Gold derivatives of eight rare-earth-metal-rich tellurides: Monoclinic R7Au2Te2 and orthorhombic R 6AuTe2 types

Article abstract of DOI:10.1021/ic202342v

Two series of rare-earth-metal (R) compounds, R₇Au ₂Te₂ (R = Tb, Dy, Ho) and R₆AuTe₂ (R = Sc, Y, Dy, Ho, Lu), have been synthesized by high-temperature techniques and characterized by X-ray diffraction analyses as monoclinic Er₇Au ₂Te₂-type and orthorhombic Sc₆PdTe ₂-type structures, respectively. Single-crystal diffraction results are reported for Ho₇Au₂Te₂, Lu ₆AuTe₂, Sc₆Au_0.856(2)Te₂, and Sc₆Au_0.892(3)Te₂. The structure of Ho ₇Au₂Te₂ consists of columns of Au-centered tricapped trigonal prisms (TCTPs) of Ho condensed into 2D zigzag sheets that are interbridged by Te and additional Ho to form the 3D network. The structure of Lu₆AuTe₂ is built of pairs of Au-centered Lu TCTP chains condensed with double Lu octahedra in chains into 2D zigzag sheets that are separated by Te atoms. Tight binding-linear muffin-tin orbital-atomic sphere approximation electronic structure calculations on Lu₆AuTe ₂ indicate a metallic property. The principal polar Lu-Au and Lu-Te interactions constitute 75% of the total Hamilton populations, in contrast to the small values for Lu-Lu bonding even though these comprise the majority of the atoms. A comparison of the theoretical results for Lu₆AuTe ₂ with those for isotypic Lu₆AgTe₂ and Lu ₆CuTe₂ provides clear evidence of the greater relativistic effects in the bonding of Au. The parallels and noteworthy contrasts between Ho₇Au₂Te₂ (35 valence electrons) and the isotypic but much electron-richer Nb₇P₄ (55 valence electrons) are analyzed and discussed.

Full text of DOI:10.1021/ic202342v

Article
pubs.acs.org/IC
Gold Derivatives of Eight Rare-Earth-Metal-Rich Tellurides:
Monoclinic R Au Te and Orthorhombic R AuTe Types
7
2
2
6
2
Ping Chai and John D. Corbett*
Department of Chemistry, Iowa State University, Ames, Iowa 50011, United States
S Supporting Information
*
ABSTRACT: Two series of rare-earth-metal (R) compounds,
R Au Te (R = Tb, Dy, Ho) and R AuTe (R = Sc, Y, Dy, Ho,
7
2
2
6
2
Lu), have been synthesized by high-temperature techniques
and characterized by X-ray diffraction analyses as monoclinic
Er Au Te -type and orthorhombic Sc PdTe -type structures,
7
2
2
6
2
respectively. Single-crystal diffraction results are reported for
Ho Au Te , Lu AuTe , Sc Au Te , and Sc Au0.892(3)Te2.
7
2
2
6
2
6
0.856(2)
2
6
The structure of Ho Au Te consists of columns of Au-
7
2
2
centered tricapped trigonal prisms (TCTPs) of Ho condensed
into 2D zigzag sheets that are interbridged by Te and additional Ho to form the 3D network. The structure of Lu AuTe is built
6
2
of pairs of Au-centered Lu TCTP chains condensed with double Lu octahedra in chains into 2D zigzag sheets that are separated
by Te atoms. Tight binding−linear muffin-tin orbital−atomic sphere approximation electronic structure calculations on
Lu AuTe indicate a metallic property. The principal polar Lu−Au and Lu−Te interactions constitute 75% of the total Hamilton
6
2
populations, in contrast to the small values for Lu−Lu bonding even though these comprise the majority of the atoms. A
comparison of the theoretical results for Lu AuTe with those for isotypic Lu AgTe and Lu CuTe provides clear evidence of
6
2
6
2
6
2
the greater relativistic effects in the bonding of Au. The parallels and noteworthy contrasts between Ho Au Te (35 valence
7
2
2
electrons) and the isotypic but much electron-richer Nb P (55 valence electrons) are analyzed and discussed.
7
4
INTRODUCTION
The incorporation of gold (Au) into different systems,
including Zintl phases and icosahedral quasicrystal approx-
imants, has frequently led to new structures with novel
■
Research into metal-rich compounds has been a consistent
highlight in solid-state chemistry because of their diverse
structures and, hence, the importance of understanding the
relationships among composition, structure, and bonding.
Among these, metal-rich pnictides, chalcogenides, and halides
of the early transition metals have attracted much attention
17−19
bonding,
in response to Au’s unique electronic proper-
20
1
,2
ties. The participation of Au can simultaneously afford low
10
vec as well as the substantial involvement of 5d states in
bonding because of large relativistic effects. Au thereby exhibits
a high (Mulliken) electronegativity and thus enhanced polar
bond characteristics relative to other metals. These special
properties have motivated us to incorporate Au into additional
rare-earth-metal-rich telluride systems, inasmuch as only a few
examples have been synthesized to date: Er Au Te ,
3
−6
during the past two decades.
Of special interest are the
electron-poorer rare-earth-metal-rich tellurides because they
display a wide variety of structural motifs, isolated metal
clusters, 1D metal chains, 2D sheets, and 3D networks.
7
2
2
Although many binary rare-earth-metal-rich tellurides have
been discovered, the more significant advances occurred when
the late transition metals were incorporated into the R-based
regions as interstitials. The cluster condensation that follows R-
richness and tellurium (Te)-poorness leads to numerous
Lu Au Te , Y Au Te and R Au Te (R = Lu, Ho, Dy,
7
2
2
7
2
2
5
2
2
9
,16,21
Y).
The present paper reports three more Au derivatives
of the first type, R Au Te (R = Tb, Dy, Ho), five new Au
7
2
2
members for R AuTe (R = Sc, Y, Dy, Ho, Lu), and details of
6
2
their structures and bonding.
7
complex structures, such as orthorhombic Er Ni Te and
7
2
2
8
9
EXPERIMENTAL SECTION
Lu Z Te (Z = Ni, Pd, Pt), monoclinic Er Au Te , hexagonal
■
7
2
2
7
2
2
1
0,11
Synthesis. All materials were handled in argon-filled gloveboxes
with moisture levels below 1 ppm (by volume). The elements were
used as received: Sc, Y, Tb, Dy, Ho, and Lu (99.95%, Ames
Laboratory), Au (99.995%, Ames Laboratory), and Te pieces (99.99%,
Aldrich). To reduce the Te activity in subsequent reactions, all of the
syntheses began with the preparation of RTe, for which the elements
in a 1:1 molar ratio were sealed inside a silica tube under a high
R6ZTe2 (R = Sc, Dy; Z = Mn, Fe, Co, Ni),
and
1
2,13
14
orthorhombic Sc ZTe (Z = Pd, Ag, Cu, Cd),
Sc Ni Te ,
5 2 2
6
2
15
16
Y Z Te (Z = Fe, Co, Ni), and Lu Au Te , etc. The strong
5
2
2
5
2
2
heterometal interactions between the rare-earth and late
transition metals play significant roles in the diversity of their
structures, which are, in turn, considerably dependent on the
atom sizes, the valence electron concentrations (vec), the
proportion of host R to centered Z, and, of course, their relative
thermodynamic stabilities.
Received: October 29, 2011
Published: February 24, 2012
©
2012 American Chemical Society
3548
dx.doi.org/10.1021/ic202342v | Inorg. Chem. 2012, 51, 3548−3556

Inorganic Chemistry
Article
Table 1. Lattice Parameters Refined from Guinier Powder Data for R Au Te (R = Tb, Dy, Ho) and R AuTe (R = Sc, Y, Dy,
7
2
2
6
2
Ho, Lu) Phases
compound
Tb Au Te
a/Å
b/Å
c/Å
β
volume/Å3
no. of lines used
18.215(3)
18.142(4)
17.978(3)
20.191(4)
21.634(5)
21.610(5)
21.420(5)
21.0731(5)
4.0279(4)
4.0134(7)
4.0073(6)
3.9199(7)
4.0931(6)
4.0667(8)
4.0620(8)
4.003(1)
17.164(4)
17.058(2)
16.997(3)
10.745(1)
11.507(1)
11.438(1)
11.391(1)
11.267(3)
104.37(1)
104.46(1)
104.427(2)
1219.8(2)
1202.7(2)
1185.9(3)
850.5(2)
1019.0(2)
1005.2(2)
991.1(2)
950.5(3)
9
9
7
2
2
Dy Au Te
2
7
2
Ho Au Te
2
10
11
10
9
7
2
Sc Au0.89Te2
6
Y6AuTe2
Dy AuTe2
6
Ho AuTe2
6
9
Lu AuTe2
6
9
Table 2. Selected Crystal and Refinement Data for Ho Au Te , Lu AuTe , Sc Au Te , and Sc Au0.89Te2
7
2
2
6
2
6
0.85
2
6
empirical formula
cryst syst
space group, Z
a (Å)
Ho Au Te
Lu AuTe2
Sc Au0.85Te2
Sc Au0.89Te2
7
2
2
6
6
6
monoclinic
C2/m (No. 12), 4
17.978(3)
4.0073(6)
16.997(3)
104.427(2)
1185.9(3)
10.102
orthorhombic
orthorhombic
orthorhombic
Pnma (No. 62), 4
20.191(4)
Pnma (No. 62), 4
21.0731(5)
4.003(1)
Pnma (No. 62), 4
20.110(2)
b (Å)
3.8869(5)
3.9199(7)
c (Å)
11.267(3)
10.705(1)
10.745(1)
β (deg)
3
volume (Å )
950.5(3)
10.496
836.8(2)
5.500
850.5(2)
5.473
3
dcalcd (g/cm )
μ (mm−1)
75.495
82.994
26.193
26.459
index ranges
−23 ≤ h ≤ 23, −5 ≤ k ≤ 5, −22 ≤ −26 ≤ h ≤ 26, −5 ≤ k ≤ 5, −14 −26 ≤ h ≤ 26, −5 ≤ k ≤ 5, −14 −26 ≤ h ≤ 26, −5 ≤ k ≤ 5, −13
l ≤ 22 ≤ l ≤ 14 ≤ l ≤ 14 ≤ l ≤ 13
reflns collcd
5204 7913 7053 7055
indep obsd reflns
1299 (Rint = 0.0515)
1299/69
1026 (Rint = 0.0706)
1026/55
996 (Rint = 0.0585)
996/56
915 (Rint = 0.0721)
915/56
data/param
GOF on F2
1.026
1.048
1.125
1.027
R indexes [I >
2σ(I)]
R1 = 0.0364, wR2 = 0.0716
R1 = 0.0375, wR2 = 0.0794
R1 = 0.0328, wR2 = 0.0640
R1 = 0.0345, wR2 = 0.0649
R indexes (all data) R1 = 0.0506, wR2 = 0.0771
R1 = 0.0521, wR2 = 0.0844
R1 = 0.0427, wR2 = 0.0681
R1 = 0.0505, wR2 = 0.0709
largest diff peak,
3.17 [0.94 Å from Ho6], −2.87
3.44 [1.06 Å from Lu2], −3.87
1.66 [0.98 Å from Au], −1.30
1.94 [0.98 Å from Au], −2.15
3
hole (e/Å )
[0.98 Å from Ho1]
[1.16 Å from Te1]
[1.43 Å from Sc5]
[1.85 Å from Au]
vacuum, slowly heated to 450 °C, held for 12 h, then reacted at 900 °C
for 72 h, and finally cooled radiatively. All of the RTe products were
single phases when examined by Guinier powder X-ray diffraction
led to no additional line shifts but more impurity phases. The Guinier
powder XRD patterns from Sc Au Te and Sc Au Te and
that calculated from the structure of the latter are provided in Figure
S1 in the Supporting Information; the shift with the Au content is
clear.
6
0.856(2)
2
6
0.892(3)
2
(
XRD).
The appropriate amounts of RTe, R, and Au on a ∼300 mg scale for
R:Au:Te = 7:2:2 stoichiometries were pressed into 10-mm-diameter
pellets with the aid of a hydraulic press (Specac) in the glovebox.
These were arc-melted for 15 s with ∼20 A current on a copper hearth
after zirconium shot had first been melted to further purify the argon
atmosphere. The samples were subsequently turned over and arc-
melted again to improve the homogeneity. The weight losses during
arc melting were under 5%. The buttons were sealed into tantalum
tubes and then into evacuated fused silica jackets. These were heated
at 1000 °C for 400 h and then allowed to cool radiatively inside the
furnace. The isostructural compounds R Au Te (R = Tb, Dy, Ho)
Powder XRD. All of the products were analyzed according to
powder XRD patterns recorded in the 2θ range of 4−100° for 30 min
with the aid of a Huber Guinier 670 image diffractometer with Cu Kα
1
radiation. Samples were ground to a fine powder and evenly
distributed between two Mylar films with the aid of a little petrolatum,
which were held between two aluminum rings of the sample holder.
Table 1 lists the lattice parameters for all of the compounds as
obtained by least-squares refinements of the measured and indexed
lines over 2θ = 10−50°.
Single-Crystal Diffraction Studies. Good-quality crystals of
Ho Au Te , Lu AuTe , and Sc Au Te were selected for single-crystal
7
2
2
with yields above ∼90% plus some RTe impurity phases were obtained
according to powder XRD pattern data. Reactions with scandium (Sc)
were unproductive, giving only some mixed binary phases.
7
2
2
6
2
6
x
2
XRD data collection on a Bruker AXS SMART APEX CCD-based X-
ray diffractometer equipped with monochromatized Mo Kα radiation.
Sets of 606 frames with exposure times of 10 s/frame were collected at
room temperature for each. The intensities were integrated with
The R AuTe (R = Y, Dy, Ho, Lu) phases were obtained according
6
2
to the same process, and initial elemental stoichiometries R:Au:Te =
:1:2. The final annealing step was at 1050 °C for 10 days. All
2
2
6
SAINTPLUS, and absorption corrections were applied with the
23
reactions resulted in the target stoichiometric phases in ∼90% yields
with small amounts of RTe as impurities according to powder XRD
patterns. On the other hand, the product from the parallel reaction
package program SADABS. The XPREP subprogram in the
24
SHELXTL software package was used for the space group
determination and averaging, and the structures were solved by direct
2
with Sc:Au:Te = 6:1:2 gave a refined composition of Sc Au0.856(2)Te2
methods and refined by a full-matrix least-squares method on Fo with
6
by single-crystal refinement, that is, with ∼14% less Au than expected.
Reactions with additional Au were thereby performed. The powder
XRD patterns showed that the line positions shifted to lower angles
with increased Au, which suggested an increase of the Au content in
the phase. A Au content of ∼89(3)% was refined after the loaded
proportion was changed to Sc:Au:Te = 6:1.3:2 (below). Further, Au
the aid of SHELXTL-6.10. The cell and refinement data for four
structures are given in Table 1.
Ho Au Te . The powder XRD patterns revealed that Ho Au Te is
7
2
2
7
2
2
isostructural with the monoclinic Er Au Te . Therefore, the space
7
2
2
group C2/m was selected directly, and the atom sites were assigned
according to the data for Er Au Te . The final anisotropic refinement
7
2
2
3
549
dx.doi.org/10.1021/ic202342v | Inorg. Chem. 2012, 51, 3548−3556

Inorganic Chemistry
Article
2
4
Table 3. Atomic Coordinates and Isotropic Equivalent Displacement Parameters (Å × 10 ) for Ho Au Te and Lu AuTe
7
2
2
6
2
a
atom
Wyckoff
symmetry
x
y
z
Ueq
Ho Au Te
2
7
2
Au1
Au2
Ho1
Ho2
Ho3
Ho4
Ho5
Ho6
Ho7
Ho8
Te1
Te2
4i
4i
4i
4i
4i
4i
4i
4i
2d
2a
4i
4i
m
0.17125(5)
0.38078(5)
0.00084(6)
0.17599(6)
0.19070(6)
0.33751(6)
0.57403(6)
0.78129(6)
0
0
0.11128(6)
0.60065(6)
0.33326(6)
0.53257(7)
0.31877(7)
0.02033(7)
0.17184(7)
0.19476(6)
0.5
152(2)
160(2)
110(2)
130(2)
125(2)
127(2)
130(2)
115(2)
119(3)
119(3)
101(3)
106(3)
m
0
m
0
m
0
m
0
m
0
m
0
m
0
2/m
2/m
m
0.5
0
0
0
0.36639(8)
0.06506(8)
0
0.33737(9)
0.85018(9)
m
0
Lu AuTe2
6
Au
4c
4c
4c
4c
4c
4c
4c
4c
4c
m
m
m
m
m
m
m
m
m
0.42410(5)
0.10868(5)
0.15787(8)
0.26765(5)
0.47411(5)
0.35986(5)
0.00667(5)
0.23678(7)
0.12595(7)
1/4
1/4
1/4
1/4
1/4
1/4
1/4
1/4
1/4
0.63237(8)
0.31371(9)
0.02941(9)
0.73968(9)
0.37815(9)
0.06222(9)
0.6114(1)
0.4565(1)
0.7681(1)
104(2)
114(2)
76(2)
87(2)
88(2)
84(2)
142(3)
74(3)
79(3)
Lu1
Lu 2
Lu 3
Lu 4
Lu 5
Lu 6
Te1
Te2
a
Ueq is defined as one-third of the trace of the orthogonalized U tensor.
ij
a
2
Table 4. Interatomic Distances (Å) and ICOHP Values [eV/bond·mol] in Lu AuTe
6
bond
n
distance
ICOHP
bond
n
distance
ICOHP
Au−Lu2
Au−Lu4
Au−Lu1
Au−Lu4
Te1−Lu5
Te1−Lu2
Te1−Lu1
Te1−Lu3
Te1−Lu3
Te2−Lu3
Te2−Lu2
Te2−Lu6
Te2−Lu5
8
8
8
4
8
8
4
8
4
4
4
4
8
2.887(1)
2.936(1)
2.942(1)
3.052(1)
3.094(1)
3.100(1)
3.143(2)
3.160(1)
3.256(2)
3.003(2)
3.020(2)
3.072(2)
3.079(2)
1.17
1.04
1.20
0.95
0.83
0.76
0.72
0.76
0.47
0.93
0.96
0.81
0.76
Lu6−Lu6
Lu1−Lu6
Lu1−Lu2
Lu1−Lu3
Lu2−Lu3
Lu5−Lu6
Lu1−Lu5
Lu1−Lu4
Lu4−Lu4
Lu4−Lu6
Lu5−Lu6
Lu2−Lu4
Te2−Lu4
4
8
4
8
8
8
8
4
4
8
4
8
8
3.223(1)
3.260(1)
3.367(2)
3.390(1)
3.476(1)
3.496(1)
3.505(1)
3.566(2)
3.569(1)
3.634(2)
3.662(2)
3.827(1)
3.161(1)
0.80
0.68
0.34
0.30
0.22
0.33
0.34
0.34
0.17
0.18
0.56
0.08
0.64
a
n is the number of interactions of each type per cell.
converged at R1 = 3.64% and wR2 = 7.16% for the stoichiometric
composition Ho Au Te . The difference Fourier map showed
featureless residual peaks of +3.17 and −2.87 e/Å that were 0.94
and 0.98 Å from Ho6 and Ho1, respectively.
12.83%. The large wR2 suggested that something was abnormal, and
careful observations indicated unusually large anisotropic displacement
7
2
2
3
parameters (U ) for Au. Subsequent refinements that allowed the Au
ii
occupancy to vary led to a large drop of wR2. The final refinement
converged at R1 = 3.28% and wR2 = 6.40% for the composition
Lu AuTe . The powder XRD patterns indicated that Lu AuTe is
6
2
6
2
isostructural with the orthorhombic Sc PdTe , so space group Pnma
6
2
Sc Au
Te , in which the Au site was occupied by 85.6(2)% Au.
6
0.856(2) 2
was chosen. A total of 3 out of the 12 sites obtained from direct
methods were immediately deleted because of the unreasonable
interatomic distances or displacement parameters. The remaining sites
were assigned as one Au, two Te, and six Lu according to the
differences in the displacement parameters. Convergence was achieved
with an R1 value of 5.3%. Further anisotropic refinement converged at
R1 = 3.75% and wR2 = 7.94% with the largest Fourier difference map
3
The only difference Fourier peaks were about ±2 e/Å and near Au
atoms. The same refinement procedures were used for a product that
had been loaded with 30% excess Au, Sc:Au:Te = 6:1.3:2. The final
refinement converged at R1 = 3.45% and wR2 = 6.49% at the
composition Sc Au
Te , in which the occupancy of the Au site
2
6
0.892(3)
had been increased by 4.2%.
3
Some crystallographic and refinement data for the four compounds
are listed in Table 2. The corresponding atomic coordinates
residuals of +3.44 and −3.87 e/Å for 1.06 and 1.16 Å from Lu2 and
Te1, respectively.
2
5
standardized with STRUCTURE TIDY for the first two phases are
given in Table 3 together with the isotropic-equivalent displacement
parameters. Selected interatomic distances and weighted integration of
the crystal orbital Hamilton population (ICOHP) data for Lu AuTe
Sc Au Te and Sc Au Te . The powder XRD patterns following
6
0.85
2
6
0.89
2
reactions of normal 6:1:2 loadings indicated the same orthorhombic
Sc PdTe -type structure. Therefore, the same procedures were used,
6
2
and the anisotropic refinement converged at R1 = 4.82% and wR2 =
6
2
3
550
dx.doi.org/10.1021/ic202342v | Inorg. Chem. 2012, 51, 3548−3556

Inorganic Chemistry
Article
Figure 1. ∼[010] view of Ho Au Te . Ho, Au, and Te atoms are gray, orange, and green spheres, respectively. The red lines are only to guide the eye
7
2
2
along the chain segments.
are shown in Table 4. The remaining Sc results are available in Tables
S1 and S2 and in the CIF outputs in the Supporting Information.
Theoretical Calculations. Tight-binding electronic structure
calculations were performed for Ho Au Te and Lu AuTe according
Ho7 and Ho8 bridge between the 2D zigzag sheets along [10−
] and [101] to form the 3D network, accompanied by Te1
and Te2 atoms that fill the cavities between the sheets. Figure 2
shows the bonding environments of Te1 and Te2 atoms, which
1
7
2
2
6
2
to the linear-muffin-tin-orbital (LMTO) method in the atomic sphere
2
6
approximation (ASA). The radii of the Wigner−Seitz (WS) spheres
were assigned automatically so that the overlapping potentials would
2
7
be the best possible approximations to the full potentials. For
Ho Au Te , 19 additional empty spheres (ESs) were introduced per
7
2
2
cell (Z = 4) within the limit of 18% overlap between an ES and any
atom-centered sphere. The WS radii were as follows: Ho, 3.28−3.48
Å; Au, 3.02−3.03 Å; Te, 3.29−3.36 Å; ES, 1.21−2.27 Å. The basis set
included Ho 6s/(6p)/5d with Ho 4f11 treated as the core, Au 6s/6p/
28,29
5
d/(5f), and Te 5s/5p/(5d)/(4f) (downfolded
orbitals are in
parentheses). Exchange and correlation were treated in a local density
approximation, and scalar relativistic effects were included. Reciprocal
space integrations were performed with the aid of the tetrahedron
method.
For Lu AuTe (Z = 4), 12 additional ESs were introduced within
6
2
the same limitations. The WS radii were similar: Lu, 3.02−3.42 Å; Au,
.02 Å; Te, 3.22−3.31 Å; ES, 1.27−2.26 Å. These calculations
3
14
employed the basis set of Lu 6s/(6p)/5d (4f as the core), Au 6s/6p/
d/(5f), and Te 5s/5p/(5d)/(4f). Exchange and correlation were
5
again treated in a local density approximation, and scalar relativistic
effects were included. For bonding analysis, the energy contributions
of filled electronic states for all Lu−Lu, Lu−Au, and Lu−Te contacts
were calculated according to the crystal orbital Hamilton population
30
(
COHP) method. Weighted integration of COHP data for each
bond type over all filled states yielded ICOHP, the Hamilton overlap
populations. The tight binding (TB)−LMTO−ASA calculations on
Lu AgTe and Lu CuTe were also carried out for comparison because
6
2
6
2
Figure 2. Section of an ∼ [100] view of the Te1- and Te2-centered
the previous theoretical consideration for this 6−1−2-type phase was
1
3
polyhedra in Ho Au Te .
7
2
2
performed at an extended Huckel−TB level and only for Sc AgTe .
̈
6
2
31
The crystal data used were from the original literature, and the
calculation procedures were the same as those for Lu AuTe .
center bicapped and monocapped Ho TPs, respectively. The
former bicapped prisms parallel the Au-centered TCTP and
likewise generate confacial columns along b. The latter
monocapped versions lie perpendicular to the former and
create 1D chains along b by sharing only Ho4−Ho5 edges.
6
2
RESULTS AND DISCUSSION
Structural Description of Ho Au Te . Ho Au Te
2
■
7
2
2
7
2
9
crystallizes in a Er Au Te -type structure in space group C2/
7
2
2
9
Compared with the member Er Au Te , the present
m. The ∼[010] view of the structure is shown in Figure 1. Au
atoms are located in tricapped trigonal prisms (TCTPs) that
share trigonal faces to construct infinite 1D columns along b.
The TCTPs further interconnect with b/2 displacements that
lead to alternation of the TP and TC functions and to 2D
zigzag sheets along c. Au atoms are distinguished as Au1 and
Au2 by their different roles in the zigzag sheets. The Ho2, Ho3,
Ho4, and Ho5 members play bifunctional roles: the vertexes of
the TP are face-capping atoms on the rectangular face in its
neighboring TCTP and vice versa. Finally, the capping atoms
7
2
2
Ho Au Te exhibits only general expansions of the distances
7
2
2
and the lattice because of the larger size of Ho.
This monoclinic Ho Au Te is a polytype (polymorph) of
7
2
2
the older orthorhombic Er Ni Te . For comparison, the
7
2
2
21
isotypic Dy
following because iridium (Ir) is closer in size to Au in the
present phase. The structure of orthorhombic Dy Ir Te is
shown in Figure 3. The 4 × 2 zigzag Ho Au sheets derive from
Ir
7
Te
is chosen to represent the latter in the
2
2
7
2
2
7
2
the slightly puckered Dy Ir sheets in Dy Ir Te . The Ho Au
7
2
7
2
2
7
2
3
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Figure 3. ∼[100] view of Dy Ir Te . Dy, Ir, and Te atoms are gray,
7
2
2
violet, and green, respectively.
sheets are bridged by Ho7 and Ho8 with inversion symmetry,
different from those in Dy Ir , which are connected by bridging
7
2
Dy4 with mirror symmetry. This packing gives Ho7 and Ho8
one fewer Te neighbor than Dy4 had, but the connection of
Ho Au sheets becomes more efficient and dense than that of
Figure 4. ∼[010] view of Lu AuTe . Lu, Au, and Te atoms are
6
2
7
2
represented by red, orange, and green spheres, respectively.
Dy Ir sheets. The average Ho7,8−Ho separation is 3.57 Å, in
7
2
contrast to the corresponding average Dy4−Dy bond of 3.73 Å.
Moreover, the Ho7,8−Au distance average is 3.13 Å, which is
of these interstitials exhibit an appreciable range, 1.439 Å for
33
Au, 1.442 Å for the slightly larger Ag, and 1.276 Å for Cu,
0
.67 Å less than the Dy4−Ir average. These indicate that a
which further indicate the flexible features of this orthorhombic
structure. The heteroatomic separations within the TPs
naturally also change: 2.89 to 2.94 Å for d(Au−Lu), 2.92 to
much better packing of Ho Au sheets is achieved, given that
7
2
there is only about a 0.02 Å decrease of the standard metallic
33
radii from Dy to Ho plus a 0.08 Å decrease from Au to Ir.
The closer packing of Ho Au sheets should be associated with
2
.98 Å for d(Ag−Lu), and 2.86 to 2.92 Å for d(Cu−Lu). (Note
7
2
the characteristic decrease in the last step.) Simultaneously, the
Lu−Lu edges within the TCTP decrease with shrinkage of the
interstitial. However, the remaining Lu−Lu bonds within the
octahedra and the Lu−Te distances are comparable in all three
compounds. These features reveal that the change from Au to
Ag and Cu results mainly in the irregular contraction of the
TCTP; the average d(Au−Lu), d(Ag−Lu), and d(Cu−Lu)
fewer Te neighbors to Ho7 and Ho8 compared with those
about Dy4 inasmuch as an increased number of Te neighbors
(
covalence) is known to detract from the bonding between
32
rare-earth-metal atoms. In other words, the 4 × 2 zigzag
Ho Au sheets achieve more efficient packing at the expense of
7
2
one fewer Te neighbor for each of the bridging Ho7 and Ho8.
Ho Au Te exhibits a larger difference in the range of d(R−R)
7
2
2
(
calculated with inclusion of the distance between the
than Dy Ir Te does, from 3.474(1) to 4.009(1) Å versus from
7
2
2
interstitial and capping Lu4) are 2.95, 2.98, and 2.93 Å,
respectively. The Au−Lu bond lengths fall between those for
Ag−Lu and Cu−Lu rather than much closer to Ag−Lu, as was
expected from the slight difference in standard radii between
3
.466(2) to 3.811(2) Å. On the other hand, the ranges of d(R−
Z) and d(R−Te) are comparable in the two compounds.
Structural Description of Lu AuTe . Lu AuTe crystal-
6
2
6
2
12
lizes in a Sc PdTe -type structure in space group Pnma.
6
2
33
Au and Ag (0.003 Å) and larger differences between Au and
Cu (0.163 Å). The shorter Au−Lu bonds should be attributed
to the relativistic effects in Au bonding, which is also reflected
in the calculation results shown below.
Figure 4 shows the ∼[010] section of its structure along the
1
3
short b axis. All atoms lie on mirror planes at b = / or / . The
4
4
Au-centered TCTPs of Lu form infinite 1D chains along b with
shared trigonal faces, the same building block as that in
Ho Au Te . The double TCTP (dTCTP) chains are further
Stoichiometries of Sc Au Te . An unusual result
6
1−x
2
7
2
2
appeared following the first attempt to synthesize the analogous
Sc AuTe ; the refined composition was clearly substoichio-
condensed with b/2 displacements via the shared bifunctional
zigzag chains of Lu4 atoms, with inversion points at the centers
of the Lu4−Lu4 bonds. The Lu atoms (1−[5,5]−[6,6]−6)
form a chain of trans-edge-sharing octahedra along b, which
then interconnect via shared Lu6−Lu6 edges to give double
octahedral chains with b/2 displacements. Inversion centers
also lie at the centers of all Lu6−Lu6 edges. The dTCTP and
double octahedral chains further interconnect alternately by
sharing Lu1−Lu6 edges to generate the 2D zigzag sheets along
c, which are separated by Te atoms along a to generate the 3D
structure. Te1 and Te2 atoms center bicapped and mono-
capped TP chains of Lu along b, respectively, the same as those
in Ho Au Te (Figure 2).
6
2
metric, Sc Au
Te , with 14% less Au than expected. This is
6
0.856(2)
2
the first such example among what appears to be a relatively
stable series of diverse R−Au−Te phases. Reactions with
increased Au proportions showed clear shifts of their powder
pattern lines to lower angles. A single crystal from a similar
reaction loaded as Sc:Au:Te = 6:1.3:2 refined as
Sc Au0.892(3)Te (Table 1), with a 4.2% increase in the Au
6 2
content. Powder patterns of these two samples and that
calculated from the result of the second refinement are shown
in Figure S1 in the Supporting Information, and the customary
refinement results, in Tables S1 and S2 in the Supporting
Information. Still greater Au contents gave no significant line
shifts but increases in byproducts.
7
2
2
Lu AuTe2 is isostructural with its ternary neighbors
6
31
Lu AgTe and Lu CuTe . The standard metallic radii (r12)
6
2
6
2
3
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Inorganic Chemistry
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Figure 5. TB−LMTO−ASA electronic structure results for Lu AuTe . (a) Total DOS and pDOS curves for Lu, Au, and Te. (b) DOS curves for
6
2
separate Lu atoms. The data for Lu1, Lu2, and Lu4 are raised in order to distinguish them. (c and d) Orbital pDOS data for Lu, Au, and Te. (f)
COHP (eV/bond·mol). The dashed lines mark the Fermi energy (E ). (The comparable data for Ho Au Te appear in Figure S2 in the Supporting
F
7
2
2
Information.)
This thermodynamic effect could be caused by the smaller
size of (or lower R−R bond strength for) Sc relative to those of
the heavy rare-earth elements (Δr ∼ 0.14 Å) and the resulting
strain (bond lengthening) in the host lattice. Note the 1.63%
increase in the cell volumes and the corresponding decrease in
the crystal densities that accompany the inclusion of more Au
Figure 5a−f as a function of the energy (eV). Three clearly
separated bands are found in the total DOS. The lowest-energy
band around −12 eV originates mainly from Te 5s. Among the
middle bands within ∼−7 to −2.5 eV, the lower parts are
dominated by Au 6s/5d and the higher parts originate about
evenly from Te 6p and Lu 6s/6p/5d. A major number of Lu 5d
(Table 1), not the opposite that would be expected on filling
states that continue well above E are common evidence for the
F
nominally free space in a lattice. These results clearly support
the idea of a size incompatibility between an increased
interstitial Au content and the Sc cluster, a matrix effect.
Theoretical Results. The electronic densities of states
oxidation of Lu by Au and Te, and these also generate a
metallic characteristic for Lu AuTe .
6
2
The orbital pDOS of Lu shows mixed contributions of the 6s,
6p, and 5d states within −13 to −11.5 and −7 eV to E , above
F
(
DOS) and COHP plots for Ho Au Te are shown in Figure
which the distribution is predominantly from 5d. The former
7
2
2
S2 in the Supporting Information. These are very similar to
matches distributions of Te 6s and its Lu neighbors. The Lu−
9
those reported for Er Au Te ; therefore they will not be
Lu COHP results from −2 eV to beyond E correlate with the
7
2
2
F
discussed further except for comparisons with parallel data for
Nb7P4 below. The theoretical results for orthorhombic
Lu AuTe have been analyzed considering that the previous
Lu 5d distribution very well, which reflect the main
contributions of 5d−5d interactions. The Lu−Au COHP is
skewed toward lower region Au 6s/5d states and higher Au 6p.
The Au 5d states lie between ∼−7 and −3 eV with a large and
quite sharp peak centered at ∼−5.5 eV. Particularly, the Au 6s
states fall within the energy range of the Au 5d band, following
their obvious relativistic mixing in Au bonding. The Au 6p
6
2
theoretical treatment of the isostructural Sc AgTe was
6
2
1
3
̈
performed only at the extended Huckel level. The total
DOS, the orbital partial DOS (pDOS) for each atom, and the
COHP data for different pairwise interactions are plotted in
3
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Table 5. Average ICOHP for Each Bond Type and the Sum per Unit Cell in Lu AuTe , Lu AgTe , and Lu CuTe (Pnma, Z = 4)
6
2
6
2
6
2
Lu AuTe
6
Lu AgTe2
6
Lu CuTe2
6
2
Lu−Lu
Lu−Au
Lu−Te
Lu−Lu
Lu−Ag
Lu−Te
Lu−Lu
0.40
76
Lu−Cu
Lu−Te
ICOHP (eV/bond· mol)
bonds/cell
0.34
76
1.11
28
0.76
60
0.38
76
0.98
28
0.85
60
0.83
28
0.82
60
ICOHP (eV/cell)
25.8
25.2
31.1
30.3
45.6
44.5
28.9
26.9
27.4
25.6
51.0
47.5
30.4
29.6
23.2
22.5
49.2
47.9
%
contribution
states are clearly separated from the 6s,5d states and lie from
2.5 eV to above E , reflecting the further reduction of Au. The
populations, a small radius for its position, and an enhanced
36
−
electronegativity, 5.77 eV, which places it near Te and Se.
Ho Au Te and Nb P , the Same but Different. A clear
F
strong peaks for Lu1, Lu2, and Lu4 at ∼−5.5 eV match the
corresponding Lu−Au COHP very well, marking significant
bonding between Au and those neighbors. The Lu−Te COHP
is skewed toward Te and the lower-lying bands from
interactions between Te 6s and Lu 6s, 6p, and 5d, whereas
the higher bands arise from interactions of Te 6p with Lu 6s,
7
2
2
7 4
isotypism and some interesting chemical differences exist
9
between Ho Au Te , Lu Au Te , etc., and the long-known
7
2
2
7
2
2
37
Nb P . A priori attempts to design such ternary or higher
7
4
polytypes starting from suitably flexible binary (or other)
phases that have multiple sites for the same atom types are
generally complicated and limited by chemical, thermodynamic,
and crystallographic challenges. Rather, accidents and luck (and
Pearson’s number indexes) usually find them for us. The
complex binary metal-rich phosphide Nb P was, of course,
6p, and 5d. The Au and Te states are almost filled, matching
the substantial oxidation of Lu that is reflected in the
distribution of the less penetrating Lu 5d orbitals. Such polar
R−Z and R−Te interactions are common in many other related
7
4
9
,16,34,35
metal-rich compounds.
found accidentally, and the structure, a major accomplishment
37
The energy-weighted integrals of the COHP data
at that time, was published by Rundquist in 1966. The
present and recent R Au Te (R = Tb, Dy, Ho, Er, Lu)
(−ICOHP) are better measures of relative bond overlap
7
2
2
populations. The distances and molar ICOHP data for all
bonds are listed in Table 4, whereas the corresponding average
ICOHP for each bond type and the sum for each over the cell
are given in Table 5. The average Lu−Au interaction in
Lu AuTe is the largest, 1.11 eV/bond·mol, relative to Lu−Te,
examples are the first ternary examples and were again
discovered by chance, not planning. The contrasts in the
characteristics of the respective atom types are particularly
noteworthy.
The metal polyhedra about all four independent anions in
the two series are more or less similar, augmented TPs that are
not highly constrained by the C2/m symmetry (Figure 1). The
relatively undifferentiated polyhedra in the phosphide presum-
ably become split when P1 and P2 atoms are replaced by the
larger, electron-poorer, and more strongly bonded Au1 and
Au2. Some distortions among bonds to the face-capping R
atoms are noticeable, but both are classical TCTP if somewhat
longer distances are included in the counts. Conversions of the
Nb−P3 and Nb−P4 polyhedra to 0.55 Å longer Ho−Te
linkages are less distinctive; in both cases, the coordination
about these two are limited to 8- and 7-fold because the next-
nearest atoms in this structure are additional Te.
6
2
0
.76 eV, and Lu−Lu, 0.34 eV. Obviously, heteroatomic bonding
plays significant roles. The numbers of Lu−Au, Lu−Te, and
Lu−Lu bonds per cell vary as 28, 60, and 76, giving cumulative
ICOHP values of 31.1, 45.6, and 25.8 eV, respectively. The
large multiplicity of the Lu−Lu bonds somewhat offsets their
much smaller average ICOHP values, but they still comprise
the smallest contribution, ∼25% to the total ICOHP. On the
other hand, the polar Lu−Au and Lu−Te populations together
provide 75% of the total, a common feature in the other metal-
rich tellurides. The more frequent Lu−Te bonding makes a
45% contribution to the total, whereas only 30% comes from
Lu−Au bonding. The corresponding lower Lu−Au bond
proportion in this Au-poorer example makes the Lu−Au
bonding distinctively less with respect to the Lu−Te
components, in contrast to the R−Au interactions that
Substantial chemical changes are noteworthy because the 55
valence electrons per formula unit Nb P (7 × 5 + 5 × 4) are
7
4
converted to the 35-electron R Au Te (7 × 3 + 2 × 1 + 2 × 6),
7
2
2
2
,9,16
dominate in many related structures.
The analogous Lu AgTe and Lu CuTe provide further
a 36% reduction. The change in the cation oxidation states and
the Au substitution are the major parts of the difference.
Certainly, the introduction of Au atoms for P1 and P2 have the
more substantial effects on bonding, Au acting as a sort of early
post-transition element with vacant s and p orbitals but with
nearby penultimate 5d10 states that also provide substantial
contributions to the bonding, particularly with the open 5d
bands (as well as 6s and 6p) on R (Figure 5). The ternary
phase yields ICOHP values comparable to those for the
phosphide (to the extent that the two sets of results can be
compared meaningfully), but the ternary is likely the more
6
2
6
2
evidence for the relativistic effects of Au in Lu AuTe . Figures
6
2
S3 and S4 in the Supporting Information contain DOS and
COHP data for Lu AgTe and Lu CuTe , and the ICOHP for
6
2
6
2
each bond type and the sum for each phase are listed in Table
. The three compounds exhibit very similar electronic
structure distributions except for the differences in the group
5
1
1 member contributions. Similarly, the average Lu−Au, Lu−
Ag, and Lu−Cu populations individually remain the largest of
the three terms for each, decreasing in that order. In Lu CuTe ,
6
2
36
the average Lu−Cu interaction (0.83 eV) is substantially equal
to that for Lu−Te (0.82 eV), and the cumulative Lu−Ag and
Lu−Cu populations become smaller than those for parallel
homoatomic Lu−Lu terms. The large contribution of Lu−Au
can be ascribed to the related polar character. The dominance/
importance of Au in such parallel population comparisons can
in different but interrelated ways be attributed to greater ns/(n
polar in terms of Mulliken electronegativities, largely because
of Au.
Differences in certain pDOS distributions in the two phase
types are most noteworthy (Figure 6). Displacement of
substantial fractions of the 5d states on R atoms to above EF
is a familiar signal of its relative oxidation in these and other R−
Au−Te compounds, whereas the greater occupation of the Nb
4d valence states on this less oxidized metal is directly related
−
1)d mixing, greater relativistic effects, larger bond
3
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Inorganic Chemistry
Article
CONCLUSIONS
■
R Au Te (R = Tb, Dy, Ho) and R AuTe (R = Sc, Y, Dy, Ho,
7
2
2
6
2
Lu) are isostructural with Er Au Te and Sc PdTe , respec-
7
2
2
6
2
tively. The first structure consists of zigzag metal layers along c
that are generated by condensation of Au-centered TCTPs of
rare-earth metal into chains that are further interbridged by
rare-earth metal and Te atoms to form a 3D network. The
second structure type contains heterometallic zigzag sheets in
bc planes that are separated by isolated Te atoms along a. The
sheets are made of double Au-centered chains of TCTP R
atoms that are interbonded with double chains of R octahedra
along b. The Lu AuTe example again exhibits the particular
6
2
effects of the substantial reduction of Au and the corresponding
oxidation of Lu. The individual Lu−Au interactions make the
largest contributions to the bond populations even though Au
constitutes only 11 atom % of the compound.
ASSOCIATED CONTENT
Supporting Information
■
*
S
Detailed crystallographic data in CIF format, powder pattern
comparisons and tables of crystallographic results for
Sc Au0.85Te2 and Sc Au Te , anisotropic displacement
6
6
0.89
2
parameters for Ho Au Te and Lu AuTe , TB−LMTO−ASA
7
2
2
6
2
results for Ho Au Te , Lu AgTe , Lu CuTe , and Nb P , and a
7
2
2
6
2
6
2
7 4
COHP comparison between the first and last phases. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
E-mail: jcorbett@iastate.edu.
■
*
Notes
Figure 6. Comparisons of the DOS of the more important Nb 4d and
Ho 5d states and of the (d), s, and p orbital states on P versus Au
showing how similar these distributions are when compared for the
same electron count and scale. The quite similar but higher-lying Nb
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Qisheng Lin for the LMTO calculations on
Nb P . This research was supported by the U.S. National
4
d states reflect its higher valence electron count and E , and these are
F
followed by the anion 3p/6p states on P/Au (green). The unmarked
Ho 6s/6p distributions follow those of Ho 5d quite well, in parallel
with lower-lying Nb 5s and 5p states at the top (blue and blue green).
The Au 5d state is the new feature.
7 4
Science Foundation, Solid State Chemistry, via Grant DMR-
0853732. All of the work was performed in the facilities of
Ames Laboratory, U.S. Department of Energy.
its larger electron count and higher E . This can be readily seen
in the comparative DOS data for the two phases around and
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4
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