Thermoelastic equation of state of boron suboxide $\mathrm{B_{6}O}$ up to 6 GPa and 2700 K: Simplified Anderson-Grüneisen model and thermodynamic consistency

The p-V-T equation of state of superhard boron suboxide $B_{6}O$ has been measured up to 6 GPa and 2700 K using multianvil technique and synchrotron X-ray diffraction. To fit the experimental data, the theoretical p-V-T equation of state has been derived in approximation of the constant value of the Anderson-Grüneisen parameter $\delta$$_{T}$. Bulk modulus B$_{0}$ =181 GPa and its first pressure derivative $B_{0}$$^\prime$ = 6 at 300 K; two parameters describing thermal expansion at 0.1 MPa, i.e., a = 1.4 × $10^{−5} K^{−1}$ and b = 5 × $10^{−9} K^{−2}$ as well as $\delta$$_{T}$ = 6 have been included in the model. The good agreement between fitted and experimental isobars has been achieved to the absolute volume changes up to 5% as compared to the volume at standard conditions, V$_{0}$. The fitted thermal expansion at 0.1 MPa well consistent with the experimental data as well as with ambient-pressure heat capacity c$_{p}$ , bulk modulus B$_{0}$ and $\delta$$_{T}$ describing its evolution with volume and temperature have been discussed. The fitted value of Grüneisen parameter $\gamma$ = 0.85 in agreement with previous empiric estimations for $B_{6}O$ and experimental values for other boron-rich solids has been reported