Mg5.23Sm0.77Sb4: An ordered superstructure derived from the Mg3Sb2 structure type

Article abstract of DOI:10.1021/ic060835w

The ternary polar intermetallic phase Mg_5.231(8)Sm _0.769(8)Sb₄ has been obtained from solid-state reactions at 700-850°C in sealed Ta or Nb containers when the synthetic conditions took into account its characteristic incongruent melting point. The compound crystallizes in the trigonal space group P3 (Z = 1) with a = 4.618(1) A and c = 14.902(6) A in a structure that derives from that of Mg ₃Sb₂ (anti-La₂O₃ type). This composition appears to be near the lower limit of Sm content, and solutions with appreciably higher Sm contents are also stable [Mg_6-xSm _xSb₄, x ≤ 1]. The result provides the first example of a superstructure of a Mg₃Sb₂-like structure with a doubled c axis induced by ordering a mixture of the larger divalent Sm and Mg ions separately within alternate layers of octahedral sites. Still larger proportions of Sm also give rise to a second solid solution region in the parent Mg ₃Sb₂ type structure (P3m1), Mg_3-ySm _ySb₄, 0 ≤ y ≤ 1. Retention of the same 3e ^- valence electron counts per antimony in all of these phases suggests that the compounds remain electron precise and Zintl phases. Analogous compounds with Ca, Sr, or Ba substitution have evidently not been investigated.

Full text of DOI:10.1021/ic060835w

Inorg. Chem. 2006, 45, 8175−8178
Mg5.23Sm0.77Sb4: an Ordered Superstructure Derived from the Mg3Sb2
Structure Type
Shalabh Gupta,†,‡ Ashok K. Ganguli,*,†,‡ and John D. Corbett*,†
Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011, and Department of
Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
Received May 15, 2006
The ternary polar intermetallic phase Mg5.231(8)Sm0.769(8)Sb4 has been obtained from solid-state reactions at 700
850 C in sealed Ta or Nb containers when the synthetic conditions took into account its characteristic incongruent
melting point. The compound crystallizes in the trigonal space group P3 (Z 1) with a 4.618(1) Å and c
−
°
h
)
)
)
14.902(6) Å in a structure that derives from that of Mg3Sb2 (anti-La2O3 type). This composition appears to be near
the lower limit of Sm content, and solutions with appreciably higher Sm contents are also stable [Mg6 xSmxSb4, x
-
e
1]. The result provides the first example of a superstructure of a Mg3Sb2-like structure with a doubled c axis
induced by ordering a mixture of the larger divalent Sm and Mg ions separately within alternate layers of octahedral
sites. Still larger proportions of Sm also give rise to a second solid solution region in the parent Mg3Sb2 type
structure (P3hm1), Mg3 ySmySb4, 0 e y e
1. Retention of the same 3e- valence electron counts per antimony in
-
all of these phases suggests that the compounds remain electron precise and Zintl phases. Analogous compounds
with Ca, Sr, or Ba substitution have evidently not been investigated.
1. Introduction
new concepts, such as metallic Zintl phases7,8 and the diverse
effects of packing and nonclassical bonding.9-11 It has been
noted in the literature12 that the structure and stabilities of
the ionic cluster salts may be influenced to a major degree
by packing, electronic effects, and Madelung energies. In
this regard, mixed cations have often been found to be
instrumental in stabilizing a diverse variety of anionic cluster
species in new and unusual structures in closely related
systems.1
Significant research activity continues in the broad area
of polar intermetallics because interesting and hitherto
unknown structure types with novel bonding patterns are
quite often encountered.1-3 Another pressing reason for
activity in this area, which remains highly exploratory in
nature, is because of the interesting electronic properties of
some examples. Of special interest to various investigators
are the stannides and antimonides as some exhibit promising
thermoelectric4,5 and magnetic properties.6 Typical polar
intermetallics are formed between electropositive elements
(alkali or alkaline-earth metals) and the elements around the
Zintl border (groups 13-15). These may exhibit a rich
variety of structural features that require understanding of
In this report, we describe a new mixed cation superstruc-
ture derived from Mg3Sb2 by ordered substitution of up to
one-half of the octahedrally coordinated Mg by dipositive
Sm. Ternary compounds with a Mg3Sb2 structure type are
known mainly with substitutions in all of the octahedra metal
layers, such as in AeMg2Sb2 in which Ae is Ca, Sr, or Ba.13
Potential derivatives with trivalent rare-earth elements would
of course require other substitutions as well in order to retain
closed-shell stability, as found in the CaAl2Tt2 analogues for
* To whom correspondence should be addressed. Email:
jcorbett@iastate.edu (J.D.C.); ashok@chemistry.iitd.ernet.in (A.K.G.).
† Iowa State University.
‡ Indian Institute of Technology Delhi.
(1) Corbett, J. D. Angew. Chem., Int. Ed. 2000, 39, 670.
(2) Corbett, J. D. In Chemistry, Structure and Bonding of Zintl Phases
and Ions; Kauzlarich, S., Ed., VCH: New York, 1996; Chapter 3.
(3) Schaefer, H. Annu. ReV. Mater. Sci. 1985, 15, 1.
(4) Gascoin, F.; Ottensmann, S.; Stark, D.; Haile, S. M.; Snyder, G. J.
AdV. Funct. Mater. 2005, 15, 1860.
(7) Nesper, R. Prog. Solid State Chem. 1990, 20, 1.
(8) Nesper, R. Angew Chem., Int. Ed. Engl. 1991, 30, 789.
(9) Corbett, J. D. Chem. ReV. 1985, 85, 383.
(10) Sevov, S. C.; Corbett, J. D. Inorg. Chem. 1991, 30, 4875.
(11) Dong, Z.-C.; Corbett, J. D. J. Am. Chem. Soc. 1994, 116, 3429.
(12) Ganguli, A. K.; Corbett, J. D.; Ko¨ckerling, M. J. Am. Chem. Soc.
1998, 120, 1223.
(5) Dashjav, E.; Szczepenowska, A.; Kleinke, H. J. Mater. Chem. 2002,
12, 345.
(6) Crerar, S, J.; Deakin, L.; Mar, A. Chem. Mater. 2005, 17, 2780.
(13) Deller, K.; Eisenmann, B. Z. Naturforsch. 1977, 32B, 612.
10.1021/ic060835w CCC: $33.50
Published on Web 09/02/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 20, 2006 8175

Gupta et al.
Tt ) Si, Ge.14 Ternary antimonide examples with rare-earth
elements that reinforced our interests in this area were the
three intergrowth structures reported by Ganguli, et al.15 in
the Mg-La-Sb system. These novel Zintl phases can be
described in terms of coherent intergrowths between either
Mg5Sb4 or Mg2Sb2 units derived from the Mg3Sb2 structure
and Lan+1Sbn slabs from LaSb (NaCl-type structure). The
longer and hence presumably weaker Mg(oct)-Sb bonds at
3.11 Å in comparison with stronger Mg(tetrah)-Sb bonds
that average 2.84 Å provide the required flexibility for the
rare-earth element substitution at the former site. However,
the observed (001)-(111) intergrowth was believed to be
additionally constrained to LnSb phases that have a/x2
values comparable to the a dimension of Mg3Sb2 (4.56 Å).
It was realized that further possibilities of deriving novel
ternary phases in this kind of system were not exhausted,
and this was the sole motivation behind the present work.
One possibility would be to substitute tin for antimony to
eliminate the cation defects, and another more obvious one
would be similar intergrowths with other rare-earth metal
antimonides. However, the latter proved unsuccessful for Ln
) Pr, Nd, Gd, and Tb, only Mg3Sb2 and LnSb forming as
major products. Lattice mismatch that induced strain was
thought to preclude intergrowths with these other trivalent
rare-earth element antimonides. On the other hand, attempted
substitution of divalent example Sm did afford the new
superstructure variation of Mg3Sb2 reported here.
600 °C yielded none of the new phase. Subsequently, a series of
three stoichiometries, that of the crystal composition and two that
were richer in Sm and poorer in Mg, were studied under three
different reaction conditions in order to obtain the new phase more
cleanly and in higher yields. Slow cooling from 850 °C yielded
moderate amounts of the product, some with noticeably large cells.
However, quenching of the samples from 950 °C followed by 10
day equilibrations at 700 °C gave the highest yields, in particular
85% 5-1-4 plus only SmSb from a Mg4Sm2Sb4 composition, the
latter suggesting that this loading was too rich in Sm. At the same
time, the cell volumes were observed to increase appreciably, from
275.3 Å3 for the structurally refined product through intermediate
values above 290 Å3 to the maximum seen for the last sample,
292.6 Å3 (a ) 4.686 Å, c ) 15.388 Å). The new phase is not very
sensitive to air at room temperature.
Substitution of Yb in the smaller parent structure was also
observed with different stoichiometries. However, Lu instead in
similar proportions and conditions did not lead to the superstructure
phase. Instead, only Mg3Sb2 and LuSb were formed, which suggests
that there are likely dipositive charge limitations to stabilizing
substitutional products in this structure.
2.2. Powder X-ray Diffraction. The powder diffraction patterns
of all products obtained with the aid of a Huber 670 camera and
Cu KR1 radiation were used for phase identification. Samples were
mounted inside a nitrogen-filled glovebox between two Mylar sheets
by means of a block-and-ring assembly and some vacuum grease
to hold and to seal the sample from the air. The presence of the
title and other phases were established by matching the observed
patterns with those of known phases generated from their lattice
parameters, space groups, and atomic positions, the last for the title
phase according to the single-crystal analysis.
2. Experimental Section
2.3. Single-Crystal Structure Determination. A few small black
irregular crystals of the title compound were isolated from the
2.1. Syntheses. Single crystals of a Mg5.231(8)Sm0.769(8)Sb4
(nominally 5-1-4) composition according to later single-crystal
analysis were obtained after a mixture of the high-purity elements
as Mg turnings (99.99%, Ames Laboratory), Sm powder (Research
Chemicals, 99.9%), and Sb chunks (99.99%, Johnson-Matthey)
was allowed to react at high temperature in Ta or Nb tubing (1/4
in. diam) welded under an Ar atmosphere. The tubes were further
jacketed in evacuated and sealed silica containers and heated in
resistance furnaces. The new phase was first recognized via high-
quality Guinier patterns obtained with the aid of a Huber 670
Guinier powder camera equipped with an area detector. The 5-1-4
phase was present in only ca. 30% yield following a reaction of
the nominal composition Mg2.5Sm0.5Sb2 in Ta at 850 °C for 7 days
followed by radiative cooling. However, four other phases were
also present, Mg3Sb2 (∼15%), Ta5Sb4 (∼10%), SmSb (∼20%), and
Sm3-xMgxSb2 (∼25%). The last, presumably with random substitu-
tion of Sm for Mg, was recognized (in several instances) from its
Mg3Sb2-like powder pattern, mainly through an increase in d values
of the reflections. This particular system was possibly not at
equilibrium. (The Ta5Sb4 side product from attack of the Ta
container was, as before in related systems,16 avoided thereafter
by the use of Nb.) An identical composition Mg2.5Sm0.5Sb2 yielded
∼80% of the new (5-1-4) phase along with ∼20% SmSb when
the elemental components were allowed to react at 700 °C for 10
days in a Nb container. On the other hand, the same reaction mixture
quenched from 850 or 1000 °C followed by annealing at 400 or
products from the first reaction (with a loaded composition Mg2.5
-
Sm0.5Sb2) and placed inside glass capillaries (0.3 mm) under a
nitrogen atmosphere. These were flame-sealed, and the crystals were
checked for singularity. An irregularly shaped one with approximate
dimensions of 0.08 mm × 0.07 mm × 0.07 mm was used for the
X-ray crystallographic analysis at 293 (2) K. Data were collected
on a Bruker SMART17 APEX CCD system equipped with an area
detector, a graphite monochromator, and a Mo KR fine-focus sealed
tube (λ ) 0.71073 Å). The detector was placed at a distance of
4.995 cm from the crystal. A total of 1800 frames were collected
for the indicated trigonal cell with ω - 2Θ scans, a scan width of
0.3°, and an exposure of 10 s/frame. The total data collection time
was approximately 8 h. The frames were integrated by means of
the Bruker SAINT software package17 and the narrow-frame
integration algorithm. The process yielded a total of 1865 reflections
to a maximum θ angle of 28.20° (0.75 Å resolution), of which 433
were independent and observed [I > 2σ(I)] (completeness ) 91.9%,
Rint ) 10.44%). Analysis of the data showed negligible decay
during the measurements. Data were corrected for absorption effects
with the aid of the multiscan technique SADABS.18 Some data
collection and refinement parameters are tabulated in Table 1.
The structure was solved by direct methods and refined by least-
2
squares methods on F0 with the Bruker SHELXTL (Version 6.1)
software package.19 Space group P3h (No. 147) was chosen on the
basis of the lack of any systematic absences and the E value
(14) Gladyshevskii, E. J.; Kripyakevich, P. I.; Bodak, O. I. Ukr. Phys. J.
1967, 12, 447.
(17) Bruker SMART and SAINT; Bruker AXS Inc.: Madison, WI, 1998.
(18) Sheldrick, G. M. SADABS user guide; University of Go¨ttingen:
Go¨ttingen, Germany, 1997.
(19) Sheldrick, G. M. SHELXTL, ver. 6.10; Bruker AXS, Inc.: Madison,
WI, 2001.
(15) Ganguli, A. K.; Kwon, Y, -U.; Corbett, J. D. Inorg. Chem. 1993, 32,
4354.
(16) Ganguli, A. K.; Gupta, S.; Corbett, J. D. Inorg. Chem., 2006, 45, 196.
8176 Inorganic Chemistry, Vol. 45, No. 20, 2006

Mg5.23Sm0.77Sb4
Table 1. Some Crystal and Structure Refinement Data for
Mg5.231(8)Sm0.769(8)Sb4
fw
729.78
crystal system
space group, Z
unit cell dimens (Å)
trigonal
P3h (No. 147), 1
a ) 4.619(1)
c ) 14.902(6)
275.3(1)
4.401
15.147
vol (Å3)
d
calc (Mg/m3)
µ (Mo KR) (mm-1
)
data/restraints/params
final R indicesa [I > 2σ(I)a]
R1, wR2
433/0/18
0.0480, 0.1028
a R1 ) ∑(|Fo - Fc|)/∑(|F0|), wR2 ) [∑w(Fo - Fc )/∑wFo ]1/2
2
2
4
Table 2. Atomic Coordinates (×104) and Equivalent Isotropic
Displacement Parameters (×103 Å2) for Mg5.231(8)Sm0.769(8)Sb4
a
Wykoff
x
y
z
U(eq)
occupancy * 1
Sm/Mg1
Sb1
Sb2
Mg2
Mg3
Mg4
1a
2d
2d
2d
2d
1b
0
0
0
14(1)
0.769(8)/0.231
1/3 2/3 3889(1) 19(1)
1/3 2/3 1301(1) 19(1)
1/3 2/3 1927(6) 24(2)
1/3 2/3 3264(5) 24(2)
0
0
5000
40(3)
a U(eq) is defined as one-third of the trace of the orthogonalized Uij tensor.
Figure 1. ∼[110] projections of portions of the parent structure Mg3Sb2
(left) and of the superstructure Mg5SmSb4 (limiting composition) (right).
Atom positions are marked as dark blue, Sb; medium blue, Mg in tetrahedral
sites; light blue, Mg in octahedral sites; pale blue, Sm (larger) in octahedral
sites. The most stable arrangement when the large Sm substitutes for Mg
in octahedral sites in trigonal Mg3Sb2 is for Sm to occupy a majority of
octahedral sites in alternate layers in a 2c superstructure, as shown.
Table 3. Important Interatomic Distances (Å)
bond
distance
Sm/Mg1-Sb2
Mg2-Sb1
Mg2-Sb2
Mg3-Sb1
Mg3-Sb2
Mg4-Sb1
×6
×1
×3
×3
×1
×6
3.297(1)
2.928(8)
2.823(3)
2.824(2)
2.925(8)
3.138(1)
system15 were not realized in the Mg-Sm-Sb system.
Rather, this system yields what is, to the best of our
knowledge, the first example of a simpler member of the
earlier series and not an intergrowth in the true sense. The
new compound exemplifies the first superstructure of an Mg3-
Sb2-type system, in this case induced by an ordered substitu-
tion of the larger dipositive rare-earth element Sm for Mg.
The changes in the Mg3Sb2 structure type on Sm substitu-
tion are illustrated in Figure 1 in ∼[110] projections. The
parent structure20 on the left contains hcp slabs of Mg2Sb2
(dark blue) in which all tetrahedral cavities are filled by Mg
(blue). The last third of the Mg (light blue) then occupies
all of the nominally octahedral cavities between these four-
layer slabs. On addition of some Sm, the refined structure
for the ternary example Mg5.231(8)Sm0.769(8)Sb4 on the right,
Figure 1, now has 77(1)% of the larger Sm (plus 23% Mg)
ordered in one-half of the former interslab layers of
octahedral Mg, thereby generating the 2-fold superstructure
along c in P3h. (The numbering of the atoms follows the z
coordinates, Table 2.) The observed difference in average
Mg-Sb distances for the two types of Mg environments
(about Mg4 vs Mg2, 3), 0.27 Å, is somewhat larger than
that according to standard cation crystal radii for Mg21 (and
a fixed value for Sb3-), 0.15 Å, which is some measure of
the strain or accommodation in this arrangement. This
particular composition, Mg5.231(8)Sm0.769(8)Sb4, was obtained
in only 30% yield from the first synthetic success, in which
the composition Mg5SmSb4 had been heated in Ta at 850
°C for 7 days and then cooled. Better and cleaner yields, up
statistics. No inconsistent equivalents were observed for the chosen
space group. The successful solution of the structure and its final
refinement affirmed this selection. No satisfactory solution could
be obtained in the higher symmetry space group P3hm1 to which
Mg3Sb2 belongs. The initial model correctly revealed the positional
coordinates for Sm and Sb, but three improbable Mg positions also
so generated were deleted during the refinement. Rather, the Mg
positions were determined from a difference Fourier map. Two of
the most intense residual peaks generated after the first few cycles
of refinement were assigned as Mg, and the model was refined for
a few more cycles. At this stage, all the atoms were refined
anisotropically which led to well-behaved ellipsoids for all atoms.
However, high values of R1 and wR2 (10% and 16%, respectively)
and large anisotropic displacement parameters for Sm suggested
that this site might be either partially occupied or co-occupied by
Mg. The latter was employed in the final model, as it provided a
slightly better fit over the former; furthermore, vacancies on this
dipositive cation site would not be reasonable either considering
the Sb proportion. The final anisotropic full-matrix least-squares
2
refinement on Fo with 18 variables converged at R1 ) 4.8% for
the 433 observed data and wR2 ) 10.66% for all data. The final
positional parameters and isotropic-equivalent displacement pa-
rameters for all atoms are collected in Table 2. More complete data
are given in Supporting Information.
3. Results and Discussion
Intergrowth structures of Mg3Sb2 with rock-salt layers of
LaSb that were demonstrated in our earlier report for three
classic examples in the ternary Mg-La-Sb intermetallic
(20) Zintl, E.; Husemann, E. Z. Phys. Chem. 1933, 21B, 138.
(21) Shannon, R. D. Acta Crystallogr. 1976, 32A, 751.
Inorganic Chemistry, Vol. 45, No. 20, 2006 8177

Gupta et al.
to 85%, were obtained after further explorations in which
an initial reaction at high temperature was followed either
by slow cooling or, best, by annealing the quenched product
around 700 °C (see Experimental Section). The sequence is
probably necessary because the product melts incongruently.
At the same time, an increase of the lattice volume of up to
17.3 Å3 (0.62%) above that for the single-crystal result was
observed from a reaction composition Mg4Sm2Sb4 quenched
from 950 °C and annealed for some time at 700 °C.
Obviously, this increase must arise from an increase in Sm
content on the substituted sites, which may approach the full
limit Mg5SmSb4, but not to completion in this case inasmuch
as SmSb was the other product (more details are given in
the Experimental Section).
Furthermore, products from the same series of reactions
without excess Sm also exhibited a shifted subcell pattern
of a second solution phase Mg3-ySmySb2, evidently reflecting
a uniform substitution of Sm on all octahedral sites in the
parent structure after the supercell phase approached satura-
tion. The same shifted patterns were also seen in the absence
of superstructure phase in earlier reactions that were evidently
not at equilibrium. The largest increase seen in the subcell
amounted to only 2.2 Å3 (a ≈ 4.586 Å, c ≈ 7.29 Å) or 1.93%
under the conditions explored, but higher values should be
possible. (Substitutions of Yb in the same parent structure
were also observed (along with Yb11Sb10), but compositions
necessary to examine the superstructure formation (or not)
were not loaded.) The characteristics of the Sm-substituted
phase with the smaller lattice were not studied further.
However, its relative stability appears to be appreciably less
than that of the superstructure solution inasmuch as substitu-
tions appeared in the smaller cell only after the new ordered
phase was close to its Sm limit. Likewise, Sm distributions
at intermediate compositions were seen to favor of the larger
cell inasmuch as this phase was also richer in Sm than the
proportion loaded. It is reasonable that the superstructure
should be better able to accommodate the difference in cation
radii (below) than a mixture on the unaltered octahedral sites
of the parent.
would correspond to that of the ordered supercell, ∼Mg5-
AeSb4, have evidently not been investigated for these
systems.
Returning to the superstructure phase, the parameters
reflect the large difference in standard six-coordinate crystal
radii21 for these two elements, 0.45 or 0.35 Å with 23% Mg
mixing; yet the experimental difference in radii within the
two types of octahedral sites is in fact only 0.16 Å. Other
dimensional changes on formation of the ternary structure
do not suggest much further strain or compromises. The small
increase in a accompanying the compositional change, 0.06
Å, is reflected in small < ∼2° flattening of the Sb-Mg-Sb
angles about both Mg types to Sb atoms in the same layer,
that is, angles that have a component in c, but not so for
those in the other direction. Likewise, Mg-Sb distances in
the common part of the two structures show changes of
e0.02 Å. On the other hand, although those angles in the
octahedra around Mg in the parent structure that have a c
component are increased beyond 90° by ∼5°, these angles
decrease (narrow) by ∼6° on Sm substitution. This remains
the largest overall change accompanying the substitution,
corresponding to a simple expansion of 0.42 Å along c
beyond a doubling of that axis. However, a double-sized and
fairly uniform displacement ellipsoid for Mg4 in the remain-
ing layered octahedra (∼40 × 103 Å2, Table 2) suggests some
softness or fluxionality in this region.
There is little doubt that alternate substitution of Eu2+ or,
particularly, the smaller Yb2+ should be possible in the new
structure, and perhaps other dipositive examples as well.
These indeed may further open up rich possibilities for more
tuning of properties important to thermoelectric effects, etc.
in this structure type. Presumably all of these compounds
with only dipositive cations are Zintl phases and semicon-
ductors since creation of cation vacancies would be expected
to be energetically unfavorable.
Acknowledgment. The principal part of this research was
supported by the NSF, Solid State Chemistry, via Grants No.
DMR-0129785 and DMR-0444657 and was carried out in
the facilities of the Ames Laboratory, U. S. Department of
Energy, Iowa State University. The authors also thank the
Department of Science and Technology, Government of India
and IIT Delhi for the funding of the Bruker SMART APEX
X-ray diffractometer.
More or less complete substitution in all octahedra are
known in Mg2AeSb2 and related pnictides for Ae ) Ca-
Ba,13 but results with less Ae were not noted. In the extreme
case, the Ba2+ radius is even larger than that of Sm2+;
however, it’s not clear from the isotropic U values reported
and the lack of synthetic details whether mixed occupancies
of the octahedral sites with some Mg can be excluded,
particularly because Ca and Sr refined with smaller values
of Uiso than did Mg or Sb. Products of a composition that
Supporting Information Available: Tables of detailed struc-
tural and refinement data and the cif file. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC060835W
8178 Inorganic Chemistry, Vol. 45, No. 20, 2006