CRYSTAL STRUCTURE, EQUATION OF STATE AND PRESSURE-INDUCED PHASE TRANSITION(S) IN B4C: IN-SITU SYNCHROTRON XRD AND RAMAN STUDY TO 55 GPA

Boron carbides have stimulated strong interest not only because of their potential armor and industrial applications as high-temperature semiconductors or hard materials, but also because of their unusual crystal-chemical, structural and bonding features, which influence their structure-property relationships. Recent measurements of the elastic and vibrational properties of B4C under compression up to to 55 GPa are reported and discussed in the context of the changes in structure and bonding. The results from ultrasonic, synchrotron X-ray powder diffraction (XRD), and Raman scattering studies in a diamond-anvil cell, in conjunction with single-crystal XRD at ambient conditions, are compared with shock-wave data in order to understand the compression behavior, to establish the equations of state (pressure-volume relationships), and to verify the previously reported and predicted phase transition(s)/distortional changes. The effects of the in-situ laser-annealing/heating of the sample (to ~ 1500 °C) and the evolution of stress within the sample in the diamond-anvil cell are addressed in interpreting the pressure-induced structural change(s) indicated by XRD measurements. Preliminary results from the in-situ electrical resistivity measurements and optical studies are also presented.