Ligang Bai

Pressure-induced phase transition in cubic Lu2O3

The phase transition in cubic Lu2O3 has been investigated by angle dispersive x-ray diffraction and Raman scattering in a diamond anvil cell up to 46.8 GPa and 40.2 GPa, respectively. The diffraction data indicated that a phase transition from the cubic to a monoclinic structure started at 12.7 GPa and completed at 18.2 GPa. This high pressure monoclinic phase is stable up to at least 46.8 GPa and can be quenched to ambient conditions. This irreversible cubic to monoclinic structural transformation has also been confirmed by Raman scattering measurements. A third-order Birch–Murnaghan fit based on the observed pressure-volume data yields a zero pressure bulk modulus of B0=214(6) GPa, its pressure derivative B0′=9(1) for the low-pressure cubic phase; and B0=218(13) GPa, B0′=2.3(3) for the high pressure monoclinic phase, respectively. The mode Gruneisen parameters of different Raman modes for both cubic and monoclinic phases have also been determined.

Pressure-induced phase transformations in cubic Gd2O3

High-pressure transformation and compression behavior of Gd2O3 were investigated using synchrotron radiation x-ray diffraction in a diamond anvil cell up to 44 GPa at room temperature. The structural transformation from a cubic to a monoclinic phase occurred during the sample precompression process. Phase transitions from both the cubic and the monoclinic polymorphs to a hexagonal structure were observed. The hexagonal phase was stable up to the highest pressure in this study and was not quenchable and transformed to a monoclinic phase after pressure release. The bulk moduli of Gd2O3 for the cubic, monoclinic, and hexagonal phases were obtained by fitting the compression data to the Birch–Murnaghan equation of state. Moreover, an anomaly of the hexagonal type Gd2O3 was observed.

Phase transformation of Ho2O3 at high pressure

The structural stability of cubic Ho2O3 under high pressure has been investigated by angle-dispersive x-ray diffraction (ADXD) in a diamond anvil cell up to 63.0 GPa at room temperature. The diffraction data reveal two structural transformations on compression. The structural transformation from a cubic to a monoclinic structure starts at 8.9 GPa and is complete at 16.3 GPa with a ∼8.1% volume collapse. A hexagonal phase begins to appear at ∼14.8 GPa and becomes dominant at 26.4 GPa. This high-pressure hexagonal phase with a small amount of retained monoclinic phase is stable up to the highest pressure of 63.0 GPa in this study. After release of pressure, the hexagonal phase transforms to a monoclinic structure. A third-order Birch-Murnaghan fit yields zero pressure bulk moduli (B0) of 206(3), 200(7) and 204(19) GPa and their pressure derivatives (B0’) of 4.8(4), 2.1(4), 3.8(5) for the cubic, monoclinic and hexagonal phases, respectively. Comparing with other rare-earth sesquioxides, it is suggested that ...

Phase transformation of Ho[subscript 2]O[subscript 3] at high pressure

The structural stability of cubic Ho2O3 under high pressure has been investigated by angle-dispersive x-ray diffraction (ADXD) in a diamond anvil cell up to 63.0 GPa at room temperature. The diffraction data reveal two structural transformations on compression. The structural transformation from a cubic to a monoclinic structure starts at 8.9 GPa and is complete at 16.3 GPa with a ∼8.1% volume collapse. A hexagonal phase begins to appear at ∼14.8 GPa and becomes dominant at 26.4 GPa. This high-pressure hexagonal phase with a small amount of retained monoclinic phase is stable up to the highest pressure of 63.0 GPa in this study. After release of pressure, the hexagonal phase transforms to a monoclinic structure. A third-order Birch-Murnaghan fit yields zero pressure bulk moduli (B0) of 206(3), 200(7) and 204(19) GPa and their pressure derivatives (B0’) of 4.8(4), 2.1(4), 3.8(5) for the cubic, monoclinic and hexagonal phases, respectively. Comparing with other rare-earth sesquioxides, it is suggested that ...

Strength and structural phase transitions of gadolinium at high pressure from radial X-ray diffraction

Lattice strength and structural phase transitions of gadolinium (Gd) were determined under nonhydrostatic compression up to 55 GPa using an angle-dispersive radial x-ray diffraction technique in a diamond-anvil cell at room temperature. Three new phases of fcc structure, dfcc structure, and new monoclinic structure were observed at 25 GPa, 34 GPa, and 53 GPa, respectively. The radial x-ray diffraction data yield a bulk modulus K0 = 36(1) GPa with its pressure derivate K0′ = 3.8(1) at the azimuthal angle between the diamond cell loading axis and the diffraction plane normal and diffraction plane ψ = 54.7°. With K0′ fixed at 4, the derived K0 is 34(1) GPa. In addition, analysis of diffraction data with lattice strain theory indicates that the ratio of differential stress to shear modulus (t/G) ranges from 0.011 to 0.014 at pressures of 12–55 GPa. Together with estimated high-pressure shear moduli, our results show that Gd can support a maximum differential stress of 0.41 GPa, while it starts to yield to pla...

Anomalous compression behaviour in Nd2O3 studied by x-ray diffraction and Raman spectroscopy

The structural stability of hexagonal Nd2O3 under pressure has been investigated by in situ synchrotron angle dispersive x-ray diffraction and Raman spectroscopy up to 53.1 GPa and 37.0 GPa, respectively. Rietveld analysis of the x-ray diffraction data indicate that the hexagonal Nd2O3 undergoes an isostructural phase transition in the pressure range from 10.2 to 20.3 GPa, accompanied by anomalous lattice compressibility and pressure-volume curve. A third-order Birch-Murnaghan fit based on the observed Pressure-Volume data yields zero pressure bulk moduli (B0) of 142(4) and 183(6) GPa for the low and high pressure hexagonal phases, respectively. Raman spectroscopy confirms this isostructural transition, the pressure dependence of the Raman modes display noticeable breaks in the pressure range of 9.7-20.9 GPa, which is consistent with the change of Nd-O bond length. The pressure coefficients of Raman peaks and the mode Gruneisen parameters of different Raman modes were also determined.

Strength and equation of state of molybdenum triboride under 80 GPa pressure, using radial X-ray diffraction

ABSTRACT The strength and equation of state of molybdenum triboride have been determined under nonhydrostatic compression up to 80 GPa, using an angle-dispersive radial X-ray diffraction technique in a diamond anvil cell (DAC). The RXD data yield a bulk modulus and its pressure derivative as K0 = 342(6) GPa with K0′ = 2.11(17) at ψ = 54.7°. Analysis of diffraction data using the strain theory indicates that the ratio of differential stress to shear modulus (t/G) ranges from 0.002 to 0.050 at pressures of 4–80 GPa. Together with theoretical results on the high pressure shear modulus, our results here show that molybdenum triboride sample under uniaxial compression can support a differential stress of ∼10 GPa when it started to yield with plastic deformation at ∼30 GPa. In addition, we draw a conclusion that MoB3 is not a superhard material but a hard material.