Yuichi Noguchi

Yield properties, phase transition, and equation of state of aluminum nitride (AlN) under shock compression up to 150 GPa

Inclined-mirror Hugoniot measurements were performed on pure AlN polycrystals in the pressure range up to 150 GPa to study the yield properties, phase transition, and equation of state. The Hugoniot-elastic limit (HEL) stress was approximately 9.4 GPa. Above the HEL, the Hugoniot data converged to a static compression curve despite the high thermal conductivity, which indicated that the thermal property is not an important factor in determining the shock yield property. The phase transformation from wurtzite-type (B4) to rock salt-type (B1) structure took place at approximately 19.4 GPa, and was completed by about 75 GPa. The corrected transition pressure at 298 K was 19.2 GPa. Shock velocity (Us) versus particle velocity (Up) relation of the final phase was given by Us=3.27+1.81Up km/s. The Birch–Murnaghan fitting curve of the calculated isothermal compression curve of the B1-type phase roughly coincided with the recent static x-ray diffraction data up to over 100 GPa. The Gruneisen parameter, bulk modul...

High-pressure phase transformation of corundum (α-Al2O3) observed under shock compression

A discontinuous volume change was observed on the Hugoniot curve of corundum (α-Al2O3) along the axis by the inclined-mirror streak photographic technique, probably due to a high-pressure phase transformation. The transition pressure was measured to be 79 GPa, where shock temparature was estimated to be 860°C, in good agreement with a fully optimized quantum mechanical calculation. The volume change at the transition point was considered to be larger than 3%, which was larger than the theoretically predicted value of the corundum-Rh2O3 (II) transition, and was consistent with the recent static compression result by using diamond-anvil cell. The occurence of the phase transformation in alumina under shock compression presents significant implication in earth interior science or ceramic science, and also seriously affects the interpretation of static and shock compresion experiments at ultra-high pressures.

Phase transitions of MnO to 137 GPa

An in situ x-ray study up to 137 GPa using a diamond anvil cell combined with synchrotron x-ray radiation has revealed three new phase transitions in MnO. The rhombohedral distortion from a B1 structure starts at about 30 GPa. The volume compression curve of the distorted phase is in good agreement with recent shock compression experiments, suggesting the possibility of a paramagnetic–antiferromagnetic transition associated with the increase of the Neel temperature (TN). A drastic change in the x-ray pattern was observed at about 90 and 120 GPa. The transition pressure of 90 GPa is also consistent with shock compression data. The phases above 90 GPa were expected to be metallic based on the highly reflective nature of the sample. The crystal structure of the high pressure phase above 120 GPa was successfully explained by a B8 (NiAs) structure as expected based on recent first principles calculations.

Shock‐induced phase transition of MnO around 90GPa

Shock compression experiments on single crystal MnO grown by the Verneuil method have been carried out for the pressure range of 41–114GPa using both propellant and two-stage light-gas guns. The shock states, recorded with the inclined mirror method, were analyzed by the free-surface approximation and impedance-match solution. The observed shock compression curve of MnO with the rock salt structure(B1) is found to be slightly more compressible than that measured statically to 60GPa using a diamond-anvil cell. We observed a phase transition around 90GPa with a volume decrease of approximately 8%. The present phase transition pressure falls on a trend of B1-B2(CsCl-type structure) transformation among alkaline earth metal monoxides, rather than transformation to the B8(NiAs-type structure) that wustite (FeO) demonstrates.

High-Pressure Phase Transformation of Cobalt Monoxide Due to Electronic Transition

High-pressure behavior of rocksalt-type CoO was investigated by shock and static experiments up to 106 and 131 GPa, respectively. A phase transition was detected at 81±1 GPa from the measurements of shock compression curve. X-ray diffraction study under static high-pressure revealed two phase transitions at about 80 GPa and 120 GPa. The crystal structure of both high-pressure phases was closely related to the rocksalt structure. The volume decrease accompanied by the phase transition was estimated to be about 5%. The high-pressure phase was reversibly transformed to the ambient phase on unloading. From comparison of recent first principle calculations, electronic transition with magnetic collapse in Co2+ was inferred.

Shock-induced phase transition of MnO and several other transition metal oxides

Shock compression measurements of MnO have been carried out up to 120 GPa, and a phase transition with a volume decrease, −ΔV/V0, of about 6 percent is observed at 90±3 GPa. Release adiabat measurements revealed that the volume difference between the high pressure phase and the rocksalt (B1) phase was estimated to be about 23 percent. Although the cesium chloride (B2) structure is a possible candidate for the high pressure phase from the systematics obtained for the B1-B2 transformation in alkaline earth monoxides, the observed large volume change suggests a drastic change in the electronic configuration of manganese ions at very high pressures. Metallic reflectivity observed in recent in situ observation of the high pressure phase in a diamond anvil cell and a metallic nickel arsenide (B8) structure predicted for the high pressure phase by theoretical calculations lend support for this interpretation. Comparison with the high pressure behavior of other transition metal oxides is also made.

Hugoniot measurement of alumina (Al2O3) single crystal in the pressure range up to over 100 GPa

A high-pressure phase transition of Al2O3 from the corundum structure to the Rh2O3(II) structure at over 70 GPa has been theoretically predicted, while sufficient experimental evidences have not been reported. In this study, the Hugoniot-measurement experiments were performed on a corundum-structure Al2O3 single crystal to obtain the informations about the high-pressure polymorph. The Hugoniot parameters were measured by the inclined-mirror method in the pressure range up to over 100 GPa. A possible kink due to phase transition was observed on the inclined-mirror images in the pressure range higher than 70 GPa.