Sn-flux syntheses, characterizations and bonding analyses of OsB and TiB2

Article abstract of DOI:10.1016/j.jallcom.2009.09.125

Single-phase crystalline powder of the technologically important TiB₂ and powder of OsB have been successfully synthesized at moderate temperature (900 °C) using metallic tin as a melting agent. The structural relationship between both phases is discussed. COHP bonding analyses show strong B-B interactions in TiB₂ but no such in OsB. The bonding situation in both phases is also compared with that of Ti_1.6Os_2.4B₂ and Ti_1.6Os_1.4RuB₂ phases. TiB₂ and OsB are metallic conductors according to DOS curves.

Full text of DOI:10.1016/j.jallcom.2009.09.125

Journal of Alloys and Compounds 489 (2010) 339–342
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Sn-flux syntheses, characterizations and bonding analyses of OsB and TiB2
Boniface P.T. Fokwa∗, Patrick R.N. Misse, Michael Gilleßen, Richard Dronskowski
Institute of Inorganic Chemistry, RWTH Aachen University, D-52056 Aachen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Single-phase crystalline powder of the technologically important TiB2 and powder of OsB have been
successfully synthesized at moderate temperature (900 ◦C) using metallic tin as a melting agent. The
structural relationship between both phases is discussed. COHP bonding analyses show strong B–B inter-
actions in TiB2 but no such in OsB. The bonding situation in both phases is also compared with that of
Ti1.6Os2.4B2 and Ti1.6Os1.4RuB2 phases. TiB2 and OsB are metallic conductors according to DOS curves.
© 2009 Elsevier B.V. All rights reserved.
Received 24 June 2009
Received in revised form
14 September 2009
Accepted 22 September 2009
Available online 30 September 2009
Keywords:
Binary borides
Tin flux synthesis
Crystal structure
COHP bonding
1. Introduction
known for more than six decades, its synthesis is still unsatis-
factory, and several synthetic routes have been proposed [12,13].
In recent years binary metal borides have gained an increas-
ing interest both theoretically and experimentally, owing to their
remarkable physical, chemical and mechanical properties. In par-
ticular, superhard materials are of great industrial importance with
applications such as abrasives, coatings and cutting tools [1].
Lately, borides of osmium have been the focus mainly within
the theoretical community, because it has been found that metal-
lic osmium has a bulk modulus close to that of diamond [2–6], but
with a much lower hardness because of its pure (and weak) metallic
bonds. Therefore increasing the bond strength of osmium by adding
light elements (boron, carbon) may increase the hardness of the so-
obtained material compared to that of pure osmium. However, only
three phases (OsB1.1, Os2B3 and OsB2) were discovered in the Os–B
system by the time [7]. A few years ago, OsB was predicted to be sta-
ble in the WC-type structure [8,9], confirming an earlier theoretical
prediction [10]. More importantly its mechanical properties were
also predicted [8,9]. In the last year (2008) the synthesis of OsB was
achieved by arc-melting the elements, and the predicted structural
and mechanical properties (hard and ultra-incompressible) were
confirmed [11].
The synthesis of TiB2 powder is mostly based on high-temperature
methods, for example, the carbothermal reduction of titanium
oxide (rutile) and boric acid with temperatures above 1800 ◦C [12].
This reaction is, however, highly endothermic and therefore makes
the commercial TiB2 synthesis very expensive as a consequence
of the very high operating temperature. Consequently, a lower-
temperature route for preparing TiB2 is imperatively needed. In the
last year (2008) two different methods for the preparation of TiB2
powders were reported, but both were performed at temperatures
up to 1400 ◦C [14,15].
Syntheses in metal fluxes have a huge potential for generating
new intermetallic species at moderate temperatures, and recently
many new phases, sometimes with very unusual compositions,
have been prepared by this method [16]. Although TiB2 was already
synthesized by this method using different metal flux agents (alu-
minum [17], iron, nickel, cobalt [18] and copper [19]), its synthesis
was carried out at temperatures of at least 1200 ◦C. The present
report describes the results of the syntheses at a moderate tem-
perature as regards to both the technologically important titanium
diboride and the recently discovered ultra-incompressible osmium
boride using metallic tin as a flux-melting agent. In addition, the
crystal structures of both phases are compared, and their bonding
situation is quantified using COHP bonding analyses.
On the other hand, more phases (Ti2B, TiB, Ti3B4, TiB2, Ti0.84B25
and Ti2B103.3) have been discovered in the Ti–B system [7]. Poly-
crystalline ceramics based on TiB2 have received considerable
attention as electrodes for liquid aluminum, cutting tools, and wall
materials for nuclear fusion [12]. Although TiB2 has now been
2. Experimental
2.1. Synthesis and EDX analysis
∗
Corresponding author. Fax: +49 2418092642.
E-mail address: boniface.fokwa@ac.rwth-aachen.de (B.P.T. Fokwa).
OsB and TiB2 were synthesized in a tin flux. The elemental powders of osmium
(powder, 99.9%, Degussa) or titanium (powder, 99.9%, Degussa) and crystalline
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.09.125

340
B.P.T. Fokwa et al. / Journal of Alloys and Compounds 489 (2010) 339–342
Table 1
Rietveld refinement results of the X-ray powder pattern of OsB product obtained
from Sn-flux synthesis.
Formula; molecular weight
OsB; 201 g/mol
Measurement range; increment
Temperature
6◦ ≤ 2Â ≤ 100◦; 0.005◦
298 K
Structure refinement
Profile function
Lattice parameters
RIETVELD, least squares method
Pseudo-VOIGT
a = 2.8767 (2) Å
c = 2.8718 (2) Å
P–6m2
Space group
RBragg
0.048
RF
0.090
Phase yield
≈ 80% (Os: ≈ 20%)
boron (pieces, 99.999%, Alfa Aesar) were weighed in the respective ratios according
to their formulas. After weighing, the powders were mixed by a dry process using a
mortar and a pestle. Pure powder of tin (99.99%, Merck) was used as a flux-melting
agent and added in a TiB2:2Sn or OsB:2Sn weight ratio. The mixed powders were
put into an alumina tube and placed in a quartz glass ampule. After evacuation
the ampule was filled with argon and positioned in a programmable stainless-steel
tube-furnace for reaction. The heating conditions in the Ar atmosphere were as
follows.
Fig. 1. Rietveld refinement of the X-ray powder pattern of the OsB-product showing
measured and fitted intensities (top), the position of the Bragg peaks for OsB and Os
(middle), and the difference intensity curve (bottom).
temperature of synthesis (900 ◦C) which is the lowest for a TiB2
synthesis until now. In fact using the above mentioned flux agents
the temperature of synthesis was 1200 ◦C (Cu flux), 1450 ◦C (Al flux)
and 1600–1750 ◦C (Fe, Co, Ni fluxes).
First, the mixture was heated slightly above the melting point of tin (300 ◦C)
with a heating rate of 10 ◦C min−1. After holding this temperature for 12 h, the sam-
ple was slowly heated to 900 ◦C with a heating rate of 2 ◦C min−1, held for 48 h
and then slowly cooled to room temperature with a cooling rate of 2 ◦C min−1
.
The obtained powders were leached out into a small beaker using concentrated
hydrochloric acid (37%) at room temperature to remove the flux-melting agent (Sn).
Finally, deionized water and acetone, respectively, were added to wash the remain-
ing solid. Dark-gray and black fine powders were obtained as final products for OsB
and TiB2, respectively.
The chemical compositions were checked by energy dispersive X-ray analysis
(EDX) on a high-resolution, low-energy SEM of the type LEO 1530 (Oberkochen,
Germany) equipped with an EDX system of the type INCA (Oxford, England). No
other metal than those used was found from these EDX analyses. In the case of
TiB2 no detectable amount of tin could be recorded in the product which had been
preliminarily treated with concentrated hydrochloric acid.
OsB crystallizes with the WC-type structure (space group
P–6m2). The structure is built up of face-sharing trigonal prisms
of osmium atoms (see Fig. 3a), but only one out of two trigonal
prisms can be occupied by boron because the distance between
both prism centers is too short (≈1.6 Å) for a B–B bond (e.g. 1.8–1.9 Å
in orthorhombic OsB2 [29]). Therefore only two types of bonding
interactions should be present in the structure: Os–B and Os–Os.
According to the COHP curves (see Fig. 4a, left) the Os–B inter-
actions are optimized in OsB: bonding orbitals are filled and
antibonding orbitals are empty. The Os–Os pair, however, shows
antibonding orbitals near and at the Fermi level. Therefore this
COHP bonding analysis suggests that it is the Os–B interaction
that creates the structural stability of OsB, as expected. In agree-
ment with this assessment the energy integral ICOHP for an
2.2. Structural characterization
X-ray powder diffraction (XRD) patterns were recorded on a Huber-Guinier 670
X-ray diffractometer with Cu K␣1 radiation (ꢀ = 1.54059 Å) and a Ge monochroma-
tor. The lattice parameters and the atomic positions were refined (see Table 1) on
the basis of powder data using the program Fullprof [20].
2.3. Theoretical methodology
Electronic-structure calculations were carried out based on the all-electron
scalar-relativistic Linear Muffin-Tin Orbital (LMTO) theory [21,22] in its tight-
binding representation [23] using the TB-LMTO-ASA 4.7 program [24]. For the
density-functional parametrization of exchange and correlation, the scheme by
Vosko, Wilk, and Nusair [25] was utilized and augmented with gradient corrections
[26] such as to follow the generalized-gradient approximation (GGA). A 24 × 24 × 16
k-mesh for OsB and a 19 × 19 × 25 k-mesh for TiB2 was used. The chemical bonding
was investigated using the Crystal Orbital Hamilton Population (COHP) analysis [27],
which is an energy-resolved partitioning technique of the band structure energy
(sum of the Kohn–Sham eigenvalues) in terms of atomic and bonding contributions
[28].
3. Results and discussion
Figs. 1 and 2 show the Rietveld refinement curve for OsB and
comparative X-ray powder patterns of the TiB2 product, respec-
tively. TiB2 was single phase (Fig. 2), whereas the OsB product
contains unreacted elemental osmium (about 20%). The lattice
parameters obtained, a = 2.8767(2) Å and c = 2.8718(2) Å for OsB
and a = 3.0295(5) Å and c = 3.2291(4) Å for TiB2, are in very good
agreement with the reported values.
OsB was synthesized for the first time using a metal flux method
(Sn flux), in contrast to TiB2 which was already successfully syn-
thesized by this method (other authors) but using different metal
flux agents (aluminum [17], iron, nickel, cobalt [18] and copper
[19]). However, the main advantage of the Sn-flux synthesis is the
Fig. 2. Comparisons of calculated and measured X-ray powder patterns from the
TiB2 synthesis (using a Sn-Flux). Single-phase TiB2 was obtained (second pattern
from bottom).

B.P.T. Fokwa et al. / Journal of Alloys and Compounds 489 (2010) 339–342
341
Os–B contact (−3.20 eV/bond) is more than twice as large as the
value obtained for the Os–Os contact (−1.09 eV/bond). The ICOHP
value (−3.20 eV/bond) of the Os–B interaction is also compara-
ble with the average values (−2.82 eV/bond) and (−3.00 eV/bond)
obtained, respectively, in the ternary Ti1.6Os2.4B2 [30] (electronic
structure calculations made in this work) and in the quaternary
Ti1.6Os1.4RuB2 [31]. The very low ICOHP value (−0.05 eV/bond)
obtained for the average B–B distances shows the insignificance
of the B–B interaction, as could be expected for the rather long B–B
distance (2.87 Å). Thus, the COHP (and ICOHP) analyses confirm the
electron localization function (ELF) analyses, which indicate par-
tially covalent bonding between Os and B and no bonding between
the boron atoms [11]. Nonetheless, no bonding analysis was carried
out by the authors for the Os–Os distance. The COHP analyses show
that these interactions are very significant and therefore contribute
to weaken the Os–B interaction, which according to the ELF analy-
sis is only partially covalent bonding. Because of the non-vanishing
DOS at the Fermi level (see Fig. 4a, right), OsB is predicted to be a
good metallic conductor.
Let us now switch to the other binary boride (TiB2) and compare
its structure with that of OsB. TiB2 crystallizes within the AlB2-type
structure (space group P6/mmm), which is also related to OsB struc-
ture (WC-type). The difference between the OsB and TiB2 structures
probably originates from the size difference between osmium and
titanium. In fact, because the titanium atomic radius (ra = 1.448 Å)
is much larger than that of osmium (ra = 1.338 Å), the trigonal prism
built up by the titanium atoms is also much larger than that of the
osmium atoms. This argument is further reinforced by the fact that
TiB crystallizes with both the NaCl and FeB structure types [32,33]
but not within the WC-type structure like OsB. On the other hand,
OsB2 crystallizes within the orthorhombic RuB2-type structure [29]
instead of the hexagonal AlB2-type structure like TiB2. The valence
electron (VE) count also plays an important role as was demon-
strated in an earlier work [34] on the WC-type structure. In this
work it was found that phases preferentially adopt the WC-type
structure when their valence electron count is ten or more (OsB has
11 VE), whereas phases with nine or less valence electrons prefers
the NaCl structure (TiB has 7 VE). In contrast to the OsB structure,
the centers of the face-connected prisms are at a sufficient dis-
tance to allow for a B–B bonding in the TiB2 structure, leading to
the formation of a boron layer. Consequently, the structure of TiB2
is built up of boron-filled face-sharing trigonal prisms of titanium
atoms (see Fig. 3b). Thus, there should be three types of bonds in
the structure: Ti–Ti, Ti–B, and B–B. From the COHP curves in Fig. 4b
(left) it is obvious that Ti–Ti and Ti–B interactions are optimized in
TiB2: bonding orbitals are filled and antibonding orbitals are empty.
Nonetheless, the COHP curve of the B–B interaction shows virtual
bonding levels seen in the conduction band about 1 eV above the
Fermi level (Fig. 4b, left). The B–B bonding is very strong, with the
highest energy integral (ICOHP = −4.04 eV/bond). A trigonal planar
Fig. 3. Projection of the crystal structures of OsB (a) and TiB2 (b) nearly along [0 0 1].
The solid black parallelepipeds represent the unit cells.
B4 unit, which can be the starting point for building the boron layer
in TiB2 (see Fig. 5), was found for the first time in Ti1.6Os1.4RuB2
[31] and later confirmed in isotypic Ti1.6Os2.4B2 [30]. COHP anal-
yses of this B4 unit show strong bonding with ICOHP values of
−3.37 eV/bond in the ternary Ti1.6Os2.4B2 and −3.13 eV/bond in the
quaternary Ti1.6Os1.4RuB2. This suggests that the B–B interactions
in the boron layer of TiB2 are even stronger than those present in
the B4 unit, with an energy gain of −0.67 eV/bond (approximately
−65 kJ/mol, if compared with the B4 unit present in the ternary
Ti1.6Os2.4B2). This could have been expected because in the B4 unit
the B–B bond is slightly weakened by its (small) partial ionic char-
acter, whereas in the boron layer the B–B bond is purely covalent.
The occurrence of B–B bonding interactions has weakened the Ti–B
bond for which an ICOHP value of only −1.80 eV/bond is observed.
The fact that most of the electron density has been engaged in the
formation of the Ti–B and B–B bonds leaves only weak bonding
Fig. 4. (a) COHP bonding analysis of Os–B and Os–Os interactions present in OsB
(left) and its partial (filled) and total DOS curve (right); (b) COHP bonding analysis
of B–B, Ti–B and Ti–Ti interactions present in TiB2 (left) and its partial (filled) and
total DOS curves (right). The Fermi level has been set to the energy zero.

342
B.P.T. Fokwa et al. / Journal of Alloys and Compounds 489 (2010) 339–342
References
[1] V.V. Brazhkin, A.G. Lyapin, R.J. Hemley, Phil. Mag. 82 (2002) 231.
[2] H. Cynn, J.E. Klepeis, C.-S. Yoo, D.A. Young, Phys. Rev. Lett. 88 (2002)
135701.
[3] F. Occelli, D.L. Farber, J. Badro, C.M. Aracne, D.M. Teter, M. Hanfland, B. Canny,
B. Couzinet, Phys. Rev. Lett. 93 (2004) 095502.
[4] T. Kenichi, Phys. Rev. B 70 (2004) 012101.
[5] B.R. Sahu, L. Kleinman, Phys. Rev. B 72 (2005) 113106.
[6] R.W. Cumberland, M.B. Weinberger, J.J. Gilman, S.M. Clark, S.H. Tolbert, R.B.
Kaner, J. Am. Chem. Soc. 127 (2005) 7264.
[7] P. Villars, K. Cenzual, Pearson’s Crystal Structure Database for Inorganic Com-
pounds (on CD-ROM), Version 1.0, Materials Park, OH, USA, 2007/8.
[8] H. Gou, L. Hou, J. Zhang, H. Li, G. Sun, F. Gao, Appl. Phys. Lett. 88 (2006)
221904.
[9] Y. Liang, J. Zhao, B. Zhang, Solid State Commun. 146 (2008) 450.
[10] (a) D.L. Novikov, A.L. Ivanovskii, V.A. Gubanov, Zh. Neorg. Khim 33 (1988) 2673;
(b) R.M. Minyaev, R. Hoffmann, Chem. Mater. 3 (1991) 547.
[11] Q. Gu, G. Krauss, W. Steurer, Adv. Mater 20 (2008) 3620.
[12] P. Schwartzkopf, R. Kieffer, Refractory Hard Metals: Borides, Carbides, Nitrides
and Silicides, Macmillan, New York, 1953.
Fig. 5. The trigonal planar B4 unit in Ti1.6Os2.4B2 (left) is the fundamental starting
motif for generating the boron layer structure in TiB2 (right). The process leads to
an energy gain of roughly −65 kJ/mol.
[13] L. Chen, Y. Gu, Y. Qian, L. Shi, Z. Yang, J. Ma, Mater. Res. Bull. 39 (2004)
609.
[14] M.X. Guo, M.P. Wang, K. Shen, L.F. Cao, Z. Li, Z. Zhang, J. Alloys Compd. 460
(2008) 585.
[15] L. Baca, N. Stelzer, J. Eur. Ceram. Soc. 28 (2008) 907.
[16] M.G. Kanatzidis, R. Pöttgen, W. Jeitschko, Angew. Chem. Int. Ed. 441 (2005)
6996.
[17] I. Higashi, T. Atoda, J. Cryst. Growth 7 (1970) 251.
[18] K. Nakano, H. Hayashi, J. Cryst. Growth 24/25 (1974) 679.
[19] S. Dallaire, J.-G. Legoux, Mater. Sci. Eng. A 183 (1994) 139.
[20] J. Rodriguez-Carvajal, FULLPROF, Version 2.8, ILL, July 2004, unpubished.
[21] O.K. Andersen, Phys. Rev. B 12 (1975) 3060.
between the titanium atoms. Indeed an average energy integral
(ICOHP) of only −0.82 eV/bond is found for the Ti–Ti interaction.
The bonding in TiB2 was also studied by Vajeeston et al. [35] and
Han et al. [36] using, respectively, the TB-LMTO-ASA method and
first-principles total-energy pseudopotential method, both based
on density-functional theory. They found a combination of ionic,
covalent and metallic bonding in TiB2. The DOS curve of TiB2 (see
Fig. 4b) shows a pseudogap at the Fermi level, but some occupied
states are still present indicating weak metallic conductivity. The
electronic band structure (not shown) reveals anisotropic electri-
cal conductivity (in the titanium layer) for TiB2 originating from the
titanium d electrons.
[22] O.K. Andersen, in: P. Phariseau, W.M. Temmerman (Eds.), The Electronic Struc-
ture of Complex Systems, Plenum, New York, 1984.
[23] O.K. Andersen, O. Jepsen, Phys. Rev. Lett. 53 (1984) 2571.
[24] G. Krier, O. Jepsen, A.O. Burkhardt, K. Andersen, The TB-LMTO-ASA program,
version 4.7, Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany.
[25] S.H. Vosko, L. Wilk, M. Nusair, Can. J. Phys. 58 (1980) 1200.
[26] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, C. Fiolhais,
Phys. Rev. B 46 (1992) 6671.
4. Conclusion
We have been able to synthesize the binary transition-metal
borides OsB and TiB2 using metallic tin as a flux agent. Phase-pure
TiB2 was obtained as a very fine polycrystalline powder, whereas
OsB contains some amount of unreacted elemental osmium. COHP
bonding analyses show strong B–B bonding in TiB2 but no B–B inter-
actions in OsB, as expected. DOS curves indicate that both phases
are metallic conductors.
[27] R. Dronskowski, P.E. Blöchl, J. Phys. Chem. 97 (1993) 8617.
[28] R. Dronskowski, Computational Chemistry of Solid State Materials, Wiley-VCH,
Weinheim, New York, 2005.
[29] B. Aronsson, Acta Chem. Scand. 17 (1963) 2036.
[30] B.P.T. Fokwa, R. Dronskowski, Z. Anorg. Allg. Chem. 634 (2008) 1955.
[31] B.P.T. Fokwa, J. von Appen, R. Dronskowski, Chem. Commun. (2005)
4419.
[32] I.V. Brettser-Portnov, V.V. Pakholkov, S.I. Alyamovskii, Y.G. Zainulin, Russ. J.
Inorg. Chem. 35 (1990) 962.
[33] B.F. Decker, J.S. Kasper, Acta Crystallogr. 7 (1954) 77.
[34] S.D. Wijeyesekera, R. Hoffmann, Organometallics 3 (1984) 949.
[35] P. Vajeeston, P. Ravindran, C. Ravi, R. Asokamani, Phys. Rev. B 63 (2001)
045115.
Acknowledgments
The authors thank Deutsche Forschungsgemeinschaft for finan-
cial support and Resi Zaunbrecher (IPC, RWTH Aachen) for the EDX
analyses.
[36] Y.F. Han, Y.B. Dai, D. Shu, J. Wang, B.D. Sun, J. Alloys Compd. 438 (2007)
327.