High-pressure phase of NaBH₄: Crystal structure from synchrotron powder diffraction data

The ambient pressure fcc phase of NaBH₄ transforms into a tetragonal P-42₁c near 6 GPa and then into an orthorhombic Pnma phase above 8 GPa. While the structures of the cubic and tetragonal phases are known, the structural information for the orthorhombic phase was limited to the space group and cell parameters. Here, we present synchrotron powder diffraction data for NaBH₄ collected in a diamond anvil cell up to 11.2 GPa. A BaSO₄-type structure of the orthorhombic high-pressure phase was solved ab initio from diffraction data and refined by the Rietveld method. Both tetragonal and orthorhombic phases show pronounced preferred orientation of crystallites with respect to the compression direction.