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K3Nb is beta Cu3Ti-like structured and crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. K is bonded in a 4-coordinate geometry to four equivalent Nb atoms. There are two shorter (3.99 Å) and two longer (4.15 Å) K–Nb bond lengths. Nb is bonded to twelve equivalent K atoms to form a mixture of face and corner-sharing NbK12 cuboctahedra.
K3Nb is alpha bismuth trifluoride structured and crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. there are two inequivalent K sites. In the first K site, K is bonded to four equivalent K and four equivalent Nb atoms to form distorted KK4Nb4 cuboctahedra that share corners with eight equivalent NbK12 cuboctahedra, corners with sixteen KK4Nb4 cuboctahedra, edges with four equivalent NbK12 cuboctahedra, edges with sixteen KK4Nb4 cuboctahedra, and faces with six equivalent KK4Nb4 cuboctahedra. All K–K bond lengths are 4.05 Å. All K–Nb bond lengths are 4.05 Å. In the second K site, K is bonded to eight equivalent K and four equivalent Nb atoms to form KK8Nb4 cuboctahedra that share corners with eight equivalent NbK12 cuboctahedra, corners with twenty KK4Nb4 cuboctahedra, edges with sixteen KK4Nb4 cuboctahedra, faces with four equivalent KK8Nb4 cuboctahedra, and faces with six equivalent NbK12 cuboctahedra. All K–Nb bond lengths are 4.19 Å. Nb is bonded to twelve K atoms to form NbK12 cuboctahedra that share corners with four equivalent NbK12 cuboctahedra, corners with twenty-four KK4Nb4 cuboctahedra, edges with eight equivalent KK4Nb4 cuboctahedra, edges with eight equivalent NbK12 cuboctahedra, faces with four equivalent NbK12 cuboctahedra, and faces with six equivalent KK8Nb4 cuboctahedra.