of zero-phonon line is unusually large and its frequency decreases upon lowering the temperature. The local configuration instability of V3+ ions in the 3T1g ground state caused by soft TO1 temperature-dependent phonon mode present in SrTiO3 is considered as a possible source of the observed unusual temperature shift.

K1-xLixTaO3 (x = 0.043, 0.08) crystals, characterized by pyroelectric current with calculated spontaneous polarization and zero-field second-harmonic generation, have been studied by broadband dielectric spectroscopy, including time-domain terahertz transmission and infrared (IR) reflectivity, and by polarized Raman spectroscopy in the 10-300 K temperature range. This multiexperimental approach has proven the percolative nature of the ferroelectric (FE) transition at low temperatures and demonstrated that the FE phase is inherently inhomogeneous and displays coexistence of FE and relaxor regions. Thanks to the very broad frequency range studied (from 1 Hz to 20 THz), the relevant excitations were identified and fitted in the dielectric response of both crystals: three relaxations, a central mode (CM), and a soft mode (SM) that splits into three components on cooling. Two Cole-Cole relaxations (assigned to flipping of polar nanoregions around the Li+ ions by pi/2 and pi, in agreement with the known literature), thermally activated below similar to 150 K, but staying in the gigahertz range at higher temperatures, do not show any frequency anomaly at the FE transition and are therefore related to the non-FE parts of the sample volume. A third thermally activated relaxation of unusually slow dynamics was revealed at low frequencies and preliminary assigned to an expected critical relaxation connected with the percolative nature of the FE phase transition. The IR SM, which undergoes much less softening than in the undoped KTaO3, splits into three components below the FE transition. Two higher-frequency components correspond to the FE volume part of the crystals assigned to the split A(1) and E modes due to the cubic-tetragonal transition. The third low-frequency component is assigned to the non-FE (relaxor) volume part. Our assignment was confirmed by modeling the terahertz-IR response of the SM using the Bruggeman model within the effective medium approach. Below the SM response, an additional CM in the 10(11) Hz range in the whole temperature range is inferred from the fits.

We report on the studies of the synthesis, structural, and magnetic properties of undoped SrTiO3 (STO), carbon-doped STO:C, and reduced STO STO:R nanoparticles. Fine (~20–30 nm) and coarse (~100 nm) nanoparticles with a single phase of cubic perovskite-type structure were sintered by thermal decomposition of SrTiO(C2O4)2. Magnetization loops of fine STO:C and STO:R nanoparticles at low temperatures and an almost linear decrease in magnetization with temperature indicate the realization of a soft, ferromagnetic state in them, with a pronounced disorder effect characteristic of doped dilute magnetic semiconductors. Oxidation and particle size increase suppress the magnetic manifestations, demonstrating the importance of surface-related defects and oxygen deficiency in the emergence of magnetism. It was found that oxygen vacancies and doping with carbon make similar contributions to the magnetization, while complementary electron paramagnetic resonance, together with magnetization measurement studies, show that the most probable state of oxygen vacancies, which determine the appearance of magnetic properties, are charged F+ oxygen vacancies and C-impurity centers, which tend to segregate on the surface of nanoparticles.

We present a study of KTaO3:Er (approximate to 0.05%) single crystals based on a multifaceted approach including the use of optical absorption, far-infrared reflectivity, electron paramagnetic resonance, photoluminescence spectra, and ab initio simulations. We describe briefly the fundamental consequences of Er doping, in particular, stiffening of TO1 soft mode of KTaO3. We provide information about energy level structures controlling f - f optical transitions in Er3+, on formation of minor cubic octahedral and major orthorhombic Er3+ centers. It is revealed that the temperature shift of narrow zero-phonon emission lines is strikingly unusual being much larger than that typical for trivalent rare-earth impurities.

Electron paramagnetic resonance studies of the impurity Fe^3+ and Mn^4+ centers indicate that the symmetry of the crystal structure of millimeter-scale strontium titanate plates above 105 K lowers in the bulk from cubic to tetragonal, different from that of the conventional low-temperature antiferrodistortive phase. It is shown that the effect does not originate from the residual stress; its magnitude (observed tetragonal distortion) and particular manifestations depend on plate orientation, surface quality and the geometry of a sample. Peculiarities of the observed phenomenon are presented and possible scenario of its realization is discussed.

The electric field effect in electron spin resonance spectra of Fe3+ and Mn4+ impurity ions in thin oriented single-crystal plates of strontium titanate has been studied; the crystal structure of these samples at T > 105 K is tetragonal (i.e., differs from both cubic and antiferrodistortive tetragonal structure that is inherent in SrTiO3 at T < 105 K). It is shown that the electric field effects for the investigated centers are quadratic with respect to the applied field and have identical signs and orders of magnitude, thus indicating a nonpolar character of the observed high-temperature tetragonal state. An analysis of the dependences of the axiality value on the applied field indicates that the electric field effect is not related to electrostriction and caused by modification of the wave function of the ground state of impurity centers.

The results of the study of the electric field effect in the electron paramagnetic resonance spectra of the Fe3+ and Mn4+ impurity ions in thin oriented strontium titanate single-crystal plates are presented. The crystal structure of plates at T> 105 K turned out to be tetragonal, different from both the cubic and the antiferrodistortive tetragonal, inherent to SrTiO3 at T < 105 K. It is shown that for the studied centers, the electric field effect is quadratic in the applied electric field and has the same sign and order of magnitude, indicating the nonpolar origin of the observed high temperature tetragonal state. Analysis of the dependences of the axiality value on the applied field indicates that the electric field effect does not originate from the electrostriction and is due to the modification the wavefunction of the ground state of the impurity centers.

Abstract In KTaO_3:Er crystals, we observed a temperature shift, unusual for rare earth impurities, for narrow zero-phonon luminescence lines, due to the ^4 S _3/2 → ^4 I _13/2, ^4 F _9/2 → ^4 I _15/2, and ^4 S _3/2 → ^4 I _15/2 transitions in Er^3+ impurity ions. The magnitude of the shifts turned out to be comparable with the large shifts of the R lines of the Cr^3+ luminescence in SrTiO_3:Cr and KTaO_3:Cr crystals, that is, substantially larger than that for the three-charged rare earth ions usually observed in the   f – f   luminescence spectra in ionic crystals.

In KTaO3:Er crystals, we observed a temperature shift, unusual for rare earth impurities, for narrow zero-phonon luminescence lines, due to the 4S3/2 → 4I13/2, 4F9/2 → 4I15/2, and 4S3/2 → 4I15/2 transitions in Er3+ impurity ions. The magnitude of the shifts turned out to be comparable with the large shifts of the R lines of the Cr3+ luminescence in SrTiO3:Cr and KTaO3:Cr crystals, that is, substantially larger than that for the three-charged rare earth ions usually observed in the  f–f  luminescence spectra in ionic crystals.

The temperature dependence of the principal values of the refraction of light (specific optical retardation) in Sr1-xCaxTiO3 with x=0.014, SrTi((O0.03O0.97)-O-16-O-18)(3) and KTa1-xNbxO3 with x=0.018 single crystals have been measured and the magnitudes and temperature variations of polar contributions to the refraction were derived. From polar contributions to the refraction on the basis of the method developed by the authors the magnitude of the root mean square polarization fluctuations characterizing the short-range polar order in the low temperature ferroelectric phase were determined in the crystals under study.

Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) studies were performed on heavily manganese doped SrTiO3 crystals. At low temperatures (T≈6K), the ENDOR measurements on MnTi4+ show the presence of electric field gradient associated with the electrical polarization. The observation of EPR signals of rhombic Mn2+ complexes remotely compensated in the SrTiO3 lattice is discussed in connection with local disorder in which Mn2+ appears due to the reduction process.

We have examined temperature changes of the light refraction, birefringence, dielectric permittivity, and dielectric hysteresis loops in Sr1 – xCaxTiO3 single crystals with x = 0.014 (SCT-1.4). The dielectric properties of Sr1 – xCaxTiO3 with x = 0.007 (SCT-0.7) have been studied. We have performed ab initio calculations of equilibrium structures and total energies for three low-temperature phases of SrTiO3 and CaTiO3, based on which we have determined an expected symmetry of the ground state of their solid solution and spontaneous polarization directions in a calcium-induced ferroelectric phase in Sr1 – xCaxTiO3. In SCT-1.4, we have separated a spontaneous contribution to the light refraction, which arises due to the spontaneous electrooptical effect caused by the spontaneous polarization and its fluctuations. From the spontaneous contribution to the light refraction, based on a previously developed our phenological approach, we have quantitatively determined for the first time the values and temperature dependences of root-mean-square fluctuations of the order parameter—the polarization Psh = 〈P fl 2 〉1/2(short-range, local polar order) in the ferroelectric phase. From optical and dielectric measurements in SCT-1.4, the average value of spontaneous polarization Ps (the contribution from the long-range order) has been determined. We have estimated the values of Psh and Ps, which characterize the short- and long-range orders in the ferroelectric phase of SCT-0.7. Separate determination of the values and temperature dependences of Ps and Psh (which considerably exceeds the value of Ps in the ordered phase) has allowed us to reveal on a quantitative level new particular features of the formation of the induced polar phase in Sr1 – xCaxTiO3.

A study of high-quality SrTiO3 single crystals doped with the Mn4+ ions in the cubic phase (T > 105 K) with X-band electron paramagnetic resonance reveals direct correspondence between a shape of a sample and magnetic anisotropy of the impurity Mn4+ centers. In particular, for a sample with the shape of a square base rectangular prism, a size of (a x a x h) and faces perpendicular to the < 100 > crystallographic directions, zero-field splitting parameter D is approximately proportional to (a/h - 1) quantity. Temperature dependence of D indicates that this peculiar symmetry lowering is a feature characteristic for the cubic Fm3m phase of the strontium titanate. Diminishing of the D value with the decrease of the surface roughness for a thin (001)-oriented SrTiO3:Mn platelet shows that the observed effect originates from the sample surface.

The magnitudes and temperature dependences of the root-mean-square of polarization fluctuations, which characterizes short-range ordering below TC, are determined for the first time for Sr0.986Ca0.014TiO3 single crystals, using temperature measurements of light refraction made in a manner developed by the authors. Ab initio calculations of the equilibrium structures and total energies of three lowtemperature phases for SrTiO3 and CaTiO3 are performed and the symmetry of the ground state of their solid solution is identified on the basis of the obtained results. Features of short- and long-range ordering in the Ca2+-induced ferroelectric phase of the Sr1 − xCa x TiO3 system are discussed.

AbstractWe have examined temperature changes of the light refraction, birefringence, dielectric permittivity, and dielectric hysteresis loops in Sr_1 – x Ca_ x TiO_3 single crystals with x = 0.014 (SCT-1.4). The dielectric properties of Sr_1 – x Ca_ x TiO_3 with x = 0.007 (SCT-0.7) have been studied. We have performed ab initio calculations of equilibrium structures and total energies for three low-temperature phases of SrTiO_3 and CaTiO_3, based on which we have determined an expected symmetry of the ground state of their solid solution and spontaneous polarization directions in a calcium-induced ferroelectric phase in Sr_1 – x Ca_ x TiO_3. In SCT-1.4, we have separated a spontaneous contribution to the light refraction, which arises due to the spontaneous electrooptical effect caused by the spontaneous polarization and its fluctuations. From the spontaneous contribution to the light refraction, based on a previously developed our phenological approach, we have quantitatively determined for the first time the values and temperature dependences of root-mean-square fluctuations of the order parameter—the polarization P _sh = 〈 P _fl ^2 〉^1/2(short-range, local polar order) in the ferroelectric phase. From optical and dielectric measurements in SCT-1.4, the average value of spontaneous polarization P _ s (the contribution from the long-range order) has been determined. We have estimated the values of P _sh and P _ s , which characterize the short- and long-range orders in the ferroelectric phase of SCT-0.7. Separate determination of the values and temperature dependences of P _ s and P _sh (which considerably exceeds the value of P _ s in the ordered phase) has allowed us to reveal on a quantitative level new particular features of the formation of the induced polar phase in Sr_1 – x Ca_ x TiO_3.

Short- and long-range impurity-induced polar ordering in Sr1-xCaxTiO3 (x = 0.014) single crystals was investigated and discussed on the basis of light refraction, morphic birefringence and temperature-dependent dielectric hysteresis loop measurements. Anew method for the calculation of the short-range polar order contribution P-sh in ferroelectric (FE) phases below T-C from light refraction measurements was applied. This method considers optical indicatrix perturbation due to polar ordering taking into account polarization fluctuations. The magnitudes and temperature dependences of P-sh originated by spatial fluctuations and long-range spontaneous polarization P-s were determined. These results allowed us to obtain quantitatively and to compare short (P-sh)-and long (P-s) range polar order contributions to the formation of impurity-induced FE phase in quantum paraelectrics.

Strontium titanate is a model quantum paraelectric in which, in the region of dominating quantum statistics, the ferroelectric instability is inhibited due to nearly complete compensation of the harmonic contribution into ferroelectric soft mode frequency by the zero-point motion contribution. The enhancement of atomic masses by the substitution of O-16 with O-18 decreases the zero-point atomic motion, and low-T ferroelectricity in (SrTiO3)-O-18 is realized. In this study we report on the local structure of Ti in (SrTiO3)-O-16 and (SrTiO3)-O-18 investigated by Extended X-ray Absorption Fine Structure measurements in the temperature range 6 - 300 K.