The effect of heating on the domain structure of beta-eucryptite, LiAlSiO₄

Abstract

Transmission electron microscopy of β-eucryptite revealed the occurrence of antiphase domains with the antiphase vector 1/2 a. This vector relates (Si, Al)O₄-tetrahedra which are chemically equivalent, i.e. either SiO₄ or AlO₄ (in an ordered structure); therefore, it leaves the Si/Al long-range order undisturbed. The size of the domains depends on the temperature history of the crystals. Antiphase domains with the antiphase vector 1/2 c have not been observed. In situ heating experiments showed that the superstructure reflections of the type h+k=2n+1, l=2n+1 (“a-reflections”) rapidly lose their intensity and become unobservable at temperatures around 550° C (furnace, not specimen temperature) as also known from X-ray studies of β-eucryptite at 460° C (specimen temperature). This change is reversible. Correspondingly, the antiphase domain boundaries which are imaged with the a-reflections disappear at these temperatures. Upon cooling, the antiphase domain boundaries reappear at the same positions as before. It is concluded that position, shape and size of the antiphase domains is controlled by the number and distribution of local defects in the Si/Al order.