A history of the past researches on materials having nano spaces inside is described in connection to the present status and the future prospect. Specific features of such a family of nano materials are categorized and some of the important issues are discussed in more details by showing experimental examples. Atoms and molecules confined inside the nano spaces are focused from the viewpoint of anharmonic oscillations or energy non-dissipative isolation from outside. Glass-like low thermal conductivity and superconductivity appearing in this system are discussed on a basis of phonons.
The use of high pressure and high temperature conditions is essential in the preparation of silicon clathrate compounds with electron excess compositions and related Si-rich binary compounds. The preparation conditions and crystal structures of Ba8Si46, Ba24Si100, BaSi6, LaSi5, LaSi10, NaxSi136 (x>24) and related Zintl silicides are discussed. These are the systems rich in superconductors.
This article reviews structural phase transitions of Zintl-phase silicide BaSi2 at high pressures and high temperatures and a formation of Si clathrate Ba8Si46 from an 8:30 molar mixture of BaSi2 and Si at high pressures and high temperatures. At pressures ranging from 0 to 10 GPa and temperatures ranging from 300 to 1300 K, BaSi2 has four phases: BaSi2 phase, EuGe2 phase, SrSi2 phase, and BaSi2-IV phase. When heated at approximately 5 GPa, BaSi2 undergoes the three structural phase transitions: the BaSi2-to-EuGe2, the EuGe2-to-SrSi2, and the SrSi2-to-BaSi2-IV transitions. At 4.3 GPa, Ba8Si46 is formed by a solid state reaction of an 8:30 molar mixture of SrSi2-phase BaSi2 and Si after BaSi2 undergoes the BaSi2-to-EuGe2 and the EuGe2-to-SrSi2 transitions. The structural phase transitions of BaSi2 and formation process of Ba8Si46 are discussed comparing it with structural phase transitions of SrSi2 and of an 8:30 mixture of SrSi2-pahse SrSi2 and Si.
The pressure effects of physical/chemical properties of a material are important to realize a relationship between its structure and the properties. Characteristic properties exhibited in some materials with nano spaces are particularly affected by pressure because the densification occurs selectively around the porous or weak-bonded portions in their structures. However, pressure distribution and shear stress often hides a critical phenomenon caused by pressure. Highly hydrostatic helium pressure medium can solve the problem and provide high quality experimental data. This report introduces some experimental examples, a high-pressure Raman scattering spectroscopy on α-boron and a powder x-ray diffraction study on type-I′ Ge-clathrate Ba24Ge100, using the helium pressure.
A review is given for the properties of boron and boron-rich solids, whereas the special roles of high pressure research are emphasized through recent progresses in this exotic material. The topics discussed encompass phase transitions, metal/insulator transition, and doping issue. Hardness and internal flexibility along with electron deficiency are key characters of boron solids. The direct metal-transition by band overlap which α-boron exhibits at high pressure is understood on this ground. Characterizing materials from electronic stiffness and internal degree of freedom provides a good guideline for high-pressure material researches.
In this article, we have developed pressure loading technique to extend the loading density of guest alkali metals in nanoporous crystals of zeolites. Zeolite crystals have solid framework and open regular nanospace. In the nanocages of zeolite crystals, s-electrons provided by the guest alkali metals occupy quantum electronic states of clusters successively. Under the pressure by the use of the pressure medium of alkali metals, we have observed significant change in the magnetic properties, indicating that the pressure loading increases loading densities which are closely related to the s-electron occupation at the quantum states of clusters.
This review describes a development of an angle-dispersive step-scan diffraction technique, in which a series of energy-dispersive diffraction data are collected at various 2θ angles. The technique was developed by the GeoSoilEnviroCARS, the University of Chicago in the United States, while experiments were conducted at the SPring-8, Japan. Resultant 2-dimensional diffraction data enable us to recognize very small peaks, such as superlattice reflections. This technique is significantly useful to determine crystal symmetry from powder diffraction when X-ray access to sample is limited.