The Li3YCl6 sample crystallized in the trigonal 𝑃3 ̅ 𝑚1 (#164) space group, as does Li3YBrxCl6-x with x=1.5. The materials with x>1.5, as well as the full bromide Li3YBr6, crystallize in monoclinic 𝐶2/𝑚 (#12) space group. Due to the larger anion radius of Br, the lattice parameters increase as Br is added to the system (Figure ). Between the compositions Li3YBrxCl6-x x=1.5 and x=3, where the phase transition occurs, the volume of the unit cell leads to the formation of platelets (SI Figure ), which corresponds to the disappearance of the Bragg peaks between 2θ=18-25° (SI Figure ). This suggests that the anisotropic, platelet-like morphology, which can show as preferred orientation due to non-random orientation of the powder when the x-ray diffraction data is measured in Bragg-Brentano geometry, could be at least a source of the discrepancy. However, recent literature has reported the occurrence of stacking faults in Li3YCl6 17 and Li3HoBrxI6-x , presenting an alternative origin for the discrepancy (SI Text 1). As described in ref.