E. S. Gagarina

Structural instabilities, incommensurate modulations and P and Q phases in sodium niobate in the temperature range 300–500 K

The relation between the phase transition in the vicinity of 420 K discovered earlier and the formation of an incommensurate phase in the temperature range 410–460 K is established based on the X-ray, dielectric, and dilatometric studies of sodium niobate (NaNbO3) single crystals and ceramics. It is also established that this phase is characterized by temperature and temporal instabilities. Anomalies in some physical characteristics in the vicinity of 350 K are revealed. It is also shown that the thermodynamic history of the samples is important for the coexistence of the regions of the ferroelectric Q and antiferroelectric P phases in NaNbO3.

Spontaneous phase transition from relaxor to macrodomain ferroelectric state in single-crystal PbSc0.5Nb0.5O3-BaSc0.5Nb0.5O3 solid solutions

The cooling of Pb1−xBaxSc0.5Nb0.5O3 solid solutions with x≤0.04 leads to a spontaneous transition from a relaxor to a macrodomain ferroelectric state, accompanied by anomalous variation of the dielectric and optical properties of the material. As the barium content in the system increases, the relaxor state becomes more stable and eventually “freezes” at x≈0.05. The crystals with x=0.06 exhibited the appearance of a macrodomain ferroelectric phase induced both by an external electric field with a strength of 1.5 kV/cm and by an internal electric field formed in the course of dielectric aging.

Crystallographic shear in niobium oxides of different compositions

Defect structures related to crystallographic displacement are revealed in niobium oxides of different composition (Nb2O5, NaNbO3, and solid solutions based on NaNbO3) by X-ray powder diffraction, X-ray fluorescence spectroscopy, and Raman scattering. It is suggested that lattice shear in NaNbO3 and NaNbO3 based solid solutions lead to the block structure of crystals. The effect of the thermodynamic history of objects under study on the formation of a defect structure of this type is demonstrated.

X-ray and dielectric studies of liquid-phase sintered PBB1/2 3+B1/2 5+O3 ceramics with differing degree of compositional ordering

Abstract Ceramics of several PbB1/2 3+ B1/2 5+O3 perovskites (B3+-In, Sc, Yb, Lu, Er, Tm; B5+-Nb, Ta) with differing degree of compositional ordering were obtained using Li2CO3 additions. Ordering process in Li-doped ceramics is greatly enhanced due to recrystallization of grains in the presence of liquid phase. The properties of Li—doped ceramics are similar to those of crystals grown from the flux at the same temperatures. In some of the perovskites studied compositional order-disorder phase transitions and the transformation of antiferroelectric phase transition into ferroelectric one were observed.

Comparative study of cation ordering effects in single crystals of 1:1 and 1:2 complex perovskites solid solutions

Solid solution crystals of ternary 1:1 (PbSc1/2Ta1/2O3-PbSc1/2Nb1/2O3) and 1:2 (PbMg1/3Nb2/3O3-PbSc1/2Nb1/2O3) perovskites were grown from the flux. X-ray supercell reflections due to B-cation ordering were observed for as-grown crystals. The same regularities of cation ordering during the crystal growth were found to be valid for both systems: the disordered structure is formed first even when the crystallization temperature is much lower than the temperature T t of the compositional order-disorder phase transition and the disordered state is non-equilibrium. The ordering then starts via a diffusion-type mechanism, its rate depending greatly on the relation of crystallization temperature and T t .

The growth and study of PbSc0.5Nb0.5O3-BaSc0.5Nb0.5 O3 solid solution crystals

Pb1-xBaxSc0.5Nb0.5O3 solid solution crystals with 0 ≤ X ≤ 0.6 have been grown by the flux method. The spontaneous transition from relaxor to the normal ferroelectric state is observed in mixed crystals with 0≤x≤0.06, while compositions with x≥ 0.10 show the usual relaxor behavior. In contrast to ceramics, the concentration dependence in crystals of the temperature Tm of dielectric permittivity maximum does not saturate over the whole Ba concentration range investigated.

T-x-s phase diagram of compositionally orderable (1-x) PbSc1/2Nb1/2O3-xPbSc1/2Ta1/2O3 solid solution

Abstract (1-x) PbSc1/2Nb1/2O3-xPbSc1/2Ta1/2O3 solid solution crystals have been grown in the whole range of compositions by spontaneous crystallization. The temperature Tt of compositional order-disorder phase transition was found to be a linear function of composition ′ The ferroelectric phase transition temperature Tc also depends linearly on ′ both for crystals with high and low degree of ordering s. On the base of the results obtained a tentative T-′—s phase diagram for (1-x) PbSc1/2Nb1/2O3-xPbSc1/2Ta1/2O3 solid solutions have been calculated.

Domain structure and related phenomena in PbYb0.5Nb0.5O3 crystals

Abstract The domain structures, their formation and reconstruction have been analysed that are realized at structural phase transitions in PbYb0.5Nb0.5O3 crystals. Some interesting features of the orientation behavior of S-type domain (twin) boundaries have been established and interpreted.

Growth and study of single crystals of the (Na,Li)Nbo3 solid solutions

Plateletlike transparent (Na,Li)NbO3 (NLN) single crystals with up to 7 mol % LiNbO3 possessing a perovskite structure are synthesized by the method of spontaneous crystallization from NaBO2 flux. Based on the X-ray diffraction data and dielectric measurements of these single crystals, the phase x-T diagram of these solid solutions is constructed in the vicinity of the NaNbO3 composition. It differs from the x-T diagrams established for the Na1 − xLixNbO3 ceramic.

Domain structure of Na1 − xLixNbO3 crystals

The domain structure of single-crystal and ceramic samples of Na1 − xLixNbO3 solid solutions (at x ≤ 0.14) in the orthorhombic ferroelectric and antiferroelectric phases at room temperature is investigated by optical and electron microscopies and X-ray diffraction. The characteristic feature of the domain structure is the formation of 90° complexes consisting of laminar domains with a specific orientation relative to the lattice of the initial cubic phase. Consideration is given to the specific features in packing of these complexes and typical configurations of domains in the crystals. Observations revealed that the domain structure can involve 90°, 60°, 120°, and 180° boundaries, as well as (hhl) boundaries of the S type whose orientation depends on the cell distortion and changes with a variation in x. The indices of these boundaries are determined. The density of 180° boundaries in the ferroelectric phase is very low compared to that of non-90° boundaries.