Crystallographic studies of complex oxides
Structural studies of four lithium-containing complex oxide systems have been undertaken by powder and/or single crystal diffraction techniques, using either X-rays or neutrons as the radiation source. Phase M, a solid solution phase of unknown structure in the Li2O-Nb2O5-TiO2 system, was studied by single crystal and powder X-ray diffraction. Single crystal photographs revealed a LiNbO3-like subcell and an incommensurate supercell, the incommensurate periodicity varying greatly with solid solution composition. From consideration of the solid solution substitution formula, a mechanism for the supercell formation vadjust vfill ejectis proposed. The crystal structures of Li4SiO4 and α solid solutions, of general formula Li_4-3x(Ga,Al)_xSiO_4, 0< x< 0.5, in the systems Li_2O-(Ga,Al)_2O_3-SiO_2, were determined by Rietveld refinement of powder neutron diffraction data. The structures have distorted tetragonal packed anions, with gallium or aluminium atoms substituting on only one set of lithium sites. A comparison is made between the three solid solution structures determined and those of γ-LiAlO2, β-BeO and rutile, and the nature of the Li_4SiO_4(s.s.) to α(s.s.) phase transition is discussed. A re-investigation of the LiTaO3-CaZrO3 phase diagram was carried out and the solid solution limits re-determined. The crystal structures of two CaZrO3 solid solutions, one on either side of an observed immiscibility gap, were determined by Rietveld refinement of powder neutron diffraction data and possible causes of the solid solution immiscibility discussed. A comparison is made between these perovskite-like structures and phases X and Y, two recently discovered new phases in the systems Li2O-(Ta,Nb)2O5-ZrO2, some related proton-exchanged LiTaO3 and LiNbO3 phases and MnF3. The crystal structure of metastable lithium disilicate, Li2Si2O5, formed by ion exchange of sodium disilicate, was solved 'ab initio' from single crystal X-ray diffraction data. It is isostructural with the α-sodium disilicate starting material. The structure was also determined by Rietveld refinement of powder synchrotron X-ray diffraction data collected on the 8.3 diffractometer at the Daresbury SRS. A comparison is made between the single crystal and powder structures and the power of the Rietveld method in crystal structure determination discussed.