In the low-temperature form of CrCl₃, every second plane of octahedral holes is occupied. What is the filling fraction of the occupied planes? Check the result by a sideon view of the closest-packed layers perpendicular to their subsequent shifts (R3 in hexagonal setting; a = 5.94, c = 17.3 Å, Cr in 6c at 0 0 ⅓, Cl in 18f at ⅔ 0 0.0757). What is the type of densest packing?