Structural characterization of a new acentric protonated garnet: Li _(6-x)H_xCaLa_2Nb_2O_(12)

摘要

A new acentric protonated garnet Li_6-xH xCaLa_2Nb _2O_(12) has been synthesized and structurally characterized from Rietveld refinement of high-resolution neutron diffraction data. This phase can be prepared by Li~+/H~+ exchange on the mother garnet Li_6CaLa_2Nb_2O_(12) in acetic acid heated at reflux for 4 days, conditions determined after several tests varying acid solution, time, and temperature. Li_(6-x)H_xCaLa _2Nb_2O_(12) crystallizes oup I43d (no. 220) with the cell parameter a = 12.8040(3) ?. The noncentrosymmetry has been confirmed from unambiguous results obtained by second harmonic generation test as well as from transmission electron microscopy study (selected area electron diffraction). The Li~+/H~+ exchange corresponds to a topotactic reaction since this new protonated phase is built from the garnet framework CaLa_2Nb_2O_(12)~(6-) like its mother form Li_6CaLa_2Nb_2O_(12) with lithium and proton cations distributed on different sites.