| Structural transformations, thermal expansion, and thermochemistry of stuffed quartz phases in the Li{dollar}sb{lcub}rm 1-x{rcub}{dollar}Al{dollar}sb{lcub}rm 1-x{rcub}{dollar}Si{dollar}sb{lcub}rm 1+x{rcub}{dollar}O{dollar}sb4{dollar} system were investigated by high- and low-temperature powder synchrotron X-ray and neutron diffraction, and high-temperature drop-solution calorimetry. Rietveld analysis of diffraction data reveals an Al/Si order-disorder transition at x = {dollar}sim{dollar}0.3 and a {dollar}beta{dollar}-{dollar}alpha{dollar} displacive transformation at {dollar}sim{dollar}0.65. Although the stuffed {dollar}beta{dollar}-quartz compounds (0 {dollar}le{dollar} x {dollar}< sim{dollar}0.65) expand within (001) and contract along c with increasing temperature, this thermal anisotropy is more pronounced for 0 {dollar}le{dollar} x {dollar}< sim{dollar}0.3 than for {dollar}sim{dollar}0.3 {dollar}le{dollar} x {dollar}< sim{dollar}0.65. The disparity can be attributed to tetrahedral tilting that only occurs in the ordered structures. The {dollar}alpha{dollar}-quartz-stuffed phases ({dollar}sim{dollar}0.65 {dollar}le{dollar} x {dollar}le{dollar} 1) exhibit positive thermal expansion along both a and c, which arises from a rotation of rigid Si(Al)-tetrahedra about {dollar}langle{dollar}100{dollar}rangle.{dollar} As Li+Al content increases, the critical temperature of the {dollar}alpha{dollar}-{dollar}beta{dollar} quartz transition decreases linearly, and the first-order character of this transition becomes weaker. Variations in the enthalpy of drop-solution with composition show a trend change at x = {dollar}sim{dollar}0.3 that corresponds to the Al/Si order-disorder transition. However, since the enthalpy of the {dollar}alpha{dollar}-{dollar}beta{dollar} transformation is rather small, no significant discontinuity is observed at x = {dollar}sim{dollar}0.65.; The {dollar}alpha{dollar}-{dollar}beta{dollar} quartz transition, the P 1-I 1 anorthite transition, and the order-disorder transition of Li ions in {dollar}beta{dollar}-eucryptite were examined by in situ transmission electron microscopy. Structural defects appear to interact strongly with displacive transitions as manifested by the memory effects associated with the transition-induced domains. Dauphine twins in {dollar}alpha{dollar}-quartz are pinned by both extended and point defects, and an Arrhenius analysis of the memory degrading behavior involving point defects yields an activation energy for point defect diffusion of 68.5 {dollar}pm{dollar} 4.1 kJ/mol. By contrast, c-antiphase domain boundaries of anorthite exhibit a nearly perfect memory, because they are fixed by localized Al/Si disorder. Electron diffraction analysis of {dollar}beta{dollar}-eucryptite reveals modulated structures that result from superperiodic stacking of two structural units with different Li configurations parallel to (100). Upon heating, these structures transform to the modification with a disordered Li distribution due to positional disordering of Li along the structural channels. |
