Lithium iodate: Phase transitions revisited

Abstract

The sequence of phase transitions of lithium iodate powders has been investigated by neutron thermodiffractometry. Upon heating, powders obtained by grinding hexagonal crystals exhibit the well known α → γ → β sequence. The intermediate orthorhombic phase γ is never observed as a single phase but always coexists either with α or with β-LiIO 3. The transition temperatures and the α → γ transformation rate are found to be strongly dependent on the crystallite dimension, especially for grain sizes smaller than 100 μm. The hysteresis of the reversible α ⇄ γ transition, the kinetics of both α → γ and γ → β transitions, and the influence of proton contamination are also investigated. Contrary to this, natural powders obtained by fast evaporation of an aqueous solution do not exhibit any intermediate phase and transform directly from α to the β tetragonal modification. Further DTA and X-ray investigations show that the occurrence of the γ phase is related to the mechanical treatment of the sample (grinding, polishing…), i.e. to the presence of defects. Previous literature data are reviewed and the apparent discrepancies concerning the transition processes are elucidated on the basis of the present results.