J. Phys. Chem. 1994,98, 9350-9353
9350
Gas Phase Antimony/Magnesium/Oxygen Clusterst
H. T. Deng, Y. Okada, M. Foltin, and A. W. Castleman, Jr.’
Department of Chemistry, 152 Davey Laboratory, The Pennsylvania State University,
University Park, Pennsylvania 16802
Received: April 29, 1994@
Antimony/magnesium/oxygen clusters are produced by a gas aggregation source, in which a mixture of antimony
and magnesium is vaporized and reacted with N2O introduced in helium carrier gas. The resulting product
distribution is detected by a time-of-flight mass spectrometer following ionization with a KrF excimer laser.
Four types of cluster products are observed: Sb,+, SbxMgyOZ+,Sb,Mgy+, and MgyO,+. The mass spectral
intensity distributions display enhanced abundances for MgzO+, S b 2 ~ M g 3 0 + Sbl4Mg20+,
,
Sb4Mg+, SbsMg2+, and SbsMg2+. The experimental observation of Mg20+ and Mg30+ shows that the suboxides of group
2 are stable species, consistent with theoretical predictions. The binding abilities of antimony clusters tomagnesium
and magnesium oxides are found to be dependent on cluster size. When the number of antimony atoms in the
clusters is smaller than 6, SbxMgyO:+ are the main products dominating the mass distribution. On the other
hand, when the cluster size of Sb, is larger than 6, only S b / M g alloy clusters are observed. The unusual
stabilities of Sbz+Mg30+ and Sbl+MgzO+ clusters are evidently due to the formation of covalent bonds between
S b and M g atoms. In Sb4Mg+, SbsMg2+, and SbaMgz+ alloy clusters, however, the M g atom donates two
electrons to the skeleton of the S b clusters in order to satisfy Wade’s rules. The structures of these stable clusters
can be predicted by the polyhedral skeletal electronic pair theory.
Introduction
The abundance distributions of clusters obtained through mass
spectrometry directly reflect their stabilities and easeof ionization
and are generally related to their electronic and/or geometric
structures. The enhanced abundances are referred to as magic
numbers and in the case of metal and metal alloy clusters are
sometimes well explained by the jellium (electron) shell
In other cases these metal clusters, including clusters of metal
oxides and other metal compounds, are accounted for by invoking
the polyhedral skeletal electronic pair theory (PSEPT)3-5and/or
geometric models.69’ For example, the jellium model is successfully applied to explain the magic numbers of alkali metals, coinage
metals, and group 13 metal clusters. However, the electronic
structures no longer dominate the mass spectra distributions for
group 14 and 15 metal clusters due to their large number of
valence electrons, and PSEPT has been proposed to explain the
magic numbers. For lead clusters, their stabilities are consistent
with considerations of their expected geometric structures.
Studies of alloy clusters prepared by substituting various atoms
into the clusters and examining their stability provide important
information about the electronic and geometric properties of these
cluster systems. In the case of alloys among metals which
individually display substantial free electron behavior,s the
observed magic numbers still obey the jellium shell model. For
Cu/Pb alloy clusters, either electronic properties or geometric
structures can account for their observed stabilities and mass
spectral abundances, depending on the relative compositions of
Cu and Pb. Previously, alloy clusters containing group 14 and
15 metals have been studied in detail. Cs/Sn and Cs/Pb,lsl3
Na/Bi,I4 and Na/Sbls clusters have been reported to have magic
numbers for stoichiometries corresponding to known Zintl ions
of these post-transition metals. It is suggested that the alkali
metal atoms merely donate electrons to alloy clusters and have
no influence on their geometric structures. Studies of Sn/Bi and
Pb/Sb,I6J7 Sn/As,I7 and Bi/Sbls systems show that magic
numbers arise for species that are isoelectronic with stable
analogues of known Zintl ions in the condensed phase, and both
atoms participate in forming the geometric structures of those
Dedicated to C. N. R. Rao on the occasion of his 60th birthday.
Abstract published in Advance ACS Absfracts, August 15, 1994.
0022-3654/94/2098-9350$04.50/0
clusters. The same kind of Zintl ions are also found in alloy
clusters of group 13 and 15,Ig group 14 and 16,20and group 15
and 16.20 As for Cu/Sb and Cu/Bi alloy clusters,2l two types
of magic numbers are observed and consistent with the jellium
model and PSEPT, respectively.
Heretofore, no studies have been reported on the alloy clusters
of group 2 and group 15. Group 2 metal atoms have an electronic
closed shell structure (nsz), and it has not been known whether
this may have some different influence on the stability of the
alloy clusters compared to those comprised of metals of other
groups. One objective of the present work is to examine the mass
spectral distributions of Mg/Sb alloy clusters and to see whether
the jellium model or PSEPT can be applied to explain the magic
numbers of this system.
In the past few years, numerous studies have been done on
oxide c l ~ s t e r s . ~ ~Martin10J2.28-30
-2~
studied the CsO and CaO,
BaO clusters and calculated the structures of C S ( C S ~ O ) ~ +
clusters.IO A1,0, clusters were studied by using CID and other
meth0ds.3393~ The reactivities of small transition metal oxide
c l u s t e r ~ 3 and
~ 3 ~mixed oxide clusters39have also been investigated.
MgO clusters have been studied by our group in detai1.4M3
Detailed ab initio calculations on Li30, Li40, Na30, and N a 4 0
suggest that M 3 0 molecules prefer a D3h structure while M 4 0
prefers a Td symmetry structure; the existence of these species
has been proven by experimental ob~ervation.~1-32-~7-49
These
suboxides have unusual stoichiometries which suggest violation
of the octet rule. Due to the similar properties of magnesium and
sodium, the structure and binding energies of magnesium
suboxides have also been of interest. They also have been studied
through a b initio calculationsg which have shown that Mg30 and
Mg40 are stable species and prefer to have D3h and DMsymmetries,
respectively. However, to the best of our knowledge, there is no
experimental evidence for the existence of Mg30 and Mg40.
Attempting to observe stable hypervalent magnesium oxides
comprised another objective of the present investigation.
The third objective of the work is to examine the stabilities of
Sb-Mg-0 clusters. Studies of metal oxide clusters have been
very important because many important catalysts consist of metal
particles dispersed on oxide s ~ r f a c e s . ~ ~There
*s
are strong
interactions between the metal particles and the oxide particles
0 1994 American Chemical Society
Gas Phase Antimony/Magnesium/Oxygen Clusters
The Journal of Physical Chemistry, Vol. 98, No. 37, 1994 9351
in these systems.46 Studies of metal-oxide clusters will be helpful
in understanding the structures and binding energies of these
systems. Compared with antimony, which cannot be oxidized at
ambient temperature, magnesium can readily form oxides.
Through analysis of the Sb/Mg/O cluster distributions, we can
find out whether the cluster size influences the binding ability of
antimony clusters with magnesium oxides.
Herein we report recent results on the system comprised of
Sb/Mg/O clusters and address the findings to the objectives
mentioned above.
Experimental Section
Details of the apparatus are described elsewhere.' Briefly, a
gas aggregation sources in used in the present work. A mixture
of antimony and magnesium is vaporized and is then reacted with
NzO, which is carried in helium gas. Inside a liquid nitrogencooled source chamber, the S b and Mg metal mixture is resistively
heated in a boron-nitridecrucible to 750-850 O C and evaporated
into a flow of He gas (5.0 slm) with 15 sccm of NzO added. A
0.062 in. hole in the apex of a cone serves to sample a small
fraction of the flow, and most of the gas in pumped out in front
of the cone. The clusters are then introduced into a low-pressure
ionization region between electrostatic grids of a time-of-flight
mass spectrometer.
The neutral clusters are photoionized by a slightly focused
beam of light from a KrF excimer laser (4.98 eV). The laser
beam (pulse width 20 ns) is focused with a convex lens to obtain
170-330 mJ/(cm2 pulse) in the ionization region, enabling
detection of the desired cluster ions. The ionized clusters are
accelerated with double stage acceleration grids and detected by
a microchannel plate particle multiplier (MCP) after traversing
a 186 cm differentially pumped field-free region. The ion signals
from the MCP are accumulated using a 100 MHz transient
digitizer and the time-of-flight spectra are transferred to a
microcomputer for analysis.
Results and Discussion
Before studying the metal-oxide cluster system, pure S b metal
and Mg metal were examined individually to probe their
distributions and magic numbers. Under the present experimental
conditions, no pure Mg clusters are observed beyond Mg2+, even
without addition of N20 reactant gas. N20 can greatly enhance
the intensities of Mg and Mg/O clusters. The peak intensity
observed for MgzO+ clusters is unusually high, in agreement
with the observations by Ziemann et alaw3 As for S b clusters,
magic numbers are observed at n = 5 and 7,which have 4n +
4 and 4n 6 skeletal electrons corresponding to nido and arachno
structures,50 respectively. In contrast to magnesium, antimony
does not react with N20 under the conditions of the present
experiments.
In order to investigate cluster distributions of SbxMgyO+,pure
S b and Mg was mixed at a 1 :1 ratio. Without NzO, the intensities
of MgO and S b clusters are very small, and no Sb,Mgy+ (except
SbMg) or Sb,MgyO+ clusters are observed. After 15 sccm of
N2O is introduced into the flow tube, reaction products are
observed; the recorded mass spectra are displayed in Figures 1-3
for different mass regions. It is found that further increasing the
concentration of N20 to 600 sccm has no significant impact on
the resulting mass distribution.
From Figure 1, both Mg30+ and MgzO+ are observed, and the
peak intensity of MgzO+ is unusually high. This is the first
observation of suboxides of group 2 metals of which we are aware,
showing that the Mg30+and MgzO+ are stable species in the gas
phase. Although only cations can be detected in the present
experiment, little fragmentation is found in these type of strongly
bonded systems. Furthermore, considering the low pressure in
the region after ionization, collision-induced dissociation is not
expected to be very significant. Hence, it is reasonable to imply
450
560
Figure 1.
670
780
890
FLIGHT TIME ( m i c r o s e c o n d s )
IO00
Time-of-flightof mass spectrum of Sb/Mg/O clusters for the
mass range from 20 to 100.
1380
1100
Figure 2.
1660
I940
2220
FLIGHT TIME ( m i c r o s e c o n d s )
2500
Time-of-flightof mass spectrum of Sb/Mg/O clusters for the
mass range from 200 to 600. The peak marked by * is due to an impurity.
- -K
IX
+
2500
2700
2900
3100
3300
FLIGHT TIME ( m i c r o s e c o n d s )
Figure 3. Time-of-flightof mass spectrum of Sb/Mg/O
mass range from 600 to 1200.
3500
clusters for the
that the neutral Mg30 and MgzO are also stable species which
can be ionized and lead to the observed species Mg30+ and MgzO+.
Magnesium-magnesium bonding contributes significantly to the
stability of M g 3 0 according to a b initio calc~lations.~
In Figures 2 and 3,three types of clusters are observed, namely,
Sb,+, Sb,Mgy+, and Sb,Mg,O,+. Allof theobservedcombinations
between Sb, and Mg,O, are listed in Table 1. It is clear that (i)
all Sb, clusters can combine with one or two Mg atoms to form
9352
The Journal of Physical Chemistry, Vol. 98, No. 37, 1994
TABLE 1: Experimentally Observed Combinations of Sb,
and M g P l Clusters'
Sb Sb2 Sb3 Sb4 Sbs Sba Sb, Sbs Sb9
Mg
+ + + + + + + + + +
Mg2
+ + + +++++ + +
+ + + +
Mg3
+ + + +
Mg4
MgO
+ + +
Mg20 ++ ++ ++ +
Mg,O
+ ++ ++ ++ +
+ +
Mg3O2 +
Mg4O +
+ ++
Mg02 +
+
a + = combining clusters; ++ = combining clusters exhibiting magic
numbers.
Sbl-9Mgl-2 cluster ions (except for SbMg2 clusters); (ii) when
the size of the S b clusters is smaller than six, Sb,MgyO, are
observed in the mass spectrum but no Sbl-5Mg3-4 clusters are
present; (iii) when the size of S b clusters is larger than five, all
S b clusters can bind one to four Mg atoms but no Sb,MgyO,
clusters are formed, which indicates that Sb,Mgy clusters are not
reactive with N2O. Therefore, it is reasonable to suggest that the
Sb,MgyOz+ clusters come from the combination of Sb, with
MgyOz clusters. The experimental findings indicate that the
binding abilities of Sb, clusters to MgyO, clusters changes at a
duster size of six. It is known that the reactivities of clusters
vary with size in many cluster systems. The above results show
that small Sb, ( x < 6 ) clusters facilely bind to suboxides, while
the large clusters mainly accept the electrons from magnesium.
The magic numbers in Sb,MgyO+ clusters appear in the form
of Sb24Mg30+ and Sb14MgzO+. When x = 5 , SbsMg40+
clusters have an unusually high peak intensity. It seems that
Mg2O and M g 3 0 are reactive and may combine with the small
antimony clusters. Ab initio calculations show that stable
geometric structures of MgzO and M g 3 0 are linear and planar
structures, respectively, with the oxygen atom in the center. The
partial charges are 0.9 and 0.66 for Mg atoms in Mg2O and
Mg30, re~pectively.~
According to these structures, the oxygen
atom in Mg30+and MgzO+ clusters draws the electrons from the
Mg atoms and the Mg atoms still have one electron left which
can be used to share with other atoms. In the Sb, clusters with
small size, nonbonding electrons mainly localize on each S b atom,
causing these S b atoms to have dangling bonds and to form
covalent bonds with polarized Mg atoms. This is the reason that
Sb,MgyO+ clusters have unusually high intensities. However,
nonbonding electrons in S b clusters have higher delocalization
with an increase of cluster size. Therefore, large S b clusters
mainly can accept electrons from Mg rather than combine with
Mg atoms or Mg suboxides. Considering the influence of
geometric structure, Sb3+ clusters have a planar triangle and
Sb4+ clusters have a tetrahedral s t r u ~ t u r e . They
~ ~ all have a
triangular plane which can combine three Mg atoms of Mg30+
clusters, so the stable geometric structures also favor the formation
of Sb34Mg30+ clusters in gas phase. The same considerations
can be applied to SbsMg,O+ clusters. Unlike Na10 and Li40,
which can exist in the gas phase, Mg40+ is not observed in the
mass distribution and the calculated structure of M g 4 0 has D2d
symmetry. The structure of Sb5+ is theoreticallys1suggested to
have a square pyramid structure; therefore, the covalent bonds
are formed between S b atoms and Mg atoms of both clusters
leading to the formation of Sb5Mg40.
From Table 1,it is found that the number of Mg atoms increases
in Sb,Mgy+ clusters with an increase in the size of S b clusters.
Compared to the Mg atom, the S b atom is larger, more polarizable,
forms stable dimers, and easily forms clusters. The space occupied
by the Sbclusters determines the number of Mg atoms which can
be taken. This reasoning suggests that the larger the cluster size,
the larger the backbone and the more Mg atoms that can be
accommodated. The magic numbers for Sb,Mgy+ clusters appear
Deng et al.
a t Sb4Mg+, SbsMg2+, and SbsMgz+. In the mass spectral
distribution, Sbs' is not observed, although Sb6+ clusters have
the expected number of electrons which satisfy Wade's rules.
The reason that SbdMg+ is chosen to be the magic number is that
the intensity of Sb.+Mg+is unusually high compared to the other
Sb,Mg peaks. If the Mg atom donates two electrons to this
system, the valence electrons can be counted as 5x 2y. This
yields 22 electrons for the Sb4Mg neutral cluster. This number
does not correspond to a jellium shell closing but does correspond
to the known Zintl ion Sb42-, which has 4n + 6 skeletal electrons
and an anachno structure. When one Mg atom attaches to this
anachno structure and takes a vertex position, a square pyramid
structure can be formed and represented as the structure of SbdMg.
The same ideas are applied to the SbsMgz+ cationic and Sb6Mgz neutral clusters. The SbsMgz+ cation can be considered as
an isoelectronic species to Sbs3-, which has 28 (4n + 8) electrons
and the hypo structure. When two Mg atoms attach to Sb5, a
pentagonal pyramid structure can be suggested for Sb5Mgz+.
SbsMg2 neutral clusters have 34 electrons, corresponding to Sbs3anions which have 4n 10 skeletal electrons and also can be
represented as a deltahedral structure. Finally, in passing it is
worth noting that 34 electrons is a jellium shell closing for the
WoodsSaxon potential.52
+
+
Conclusions
Four kinds of clusters MgyOp+,Sb,+, Sb,MgyO+ and Sb,Mgy+
are formed by vaporizing a Sb-Mg mixture in a gas aggregation
source and allowing it to undergo oxidation. The observation of
Mg30+and MgzO+ in the mass spectra proves that the suboxides
of group 2 are stable species. This provides experimental
confirmation of earlier theoretical predictions.
In the case of mixed metal oxide systems, we have observed
that certain ones are especially prominent. Generally, Sbclusters
are found to facilely combine with the Mg suboxides and Mg.
Thesizeof the Sb,clusters directly determines the binding abilities
to Mg or Mg-0 clusters. Magic numbers for Sb,MgyO+ clusters
are observed in SbzdMg30+ and SblAMg20+,which are related
to the formation of covalent bonds between S b and Mg atoms.
Finally, as far as Sb-Mg interactions are concerned, certain
species are found to be particularly stable. The Mg atom donates
two electrons to form very stable Sb4Mg+, SbsMg2+, and Sb6MgZ+ clusters. The total number of electrons in these clusters
satisfies Wade's rules, and the cluster structures can be predicted
by the polyhedral skeletal electron pair theory which accounts
for their appearance as magic numbers in the experiments.
Acknowledgment. The authors thank Dr. Y. Yamada and Dr.
S. Wei for helpful discussions. Financial support by the Division
of Chemical Sciences, Office of Basic Energy Sciences, Office
of Energy Research of the U S . Department of Energy, Grant
DE-FG02-92ER14258, is gratefully acknowledged.
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