Preparation and crystal structure of Li6Zr2O7 and Li6Hf2O7

Li₆Zr₂O₇ was obtained by annealing an intimate mixture of LiOH · H₂O and freshly prepared ZrO₂ in a stream of argon. It is monoclinic: C2/c, a = 1 044.5(1), b = 598.9(1), c = 1 020.0(1) pm, β = 100.26(1)°, Z = 4, R = 0.016 for 1 218 F values and 55 variables. The structure is closely related to that of NaCl with an ordered distribution of the metal atoms on the sodium sites while the oxygen atoms occupy seven eighths of the chlorine positions. Li has square pyramidal, Zr octahedral oxygen coordination. The corresponding Hf compound is isotypic: a = 1 040.2(1), b = 596,2(1), c = 1 015.0(1) pm, β = 100.36(1)°. ⁷Li nuclear magnetic resonance spectra of this compound give no indication for a high mobility of the Li⁺ ions.