In this article, recent advances in the fundamental and applied high-pressure synthesis of rare-earth borides, especially RB12, were reviewed. High-pressure equipment using the Walker module has been first introduced to Ibaraki University. It seems very convenient for primary users of high-pressure synthesis to use the equipment. We expect novel materials which have exotic physical properties.
Hydrogen storage technique is one of the key technologies to create hydrogen energy society. In this article, high-pressure synthesis of novel alloys and hydrides for hydrogen media was reviewed. New intermetallic compounds and hydrides were synthesized by using multi-cubic anvil system. For example, new MgNi intermetallic compound with a CuTi type structure was synthesized at 6 GPa and 300℃. This compound was stable in ambient pressure and hydrogenated/dehydrogenated reversibly.
We searched for new superconductors using cubic-anvil-type high-pressure apparatus. Samples including valence skip elements such as Bi, Sb were intensively investigated. As a result, we have found new superconductors of LaBi3 and Ba2Bi3 with the superconducting transition temperature (Tc) of 7.3 K and 4.4 K, respectively. We also succeeded in enhancing Tc of Au-Sb alloys and SrBi3 up to 8.1 K and 9.0 K, respectively, by controlling their chemical compositions. In this article, we explain the properties of the superconductors and the research processes that led to the discovery of the new superconductors.
High-pressure X-ray diffraction study by using synchrotron radiation source was carried out to determine the synthetic condition of high-pressure synthesis. We also open our cell assembly of multi anvil apparatus which is readily available for beginners of high-pressure experiment. The synthetic condition of Mg2Si, which is expected to be inexpensive thermoelectric material, was explored by means of X-ray diffraction study. High-pressure synthesis of Mg2Si was performed by piston cylinder equipment. Thermoelectric properties of Mg2Si including Seebeck coefficient and electrical conductivity were measured at room pressure and high temperatures. A simple measurement system we develop enables us to measure the thermoelectric properties of small-sized synthesized sample under pressure.
Recent advances in novel perovskite oxide catalysts synthesized under high pressure are reviewed. The simple Fe4+-perovskite oxides CaFeO3 and SrFeO3 demonstrate high catalytic activity for oxygen evolution reaction (OER) compared with the isoelectronic compound LaMnO3. The quadruple Fe4+-perovskite oxide CaCu3Fe4O12 displays a high stability in OER conditions, together with its excellent catalytic activity comparable to or exceeding state-of-the-art OER catalysts. The catalytic activity and stability for CaCu3Fe4O12 are attributed to the characteristic crystal structure of the quadruple perovskite.
We have studied the antiperovskite superconductors and layered Ni oxide Nd3.5Sm0.5Ni3O8 which has very similar crystal and electrical structure with high-Tc cuprate. In both systems, the materials are not high-pressure stabilized phases. For synthesis of antiperovskite material, high-pressure apparatus has been used to avoid the evaporation from the starting material with high vapor pressure. For layered Ni oxide, high-pressure apparatus has been utilized to make high density powder compact for the electrical resistivity measurement, because Nd3.5Sm0.5Ni3O8 can be obtained only with powder form. In this article, the research of layered Ni oxide Nd3.5Sm0.5Ni3O8 system is introduced.