[en] We experimentally show that, in contrast to the data having been collected so far, some single crystals of NaNbO₃ exhibit a dielectric permittivity of several thousand, at low T, and this value is saturated when approaching 0 K on cooling. Other sodium niobate crystals (having larger dielectric losses) present a first-order phase transition to a ferroelectric phase on cooling (at 80-200 K). The width of the thermal hysteresis in these crystals increases when the temperature of the phase transition obtained on heating decreases. The dielectric permittivity at the phase transition obtained on cooling shows a tendency to increase and saturate, when the thermal hysteresis increases. We identify the ground state of the sodium niobate crystal exhibiting the smallest dielectric losses (in the studied set of crystals) as a novel quantum paraelectric state coexisting with a metastable ferroelectric state. In principle, the crystal presenting the state of quantum paraelectricity can be considered as having the largest (among the crystals studied) thermal hysteresis, for which the low boundary is below 0 K. (fast track communication)

[en] We report an x-ray diffraction (XRD) and a Raman-scattering investigation of ferroelectric/paraelectric superlattices [BaTiO₃] _(1−x)Λ/[BaZrO₃]_xΛ for which the composition varied, 0.15 ≤ x ≤ 0.85, while the superlattice (SL) modulation period Λ was kept constant at about 100 Å. The samples were epitaxially grown by pulsed laser deposition on MgO substrates buffered with La_0.5Sr_0.5CoO₃. Based on the XRD analysis and on polarized Raman spectra, we have showed that the large strain in SLs induced ferroelectricity in BaZrO₃ (BZ) for all SLs, a material that is paraelectric in the bulk form at any temperature and in the single film. The induced polar axis in BZ layers is perpendicular to the plane of substrate while BaTiO₃ (BT) layers exhibit in-plane polar orientation. Raman spectroscopy revealed a lattice ordering in SLs due to the misfit strain generated by the large lattice mismatch between the alternating BZ and BT layers. This strain induced a huge upward frequency of the lowest E(1TO) soft mode from 60 cm⁻¹ in the BT single film to 215 cm⁻¹ in the SL with x = 0.85. These results show that in spite of relatively large periodicity of SLs, they are highly constrained and the variation of BZ ratio allowed modifying strains between layers. The temperature dependence of the Raman spectra for BT_0.3Λ/BZ_0.7Λ and BT_0.7Λ/BZ_0.3Λ samples revealed giant shift of the ferroelectric phase transition. The phase transition temperature was found to be upshifted by about 300 °C with respect to BT single crystal.

[en] 

Abstract—

: The concentration dependences of the magnetic phase transition temperature T_N, determined based on the changes in Mössbauer spectra, were investigated for a BiFeO₃ solid solution with ordered perovskite PbFe_0.5Sb_0.5O₃ and for a disordered BiFe_1 – xCr_xO₃ solid solution. It is established that the magnetic-order type in BiFe_1 – xCr_xO₃ changes from antiferromagnetic to spin-glass at a higher degree of Fe-sublattice dilution in comparison with the PbFe_0.5Nb_0.5O₃-based solid solutions presumably because of the smaller BiFeO₃ lattice parameter. Due to the local ordering of Fe³⁺ and Sb⁵⁺ ions, the T_N value decreases more rapidly (as compared to BiFe_1 – xCr_xO₃) with a decrease in the Fe³⁺ content in the lattice of (1 – x)BiFeO₃– xPbFe_0.5Sb_0.5O₃ solid solutions.

[en] Dielectric permittivity measurements show that (1- x)NaNbO₃-(x)Gd_1/3NbO₃ crystals, at x = 0.09, exhibit a first-order phase transition near 150 deg. C that is diffuse over a wide temperature interval. The area of the ε'(T) thermal hysteresis loop was found to depend both on annealing and cooling temperatures. Optical studies in polarized light show a decrease of the domain sizes at this phase transition. The results obtained are fitted assuming a spatial distribution of Curie temperatures. (letter to the editor)

[en] We have fabricated new perovskite multiferroic PbFe_1/2Sb_1/2O₃ with a high degree (up to 0.9) of chemical ordering and unexpectedly high-temperature magnetic relaxor properties, which can barely be described within concepts of conventional spin glass physics. Notably, we found that the field-temperature phase diagram of this material, in the extremely wide temperature interval, contains the de Almeida–Thouless-type critical line, which has been the subject of long debates regarding its possible experimental realization. We explain our findings by the creation, at high temperatures of not less than 250 K, of giant superspins (SSs), owing, curiously enough, to the antiferromagnetic superexchange interaction. We show that these SSs are capable of strong high-temperature magnetic relaxation in the relaxor phase, down to about 150 K, where they transform into a SS glass phase. On further cooling, the material experiences another striking transition, this time, into an ordinary (single-spin) antiferromagnetic phase. We comprehensively analyze the above complex physical picture in terms of three complimentary theoretical approaches. Namely, the ab initio calculations elucidate the microscopic mechanism of giant SS formation, the high-temperature expansion accounts for the morphology of these clusters, and the random field approach provides the description of disorder-related characteristics. (paper)