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PmBr3 crystallizes in the hexagonal P6_3/mmc space group. The structure is one-dimensional and consists of two PmBr3 ribbons oriented in the (0, 0, 1) direction. Pm3+ is bonded to six equivalent Br1- atoms to form distorted face-sharing PmBr6 pentagonal pyramids. All Pm–Br bond lengths are 2.90 Å. Br1- is bonded in an L-shaped geometry to two equivalent Pm3+ atoms.
PmBr2 is trigonal omega-like structured and crystallizes in the tetragonal P4_2/mnm space group. The structure is three-dimensional. Pm is bonded to six equivalent Br atoms to form a mixture of edge and corner-sharing PmBr6 octahedra. The corner-sharing octahedral tilt angles are 39°. All Pm–Br bond lengths are 2.98 Å. Br is bonded in a 3-coordinate geometry to three equivalent Pm atoms.