High pressure polymorphism of LiBH(4) and of NaBH(4)

The pressure-induced structural changes in LiBH(4) and in NaBH(4) have been investigated experimentally up to 290 GPa by coupling Raman spectroscopy, infrared absorption spectroscopy and synchrotron X-ray diffraction. This data set is also analysed in the light of Density Functional Theory calculations performed up to 600 GPa. The [BH(4)](−) unit appears to be remarkably resistant under pressure. NaBH(4) remains stable in the known Pnma γ-phase up to 200 GPa and calculations predict a transition to a metallic polymeric C2/c phase at about 480 GPa. LiBH(4) is confirmed to exhibit a richer polymorphism. A new Pnma orthorhombic phase VI is found to be stable above 60 GPa and there are hints of a possible phase VII above 160 GPa. DFT calculations predict that two other high pressure LiBH(4) phases should appear at about 290 and 428 GPa. A very slight solubility of H(2) inside phases II, III and V of LiBH(4) is observed. A NaBH(4)(H(2))(0.5) complex is predicted to be stable above 150 GPa.