A dynamical model for the P1-I1 phase transition in anorthite, CaAl2Si2O8. I: Evidence from electron microscopy

摘要: Electron diffraction and electron microscopic evidence is presented for a dynamical reversible $$P\bar 1 - I\bar 1$$ phase transition in anorthite at T c=516 K. Antiphase boundaries with displacement vector, R=1/2[111] become unstable T c, while other antiphase boundary loops the same vector are formed. These interfaces very mobile vibrate frequency which increases strongly temperature. At temperatures considerably above shimmering effect observed on imaging dark field using diffuse c reflections. observations agreement interpretation of high temperature body-centered as statistical average small type domains primitive anorthite. We propose that originate from ordered anti-ordered configurations around Ca2+ ions (ooo) (oio) [likewise (zoo) (zio)] positions. The model involves two-stage mechanism: softmode mechanism causing aluminosilicate framework to approach symmetry, followed by an orderdisorder ion configurations. Close fluctuations set breathing motion lattice vibrations cause Ca Ca(oio) Ca(zoo) Ca(zio)] configuration dynamically interchange through intermediate $$I\bar configuration. nature comparable α — β quartz.