I. P. Bykov

Influence of impurities on the properties of rare-earth-doped barium- titanate ceramics

Investigations of impurity centers, electrical resistivity and microstructure of BaTiO3 ceramics doped with rare-earth ions Y, La, Nd, Sm, Dy and Lu at concentrations x = 0.001–0.005 were carried out. Electron paramagnetic resonance, X-ray diffraction and electron microscopy were used for measurements. The most intense EPR lines were shown to belong to paramagnetic complexes Fe3+–VO and Ti3+–Ln3+ (Ln = rare-earth ion, VO = oxygen vacancy). A change in symmetry of the center Fe3+–VO at the transition temperature from the ferroelectric to paraelectric phase has been revealed for the first time. Measurements of the dependence of EPR line intensities and electrical resistivity with rare-earth ion concentrations were performed. The observed correlation in their behaviour showed an essential role of the identified paramagnetic complexes in the appearance of BaTiO3 ceramic semiconducting properties and the positive temperature coefficient of resistance (PTCR) effect. The latter effect was at a maximum for x ≈ xc where xc ≈ 0.002–0.003 is the critical rare-earth ion concentration which determines the excess charge compensation mechanism. Up to xc, the rare earths investigated, (except for the small ion Lu), substitute for barium, and the main compensation mechanism is an electronic mechanism. At high concentrations (x > xc) in the case of large ions (e.g. La), substitution is at barium sites, with the creation of titanium vacancies, whereas intermediate ions (e.g. Y) begin to substitute for titanium. The influence of impurities on the BaTiO3 microstructure, including the grain sizes, is discussed.

Central-Peak Components and Polar Soft Mode in Relaxor PbMg1/3Nb2/3O3 Crystals

Dielectric response of relaxor PMN single crystals was investigated in a broad frequency range 100 Hz–100 THz by a combination of dielectric spectroscopy (100 Hz–1 GHz, 10–700 K), time-domain THz spectroscopy and infrared (IR) reflectivity (5–3000 cm−1, 10–300 K). Polar TO1 phonon (soft mode) is resolved in THz and IR spectra at all temperatures studied, follows the Cochran law and softens towards the Burns temperature on heating. The central mode (dielectric relaxation) splits below room temperature into two components. First one appears in the THz range and is assigned as IR activated phonon density of states due to the breaking of local symmetry as a consequence of polar nano-clusters. Second relaxation originates from cluster breathing and flipping. It anomalously broadens on cooling and leads to frequency-independent dielectric loss between 100 Hz–100 GHz below 200 K in the non-ergodic phase.

ESR investigation of yttria stabilized zirconia powders with nanosize particles

Investigations of undoped zirconia oxide powders and stabilized by 3% or 8% of yttria with nanosize particles were carried out by electron spin resonance (ESR) method. Sizes of the particles from different series of samples were 8 nm and 30 nm. The most intensive resonance lines with g-factors 4.25 and 1.97 were observed in all the samples. The characteristic feature of the line with g = 1.97 was its complex asymmetrical form with broad shoulder to the side of smaller g-factor value. Computer analysis had shown that the line is composed from several lines. It was shown that each paramagnetic centers spectrum contains two lines: one from the bulk and another shifted to smaller g-factor side from interface region of nanosize grains. The integral intensity of the later line increases since the size of nanoparticle decrease.

The photoinduced Ti3+ centre in SrTiO3

We report the results of an electron spin resonance study of Ti3+ centres in SrTiO3 single crystals. The Ti3+ centres are created in perturbed regular Ti4+ sites by trapping a photoelectron from the conduction band after ultraviolet irradiation of the sample at low temperature (T Tc (Tc ≈ 105 K corresponds to the temperature of the cubic–tetragonal phase transition), the Ti3+ centre exhibits an orthorhombic symmetry of the g-tensor with its principal axes oriented exactly along 001 and 110 crystal directions: g[110] = 1.9920, g[10] = 1.9375, g[001] = 1.8843. At T < Tc, due to a structural phase transition, two of the Ti3+ principal axes are tilted relative to the 110 directions by up to ±8o in the (001) crystal plane. The spectroscopic data are explained assuming Jahn–Teller orthorhombic distortions of the oxygen octahedron with non-linear T2g × (eg + t2g) vibronic coupling.

Dynamic of Nb ions in PMN diffused phase transition region and its NMR investigation

Abstract The investigation of 93Nb NMR spectra in PMN crystal at 130 K < T < 430 K, v L = 49 MHz and 73 MHz have been carried out. The temperature dependence of line width and spin lattice relaxation time was measured. The peculiarities of these dependences were shown to be connected with Nb ions motion, which was described by the Vogel-Fulcher law in accordance with glassy character of PMN diffused phase transition. NMR spectra modeling by Monte-Carlo numerical method gave evidence about the Nb ions displacement only along [111] directions and its value is x 0 = 0.09 A with dispersion [sgrave] = 0.01 A at T = 260 K. NMR data gave no evidence of 1:1 regions existence in PMN crystal.

Impurities in nominally pure KTaO3: evidence from electron spin resonance

The ESR investigation of nominally pure KTaO3 was carried out at 4.2 K<T<77 K. Two ESR spectra were observed for the first time. The first spectrum with cubic symmetry was shown to be that of Gd3+ substituted for K+. The analysis of the spectrum line intensities and widths gave no evidence about the quasi-static displacements of tantalum ions in the lattice assumed earlier. The second spectrum had axial symmetry and its source was identified as a new off-centre impurity ion Fe+ substituted for K+ in KTaO3. The surrounding oxygen ion displacements were shown to be the main reason for the Fe+ ESR spectra axiality. The estimation of these displacements Delta x was carried out in the framework of crystalline-field theory and appeared to be about 0.2 AA.

Local configurational instability of in

The ESR spectra of impurity in single crystals are studied in the 4.2 - 300 K temperature range. It is shown that substitutes for with a charge compensation in distant spheres. The axial-symmetry crystal-field parameter is negative and exhibits unusual (for a deep tetragonal phase) temperature behaviour: increases linearly as the temperature is reduced to = 170 - 180 K and then for its value strongly decreases. It is shown that the reason for this decreasing is the displacement of from the central position in the oxygen cage which is occupied by for . A possible connection of this phenomenon with a small lattice distortion at T = 170 - 180 K, which was observed earlier, is discussed.

Peculiarities of the radiospectroscopy line shape in nanomaterials

The theory of the radiospectroscopy (nuclear magnetic resonance [NMR] and electron spin resonance [ESR]) line shape for nanomaterials is developed. The consideration was performed in the core and shell models which are, respectively, the nanoparticle regions unperturbed and perturbed by the surface influence. The shift of the resonance frequency by the surface tension was taken into account. The homogeneously broadened line shape was supposed to be Gaussian or Lorentzian. Inhomogeneous broadening of lines via the distribution of nanoparticle sizes was calculated for several forms of the size distribution function. The splitting of radiospectroscopy spectra into two lines decreases with particle sizes, which looks like that in the bulk and on the surface. It was shown to be the characteristic feature of nanomaterial spectra. The changing of these lines’ intensity and width with the change of the distribution function parameters and the particle size decrease was considered. The comparison of the theory with NMR spectra of17O and25Mg observed recently in nanocrystalline MgO is performed. The calculations fit pretty good the observed size dependence of the line shape, intensity and width.

The charge state of manganese, titanium and chromium ions and its influence on the properties of the lead titanate-zirconate solid solutions

Abstract EPR spectra of pure and doped with Mn, Cr, La, Nb, Sb PZT ceramics and macroscopic parameters (σ, tanδ, ϵ) of PZT doped with Mn were measured. It is shown that the mechanism of the compensation of excessive charge of impurity ions depends on the value of impurity concentration. Anomalous conductivity in pure and doped with less than 0.5% of Mn PZT is caused by Pb2+ overcharging to Pb4+.

NMR investigation of crystals with diffused phase transitions

Abstract The NMR spectra of PMN and PSN were measured at T⩾400K. It was shown that only 1/10 part of PMN volume had the ideal structure which agrees with chemical formula. The displacement of Sc ions along [001] type direction was supposed in the regions with random distribution of Sc and Nb in PSN