Relaxation processes in the incommensurate phase of a crystal with defects

The nature of the relaxation of the incommensurate superstructure of a ferroelectric to the equilibrium state is investigated experimentally. It is shown that near a phase transition the temperature dependence of the relaxation time of the incommensurate phase of the defective crystal is exponential. This law agrees qualitatively with the notion of domain wall motion in an inhomogeneous medium containing “random local phase-transition temperature” type defects. Fiz. Tverd. Tela (St. Petersburg) 41, 513–515 (March 1999)     

AC conductivity of BiFeO<Subscript>3</Subscript> ceramics obtained by spark plasma sintering of nanopowder

The structure and electrical properties of BiFeO₃ ceramics obtained by spark plasma sintering of a nanopowder are investigated. The nanopowder was synthesized by burning of an organic nitrate precursor. The ac conductivity was measured in a frequency range of 1 kHz–10 MHz in a temperature interval of 25–500°C. It is established that the temperature conductivity coefficients above and below ~350°C significantly differ with both alternating and direct currents. The frequency dependence of the conductivity obeys the Jonscher power law σ ~ ω ^s , where s < 1. The interpretation of this behavior is given in the framework of the model of correlated hops of charge carriers over potential barriers. It is assumed that the hopping mechanism is realized between Fe²⁺ and Fe³⁺ ions in ceramic grains. The role of oxygen vacancies in the conduction is also discussed.     

Specific features of the thermal physical properties of relaxor ceramics based on lead zirconate titanate

The heat capacity and thermal expansion of ferroelectric relaxors based on lead zirconate titanate are studied near the diffuse phase transition. It is shown that no spontaneous phase transitions from the paraelectric phase to the ferroelectric phase and from the relaxor state to the normal ferroelectric state occur in an ensemble of nanometer-sized polar regions. It is noted that the transitions can be caused only by external electric fields or storage for a fairly long time.     

Structure and Dielectric Properties of Bi1 – xSmxFeO3 Nanostructured Ceramics

Physics of the Solid State - Structure and dielectric properties of polycrystalline multiferroics of Bi1&nbsp;–&nbsp;xSmxFeO3 (x = 0–0.2) system are studied. Using X-ray...     

Thermal properties of PZT-based ferroelectric ceramics

The thermal properties of piezoelectric ceramics (PKR-8, PKR-7M) based on lead zirconate titanate solid solutions Pb(Ti, Zr)O₃ were studied over the temperature range 300–800 K. The thermal conductivity and thermal expansion coefficients were found to exhibit an anomalous behavior in the region of the ferroelectric phase transition.     

Heat capacity of BiFeO3-based multiferroics

The heat capacity of Bi_{1 ? x} Re _x FeO₃ (Re = La, Eu, Ho; x = 0, x = 0.05) multiferroics has been studied in the temperature range of 120–800 K. The substitution of a small amount of rare-earth elements for bismuth leads to a significant increase in the heat capacity in the broad temperature range studied. It is established that the temperature dependence of the excess heat capacity is related to the Schottky effect for three-level states certain that appear as a result of structure distortions in the rare-earth-doped compositions.     

Slow stage in the evolution of an incommensurate ferroelectric superlattice

The nature of the slow evolution of a soliton system in an incommensurate phase of a ferroelectric is investigated experimentally. It is shown that the duration of the time interval in which the anomalous permittivity and the corresponding soliton spacings are governed by a logarithmic law increases from a few minutes to several hours as the ferroelectric phase transition is approached. Fiz. Tverd. Tela (St. Petersburg) 40, 2101–2102 (November 1998)     

Stabilization of Ferromagnetism in BiFeO<Subscript>3</Subscript>:Ho at Hydrostatic Pressure

The isothermal magnetization of the Bi_{1 – x}Ho _x FeO₃ (x = 0?0.2) multiferroic has been studied at a hydrostatic pressure up to 9 GPa in the range of room temperatures. A new anomaly at P_C ≈ 3.81 GPa related to intermediate phases between the structural transition R3c → Pnma has been found against the background of the pressure-induced antiferromagnetic ordering in BiFeO₃ (BFO) at P ≈ 2.59 GPa. It is established that the ferromagnetic behavior under pressure depends on the Ho impurity concentration: P_C decreases at 0.05 ≤ x ≤ 0.1 because of the decrease in R3c bond lengths in the structure, and the stabilization of ferromagnetism is implemented at 0.1 ≤ x ≤ 0.2 probably because of the coexistence of the R3c and Pnma phases. The results of studies indicate that, in Bi_{1 – x}Ho _x FeO₃ with x = 0.2, the transition pressure P_C = 3.7 GPa exceeds the values for BFO doped with other 4f elements (Eu, Y, Sm) in the region R3c → Pnma of the transition.     

Specific features of the thermal,magnetic, and dielectric properties of multiferroics BiFeO<Subscript>3</Subscript> and Bi<Subscript>0.95</Subscript>La<Subscript>0.05</Subscript>FeO<Subscript>3</Subscript>

Thermophysical, magnetic, and dielectric properties of multiferroic BiFeO₃ and Bi_0.95La_0.05FeO₃ ceramic compounds were comprehensively studied. Anomalies of the permittivity near an antiferromagnetic phase transition related to the structural variations were detected. The temperature T _N was determined from the temperature dependences of the thermal expansion coefficient, heat capacity, and differential susceptibility. It is shown that the transition point is shifted to higher temperatures as the rare-earth La ion substitutes for Bi. It is established that an insignificant substitution of lanthanum for bismuth enhances the magnetic properties of bismuth ferrite and the magnetodielectric effect.     

The Schottky Effect in Bi0.95Sm0.05FeO3

Physics of the Solid State - The temperature dependence of the specific heat of the Bi0.95Sm0.05FeO3 multiferroic has been studied in the temperature range 130–780 K. The substitution of...