METASTABLE STATE WITH HIGH ELECTRIC CONDUCTIVITY IN LiNaTaNbO SYNTHESIZED AT HIGH PRESSURE

Представлены результаты исследования сегнетоэлектрического твердого раствора LiNaTaNbO со структурой перовскита, основанного на ниобате натрия и синтезированного в условиях высокого давления и температуры. Методом импеданс спектроскопии в области температур 290 - 800 К были определены значения удельных проводимостей на постоянном токе, энергии активации носителей заряда и реальная часть диэлектрической проницаемости. Показана эволюция температурных аномалий удельной проводимости и диэлектрической проницаемости при термоциклировании. Обнаруженые эффекты связанны со структурными фазовыми переходами, определена температура Кюри. LiNaTaNbOпретерпевает фазовый переход второго рода. Установлено, что в LiNaTaNbO образуется метастабильная фаза, обладающая высокой электропроводностью в области комнатной температуры. При нагреве выше температуры Кюри данная фаза разрушается. Обсуждаются возможные механизмы обнаруженных явлений. The results are presented of a study of a ferroelectric solid solution LiNaTaNbO with a perovskite structure based on sodium niobate and synthesized under high pressure and temperature. In the temperature range of 290-800 K, the values of the specific conductivity at direct current, the activation energy of charge carriers, and the real part of the dielectric constant were determined by the method of impedance spectroscopy. Evolution of temperature anomalies of specific conductivity and dielectric constant during thermal cycling is shown. The observed effects are associated with structural phase transitions, and the Curie temperature is determined. The LiNaTaNbO undergoes a second-order phase transition. It was found that a metastable phase is formed in LiNaTaNbO, which has a high electrical conductivity at the room temperature. When heated above the Curie temperature, this phase is destroyed. Possible mechanisms of the discovered phenomena are discussed.

Transformation of the Metastable Al6Fe Intermetallic Phase during Homogenization of a Binary Al-Fe Alloy

Within the scope of this research the transformation of the Al6Fe metastable phase was analyzed via Differential Scanning Calorimetry (DSC), optical and Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). A binary Al-Fe1.1 low-impurity alloy was produced with refined raw materials in a controlled environment. With a cooling rate of 35 K/s, solidification of the Al6Fe metastable phase was achieved. The samples were homogenized at 600 °C for 2–24 h. Results of a qualitative analysis of metallographic samples show that the transformation began on grain boundaries, forming an Fe-phase free region, but after 2 h began to take place within the eutectic region. The transformation is mostly complete after 12 h, but after 24 h of homogenization it is fully complete as all samples, except the 24 h homogenized one, contain both the metastable Al6Fe and the stable Al13Fe4 phase.

Thermal transitions in metastable Cu – 68.5 at. % Co alloy.

Arc melted and drop tube processed Cu – 68.5 at. % Co alloy has been subjected to differential thermal analysis (DTA). The liquidus temperature determined from the DTA curves in the arc melt sample (1664 K) was found to be close to phase diagram estimate of 1662 K. In contrast as a result of liquid phase separation in the drop tube samples, the values obtained in the powders were much lower mainly because the compositions of the demixed phases vary from that of the parent melt. The liquidus temperature of the 850 + μm powders was 1632 K while that of the < 38 μm sieve size powder was 1616 K. This variance is due to the asymmetric nature of the metastable phase diagram of the system.

Perspectives on Pyrochlores, Defect Fluorites, and Related Compounds: Building Blocks for Chemical Diversity and Functionality

In this article we provide some perspectives on a range of pyrochlore and defect fluorite type compounds with nominal A2B2O7, A2BO5, ABC2O7, and other stoichiometries. Typically, the phase transformations and stability fields in these systems are mapped as a function of the ionic radii of the A and B-site cations, e.g., the A/B cation radius ratio (rA/rB). This provides a useful guide to compatible structures and compositions for the development of advanced materials. Pyrochlore commonly transforms to a defect fluorite structure at high temperature in many systems; however, it is not uncommon to observe defect fluorite as the initial metastable phase at low temperature. The patterns of order-disorder observed in these materials are primarily due to the energetics of layer stacking, the defect formation and migration energies of cations and anions, or modulations of the parent cubic structure in 3 + n dimensional space. The first lead to predominantly non-cubic derivatives of the parent defect fluorite structure (e.g., zirconolite polytypes), the second control the order-disorder processes, and the latter lead to a variety of subtle additional scattering features within the cubic parent structure. Although the energetics of cation disorder and anion-vacancy disorder have become more accessible via atomistic approaches (e.g., MD and DFT), we continue to find interesting physical-chemical problems in these materials. For example, although there are significant differences in composition (Tb/Zr ratio and O content) between Tb2Zr2O7 and Tb2ZrO5, both of which are defect fluorites, we note that the modulations found in these two compounds by electron scattering are virtually identical with regard to the direction and magnitude of displacement from the normal Bragg diffracted beams. This suggests that neither the A/B cation ratio nor the oxygen stoichiometry have a significant effect on the modulations. The general observations on the systems of compounds noted in this paper rest primarily in the context of industrial materials for nuclear waste disposal, potential applications in inert matrix fuel designs, and other important technological applications such as ionic conductivity, electrical conductivity, and magnetism. Scientific advances in these areas have been underpinned by recent advances in ion irradiation, synchrotron X-ray, neutron scattering, and modelling and simulation capabilities. Furthermore, there has been some renewed interest in natural samples, e.g., Th-U zirconolite and pyrochlore as analogues for potential host phases in nuclear waste forms. In particular, the natural pyrochlores have provided additional details with regard to radiation damage ingrowth, percolation transitions, and the relationships between accumulated dose and physical properties including hardness, elastic modulus. Specific details of the thermal annealing of these samples have also been elucidated in considerable detail.