Deformation twinning in monazite

Abstract

Polycrystalline monazite (LaPO 4) was deformed at room temperature by a spherical indenter. Deformation twins were identified by TEM in 70 grains. Five twin planes were found: (1 0 0) was by far the most common; (0 0 1) and (1 2 0) were less common; (1 2 2 ̄ ) was rare, and kinks in (1 2 0) twins were identified as irrational ′(4 8 3)′ twin planes. The twinning modes on these planes were inferred from the expression of twinning shear at free surfaces, predictions of classical deformation twinning theory, and various considerations of twin morphology and crystal structure. Atomic shuffle calculations that allow formation of either a glide plane or a mirror plane at the twin interface were used to analyze twin modes. The inferred twin modes all have small atomic shuffles. For (0 0 1) twins, the smallest shuffles were obtained with a glide plane at the interface, with displacement vector R = 1 2 [010 ] . The results do not uniquely define a twin mode on (1 0 0), leaving open the possibility of more than one mode operating on this plane. Factors that may determine the operative deformation twinning modes are discussed. Crystal structure considerations suggest that the relative abundance of twinning modes may correlate with low shear modulus on the twin plane in the direction of twinning shear, and with a possible low-energy interface structure consisting of a layer of xenotime of one half-unit-cell thickness that could form at (1 0 0) and (0 0 1) twins. The three most common twins have low strains to low ∑ coincidence site lattices (CSLs).