Hectoborides: Crystal structure of NdB65 and ThB60

The crystal structures of the "hectoborides" NdB 65 and ThB 60 have been elucidated from single crystal X-ray structure determination supported by TEM-SAED analyses. Both compounds crystallize in a face-centred cubic lattice with space group Fm c (No. 226) ( a NdB65 = 2.35385(4) nm, R F = 0.0491; a ThB60 = 2.35170(5) nm, R F = 0.0489) isotypic with the aristo-type of hectoborides, namely YB 66 . With respect to the structure type of YB 66 in its original version by Richards and Kaspar [2], we observe for both compounds, NdB 65 and ThB 60 , a split position (total occupancies ∼0.5) for the metal atoms in site 48f as well as a second metal atom position in site 8a at a reduced occupancy; furthermore, we only arrive at seven B-sites 192j and 5 B-sites 96i , but did not find a B-atom (B13) in site 64g (x = 0.23, x, x). In contrast to the earlier structure determination of ThB 66 (O) [1971Naslain] our WDX analysis did not present any hints for oxygen contamination, therefore site 8a (¼,¼,¼) hosts a Th atom at low occupancy of 0.12(1) Th instead. Similarly, the 8 a site is also partially occupied by 0.04(1) Nd in NdB 65 . Whereas the present investigation removes all doubts on the formation of NdB 65 (YB 66 -type), our analysis did not reveal the formation of isotypic "PrB ∼66 ". On the basis of our SEM data, revised phase relations for the three systems Nd-B, Pr-B and Th-B have been designed for the boron-rich part (>85 at.% B). Whereas at room temperature hardness of ThB 60 (23.8 GPa) and for NdB 65 (24.3 GPa) fit well among hardness data for the corresponding rare earths hectoborides, the by-product material ThB 99 (βB-type) with 42 GPa is a superhard material. • Single crystal X-ray analyses: NdB 65 and ThB 60 (YB 66 -type). • Metal atoms in 8a site replaced B13 atoms in 64g site. • Vickers hardness and fracture toughness of NdB 65 and ThB 60 . • Superhard ThB 99 (Th in ßB). • Phase relations (>85 at.% B) for the systems Nd-B, Pr-B, Th-B.