Li Yan-Chun

The Phase Transition of Eu2O3 under High Pressures

Pressure-induced phase transition of cubic Eu2O3 is studied by angle-dispersive x-ray diffraction (ADXD) up to 42.3 GPa at room temperature. A structural transformation from a cubic phase to a hexagonal phase is observed, which starts at 5.0 GPa and finishes at about 13.1 GPa. The phase transition leads to a volume collapse of 9.0% at 8.6 GPa. The hexagonal phase of Eu2O3 maintains stable up to the highest experiment pressure. After release of pressure, the high-pressure phase transforms to a monoclinic phase. The pressure-volume data are fitted with the Birch-Murnaghan equation of state. The bulk moduli obtained upon compression from the fitting are 145(2) GPa and 151(6) GPa for the cubic and hexagonal phases, respectively, when their first pressure derivatives are fixed at 4.

Isostructural Phase Transition of TiN under High Pressure

In situ high-pressure energy dispersive x-ray diffraction experiments on polycrystalline powder TiN with NaCl-type structure have been conducted with the pressure up to 30.1 GPa by using the diamond anvil cell instrument with synchrotron radiation at room tempearture. The experimental results suggested that an isostructural phase transition might exist at about 7 GPa as revealed by the discontinuity of V/V0 with pressure.In situ high-pressure energy dispersive x-ray diffraction experiments on polycrystalline powder TiN with NaCl-type structure have been conducted with the pressure up to 30.1 GPa by using a diamond anvil cell instrument with synchrotron radiation at room temperature. The experimental results suggest that an isostructural phase transition might exist at about 7 GPa as revealed by the discontinuity of V/V0 with pressure.

Phase Transition of Graphitic-C3N4 under High Pressure by In Situ Resistance Measurement in a Diamond Anvil Cell

In situ resistance measurement of Graphitic-C3N4 has been performed under high pressure in a diamond anvil cell. The result reveals that there are changes of electron transport behaviour. As the pressure increases from ambient to 30 GPa, three abnormal resistance changes can be found at room temperature and two are found at 77 K. The abnormal resistance dropped at 5 GPa is close to the phase transition pressure from the Pm2 structure to the p structure predicted by Lowther et al. [Phys. Rev. B 59 (1999) 11683] Another abnormal change of resistance at 12 GPa is related to the phase transition from g-C3N4 to cubic-C3N4 [Teter and Hemley, Science 271 (1996) 53].

In situ X-ray diffraction investigation of compression behavior in Gd40Y16Al24Co20 bulk metallic glass under high pressure with synchrotron radiation

The compression behavior of the heavy RE-based BMG Gd40Y16Al24Co20 under high pressure has been investigated by in situ high pressure angle dispersive X-ray diffraction measurements using synchrotron radiation in the pressure range of 0~33.42 GPa at room temperature. By fitting the static equation of state at room temperature, we find the value of bulk modulus B is 61.27±4 GPa which is in good agreement with the experimental study by pulse-echo techniques of 58 GPa. The results show that the amorphous structure in the heavy RE-based BMG Gd40Y16Al24Co20 keeps quite stable up to 33.42 GPa although its compressibility is as large as about 33%. The coexistence of normal local structure similar to that of other BMGs and covalent bond structure similar to those of oxide glasses may be the reason for the anomalous property under high pressure of the Gd40Y16Al24Co20 BMG.

Pressure-induced phase transition of wulfenite

The in-situ high-pressure structures of wulfenite have been investigated by means of angular dispersive X-ray diffraction with diamond anvil cell and synchrotron radiation. In the pressure up to 22.9 GPa, a pressure-induced scheelite-to-fergusonite transition is observed at about 10.6 GPa. The pressure dependence for the lattice parameters of wulfenite is reported, and the axial compression coefficients Ka0 = −1.36 × 10−3 GPa−1 and Kc0 = −2.78×10−3 GPa−1 are given. The room-temperature isothermal bulk modulus is also obtained by fitting the P-V data using the Murnaghan equation of state.

Phase Transition and EOS of Marmatite (Zn0.76Fe0.23S) up to 623 K and 17 GPa

In situ energy dispersive x-ray diffraction for natural marmatite (Zn0.76Fe0.23S) is performed up to 17.7 GPa and 623 K. It is fitted by the Birch–Murnaghan equation of state (EOS) that K0 and α0 for marmatite are 85(3)GPa and 0.79(16)×10−4 K−1, respectively. Fe2+ isomorphic replacing to Zn2+ in natural crystal is responsible for high bulk modulus and thermal expansivity of marmatite. Temperature derivative of bulk modulus (∂K/∂T)P for marmatite is fitted to be -0.044(23) GPaK−1. The unambiguous B3–B1 phase boundaries for marmatite are determined to be Pupper(GPa) = 15.50−0.016T(°C) and Plower(GPa)=9.94–0.012T(°C) at 300–623 K.

In-Situ Conductivity Measurement of BaF2 under High Pressure and High Temperature

We perform the in-situ conductivity measurement on BaF2 at high pressure using a microcircuit fabricated on a diamond anvil cell. The results show that BaF2 initially exhibits the electrical property of an insulator at pressure below 25 GPa, it transforms to a wide energy gap semiconductor at pressure from 25 to 30 GPa, and the conductivity increases gradually with increasing pressure from 30 GPa. However, the metallization predicted by theoretical calculation at 30–33 GPa cannot be observed. In addition, we measure the temperature dependence of the conductivity at several pressures and obtain the relationship between the energy gap and pressure. Based on the experimental data, it is predicted that BaF2 would transform to a metal at about 87 GPa and ambient temperature. The conductivity of BaF2 reaches the order of 10−3Ω−1cm−1 at 37 GPa and 2400 K, the superionic conduction is not observed during the experiments, indicating the application of pressure elevates greatly the transition temperature of the superionic conduction.

Compressibility of lattice parameters of several layered compounds

The lattice parameters of AlB2, MgB2 and TiB2 under pressures are determined with a high-energy synchrotron source in a diamond anvil cell. The experimental results indicate that these three compounds have different mechanical behaviour under pressures, TiB2 is the hardest and MgB2 is the softest among the three materials. The phenomena are explained in terms of bonding strength in the crystal. Our results may be helpful for understating the decrease of the superconducting transition temperature of MgB2 under pressures.

Pressure-induced phase transition in BaTiO(3) nanocrystals

The crystal structure and electric properties of BaTiO(3) nanocrystals are studied by in situ high-pressure synchrotron radiation x-ray powder diffraction. The phase transition takes place not only in the samples of BaTiO(3) 14 nanocrystals that are tetragonal phase with grain sizes more than 100 nm, but also in the samples of BaTiO(3) nanocrystals that are cubic phase with grain sizes less than 100 nm. The pressures of phase transition are found to increase with decrease of the grain size from about 4 to 10 GPa, for crystallites ranging from 200 to 10,nm in radius. The bulk moduli are calculated according to Birch-Murnaghan state equation before and after the phase transition.

Structural and magnetic transition in stainless steel Fe-21Cr-6Ni-9Mn up to 250 GPa

Stainless steel Fe-21Cr-6Ni-9Mn (SS 21-6-9), with similar to 21% Cr, similar to 6% Ni, and similar to 9% Mn in weight percentage, has wide applications in extensive fields. In the present study, SS 21-6-9 is compressed up to 250 GPa, and its crystal structures and compressive behaviors are investigated simultaneously using the synchrotron angle-dispersive x-ray diffraction technique. The SS 21-6-9 undergoes a structural phase transition from fcc to hcp structure at similar to 12.8 GPa with neglectable volume collapse within the determination error under the quasi-hydrostatic environment. The hcp structure remains stable up to the highest pressure of 250 GPa in the present experiments. The antiferromagnetic-to-nonmagnetic state transition of hcp SS 21-6-9 with the changes of inconspicuous density and structure, is discovered at similar to 50 GPa, and revealed by the significant change in c/a ratio. The hcp SS-21-6-9 is compressive anisotropic: it is more compressive in the c-axis direction than in the a-axis direction. Both the equations of states (EOSs) of fcc and hcp SS 21-6-9, which are in accordance with those of fcc and hcp pure irons respectively, are also presented. Furthermore, the c/a ratio of hcp SS 21-6-9 at infinite compression, R infinity, is consistent with the values of pure iron and Fe-10Ni alloy.