R6TT′2, New Variants of the Fe2P Structure Type. Sc6TTe2 (T = Ru, Os, Rh, Ir), Lu 6MoSb2, and the Anti-typic Sc6Te 0.80Bi1.68

Article abstract of DOI:10.1021/ic0302581

The Fe₂P structure (P62m) features two 3-fold Fe positions and both 2-fold and 1-fold P sites, and variations in occupancies of the latter pair yield the reported diversity of results. The known Sc₆TTe ₂ examples for T = Fe-Ni are herein extended to four heavier transition metal T derivatives. An attempt to synthesize bismuth analogues led to the novel inverse derivative in which fractional Te (vice T) occupies the smaller tricapped trigonal prismatic (TTP) Sc polyhedron, and Bi rather than Te occurs in the larger TTP of Sc, with parallel reversal of polarity in the bonding. The reported Lu₈Te, which is distributed as Lu ₆TeLu₂, is the only example in which a transition metal occupies the normal 2-fold P or Te non-metal position, with corresponding large effects on the bonding. Lutetium otherwise does not form R₆TTe ₂ analogues, but the novel Lu₆MoSb₂ isotype occurs instead. Extended Hueckel calculations are presented for five examples, and the structural and bonding regularities and varieties are discussed further.

Full text of DOI:10.1021/ic0302581

Inorg. Chem. 2004, 43, 436−442
R TT′ , New Variants of the Fe P Structure Type. Sc TTe (T ) Ru, Os,
6
2
2
6
2
Rh, Ir), Lu MoSb , and the Anti-typic Sc Te Bi
6
2
6
0.80 1.68
Ling Chen and John D. Corbett*
Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011
Received August 13, 2003
The Fe
2
P structure (P 6h 2m) features two 3-fold Fe positions and both 2-fold and 1-fold P sites, and variations in
TTe examples for T ) Fe−Ni
occupancies of the latter pair yield the reported diversity of results. The known Sc
6
2
are herein extended to four heavier transition metal T derivatives. An attempt to synthesize bismuth analogues led
to the novel inverse derivative in which fractional Te (vice T) occupies the smaller tricapped trigonal prismatic
(
TTP) Sc polyhedron, and Bi rather than Te occurs in the larger TTP of Sc, with parallel reversal of polarity in the
bonding. The reported Lu Te, which is distributed as Lu TeLu , is the only example in which a transition metal
occupies the normal 2-fold P or Te non-metal position, with corresponding large effects on the bonding. Lutetium
otherwise does not form R TTe analogues, but the novel Lu MoSb isotype occurs instead. Extended H u¨ ckel
8
6
2
6
2
6
2
calculations are presented for five examples, and the structural and bonding regularities and varieties are discussed
further.
8
Introduction
hexagonal Fe
Dy TTe , T ) Fe-Ni. To these can be added the zirconium
analogues Zr
2 6 2
P-type structures, Sc TTe , T ) Fe - Ni, and
9
6
2
Metal-rich compounds formed between an early transition
metal, a late transition metal, and a late main-group element
1
0
6
TTe
2
6
, T ) Mn - Ni, Ru, Pt and Hf Ni1-xSb2+x,
1
1
x ∼ 0.25.
(non-metal) exhibit a variety of novel stoichiometries,
The well-known hexagonal Fe
2
P-type structure12,13 is
exhibited by hundreds of alloys and intermetallic compounds.
One distinctive feature of this structure type is that both the
iron and phosphorus sites are double, viz., Fe(I), Wyckoff
structures, and bonding. All of these appear to reflect the
unusually strong bonding that occurs between early and late
transition metals, as first noted by Brewer in intermetallic
1
systems. A large group of such ternary phases occur as
6 3
site 3f, Fe(II) 3g, P(I) 1b, and P(II) 2c or, literally, as Fe P .
metal-rich halides, both as isolated clusters and in extended
2
,3
Thus, many mixed but ordered compositions are possible in
higher order compounds. Mixed metals on the two Fe sites
chain motifs. Chalcogen and pnictogen compounds with
early-late transition metal combinations yield a completely
different group of compounds that are generally more metal-
rich and, accordingly, more two- or three-dimensional in
metal-metal bonding. The largest group has been reported
among ternary compounds of the rare-earth elements, scan-
1
4,15
appear more common, and phases such as ZrNbP
and
1
6
RAgGe, R ) Y, Sm, Gd-Lu, occur as ordered inter-
1
7
metallics.
Compounds in which the two phosphorus sites are dis-
tinguished by different elements are principally an inter-
dium in particular, with tellurium. These include the families
4
of orthorhombic Sc
5
Ni
2
Te
2
, orthorhombic Y
5
T
2
Te
2
, T ) Fe,
(
(
7) Chen, L.; Corbett, J. D. J. Am. Chem. Soc. 2003, 125, 1170.
8) Maggard, P. A.; Corbett, J. D. Inorg. Chem. 2000, 39, 4143.
5
Co, Ni (with metal columns rather than sheets), orthorhom-
5
6
bic Sc
6
TTe
2
, T ) Pd, Ag, Cu, Cd, the tetragonal chain
(9) Bestaoui, N.; Herle, S.; Corbett, J. D. J. Solid State Chem. 2000, 155,
9
.
7
phase Sc14
T
3
Te
8
, T ) Ru, Os, and two groups with the
(
(
10) Wang, C.; Hughbanks, T. Inorg. Chem. 1996, 35, 6987.
11) Kleinke, H. J. Alloys Compd. 1998, 270, 136.
*
Author to whom correspondence should be addressed. E-mail: jdc@
(12) Hyde, B. G.; Andersson, S. Inorganic Crystal Structures; J. Wiley:
New York, 1989; p 88.
ameslab.gov.
(
(
(
(
(
(
1) Brewer, L.; Wengert, P. R. Metall. Trans. 1973, 4, 83.
2) Corbett, J. D. J. Alloys Compd. 1995, 229, 10.
3) Corbett, J. D. Inorg. Chem. 2000, 39, 5178.
4) Maggard, P. A.; Corbett, J. D. Inorg. Chem. 1999, 38, 1945.
5) Maggard, P. A.; Corbett, J. D. J. Am. Chem. Soc. 2000, 122, 10740.
6) Chen, L.; Corbett, J. D. Inorg. Chem. 2002, 41, 2146.
(13) Rundqvist, S.; Jellinek, F. Acta Chem. Scand. 1959, 13, 425.
(14) Miller, G.; Cheng, J. Inorg. Chem. 1995, 34, 2962.
(15) Marking, G.; Franzen, H. F. J. Alloys Compd. 1994, 204, L16.
(16) Gibson, G.; P o¨ ttgen, R.; Kremer, R. K.; Simon, A.; Ziebeck, K. R. A.
J. Alloys Compd. 1996, 239, 34.
(17) Dwight, A. E. J. Less-Common Met. 1973, 30, 1.
436 Inorganic Chemistry, Vol. 43, No. 2, 2004
10.1021/ic0302581 CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/23/2003

R
6
TT′
2 2
, New Variants of the Fe P Structure Type
Table 1. Some Reaction Compositions, Conditions, and Products
loading
reactionsa
source; product phase analysisb
Sc6RuTe2
Sc6OsTe2
Sc6RhTe2
arc melt; HTF, 1135, 1 w
arc melt; HTF, 1300, 1 w
arc melt;
HTF; >95% Sc6RuTe2 (Fe2P)
c
HTF; 85% Sc6OsTe (Fe2P), 15% ScTe (NiAs)
c
90% Sc6RhTe2 (Fe2P), 10% ScTe (NiAs)
HTF, 1300, 48 h
arc melt;
HTF, 1300, 48 h
arc melt
85% Sc6RhTe2 (Fe2P), 15% ScTe (NiAs)
>95% Sc6IrTe2 (Fe2P)
Sc6IrTe2
Sc6PtTe2
>90% Sc6IrTe2 (Fe2P), ∼10% ScTe (NiAs)
d
90% Sc6PtTe2 (Sc6PdTe2 type ), 10% ScTe (NiAs)
HTF, 1300, 48 h
tube furn., 1050, 72 h
arc melt; HTF, 1200, -
85% Sc6PtTe2, 15% ScTe (NiAs)
d
Sc6PdTe2
Lu7Sb
95% Sc6PdTe2 (Sc6PdTe2 type )
e
c
90% Lu6MoSb2 (Fe2P type)
a
HTF ) high-temperature vacuum furnace, °C. All products identified by Guinier X-ray powder diffraction. c Data crystal for X-ray diffraction solution
b
d
e
as well. Reference 5. See text.
metallic example, such as Zr
6
CoAl 18
2
and the R
6
TT′
2
types
the Bi reaction) at high temperatures. (Some may still erode the
Ta tubing as well as the Mo foil itself, but to a much smaller extent.)
Some slow decomposition with time at 1300 °C is evident in many
systems (Table 1).
noted above in which R may be Sc, Y, or Dy, T is a late
transition metal, and T′ is Te or, in one case, Sb, although
examples with other R or other main-group elements are
presumably feasible. Here, we report some additional diverse
6 2
The best way to synthesize pure Sc MTe phases is via annealing
at 1000 °C for 1 week without arc-melting and with the Ta held
within sealed evacuated silica jackets and heated in conventional
tube furnaces. The best way to grow diffraction-size single crystals
is via annealing the arc-melted products at 1300 °C for 48 h
followed by slow cooling to room temperature. The latter was done
compounds of the Fe
more of the structural and electronic flexibility of the parent
type: (1) Sc TTe with T being the 4d or 5d elements Ru,
Os, Rh, and Ir as derivatives of Sc
the first example of a lutetium antimonide under the
circumstance that Lu TTe analogues do not appear to be
stable, and (3) the anti-type example Sc Te
2
P-type family that nicely demonstrate
6
2
8
6
2 6 2
FeTe , (2) Lu MoSb as
in a graphite-heated vacuum furnace (if >1100 °C) with a residual
pressure less than 10-6 Torr. which also eliminates any possibility
6
2
6
x
Bi2x (x ∼ 0.8) in
of hydrogen impurities. Usually, the samples partially melted in
the early stages, and small crystals that were suitable for single-
crystal X-ray diffraction could be picked from the product or from
the inner Mo surface. The yields of the target compounds according
to relative intensities of the Guinier powder diffraction components
were generally pretty high (>85%). All the reactions with Ru, Os,
Rh, and Ir succeeded on the first try. All the compounds are stable
in air at room temperature for a couple of months, and crystals of
which only main-group elements occupy both phosphorus
sites and with a reversal of the usual disposition according
to electronegativity. Another anti-example occurs in the
19
newly discovered Lu
8
Te. This has a parallel distribution
as Lu TeLu , but the bonding has not been considered before
6
2
in any detail.
Sc
were also established in the Sc-Pd-Te and Sc-Pt-Te systems,
but these form the orthorhombic Sc TTe (Sc PdTe type, derived
(T ) Pd, Pt). The last is
interesting in that the parent binary member Y Te cannot be
6
Te0.8Bi1.6 are so stable for more than a year. Similar reactions
Experimental Section
Syntheses. All reactions were loaded in a glovebox filled with
Ar. The rare-earth metals Sc (chunk) and Lu (powder) were used
as supplied from Ames Laboratory (99.99%), the late transition
metals Ru, Os, Rh, and Ir as powders from Alfa (>99.5%), and Te
ingots, Bi pieces and Sb powder were from Aldrich (99.99%). The
purities of all starting elements were checked by EDS analyses as
well.
All compounds were synthesized by typical solid-state chemistry
methods on a scale of about 300 mg total. Some details are listed
in Table 1. For each reaction, a pressed /
6
2
6
2
6
20
2 6 2
from Sc Te ) instead, as do Y TTe
2
21
synthesized, in contrast to Sc
A wide variety of similar reactions in the Sc-Bi-Te system all
failed to give any comparable Sc BiTe -type phase, producing
cubes, ScTe (NaCl) sheets, and
leftover Sc. Nonetheless, a few rod crystals that gave the structure
of, and analyzed as, the anti-type Sc Te0.80Bi1.68 (below) were picked
2
Te.
6
2
22
5 3
instead orthorhombic Sc Bi
6
1
4
in. diameter pellet of
from the surface of the Mo foil after reaction of the arc-melted
the appropriate mixture was first arc-melted for 20 s with ∼40 amp
current on a water-cooled copper hearth in the Ar atmosphere within
a glovebox after Zr shot had first been melted to purify further the
atmosphere. (The minimum current was utilized for the Bi reaction.)
The sample pellet was subsequently inverted and arc-melted again
to promote homogenization. The total weight losses after arc-
melting because of volatilization were under 4% unless noted. The
buttons were crushed into smaller pieces in an agate mortar and
then ground into a fine black powder for Guinier X-ray powder
examinations so as to identify crystalline phases at that point. The
powders were then pelletized again, wrapped in molybdenum foil,
and sealed into tantalum tubing. The molybdenum foil helped to
protect (at least) the inner wall of the Ta container from direct
reaction with the late transition metal components (and with Te in
button at 1300 °C for 48 h. This and the Ta inner wall were also
covered with well dispersed smaller crystals, probably Sc Bi .
5 3
Variations in reaction temperature or prior treatment were not as
useful. The physical separation of the leftover Sc from the initial
reaction (2.5 mol of Sc/mol of Sc
composition) appeared to be a major limitation. We assume that
the well-isolated rods of Sc Te0.8Bi1.6 grew instead by a customarily
5 3 6 2
Bi for a loaded Sc TeBi
6
not-very-efficient, dynamic vapor phase transport reaction, doubt-
2
lessly autogenous in nature, as with ZrCl. Compositions determined
6
by single-crystal structural refinements of two crystals were Sc -
Te0.79(1)Bi1.63(1) and Sc Te0.797(5)Bi1.682(8) which are in accord with
6
the EDS results for the latter, in at. %. Calcd [found]: Sc, 70.8
(
(
20) Chen, L.; Corbett, J. D. Unpublished research.
21) Maggard, P. A.; Corbett, J. D. Angew. Chem., Int. Ed. Engl. 1997,
36, 1974.
(
18) Krypyakevich, P. I.; Burnashova, V. V.; Markiv, V. Ya. DepoV. Akad.
Nauk. Ukr. RSR, Ser. A.: Fiz-Tekhn. Met. Nauk. 1970, 32, 828.
19) Chen, L.; Corbett, J. D. J. Am. Chem. Soc. 2003, 125, 7794.
(22) Haase, M.; Block, H.; Jeitschko, W. Z. Anorg. Allg. Chem. 2001, 627,
(
1941.
Inorganic Chemistry, Vol. 43, No. 2, 2004 437

Chen and Corbett
Table 2. Lattice Dimensions of R6TT′2 (Fe2P-Type) Phases, R ) Sc,
Lu; T ) Ru, Os, Rh, Ir, Te, Mo; T′ ) Te, Sb, Bi, Lu
compounds
a (Å)
b (Å)
V (Å3)
a
Sc6RuTe2
Sc6OsTe2
7.681(1)
7.627(2)
7.718(1)
7.681(8)
7.6821(3)
9.000(3)
7.935(1)
3.844(2)
3.864(1)
3.8379(7)
3.853(4)
4.0815(4)
3.687(2)
4.2630(9)
196.4(1)
194.67(9)
197.98(5)
196.8(4)
208.60(2)
258.6(2)
232.43(7)
b
b
Sc6RhTe2
a
Sc6IrTe2
b
Sc6Te0.797(5)Bi1.680(8)
b,c
Lu6TeLu2
b
Lu6MoSb2
a
Lattice parameters determined from Guinier powder data, g12 lines
b
indexed. Lattice parameters and composition determined from single-
crystal data. Reference 19.
c
[74.0(6)]; Te, 9.40 [9.3(8)]; Bi, 19.8 [16.7(8)]. This was equivalent
to Sc6.00(5)Te0.75(7)Bi1.35(7). The latter examination should eliminate
all significantly stabilizing impurity elements heavier than B,
whereas the high vacuum precludes a hydride. Otherwise, we do
not understand the substoichiometry.
Figure 1. [001] section of the hexagonal R6TT′2 structure with the cell
marked. Key: red, (3f) Sc or Lu; pink, (3g) Sc or Lu; yellow, (1b) Ru, Rh,
Os, Ir, Mo or Te; green, (2c) Te, Bi, or Sb. All of the metal-metal contacts
are marked up through 5.0 Å.
Following the surprising discovery of Lu
led us to load parallel Lu-Sb reactions. The compositions Lu
and Lu Sb both gave a major Fe P-type product by powder
diffraction analysis, tentatively Lu Sb, but single crystal structural
8
Te,19 natural curiosity
Results and Discussion
7
Sb
9
2
2
Crystal Structure. The overall structure of most ternary
R TT′ (Fe P-based) compounds projected along [001] is
6 2 2
shown in Figure 1, with a bond cutoff in the drawing of 5.0
Å. All atoms lie on mirror planes, the 3f and 2c (red, green)
2
refinements of crystals from both reactions gave only about 75%
Sb occupancy of the 1b site. But a test for phase breadth via four
more reactions loaded between 31 and 38 at. % Sb instead all gave
20
23
atoms at z ) 0, and the 3g and 1b (pink, yellow) atoms at z
as the main products Lu
7
Sb
3
7 3
(Sc As type ) plus LuSb (NaCl)
1
)
2
/ . In the parent structure, 3f and 3g are occupied by Fe,
in the powder patterns. An EDS check helped us to clarify that the
apparent Sb-deficient occupancy of the 1c site from single-crystal
and the independent 2c and 1b both contain P, but in an
appreciable range of ternary members, the 3f and 3g (red,
pink) sites are usually occupied by the same early metal,
the 1b site (yellow) is usually favored for a later transition
metal, and, up to recent times, 2c (green) always belonged
results were instead those for Lu
6 2
MoSb [at. % calcd [found]: Lu,
6
6.7 [62(4)]; Mo, 11.1 [11.1(6)]; Sb, 22.2 [27(2)]], or Sc6.0(4)
-
Mo1.07(6)Sb2.6(2). The two reactions that gave this phase also produced
distinctly brittle Mo foil, the Mo source. Other T explorations
yielded a series of new orthorhombic Lu T Te
7 2 2
phases.24
to a main group element. This gives the families R
6
TTe
2
(R
X-ray Crystallography. All single-crystal data sets were col-
lected at room temperature with the aid of a Bruker AXS SMART
APEX CCD-based X-ray diffractometer and monochromatized Mo
KR radiation. Lattice constants are given in Table 2, and some
crystallographic details are listed in Table 3. Given the clear powder
8
9
)
Sc, Dy; T ) Fe, Co, Ni), Zr
6
TTe
, and Hf TSb
To these are here added Sc TTe , T ) Ru, Rh, Os, and Ir,
and Lu MoSb
The general motif of this R
sconsists of centered tricapped trigonal prisms (TTP)
2
(T ) Mn-Ni, Ru,
(T ) Fe,Co,Ni).
10
18
27
11
Pt), Zr
6
CoAl
2
, Zr
6
FeSn
2
6
2
6
2
6
2
.
6
2
TT′ structure typesFigure
25,26
pattern identifications, most structures were solved
assuming
1
the Fe P-type space group P 6h 2m. In accord, the mean values of
2
2
further condensed to form the 3-D hexagonal structure. There
are two kinds of confacial TTPs: the smaller metal one (red)
centered by a (yellow) (T) atom and two larger metal ones
(pink) centered by the green (T′) element. Both share their
triangular faces with like polyhedra to generate chains along
|
E - 1| in all cases strongly suggested that the structures were
noncentrosymmetric. Table 4 lists the positional and displacement
parameters and site occupancies (*1) for the four representative
compounds in the standard setting.13 Both Sc
6 2 6
OsTe and Sc -
Te0.797(5)Bi1.682(8) crystals were racemic twins, the components
twinning under the law [-1 0 0, 0 -1 0, 0 0 -1] with minor
[001]. Each rectangular face of the trigonal prisms is outer-
components of 18.01% and 18.48%, respectively. For Sc
although Rh at the 1b site has a large isotropic parameter compared
with that for Te, this is not the Fe P-type binary compound “Sc
Te ” according to the high synthetic yield. The occupancy of Rh
6 2
RhTe ,
capped by the other R element type. Finally, the 2c-centered
TTP are interconnected with 1b-centered TTPs through
relatively short (strong) inner-outer R-R interactions.
Usually, the 1b site (yellow) surrounded by the smaller
trigonal prism is occupied by a relatively smaller, late
transition metal (T ) Mn, Fe - Ni, Ru, Rh, Os, Ir). Data on
the six new compounds reported here are given in Tables 1
and 2 and, for the four structures refined, in Tables 3 and 4.
Table 5 summarizes specific and average bond distances of
different types in the latter four R TT′ compounds.
2
6
-
3
freely refined to 100.7(7)% with this thermal parameter; in addition,
a binary compound with this structure is not known. The same thing
happens with Sc
full occupancy. In the case of Sc
synthesis (above) both gave essentially the same results: Sc
Te0.797(5)Bi1.682(8). Also, the Bi:Te ratio was strongly
supported by EDS results (above) from one of the single crystals.
For Lu MoSb , once the presence of Mo was clarified by EDS,
the structure was refined without event.
6 2
OsTe , a larger 1b site thermal parameter but with
6
Te Bi2x, two crystals from the
x
6
Te0.79(1)-
Bi1.63(1) and Sc
6
6
2
6
2
6
The antitype Sc Te0.8Bi1.6 shows a very nice site prefer-
ence, which may simply arise because Bi is larger than Te
(23) Berger, R.; Nolaeng, B. I.; Tergenius, L. E. Acta Chem. Scand. 1981,
A35, 679.
(25) SHELXTL6.10. Bruker AXS, Inc.: Madison, WI, 2000.
(26) Blessing, R. H. Acta Crystallogr. 1995, A51, 33.
(24) Chen, L.; Corbett, J. D. Inorg. Chem., accepted.
(27) Kwon, Y.-U.; Sevov, S. C.; Corbett, J. D. Chem. Mater. 1990, 2, 550.
438 Inorganic Chemistry, Vol. 43, No. 2, 2004

R
6
TT′
2 2
, New Variants of the Fe P Structure Type
Table 3. Some Data Collection and Refinement Parametersa
empirical formula
fw
Sc6RhTe2
627.87
Sc6OsTe2
715.16
Sc6Te0.797(5)Bi1.682(8)
722.9
Lu6MoSb2
1389.26
space group, Z
P 6h 2m (No. 189),1
P 6h 2m (No. 189),1
P 6h 2m (No. 189),1
P 6h 2m (No. 189),1
1
abs. coeff, mm
13.976
28.397
49.914
69.973
3
dcalc, Mg/m
5.266
6.100
5.76
9.925
R1, wR2 [I > 2σI]
R1, wR2 (all data)
0.0139, 0.0333
0.0139, 0.0333
0.0166, 0.0489
0.0167, 0.0490
0.0105, 0.0240
0.0105, 0.0240
0.0176,0.0391
0.0176, 0.0391
b
a
Lattice parameters in Table 2. b In the majority of cases, all allowed reflections were also observed (>2σI).
Table 4. Positional and Equivalent Isotropic Displacement Parameters
3
(
×10 ) for Four Sc6TT′2 Structures
atom Wyckoff site
x
y
z
U(eq) site occ. (*1)
Sc6RhTe2
Sc1
Sc2
Te
3f
0.2420(2)
0.6068(2)
/3
0
0
/2
14(1)
16(1)
12(1)
24(1)
1
3g
2c
1b
0
1
2
/3
0
1
Rh
0
0
/2
Sc6OsTe2a
Sc1
Sc2
Te
3f
0.2386(3)
0
0
/2
7(1)
8(1)
5(1)
1
3g
2c
1b
0.6126(4)
0
1
2
/3
/3
0
Figure 2. Groups of diverse intermetallic compounds with an Fe2P
structure type that are discussed in the present paper. The arrows indicate
how the compounds are derived from the Sc6FeTe2 parent through chemical
substitution. 3f, 3g, 1b, and 2c are Wyckoff symbols for the four independent
crystallographic sites.
1
Os
0
0
/2
16(1)
Sc6Te0.797(5)Bi1.682(8)a
Sc1
Sc2
Bi
3f
0.2373(2)
0.6076(2)
0
0
10(1)
9(1)
10(1)
10(1)
1
3g
2c
1b
0
/2
1
2
/3
/3
0
0.841(4)
0.797(5)
1
Te
0
0
/2
crystal structure can also be interchanged, e.g., with Te and
Bi, to yield the evidently novel antitype Sc Te0.8Bi1.6.
6
Substitution of Sc by Lu (or Dy) is not a surprise, but the
fact that Lu atoms themselves can also occupy the 2c site in
Lu6MoSb2a
Lu1
Lu2
Sb
3f
0.2407(1)
0.6041(1)
0
0
0
7(1)
8(1)
8(1)
9(1)
1
3g
2c
1b
/2
1
2
/3
/3
0
1
Mo
0
0
/2
Lu
also the electronic flexibility of this type of structure. Again,
the discovery of the electron poorer Lu MoSb , the first
6 2
TeLu is, illustrating not only a presumed size effect but
a
Refined component has been converted to standard setting.
6
2
28
(1.51 vs 1.37 Å in metallic radii ) and then naturally fits
lutetium antimonide, also shows that Te and Sb have similar
the larger TTP better. (Note that both main-group element
sites are only about 80% occupied.) From Os to Bi, the
trigonal faces of two TTP do not change much (Table 5),
but they are 0.22 Å farther apart along the c direction with
Bi owing to differences in something akin to the van der
Waals radii. This size-determined site preference is evidently
chemical properties in this metal-rich intermetallic system.
8
As described before, the Sc
6
TTe
2
(M ) Fe, Co, Ni)
compounds exhibit a dominant one-dimensionality of metal-
metal bonding within the TTP chain along the c direction in
terms of bond distance as well as Mulliken overlap popula-
19
6 2
tions (MOP). In contrast, Zr TTe (T ) Mn-Ni, Ru, Pt)
again the case with Lu
principal metallic element also occupying the nominally
interstitial 2c site. In the case of Lu MoSb , the Lu1-Lu1
8
Te, the first example with the
1
0
phases with electron-richer transition metals have metal-
metal bonding arrays that have been described as fully three-
dimensional, corresponding to greater filling of the broad
conduction band. However, the opposite trend is seen on
6
2
interaction is the shortest among all Lu-Lu distances, in
parallel with the strong Lu-Mo interactions in the Mo-
centered TTP of Lu1 at essentially the sum of single bond
metallic radii.28 These unusual site occupancies are well
8 6 2
comparing the electron-poor Lu Te (Lu TeLu , VEC ) 30)
with Sc OsTe (VEC ) 38), in which the metal-metal array
6
2
in the former is more three-dimensional because of the
greater delocalization achieved with the metallic lutetium in
the 2c sites provides stronger metal-metal bonding. (This
aspect is considered in the next section.)
reflected in bonding differences as well (see below).
Figure 2 schematically summarizes the relationships
among all of these Fe
2
P-type intermetallics relative to the
(Ru/Os)Te phases are
parent Sc FeTe . The subgroup Sc
6
2
6
2
isoelectronic compounds, and these T elements can also be
substituted by Co, Rh, or Ir, their respective neighbors in
the periodic table. Some of the same substitutions are found
Theoretical Calculations and Comparisons. To gain
further understanding of the characteristics and differences
among these compounds, extended H u¨ ckel calculations were
carried out within the tight-binding approximation for the
with the Dy
electron-richer Pt and Pd examples of Sc
crystallize in the Fe P structure, but in an orthorhombic sheet
structure related to that of Sc
evidence that the types of atoms at 2c and 1b sites in the
6
TTe
2
subgroup.9 Interestingly, the slightly
TTe do not
6
2
6 2 6 2 6 2 6 2 8
Sc RuTe , Sc OsTe , “Sc TeBi ”, Lu MoSb , and Lu Te
2
29
examples with the aid of the CAESAR program. To make
the results more appropriate to the charge distributions in
these unconventional compounds and also more comparable
6,21
2
Te. We report here the first
to each other, interated Hii parameters of Sc and Te were
(
28) Pauling, L. Nature of the Chemical Bond, 3rd ed.; Cornell University
8
Press: Ithaca, NY, 1960; p 403.
taken from Sc
6
FeTe
2
, whereas such data for Fe, Ru, Os,
Inorganic Chemistry, Vol. 43, No. 2, 2004 439

Chen and Corbett
Table 5. Selected and Average Bond Distances (Å) in Some R6TT′2 Compounds
atom pairs
Sc6RhTe2
Sc6OsTe2
3.152(4)
4.095(4)
3.30
Sc6Te0.80(1)Bi1.68(1)
Lu6MoSb2
3.308(1)
4.217(1)
3.51
Lu8Tea
R1-R1
3.235(3)
4.114(3)
3.32
3.158(2)
4.096(2)
3.39
4.318(2)
4.835(3)
3.59
R2-R2
R1-R2, av
i-o, small TTP
o-i, large TTP
R1-T (3f-1b)
R1-T′ (3f-2c)
R2-T (3g-1b)
R2-T′ (3g-2c)
3.273(1) (×4)
3.407(2) (×2)
2.678(1)
2.988(1)
3.035(2)
3.223(2) (×4)
3.448(3) (×2)
2.654(2)
2.970(2)
2.951(3)
3.3287(8) (×4)
3.501(2) (×2)
2.7365(8)
2.9983(8)
3.015(1)
3.4728(6) (×4)
3.5856(8) (×2)
2.8619(6)
3.0790(6)
3.1416(8)
3.2360(4)
3.611(2) (×4)
3.545(2) (×2)
3.100(2)
3.283(2)
3.479(2)
3.345(1)
3.0537(6)
3.054(1)
3.1250(4)
a
Lu6TeLu2, ref 19.
Figure 3. DOS (upper) and COOP (lower) data for R6TT′2 compounds: (a) Sc6OsTe2; (b) Sc6TeBi2; (c) Lu6MoSb2; (d) Lu8Te (Lu6TeLu2). The horizontal
dashed lines mark the Fermi levels.
and Lu were obtained from a full charge-iterative calculations
in which orbital energy parameters for these atoms were
varied to self-consistency as a function of Mulliken charge
transfer. The Sb and Bi parameters and the standard orbital
exponents were taken from Alvarez, and Mo parameters
were estimated from those of Zr10 and Ru. All parameters
used are listed in Supporting Information in Table S3.
Figure 3 shows the total and some partial densities-of-
states (DOS) (top) and COOP (crystal orbital overlap
interactions. The four small peaks around -10.5 to -8.5
eV represent mainly Os(s)-Sc1p bonding states, whereas
the pronounced peak at -7.0 eV originates mainly from Os
d and Sc1 d states, with smaller contributions from Sc2 )
30
F
d. The states above E are mainly Sc d states with some Os
d and are initially bonding, as is usual in these electron-
poor phases. There is a clear distinction between nearest-
neighbor Sc1-Os intermetallic bonding interactions, which
fall just below (and above) E
bonding which falls lower (-12 to -14 eV) with a counter
antibonding states above E . (This effect is familiar in many
metal-rich Sc-Te phases, in which it is reflected in lower
F
, and the more polar Sc2-Te
population) (bottom) plots for four of the R
the energy range of -14.0 to -2.0 eV. For Sc
the states from -14.0 to -10.5 eV correspond to Te(p)-Sc
6
TT′
2
phases in
6
OsTe (a),
2
F
5,8
Sc-Sc MOP values for those Sc with Te neighbors. ) This
distinction is clear in the charges deduced for 3d Sc TTe
(
29) Ren, J.; Liang, W.; Whangbo, M.-H. CAESAR for Windows; Prime-
Color Software, Inc., North Carolina State University: Raleigh, NC,
6
2
1
995.
phases with T ) Fe, Ru, and Os according to the Mulliken
approximation, Table 6, namely Sc1 (-0.7 to -0.6 eV) vs
(
30) Alvarez, A. Tables of Parameters for Extended H u¨ ckel Calculations,
Parts 1 and 2; Barcelona, Spain, 1987.
440 Inorganic Chemistry, Vol. 43, No. 2, 2004

6 2 2
R TT′ , New Variants of the Fe P Structure Type
Table 6. Effective Atom Charges in Some R6TT′2 Phases as Calculated
by Extended H u¨ ckel Means
Table 7. Selected Metal-Metal Distances (Å) and Mulliken Overlap
Populations (MOP)
Sc6FeTe2a
Sc6RuTe2
Sc6OsTe2
atom 1-atom 2
R1-R1, basal
R1-R1 along c
R2-R2, basal
R2-R2, along c
R1-R2 (av)
i-o small TTP
o-i large TTP
R1-T′(R3)
Sc6OsTe2
MOP
Lu8Te
MOP
Wyckoff site
atom
charge
atom
charge
atom
charge
3.152(4)
3.864(1)
4.114(3)
3.864(1)
3.30
0.163
0.109
0.000
-0.003
0.095
0.125
0.033
4.318(2)
3.687(2)
4.835(3)
3.687(2)
3.59
3.611(2)
3.545(2)
3.283(2)
3.345(1)
3.687(2)
0.032
0.159
0.045
0.272
0.240
0.210
0.300
0.380
0.353
0.242
3
3
2
1
f
Sc1
Sc2
Te
-0.71
0.47
-0.62
1.98
Sc1
Sc2
Te
-0.70
0.48
-0.61
1.92
Sc1
Sc2
Te
-0.58
0.69
-0.60
0.86
g
c
b
Fe
Ru
Os
3.223(2)
3.448(3)
Sc6TeBi2b
Lu6TeLu2
Lu6MoSb2
Wyckoff site
atom
charge
atom
charge
atom
charge
R2-T′(R3)
T′-T′(R3BR3) (along c)
3
3
2
1
f
Sc1
Sc2
Bi
-0.08
0.05
0.21
Lu1
Lu2
Lu3
Te
0.29
-0.01
-0.32
-0.2
Lu1
Lu2
Sb
-0.41
0.29
-0.89
2.14
g
c
b
Some selected metal-metal distances and MOP data for
the two representatives Sc OsTe (VEC ) 38) and Lu TeLu
VEC ) 30) are listed in Table 7 for comparison. As before,
Te
-0.36
Mo
6
2
6
2
a
Reference 8. b Calculated with full occupancies.
(
6 2
the metal bonding in Sc OsTe is expected to be more one-
Sc2 (+0.5 to +0.7 eV). Naturally, the most electronegative
element Te has a negative charge, whereas the electron-rich
T ) Fe, Ru, and Os have positive effective charges, +2.0
dropping to +0.9 eV as the mixing with Sc becomes notably
larger and the interactions become less polar with heavier
period d elements. (Interestingly, these Sc1-T charge
transfers are in the direction first postulated by Brewer and
Wengert from an acid-base viewpoint.)
6
The novel anti-typic Sc Te0.8Bi1.6 (calculated for full rather
than ∼80% occupancies) exhibits a broader and less polar
Bi(6p)-Sc2-based band at midenergies, whereas Te in the
Sc1 TTP is now the more polar. Both effects level the
approximate charges with only Te being somewhat negative
dimensional character in the TTP chain. The strongest
bonding lies in the small TTP chain containing Os, namely,
within the trigonal base, the i-o capping interactions and
the R1-R1 interactions along c in decreasing magnitude.
The large trigonal prism has nearly no metal-metal bonding.
In contrast, the spread of bonding into a 3D metal network
1
character in Lu Te is obvious. The strongest Lu-Lu bonding
8
is within the larger Lu3-centered TTP; the weakest is in the
small TTP around Te with a MOP only ∼8% of the largest
one (0.032 vs 0.380). (Of course, the Coulomb contributions
are doubtlessly in inverted order.) As a simple scale of 3D
bonding character, the inter-TTP-chain interactions in these
and with Bi as a modest electron donor. E
with the observed occupancies, ∼Sc
band approximation), and only some nonbonding Sc-Te,
Sc-Bi states are emptied. The conversion to Lu MoSb
shows two notable effects. First, the Lu-Lu interactions are
strong and give rise to broader Lu-Lu bonding states,
F
is barely lowered
Te0.80Bi1.68 (in a rigid
8
two examples have distinguishable differences; that in Lu -
Te (Lu2-Lu1, 3.61 Å, MOP 0.210) has a MOP value around
55% of that for the largest Lu-Lu interaction, whereas that
in Sc OsTe (Sc1-Sc2, 3.45 Å, MOP 0.033) is just ∼20%
6
6
2
6
2
of the largest. The reason is more likely to be that Te at 2c
sites is more electronegative and at lower energy, which
drains more electron density from the Sc-Sc metal network
than Lu3 does from the Lu-Lu network. The more polar
Sc-Te interactions help the localization of the Sc-Sc
interactions.
2
paralleling the similar behavior already discussed for Dy -
31
Te vs Sc Te and broadening the empty portion of Lu-Lu
2
bonding states (COOP) to -2 eV. The Lu-Sb(p) band and
COOP are with better mixing lower and broader than, e.g.,
for Sc-Te, and the Lu-Mo band is broader, again giving
appreciable charges to Sb and Mo, -0.9 and +2.1, respec-
tively.
Conclusions
8 6 2
Finally Lu Te or, better for comparison, Lu TeLu shows
the largest changes (Figure 3d). The much broader conduc-
tion band and Lu-Lu COOP curves are striking, as the Lu-
Both the crystal and electronic structures show that the
d and 5d derivatives of Sc FeTe , namely Sc TTe (T )
6 2 6 2
Ru, Os, Ru, Ir), keep a dominant 1D TTP-chain character in
the metal-metal bonding, but the Sc-T heteroatomic
interactions become less polar with the heavier d period
4
Lu states here (and for Lu
6 2
MoSb ) remain bonding to well
above E . The lowest lying Lu-Te states correspond to
F
particularly strong interactions with Lu1, the only close
neighbor to Te, around -11 eV, and the weaker (longer)
Te-Lu2 bonding. Again, the surface Lu1 atoms (with Te
neighbors) tend to have positive charge, whereas all the inner
atoms have stronger Lu-Lu bonding and relatively negative
elements. The novel anti-typic Sc
determined site preference, as does Lu
6
Te0.8Bi1.6 shows a nice size-
Te. The latter is also
8
the first example in which the principal metallic element also
occupies the interstitial site that is usually favored for a non-
metal, and its metal-metal bonding tends to become more
6
charges. Naturally, Te has a negative charge. The innermost
3
D with a lower VEC and fewer polar metal-non-metal
interactions. The heavier variants of Fe P family, the
electron-poorer Lu Te and Lu MoSb , show strong Lu-Lu
interactions which give rise to broader Lu-Lu bonding
bands. The diversity among Fe P structure type members in
atom Lu3, which is not a Te neighbor, has the most negative
charge, -0.32, and, in parallel, all of the strongest Lu-Lu
bonding interactions.
2
8
6
2
(
31) Herle, P. S.; Corbett, J. D. Inorg. Chem. 2001, 40, 1858.
2
Inorganic Chemistry, Vol. 43, No. 2, 2004 441

Chen and Corbett
terms of stoichiometries, electron counts, and bonding
characteristics as well as in the chemical resemblance of
neighboring elements, even metals and non-metals therein,
illustrates some of the fascinating character of solid-state
chemistry.
Acknowledgment. We thank Marina Vondrova for as-
sistance with the Sc TTe reactions. This research was
6 2
supported by the National Science Foundation, Solid State
Chemistry, via Grants DMR-9809850 and -0129785 and was
carried out in the facilities of the Ames Laboratory, U.S.
Department of Energy.
These results beg for some predictive output, i.e., regarding
the stabilities of Lu
8
Sb, Sc
6
SbTe
2
, Sc
6
TSb, and the like.
Supporting Information Available: Tables of X-ray data
collection and refinement results, anisotropic displacement param-
eters, and parameters used in extended H u¨ ckel calculations. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Unfortunately, the answers always depend on the relative
stabilities of alternate products as well, many of which may
involve now unknown compositions and structures. The
synthetic experiment often gives the answer much more
quickly and reliably.
IC0302581
442 Inorganic Chemistry, Vol. 43, No. 2, 2004