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.

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.

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.

The expressions for the spontaneous polar contribution δ n i s to the principal values of the refractive index due to the quadratic electro-optic effect in ferroelectrics have been considered within the phenomenological approach taking into account the polarization fluctuations. A method has been proposed for calculating the magnitude and temperature dependence of the root-mean-square fluctuations of the polarization (short-range local polar order) P sh = 〈 P fl 2 〉 1/2 below the ferroelectric transition temperature T c from temperature changes in the spontaneous polar contribution δ n i s ( T ) if the average spontaneous polarization P s = 〈 P 〉 characterizing the long-range order is determined from independent measurements (for example, from dielectric hysteresis loops). For the case of isotropic fluctuations, the proposed method has made it possible to calculate P sh and P s only from refractometric measurements. It has been shown that, upon interferometric measurements, the method developed in this work allows calculating P sh and P s directly from the measured temperature and electric-field changes in the relative optical path (the specific optical retardation) of the light.

The results of the investigation of the quadratic electro-optic effect in Sr1-x Ca (x) TiO3 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 tau a parts per thousand 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.

Short and long range impurity-induced polar ordering in Sr1-xCaxTiO3 with x = 0.014 (SCT-1.4) and SrTi(16O0.0318O0.97)3 (STO-18) single crystals were investigated and discussed on the basis of the light refraction temperature dependences measurements. For SCT-1.4 the temperature dependences of the morphic birefringence and dielectric hysteresis loops were measured. Within the optical indicatrix perturbation approach a method for calculation from polar contributions to the refraction of light of short-range polarization contribution Psh originated by spatial fluctuations is developed for the systems with the coexistence of long and short range polar ordering. The magnitudes and temperature dependences of Psh and long-range spontaneous polarization Ps have been determined in SCT-1.4 and STO-18. The results allowed to characterize quantitatively and to compare contributions of short (Psh) and long (Ps) range ordering in the formation of impurity-induced polar phase in SrTiO3.

We used optical ellipsometry to study and elucidate the nature of the negative thermo-optic effect in KTaO3 single crystals, which consists of an increasing refraction index magnitude in the visible region upon cooling (dn/dT < 0). The optical constant functions were determined within the 1–6 eV region. The energies of the band edge indirect and direct optical transitions and their temperature evolutions in the region of 80–300 K were carefully investigated. The absorption edge in the region of the lowest indirect R → Γ optical transitions, 3.7–4.2 eV, was observed to shift to higher energies upon cooling. Simultaneously, an increase and ‘red shift’ of the optical absorption in the region of the direct X5'upper → X3 transitions (4.3–4.8 eV) were observed, which should be responsible for the nature of the observed negative thermo-optics in KTaO3.

The temperature dependences of the dielectric constant and dielectric hysteresis loops in ceramic samples of (1 − x )SrTiO 3− x KNbO 3 and (1 − x )SrTiO 3− x KTaO 3 (0 ≤ x ≤ 0.3) solid solutions prepared using different heat treatments have been investigated. Phase diagrams of the studied solid solutions have been constructed in the T - x coordinates. It has been shown that, after quenching of samples (spontaneous cooling at room temperature after long-term heating at the sintering temperature of the ceramic samples), the temperature of the induced phase transition increases because of the weakening of random electric fields associated with nonisovalent impurities due to their “frozen” nonequilibrium redistribution. For small concentrations x , strong dielectric relaxation is observed in the temperature range of 150–250 K. A model of relaxing centers, which is based on the local charge compensation of heterovalent impurities, has been proposed.

Temperature dependences of the dielectric permittivity and thermal treatment effect have been studied for (1-x)SrTiO3-(x)KTaO3 and (1-x)SrTiO3-(x)KNbO3 (0 <= x <= 0.2) solid solutions. For low concentrations x, strong dielectric relaxation was found in the temperature range of 150-250 K. A relaxing centers model based on the local charge compensation of heterovalent impurities is discussed.

This issue of FERROELECTRICS is devoted to the jubilee of Prof. Dr. Wolfgang Kleemann. Wolfgang Kleemann is a well-known scientist working in the field of physics of ferroelectrics and magnetic mat...

The optical functions of strontium titanate (SrTiO3) (STO) single crystals were determined with spectroscopic ellipsometry in the 1-6 eV spectral region and the 4.2-300 K temperature region. On approaching the band gap edge, the optical absorption shows Urbach tail behavior followed by the onset of indirect optical transitions starting at similar to 3.2 eV and extending up to similar to 3.68 eV; then direct transitions are very well seen. The refractive index and its temperature and spectral dependences were obtained. In the visible region, the refractive index rises on cooling (negative thermooptic effect) down to 50 K, saturating at lower temperatures. The energy of indirect interband optical transitions increases on cooling up to 3(23 eV at 100 K. The observed magnitude partial derivative E/partial derivative T = -1.7 x 10(-4) eV K-1 is typical of ionic crystals. Surprisingly, the absorption edge in the region of the lowest direct interband optical transitions 3.7-4 eV shifts to lower energies on cooling. It is concluded that the presence of this 'soft electronic band' together with the rise in absorption intensity at 4.8 eV is responsible for the increase of refractive index on cooling, elucidating the origin of the negative proper thermooptics in STO.

Spectral dependences of the relative permittivity ɛ = ɛ1 − iɛ2 of a uniaxial weak ferromagnet FeBO3 are measured for two principal polarizations in the energy range 0.6–5.6 eV. The positions have been determined for the charge-transfer transitions that make the main contribution to absorption above the bandgap and determine the refractive-index dispersion below the bandgap. The isotropic magnetic contribution to the refractive index has been detected by studying the temperature dependence of the refractive index in the range 100–700 K; its value (≈2 × 10−2) is found to be record high for magnetic dielectrics. The energy shift of the effective oscillator, which characterizes the shift of the positions of the charge-transfer transitions due to magnetic ordering, is determined from these data within the framework of a single-oscillator model. The value of the exchange striction in FeBO3 is determined from optical measurements.

The effect of the antiferrodistorsive phase transition observed at ∼ 105 K on the photo-carriers transport and band structure of nominally pure SrTiO3 single crystals was investigated. Simultaneously the ab-initio analysis based on the DFT-HF hybrid approach was performed in order to elucidate the atomic and electronic structure changes. It was found that the structural transition play a minor role in photo-charge transport. However, in highly resistive specimens at low UV-light intensities or under visible-light, 514 nm excitation, weak photoconductivity features are observed in the transition region. Besides, in this region the temperature maximum was found for the photoconductivity spectra. Experimental results are discussed and agree well with theoretical predictions.

We suggest considering the Burns temperature in lead containing relaxor ferroelectrics as the temperature, at which Pb becomes off-center and nearly randomly occupies positions around the center on a sphere with the radius, which increases with the temperature decrease. The significance of the Burns temperature, in this approach, is the crossover between the soft-mode and order-disorder dynamics. The lattice parameter, birefringence and neutron scattering experiments are analyzed in the light of this model.

A model of polar clusters (PC) and of chemical fluctuation clusters (CFC) in PMN-like relaxors is proposed. The model is mainly based on three phenomena: on a charge transfer state formation corresponding to charge transfer reorienting dipole states induced by Mg-Nb disorder, on an appearance of electronic and hole polarons in CFC due to charge compensation, and on in-cluster Local Configuration Instability effects. The origin of co-operative phenomena, of the giant piezoelectric effect and characteristic dielectric losses, including Vogel-Fulcher law as well as peculiarities of EPR and NMR do find an explanation within this model. It is shown that mobile polaron clusters play an important role in the formation of PC reorienting polarization. The presence of small polarons in PMN was confirmed in this work by detection of the characteristic IR-absorption accompanied by light-induced polaron transport.