High-Pressure Investigation on Calcium Borohydride

Structural properties of calcium borohydride, Ca(BH 4 ) 2, containing α and β polymorphs are investigated under pressure by in situ synchrotron X-ray diffraction up to 13 GPa and by Raman spectroscopy up to 25 GPa. Results show that the β phase transforms to a highly disordered structure above 10.2 GPa, whereas the α phase is stable in the pressure range studied. Rietveld analysis of the X-ray diffraction pattern collected at ambient conditions shows that the α phase is an orthorhombic F 2 dd structure with lattice parameters a = 8.82(7) Å, b = 13.05 (10) Å, c = 7.404(5) Å, and V = 851.8(10) Å 3 (64.13 cm 3 /mol). The β phase was checked for all the suggested tetragonal structures, and the best match is obtained with space group P 4 2 / m; the lattice parameters are a = 6.88(4) Å, c = 4.32(5) Å, and V = 204.82(18) Å 3 (61.68 cm 3 /mol). Pressure volume data obtained for α-Ca(BH 4 ) 2 are fitted with the third-order Birch−Murnaghan equation of state that yields a bulk modulus value of K 0 = 22.95(4) GPa with K 0 ′ = 2.63(0.5).