  It is well known that diamond does not deform plastically at room temperature
and usually fails in catastrophic brittle fracture. Here we demonstrate
room-temperature dislocation plasticity in sub-micrometer sized diamond pillars
by in-situ mechanical testing in the transmission electron microscope. We
document in unprecedented details of spatio-temporal features of the
dislocations introduced by the confinement-free compression, including
dislocation generation and propagation. Atom-resolved observations with
tomographic reconstructions show unequivocally that mixed-type dislocations
with Burgers vectors of 1/2<110> are activated in the non-close-packed {001}
planes of diamond under uniaxial compression of <111> and <110> directions,
respectively, while being activated in the {111} planes under the <100>
directional loading, indicating orientation-dependent dislocation plasticity.
These results provide new insights into the mechanical behavior of diamond and
stimulate reconsideration of the basic deformation mechanism in diamond as well
as in other brittle covalent crystals at low temperatures.
