Hectoborides: Crystal structure of NdB₆₅ and ThB₆₀

Author(s)

Gerda Rogl, Vilma Bursikova, Jiri Bursik, Gerald Giester, Henri Noel, Peter Franz Rogl

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 Fm3c (No. 226) (a_NdB65 = 2.35385(4) nm, RF = 0.0491; a_ThB60 = 2.35170(5) nm, RF = 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 Batom (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.

Organisation(s)

Department of Materials Chemistry, Department of Mineralogy and Crystallography

External organisation(s)

Masaryk University, Czech Academy of Sciences

Journal

Solid State Sciences

Volume

162

Pages

1-9

No. of pages

9

ISSN

1293-2558

DOI

https://doi.org/10.1016/j.solidstatesciences.2025.107826

Publication date

2025

Peer reviewed

Yes

Austrian Fields of Science 2012

105116 Mineralogy, 105113 Crystallography

Keywords

ASJC Scopus subject areas

General Chemistry, Condensed Matter Physics, General Materials Science

Portal url

https://ucrisportal.univie.ac.at/en/publications/e4f12267-1b9b-47d4-9a9a-973c2d09e72b