Ion exchange in manganese dioxide spinel: Proton, deuteron, and lithium sites determined from neutron powder diffraction data

The structures of deuterated, protonated, and relithiated forms of MnO2 spinel, obtained by ion exchange of Li ions with D or H in a well-crystallized spinel Li1.27Mn1.73O4 compound, have been determined by powder neutron diffraction. Refinements were carried out using the Rietveld method of powder profile analysis in the space group Fd3m. Li ions are removed from both tetrahedral 8a and octahedral 16d sites when Li1.27Mn1.73O4 is contacted with DCl or HCl solution. The neutron diffraction data show that deuterium and hydrogen ions are predominantly incorporated into the crystal as -OD and -OH by bonding to lattice oxygen atoms without any other major change to the MnO2 spinel structure. The -OD and -OH groups are directed into the interstitial space of the 8a tetrahedra, but with an orientation which suggests that the deuteron or proton site is favored for occupation only when the nearest neighbor octahedral 16d site contains no Mn ion. When the protonated compound is contacted with LiOH solution, reinserted Li ions relocate on tetrahedral 8a sites in preference to vacant octahedral 16d sites in the MnO2 framework.