Photoconductivity and the structural phase transition in SrTiO3

Detailed photoconductivity measurements have been performed in nominally pure SrTiO₃ in order to elucidate the effect of the antiferrodistorsive cubic-tetragonal phase transition. Small features in the photoconductivity’s temperature dependence in the phase transition region were found using low intensity interband UV or 514 nm light illumination. Such features are associated with a transformation of the defect system controlling the photoconductivity. At the same time, the temperature behavior of the photoconductivity spectral maximum reveals a rather unusual feature which is connected with changes in the absorption band edge structure in the phase transition region.     

Li and Nb codoping in dielectric properties and phase transitions of KTaO3: Reentrant dipolar glass and long-range dipole ordering

Abstract

The highly polarizable perovskite-type oxide, KTaO₃ doped simultaneously with Li⁺ and Nb⁵⁺ (K_1?xLi_xTa_1?yNb_yO₃, KLTN), reveals unexpected properties and ordering effects. Studies of the dielectric permittivity ?'(T, f) (10—300K, 100Hz-1 MHz) for x = 0.0014 and y = 0.024 show collective dipolar ordering effects with a transition from paraelectric into a mixed phase (coexisting dipole-glass-like and long-range ordered ferroelectric phases) taking place near 39 K. At 15 K another phase transition into a reentrant dipolar glass-like state is observed. Such a sequence of transitions and the existence of a reentrant glass state are unknown for electrical dipolar systems.     

Study of absorption spectra of Er<Superscript>3+</Superscript> in KTaO<Subscript>3</Subscript> crystals

The first results of the study of optical absorption spectra of KTaO₃: Er³⁺ crystals are presented. In the 350–660-nm region, lines are observed deriving from intraconfigurational electronic transitions from the ⁴ I _15/2 ground state to levels of the ⁴ F _9/2, ⁴ S _3/2, ² H _11/2, ⁴ F _7/2, ⁴ F _5/2(⁴ F _3/2), ² G _9/2, and ⁴ G _11/2 excited states of the Er³⁺ ions. A comprehensive study of transitions to the ⁴ F _9/2, ⁴ S _3/2, ² H _11/2, and ⁴ F _7/2 levels at 77 K is carried out. The number of lines observed for the above transitions fits the theoretically possible number for ?-? electronic transitions in Er³⁺ ions in the cubic crystal field. In the case of a differently charged substituted ion, this situation occurs only under nonlocal impurity charge compensation. The energies of the excited state levels for the transitions under study are determined.     

Infrared and ESR spectroscopy of KTaO3 : Er

Infrared reflection and ESR spectra of KTaO₃ : Er³⁺ single crystals have been investigated. The frequency of the lowest optical phonon mode TO₁ has been found to increase as compared to undoped KTaO₃ crystals, which indicates the suppression of the ferroelectric instability of the system. The ESR spectra have revealed the presence of two Er³⁺ centers of different (cubic and non-cubic) symmetries in KTaO₃ : Er³⁺.     

High-frequency electron paramagnetic resonance investigation of Mn3+ centers in SrTiO3

A high-frequency (49 and 70 GHz) electron paramagnetic resonance (EPR) study is reported on SrTiO₃ single crystals doped with 0.5-at% of manganese. EPR spectra originating from the S=2 ground state of Mn³⁺ ions are shown to belong to the different types of Jahn–Teller centers that arise as a result of varied ordering of the oxygen vacancies due to the reducing process. The present results indicate the strong covalent reduction of the density of unpaired spins (Fermi term) at the manganese nucleus for the various types of centers.     

Dielectric permittivity and phase transitions in the SrTiO3-KTaO3 system

The dielectric permittivity and losses of the (SrTiO₃)_0.85(KTaO₃)_0.15 solid solution have revealed an unusual behavior at temperatures of 5–300 K and frequencies ranging from 100 Hz to 1 MHz. The dielectric permittivity exhibits a broad maximum at ≈40 K, which obeys the Curie-Weiss law in the high-temperature wing, and the onset of low-temperature relaxation contributions. Besides, while for T>40 K it is a fully ergodic, uniformly ordering system, the uniform polar ordering is replaced with decreasing temperature by two low-temperature phase transitions to a glass-like state. The proposed mechanism for the dielectric relaxation and the observed phase transformations involves reorientation and ordering in the system of electron and hole polarons, which form in the course of charge compensation of the heterovalent ions Sr²⁺, K⁺ and Ti⁴⁺, Ta⁵⁺ distributed randomly in the sublattices. Fiz. Tverd. Tela (St. Petersburg) 39, 2040–2045 (November 1997)     

Effect of external electric field on R line absorption spectra in LiNbO3:Cr3+ crystals

The effect of an external electric field on the R absorption lines of LiNbO₃:Cr³⁺ crystals has been studied by a high-sensitivity differential technique at 77 K. Linear Stark effect has been observed in a field directed along the trigonal crystal axis. No effect of the field was observed when directed perpendicular to this axis. These characteristics of the Stark effect provide a convincing argument for the electric dipole moments of Cr³⁺ centers being oriented along the trigonal C ₃ axis of the crystal and indicate that the chromium centers in LiNbO₃ have C ₃ symmetry. Fiz. Tverd. Tela (St. Petersburg) 39, 2053–2056 (November 1997)     

Diffusion of particles over dynamically disordered lattice

Using kinetic Monte Carlo simulations we have investigated the diffusion of particles over a lattice with moving atoms in the framework of a simple lattice-gas model of a reconstructive surface. The particle migration over the static and dynamic lattices differs considerably. The dynamic lattice reconstruction changes substantially the particle diffusion coefficient. The Arrhenius dependencies are quantitatively different. An interesting peculiarity of the particle migration is the formation of defectons--local lattice deformations around the adsorbed particles. In certain ranges of the system parameters (jump rates of the substrate atoms and adsorbed particles) the adsorbed particles cause local displacements of the nearest substrate atoms, resulting in deeper adsorption sites and lower jump rates. Such particle self-trapping produces a characteristic minimum on the Arrhenius dependencies.     

The transition from dynamics to statics in the electron-spin-resonance spectra of impurity Mn2+ ions in strontium titanate

The electron-spin-resonance (ESR) spectra of SrTiO₃:Mn single crystals have been investigated. Results unambiguously indicate that the impurity center formed by an Mn²⁺ ion has a dynamic nature. In the high temperature range (T > 100 K), ESR spectra of Mn²⁺ ions reveal cubic symmetry; the spectrum is found to broaden significantly with a decrease in temperature. Upon cooling to T < 10 K, low-symmetry centers of Mn²⁺ ions with a strong orientational dependence emerge in the spectra. Temperature evolution of the ESR spectrum can be described within the model of a dynamic off-center Mn²⁺ ion substituting for the Sr²⁺ ion, with a transition to the static regime at low temperatures with an average localization energy of ～2.4 ± 0.4 meV for Mn²⁺ centers due to random deformations.