Polycrystalline monazite (LaPO4) was deformed at room temperature by using a spherical indenter. Deformation twins were identified, by transmission electron microscopy, in 70 grains. Five twin planes were found, of which (100) was very much the most common. The (001) and (120) planes were less common, and (12¯2) was rare. Kinks in (120) twins were identified as being irrational so-called (483) twin planes. The twinning modes on these planes were deduced from the appearance of the twinning shear at free surfaces, from the predictions of classical deformation-twinning theory and from various considerations of twin morphology and crystal structure. Atomic shuffle calculations that allowed for the formation of a glide plane or a mirror plane at the twin interface were used to analyze the twin modes. The deduced twin modes all had small atomic shuffles. For (001) twins, the smallest shuffles were found for a glide plane at the interface; with a displacement vector of ½[010]. The results did not uniquely define a twin mode on (100). This offered the possibility of more than one mode operating on this plane. Crystal structure considerations suggested that the relative abundance of twinning modes could be related to a low shear modulus on the twin plane in the direction of twinning shear, and possibly with a low-energy interface structure that consisted of a layer of xenotime of one half unit-cell thickness that formed at (100) and (001) twins.

Deformation Twinning in Monazite. R.S.Hay, D.B.Marshall: Acta Materialia, 2003, 51[18], 5235-54