Publication details
Hectoborides: Crystal structure of NdB65 and ThB60
| Authors | |
|---|---|
| Year of publication | 2025 |
| Type | Article in Periodical |
| Magazine / Source | Solid State Sciences |
| MU Faculty or unit | |
| Citation | |
| web | https://doi.org/10.1016/j.solidstatesciences.2025.107826 |
| Doi | http://dx.doi.org/10.1016/j.solidstatesciences.2025.107826 |
| Keywords | Crystal structure; X-ray structure analysis; TEM; Phase diagram; Hardness |
| Description | 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 (3) over barc (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 (1/4,1/4,1/4) 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 "PrB66". 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 (beta B-type) with 42 GPa is a superhard material. |