Crystal structures of the metal diborides ReB2, RuB2, and OsB2 from neutron powder diffraction

Article abstract of DOI:10.1002/zaac.201000101

Because of the very small scattering power of the light element, the crystal structures of metal borides that contain heavy metal atoms are difficult to determine unambiguously from X-ray diffraction data only. Using neutron diffraction methods and applying them to isotopically enriched ¹¹B boride powders, the crystal structures of ReB₂, RuB₂, and OsB₂ were re-determined and analysed with respect to the boron atom arrangement. In accordance with the findings from X-ray diffraction experiments, the structures exhibit corrugated boron atom layers of conjugated six-rings, either seat-like (ReB₂) or boatlike (RuB₂, OsB ₂). ReB₂ crystallises in the hexagonal crystal system, space group P6₃/mmc (no. 194, a = 290.05(1) pm, c = 747.72(1) pm); OsB₂ and RuB₂ are isostructural and crystallise orthorhombically, space group Pmmn (no. 59, a = 464.479(5) pm, b = 286.515(3) pm, c = 404.560(6) pm (RuB₂); a = 468.408(5) pm, b = 287.255(3) pm, c = 407.693(6) pm (OsB₂)). Boron-boron distances vary between 181.7 and 189.9 pm. For RuB₂ and OsB₂, shortest metal-boron distances range from 217.2 to 217.3 pm, indicating a covalent interaction between metal and boron, compared to 222.7 pm for ReB₂. Metal- metal distances are between 286.5 pm and 302.2 pm. All three compounds have been described as very hard or incompressible materials.

Full text of DOI:10.1002/zaac.201000101

ARTICLE
DOI: 10.1002/zaac.201000101
Crystal Structures of the Metal Diborides ReB2, RuB2, and OsB2 from
Neutron Powder Diffraction
Michael Frotscher,[a] Markus Hölzel,[b] and Barbara Albert*[a]
Dedicated to Professor Bernd Harbrecht on the Occasion of His 60th Birthday
Keywords: Boron; Neutron diffraction; Osmium; Rhenium; Ruthenium
Abstract. Because of the very small scattering power of the light ele- space group P63/mmc (no. 194, a = 290.05(1) pm, c = 747.72(1) pm);
ment, the crystal structures of metal borides that contain heavy metal OsB2 and RuB2 are isostructural and crystallise orthorhombically,
atoms are difficult to determine unambiguously from X-ray diffraction space group Pmmn (no. 59, a = 464.479(5) pm, b = 286.515(3) pm,
data only. Using neutron diffraction methods and applying them to c = 404.560(6) pm (RuB2); a = 468.408(5) pm, b = 287.255(3) pm,
isotopically enriched 11B boride powders, the crystal structures of c = 407.693(6) pm (OsB2)). Boron–boron distances vary between
ReB2, RuB2, and OsB2 were re-determined and analysed with respect 181.7 and 189.9 pm. For RuB2 and OsB2, shortest metal-boron distan-
to the boron atom arrangement. In accordance with the findings from ces range from 217.2 to 217.3 pm, indicating a covalent interaction
X-ray diffraction experiments, the structures exhibit corrugated boron between metal and boron, compared to 222.7 pm for ReB2. Metal–
atom layers of conjugated six-rings, either seat-like (ReB2) or boat- metal distances are between 286.5 pm and 302.2 pm. All three com-
like (RuB2, OsB2). ReB2 crystallises in the hexagonal crystal system, pounds have been described as very hard or incompressible materials.
from X-ray diffraction data. Neutron diffraction methods have
Introduction
now been applied to isotopically enriched 11B boride powders
The structures of ruthenium, osmium and rhenium diboride
to re-determine the crystal structures of ReB2, RuB2, and OsB2,
have been known since 1962, when Aronsson et al. [1] and
special attention being paid to the boron atom arrangement. To
Roof and Kempter [2] described RuB2 and OsB2, whereas La
the best of our knowledge, this is the first time neutron diffrac-
Placa and Post published the structure of ReB2 [3], which had
tion has been applied to these metal diborides. We will discuss
formerly been falsely assigned (“ReB3”) [4]. The MB2 (M =
the different layer structures of diborides in the light of the
Re, Ru, Os) structures were determined by single-crystal X-
new findings.
ray diffraction. It was not until very recently that these com-
pounds raised fresh interest and were described as super-hard
Results and Discussion
or ultra-incompressible [5–14]. RuB2 and OsB2 are known to
become superconducting at TC = 1.7 K [15] (Ru) and 2.1 K
[16] (Os). Several first principle studies were undertaken to
analyse the anisotropic hardness of OsB2, RuB2, and ReB2
[17–26]. However, no up-to-date structural data and especially
no accurate boron atom positions became available. Only very
recently did Zogal et al. publish new single-crystal X-ray data
on one of the three compounds under discussion, ReB2 [27].
Boride structures that consist of heavy metal atoms next to
light boron atoms are difficult to be solved unambiguously
Mono-phase samples of ReB2, RuB2 and OsB2 were obtained
from melting the elements at 2500–2600 K (1 h) under argon
in an induction furnace. After ball-milling, the powders were
subjected to laboratory X-ray data collection (Co-Kα1 radia-
tion) and neutron powder diffractometry at the SPODI beam-
line of the FRM2 reactor (Garching, Germany). Rietveld re-
finements (Programme GSAS [28] were performed by using
data from the literature [1, 3] as starting models. Both X-ray
and neutron data sets for all three samples were refined sepa-
rately. The refinement results are comprised in Table 1, Ta-
ble 2, Table 3, and Table 4. The structures proved to be essen-
tially identical to earlier published models, with small
deviations in the boron atom arrangement.
ReB2 (Figure 1) is characterised by alternating layers of
metal atoms and boron atoms, the latter forming condensed
six-rings in chair-like conformation. Each boron atom has three
boron neighbours at distances of 182.10(5) (182(2) pm in
[27]). Rhenium has eight boron neighbours, all of them at dis-
tances between 222 pm and 226 pm. The coordination polyhe-
* Prof. Dr. B. Albert
Fax: +49-6151-16-6029
E-Mail: albert@ac.chemie.tu-darmstadt.de
[a] Eduard Zintl-Institut für Anorganische und Physikalische Chemie
Fachbereich Chemie
Technische Universität Darmstadt
Petersenstr. 18
64287 Darmstadt, Germany
[b] Fachbereich Material- und Geowissenschaften
Technische Universität Darmstadt
Petersenstr. 18
64287 Darmstadt, Germany
Z. Anorg. Allg. Chem. 2010, 636, 1783–1786
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1783

M. Frotscher, M. Hölzel, B. Albert
ARTICLE
Table 1. Structure determination and crystallographic data for ReB2.
dron of rhenium could be described as a trigonal prism of bo-
ron atoms with both triangular faces capped with additional
boron atoms. The shortest Re–B distances are 222.70(6) pm
(2x) (vs. 222(2) pm in [27]). The metal atom arrangement can
be de derived from a hexagonal close packing, with each rhe-
nium atom coordinated by six rhenium neighbours within the
planar layer (Re–Re distances are 290.058(1) pm vs.
289.82 pm in [27]) and an AB stacking sequence of these
metal atom layers perpendicular to the c axis.
The ReB2 structure is similar to that of AlB2 with its planar
six-ring sheets of boron atoms and planar metal atom layers,
although the latter exhibits a metal atom arrangement that has
a stacking sequence of AA. The AlB2 structure type is known
for diborides of magnesium, aluminium, yttrium, molybdenum,
tungsten and many others. There are further modifications of
MoB2 and WB2, better known as W2B4 and Mo2B4 [29], which
are even more similar to ReB2. Their crystal structures again
consist of alternating layers of metal and boron atoms; hence
the boron atom layers are alternating planar (as in AlB2) or
corrugated (as in ReB2).
ReB2
X-ray
Neutron
Temperature /K
Crystal system
Space group
Lattice parameters
a, b /pm
293(2)
hexagonal
P63/mmc (No. 194)
290.059(2)
747.745(6)
54.482(0)
12.669
290.05(1)
747.72(1)
54.48(1)
12.671
c /pm
V /Å3
Calculated density
/g·cm–3
2θ range /°
25 < 2θ < 110
17 measured
5 refined
20 < 2θ < 152
35 measured
6 refined
–
No. reflections
Structure parameters
a)
ρmin./max.
–1.72/1.17
Residuals
Rp: 0.0502;
Rp: 0.0439;
R
wp: 0.0583;
Rwp: 0.0566;
χ2: 1.651
χ2: 3.836
a) Maximum residual electron density is found at the origin of the
cell, but residual neutron scattering is very low at that position.
OsB2 and RuB2 are isostructural to each other (Figure 2).
Here, both the layers of metal atoms and those of the boron
atoms are undulated. The boron-atom sheets consist of boat-
like six-rings, again conjugated. The metal atom arrangement
is that of corrugated hexagonal sheets, stacking sequence AA
perpendicular to the c axis. Each metal atom has two plus four
metal atom neighbours within the layer.
For OsB2, short Os–B distances have been described to be
essential for the mechanical properties. Here, we observed val-
ues between 217.2(2) pm and 229.3(3) pm, compared to a
range of 215.6–231.8 pm according to [1]. Again, like in ReB2,
each metal atom has eight boron neighbours, but the coordina-
tion polyhedron formed is different from that in the rhenium
compound. Each osmium atom is surrounded by 2+4+2 boron
atoms, forming an irregular polyhedron which resembles to a
distorted trigonal trapezohedron. Within the corrugated boron
atom layer, we found the two B–B distances larger than previ-
ously described. For OsB2 they are 182.0(2) pm and 189.9 (2)
pm (2x) vs. 179.9 pm and 187.8 pm according to [1]. For
RuB2, they had been described as 177.4 and 190.2 pm; neutron
diffraction data now led to values of 181.7(2) pm and 188.8(2)
pm (2x).
Table 2. Structure determination and crystallographic data for RuB2.
RuB2
X-ray
Neutron
Temperature /K
Crystal system
Space group
Lattice parameters
a /pm
293(2)
orthorhombic
Pmmn (No. 59)
464.569(5)
286.559(3)
404.605(6)
53.863(1)
7.565
464.479(5)
b /pm
286.515(3)
404.560(6)
53.839(1)
7.565
c /pm
V /Å3
Calculated density
/g·cm–3
2θ range /°
No. reflections
Structure parameters
ρmin./max.
15 < 2θ < 110
34 measured
10 refined
15 < 2θ < 150
77 measured
10 refined
–
–0.70/1.07
Residuals
Rp: 0.0559;
Rp: 0.0621;
R
wp: 0.0704;
Rwp: 0.0843;
χ2: 1.518
χ2: 14.90
Table 3. Structure determination and crystallographic data for OsB2.
OsB2
X-ray
Neutron
Temperature /K
Crystal system
Space group
Lattice parameters
a /pm
293(2)
orthorhombic
Conclusions
Pmmn (No. 59)
For the first time, the crystal structures of ReB2, RuB2 and
OsB2 were investigated using neutron diffraction, resulting in
an accurate localisation of the boron atoms.
468.450(6)
287.372(4)
407.742(6)
54.890(1)
12.816
468.408(5)
b /pm
287.255(3)
407.693(6)
54.856(1)
12.826
c /pm
V /Å3
Calculated density
Experimental Section
Synthesis
Pure crystalline boron powder (Chemotrade, > 99 % 11B-enriched for
neutron diffraction) was mixed with rhenium (Degussa, 99.9 %), os-
mium (American Elements, 99.99 %), or ruthenium (W.C. Heraeus)
powder in stoichiometric ratios and pressed into pellets using a hydrau-
lic press. All samples were prepared in an induction furnace using a
/g·cm–3
2θ range /°
No. reflections
Structure parameters
ρmin./max.
15 < 2θ < 110
34 measured
9 refined
15 < 2θ < 152
77 measured
10 refined
–
–2.18/2.28
Residuals
Rp: 0.0616;
Rp: 0.0411;
R
wp: 0.0737;
Rwp: 0.0551;
χ2: 1.857
χ2: 11.95
1784
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© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2010, 1783–1786

Crystal Structures of the Metal Diborides ReB2, RuB2, and OsB2
Table 4. Positional and displacement parameters of atoms in ReB2, RuB2, OsB2. Uiso values (pm2) are defined as one third of track of the
orthogonalised tensor Uij. Figures in brackets are standard deviations that refer to the last digit.
Atom Wyckoff site
ReB2
x
y
z
Uiso or U11, U22, U33, U12, U13, U23
Re
B
2c
4f
1/3
1/3
2/3 1/4
2/3 0.54783(8)
0.0039(2), 0.0039(2), 0.0038(3), 0.0019(1), 0, 0
0.0005(2)
RuB2
Ru
B
OsB2
Os
B
2a
4f
1/4
0.0544(2)
1/4 0.1505(5)
1/4 0.6385(3)
0.0185(4), 0.0243(5), 0.0392(7), 0, 0, 0
0.0161(2)
2a
4f
1/4
0.0557(2)
1/4 0.1545(3)
1/4 0.6325(4)
0.0251(4), 0.0326(5), 0.0296(5), 0, 0, 0
0.016(2)
Figure 1. Crystal structure of ReB2 (light grey: boron atoms, dark
grey: metal atoms, Programme DIAMOND [30]).
Figure 2. Crystal structures of OsB2 and RuB2 (light grey: boron at-
oms, dark grey: metal atoms, Programme DIAMOND [30]).
combination of an inner reaction crucible (hexagonal boron nitride)
and an outer inductive coupling crucible (glassy carbon) under purified
argon by melting the starting materials at temperatures of T = 2600 K (y axis) are given in arbitrary units.
Figure 3. Traces of the neutron powder patterns of ReB2, RuB2, and
OsB2 (+: observed, –: calculated, bottom: difference curve). Intensities
Z. Anorg. Allg. Chem. 2010, 1783–1786
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
1785

M. Frotscher, M. Hölzel, B. Albert
ARTICLE
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Further details of the structure investigations may be obtained from the
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Acknowledgement
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Received: February 22, 2010
Published Online: June 22, 2010
1786
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© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2010, 1783–1786