The crystal structure of PrBN 2 and isotypic compounds REBN 2 ( RE = Nd, Sm, Gd)

Abstract

New compounds REBN 2 have been synthesized by direct sintering of mixtures of hex-BN and the rare earth metal under nitrogen at 1800°C. The crystal structure of PrBN 2 has been determined from single crystal X-ray counter data. PrBN 2 crystallizes in a unique structure type with the noncentrosymmetric rhombohedral space group R3 c - C 6 3 v , No. 161. The hexagonal lattice parameters are a = 1.21144(13) nm and c = 0.70126(15) nm; Z = 18. For 388 reflections (| F 0| > 3 σ) the residual values are R = Σ|ΔF| Σ|F 0‖ = 0.038, R w = 0.028 . The structure type of PrBN 2 is a combination of the typical net-type structural units B 3N(1) 3 of hex-BN and of irregular bipyramidal units ‖Pr 5B‖ centered by the N(2) nitrogen atoms. In accordance with the typical arrangement of B and N atoms in hex-BN, there are no direct BB or NN contacts in PrBN 2. With respect to the structural chemical building blocks, the crystal structure of PrBN 2 is intermediate between the hexagonal BN type and structure types such as Pr 3B 2N 4. Isotypic compounds REBN 2 were synthesized with the early rare earth metals Nd, Sm, and Gd. GdBN 2 was only observed at temperatures above 1400°C. No REBN 2 phase was found with the smaller rare earth members Tb → Lu, Sc, and yttrium.