In this article, current status of high-pressure synthesis of boron nitride single crystals and their properties were reviewed. In the viewpoint of the nature of the wide band gap semiconductor, cBN has promising potential so as to be easily fabricated p-n domain. The intrinsic properties of cBN has not yet been, however, realized due to their defects and impurities. By using Ba-BN solvent system, high purity single crystals of cubic and also hexagonal BN can be obtained. In particular, high purity hBN single crystals exhibit promising nature as a new candidate of deep ultraviolet-light emitter. Further study to realize semiconducting nature in hBN is important for the future work.
High-pressure synthesis is a powerful technique in searching for new materials. We have found a large number of functional oxides using this technique. In this article, three topics will be reviewed from our recent works. These are search for magnetic ferroelectrics in Bi, Pb-3d transition metal perovskites, single crystal growth of oxychloride superconductor, Ca2-xNaxCuO2Cl2, and pressure-induced structural transition in spin ladder compound, SrCu2O3.
High pressure technique still appears to be an important tool for the synthesis of advanced new materials. The current topics of high pressure synthesis of intermetallic phases, achieved in our laboratory, are presented and discussed in terms of crystal chemical aspect. The phase transition sequences in transition metal dipnictides and dichalcogenides are schematically summarized. The other topics include the synthesis of novel macro-tetrahedral compounds in boro-sulfide system and the atom insertion into the cage of the skutterudite host lattice.
In this article we describe the possibility of the improvement of thermoelectric properties of polycrystalline materials through means of high pressure sintering. We made an attempt to reduce the thermal conductivity, which is one of the important parameter of thermoelectric properties, by the high pressure sintering of nano-crystalline particles. It was confirmed that the thermal conductivity of ZnO, CoSb3, and Bi2Te3 are actually reduced by using this method and the dimensionless figure of merits of these materials efficiency at room temperature are improved remarkably.
High-pressure synthesis under giga-pascal hydrogen is attractive method to discover new metal hydrides with high hydrogen storage capacity. Recently, we have succeeded to prepare a series of new hydrides Mg7MHx (M=Ti, V, Nb, Hf, Ta, etc) with a cubic super-lattice structure (Ca7Ge-type) at 8 GPa and 600°C. One of them, Mg7TiHx had the theoretical hydrogen capacity of 6.9 mass%, and exhibited lower hydrogen desorption temperatures of ca 150°C than MgH2. The detailed crystal structure analysis showed that the Mg-H interatomic distance in Mg7TiHx was longer than that in MgH2, explaining the lowered desorption temperature.
The history of challenges for diamond anvil cell NMR is reviewed in this article. Pros and cons of the newly emerging method are discussed first. All previous works are reviewed briefly. Our state-of-the-art techniques are presented in detail. These are (i) a new DAC design for large-volume sample to be compressed, (ii) a new RF probe design for higher sensitivity, and (iii) a magnetically tuned gasket for better static field. Using them, we have obtained high-resolution NMR spectra of liquid methanol to 4.5 GPa that is factor four larger than previous studies. These results and their implication are presented.
The results of electrical resistivity measurements for β’-(BEDT-TTF)2ICl2 under extremely high pressures are reviewed. We have found that the ambient-pressure Mott insulator, β’-(BEDT-TTF)2ICl2, shows superconductivity under extreme pressures above 7.0 GPa. The superconducting temperature TC which reached 14.2 K as an onset renewed the previous records of the organic charge-transfer complexes. The cubic anvil high pressure cell and the turn-buckle type diamond anvil high pressure cell which were used for electrical measurements up to 11 GPa are also introduced in this article.