The crystal structures of the trigonal and monoclinic phases have distinct structural characteristics (Figure ). The trigonal structure with the compositions Li3YBrxCl6-x at x=0 and 1.5 is built by hexagonal close-packing-like arrangement of the halogen atoms, with the Y and Li in octahedral positions. The Rietveld refinement shows that the Cl and Br atoms distribute statically across the halogen sites (as consistent with literature, see ref. ) and small differences in the Y and Li occupancies (SI tables S4-S5). The monoclinic phase with the compositions Li3YBrxCl6-x at x = 3, 4.5 and 6 on the other hand, shows cubic close packing of the halogen atoms. Similar to the trigonal phase, the Cl and Br distribute statistically across the halogen sites and the Y and Li reside on octahedral sites. Also here, there appear to be small redistributions of the Li and Y, but there is no clear trend as a function of x (SI tables S1-S3).  With the introduction of more (larger) Br, the lattice expands, rendering larger polyhedral faces ("bottlenecks") for the Li to jump through .