High-pressure phase of $\mathrm{Na}\mathrm{B}{\mathrm{H}}_4$: Crystal structure from synchrotron powder diffraction data

The ambient pressure fcc phase of $\mathrm{Na}\mathrm{B}{\mathrm{H}}_4$ transforms into a tetragonal $P\text{−}{42}_{1}c$ near $6\mathrm{GPa}$ and then into an orthorhombic $Pnma$ phase above $8\mathrm{GPa}$. 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 $\mathrm{Na}\mathrm{B}{\mathrm{H}}_4$ collected in a diamond anvil cell up to $11.2\mathrm{GPa}$. A $\mathrm{Ba}\mathrm{S}{\mathrm{O}}_4$-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.

Figure 1

(Color online) Background-subtracted Rietveld refinement profile for the $Pnma$ phase of $\mathrm{Na}\mathrm{B}{\mathrm{H}}_4$ at $11.2\mathrm{GPa}$. A raw two-dimensional image is shown in the inset. $\lambda =0.71136\mathrm{\AA }$.

Figure 2

(Color online) Crystal structure of $\gamma \text{−}\mathrm{Na}\mathrm{B}{\mathrm{H}}_4$, space group $Pnma$.

Figure 3

XRD patterns recorded upon decompression of $\mathrm{Na}\mathrm{B}{\mathrm{H}}_4$.