Revisiting the High-Pressure Behaviors of Zirconium: Nonhydrostaticity Promoting the Phase Transitions and Absence of the Isostructural Phase Transition in ß-Zirconium.

Zirconium (Zr) is an important industrial metal that is widely used in nuclear engineering, chemical engineering, and space and aeronautic engineering because of its unique properties. The high-pressure behaviors of Zr have been widely investigated in the past several decades. However, the controversies still remain in terms of the phase transition (PT) pressures and the isostructural PT in ß-Zr: why the PT pressure in Zr is so scattered, and whether the ß to ß' PT exists. In the present study, to address these two issues, the Zr sample with ultra-high purity (>99.99%) was quasi-hydrostatically compressed up to ~70 GPa. We discovered that both the purity and the stress state of the sample (the grade of hydrostaticity/nonhydrosaticity) affect the PT pressure of Zr, while the stress state is the dominant factor, the nonhydrostaticity significantly promotes the PT of Zr. We also propose two reasons why the ß-ß' isostructural PT was absent in the subsequent and present experiments, which call for further investigation of Zr under quasi-compression up to 200 GPa or even higher pressures.

[Fluorescent Characteristics of Strontium Tetraborate (SrB4O7) Doped with Divalent Lanthanide Elements].

Strontium borate doping with different lanthanide bivalent ions and concentration (SrB4O : Re2+) were synthesized by the high-temperature solid state method. The fluorescent spectral characteristics of SrB4O7 : Re2+ were investigated by the non-polarization con-foucus fluorescence/raman measurement system built by us. The results indicate that the fluorescent spectral characteristics of SrB4O7 : Re2+ is very similar to that of SrB4O7 : Sm2+. The most strong fluorescence line (0-0 line) arises from 5D0 - 7F0 electron transition and the wavelength is 685.41 nm. In addition, two fluorescent bands coming from 5D0 - 7F1 and 5D0 -7F2 electron transition are observed near 700 and 730 nm, respectively. The intensity of 0-0 line of SrB4O7 : Re2+ is at least a magnitude smaller than that of SrB4O7 : Sm2+. A further study on the fluorescent spectrums of SrB4O7 : Re2+ shows that the doping elements and concentration both are the key points that affect the intensity of the fluorescent peaks, which directly decide the amount of Re2+ concerned with irradiance.

[High temperature Raman spectrum study of LiTaO3].

The high temperature Raman spectrum of LiTaO3 was studied from 298 to 948 K by micro-confocal Raman technique and the mechanism of phase transition from ferroelectric phase to paraelectric phase was studied by analyzing the high wave number Raman peaks of LiTaO3. The experimental results show that all the Raman peaks except 466.7 cm(-1) have a remarkable Raman shift to low frequency when increasing temperature. The full width at half maximum (FWHM) of all Raman peaks increases with increasing temperature, but the intensity weakens. At about 933 K, the phase transition is observed since the three Raman peaks disappear. And at about this temperature, the FWHM of 359.5, 385.0 and 466.7 cm(-1) shows an obviously super-linear increasing. From the experimental results, we conclude that the phase transition is a mixed phase transition, and the phase transition is reversible.

Yield strength of molybdenum at high pressures.

In the diamond anvil cell technology, the pressure gradient approach is one of the three major methods in determining the yield strength for various materials at high pressures. In the present work, by in situ measuring the thickness of the sample foil, we have improved the traditional technique in this method. Based on this modification, the yield strength of molybdenum at pressures has been measured. Our main experimental conclusions are as follows: (1) The measured yield strength data for three samples with different initial thickness (100, 250, and 500 microm) are in good agreement above a peak pressure of 10 GPa. (2) The measured yield strength can be fitted into a linear formula Y=0.48(+/-0.19)+0.14(+/-0.01)P (Y and P denote the yield strength and local pressure, respectively, both of them are in gigapascals) in the local pressure range of 8-21 GPa. This result is in good agreement with both Y=0.46+0.13P determined in the pressure range of 5-24 GPa measured by the radial x-ray diffraction technique and the previous shock wave data below 10 GPa. (3) The zero-pressure yield strength of Mo is 0.5 GPa when we extrapolate our experimental data into the ambient pressure. It is close to the tensile strength of 0.7 GPa determined by Bridgman [Phys. Rev. 48, 825 (1934)] previously. The modified method described in this article therefore provides the confidence in determination of the yield strength at high pressures.