Progress in research on pressure-induced abnormal order-disorder phase transition

[ Instrument R & D of Instrument Network ] High pressure is an important technical means to develop new materials with special properties. High pressure can synthesize many new structures that cannot be formed under conventional conditions, and exist in a metastable phase under normal pressure. High pressure exists widely in nature, for example, the celestial body is in a high pressure state, and studying the pressure-related material state is an important prerequisite for understanding the physical substance of nature.

The perovskite structure is an important structural carrier of functional materials and the main structural form of mantle material that accounts for the largest proportion in the earth. It has special significance for high-pressure science. The team of Jin Changqing of the Institute of Physics, Chinese Academy of Sciences / National Laboratory for Condensed Matter Physics, Beijing, has conducted a high-pressure development of new perovskite-like materials for a long time. Through high-pressure technological innovation, we have designed and developed a variety of structural elements containing perovskite Functional new materials.

Perovskite refers to a class of ceramic oxides whose molecular formula is ABO3; such oxides were first discovered as calcium titanate (CaTiO3) compounds present in perovskites, hence the name. Because of the many structural characteristics of such compounds, they are widely used and studied in condensed matter physics, so physicists and chemists often refer to them as the ratio of each compound in the molecular formula (1: 1: 3), so The name "113 Structure". Cube-shaped crystal form. Cubic crystals often have stripes with parallel prisms, which is the result of producing polycrystalline twin crystals when the high-temperature variant is transformed into a low-temperature variant.

Order-disorder transformation has always been the frontier and difficulty of material science. The order of atoms in the material can directly determine the performance parameters such as crystal structure, stability, magnetism, thermal conductivity, electrical conductivity, and elastic modulus, which are extremely important for the performance of the material. Impact. People usually use temperature and components to control the order of materials. As a thermodynamic variable independent of temperature and components, the effect of pressure on the order-disorder transition of materials is also significant.

Thermodynamic variables (thermodynamic variables) are physical quantities that determine the macroscopic state of a thermodynamic system and a few macroscopically measurable properties, such as temperature, volume, pressure, and composition. For a given thermodynamic system, how many variables are needed to determine its state is determined by experiment. For example, experiments prove that for a certain amount of a certain gas, only two variables in temperature, volume, and pressure are needed to determine its state. The thermodynamic variables do not include the mechanical variables such as the overall speed and position of the system.

Recently, Jin Changqing's team has made new progress in the research of high-pressure synthesis of B-position double perovskite structural functional materials. For most known materials, pressure will increase the structural coordination number and tend to order the material. The B-site double perovskite (A2B'B''O6) is filled with the same proportion of two ions (B'B '') in the B-site. Previously, in the double-perovskite material, only the pressure increased and the B-site was ordered. According to the report of degree, the explanation for this phenomenon is that the disorder of B site will increase the unit cell volume, making the disordered structure unable to exist stably under high pressure. Jin Changqing's associate researcher Deng Zheng and Dr. Li Wenmin and Zhao Jianfa successfully developed the B-order ordered double perovskite new material Y2CoIrO6 using high-pressure technology, and for the first time discovered the B-order disordered phase change caused by the synthetic pressure.

With the development of computing technology, it has become a reality to obtain chemical equilibrium data and other thermodynamic data by solving complex models. A large number of excellent phase diagram calculation software based on Gibbs energy minimization and the development of supporting databases have made great progress , And a new interdisciplinary subject has been derived from it. Thermodynamic calculations are very important. Whether it is process development or engineering design, thermodynamic calculations are required. But most of the more complicated calculations are done by computer, and the chances of hand calculation are getting less and less, especially for engineering design, if the larger project is calculated by hand, the workload is too large and time is not allowed. But basic thermodynamic calculations should still be possible!

By experimenting with different synthetic pressure conditions, they found that as the synthetic pressure increased, the B-site ions of the new perovskite material Y2CoIrO6 showed low-pressure order, partial order in medium pressure, and complete disorder until 15GPa (1GPa ~ 10,000 atmospheres) . The pressure of disorder appears to be equivalent to the boundary between the upper mantle and the lower mantle. This boundary is the watershed where the mantle forms the perovskite structure, and the lower mantle is full of perovskite minerals. At the same time, the order-to-disorder transition caused the material's magnetism to evolve from long-range ferrimagnetic to short-range spin-glass state. They found that the main reason for this abnormal pressure-induced disorder is that the strength of the two ionic chemical bonds at the B site (that is, the degree of orbital hybridization) forms a unique combination morphology under high pressure conditions, resulting in a disordered structure with smaller crystal Cell volume. This conclusion is supported by thermodynamic models and theoretical calculations.

Source: Encyclopedia, Institute of Physics