Research

The study of phase transitions in solid-state materials is an interdisciplinary field that has developed over a long period of time, attracting the interest of many researchers in both chemistry and physics, starting with the origin of interactions, the formation of short-range order, its development into long-range order, and the dynamics of aggregates dominated by long-range order.

The main focus of research in our laboratory is in the field of solid-state chemistry and solid-state physics. The aim is to control the bistability of materials through the material synthesis, to find phase transitions (switching phenomena) that have never been observed before, and to find metastable phases that have not been found before and to search for new physical properties.

Our concept is to understand the bistability and phase transitions of materials from the fundamental point of view and to find universality in many examples for the development of materials, and we conduct research daily.

Cyanide metal complexes and molecular-based magnets

Among molecular-based magnet, particular attention has been paid to cyanide-based metal complexes. We have reported various phase transition phenomena, such as charge-transfer structural phase transitions (temperature- and light-induced colour switching), light-induced phase collapse (optical phase transition from metastable to stable phase) and visible light-reversible photo-ferromagnetic-antiferromagnetic transitions (magnets are switched ON and OFF by blue and green light irradiations).

The relationship between phase transitions and phonon modes is also studied. For example, we have shown that there are specific phonon modes that induce charge transfer. Furthermore, we have used the temperature dependence of the formation enthalpy obtained from ab initio band calculations and the thermodynamic parameters (enthalpy, entropy, Gibbs free energy, etc.) obtained from ab initio phonon mode calculations to develop a computational method to discuss the possibility of phase transitions. In addition, we are everyday preparing materials, to synthesis new magnetic complexes and investigating their magnetic and switching properties.

Titanium oxide

There are many types of titanium dioxide, but our research group studies Ti3O5 and Ti4O7. In particular, lambda-Ti3O5 is a new type of titanium dioxide, first reported by our research group1 in 2010, and is a structure that can only exist in nanoparticles and nanocrystals. It is the only metal oxide that exhibits reversible photoswitching (photo-induced phase transition) at room temperature. Based on the pressure-induced phase transition, our research group2 has also reported that it exhibits unique thermal properties, namely pressure-responsive thermal storage properties. In our laboratory, the properties of lambda-type Ti3O5 have been controlled through material synthesis and various fractional measurements have been carried out to physically elucidate the phase transition phenomena. On the other hand, Ti4O7 is an interesting material that shows the highest electrical conductivity as titanium dioxide and exhibits a two-step phase transition from metal-disordered semiconductor-ordered semiconductor when the temperature is decrease from room temperature. In our laboratory, the physical properties of Ti4O7 are controlled by changing the particle size and morphology.

1 University of Tokyo, Ohkoshi Group, which belonged at the time. (S. Ohkoshi, et al., Nature Chem., 2, 539-545 (2010).)
2 Collaborative research with Ohkoshi Laboratory, University of Tokyo. (H. Tokoro, S. Ohkoshi, et al., Nature Commun., 6, 7037 (2015).)

Iron oxide, epsilon-type ferrite

We focus on epsilon-type iron oxide materials3 that exhibit a giant magnetic coercive force, which was first reported in 2004. We are investigating the origin of the anisotropic crystal growth of epsilon-type iron oxide and controlling its magnetic properties by varying the particle size. Furthermore, their potential as magnetic force microscopy (MFM) probes is investigated.

3 J. Jin, S. Ohkoshi, K. Hashimoto, Adv. Mater., 16, 48-51 (2004).

Single-molecule magnets showing diverse functionalities

As the demand for multifunctional sensors and devices increases, research on multifunctional materials, in which diverse functionalities are incorporated into a single material, has been active. We aim to construct single-molecule magnets, which show magnetic memory effect at sub-nanometer level, by utilizing lanthanide complexes and expand their physical functionalities.

For example, we reported a CoIII-YbIII-CoIII trinuclear complex, which behaves as single-molecule magnet and exhibits YbIII-centered near-infrared luminescence functioning as optical thermometer. The compound also shows superionic proton conductivity due to its abundance of H5O2+ cations. Thus, it is a first example that incorporates three diverse physical functionalities (optical, magnetic, and electrical) within a single-phase material.