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 MnO 2 spinel, obtained by ion exchange of Li ions with D or H in a well-crystallized spinel Li 1.27 Mn 1.73 O 4 compound, have been determined by powder neutron diffraction. Refinements were carried out using the Rietveld method of powder profile analysis in the space group Fd 3 m . Li ions are removed from both tetrahedral 8 a and octahedral 16 d sites when Li 1.27 Mn 1.73 O 4 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 MnO 2 spinel structure. The −OD and −OH groups are directed into the interstitial space of the 8 a tetrahedra, but with an orientation which suggests that the deuteron or proton site is favored for occupation only when the nearest neighbor octahedral 16 d site contains no Mn ion. When the protonated compound is contacted with LiOH solution, reinserted Li ions relocate on tetrahedral 8 a sites in preference to vacant octahedral 16 d sites in the MnO 2 framework.