Novel condensation of Au-centered trigonal prisms in rare-earth-metal-rich tellurides: Er7Au2Te2 and Lu7Au 2Te2

Article abstract of DOI:10.1039/c000650p

A new monoclinic structure occurs for Er₇Au₂Te ₂ according to X-ray diffraction analysis of single crystals grown at 1200 °C: C2/m, Z = 4, a = 17.8310(9) A, b = 3.9819(5) A, c = 16.9089(9) A, β = 104.361(4)°. The isostructural Lu ₇Au₂Te₂ also exists according to X-ray powder pattern means, a = 17.536(4) A, b = 3.9719(4) A, c = 16.695(2) A, β = 104.33(1)°. The structure contains zigzag chains of condensed, Au-centered tricapped trigonal prisms (TCTP) of Er along c that also share basal faces along b to generate puckered sheets. Further bi-face-capping Er atoms between these generate the three dimensional network along a, with tellurium in cavities outlined by augmented trigonal prismatic Er polyhedra. Bonding analysis via LMTO-DFT methods reveal very significant Er-Au bonding interactions, as quantified by their energy-weighted Hamilton overlap populations (-ICOHP), ～49% of the total for all interactions. These and similar Er-Te contributions sharply contrast with the small Er-Er population, only ～14% of the total in spite of the high proportion of Er-Er contacts. The strong polar bonding of Er to the electronegative Au and Te leaves Er relatively oxidized, with many of its 5d states falling above the Fermi level and empty. The contradiction with customary representations of structures that highlight rare-earth metal clusters is manifest. The large Er-Au Hamilton overlap population is in accord with the strong bonding between early and late transition metals first noted by Brewer in 1973. The relationship of this structure to the more distorted orthorhombic (Imm2) structure type of neighboring Dy₇Ir₂Te₂ is considered.

Full text of DOI:10.1039/c000650p

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www.rsc.org/dalton | Dalton Transactions
Novel condensation of Au-centered trigonal prisms in rare-earth-metal-rich
tellurides: Er7Au2Te2 and Lu7Au2Te2†
Shalabh Gupta* and John D. Corbett*
Received 11th January 2010, Accepted 27th April 2010
First published as an Advance Article on the web 12th May 2010
DOI: 10.1039/c000650p
A new monoclinic structure occurs for Er7Au2Te2 according to X-ray diffraction analysis of single
◦
˚
˚
˚
crystals grown at 1200 C: C2/m, Z = 4, a = 17.8310(9) A, b = 3.9819(5) A, c = 16.9089(9) A, b =
104.361(4)◦. The isostructural Lu7Au2Te2 also exists according to X-ray powder pattern means, a =
◦
˚
˚
˚
17.536(4) A, b = 3.9719(4) A, c = 16.695(2) A, b = 104.33(1) . The structure contains zigzag chains of
condensed, Au-centered tricapped trigonal prisms (TCTP) of Er along c that also share basal faces
along b to generate puckered sheets. Further bi-face-capping Er atoms between these generate the three
dimensional network along a, with tellurium in cavities outlined by augmented trigonal prismatic Er
polyhedra. Bonding analysis via LMTO-DFT methods reveal very significant Er–Au bonding
interactions, as quantified by their energy-weighted Hamilton overlap populations (–ICOHP), ~49% of
the total for all interactions. These and similar Er–Te contributions sharply contrast with the small
Er–Er population, only ~14% of the total in spite of the high proportion of Er–Er contacts. The strong
polar bonding of Er to the electronegative Au and Te leaves Er relatively oxidized, with many of its 5d
states falling above the Fermi level and empty. The contradiction with customary representations of
structures that highlight rare-earth metal clusters is manifest. The large Er–Au Hamilton overlap
population is in accord with the strong bonding between early and late transition metals first noted by
Brewer in 1973. The relationship of this structure to the more distorted orthorhombic (Imm2) structure
type of neighboring Dy7Ir2Te2 is considered.
tellurides relative to the halides because only about half as many
Introduction
of the monoanions are now necessary to achieve about the same
electron count per network metal atom (~2). This leads to notably
greater degrees of cluster condensation of the tellurides and to
more complex, two- or three-dimensional network structures.1,10,11
A unifying feature in these and other recently reported metal-rich
ternary tellurides is a common structural motif: tricapped trigonal
prisms (TCTP) of the rare-earth metal centered by a late transition
metal as well as similar basic coordination of Te by R.9,11
Gold incorporation has recently been found to stabilize an
appreciable group of novel structures and stoichiometries among
a remarkable variety of related electron-deficient Zintl phase
relatives12–15 (polyanionic salts of active metals, counterparts of
the foregoing cationic polymetal salts16). This has given us good
incentive to examine Au reactions with the poly-rare-earth-metal
salts as well. The distinctive behavior of gold in the former
phases evidently originates with prominent relativistic effects17
that particularly stabilize (and contract) its 6s orbital, reduce the
atom’s effective radius, and therewith bring 5d states back into
a prominent bonding role. Another expression of these effects is
that they markedly increase the Mulliken electronegativity of gold
(5.77 eV), putting it in the neighborhood of Te and Se.18
An impressive number of new structures and bonding novel-
ties have been discovered in the rare-earth-metal-rich telluride
systems.1 This direction evolved from the rather spectacular results
for similarly electron-poor cluster halides of the rare-earth metals
R (group 3 and lanthanides). Seemingly regular cluster/network
electronic requirements for R bonding in both families necessitate
the inclusion of substantially electron-richer interstitial atoms (Z)
within all R cages.2,3 However, the particular stability of R clusters
centered by the late transition metals in the halide and telluride
“salts” has been qualitatively attributed to the strength of the
polar heteroatomic bonding between early and late (electron-poor
and electron-rich) transition metals (Tn) first put forth by Brewer
and Wengert.4 This concept has led to the discovery of a large
number of new phases and structures stabilized by 3d, 4d, and 5d
transition metal interstitials.1,5,6 Interestingly, the more productive
R members appear to be Sc, Y and the heavier lanthanides,
particularly Gd, Dy, Er and Lu (to date), a differentiation has
been attributed to the fact that the more tightly bound valence
orbitals in this group are closer in energy to those of the usual
Z.7–9 A more diverse structural chemistry arises for the ternary
Earlier extended-Hu¨ckel examinations of the bonding among
the halide examples have generally suggested the presence of
strong R–Z and somewhat weaker R–R bonding.19–22 However,
the various Mulliken overlap population (COOP) values are not
directly comparable, and no more quantitative studies have yet
appeared. However, LMTO-type first-principle calculations for
Department of Chemistry, Iowa State University, Ames, Iowa, USA 50011.
E-mail: jcorbett@iastate.edu, shalabh@iastate.edu
† Electronic supplementary information (ESI) available: Displacement
ellipsoid parameters, distances and individual –ICOHP data, and other
structural views. CSD reference number 420862. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c000650p
6074 | Dalton Trans., 2010, 39, 6074–6079
This journal is
The Royal Society of Chemistry 2010
©

several novel gold-stabilized Zintl-phase derivatives14,15,23,24 have
greatly illuminated new and more quantitative aspects of their
bonding in terms of (Hamilton) bond populations. The better
representation of gold’s 5d orbitals leads to distinctively enhanced
bonding relative to those of the following, largely p-bonded
components. The present article reports the first results among
the R-based halides and tellurides for (1) gold substitution in a
new structure type for Er7Au2Te2 (and Lu7Au2Te2) and (2) such ab
initio calculations as aids to understanding and representing the
bonding. Only one analogous reduced halide system is known with
gold, the fairly unremarkable cubic La3AuI3 (Ca3PI3 type) in which
gold centers condensed, nominal La6 octahedra condensed into a
spiral network.25 In contrast to the present results, the polytypic
orthorhombic Er7Ni2Te2 structure type (Imm2)26 occurs for several
of the heavier R and a fairly large number of other transition
metal interstitials: R(Z) = Dy (Pd, Ir, Pt), Er (Rh, Ir, Ni, Pd,
Pt),9 and Lu (Ni, Ru, Rh, Pd).11 The Y7Au2Te2 composition also
exists in the orthorhombic structure rather than that of the new
Er7Au2Te2.9
Powder diffraction
X-Ray powder patterns were secured at ambient temperature
with the aid of a Huber 670 Guinier (image-plate) camera and
˚
Cu-Ka radiation (l = 1.540598 A). Phase distributions on a
volume basis were estimated from the powder patterns of products
relative to those calculated according to single crystal refinements.
Lattice constants were obtained from least-squares refinements of
measured and indexed powder reflections from the stoichiometric
as-cast samples using the WinXPOW program.28
Semiquantitative microprobe analyses (EDS) of the Er7Au2Te2
single crystal used for X-ray diffraction and of several others were
performed with a JEOL 5910LV scanning electron microscope
equipped with a Noran-Vantage energy-dispersive spectrometer.
The average composition from various spots on four different
crystals, normalized with respect to Au, was Er6.94(2)Au2Te1.94(1), in
good agreement with the single crystal X-ray refinement result.
Structure determination
Dark, irregular, and metallic-looking crystals were separated from
the crushed product of the reaction composition Er8Au2Te2 and
were mounted in glass capillaries inside the glove box. A total
of 5031 reflections (3 sets of 606 frames with exposure times of
10 s per frame) covering 2.48◦ ≤ 2q ≤ 56.44◦ were collected
at room temperature with the aid of a Bruker SMART APEX
CCD diffractometer equipped with graphite-monochromatized
Experimental
Syntheses
All reactants and products were handled in nitrogen- or argon-
filled glove boxes with less than 0.4 ppm H2O levels. The general
techniques were as before.5,7,9–11 The starting materials were Ho,
Er, Tm, Yb, Lu (Ames Laboratory, 99.95% total), Au (Ames Lab-
oratory, 99.95%), and Te pieces (Aldrich, 99.999% metal basis).
Crystals of Er7Au2Te2 were first obtained during an exploratory
investigation of rare-earth metal-rich phase space in the Er–Au–Te
system. Mixtures of Er, Au and the preformed AuTe2 (>95%, from
˚
Mo-Ka radiation (l = 0.71069 A). Unit cell parameters were
determined from 990 reflections indexed on a C-centered mon-
oclinic basis. Intensity data were integrated and corrected for
Lorentz, polarization and (empirical) absorption effects with the
aid of SAINT29 and the SADABS subprograms,30 respectively. The
empirical absorption corrections proved to be adequate in spite of
the relatively high absorption coefficient, 79.9 mm-1. During the
final refinement, 1560 unique reflections [R(int) = 6.31%] were
used.
◦
reaction of the elements in a SiO2 container at 800 C for 12 h)
were arc-melted in various proportions and then annealed. One
such reaction designed to test the existence of an orthorhombic
Er7Ni2Te2 type analogue9 revealed the existence of the new phase
along with ErTe (NaCl type) (significant Er evaporation also
occurred during the arc-melting process). A better approach,
which presumably also expedited homogenization during arc-
melting, employed preformed ErAu and ErTe, as synthesized in
~95% yields from reactions of the elements as above. These and
Er metal in 2 : 2 : 4 molar proportions were cold pressed into a
pellet and arc-melted at ≤ 30 amp for about 30 s within an Ar-
filled glove box. Homogenization was improved by flipping the
pellet and repeating the process twice. This product contained
~80% of the new orthorhombic phase, as later judged by the single
crystal result, plus AuTe2. Single crystals of this Er7Au2Te2 were
obtained by annealing the product in a sealed Ta tube at 1200 ◦C
for one week in a graphite-heated vacuum furnace (Labmaster
Thermal Technology Inc. 1000-2560-FP20) at < ~10-6 torr. This
shiny black phase turns dull within 24 h on standing in moist air
at room temperature.
The structure solution was obtained by direct methods and
refined by full-matrix least-squares method on Fo2 with the aid of
SHELXTL v 6.1.31 The centrosymmetric space group C2/m was
selected for refinement according to systematic absence conditions
and intensity statistics (|E2 - 1| = 0.897). No systematic absence
violations or inconsistent equivalents were observed. A symmetry
check by PLATON indicated that no crystallographic symmetry
was missing. One out of 13 peaks calculated by direct methods
was immediately rejected on the basis of distance, and the other
12 peaks were initially assigned as Er. Convergence was achieved
at a residual of ~9%. Isotropic displacement parameters for two
positions were too small and two others were too large, clearly
indicating the probable positions of the Au and Te. A few more
refinement cycles on that basis led to a relatively featureless Fourier
map and to reasonable displacement parameters and interatomic
distances. Further anisotropic refinement led to well behaved
ellipsoids for all peak positions and R1 = 4.59%, wR2 = 9.38%
Similar reactions of Ho gave only HoTe and Ho2Au, whereas
extreme metal vaporization made analogous reactions with Tm
and Yb unproductive. However, the isostructural Lu7Au2Te2 was
obtained in >80% yield from that composition, a = 17.536(4),
(I > 2s(I)) at convergence. The largest peaks in the final Fourier
-3
˚
˚
difference map (3.98 and -3.50 e A ) were 1.1 and 1.3 A from
erbium atoms.
◦
˚
b = 3.9719(4), c = 16.695(2) A, b = 104.33(1) . More metal-
rich compositions yielded Lu2Au (Co2Si-type)27 as the major
product.
Some crystallographic and refinement data for the struc-
ture are listed in Table 1. The corresponding atomic co-
ordinates, standardized with STRUCTURE TIDY,32 and the
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6074–6079 | 6075
©

empty spheres (downfolded orbitals40,41 in parentheses); the Er
4f orbitals were treated as filled core orbitals. Reciprocal space
integrations to determine self-consistent charge densities, and
densities-of-states (DOS) curves were performed by the tetrahe-
dron method42 on grids of 282 k-points in the irreducible wedges
of the corresponding Brillouin zones. For bonding analysis, the
energy contributions of all electronic states for selected atom pairs
were calculated as a function of energy according to the crystal
orbital Hamilton population (COHP) method.43 The (average)
energy-weighted sums of these up to EF give –ICOHP values for
the individual pair-wise overlap populations. The Fermi levels are
all set to zero, and the –COHP data are plotted with respect to
energy as negative values correspond to bonding interactions.
Table 1 Selected crystal and refinement parameters for Er7Au2Te2
Empirical formula
Crystal system
Space group, Z
Er7Au2Te2
Monoclinic
C2/m, 4
17.8310(9)
3.9819(5)
16.9089(9)
104.361(4)
1163.1(5)
10.394
55.337
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
Volume/A
cal/g cm-3
r
m/mm-1
Index ranges
-23 ≤ h ≤ 23
-5 ≤ k ≤ 5
-22 ≤ l ≤ 22
5031
Reflections collected
Independent reflections
Data/parameters
1560 [R(int) = 0.0631]
1560/70
Goodness-of-fit on F2
Final R indices [I > 2s(I)]
1.061
Results and discussion
R1 = 0.0459
wR2 = 0.0938
R1 = 0.0636
wR2 = 0.1016
3.976, -3.505
Syntheses
R indices (all data)
A new monoclinic Er7Au2Te2 was discovered during exploration
of metal-rich phase space in the Er–Au–Te system by methods
very similar to those used earlier for other R–Tn–Te systems.9–11
Arc-melting a pellet pressed from suitable proportions of ErTe,
ErAu, and Er gave distinctively higher yields than from AuTe2
and Er because of the significant vapor pressure of liquid Er.44
Attempted extensions to Ho, Tm, Yb, Lu proved fruitful only as
far as the isostructural Lu7Au2Te2 (Experimental), the Tm and
Yb systems probably being irregular because of the high vapor
pressures of the metals as well as the same fundamentals that lead
to their formation of dihalides. The formation of Au compounds
only with Er or Lu for these tellurides is special inasmuch as
orthorhombic R7Tn2Te2 types are known instead for 14 examples
of the heavier Dy, Er, Tm, Lu with late transition metals (groups
8 to 10, Cu, Ag) as well as Y7Au2Te2.9,11 These are compared later.
-3
˚
Largest diff. peak and hole/e A
isotropic-equivalent displacement parameters are listed in Table 2.
More refinement data and anisotropic displacement parameters
are provided in the ESI.†
Band calculations
The electronic band structure and densities-of-states for Er7Au2Te2
were calculated self-consistently by the tight-binding, linear
muffin-tin orbital (TB-LMTO) method33–35 within the atomic
spheres approximation (ASA) with the Stuttgart code.36 Ex-
change and correlation were treated in a local spin density
approximation,37 and scalar relativistic effects were taken into
account.38 In ASA, space is filled with overlapping Wigner–
Seitz (WS) spheres at each atomic site. An automated procedure
is used to optimize the WS sphere radii, requiring that the
overlapping potential be the best possible approximation to the
full potential within the limit of 16% overlap between any atom-
centered spheres.39 The WS radii so determined were: erbium
Structure description
Fig. 1 shows a projection of the new monoclinic Er7Au2Te2 struc-
˚
ture along the short b axis (3.9819(5) A). A simplistic view of the
structure is gained by outlining all Er–Er separations between the
˚
˚
˚
˚
˚
1.72–1.82 A; gold 1.59–1.60 A; and tellurium 1.73–1.77 A. For
shortest, 3.45 A, and 3.98 A, the b axis repeat length. This rather
conventional representation helps illuminate the connectivities
within the major building blocks in the structure, but it doesn’t
recognize stronger interactions elsewhere, as discussed later. The
˚
space filling, 18 empty spheres (ES) with WS radii of 0.64–1.20 A
were also introduced. The basis set included Er 6s/(6p)/5d, Au
6s/6p/5d/(5f), Te 5s/5p/(5d)/(4f), and a 1s function for the
2
3
˚
Table 2 Atomic coordinates and isotropic-equivalent displacement parameters (A ¥ 10 ) for Er7Au2Te2
Atom
Wyckoff site, sym.
x
y
z
U(eq)a
Er1
Er2
Er3
Er4
Er5
Er6
Er7
Er8
Au1
Au2
Te1
Te2
4i
4i
4i
4i
4i
4i
2d
2a
4i
4i
4i
4i
m
0.00160(8)
0.17552(8)
0.19106(8)
0.33787(8)
0.57520(8)
0.78097(8)
0
0
0
0
0
0
0
1/2
0
0
0.33363(8)
0.53172(8)
0.31896(8)
0.01995(8)
0.17146(9)
0.19451(8)
1/2
12(1)
13(1)
13(1)
13(1)
14(1)
11(1)
14(1)
13(1)
13(1)
14(1)
12(1)
11(1)
m
m
m
m
m
2/m
2/m
m
0
0
0.17180(7)
0.38046(7)
0.0648(1)
0.3668(1)
0.11081(7)
0.60085(7)
0.8496(1)
0.3376(1)
m
0
0
0
m
m
a U(eq) is defined as one-third of the trace of the orthogonalized Uij tensor.
6076 | Dalton Trans., 2010, 39, 6074–6079
This journal is
The Royal Society of Chemistry 2010
©

atomic and orbital breakdowns (partial DOS), and the pair-wise
–COHP populations by bond type according to LMTO–ASA
calculations for Er7Au2Te2 are presented in Fig. 2 as a function
of energy. (A low lying band at -13.0 to -11.0 eV (not shown) is
largely core-like and nearly all Te s in origin.) Not surprisingly,
a substantial DOS at the Fermi level (dashed line) is indicative
of a metallic property. Major contributions here arise from Er
d ꢀ p > s and lesser Au d states, as seen from the atom-resolved
PDOS in Fig. 2(b) and 2(c). Overall, the relatively well-separated
band components in DOS and PDOS align well with the COHP
functions, allowing fairly ready assignment of all other features in
Fig. 2. (Note that the ordinate scales change downward in this.)
Fig. 1 ~[010] section of the monoclinic Er7Au2Te2 structure (C2/m): Er,
red; Te, olive, brown; Au, yellow, orange. All atoms lie on mirror planes
at y = 0, 1/2. Puckered sheets defined by zigzag chains of condensed
Au-centered TCTPs of Er are highlighted by black bonds and overlaid
golden lines. Note the bifunctional face-capping Er7 and Er8 atoms on
inversion centers.
Er7Au2Te2 is built primarily from the distorted, Au-centered,
tricapped trigonal prisms (TCTP) of Er, the same building units
as in the orthorhombic Er7Ni2Te2-type structure. These TCTPs
share basal trigonal faces to generate metal-columns along the
view direction (b), whereas contiguous columns arrayed roughly
along a are alternately displaced along the projection axis by b/2
and condensed by sharing functions. This rather common mode
produces sheets of condensed TCTP layers via bi-functional Er2,
Er3, Er4 and Er5 atoms that are both members of trigonal prisms
in one column and capping atoms on the rectangular side faces
of trigonal prisms in adjacent columns. (The side view is given
in ESI.†) However, what are slightly puckered sheets of TCTP in
the higher symmetry orthorhombic Dy7Ir2Te2 appear here as 4 ¥
2 zigzag chains of TCTP, as marked by an overlaid golden line
in Fig. 1. The effects of this new mode appear in the distinctive
TCTP environments about Au1 (yellow) and Au2 (orange), with
the three shortest Er–Er distances (7-1-3-5) and a greater range of
˚
d(Er–Er)) in the TCTP about Au2 (3.45–3.96 A) than around Au1
˚
(3.59–3.92 A). The zigzag puckered layers are further joined into
a three dimensional structure along a via bi-face-capping Er8 and
Er7 atoms that lie on inversion centers at x = 1/2, a functionality
that is also present in the Er7Ni2Te2 family. Finally, these puckered
metal layers are also held together by distant pairs of Te1 and Te2
spaced likewise along b, with a distorted monocapped trigonal
prism of Er around Te1 and a more regular bicapped trigonal
prismatic environment about Te2. Although formal Te2- anions
within such condensed poly-metal networks (and the halides in
their family of analogues) have often been concluded to just be
large spacers, ab initio calculations make clear this is not correct,
below. Note that representations or indications of Er–Au and Er–
Te bonding are as usual not marked in Fig. 1, although these turn
out to be of major importance.
Fig. 2 Densities-of-states (DOS) (a), partial DOS by atom and orbital
types (b–d), and crystal orbital Hamilton populations (–COHP, eV per
bond·mol) (e) for monoclinic Er7Au2Te2. The dashed line marks the Fermi
energy (EF). Note the ordinate scale expansion downward.
The lowest –COHP feature around -6 eV originates with
roughly equal numbers of Er 5d, 6s, and 6p states mixed with
the dominant Au 5d and 6s, Fig. 2(b) and 2(c). Its smaller width
parallels the higher binding energy. The COHP band around
-4 eV arises principally from Er (d,s,p) and Te 6p states but
with significant Au d,s contributions in the lower regions as well.
The width of the Te p band clearly denies the presence of simple
Te2- anionic states. And the bands in DOS around -2 and -1 eV,
Theoretical considerations
These results lead to bonding concepts that differ substantially
from those implied by usual pictorial description of this structure
(Fig. 1). The calculated total densities-of-states (DOS), their
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6074–6079 | 6077
©

Table 3 Individual (average) and cumulative –ICOHP components (eV/bond mol) in Er7Au2Te2, Z = 4
Atom pair
–ICOHP (avg. per bond)
No. of bonds per cell
–ICOHP (cumulative, per cell)
% contribution
Er–Er
Er–Au
Er–Te
0.174
0.846
0.699
104
72
66
17.10
60.91
46.13
14.0
48.9
37.1
Fig. 2(a), both originate with Au, principally 6p, and Er 5d but
with appreciable contributions of Te1 p as well in the lower of the
two bands. (Site-differentiated PDOS for gold show no significant
differences between Au1 and Au2, and the same applies to Er7
and Er8 relative to the rest.) Thus the bands around -4 and -2 eV
as well as around EF all feature substantial contributions from
all three elements and more delocalized bonding. The same is
reflected by the 9 to 13 nearest neighbors for each Er as well as the
nine around Au.
Au comprises nearly 50%, certainly enhanced by the substantial
relativistic effects for Au.15 The common and convenient way of
representing such structures in terms of the network defined by
a major element, as for 64% Er in Fig. 1, is certainly misleading
regarding the bonding. Similar effects of polar bonding are found
for many electron-poor and metallic Zintl-phase relatives.12–15,24,25
R7Tn2Te2 structural comparisons
Results of our recent and fairly systematic synthetic explorations
among metal-rich ternary tellurides of the late 4d and 5d transition
metals and the heavier R elements have appreciably expanded
(to 14) the collection of alternative orthorhombic Er7Ni2Te2-type
compounds.9,11 It is noteworthy that among these, Y7Au2Te2 and
four close neighbors, R7Tn2Te2 for R = Dy, Er and Tn = Ir, Pt,
appear to adopt only the orthorhombic structure, not the new
monoclinic version of Er7Au2Te2 and Lu7Au2Te2. Reasons for the
structural change between the more common Imm2 Er7Ni2Te2
structure type for the neighboring Dy7Ir2Te2 (the only one with
refined structural data) and the new C2/m Er7Au2Te2, are apt to
be manifold and complex. However, comparisons of some of the
multiple features between these two are quite helpful and definitive,
namely, that the smaller, higher symmetry orthorhombic structure
exhibits distinctly more distorted interconnections between the
TCTP sheets but no evidence that fundamental atom size differ-
ences are important (their molar volumes are not significantly
different).
Significant charge transfer among all three elements can be
recognized too. Nearly all the Te 5p states as well as the Er–Te
COHP function fall below EF (Fig. 2(d,e)). Likewise, a negative
oxidation state for gold can be readily inferred inasmuch as many
Au 6s and 6p and almost all Au 5d states lie below EF, in contrast
to its classical 6s1 valence configuration. In other words, the
orbital distributions of Er, Fig. 2(b), clearly reflect a substantial
oxidation of the element. Note that many of the Er 6s and 6p
states now lie below -2 eV, whereas a large and increasing number
of its 5d orbitals have been raised in energy, many to above EF
(COHP, Fig. 2(e)). Thus the more penetrating 6s and 6p orbitals
fall in energy faster as the metal is oxidized. The above Er–Au
dispositions are in fact general and characteristic of bonding
between pairs of early and late d elements in binary phases that
likewise behave as “polar intermetallics”, e.g., Ti–Fe and Ti–Ru.45
This same characteristic for analogous halides of rare-earth-metal
clusters stabilized by late transition metals2,5 can be qualitatively
recognized in earlier extended-Hu¨ckel (EHTB) outputs for these,
noting that this empirical method does a poor job of modeling
virtual R states above EF and that the valence d bands are often
too narrow.19
Finally, the energy-weighted sums of the COHP bond data, –
ICOHP (total Hamilton (overlap) populations) provide a much
better analysis of relative bond populations or indices and hence
approximate bond strengths, as summarized in Table 3. The
average –ICOHP value per contact (bond·mol) of each type are
listed in column 2. The structure itself exhibits fixed intrinsic
relationships between the number of each bond type (interacting
atom pairs) per cell, and the products of these and individual –
ICOHP values yield some good measures of the different pair-wise
contributions to the overall population (stability), column 4. The
percentage distributions by bond type in the last column furnish
plausible assessments of the relative weights of the separate bond
types in the phase.
A relatively small decrease in metallic radius is expected from
˚
Dy to Er, only of the order of 0.02 A. Although some radius
˚
scales among the 5d metals show an increase of ~0.08 A from Ir
to Au,46 our experiences with the more anionic states of these in
intermetallic phases containing early active metals indicate that a
˚
size decrease of the same magnitude (~0.08 A) occurs between Ir
and Au as the relativistic effects increase.47 (It is also clear that the
˚
Pt radius is ~0.08 A greater than that of Au in isotypic BaTnIn3
phases, Tn = Pt, Au, with BaAl4-type structures.48)
The new structure type with 12 independent atom sites and two
types each of Au and Te (Z = 4) is less distorted on average than
the orthorhombic Dy7Ir2Te2 (Z = 2) with seven independent atoms
and one type of TCTP. Differences in particularly the ranges of
d(R–R) and d(R–Z) in the TCTP arrays are somewhat greater for
the single TCTP type in Dy7Ir2Te2, but the distances alone are not
very different in the two. But it is particularly the intersheet R–R
bridge bonds between the novel bi-face-capping (b.f.c.) Er7 and
Er8 and the Er trigonal prisms in adjacent sheets (Fig. 1) (and
the angles therewith) that are distinctly more symmetric in the
lower symmetry monoclinic structure than in Dy7Ir2Te2. In other
words, relaxations around the two independent bridging R (as well
as, implicitly, among larger numbers of the Tn and Te) atoms
in Er7Au2Te2 allow the bridged TCTP array to pack/bond more
efficiently. The bridging Er7 and Er8 lie on inversion centers within
The results are quite striking: distinctively larger –ICOHP
values per bond are associated with the polar bonds Er–Au and
Er–Te, whereas the homoatomic bonding among the majority, but
electron-depleted, Er atoms (Fig. 2(b)) is quite small. (There are
no significant Au–Au or Te–Te contacts in the structure.) Even the
preponderance of Er–Er interactions, 49% of the total of all types,
does not overcome their small individual contributions. Thus Er–
Er makes up only 14% of the total –ICOHP value, whereas Er–
6078 | Dalton Trans., 2010, 39, 6074–6079
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The Royal Society of Chemistry 2010
©

rectangular prisms defined by Er in the adjoining slabs so the trans-
bonding Er4–Er8–Er5 angles are 180◦, Fig. 1. The fairly regular
packing in this structure also give Er7 and Er8 two additional
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6413.
˚
Au2 or Au1 neighbors (at 3.17 and 3.04 A) in the adjoining slabs,
respectively; in a reverse sense, these are also the closest face-
8 M. Ko¨ckerling and J. D. Martin, Inorg. Chem., 2001, 40, 389.
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635, 848.
˚
capping Er atoms about both TCTP around Au, by 0.3 and 0.5 A,
respectively.
On the other hand, the arrangement in higher symmetry and
smaller orthorhombic Dy7Ir2Te2 is a good deal less regular, both
in the packing within the TCTP-based sheets and around the
interbridging, face-capping Dy4 (pictured in Fig. S2, ESI†). The
trans-bridging-angles Dy1–Dy4–Dy3 are now 158◦, not 180◦, and
the opposed rectangular faces of the distorted rectangular Dy
prism about Dy4 now have a dihedral angle of 17.1◦ between
them. Furthermore, the centering Ir atoms beyond those faces
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˚
are 0.6 A or more further away than in the title phase, and the
other face-capping Ir–Dy distances about the Dy trigonal prisms
are also larger. Only the innermost features of the common R6Tn
prisms are similar in the two compounds, the Dy–Ir values being
20 Y. Park, J. D. Martin and J. D. Corbett, J. Solid State Chem., 1997, 129,
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21 S.-T. Hong, J. D. Martin and J. D. Corbett, Inorg. Chem., 1998, 37,
˚
~0.02 A larger.
3385.
We lack enough detail to ferret out the countervailing factors
that preclude this transformation from occurring in all cases.
The stronger bonding of the smaller gold may be particularly
important, but higher symmetry and smaller structures are often
favored when sensitive band features are not present around EF.
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7954.
23 B. Li and J. D. Corbett, Inorg. Chem., 2007, 46, 6022.
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26 F. Meng, C. Magliocchi and T. Hughbanks, Inorg. Chem., 2001, 40,
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Conclusions
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Strong polar Er–Au bonding arising in part from substantial
relativistic effects for Au together with major Er–Te interactions
appear in a new monoclinic network structure for (Er,Lu)7Au2Te2.
On the other hand, the large number of Er–Er contacts still
makes the smallest contributions to the overall Hamilton bond
populations. The new structure exhibits apparently more nearly
ideal packing of condensed Er(Au) tricapped trigonal prisms
than the more common orthorhombic Er7Ni2Te2-type polytype.
High coordination numbers and extensive delocalized bonding
are characteristic of the more polar electron-poor intermetallic
phases.
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Acknowledgements
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The guidance and insights of G. J. Miller are gratefully acknowl-
edged. This research has been supported by the National Science
Foundation, Solid State Chemistry, via Grants DMR-0444657 and
-0853732 and was performed in the Ames Laboratory of the U.S.
Department of Energy.
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Dalton Trans., 2010, 39, 6074–6079 | 6079
©