M. V. Talanov

Effect of sintering temperature on the density, piezodielectric response, and mechanical and elastic properties of materials of different functional groups

This paper describes the synthesis of Ba-doped PbZn1/3Nb2/3O3-PbMg1/3Nb2/3O3-PbNi1/3Nb2/3O3-PbTiO3 multicomponent solid solutions rich in PbNi1/3Nb2/3O3, which include ferroelectric relaxors, ferroelectrics with a diffuse phase transition, and classic ferroelectrics, and presents the phase diagrams along three cuts through a section near PbNi1/3Nb2/3O3. We examine the effect of sintering temperature on the density, piezodielectric response, and mechanical and elastic properties of ceramics of different functional groups; optimize the conditions for the fabrication of such ceramics, and compare the performance parameters of the materials obtained with those of their commercially available analogs.

Evolution of domain processes during the transition from classical ferroelectric to relaxor ferroelectric

Dependences of the relative permittivity on the external bias electric field of ceramics of the yPZN-mPMN-nPNN-xPT (x = 0.25–0.40) system have been studied. The evolution of domain processes during the concentration transition from classical ferroelectric to relaxor ferroelectric has been shown. Tentative conclusions on the mechanisms of electric field-induced transitions from the relaxor state to the “normal” ferroelectric state have been drawn.

Unique atom hyper‐kagome order in Na4Ir3O8 and in low‐symmetry spinel modifications

Group-theoretical and thermodynamic methods of the Landau theory of phase transitions are used to investigate the hyper-kagome atomic order in structures of ordered spinels and a spinel-like Na4Ir3O8 crystal. The formation of an atom hyper-kagome sublattice in Na4Ir3O8 is described theoretically on the basis of the archetype (hypothetical parent structure/phase) concept. The archetype structure of Na4Ir3O8 has a spinel-like structure (space group Fd\bar 3m) and composition [Na1/2Ir3/2](16d)[Na3/2](16c)O(32e)4. The critical order parameter which induces hypothetical phase transition has been stated. It is shown that the derived structure of Na4Ir3O8 is formed as a result of the displacements of Na, Ir and O atoms, and ordering of Na, Ir and O atoms, ordering dxy, dxz, dyz orbitals as well. Ordering of all atoms takes place according to the type 1:3. Ir and Na atoms form an intriguing atom order: a network of corner-shared Ir triangles called a hyper-kagome lattice. The Ir atoms form nanoclusters which are named decagons. The existence of hyper-kagome lattices in six types of ordered spinel structures is predicted theoretically. The structure mechanisms of the formation of the predicted hyper-kagome atom order in some ordered spinel phases are established. For a number of cases typical diagrams of possible crystal phase states are built in the framework of the Landau theory of phase transitions. Thermodynamical conditions of hyper-kagome order formation are discussed by means of these diagrams. The proposed theory is in accordance with experimental data.

Theory of structural phase transition in MgTi2O4

A theory of phase transition in MgTi2O4 is proposed based on a study of the order-parameter symmetry, thermodynamics, and mechanisms of formation of the atomic and orbital structure of the low-symmetry MgTi2O4 phase. The critical order parameter (which induces a phase transition) is determined. It is shown that the calculated MgTi2O4 tetragonal structure is a result of displacements of magnesium, titanium, and oxygen atoms; ordering of oxygen atoms; and the participation of dxy, dxz, and dyz orbitals. The contribution of noncritical representations to ion displacements is proven to be insignificant. The existence of various metal clusters in the tetragonal phase has been established by calculation in correspondence with experimental data. It is shown (within the Landau theory of phase transitions) that phase states can be changed as a result of both first- and second-order phase transitions: the high-symmetry phase borders two low-symmetry phases by second-order transition lines, while the border between low-symmetry phases is a first-order transition line.

Effect of barium on the structure and dielectric properties of multicomponent ceramics based on ferroelectric relaxors

High-density ceramic samples of mPb(Mg1/3Nb2/3) · yPb(Zn1/3Nb2/3) · nPb(Ni1/3Nb2/3) · xPbTiO3 (m = 0.4541, y = 0.0982, n = 0.1477, x = 0.3) solid solutions, unmodified and barium-modified, with the composition lying in a morphotropic phase region, have been prepared by a conventional ceramic processing technique. We demonstrate that barium substitution for 5% of the lead on the A site allows one to obtain ceramics containing no pyrochlore phase. Modification with barium shifts the solid solution from the morphotropic to a tetragonal phase region. In addition, barium doping increases the average grain size of the ceramic from 2–3 to 3–4 μm and changes the type of fracture from intergranular to mixed (intergranular and transgranular). Modification with barium increases the relative dielectric permittivity ɛ/ɛ0 (at E = 0) of the material by more than a factor of 2 (from 4300 to 9100). We conclude that materials based on the ceramics studied can find practical application and examine ways of further improving their piezoelectric performance.

Electric-field-induced phase transition in the relaxor ceramics based on PMN-PT

The mechanisms of formation of a plateau in dependences of the reversible permittivity on the external electric field strength in a ceramic sample of the yPZN-mPMN-nPNN — xPT (x = 0, 3; y = 0.0982; m = 0.4541; n = 0.1477) system have been analyzed using X-ray diffraction and atomic force microscopy. It has been shown that, in electric fields corresponding to the permittivity anomaly, there occurs an induced phase transition from the heterophase (tetragonal + pseudocubic) state to the single-phase (tetragonal) state. This leads to significant strains of the tetragonal unit cell (up to 0.1%) and the termination of 90° domain switching. The proposed qualitative model of the formation of the observed dielectric anomaly is based on two processes occurring with an increase in the electric field strength: the growth of polar regions of the tetragonal phase and their switching.

Crystal chemistry and formation mechanism of tetragonal MgTi2O4

Symmetry methods of the theory of second-order phase transitions are used to clarify the structural mechanism of the transition from the cubic (Fd3m) to the tetragonal (P41212) phase of magnesium titanite. The proposed mechanism is consistent with all experimental structural data reported to date for magnesium titanite. We analyze the crystal-chemical features of the structure of tetragonal MgTi2O4 and infer the presence of metallic pico- and nanostructures: two types of Ti2 dimers, two types of helices along the twofold and fourfold axes of the tetragonal cell, and two types of infinite one-dimensional chains of titanium ions.

Anion order in perovskites: a group-theoretical analysis.

Anion ordering in the structure of cubic perovskite has been investigated by the group-theoretical method. The possibility of the existence of 261 ordered low-symmetry structures, each with a unique space-group symmetry, is established. These results include five binary and 14 ternary anion superstructures. The 261 idealized anion-ordered perovskite structures are considered as aristotypes, giving rise to different derivatives. The structures of these derivatives are formed by tilting of BO6 octahedra, distortions caused by the cooperative Jahn-Teller effect and other physical effects. Some derivatives of aristotypes exist as real substances, and some as virtual ones. A classification of aristotypes of anion superstructures in perovskite is proposed: the AX class (the simultaneous ordering of A cations and anions in cubic perovskite structure), the BX class (the simultaneous ordering of B cations and anions) and the X class (the ordering of anions only in cubic perovskite structure). In most perovskites anion ordering is accompanied by cation ordering. Therefore, the main classes of anion order in perovskites are the AX and BX classes. The calculated structures of some anion superstructures are reported. Comparison of predictions and experimentally investigated anion superstructures shows coherency of theoretical and experimental results.

Dielectric spectroscopy of Pb1–xBax(Mg1/3Nb2/3)m(Zn1/3Nb2/3)y(Ni1/3Nb2/3)nTizO3 solid solutions in a wide temperature interval

The dielectric spectra of Pb1–xBax(Mg1/3Nb2/3)m(Zn1/3Nb2/3)y(Ni1/3Nb2/3)nTizO3 (x = 0–0.15, m = 0.4541, y = 0.0982, n = 0.1477, and z = 0.3) ceramic samples have been studied in wide intervals of temperature (10–873 K) and frequency of the measurement electric field (0.1–1000 kHz). It has been found that an increase in the Ba2+(x) content leads to a reduction in the phase transition temperature (from 418 K at x = 0 to 256 K at x = 0.15), to the transition from the normal ferroelectric state to the relaxor ferroelectric one (at x ≥ 0.025), and to the disappearance of temperature hysteresis of dependences of the relative dielectric permittivity. It has been hypothesized that a tricritical point is present near x ∼ 0.125 in the x–T phase diagram of the studied solid solutions.

Group-theoretical study of cationic ordering in perovskite structure

Atomic ordering in the structure of cubic perovskite has been investigated by the group-theoretical method. The possibility of existence of 13 ordered phases in the 1(a) position (including four binary, two ternary, and five quaternary cationic superstructures) and 13 phases with different types of ordering in the 1(b) position (including four binary, two ternary, and five quaternary cationic superstructures) is established. Calculated structures of some types of ordered low-symmetry perovskite modifications are reported.