Monazite (monoclinic LaPO4) slip systems at room temperature

Polycrystalline monazite (monoclinic LaPO₄) was deformed by spherical indentation at room temperature. Slip systems were identified using TEM of thin sections prepared parallel and close to the indented surface. Dislocation Burgers vectors (b) were identified by Burgers circuit closure in high resolution TEM images, supplemented by diffraction contrast where possible. A total of 441 b determinations were made in 97 grains. The most common slip systems were 001]/(010), 100]/(010) and 010]/(100). Slip on (001) was less common. Many other less common slip systems and Burgers vectors were also identified, including b = 101], 101], 011], 110] and 111]. b = 101] dislocations dissociate into ½101] partials, and b = 101] dislocations are inferred to dissociate to ½101] partials, with a low energy stacking fault of ～30 mJ/m². b = 100] dislocations may dissociate into ¼210] + ¼210] partials. b = 010] may sometimes dissociate to ½010] + ½010] partials. Other types of partial dislocations were also observed and discussed. All partial dislocations were climb dissociated. The line energies of monazite dislocations and their partials were calculated, and stacking fault structures for partial dislocations are analyzed. Satisfaction of the Von Mises criterion for full ductility most likely involves 101]/(111) and ?011?/{011} or {111} slip, but other combinations that require both b = 101] and ?011? or ?110? are possible. If deformation twinning is active, slip systems with b = ?011? or ?110? may not be necessary for full ductility.