As we entered the 21st century, mixed anion compounds, which contain several different anions, began to draw attention as new types of inorganic material. Since, compared with existing inorganic compounds such as oxides and nitrides, unique coordination and crystal structures are obtained from such mixed anion compounds, it is possible that fundamentally different, innovative functions may be created. Materials with such innovative functions are expected to be created by using several different anions with differing electronegativity and polarizability, and exploiting the exceptional ability of such anions to control chemical bonding and electronic structures. In addition, many elements that become anions have a high Clarke number, and mixed anion compounds have the possibility of becoming materials that act as a driving force for element strategies.
Most mixed anion compounds are artificial compounds created under controlled conditions, and form a group of materials totally different from single-anion compounds, including oxides, which are present in common forms on the Earth. Because of this, the chemistry of mixed anion compounds is not an extension of conventional crystal chemistry, which has developed based on mineralogy; and there are a number of undeveloped areas in this field, while no basic theory has yet been well established. It is likely, then, that new, hitherto unobserved phenomena will be discovered through research of such compounds.
In this new area of research, we will create new materials and establish a basic theory of mixed anion compounds, developing material design concepts and methodologies that will lead to practical applications in the future. In addition, we will create new functionality, discovering and developing the roots of chemical and physical functions that will meet the needs of the future society. The main aim of research topics A01 and A02 is to conduct basic research on mixed anion compounds; and thus the two groups will, respectively, synthesize and analyze such compounds. The aim of research topic A03 is to search for new chemical and physical functions unique to mixed anion compounds, with a view to applied research.
A01 “Creation of Mixed Anion Compounds: Development of Guidelines for Material Synthesis and Design”
The constituent elements of inorganic compounds are limited to a few tens of types. A new approach, differing from conventional approaches, is required to create innovative new materials. The chemical properties of nearby anions on the periodic table vary significantly; and this makes it possible to obtain unique electronic and crystal structures from mixed anion compounds containing several different anions, which cannot be obtained from single-anion compounds, and thus to add a new dimension to material design. Hitherto, there has been an overall lack of systematic development of synthesis techniques for mixed anion compounds. Thus, this research group will synthesize materials by applying various experimental and computer science techniques, with the aim of establishing synthesis techniques, controlling the anion composition and degree of anion order, and developing new structures.
The members of this research group are experts in the synthesis of inorganic compounds, and have synthesized materials with various anions. We will bring them together from the perspective of material design for mixed anion compounds, thoroughly search for mixed anion compounds, using various synthesis techniques, and develop a multiplex synthesis technique combining various related techniques. In this manner, we will create novel crystal and electronic structures in which the anion composition, local anion structure, and degree of anion order are fully controlled; and will derive innovative new functions.
With oxyhydrides, oxynitrides, acid halides, and three-anion systems as the main target, we aim to create mixed anion compounds with existing structures, such as perovskite or spinel structures, as well as new structures with interleaved blocks of different anions. Mixed anion compounds have new degrees of freedom (cis, trans, etc.) in their local structure; and there is a possibility that if these are integrated, new crystal arrangements may emerge. In this research, we will create innovative chemical and physical functions unique to mixed anions, such as electrical conductivity, optical properties, and dielectric properties, by fully controlling anion composition, as well as local anion structure, degree of anion order, and interleaved anion structure.
A02 “Understanding Mixed Anion Compounds: Analysis of Chemistry, Structure, and Electronic State”
Mixed anion compounds are a group of materials that have been rapidly developed in recent years, and have the potential to be instrumental to innovation in solid chemistry and its application and to element strategies. However, conventional research methods for oxides do not work for this group of materials; and even a method of evaluation and analysis to determine their crystal structure has yet to be established. For example, in both X-ray and neutron diffractometry of oxyfluoride (O2－-F－) mixed anion compounds, the scattering cross-sections are close to each other, making it impossible to determine the arrangement of anions. The stable presence of hydride ions (H－) in oxides has recently been widely accepted, as a result of research conducted with crystal diffractometry, solid nuclear magnetic resonance spectroscopy, and theoretical computation.
The ‘structure’ of the newly synthesized material must first be determined, in order to proceed with the synthesis process. And if the ‘state’ of the material is unknown, the source of function creation cannot be determined. The purpose of this research is to obtain information on structures unique to mixed anion systems, which will be central to the search for new mixed anion compounds and the development of functions; and to understand and predict the function of such materials. We aim to resolve the current issue of material evaluation, by applying this understanding to specific mixed anion systems; and to generalize the analytical technique and accelerate the search for materials. In collaboration with leading experts in the latest measurement techniques, including atomic-resolution analytical electron microscopy, single-crystal diffractometry, and powder diffractometry; and in orientation analysis; we will explore issues such as prompt determination of crystal structure, distinguishing oxygen from fluorine, and detecting mixed hydrides and anions with anomalous valence. Researchers who have deep experience in techniques such as X-ray absorptiometry, electron energy loss spectroscopy, solid nuclear magnetic resonance spectroscopy, the adsorption method, and mass spectrometry will work together to analyze low-dimensional materials, dynamic phenomena such as surface adsorption and desorption, and the state of elements on the surface of low crystalline materials and catalysts. We will use a means of substantiation and systematization by theoretical computation; and will predict properties by integrating, with theoretical computation, electrical, magnetic, and chemical experimental data accumulated mainly by experts in such computation, in this group and groups A01 and 03. In particular, we will develop a research structure that enables use of methods requiring high computation loads capable of handling lattice vibrations, for analyzing such things as dielectric and thermoelectric properties, as well as electronic state, thermodynamic variables, and mechanical properties; and will discover novel properties and functionality in collaboration with the A03 research. Finally, we will seek to generalize the methodology, and improve efficiency in the evaluation of new materials created in the A01 research.
A03 “Creation of New Chemical and Physical Functions of Mixed Anion Compounds”
In recent years, it has been shown that in mixed anion compounds containing several different anions such as oxygen, nitrogen, fluorine, and hydrogen, unique coordination and crystal structure structures are obtained from mixed anion compounds, which cannot be obtained from single-anion compounds such as oxides; leading to the creation of innovative functions (e.g., photocatalytic , ferroelectrical, superconductive, fluorescent, and electrochemical properties) fundamentally different from those obtained from a single-anion systems. For example, visible absorption, which was not possible with single-anion systems, has become possible with an oxynitride (GaN-ZnO) solid solution; and visible-light-driven overall water splitting, which had never been accomplished in more than 40 years of water splitting photocatalysis research, was achieved using a powder photocatalyst. It has been reported that ferroelectricity was observed for the first time in an oxynitride (SrTaO2N) thin film. It has also been reported that when part of the oxygen site of an iron arsenide superconductor is replaced with hydrogen anions, a large number of conduction electrons are doped, and a new superconducting region occurs. Mixed anion compounds, therefore, have the potential to create high functionality, exceeding even that of perovskite oxides, which have been called a “gold mine of functionalities.”
With such a function-creation approach, and a major goal of solving energy problems, which are Japan’s Achilles heel, this research group will develop new functional materials composed of mixed anion compounds, which will contribute to creating and saving energy. Specifically, a total of eight teams will collaborate organically: photocatalysis and photoelectrodes (Maeda); ion conduction (Ishihara); secondary battery active materials (Uchimoto); fluorescent materials (Tabe); dielectric, magnetic, and thin film materials (Hasegawa); thermoelectric materials (Mori); superconductivity (Matsuishi); and theoretical computation (Ushiyama). In this way, we will create innovative functions unique to mixed anion compounds. Furthermore, we will improve the traditional research approach, whereby research projects are conducted individually with little or no interaction between researchers in different fields, and develop a new area of materials chemistry focusing on mixed anions, in collaboration both with the A01 research, which is developing a high-level synthesis technique essential to improving material functionality, and with the A02 research, which focuses on the investigation of structural properties and the source of function creation.