New incommensurate structures of the borides Pr 41(Mn 4B 4) 35 and Pr 7(Re 4B 4) 6

Abstract

By means of single-crystal X-ray analysis the crystal structures of the ternary borides Pr 41(Mn 4B 4) 35 and Pr 7(Re 4B 4) 6 have been investigated and found to be incommensurate. The Pr 41(Mn 4B 4) 35 structure belongs to the incommensurate structure series based on the NdCo 4B 4 type. The structure refinements have been carried out separately for the Pr sublattice (space group I4/mm, c=0.3422(2) nm, N hkl=81, R=0.0457) and the Mn 4B 4 sublattice (space group P4 2/ ncm, c=0.4005(2) nm, N hkl=34, R=0.0292). The resultant structure unit cell is described by space group P4 22 12, a=0.7183(3) nm, c=14.024(8) nm, N hkl=134, R=0.087 (without atomic parameter refinement). The Pr 7(Re 4B 4) 6 structure is the first representative of a new incommensurate structure series with lattice parameter a≈ a NdCo4B4 × √2. The structure refinements have been performed separately for the Pr sublattice (space group F4/ mmm, c=0.3605(2) nm, N hkl=44, R=0.097) and the Re 4B 4 sublattice (space group I4 1/ amd, c=0.4201(2) nm, N hkl=137, R=0.033) and also for the complete unit cell (space group P4 c2, a=1.0566(6) nm, c=2.522(2) nm, N hkl=233, R=0.032). In both structures the Pr atoms are located in channels formed by four columns of M 4 tetrahedra along the Z axis. The M 4 tetrahedra are rotated relative to one another by an angle around the Z axis. The selection of possible space groups for the incommensurate structures, being derivatives of NdCo 4B 4, was made on the basis of the numbers of R and M 4B 4 sublattices.