High-pressure synthesis, characterization, and equation of state of cubic C-BN solid solutions

Synthesis of several samples across the cubic Cₗ (BN) ₁-ₗ solid solution (x=0. 3--0. 33, 0. 5, 0. 6) at pressures in excess of 30 GPa and temperatures above 1500 K indicates that they are isostructural with diamond and cubic BN (borazon). Measurement of the lattice parameters of C-BN samples quenched to ambient conditions shows that the solid solution between C (diamond) and cubic BN is nonideal, with unit-cell volumes up to 1% larger than predicted based on ideal mixing (Vegard's law). In addition, we have measured the zero-pressure, 300-K vibrational spectra for C₀. ₃ (BN) ₀. ₇. In the midinfrared absorption spectrum, we observe a reststrahlen band ranging from 1000--1120 cm^-1, and the Raman spectrum has a longitudinal optic mode at 1323 (2) cm^-1. Based on a comparison of the spectrum with that of diamond and cubic BN, we conclude that the bonding in cubic C-BN is partially ionic. In addition to these measurements on quenched samples, we have measured the isothermal bulk modulus of C₀. ₃₃ (BN) ₀. ₆₇ by x-ray diffraction through a diamond cell to over 100 GPa at 300 K. The bulk modulus is 355 (19) GPa, which is lower than those of diamond, cubic BN and the value predicted from ideal mixing between the end members, but is consistent with the nonideal expansion observed for the cubic C-BN lattice parameters.