Search of novel boron-rich borides
Last modified on 6 September 2002

[B12 boron icosahedron]

In metal-boron compounds bonding scheme of boron varies depending on the composition ratio of [B]/[Me]. When the composition ratio exceeds 12, boron forms B12 icosahedron and three dimensional boron framework structure would be constructed by the linkage between the B12 icosahedron. Metal atoms reside in voids of the boron framework structure.

Before our search work crystal structures of the B12 icosahedron compounds have been classified into 8 structure types of
(1) α-rhombohedral boron (B13C2),
(2) β-rhombohedral boron (MeBx, x>23),
(3) α-tetragonal boron (B48B2C2),
(4) β-tetragonal boron (β-AlB12),
(5) AlB10 or AlC4B24,
(6) YB66,
(7) NaB15 or MgAlB14,
(8) γ-AlB12.

Later BeB3 and SiB6 with new structure types were added. Thus only two kinds of rare earth icosahedral borides have been known, they are REAlB14 (MgAlB14 structure type) and REB66 (YB66 structure type).


[Rare earth boron-rich borides]

We successfully developed the YB66 soft X-ray monochromator that is now practical on synchrotron radiation soft X-ray beam lines worldwide. However, extended use on brighter beam lines using insertion devices is rather difficult because of amorphous material like low thermal conductivity of YB66. We could find many novel rare earth boron-rich borides during our search for a novel boron-rich boride that can replace YB66. They are REB25, REB50 and ScB19 as binary RE-B system compounds, and ScB15C1.6, ScB15C0.8, ScB17C0.25 and homologous series of REB15.5CN, REB22C2N and REB28.5C4 as ternary/quaternary RE-B-C/N compounds. REB25C5N2 and REB17.6Si4.6 were also found. All these are summarized in the next figure. REB25, ScB15C1.6 and REB25C5N2 are classified into the same structure type as that of MgAlB14. Structure modulations could be found for the crystal structure of REB25C5N2. All others have their own new structure type.



[Homologous boron-rich borides]

A series of compounds, REB15.5CN, REB22C2N and REB28.5C4 are homologous with B4C. B4C has a rhombohedral lattice unit formed by the B12 icosahedron. A C-B-C chain resides at the center of the lattice unit as shown in the figure (right). However, the B4C crystal structure can be understood as a layer structure also. The homologous series compounds have two basic structural units, the B12 icosahedron and the B6 octahedron. In their crystal structures the B6 octahedron layer replaces every 3rd, 4th and 5th B12 icosahedron layer of the B4C structure, respectively, as shown in the figure (below). Because of smaller size of the B6 octahedron than the B12 icosahedron rare earth elements can reside in the spaces created by the replacement. We firstly found boron-rich homologous series borides in which both B6 and B12 polyhedra coexist.



[Rare earth boron-rich borides having a Si-Si bonding]

The linkage between the B12 icosahedron forms the three-dimensional framework structure of boron-rich borides as mentioned above. In the crystal structure of REB17.6Si4.6 that includes much Si the Si-Si bonding plays an important role in order to form the framework structure as well as the B12 icosahedron linkage. Both B12 icosahedron (green) bonding and the Si-Si (violet) bonding form a complicated framework in the REB17.6Si4.6 crystal structure as shown in the figure (below). Rare earth ions (red) are arranged in the spaces of the framework structure.


[Sc-B-C- (Si) system boron-rich borides]

The ternary Sc-B-C system phase relation is most complicated in the ternary rare earth boron-rich borocarbides. There are many boron-rich phases in the boron-rich corner as shown in the figure (below). A slight composition variation can cause variety of phases such as ScB19, ScB17C0.25, ScB15C0.8 and ScB15C1.6. Some of their crystal structures are quite unique and completely different each other.



[ScB17C0.25]

For example, a projection along the c-axis of the crystal structure of ScB17C0.25 shows a beautiful pattern as shown in the figure (right). A projection along a-axis (bottom figure) reveals that ScB17C0.25 has a layer structure. Sc layer is sandwiched by the B12 icosahedron layers. ScB15C1.6 has the same boron framework structure as MgAlB14. Sc occupies the Mg site and the Al site is empty. A part of the boron sites is replaced by carbon.

[ScB15C0.8]

REB66 is known to have the highest number of atoms, about 1600 atoms, in the unit cell and ScB15C0.8 follows. The number of atoms in the unit cell of ScB15C0.8 is about 1000. It is better to introduce some one-size larger structure units than the B12 icosahedron because it is difficult to understand such complicated boron framework structure by describing the B12 icosahedron linkage only. The boron framework structure of REB66 is described by introducing a boron super icosahedron that is formed thirteen B12 icosahedra, {B12(B12)12, number of atoms=156}.

In the case of ScB15C0.8 four kinds of super cluster were introduced, two kinds of super tetrahedron T1 and T2 which are formed by four B12 icosahedra and a super octahedron O1 which is formed by six B12 icosahedra. One characteristic feature of the ScB15C0.8 crystal structure is the existence of a B10 polyhedron cluster formed by 10 boron atoms. Six B10 polyhedra form another super octahedron O2. All these are shown in the next figure.



In the super tetrahedron T1 the central Si atom tetrahedrally coordinated by four boron atoms, which is further tetrahedrally coordinated by four B12 icosahedra. The super tetrahedron T2 is commonly seen in the crystal structures of boron-rich borides. A tetrahedral unit of SiC4 that is a basic structure unit of SiC locates at the center of the super octahedron O1 and the SiC4 unit is octahedrally coordinated by six B12 icosahedra. In the super octahedron O2 four boron atoms bridge the octahedral coordination of six B10 polyhedra. Si was added for ease of single crystal growth, thus the real crystal composition of this compound is ScB12.7C0.62Si0.08. The boron framework structure is shown in the next figure where Sc atoms are not shown. The super clusters of T1, T2 and O1 are depicted as two kinds of tetrahedron and an octahedron with corresponding sizes, respectively, and the super cluster O2 is depicted as it is.



We could find variety of novel rare earth boron-rich borides. All these rare earth boron-rich borides behave as a semiconductor and they are unique with including rare earth atoms as a constituent element. Rare earth elements exist as a trivalent ion in the rare earth boron-rich borides. We are conducting physical property measurements expecting unique properties related to the existence of the rare earth trivalent ions.

References

[YB66]
1. T. Tanaka et al., "FZ crystal growth of monochromator-grade YB66 single crystals as guided by topographic and double-crystal diffraction characterization", J. Crystal Growth 192, 141 (1998).
2. T. Tanaka et al., "Nature of the 1385.6 and 1438 eV positive glitches in the transmission function of the YB66 soft-X-ray monochromator", J. Appl. Cryst. 30, 87 (1997).
3. J. Wong et al., "YB66 - a new soft X-ray monochromator for synchrotron radiation. II. characterization", J. Synchrotron Rad. 6, 1086 (1999).
4. T. Oku et al., "Digital HREM imaging of yttrium atoms in YB56 with YB66 structure", J. Solid State Chem. 135, 182 (1998).
5. T. Oku et al., "Atomic structure of YB56 studied by digital high-resolution electron microscopy and electron diffraction", J. Mater. Res. 16, 101 (2001).
6. T. Tanaka et al., "Effect of transition metal doping in YB66", J. Solid State Chem. 154, 54 (2000).
7. I. Higashi et al., "Structure refinement of YB62 and YB56 of the YB66-type structure", J. Solid State Chem. 133, 16 (1997).
[YB50]
8. T. Tanaka, S. Okada and Y. Ishizawa, "A new yttrium higher boride: YB50", J. Alloys Comp. 205, 281 (1994).
9. T. Tanaka, S. Okada and Y. Ishizawa, "Single crystal growth of a new YB50 family compound: YB44Si1.0", J. Solid State Chem. 133, 55 (1997).
10. Y. Ishizawa and T. Tanaka, "Transport phenomena of YB41Si1.2", J. Solid State Chem. 154, 229 (2000).
11. I. Higashi et al., "Crystal structure of YB41Si1.2", J. Solid State Chem. 133, 11 (1997).
[YB25]
12. T. Tanaka et al., "A new yttrium boride: YB25", J. Solid State Chem. 133, 122 (1997).
13. F. X. Zhang et al., "Incorporation of carbon atoms in rare earth boron-rich solids and formation of superstructures", J. Alloys Comp. 337, 120 (2002).
[ScB19]
14. T. Tanaka, S. Okada and V. N. Gurin, "A new scandium boride: ScB19", J. Alloys Comp. 267, 211 (1998).
15. T. Tanaka and A. Sato, "Floating zone crystal growth and structure analysis of novel ScB19 family compound, ScB19+xSiy", J. Solid State Chem. 160, 394 (2001).
[ScB17C0.25]
16. T. Tanaka, "A new scandium borocarbide: ScB17C0.25", J. Alloys Comp. 270, 132 (1998).
17. A. Leithe-Jasper et al., "A single-crystal XRD and TEM study of 'ScB17C0.25", J. Solid State Chem. 154, 130 (2000).
[Homologous phases]
18. F. X. Zhang et al., "Crystal structure of new rare -earth boron-rich solids: REB28.5C4", J. Alloys Comp. 329, 168 (2001).
19. F. X. Zhang et al., "Novel rare earth boron-rich solids", J. Solid State Chem. 159, 174 (2001).
20. F. X. Zhang et al., "Homologous phases built by boron clusters and their vibrational properties", Inorg. Chem. 40, 6948 (2001).
[REB17.6Si4.6]
21. F. X. Zhang, A. Sato and T. Tanaka, "A new boron-rich compound in the Y-B-Si ternary system", J. Solid State Chem. 164, 361 (2002).
[Sc-B-C ternary phases]
22. Y. Shi et al., "New ternary compounds Sc3B0.75C3, Sc2B1.1C3.2, ScB15C1.60 and subsolidus phase relations in the Sc-B-C system at 1700°C", J. Solid State Chem. 148, 250 (1999).
23. T. Tanaka and A. Sato, "A novel boron-rich scandium borocarbosilicide; Sc0.83-xB10.0-yC0.17+ySi0.083-z (x=0.030, y=0.36 and z=0.026): Floating zone crystal growth and structure analysis", J. Solid State Chem. 165, 148 (2002).
24. A. Leithe-Jasper, A. Sato and T. Tanaka, "Crystal structure of scandium borocarbide, ScB13C", Z. Kristallogr. NCS 216, 45 (2001).
25. Y. Shi et al., "Sc3B0.75C3, a novel scandium boron carbide", J. Alloys Comp. 298, 99 (2000).
[Sc2B1.1C3.2]
26. Y. Shi et al., "Sc2B1.1C3.2, a new rare-earth boron carbide with graphite-like layers", J. Solid State Chem. 148, 442 (1999).
27. Y. Shi, A. Leithe-Jasper and T. Tanaka, "Crystal growth of new compound Sc2B1.1C3.2 with graphite-like layers", Mol. Cryst. Liq. Cryst. 340, 271 (2000).
28. M. Onoda et al., "Structure refinement of the layered composite crystal Sc2B1.1C3.2 in a five-dimensional formalism", Acta Cryst. B57, 449 (2001).
Correspondence to:
Takaho Tanaka

AML/NIMS
Boride group