  Icosahedral nanoparticles composed of fivefold twinned tetrahedra have broad
applications. The strain relief mechanism and angular deficiency in icosahedral
multiply twinned particles are poorly understood in three dimensions. Here, we
resolved the three-dimensional atomic structures of Janus icosahedral
nanoparticles using atomic resolution electron tomography. A geometrically
fivefold face consistently corresponds to a less ordered face like two
hemispheres. We quantify rich structural variety of icosahedra including bond
orientation order, bond length, strain tensor; and packing efficiency, atom
number, solid angle of each tetrahedron. These structural characteristics
exhibit two-sided distribution. Edge dislocations near the axial atoms and
small disordered domains fill the angular deficiency. Our findings provide new
insights how the fivefold symmetry can be compensated and the
geometrically-necessary internal strains relived in multiply twinned particles.
