High-pressure high-temperature x-ray diffraction of β-boron to 30 GPa

The high-pressure/high-temperature behavior of boron has been investigated up to 30 GPa using x-ray diffraction techniques. We have investigated the isothermal compressibility of rhombohedral-105 (R-105) boron to 30.7 GPa, and determined the bulk modulus K0T to be 205(±16)GPa with K0T^'=4.3(±1.6). We found that the a direction of R-105 boron (hexagonal setting) is less compressible than c, which indicates that the B10-B-B10 chain is less rigid than the $B12$ cluster. High P-T studies were conducted using double-sided laser heating to temperatures of 3500 K. R-105 boron transforms above 10 GPa at high temperature to a tetragonal phase that remains on P-T quenching. The transformation results in both a higher density and higher entropy phase and is consistent with the formation of the T-192 phase of boron.