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 Néel 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.

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 Grüneisen parameter, bulk moduli (K0), and the pressure derivative (K0′) at zero pressure of the B1-type phase were estimated to be 1.51±0.03, 304±4 GPa, and 3.9±0.2 (K0″=−0.02), respectively.

Abstract. A discontinuous volume change was observedon the Hugoniot curve of corundum (c•-A1203) 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 860øC, 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 largerthan the theoretically predicted value of the corundumRh203 (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.

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 wüstite (FeO) demonstrates.

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