This was done by a series of calculations of the total energy as a function of volume for the tetragonal and hexagonal phases of new B-N compounds. The resulting E(V) curves, shown in Fig. , were fitted to the third-order Birch equations of state : 3 , where E 0 , V 0 , B 0 , and B ʹ are the equilibrium energy; volume; bulk modulus; and its first pressure derivative, respectively. The calculated values are summarized in Table . In the case of boron sesquinitride B 2 N 3 (Fig. ), the E(V) curve of tet-B 2 N 3 remains at slightly lower energy and higher volume than h-B 2 N 3 , but both curves remain close. Quantitatively, this is translated by close equilibrium values (Table ) due to the close densities of the two phases. Larger differences are observed for the equiatomic phases (Fig. ), where the tetragonal B 3 N 3 systematically has lower energy and larger volume than the hexagonal one. An intersection of the E(V) curves is observed at a volume of 9.74 Å 3 per BN formula unit. The corresponding pressure of 200 GPa was estimated by the Murnaghan equation using the V 0 , B 0 and B ʹ values from Table .