Hectoborides: Crystal structure of NdB65 and ThB60

Abstract

The crystal structures of the "hectoborides" NdB₆₅ and ThB₆₀ 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$\bar{3}$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 YB₆₆. With respect to the structure type of YB₆₆ in its original version by Richards and Kaspar [2], we observe for both compounds, NdB₆₅ and ThB₆₀, 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 ThB₆₆(O) [11] 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 8a site is also partially occupied by 0.04(1) Nd in NdB₆₅. Whereas the present investigation removes all doubts on the formation of NdB₆₅ (YB₆₆-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 ThB₆₀ (23.8 GPa) and for NdB₆₅ (24.3 GPa) fit well among hardness data for the corresponding rare earths hectoborides, the by-product material ThB₉₉ (βB-type) with 42 GPa is a superhard material.