Synthesis, Crystal Structure, and Properties of MgxB 50C8 or Mgx(B12)4(CBC) 2(C2)2 (x = 2.4-4)

Article abstract of DOI:10.1021/ja102659d

Single crystals of a new magnesium boride carbide Mg_xB ₅₀C₈ (x = 2.4-4) were synthesized from the elements in a metallic melt using tantalum ampules. Crystals were characterized by single crystal X-ray diffraction and electron microprobe analysis. The variation of the Mg content results from different reaction conditions. The composition Mg _～3B₅₀C₈ is by far the most favored. It fulfills the electron counting rules of Wade and Longuet-Higgins and thus explains the light-green to yellow transparent color. The structure of Mg _～3B₅₀C₈ (C2/m, Z = 1, a = 8.9384(12) A, b = 5.6514(9) A, c = 9.6021(13) A, β = 105.86(1)°) consists of B₁₂ icosahedra. The icosahedra are interconnected by four exohedral B-B bonds to layers. The layers are connected to a three-dimensional covalent network by C₂ and CBC units and further exohedral B-B bonds. The Mg sites are partially occupied. Different site occupation factors cause the various compositions and colors (Mg _2.4B₅₀C₈, brown; Mg₄B ₅₀C₈, black). The vibrational spectra show the modes of B₁₂ icosahedra and C₂ and CBC units as well. Measurements of the microhardness according to Vickers and Knoop revealed remarkably high values of H_V = 3286 (32.0 GPa) and H_K = 3165 (31.5 GPa), which exceed the values of B₄C. Optical spectra reveal a band gap of 2.7 eV for Mg_～3B₅₀C₈, in agreement to the observed color. This justifies an ionic description, and the formula can be written as (Mg²⁺)₃(B₁₂^2-) ₄(CBC⁺)₂(C₂)₂.

Full text of DOI:10.1021/ja102659d

Published on Web 09/14/2010
Synthesis, Crystal Structure, and Properties of MgxB50C8 or
Mgx(B12)4(CBC)2(C2)2 (x ) 2.4-4)
Volker Adasch,† Melanie Schroeder,‡ Dominik Kotzott,‡ Thilo Ludwig,‡
Natascha Vojteer,‡ and Harald Hillebrecht*,‡,§
DRONCO AG, Wiesenmu¨hle 1, D-95632 Wunsiedel, Germany, Institut fu¨r Anorganische und
Analytische Chemie, Albert-Ludwigs-UniVersita¨t Freiburg, Albertstrasse 21,
D-79104 Freiburg, Germany, and Freiburger Materialforschungszentrum FMF,
Stefan-Maier-Strasse 25, D-79104 Freiburg, Germany
Received April 13, 2010; E-mail: harald.hillebrecht@ac.uni-freiburg.de
Abstract: Single crystals of a new magnesium boride carbide MgxB50C8 (x ) 2.4-4) were synthesized
from the elements in a metallic melt using tantalum ampules. Crystals were characterized by single crystal
X-ray diffraction and electron microprobe analysis. The variation of the Mg content results from different
reaction conditions. The composition Mg∼3B50C8 is by far the most favored. It fulfills the electron counting
rules of Wade and Longuet-Higgins and thus explains the light-green to yellow transparent color. The
structure of Mg∼3B50C8 (C2/m, Z ) 1, a ) 8.9384(12) Å, b ) 5.6514(9) Å, c ) 9.6021(13) Å, ꢀ ) 105.86(1)°)
consists of B12 icosahedra. The icosahedra are interconnected by four exohedral B-B bonds to layers.
The layers are connected to a three-dimensional covalent network by C2 and CBC units and further exohedral
B-B bonds. The Mg sites are partially occupied. Different site occupation factors cause the various
compositions and colors (Mg2.4B50C8, brown; Mg4B50C8, black). The vibrational spectra show the modes of
B12 icosahedra and C2 and CBC units as well. Measurements of the microhardness according to Vickers
and Knoop revealed remarkably high values of HV ) 3286 (32.0 GPa) and HK ) 3165 (31.5 GPa), which
exceed the values of B4C. Optical spectra reveal a band gap of 2.7 eV for Mg∼3B50C8, in agreement to the
observed color. This justifies an ionic description, and the formula can be written as (Mg2+)3(B122-)4(CBC+)2(C2)2.
temperature thermoelectrics.8 Our previous investigations have
shown that the use of molten metals gives a synthetic access to
Introduction
Because boron-rich borides and boride carbides contain a
great variety of boron polyhedra, they form their own class of
compounds and show an unique structure chemistry1 and
physical properties.2,3 Furthermore, they are of growing interest
to a number of applications in material sciences, for example,
as high temperature materials,3 abrasives,4 composites,5 high
Tc superconductors,6 high temperature semiconductors,7 and high
new borides and boride carbides not obtained by conventional
high temperature methods.9-12 Motivated by the discovery of
superconductivity in MgB213 and the excellent material proper-
ties of Mg alloys,14 we extended this method to compounds
containing Li15,16 and Mg17-21 by use of h-BN crucibles and
tantalum ampules.
(6) (a) Gunji, S.; Kamimura, H. Phys. ReV. B 1996, 54, 13665–13673.
(b) Calandra, M.; Vast, N.; Mauri, F. Phys. ReV. B 2004, 69, 224505.
(c) Matsuda, H.; Nakayama, T.; Kimura, K.; Murakami, Y.; Suematsu,
H.; Kobayashi, M.; Higashi, I. Phys. ReV. B 1995, 52, 6102–6110.
(7) (a) Emin, D. J. Solid State Chem. 2004, 177, 1619–1623. (b) Samara,
G. A.; Tardy, H. L.; Venturini, E.; Aselage, T. L.; Emin, D. Phys.
ReV. B 1993, 48, 1468. (c) Emin, D. Phys. Today 1987, 20, 55. (d)
Mori, T. In Handbook on the Physics and Chemistry of Rare Earths;
Gschneidner, K. A., Bunzli, J. C., Pecharsky, V., Eds.; Elsevier:
Amsterdam, 2008; Vol. 38, pp 106-173.
† DRONCO AG.
‡ Albert-Ludwigs-Universita¨t Freiburg.
§ Freiburger Materialforschungszentrum FMF.
(1) (a) Matkovich, V. I. Boron and Refractory Borides; Springer-Verlag:
Berlin, 1977. (b) Lundstro¨m, T. In Encyclopedia of Inorganic
Chemistry; King, R. B., Ed.; Wiley: New York, 1994. (c) Albert, B.;
Hillebrecht, H. Angew. Chem. 2009, 121, 8794–8824; Angew. Chem.,
Int. Ed. 2009, 48, 8640–8668.
(8) Wood, C. Rep. Prog. Phys. 1988, 51, 459.
(2) (a) Werheit, H. Boron in Landolt-Bo¨rnstein; Springer-Verlag: Berlin,
1998; Vol. 41C. (b) He, J.; Wu, E.; Wang, H.; Liu, R.; Tian, Y. Phys.
ReV. Lett. 2005, 94, 015504. (c) Cao, L.; Zhang, Z; Sun, L.; Gao, C.
AdV. Mater. 2001, 13, 1701–1704.
(9) (a) Hillebrecht, H.; Meyer, F. Angew. Chem. 1996, 108, 2655; Angew.
Chem., Int. Ed. 1996, 35, 2499. (b) Hillebrecht, H.; Ade, M. Angew.
Chem. 1998, 110, 981; Angew. Chem., Int. Ed. 1998, 37, 935. (c)
Hillebrecht, H.; Gebhardt, K. Angew. Chem. 2001, 113, 1492; Angew.
Chem., Int. Ed. 2001, 40, 1445.
(3) (a) Chawla, K. K. Ceramic Composites; Chapman & Hall: London,
1993. (b) Chou, T. W. Structure and Properties of Composites in
Material Science and Technology; Verlag Chemie: Weinheim, Ger-
many, 1993; Vol. 13. (c) Riedel, R., Ed. Handbook of Ceramic Hard
Materials; Wiley-VCH: Weinheim, Germany, 2000.
(4) (a) Riedel, R. AdV. Mater. 1994, 4, 549–560. (b) Telle, R. Chem.
Unserer Zeit 1988, 22, 93–99.
(10) Adasch, V. Ph.D. Thesis, University Bayreuth, Germany, 2005.
(11) Vojteer, N. Ph.D. Thesis, University Freiburg, Germany, 2008.
(12) Schroeder, M. Ph.D. Thesis, University Freiburg, Germany, 2009.
(13) Nagamatsu, J.; Nakagawa, N.; Muranaka, T.; Zenitani, Y.; Akimitsu,
J. Nature 2001, 40, 63–64.
(14) Kainer, K. U., Ed. Magnesium Alloys and Their Applications; Wiley-
VCH, Weinheim, Germany, 2000.
(15) Vojteer, N.; Hillebrecht, H. Angew. Chem. 2006, 118, 172–175; Angew.
Chem., Int. Ed. 2006, 45, 165–168.
(5) (a) Halverson, D. C.; Pyzik, A.; Aksay, I. Ceram. Eng. Sci. Proc.
1985, 6, 736–744. (b) Pyzik, A.; Beamon, D. J. Am. Ceram. Soc. 1995,
78, 305–312.
9
10.1021/ja102659d  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 13723–13732 13723

A R T I C L E S
Adasch et al.
When we started our investigations, MgB2C222 was the only
ternary compound reported for the system Mg/B/C.23 The crystal
structure of MgB2C2 can be described as a heterographite where
boron and carbon form hexagonal layers and magnesium is
intercalated between the layers.
Recently we reported on the first boron rich boride carbides
of magnesium: Mg2B24C,20 o-MgB12C2, and m-MgB12C2.21 In
this contribution we report on MgxB50C8 (x ) 2.4-4). The
crystal structure combines the structural features of Li2B12C2
and LiB13C2.15 Besides the well-known B12 icosahedra, the
crystal structure contains C2 and linear CBC units. Therefore,
the formula can be written as Mgx(B12)4(CBC)2(C2)2. The Mg
content varies between x ) 2.4 and x ) 4.0, depending on the
conditions of the synthesis. For Mg3B50C8 the composition was
confirmed by EDX and WDX measurements on single crystals.
The microhardness is remarkably high. Hardness measurements
according to the methods of Vickers (HV ) 32.0 GPa) and
Knoop (HK ) 31.5 GPa) reveal values that are even higher than
those of B4C, so Mg3B50C8 is after diamond and c-BN is one
of the hardest materials known. A new concept for superhard
materials can be developed.
Figure 1. Upper row: Single crystals of Mg3B50C8. Lower row: Mg∼4B50C8,
crystal size between 0.2 and 0.5 mm.
Mg/B/C of 200:200:75:3. The ampule was heated under an argon
atmosphere with 100 K/h to 1573 K, held for 25 h, and cooled
with 10 K/h to 773 K and with 100 K/h to room temperature.
1.3. Mg∼2.4B50C8. Brown crystals of Mg2.4B50C8 (Figure 1) were
found as a byproduct from a batch that yielded mostly MgB9N.23b
A Mg/Cu melt was used with a ratio Mg/Cu/B/C of 300:100:75:3.
The ampule was heated under an argon atmosphere up to 1673 K,
held for 30 h, and cooled with 60 K/h to room temperature. The
formation of the nitride results from the reaction of the crucible,
maybe caused by the higher temperature.
Experimental Section
1. Synthesis. 1.1. Mg∼3B50C8. Single crystals of Mg∼3B50C8
were synthesized from the elements in a Cu/Mg melt. Cu (powder,
>250 mesh, p.A., Merck), Mg (powder, 325 mesh, 99.8%, Riedel-
de Haen), B (crystalline, ∼325 mesh, 99.7%, Alfa Aesar), and C
(graphite powder, <100 µm, 99.9%, Heraeus) were mixed in a molar
ratio of 12:4:3:0.1 and pressed into a pellet (∼3 g). The pellet was
put into a h-BN crucible and the crucible into a tantalum ampule,
which was sealed by welding with an electric arc. The ampule was
heated under an argon atmosphere up to 1573 K, held for 6 h, and
cooled with 10 K/h to 873 K and with 100 K/h to room temperature.
The ampule was opened, and the excess melt was dissolved in
concentrated nitric acid. Single crystals of Mg∼3B50C8 were
transparent platelets of light-green to yellow or light-brown color
with a size of up to 1 mm (Figure 1). As byproducts, single crystals
2. Structure Solution and Refinement. 2.1. Mg3B50C8. Single
crystal investigations of Mg3B50C8 were done with a diffractometer
equipped with Mo KR radiation and an image plate detector (IPDS
II, Fa. Stoe). A C-centered monoclinic unit cell was obtained with
a ) 8.9384(12) Å, b ) 5.6514(9) Å, c ) 9.6021(13) Å, and ꢀ )
105.86(1)°. The measurement of 4180 intensities gave a data set
of 1103 independent reflections (964 with I > 2σ(I)). Because of
the low absorption coefficient (0.20 mm-1), no correction of
absorption effects was done. The additional reflection condition hkl
with h + k ) 2n led to space group C2/m and the noncentrosym-
metric space groups C2 and Cm, respectively. The structure solution
by direct methods (SHELXL25) was started in C2/m and revealed
a structure model with 11 independent nonmetal atoms and two
Mg sites. The labeling of the atoms was done according to electron
densities and bonding distances. B and C atoms were refined with
anisotropic displacement parameters. Because partial and mixed
occupations are well-known for boron rich borides, all occupation
factors were refined separately as a free variable, but no significant
deviations from full occupation were observed. Both Mg atoms
are on general sites, but their site occupation factors are far from
unity. Additionally the Mg-Mg-distances (Mg1-Mg1, 0.465 Å;
Mg1-Mg2, 0.397/0.860 Å) are much too short for a simultaneous
occupation. The free refinement of the site occupation factors
yielded values of 0.2193(3) for Mg1 and 0.1482(15) for Mg2, which
were in agreement with the expected minimum distances. In order
to get a stable refinement, it was necessary to use isotropic and
coupled displacement parameters. Finally R-values of R1(F) ) 0.029
and wR2(I) ) 0.085 were yielded for 1096 reflections and 87 free
variables. Several single crystals were measured because of the
problems with the Mg sites. It turned out that this is a general
phenomenon in Mg∼3B50C8. Diffuse scattering (streaks) and weak
satellite reflections indicating a modulated structure and not a simple
supercell were observed for all crystals investigated. This can be
explained by the special situation of the Mg atoms (see Results
and Discussion). For the (nearly) colorless crystals the sum of the
Mg site occupation factors was always very close to the composition
10,11,24
18
of dark red MgB7
and black MgB12 were found.
Small amounts of Mg∼3B50C8 were frequently observed as
unexpected byproduct from syntheses of other borides17,18 because
commercial samples of boron can contain significant amounts of
carbon. Furthermore Mg∼3B50C8 can be formed together with other
boride carbides20,21 of Mg according to the equilibrium conditions
during the crystal growth.
1.2. Mg∼4B50C8. Black crystals of Mg4B50C8 were found as a
byproduct from a batch that yielded mostly MgB7 and
o-MgB12C2. In comparison to the previous synthesis the melt
contained more Mg. The elements were mixed with a ratio Cu/
(16) Vojteer, N.; Stauffer, J.; Hillebrecht, H.; Hofmann, K.; Panda, M.;
Albert, B. Z. Anorg. Allg. Chem. 2009, 635, 653–659.
(17) Adasch, V.; Hess, K.-U.; Ludwig, T.; Vojteer, N.; Hillebrecht, H. J.
Solid State Chem. 2006, 179, 2900–2907.
(18) Adasch, V.; Hess, K.-U.; Ludwig, T.; Vojteer, N.; Hillebrecht, H. J.
Solid State Chem. 2006, 179, 2916–2926.
(19) Ludwig, T.; Hillebrecht, H. J. Solid State Chem. 2006, 179, 1622–
1628.
(20) Adasch, V.; Hess, K.-U.; Ludwig, T.; Vojteer, N.; Hillebrecht, H. J.
Solid State Chem. 2006, 179, 2150–2157.
(21) Adasch, V.; Hess, K.-U.; Ludwig, T.; Vojteer, N.; Hillebrecht, H.
Chem.sEur. J. 2007, 13, 3450–3458.
(22) Wo¨rle, M.; Nesper, R. J. Alloys Comp. 1994, 216, 75–83.
(23) (a) Rogl, P. Phase Diagrams of Ternary Metal-Boron-Carbon
Systems; ASM: Materials Park, OH, 1998. (b) Villars, P.; Calvert, D.
Pearson’s Handbook, Crystallographic Data for Intermetallic Phases;
ASM International: Materials Park, OH, 1997.
(24) (a) Naslain, R.; Guette, A.; Hagenmu¨ller, P. J. Less-Common Met.
1976, 47, 1. (b) Guette, A.; Barret, M.; Naslain, R.; Hagenmuller, P.
J. Less-Common Met. 1981, 82, 325–334.
(25) Sheldrick, G. M. SHELXL; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
9
13724 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010

MgxB50C8 and Mgx(B12)4(CBC)2(C2)2 (x ) 2.4-4)
A R T I C L E S
Table 3. Anisotropic Displacement Parameters (in Å2) of
Table 1. Crystallographic Data and Refinement for Mg3B50C8
a
Mg∼3B50C8
parameter
Mg3B50C8
atom
U11
U22
U33
U12
U13
U23
temperature
crystal shape
color
293(2) K
platelet
C1 0.0051(2) 0.0041(3) 0.0042(3)
C2 0.0124(3) 0.0044(3) 0.0045(3)
B1 0.0069(4) 0.0063(4) 0.0063(4)
B2 0.0058(3) 0.0065(3) 0.0045(3)
B3 0.0055(3) 0.0065(3) 0.0050(3)
B4 0.0052(3) 0.0051(3) 0.0043(3)
B5 0.0054(3) 0.0044(3) 0.0043(3)
B6 0.0066(2) 0.0046(2) 0.0045(2) 0.0000(2) 0.0011(2) -0.0001(2)
B7 0.0056(2) 0.0045(2) 0.0042(2) 0.0002(2) 0.0009(2) -0.0001(2)
B8 0.0049(2) 0.0055(2) 0.0049(2) 0.0000(2) 0.0014(2) -0.0000(2)
B9 0.0053(2) 0.0055(2) 0.0044(2) 0.0004(2) 0.0010(2) -0.0000(2)
0
0
0
0
0
0
0
0.0008(2)
0.0011(2)
0.0011(3)
0.0017(2)
0.0013(2)
0.0014(2)
0.0009(2)
0
0
0
0
0
0
0
light-green
size
0.2 × 0.2 × 0.04 mm3
crystal system
space group
unit cell
monoclinic
C2/m (No. 14)
a ) 8.9384(13) Å
b ) 5.6514(9) Å
c ) 9.6021(13) Å
ꢀ ) 105.86(1)°
V ) 466.58(8) Å3
Z ) 1
dcalcd
data collection
Mo KR
2.525 g/cm3
a esd values in parentheses.
STOE IPDS II
λ ) 0.710 73 Å (graphite monochromated)
0° e ω e 180°
ψ ) 0°; ∆ω ) 2°
300 s
2.2. Mg4B50C8. Black crystals of Mg4B50C8 yielded a slightly
larger unit cell of a ) 8.942(2) Å, b ) 5.6478(8) Å, c ) 9.614(2)
Å, and ꢀ ) 105.60(3)°. The measurement of 5033 intensities gave
a data set of 983 independent reflections (868 with I > 2σ(I)). The
refinement was started with the parameters of Mg3B50C8. No
significant differences were observed for B and C atoms including
the very small and homogeneous displacement parameters, but the
site occupation factor of the Mg atoms was higher. Again, several
crystals were measured yielding similar results within small standard
deviations. The resulting compositions were between Mg∼3.5B50C8
and Mg∼4.0B50C8. Again, diffuse scattering and weak satellite
reflections were observed for all crystals. The best results were
exposure time
θ range
7° < 2θ < 70°
-14 < h < 13
-9 < k < 9
-16 < l < 15
µ
0.20 mm-1
absorption correction
Rint./Rσ
none
0.047/0.007
refinement
SHELXL;25 full-matrix least-squares
refinement on F2
3589, 1096
N(hkl) measd, unique
N′(hkl) (I > 2σ(I))
parameters refined
R-values
obtained for a crystal with a composition Mg4B50C8, i.e., sofMg1
)
964
87
0.329(6) and sofMg2 ) 0.183(6). The final R-values were R1(F) )
0.040 and wR2(I) ) 0.106 with 983 reflections and 87 free variables.
2.3. Mg2.4B50C8. The lattice parameters of the brown crystals
were smaller (a ) 8.908(2) Å, b ) 5.6623(14) Å, c ) 9.605(3) Å,
ꢀ )105.60(3)°). The measurement of 4023 intensities gave a data
set of 996 independent reflections (681 with I > 2σ(I)). The
refinement with the known model showed only small differences
for the B and C atoms, but the site of Mg2 was not occupied. The
occupation factor for the remaining Mg atom increased to 0.299(3),
and the composition was Mg2.4B50C8. It is worth mentioning that
the streaks and satellite reflections were no longer observed. The
displacement parameters were uniform and very similar to those
of Mg3B50C8, so there is no sign of significant amounts of nitrogen.
R-Values were R1(F) ) 0.042 and wR2(I) ) 0.102 with 996
reflections and 88 free variables.
Further details of the crystal structure investigations may be
obtained from the Fachinformationszentrum Karlsruhe, D-76344
Eggenstein-Leopoldshafen (Germany) (fax, (+49)724-808-666;
e-mail, crysdata@fiz-karlsruhe.de) on quoting the depository num-
bers CSD-419728 (Mg2.4B50C8), CSD 419727 (Mg3B50C8), and
CSD-419729 (Mg4B50C8).
3. Analyses/Characterization. Qualitative and quantitative analy-
ses on selected single crystals were done by EDX and WDX
measurements. Several single crystals of Mg∼3B50C8 were checked
by EDX (Jeol, JSM 6400 with Ge detector, sample fixed with
conducting glue on a graphite platelet mounted on an aluminum
sample holder). It was confirmed that magnesium is the only heavy
element (Z > 10).
By WDX (Jeol, JXA 8200) a more detailed analysis especially
in consideration of light elements (4 < Z < 11) was done to exclude
their incorporation (oxygen, nitrogen) or to ensure its presence
(carbon), respectively, because these problems frequently occur in
boron rich borides. For the WDX measurement the single crystals
used for the structure determinations by X-ray methods were fixed
in a matrix with Ag/epoxy resin. They were polished to get a clear
surface and to ensure the measurement of the interior of the crystal10
and not of the surface which may be influenced by the contact to
the melt or the single crystal isolation process. Boron, carbon, and
magnesium were detected as the only elements with Z > 4. There
were very small amounts of Cu (0.1%). The molar ratios B/C/Mg
R1(F) ) 0.0290, wR2(F2) ) 0.0853
R1 ) 0.0336
all data
weighting scheme25
extinction correction25
goodness of fit
0.0663/0.0168
0.000(11)
1.076
residual electron density
(max, min, σ)
+0.50, - 0.28, 0.08 e-/Å3
Table 2. Atomic Coordinates, Equivalent Displacement
a
Parameters (in Å2), and Site Occupation Factors of Mg∼3B50C8
atom site
x
y
z
sof
Ueq
b
c
Mg1 8j 0.7052(2) 0.3358(4)
Mg2 8j 0.7683(3) 0.2202(5)
C1 4i 0.13442(8)
C2 4i 0.02208(9)
0.5
0.5
0.2193(3)
0.0052(2) /Uiso
c
b
0.1481(15) 0.0052(2) /Uiso
0
0
0
0.94367(8) 1.000(4)
0.57526(9) 1.000(4)
0
0.60812(8) 1.000(3)
0.60018(8) 1.000(4)
0.91679(8) 0.990(4)
0.91084(8) 1.000(4)
0.0045(2)
0.0073(2)
0.0066(2)
0.0055(2)
0.0057(2)
0.0048(2)
0.0048(2)
0.0053(1)
0.0049(1)
0.0051(1)
0.0051(1)
B1 2a
0
0.997(3)
B2 4i 0.93410(9) 0.5
B3 4i 0.12581(9) 0.5
B4 4i 0.02093(9) 0.5
B5 4i 0.22294(9) 0.5
B6 8j 0.04885(6) 0.24148(11) 0.66446(6) 1.002(3)
B7 8j 0.10655(6) 0.75550(10) 0.85302(6) 1.003(3)
B8 8j 0.91773(6) 0.33789(10) 0.75755(6) 1.003(3)
B9 8j 0.74452(6) 0.83697(10) 0.75433(6) 0.990(4)
a esd values in parentheses. b Refined as free variable. c Coupled
refinement, Uiso
.
Mg3B50C8. Details for the best refinement are listed in Table 1.
Coordinates and thermal displacement parameters are given in Tables
2 and 3. Selected distances and angles are shown in Table 4.
Similar to other structure determinations of comparable com-
pounds15,17,18,20,21 the distinction between boron and carbon atoms
is easy. With the correct labeling all thermal displacement
parameters have nearly the same value. The ellipsoids show only
small deviations from ideal spheres. The only exception is the C
atom of the C2 unit in Mg4B50C8 and Mg3B50C8. This is caused by
the distribution of the Mg atoms. The very small absolute values
of Ueq indicate a strong covalent bonding and are consistent with
the high hardness of boron rich boride carbides.
9
J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010 13725

A R T I C L E S
Adasch et al.
a b
,
Table 4. Selected Distances (in (Å) and Angles (in deg) in Mg∼3B50C8
Mg1-C2
Mg1-B3
Mg1-B2
Mg1-B3
Mg1-B2
Mg1-B6
Mg1-B6
Mg1-B9
Mg1-B8
Mg1-B9
Mg1-B8
2.167(2)c
2.205(1)
2.216(1)
2.325(1)
2.333(1)
2.417(1)
2.424(1)
2.564(1)
2.596(1)
2.601(1)
2.636(1)
Mg2-B3
Mg2-B2
Mg2-B3
Mg2-B2
Mg2-B9
Mg2-C2
Mg2-B9
Mg2-B8
Mg2-B8
Mg2-B6
Mg2-B6
2.182(2)
2.192(2)
2.198(2)
2.207(2)
2.503(2)
2.512(3)c
2.529(2)
2.536(2)
2.563(2)
2.567(2)
2.575(2)
C1-B1
C1-B5
C1-B7
1.4460(7)
1.6135(11)
1.6159(7)c
C2-C2
C2-B6
C2-Mg1
C2-Mg1′
1.3895(16)
1.5943(7)c
2.169(2)c
2.170(2)c
B1-C1
1.4460(7)c
B2-B3
B2-B6
B2-B8
B2-B3
1.7179(12)
1.7921(8)c
1.8234(9)c
1.8653(11)
B3-B2
B3-B6
B3-B9
B3-B2
1.7179(12)
1.7948(8)c
1.8189(9)c
1.8653(11)
B4-B4
B4-B7
B4-B5
B4-B8
1.7369(16)
1.8174(8)c
1.8224(11)
1.8321(9)c
B5-C1
B5-B7
B5-B9
B5-B4
1.6135(11)
1.7777(7)c
1.8174(9)c
1.8224(11)
B6-C2
B6-B7
B6-B2
B6-B3
B6-B9
B6-B8
1.5943(7)
1.7414(9)
1.7921(8)
1.7948(8)
1.8089(8)
1.8174(8)
B7-C1
B7-B6
B7-B5
B7-B4
B7-B9
B7-B8
1.6135(7)
1.7414(9)
1.7777(7)
1.8174(8)
1.8255(8)
1.8400(8)
B8-B9
B8-B6
B8-B2
B8-B4
B8-B8
B8-B7
1.7548(8)
1.8174(8)
1.8234(9)
1.8321(9)
1.8323(12)
1.8400(8)
B9-B8
B9-B6
B9-B5
B9-B3
B9-B7
B9-B9
1.7548(8)
1.8089(8)
1.8174(9)
1.8189(9)
1.8255(8)
1.8425(12)
B1-C1-B3
102.52(8)
C2-C2-B2
B2-C2-B2
121.14(3)c
117.72(7)
B1-C1-B4
B3-C1-B4
B4-C1-B4
100.38(4)c
115.82(3)c
117.54(6)
C1-B1-C1
180° (by symmetry)
a Exohedral B-B distances are in italics. Mg-Mg distances are omitted. b Refinement with disorder and partial occupation. c Occurs 2×.
were found to be 81.0:13.9:4.9. Calculated values for the composi-
tion of Mg3B50C8 are 81.9:13.3:4.8.
The special properties of boron rich borides require a detailed
quantitative analysis and special care on the refinement of single
crystal data (for example, NaB15/Na2B29,26 NaB5C/KB5C/KB627).
In some previous contributions we have shown that the combination
of high quality single crystal data and WDX/EDX measurements
of single crystals yields reliable results for this special class of
compounds (Al2.7B48C2,28 MgB17,17 MgB12,18 MgB12Si2,19
Mg2B24C,20 MgB12C2,21 Li2B12C2,15 LiB13C2,15 Li2B12Si229).
4. Microhardness. Microhardness was measured with a micro-
hardness equipment MHT 10 (producer, A. Paar, Austria). A force
of 2 N was generated within 10 s and applied for 15 s. The imprints
Figure 2. Crystal structure of Mg3B50C8.
of the indenters (Vickers hardness, square pyramid; Knoop, rhombic
pyramid) were evaluated and converted into a value for the
microhardness according to the usual procedures.30
6. UV/Visible Spectra. The single-crystal-UV/vis spectra were
measured in transmission at room temperature with a JASCO V-570
UV/vis/NIR photometer in a range from 200 to 2500 nm.
5. IR and Raman Spectra. The FT-IR and FT-Raman measure-
ments were performed with a Bruker IFS66v spectrometer. The IR
sample was made of crystalline powder pressed with KBr to pellets.
Raman measurements were carried out with a Nd:YAG laser with
an output of 100 mW and 1000 scans on single crystals with a
Raman microscope. Powder samples were prepared in 0.3 mm
capillary and measured with an output of 400 mW and 5000 scans.
Results
According to the different conditions of synthesis, the Mg
content shows a significant variation: MgxB50C8 with x ) 2.4-4.
Mg∼3B50C8 (i.e., x ) 3) is accessible as a main product, while
Mg∼2.4B50C8 and Mg∼4B50C8 were only observed in small
amounts as byproduct. Therefore, we focus the discussion on
the phase with x ≈ 3. Furthermore, it will be named as Mg3B50C8
because the deviation from the integer number is very small.
Mg3B50C8. The crystal structure of Mg3B50C8 (Figure 2) is
characterized by layers of B12 icosahedra. The icosahedra are
quite regular (Figure 3). The endohedral B-B distances are
spread over a range between 1.718 and 1.843 Å (L ) 1.801
Å). Within the hexagonal layer each B12 icosahedron is
coordinated by six icosahedra forming exohedral B-B bonds
to four of them with a bond length of 1.755 Å. This explains
the pseudohexagonal metric of the ab plane and its slight
(26) (a) Albert, B.; Schmitt, K. Chem. Commun. 1998, 2373–2374. (b)
Albert, B.; Hofmann, K.; Fild, C.; Eckert, H.; Schleifer, M.; Gruehn,
R. Chem.sEur. J. 2000, 6, 2531–2536. (c) Naslain, R.; Kasper, J. S.
J. Solid State Chem. 1970, 1, 150–151.
(27) (a) Albert, B.; Schmitt, K. Chem. Mater. 1999, 11, 3406–3409. (b)
Ammar, A.; Me´ne´trier, M.; Villesuzanne, A.; Matar, S.; Chevalier,
B.; Etourneau, J.; Villeneuve, G.; Rodr´ıguez-Carvajal, J.; Koo, H.-J.;
Smirnov, A. I.; Whangbo, M.-H. Inorg. Chem. 2004, 43, 4974–4987.
(28) Meyer, D. Ph.D. Thesis, University Freiburg, Germany, 1999.
(29) Vojteer, N.; Schroeder, M.; Ro¨hr, C.; Hillebrecht, H. Chem. Eur. J.
2008, 14, 7331–7342.
(30) DIN EN 6507.
9
13726 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010

MgxB50C8 and Mgx(B12)4(CBC)2(C2)2 (x ) 2.4-4)
A R T I C L E S
LiB13C2 (1.62 Å). Concerning the centers of the icosahedra, the
CBC groups occupy octahedral voids. These (nearly) linear CBC
groups are known from other boride carbides like Al3BC3,9b
Sc2BC2,34 Lu3BC3,35 Sc3B0.75C3,36 Ca9Cl8(CBC)2,37 Ca5Cl3C2-
(CBC),38 La9Br6(CBC)3,39 and La9Br5(CBC)3.40
The two independent Mg atoms are placed in voids between
the congruent layers. The distances to the surrounding B and C
atoms (Mg-B, 2.18-2.64 Å; Mg-C, 2.17 Å) are comparable
to those of other boride carbides of magnesium.20,21 The
surroundings (Figure 5) are built up by two faces and two edges
of the B12 icosahedra and a C2 unit. The resulting coordination
number of 12 (i.e., 10 + 2) is plausible for boron rich borides.
Both Mg sites are only partially occupied (Mg1, 0.2193(3); Mg2,
0.1482(15)). The site occupation factors sum up to 2.90(3); i.e.,
3 of the 16 possible Mg positions are occupied. A detailed
discussion is given below.
Figure 3. B12-icosahedron in Mg3B50C8. Ellipsoids are drawn with 95%
probability and with mirror plane through B2/B3/B4/B5.
In total each B12 icosahedron has seven exohedral B-B-bonds
(4 + 2 + 1) and five bonds to carbon. According to the structural
features, the formula Mg3B50C8 can be written as
Mg3(B12)4(CBC)2(C2)2. Because Mg3B50C8 is nearly colorless
and transparent, an electron-precise bonding situation is likely
and the charge distribution follows well-known principles.
According to Wade,41 the icosahedra as closo-clusters with 12
exohedral 2e-2c bonds take up two electrons. Longuet-
Higgins42 showed this principle can be transferred from
molecules to extended 3D structures with a covalent network
of boron polyhedra. In Mg3B50C8, Mg as an electropositive metal
is a cation Mg2+ and supplies two electrons. The carbon atoms
of the C2 unit show four covalent bonds (one C-C double bond
and two B-C single bonds), so they can be regarded as neutral.
For the remaining CBC unit the charge +1 should be assumed
with the positive charge mainly localized on the 2-fold bonded
boron atom. A similar situation exists in LiB13C2 (or LiB12(CBC)
) Li+ (B122-)(CBC+)), where the proposed charge distribution
was confirmed by band structure calculations.43 For other boron-
rich borides like Li2B12Si229 and o-MgB12C212 it was also shown
that the formal charge of the cations is quite close to the
idealized values. Therefore, the electronic situation is described
by the formula (Mg2+)3(B122-)4(CBC+)2(C2)2, and Mg3B50C8 is
another example for the preference of electron-precise structures
in boron-rich borides, boride carbides, and boride silicides of
alkali and alkali earth metals.
deviation from the ideal value (c/a ) 1.58 * ꢀ3). This type of
layer is a frequent motif in boron-rich borides and is also
observed in o-MgB12C2,21 Mg1.13B12Si2,31 SiB3,32 Li2B12Si2,29
LiB13C2,15 and Li2B12C2.15 The layers are stacked in a sequence
AABBCC parallel to the ab plane; i.e., two congruent layers
form a double layer. The linkage between these double layers
takes place via edges of the icosahedra. This type of connection
is quite rare. Under the large number of boron-rich borides it is
only observed in m-MgB12C2,21 Mg1.13B12Si2,31 Li2B12C2,15 and
YB41Si1.2.33 The geometrical restrictions connecting icosahedra
in that way may explain the long exohedral B-B bond of 1.864
Å (B2-B3). These are the longest B-B-distances in the
structure, while the shortest ones (1.718 Å) are the distances
B2-B3 within the icosahedron. The congruent double layers
are connected by one exohedral bond (B4-B4: 1.734 Å) and
stacked in a way as it is known from closest sphere packings.
The covalent network is completed by C2 and linear CBC
units. The C2 units are between the congruent icosahedra. Each
C atom is bonded to two icosahedra (Figure 4). The bonding
angles B-C-C and B-C-C are quite close to 120°, so the
short C-C distance of 1.389 Å can be explained as a double
bond and a sp2-like situation for carbon. A similar C2 unit was
already found in Li2B12C2 (C-C, 1.374 Å), so Mg3B50C8
represents the second example.
The CBC units link the double layers of congruent icosahedra.
The central boron atom has two short distances to carbon (1.446
Å). According to the site symmetry, the C-B-C angle is 180°.
The carbon atoms of the CBC unit are tetrahedrally coordinated
by boron. One short bond occurs to the central B atom and
three longer ones (1.613-1.616 Å) occur to three different
icosahedra (Figure 4). The CB4 tetrahedra are distorted with
B-C-B angles between 100.4° and 117.5°. The B-C distances
are comparable to those of other boron-rich boride carbides
(Mg2B24C, 1.67 Å;20 o-MgB12C2, 1.65-1.66 Å; m-MgB12C2,
1.64-1.67 Å21) but slightly longer than those of the C2 unit
(1.594 Å), which may be seen as a result from the different
coordination (CN 4/CN 3) and bonding (sp3/sp2) because similar
observations were made for Li2B12C2 (B-C: 1.59 Å) and
Despite the structural similarity of the CBC units in Mg3B50C8/
LiB13C2 (B-C, 1.415 Å) and Al3BC3 (1.441 Å)/Sc2BC2 (1.475
Å)/Lu3BC3 (1.446 Å), the description is different. According
(34) (a) Halet, J.-F.; Saillard, J.-Y.; Bauer, J. J. Less-Common Met. 1990,
158, 239–250. (b) Shi, Y.; Leithe-Jasper, A.; Tanaka, T. J. Solid State
Chem. 1999, 148, 250–259.
(35) Oeckler, O.; Jardin, C.; Mattausch, H.; Simon, A.; Halet, J.-F.; Saillard,
J.-Y.; Bauer, J. Z. Anorg. Allg. Chem. 2001, 627, 1389–1394.
(36) Shi, Y.; Bourgeois, L.; Leithe-Jasper, A.; Bando, Y.; Tanaka, T. J.
Alloys Compd. 2000, 298, 99–106.
(37) Reckeweg, O.; DiSalvo, F.; Meyer, H.-J. Z. Anorg. Allg. Chem. 1998,
625, 1408–1410.
(38) Reckeweg, O.; Meyer, H.-J. Angew. Chem. 1998, 110, 3619–3621.
(39) Mattausch, H.; Simon, A. Angew. Chem. 1995, 107, 1764–1766;
Angew. Chem., Int. Ed. Engl. 1995, 34, 1633–1635.
(40) Mattausch, H.; Simon, A.; Felser, C.; Dronskowski, R. Angew. Chem.
1996, 108, 1805–1807; Angew. Chem., Int. Ed. Engl. 1996, 35, 1685-
1687.
(31) Ludwig, T.; Hillebrecht, H. Presented at the 15th International
Symposium on Boron, Borides and Related Materials, Hamburg,
Germany, August 21-26, 2005; ISBB05. Publication in preparation.
(32) (a) Hirsche, B. Presented at the Symposium on Solid State Chemistry,
Hirschegg, Austria, May/June 2002. (b) Salvador, J. R.; Blic, D.;
Mahanti, S. D.; Kanatzidis, M. G. Angew. Chem. 2003, 115, 1973–
1976; Angew. Chem., Int. Ed. 2003, 42, 1929–1931. (c) Hirsche, B.
Ph.D. Thesis, University of Bayreuth, Germany, 2005.
(41) (a) Lipscomb, W. N. AdV. Inorg. Chem. Radiochem. 1950, 1, 117. (b)
Wade, K. AdV. Inorg. Chem. Radiochem. 1976, 18, 1.
(42) Longuet-Higgins, H. C.; Roberts, M. d. V. Proc. R. Soc., Ser. A 1955,
230, 110.
(33) Higashi, I.; Tanaka, T.; Kobayashi, K.; Ishizawa, Y.; Takami, M. J.
Solid State Chem. 1997, 133, 11.
(43) Balakrishnarajan, M. M.; Pancharatna, P. D.; Hoffmann, R. New
J. Chem. 2007, 31, 473–485.
9
J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010 13727

A R T I C L E S
Adasch et al.
Figure 4. Surroundings of the C2 unit (left) and the CBC unit (right) in Mg3B50C8.
Figure 5. Surrounding of Mg in Mg3B50C8.
to the (formally) positive charge of the B atom in the CBC unit,
the distances are shortened and the force constants (frequencies
in the vibrational spectra) enlarged with respect to a B-C single
bond. Vice versa the (formally) double bond in Al3BC3 is
reduced by the high negative charge of the CBC5- unit. The
effective bond order in the CBC unit of Al3BC3 is around 1.5
based on force constants (IR/Raman) and distances (Pauling’s
formula).9b In a previous publication44 the band structure
calculations reveal CBC5- units with a significant covalent
Al-C interaction.
Mg3B50C8 continues the row of boron-rich boridecarbides of
magnesium. It represents the most complex example for a crystal
structure containing a covalent network of B12 icosahedra.
Depending from the amount of additional carbon atoms or small
building units (i.e., C2, CBC) different types of frameworks are
realized. In Mg2B24C the B12 icosahedra form a tetragonal rod
packing with isolated C atoms in tetrahedral and Mg2+ in
octahedral voids between the rods. Each icosahedron has 10
exohedral B-B bonds to neighboring icosahedra and two bonds
to carbon. In m-MgB12C2 the icosahodra show a cubic closest
packing (fcc) with carbon atoms in all tetrahedral voids and
Mg2+ in all octahedral voids. According to the higher content
of carbon, each icosahedron shows eight exohedral B-B-bonds
and four B-C bonds. In o-MgB12C2 the icosahedra have a
hexagonal primitive arrangement. Mg2+ cations and C2 units
with an extremely long C-C distance of 1.73 Å and tetrahe-
drally coordinated C atoms are located in trigonal-prismatic
voids. Here the icosahedron has six exohedral B-B bonds and
six B-C bonds.
Remarkably Mg3B50C8 combines the structural features of
Li2B12C2 and LiB13C2 in a 1 + 1 manner. In both Li compounds
the B12 icosahedra form hexagonal layers connected by four
exohedral bonds. These layers are connected by a linear CBC
unit and one exohedral B-B bond (LiB13C2) or two exohedral
bonds between the edge of the icosahedron and a C2 unit with
a C-C double bond (Li2B12C2).
Mg2.4B50C8 and Mg4B50C8. The refinement of the single crystal
data shows that the different colors of the crystals are connected
to a variation of the Mg content. The black crystals have a higher
Mg content. From the sum of the occupation factors of the two
different Mg sites it results in a Mg content between Mg3.5B50C8
and Mg4.0B50C8. Similar to Mg3B50C8 the refinement of Mg must
be done with isotropic displacement parameters. For the
distances within the covalent network of B12 icosahedra, C2 and
CBC groups are nearly unchanged, so they will not be discussed
in detail.
(44) Jardin, C.; Hillebrecht, H.; Bauer, J.; Halet, J.-F.; Saillard, J.-Y.;
Gautier, R. J. Solid State Chem. 2003, 176, 609–614.
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13728 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010

MgxB50C8 and Mgx(B12)4(CBC)2(C2)2 (x ) 2.4-4)
A R T I C L E S
rich borides (Li2B12C2, LiB13C2,15 MgB12Si2,45 Li2B12Si229) and
can be discussed by comparison. The structure of Mg3B50C8
(or Mg3(B12)4(C2)2(CBC)2) can be seen as a combination of
Li2B12C2 and LiB13C2 () LiB12(CBC)). For both Li compounds
a detailed analysis of the vibrational spectra was possible
including symmetry analysis, setup of spectra of expectation,
and calculation of the frequencies on the basis of a force field.45
It turned out that all modes are more or less coupled because
of the strong interaction with the 3D covalent structure but the
qualitative assignment of frequencies to building units is
possible, especially for the region of the valence vibrations.
The Raman spectrum of Mg3B50C8 shows a strong and sharp
mode at 1436 cm-1 which can easily be attributed to the C-C-
double bond (Li2B12C2, 1477 cm-1). The shoulders on both sides
may be caused by the ordering of magnesium (see above). The
signal at 1148 cm-1 probably corresponds to the B-C bonds
(Li2B12C2, 1194 cm-1). Obviously there is no significant
difference between the two different types of B-C bonds. The
frequencies between 900 and 1050 cm-1 result from the internal
modes of the B12 icosahedron, the exohedral B-B bonds, and
contain the symmetric valence vibration of the CBC group
(probably 1034 cm-1). The latter occurs at 1156 cm-1 in
LiB13C2; i.e., the shorter B-C distance of 1.413 Å corre-
sponds to a higher frequency. In Al3BC3 the analogous values
are 1044 cm-1 and 1.45 Å.9a For the assignment of the modes
at lower wave numbers (deformation of B12 icosahedra, lattice
modes, ...) more detailed investigations are necessary.
Figure 6. Structure of Mg3B50C8 at z ≈ 0.5.
In the brown crystals only one site is occupied and the Mg
content is significantly lower (Mg2.4B50C8). The refinement of
the Mg atom with anisotropic displacement parameters was
possible without peculiarities, and the anisotropy of the thermal
displacement parameter of C2 vanished.
Mg Sites in Mg2.4B50C8, Mg3B50C8, and Mg4B50C8. The
problems with the localization of the Mg sites and the
observation of diffuse diffraction and satellite reflections
required a more detailed investigation. Figure 6 shows the
The IR spectrum shows very strong modes around 1090 cm-1,
which can certainly be assigned to the internal vibrations of
B12 icosahedra and the exohedral B-B bonds because they are
similarly observed in all boron-rich borides. The assignment of
1
structure of Mg3B50C8 at z ≈ /2. The 16 positions can be
the signals at 1630 cm-1 to the antisymmetric valence mode of
combined into four groups. The “outer” Mg atoms are Mg1,
and the “inner” are Mg2. Within a group only one site can be
occupied. The distances between the groups are as follows:
Mg1-Mg1, 1.85 Å; Mg1-Mg2, 2.20/2.60 Å. An analysis shows
that up to four positions can be occupied if unrealistically short
Mg-Mg-distances (<2.3 Å) are excluded. Although this model
is quite plausible, difference Fourier syntheses were done for
15
the CBC group is also quite clear (LiB13C2, 1628 cm-1
;
Al3BC3, 1580 cm-1).9a The shoulder at 1165 cm-1 may result
from the exohedral B-C valence modes (Li2B12C2, 1262 cm-1;
LiB13C2, 1263 cm-1). Modes around 800 cm-1 and 500 cm-1
are similarly found in Li2B12C2 (803 and 495 cm-11) and
LiB13C2 (793 and 493 cm-1). Although the spectrum below 900
cm-1 is well resolved, a detailed assignment requires again the
extensive approach of force field based calculations. These
investigations were successfully done for Li2B12C2, LiB13C2,45
1
all the three examples. Figure 7 shows the maps for z ≈ /2.
There are well-resolved maxima for Mg2.4B50C8 with a distance
of 0.9 Å. For Mg4B50C8 and especially for Mg3B50C8 the
resolution is lower and the electron densities are higher. In all
cases the maxima of the residual electron density correlate with
the site occupation factors. In Mg2.4B50C8 the maximum is 11.3
e-/Å and 3.8 e-/Å between them. The corresponding values in
Mg3B50C8 are 13.7 and 8.0 e-/Å, and in Mg4B50C8 they are 16.3
and 5.2 e-/Å, respectively. The refinement ignoring the diffuse
amounts simulates the distribution of the Mg atoms as a
superposition of one (Mg2.4B50C8) or two (Mg3B50C8, Mg4B50C8)
Mg sites. The displacement parameters of the surrounding atoms
show that only the position of the C2 unit is significantly
influenced by Mg. This is only observed for the samples with
a higher Mg content. With lower Mg content there is enough
space for the Mg atoms. Therefore, we conclude that the crystal
structure is well-described by the refinement with a statistical
occupation of the Mg sites and the highest possible Mg content
results in the composition Mg4B50C8.
12
and o-MgB12C2 and are in progress for Mg3B50C8.
Optical Spectra. Figure 9 shows the UV-vis spectrum of a
single crystal of Mg3B50C8 measured in transmission. Between
420 and 500 nm the absorption changes significantly. The onset
point in the low energy range can be determined to a value of
491 nm, which corresponds to an optical band gap of 2.7 eV.
This value agrees very well with the crystal’s color and is in a
line with the optical properties and band gaps of other boron-
rich borides. For colorless B12P2 we measured 400 nm () 3.1
eV),12 for the yellow crystals of Li2B12Si2 and MgB12Si2 545
nm () 2.3 eV),19,29 and for red MgB7 630 nm () 1.9 eV).12
Unfortunately, a theoretical calculation of the band gap and other
bonding properties is prevented for Mg3B50C8 by the partial Mg
occupation. Usually experimental and calculated values are in
excellent agreement.12,19,29
Hardness Measurements. Values for hardness depend sig-
nificantly on the conditions in which they were determined.46
Therefore, we have measured the hardness of several well-
Vibrational Spectra. Because many boron-rich borides are
black, the measurement of IR and Raman spectra for this class
of compounds is not very common. But Mg3B50C8 is nearly
colorless, so well-resolved vibrational spectra were obtained
(Figure 8). They continue the row of other stoichiometric boron-
(45) Rotter, H. W.; Hillebrecht, H. Manuscript in preparation.
(46) Brazhkin, V.; Dubrovinskaia, N.; Nicol, M.; Novikov, N.; Rieder, R.;
Solozhenko, V.; Zhao, Y. Nat. Mater. 2004, 3, 576–577.
9
J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010 13729

A R T I C L E S
Adasch et al.
Figure 7. Difference Fourier maps at z ≈ 0.5 for Mg3B50C8 and Mg∼4B50C8 (from top to bottom).
characterized hard materials (c-BN, B4C, R-AlB12) to get a
reliable scaling. Furthermore, we have measured the micro-
hardness of several boron-rich borides to get a feeling of how
structural features influence the hardness.
9
13730 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010

MgxB50C8 and Mgx(B12)4(CBC)2(C2)2 (x ) 2.4-4)
A R T I C L E S
Figure 8. Vibrational spectra of Mg3B50C8 (IR, top; Raman, bottom).
Figure 9. UV-vis spectrum in transmission of a Mg3B50C8 single crystal.
For Mg3B50C8, microhardness measurements according to
Vickers HV and Knoop HK (see Experimental Section) revealed
values of HV ) 3286 (32.0 GPa) and HK ) 3165 (31.5 GPa).
The shapes of the imprints show the high brittleness of
Mg3B50C8 as expected for boron-rich borides (Figure 10). In
particular, the Vickers method caused cracks. An overview of
microhardness values measured with our setup is listed in Table
5. According to this, Mg3B50C8 is the hardest boron-rich boride.
An analysis of the dependence between microhardness and
additional structural units shows that elements of the first row
(B, C) favor higher hardness compared to second row elements
(Si, P), in agreement with the higher bond strength. Compared
to the other boron-rich boride carbides of Mg, the C2 unit with
double bond (and perhaps the CBC unit) seems to increase the
microhardness.
Figure 10. Microhardness measurements of Mg3B50C8: imprints according
to the methods of Vickers (top) with noticeable cracks and of Knoop
(bottom).
to the simple rules of Wade and Longuet-Higgins. The validity
of these rules was confirmed for several other boron-rich borides
with similar crystal structures by state of the art band structure
calculations.12,29 Actually calculations for Mg3B50C8 with respect
to the partial Mg occupation are in progress.
Discussion
Molten metals are useful tools for synthesis and crystal growth
of boron-rich borides. Mg3B50C8 is another example of the
versatility of this method. Furthermore, Mg3B50C8 demonstrates
the high flexibility of boron-rich borides to adopt a crystal
structure, which can be described as a network with covalent
bonds between boron polyhedra and additional “molecular”
units. In sum, the resulting network is electron-precise according
Furthermore, Mg3B50C8 is the first example of a boron-rich
boride with a significant variation of the cation content. The
deviations from the “ideal” composition, i.e., Mg3B50C8, resulted
in a significant change of the crystal’s color. In general, physical
properties can be varied in that way. Especially thermoelectric
properties are of interest because boron-rich borides are a promising
class of compounds for high temperature applications.7,8 While
n-type semiconducting phases are well-known (for example, B4C8),
p-type representatives are still missing. Here, the variation of the
Mg content in MgxB50C8 may be of special importance.
(47) Kotzott, D. Ph.D. Thesis, University Freiburg, Germany, 2009.
(48) Teter, D. M. MRS Bull. 1998, 23, 22–27.
(49) Adasch, V.; Hess, K.-U.; Hillebrecht, H. Ger. Offen. DE 102004014315
A1 20050512 CAN 142:413894 AN 2005:408030, 2005.
(50) Kotzott, D.; Hillebrecht, H. Unpublished results.
(51) Gurin, V.; Derkachenko, L. I. Prog. Cryst. Growth Charact. 1993,
27, 163–199.
The high stability of boron-rich borides whose structures
2-
based on an ionic separation in (formally) negative B12
(52) Prikhina, T. A.; Kisly, P. S. AIP Conf. Proc. 1991, 231, 590–593.
icosahedra linked by additional small fragments has recently
9
J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010 13731

A R T I C L E S
Adasch et al.
Table 5. Values for the Microhardness of Superhard Materials and Some Boron-Rich Borides Based on B12 Icosahedra
compd
HV, GPa
HK, GPa
structural units
ref
data from literature, GPa
c-BN
5108/50.0
3237/31.7
3286/32.0
4612/45.2
2787/27.3
3165/31.5
26
2485/24.0
2005/19.3
1985/19.2
2075/20.0
2134/20.9
zinc blende
CBC units
47
47
this work
21, 49
50
47
50
29
47
48-5848
28-3048
B4C/B13C2
Mg3B50C8
m-MgB12C2
o-MgB12C2
MgB7
MgB12Si2
Li2B12Si2
B12P2
C2 (1.39 Å) and CBC units
isolated C
C2 units (1.73 Å)
B2 units (2.26 Å)
isolated Si
2482/24.0
2125/20.4
2075/19.9
2027/19.5
2505/24.2
2345/22.8
3152/31.0
isolated Si
P2 units (2.24 Å)
B20 polyhedra
isolated C
R-AlB12
Al3B48C2
47
50
2351
33.652
shown up in the orthorhombic HP/HT polymorph of boron.53,54
The structure consists of B12 icosahedra and additional B2 units
that link the icosahedra to a 3D framework. For this structure
a more53 or less54 extended charge transfer is discussed.
Independent from the description of the electronic situation, the
hardness of orthorhombic boron (B28) is higher and the
compressibility lower than for R-rhombohedral boron.53,54
According to Oganov et al.,53 the reason for this is the ionicity
of the structure. Another explanation might be the substitution
of 2e-3c bonds between the icosahedra in R-rh boron by shorter
bonds to the B2 units, similar to the bonding situation in boron
carbide B4C/B13C2.
According to the findings for Mg3B50C8, an alternative
concept to obtain new (super)hard compounds is the substitution
of exohedral 2e-2c bonds by bonds to small units with multiple
bonds, for example, C2 units with double bonds, CBC+ units
with short B-C distances, or new units containing nitrogen or
oxygen. For the last we expect high hardness too because the
covalent radii of N and O are smaller than for B and C. As we
have learned from the structural chemistry of boron-rich borides,
the high variability to form an electron-precise covalent network
of B12 icosahedra and further small building units might result
in new superhard materials.
Nevertheless it is not very likely that the hardness of boron-
rich borides exceeds significantly a value of about 35 GPa
because the hardness (i.e., bond strength) of the B12 icosahedra
and the exohedral B-B bonds are a fundamental limit for this
class of compounds. This may be extended for structures, where
the exohedral B-B bonds are completely substituted by 2e-2c
bonds to C, N, or O. According to Sung et al.55 and Haines et
al.,56 short bond lengths (i.e., sum of covalent radii) and high
ionicity (difference of electronegativity) increase the hardness,
but until now no representatives are known.
The search for new (super)hard materials is still a chal-
lenge.48,55-57 Despite strong efforts, diamond is still unbeaten.
But the high “perfectness” of the diamond structure is a problem
for changing its properties (i.e., band gap, conductivity, color,
magnetic behavior, optical properties, ...). The same is true for
diamond-related structures like c-BN, BC2N,58 or BC5.59 The
usual synthetic access are HP/HT techniques according to the
thermodynamic properties as high pressure phases. This is
another drawback for a broader application and in some cases
clear characterization of these interesting materials.
Recently great interest was caused by the high hardness of
borides of 5d metals like ReB2 and OsB2.60 Though the “real”
values are still in discussion, the high hardness is obvious. It
can be explained by the strong covalent B-B bonds and the
strong metallic bonding between the high-melting 5d metals.
Additionally the high number of shell electrons results in a low
compressibility. But unfortunately the late 5d metals (i.e., Re,
Os, Ir) are among the rarest elements on earth. So applications
that demand them (for example, cutting tools) will be very
expensive.
Conclusions
In conclusion boron-rich borides synthesized from metallic
melts are very promising candidates for new hard and superhard
materials with unique physical properties and a high potential
for applications. In the case of MgxB50C8 (x ) 2.4-4) more
detailed investigations on the physical properties (conductivity,
magnetism, optical and vibrational spectra, thermoelectricity)
and band structure calculations are in progress. Especially NMR
measurements are of interest. We expect distinct different signals
for the two C atoms and a special situation for the linear
coordinated B atom of the CBC group. EPR investigations can
give insights into the influence of the Mg content on the bonding
situation in MgxB50C8 (x ) 2.4-4).
Acknowledgment. Thanks are due to the Bayerisches Geo-
Institut (BGI, Universita¨t Bayreuth, Germany) for the access to
WDX measurements and to Detlev Krausse for his support.
Supporting Information Available: Complete list of authors
for ref 54. This material is available free of charge via the
Internet at http://pubs.acs.org. Additional data for the structure
refinement are deposited at Fachinformationszentrum Karlsruhe.
JA102659D
(59) Solozhenko, V. I.; Kurakevych, O. O.; LeGodec, Y.; Mezouar, M.
Phys. ReV. Lett. 2009, 102, 015506.
(53) Oganov, A. R.; Chen, J.; Gatti, C.; Ma, Y.; Ma, Y.; Glass, C. W.;
Liu, Z.; Yu, T.; Kurakevych, O. O.; Solozhenko, V. L. Nature 2009,
457, 863–867.
(60) (a) Cumberland, R. W.; Weinberger, M. B.; Gilman, J. J.; Clark, S. M.;
Tolbert, S. H.; Kaner, R. B. J. Am. Chem. Soc. 2005, 127, 7264–
7265. (b) Kaner, R. B.; Gilman, J. J.; Tolbert, S. H. Science 2005,
308, 1268–1269. (c) Chung, H.-Y.; Weinberger, M. B.; Levine, J. B.;
Kavner, A.; Yang, J.-M.; Tolbert, S. H.; Kaner, R. B. Science 2007,
316, 436–439. (d) Qin, J.; He, D.; Wang, J.; Fang, L.; Lei, L.; Li, Y.;
Hu, J.; Kou, Z. AdV. Mater. 2008, 20, 4780–4783. (e) Chen, X.-Q.;
Fu, C. L.; Krcmar, S.; Painter, G. S. Phys. ReV. Lett. 2008, 100, 19640.
(f) Koehler, M. R.; Keppens, V.; Sales, B. C.; Jin, R.; Mandrus, D. J.
Phys. D: Appl. Phys. 2009, 42, 095414. (g) Simunek, A. Phys. ReV.
B 2009, 80, 060103.
(54) Zarechnaya, E. Yu.; et al. Phys. ReV. Lett. 2009, 102, 185501. For
complete list of authors, see Supporting Information.
(55) Sung, M.-C.; Sung, M. Mater. Chem. Phys. 1996, 43, 1–18.
(56) Haines, J.; Le´ger, J. M.; Bocquillon, G. Annu. ReV. Mater. Res. 2001,
31, 1–23.
(57) (a) Cohen, M. L. Science 1986, 234, 549. (b) Catlow, R. C. A.; Price,
G. D. Science 1990, 347, 243.
(58) Solozhenko, V. I.; Andrault, D.; Fiquet, G.; Mezouard, M.; Rubie,
D. C. Appl. Phys. Lett. 2001, 70, 1385–1387.
9
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