Takumi Kikegawa

Phase transitions of LnAs (Ln = Pr, Nd, Sm, Gd, Dy and Ho) with NaCl-type structure at high pressures

By use of synchrotron radiation the powder x-ray diffraction of lanthanide monoarsenides LnAs (Ln = Pr, Nd, Sm, Gd, Dy and Ho) with a NaCl-type structure has been studied up to 60 GPa at room temperature. First-order phase transitions with crystallographic change were found at around 27.1 GPa for PrAs, 24.2 GPa for NdAs and 32.1 GPa for SmAs. The high-pressure form of the lighter LnAs (Ln = Pr, Nd and Sm) is a tetragonal structure and can be viewed as a distorted CsCl-type structure. The atoms in the tetragonal structure are located at Ln: 0,0,0; As: 1/2,1/2,1/2. The space group is P4/mmm. Pressure-induced phase transitions of the heavier LnAs (Ln = Gd, Dy and Ho) with many f electrons occur at around 36 GPa for GdAs, 44 GPa for DyAs and 46 GPa for HoAs. The structure of these high-pressure phases is unknown. The high-pressure structural behaviour of LnAs (Ln = Pr, Nd, Sm, Gd, Dy and Ho) with the NaCl-type structure is discussed.

Pressure-induced phase transitions of lanthanide monoarsenides LaAs and LuAs with a NaCl-type structure

Abstract By use of synchrotron radiation the powder X-ray diffraction of lanthanide monoarsenides LaAs and LuAs with a NaCl-type structure has been studied up to 60 GPa at room temperature. First-order phase transitions with the crystallographic change were found at around 20 GPa for LaAs, and 57 GPa for LuAs. The high-pressure form of LaAs is a tetragonal structure and can be viewed as a distorted CsCl-type structure. The atoms in the tetragonal structure are located at La: 0, 0, 0; As: 1/2, 1/2, 1/2. The space group is P4/mmm. The structural change to the tetragonal structure occurs with the volume collapse of about 10%. The structure of these high-pressure phases of LuAs is unknown. The volume vs. pressure curves for LaAs and LuAs are fitted by a Birch equation of state. The bulk moduli of both arsenides are 92±6 GPa for LaAs and 85±3 GPa for LuAs. The high-pressure structural behavior of LaX (X=P, As and Sb) and LnAs (Ln=lanthanide) with the NaCl-type structure is discussed.

X-ray study with synchrotron radiation for P- and Sb-based skutterudite compounds at high pressures

Using synchrotron radiation, the x-ray diffraction of P- and Sb-based skutterudite compounds has systematically been studied at room temperature and high pressures. Bulk moduli of binary skutterudites CoX3 (X = P and Sb), filled skutterudites CeT4X12 (T = Fe, Ru and Os), LaRu4X12and PrRu4X12 are obtained from the volume versus pressure curves fitted by a Birch equation of state. Bulk moduli are 150–225 GPa for P-based skutterudites and 80–115 GPa for Sb-based skutterudites. The bulk modulus of the phosphides is about two times that of the corresponding antimonides. The antimonides are very compressible compared with the phosphides. The Gruneisen constants (γ) for skutterudite compounds are estimated from the compressibility and its volume derivative in the Debye approximation. The bulk modulus of CoX3 and CeT4X12 increases with increasing lattice constant, attaining a maximum for the Ru compounds. Bulk moduli for the Ru-based skutterudites are larger than those of the corresponding Fe and Os compounds. This may be closely related to the interesting physical properties in the Ru-based skutterudites. The bulk modulus of the binary skutterudites is smaller than that of the filled skutterudites. The cell volume for CeT4X12 (T = Fe, Ru and Os; X = P and Sb) decreases monotonically with increasing pressure. The valence state of Ce ion in these compounds does not change at high pressures.

Effect of nonhydrostaticity on the pressure induced phase transformation of rhombohedral boron nitride

An in situ x-ray diffraction study of room-temperature compression of rhombohedral boron nitride (rBN) was performed up to 10 GPa. Although no phase transformation of rBN was observed in a liquid pressure transmitting medium, the structure of rBN changed to become disordered within the layered stacking sequence at less than 1 GPa with the solid-state pressure transmitting medium. Further transformation to the wurtzite structure (wBN) was observed at 6 GPa and was unquenchable upon the release of pressure at room temperature. The orientation relationship of the phase transformation of rBN to wBN was compared with that of hexagonal BN to wBN.

Unusual transition phenomenon in Zr-based bulk metallic glass upon heating at high pressure

Structural changes of the bulk metallic glass (BMG) Zr41.2Ti13.8Cu12.5Ni10Be22.5 were investigated at high pressure and high temperature with synchrotron radiation x-ray diffraction (SR-XRD). In situ SR-XRD measurements confirm that heating the BMG at a pressure of 10 GPa leads to an unusual sequence of transitions. The initial amorphous solid transforms into a crystalline phase at 582 K, and then the crystalline phase transforms back to an amorphous phase at 698 K. This crystalline-to-amorphous phase transition in a BMG is driven by increasing temperature at higher density produced by high pressure.

Formation of nanocrystalline MgB2 under high pressure

The microstructural features of MgB2 at ambient pressure and high pressure have been investigated by means of in situ synchrotron radiation x-ray diffraction and transmission electron microscopy (TEM). The x-ray diffraction measurements indicated that nanocrystalline MgB2 formed in the pressure range of 26.3?30.2 GPa. TEM investigations reveal complex structure domains with evident lattice distortion in the relevant samples. The superconductivity of nanocrystalline MgB2 was measured and compared with that of the starting sample of MgB2.