La2Tel2: A new layered telluride iodide with unusual electrical properties

Article abstract of DOI:10.1021/ic061675r

A new layered metal-rich telluride halide, La₂Tel₂, has been synthesized by heating stoichiometric mixtures of Lal₃, La, and Te under argon at 900°C, and its structure has been refined from X-ray powder diffraction data. The compound crystallizes in the 3R-Lu ₂CCl₂ structure type (rhombohedral space group R3m with a = 4.5074(4) A, c = 32.528(2) A, and Z = 3). The crystal structure is composed of infinite layers of edge-sharing, Te-centered metal atom octahedra and iodine atoms separating these layers to form three close-packed I-Ln-Te-Ln-I slabs within the unit cell. The title compound is metallic at room temperature and exhibits an anomaly in the resistivity around 140 K which is closely related to changes in the a lattice parameter with temperature. The chemical bonding and metallic properties of La₂Tel₂ can be plausibly understood in terms of an ionic description (Ln³⁺)₂Te ^2-(I^-)₂(e)₂ where two electrons are delocalized in the La 5d conduction band.

Full text of DOI:10.1021/ic061675r

Inorg. Chem. 2006, 45, 10728−10733
La TeI : A New Layered Telluride Iodide with Unusual Electrical
2
2
Properties
Mikhail Ryazanov, Arndt Simon,* and Hansj u1 rgen Mattausch
Max-Planck Institut f u¨ r Festk o¨ rperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany
Received September 5, 2006
A new layered metal-rich telluride halide, La
La, and Te under argon at 900 C, and its structure has been refined from X-ray powder diffraction data. The
compound crystallizes in the 3R-Lu CCl structure type (rhombohedral space group R3m with a 4.5074(4) Å, c
32.528(2) Å, and Z 3). The crystal structure is composed of infinite layers of edge-sharing, Te-centered metal
atom octahedra and iodine atoms separating these layers to form three close-packed I Ln Te Ln I slabs within
the unit cell. The title compound is metallic at room temperature and exhibits an anomaly in the resistivity around
2 2 3
TeI , has been synthesized by heating stoichiometric mixtures of LaI ,
°
2
2
h
)
)
)
−
− − −
1
40 K which is closely related to changes in the a lattice parameter with temperature. The chemical bonding and
can be plausibly understood in terms of an ionic description (Ln3+)
Te2 (I )
-
-
metallic properties of La
2
TeI
2
2
2
(e)
2
where two electrons are delocalized in the La 5d conduction band.
Introduction
tions. A variety of such interstitial phases are known for Sc,
Y, and lanthanide halides, mostly chlorides and bromides,
1
2
The zirconium monohalides ZrCl and ZrBr and ternary
7,8
3
9
with the endohedral units Z including H, N, O, C (single
M
2
ZX
halides (X ) Cl, Br, or I) of Zr3,4 and the trivalent
2
3
,5,6
6,10
11
atoms
2
or C units ), and Ge. Normally, nitrogen and
rare-earth elements with interstitial nonmetal atoms, Z,
constitute a closely related family of layered compounds.
They all feature infinite close-packed bilayers of metal atoms
that are sandwiched by layers of halogen atoms, forming
cubic closed-packed stacking sequences X-M-M-X. Such
slabs are in turn bound with each other by weak van der
Waals forces with the translation period varying from three
oxygen atoms occupy the tetrahedral voids between metal
atoms, whereas the larger carbon and germanium atoms
prefer octahedral voids. Hydrogen atoms can reside in
7
,8
tetrahedral voids or near to the parallel faces of trigonal
4
8
antiprisms, or in both, as found for M
In addition to their peculiar chemical and structural
features, the M ZX phases exhibit striking physical proper-
ties associated with low-dimensional strongly correlated
electron systems. Therefore, the metallic M (M ) Y,
2 4 2
H X .
1
2
slabs per unit cell as in ZrCl, ZrBr, and the so-called 3R
2
2
3,5
structures of M
2
ZX
2
to only one slab as in the 1T structures
In the ZrX phases, three electrons per metal
3
,5,6
2 2
of M ZX .
2
2 2
C X
contribute to the robust bonding within the metal atom double
layers and the compounds are two-dimensional metallic
systems. On the other hand, for the more electron deficient
rare earth metals, interstitial species are required to stabilize
the structure by introducing strong metal-interstitial interac-
La; X ) Cl, Br, or I) carbide halides are low-temperature
superconductors with the maximum transition temperature
1
2
reaching T
c
x
) 11.6 K. The nonstoichiometric YH I phases
(
7) Ueno, F.; Ziebeck, K.; Mattausch, H.; Simon, A. ReV. Chim. Miner.
*
To whom correspondence should be addressed. E-mail: a.simon@
1984, 21, 804. Meyer, G.; Hwu, S.-J.; Wijeyesekera, S. D.; Corbett,
J. D. Inorg. Chem. 1986, 25, 4811. Cockcroft, J. K.; Bauhofer, W.;
Mattausch, H.; Simon, A. J. Less-Common Met. 1989, 152, 227.
fkf.mpg.de.
(
1) Izmailovich, A. S.; Troyanov, S. I.; Tsirelnikov, V. I. Russ. J. Inorg.
Chem. 1974, 19, 1597. Adolphson, D. G.; Corbett, J. D. Inorg. Chem.
(8) Mattausch, H.; Simon, A.; Ziebeck, K. J. Less-Common Met. 1985,
113, 149. Mattausch, H.; Schramm, W.; Eger, R.; Simon, A. Z. Anorg.
Allg. Chem. 1985, 530, 43. Mattausch, H.; Eger, R.; Corbett, J. D.;
Simon, A. Z. Anorg. Allg. Chem. 1992, 616, 157. Ryazanov, M.;
Simon, A.; Mattausch, H. Z. Anorg. Allg. Chem. 2004, 630, 1401.
(9) Ford, J. E.; Corbett, J. D. Inorg. Chem. 1985, 24, 4120.
(10) Schwarz, C.; Simon, A. Z. Naturforsch. 1987, B42, 935.
(11) Mattausch, H.; Zheng, C.; Ryazanov, M.; Simon A. Z. Anorg. Allg.
Chem. 2005, 631, 302.
1976, 15, 1820.
(
(
2) Daake, R. L.; Corbett, J. D. Inorg. Chem. 1977, 16, 2029.
3) Hwu, S.-J.; Ziebarth, R. P.; Winbush, S. v.; Ford, J. E.; Corbett, J. D.
Inorg. Chem. 1986, 25, 283.
(
4) Wijeyesekera, S. D.; Corbett, J. D. Solid State Commun. 1985, 54,
657.
(
(
5) Schleid, T.; Meyer, G. Z. Anorg. Allg. Chem. 1987, 552, 90.
6) Schwanitz-Sch u¨ ller, U.; Simon, A. Z. Naturforsch. 1985, B40, 710.
10728 Inorganic Chemistry, Vol. 45, No. 26, 2006
10.1021/ic061675r CCC: $33.50
© 2006 American Chemical Society
Published on Web 11/17/2006

Unusual Electrical Properties of La
2 2
TeI
undergo a continuous transition from an itinerant to a
13
localized electronic state at low temperatures, whereas the
magnetic analogues GdH I exhibit a colossal negative
magnetoresistance as large as 3 orders of magnitude at 2 K
x
13,14
for x ≈ 0.7.
Recently we succeeded in preparing layered telluride
halides Ln TeI (Ln ) La, Gd), which are the first members
of the M ZX -type compounds with tellurium atoms acting
2
2
2
2
as interstitials. To the best of our knowledge, only one
example of Te-containing ternary rare earth halides is known
so far. This is a nonstoichiometric Er17.3Te24
8
I phase that
adopts a three-dimensional structure with polycationic and
15
polyanionic fragments, derived from the cubic rocksalt type.
Here we report the synthesis, crystal structure and electrical
properties of La TeI , which exhibits a resistivity anomaly
characteristic of a charge density wave (CDW) instability.
The results of first principles electronic structure calculations
2
2
Figure 1. X-ray powder diffraction pattern for La2TeI2 taken at 293 K.
The observed pattern is denoted by “O”; the solid line corresponds to the
best Rietveld fit profile; vertical bars indicate the calculated reflection
positions for La2TeI2 (top) and LaTe (bottom). The bottom trace represents
the difference between observed and calculated profiles.
2 2
for La TeI are also reported.
Experimental Section
Table 1. Selected Data from the Structure Rietveld Refinement of
La2TeI2
Synthesis. All compounds were handled under purified Ar either
in a glovebox (H
standard Schlenk techniques. La metal pieces (99.99%; Johnson
Matthey, Germany), LaI , Te (99.999%; Aldrich), and iodine
powder (99.8%; Merck) were utilized as starting materials. LaI
2
O < 0.2 ppm; O
2
< 0.8 ppm) or by employing
fw
cryst syst
space group
temp (K)
659.22
trigonal
R 3h m (No. 166)
3
293(2)
120(1)
3
wavelength (Å)
0.7093 (Mo KR)
was prepared from the elements in an evacuated sealed silica tube
lattice dimensions (Å)
a
c
4.5074(4)
4.4981(5)
32.402(3)
567.8(2), 3
5.78
by slowly heating from 300 to 900 °C and further purified by two
32.528(2)
572.3(2), 3
5.74
-
4
subsequent sublimations at 930 °C in high vacuum (10 Torr).
La TeI was synthesized by solid-state reaction of the starting
materials (LaI /La/Te ) 2:4:3) at 900 °C for 6 days. The reaction
3
V (Å ), Z
Fcalc (g·cm-3)
2
2
-
1
µ (cm
)
227.6
229.4
3
scan range
2.5° < 2θ < 45.0°
mixture (ca. 1 g) was ground, pressed into a pellet, and then heated
in an arc-welded Ta ampule filled with argon. To prevent oxidation
of the container, the Ta tube was further sealed in an evacuated
silica jacket. After reaction, the ampule was cooled to room
temperature at a rate of 10 °C/h. The product, observed under the
microscope, contained a dark bronze colored microcrystalline
powder composed of thin golden platelets and a small amount of
bulky violet crystals which had been identified as LaTe. According
to the X-ray powder diffraction patterns, the content of the binary
phase amounted to ∼5-10 wt % (after Rietveld analysis). The
compounds obtained are very reactive with air, resulting in strongly
smelling oxidized or hydrolyzed products. EDX (energy dispersive
X-ray spectroscopy) analyses of thin plate crystals, using a Tescan
scanning electron microscope equipped with an Oxford EDX
detector, confirmed the presence of La,Te, and I elements in an
no. of collected points (N)
no. of reflns
no. of refined params (P)
4250
148
15
4250
152
15
correction for preferred orientation
March-Dollase function
G1 ) 1.079(3) G1 ) 1.114(3)
0.084 0.098
a
final R indices Rp
Rwp
Rexp
0.109
0.059
0.089
1.85
0.128
0.066
0.091
1.94
RB
GOFb
a
2
Rp ) ∑i)1,N|yi - yc,i|/∑i)1,Nyi, Rwp ) [∑i)1,Nwi|yi - yc,i| /∑i)1,Nw
2
i
1/2
2 1/2
y }] , Rexp ) [(N - P)/∑i)1,Nwy ] , RB ) ∑i|Iobs,i - Icalc,i|/∑i|Iobs,i|.
i
b
GOF ) Rwp/Rexp.
recorded in Debye-Scherrer geometry on a Stoe Stadi P diffrac-
tometer (Mo K radiation, λ ) 0.7093 Å, germanium monochro-
mator). Low-temperature XRD experiments were performed on the
same instrument equipped with a self-constructed cooling system,
the operating temperature range being between 100 and 300 K.
Lattice dimensions and atomic parameters were refined by Rietveld
R
atomic ratio of 2:1:2 (standard deviation <5 %). A number of La
2
-
TeI
2
crystals (∼30 mg) were mechanically separated under the
microscope and used for further studies.
2 2
Powder X-ray Analysis. The isolated samples of La TeI were
mixed with some glass powder and sealed in glass capillaries
diameter: 0.2 mm) filled with Ar. X-ray diffraction patterns were
16
analysis using the FullProf program. The pseudo-Voigt function
was used for simulation of the peak shapes. All reflections were
(
corrected for asymmetry (2θ < 40°) and preferred orientation effects
using the March-Dollase function. Figure 1 shows the final
Rietveld refinement pattern for a selected La TeI sample used for
2 2
further physical measurements. According to the results of a detailed
pattern matching, the sample contained a minor amount of the
(
12) Simon, A.; Mattausch, H.; Eger, R.; Kremer, R. K. Angew. Chem.
17
1991, 103, 1209. Angew. Chem., Int. Ed. Engl. 1991, 30, 1188. Simon,
A.; Yoshiasa, A.; B a¨ cker, M.; Henn, R. W.; Felser, C.; Kremer, R.
K.; Mattausch, H. Z. Anorg. Allg. Chem. 1996, 622, 123. Henn, R.
W.; Schnelle, W.; Kremer, R. K.; Simon, A. Phys. ReV. Lett. 1996,
77, 374.
2 2
secondary-phase LaTe (ca. 2%). Crystallographic data for La TeI
(
(
(
13) Ryazanov, M.; Kremer, R. K.; Simon, A.; Mattausch, H. Phys. ReV.
B 2006, 73, 0351141.
14) Bauhofer, W.; Joss, W.; Kremer, R. K.; Mattausch, H.; Simon, A. J.
Magn. Magn. Mater. 1992, 104-107, 1243.
15) St o¨ we, K. J. Solid State Chem. 1998, 139, 57.
(16) Rodriguez-Carvajal, J. FullProf 2k Version 2.50; LLB CEA-CNRS:
Gif sur Yvette, France, 2003.
(17) Dollase, W. A. J. Appl. Crystallogr. 1986, 19, 267.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10729

Ryazanov et al.
Table 2. Atomic Coordinates and Equivalent Isotropic Displacement
2
3
Parameters (Å × 10 ) for La2TeI2 at 293 and 120 K (Given in Square
Brackets)
atom
La
x/a
y/b
z/c
Ueq
0
0
0.1120(1)
32(1)
[
0.1119(1)]
0.2827(1)
0.2828(1)]
1/2
[31(1)]
38(1)
[37(1)]
30(1)
I
0
0
0
0
[
Te
[28(1)]
Table 3. Selected bond distances (Å) in La2I2Te at 293 and 120 K
Given in Square Brackets)
(
LasLa
4.507(1)
4.498(1)]
IsI
4.507(1)
[4.498(1)]
[
a
a
4
.407(4)
4.198(4)
a
[4.178(3)]a
4.507(1)
[
4.396(3)]
3.280(3)
3.272(2)]
3.152(2)
3.144(2)]
Figure 2. Projection of the structure of La2TeI2 along [110]. The stacking
sequence of close-packed layers within the unit cell is shown in the ABC
notation.
La-I
TesTe
I-Te
[
[4.498(1)]
4.585(3)
[4.573(2)]
La-Te
[
netism of the constituent atoms estimated from Pascal’s increments
a
Interlayer distances.
3+
-6
3
-
2-
-6
3
24,25
(La , -20 × 10 cm /mol; I , Te , -52 × 10 cm /mol).
are summarized in Table 1. The atomic positions and isotropic
displacement parameters are listed in Table 2; Table 3 presents
important interatomic distances in the structure.
Results and Discussion
Structural and Physical Properties. La
in the rhombohedral 3R-Lu
2
TeI
2
5
crystallizes
Electronic Structure Calculations. First-principles DFT band
2
CCl
2
-type structure with lattice
structure calculations for LaI were carried out in the local density
dimensions of a ) 4.5074(4) Å and c ) 32.528(2) Å. The
approximation using the Stuttgart TB-LMTO program.1
8,19
The
arrangement of atoms has been refined from X-ray powder
26
eigenvalue problem was solved on the basis of the von Barth-
Hedin local exchange-correlation potential20 taking all relativistic
effects into account except for the spin-orbit coupling. The k space
integration was performed using the tetrahedron method21 with 781
irreducible points within the rhombohedral Brillouin zone. Further
computational parameters are given in the Supporting Information.
In addition, a study of the crystal orbital Hamiltonian population
diffraction data by the conventional Rietveld method. The
crystal structure is built from planar layers of edge-sharing,
Te-centered octahedra of the metal atoms and I atoms
separating these layers to form three closed-packed I-Ln-
Te-Ln-I slabs within the unit cell (Figure 2). Along the c
direction, these slabs are bound to each other via van der
Waals interactions in the stacking sequence AâcRB CRbγA
BγaâC. The closest I-I contact between adjacent I-Ln-
Te-Ln-I slabs is equal to 4.19 Å. Due to the rather identical
(
COHP)22 was performed to get a better insight into the chemical
bonding of La TeI
2
2
.
Physical Properties. Electrical resistance, R, was determined
on pressed-powder pellets of 3 mm diameter (thickness h ≈ 0.5
mm) by the van der Pauw method23 in a temperature range of 10-
2-
-
scattering cross sections of Te and I , some statistical
distribution of the anions cannot be ruled out; however, this
seems improbable, as there is no sign of a range of
290 K. The samples were connected to gold wire contacts of a
2
7
homogeneity. It is interesting to note that when neglecting
the distinction between tellurium and iodine atoms, which
are actually indistinguishable by X-ray diffraction methods,
four-point probe setup using commercial silver paste (Epo-Tek
H20E, Polytec, Germany). Each R(T) curve was recorded both in
cooling and heating modes by measuring the voltage when a
constant current of 1 mA was applied. To approximately evaluate
the resistivity values, we used the van der Pauw formula F (Ω‚cm)
the arrangement in a single La
of the A-form of the rare-earth sesquioxides Ln
case, the structure of La TeI can be described as a cubic
close-packed sequence of A-Ln -type blocks.
2 2
TeI slab corresponds to that
28
2
3
O . In this
2
2
≈
R(Ω) h(cm) π/ln2.
2
O
3
Magnetization measurements were performed with a SQUID
The shortest La-La distance is 4.41 Å between adjacent
metal atom layers. The intralayer distance between La atoms
(4.51 Å) is somewhat longer than the interlayer one,
indicating weaker bonding M-M interactions within a single
magnetometer (MPMS, Quantum Design, 6325 Lusk Boulevard,
San Diego) between 2 and 350 K in dc magnetic fields H ) 1, 3,
and 5 T. A polycrystalline sample of about 40 mg was pressed
into a pellet and sealed in a silica ampule under 1 atm He exchange
gas to provide sufficient thermal contact. The magnetic data were
corrected for the sample holder contribution and for the diamag-
(
24) Carlin, R. L. Magnetochemistry; Springer-Verlag: Berlin, Heidelberg,
(
(
18) Andersen, O. K.; Jepsen, O. Phys. ReV. Lett. 1984, 53, 2571.
19) Tank, R. W.; Jepsen, O.; Burkhardt, A; Andersen, O. K. TB-LMTO-
ASA 4.7; Max-Planck Institut f u¨ r Festk o¨ rperforschung: Stuttgart,
Germany, 1998.
New York, Tokyo, 1986.
(25) The diamagnetic Pascal increments for isoelectronic I and Te2-
-
closed-shell ions were assumed to be almost identical.
(26) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65.
(
(
20) von Barth, U.; Hedin, L. J. Phys. C 1972, 5, 1629.
21) Bl o¨ chl, P. E.; Jepsen, O.; Andersen, O. K. Phys. ReV. B 1994, 49,
(27) Attempts to synthesize mixed La(I1-xTex)2 compounds by heating
appropriate mixtures of LaI2 and LaTe2 at 820 °C resulted in the
formation of stoichiometric binary and ternary phases: La2TeI2/La2I5
(x ) 0.1), LaI3/LaTe (x ) 0.2), or LaI3/La2Te3 (x ) 0.5).
16223.
(
(
22) Dronskowski, R.; Bl o¨ chl, P. E. J. Phys. Chem. 1993, 97, 8617.
23) van der Pauw, L. J. Philips Res. Rep. 1958, 13, 1-9.
(28) Aldebert, A.; Traverse, J. P. Mater. Res. Bull. 1979, 14, 303.
10730 Inorganic Chemistry, Vol. 45, No. 26, 2006

2 2
Unusual Electrical Properties of La TeI
Figure 4. LMTO band structure and electronic densities of states (DOS)
of La2TeI2 with partial contributions of lanthanum 5d states (filled area).
The inset shows the Brillouin zone for a rhombohedral structure with special
k points outlined. The dashed line marks the Fermi level.
Figure 3. (a) Temperature dependence of the electrical resistivity, F, for
La2TeI2, measured both in cooling and heating modes. T* represents the
onset temperature of a resistivity/structural transition. (b) Variation of the
lattice parameters of La2TeI2 with temperature.
humplike feature as found in F(T), whereas the c parameter
changes monotonically with decreasing temperature (see
Figure 3b). This indicates that the observed resistivity
anomaly is closely related to the variation in atomic distances
within the ab plane, i.e., the a-axis length. However, except
for this metric change, our low-temperature XRD measure-
ments between 105 and 300 K show no other structural
changes, e.g., of symmetry.
layer. According to Pauling’s concept of bond order,29 the
observed La-La distances of 4.41 and 4.51 Å correspond
to a metallic bond order of 0.019 (×3) and 0.013 (×6),
respectively. The La-I bond length is equal to 3.28 Å, which
3+
-
is slightly larger than the sum of ionic radii (La , 1.03; I ,
.20 Å). On the other hand, the La-Te distances within the
La Te units (3.15 Å) are 0.09 Å shorter than those expected
for pure ionic bonding. The bond distance relations in La
TeI can thus be plausibly understood in terms of an ionic
description, (Ln )
Magnetic susceptibility measurements reveal no anomaly
2
around T*. Starting from high-temperature ø
M
values of ∼40
6
-6
3
×
10 cm /mol, which are almost temperature independent
2
-
in the range 100-350 K, the molar magnetic susceptibility
2
3
increases to 0.003 cm /mol at 5 K. The observed paramag-
3
+
2-
-
-
2
2 2
Te (I ) (e ) , taking into account en-
netic contribution is attributed to a small contamination of
the sample by magnetic impurities. The susceptibility data
hanced cation-anion attraction effects through the highly
charged Te2 ions. The excess valence electrons contribute
to metal-metal bonding, thus implying metallic conductivity
for the title compound.
-
can be fitted to a modified Curie law, ø
the Curie term C/T accounts for the paramagnetic impurities;
the temperature-independent part (ø ) includes the diamag-
M
ï
) C/T + ø , where
ï
Figure 3a shows the temperature dependence of the
electrical resistivity for La TeI . The sample exhibits indeed
2 2
metallic behavior with a positive slope dF/dT. The resistivity
is ∼3.7 mΩ·cm at room temperature and decreases linearly
when the temperature is lowered to 200 K. However, at lower
temperatures, F shows a broad humplike feature near T* )
netic contribution ødia from the closed electronic shells and
the Pauli paramagnetic component øPauli from the conduction
electrons. The extrapolation of the data 1/T f 0 directly
-6
3
provides ø
the core diamagnetism (ødia ) -196 × 10 cm /mol), the
Pauli susceptibility of øPauli ≈ 135 × 10 cm /mol is
obtained for La TeI . The observed øPauli is approximately
ï
) -60(5) × 10 cm /mol. After subtracting
-6
-6
3
3
140 K before reaching values of 2.6 mΩ·cm at 10 K. A
2
2
similar resistivity anomaly has been observed in layered
-6
3
two times larger than a value of 76 × 10 cm /mol,
transition metal chalcogenides and bronzes with nested Fermi
estimated from the band structure calculations according to
30,31
surfaces.
Such compounds have a tendency to lower their
2
32
(
ø
Pauli
)
calcd ) N(E
F
)µ
Electronic Band Structure. La
isovalence-electronic with the ternary zirconium halide Zr
CCl , thus adopting general electronic features of the latter
compound. The electronic band structure and density of
states (DOS) for La TeI derived from the LDA calculations
are shown in Figure 4. The bonding combinations of the I
p states hybridized with the s and p states of lanthanum
B
.
electronic energy by undergoing a periodic lattice distortion,
causing a structural transition due to induced modulation of
the electronic density at the Fermi level (Charge Density
Wave instability).
2
2
TeI is isostructural and
2
-
2
33
The most remarkable feature is that the a lattice parameter
of La TeI as a function of temperature shows the same
2 2
2
2
5
(
29) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell
form the valence band manifold between -4 and -2 eV
University Press: Ithaca, NY, 1960.
(
(
30) Wilson, J. A.; Di Salvo, F. J.; Mahajan, S. AdV. Phys. 1975, 24, 117.
31) Schlenker, C.; Dumas, J.; Escribe-Filippini, C.; Guyot, H. In Low
dimensional properties of molybdenum bronzes and oxides; Schlenker,
C., Ed.; Kluwer Academic Publishers: Dordrecht, Boston, London,
(32) Ashcroft, N. W.; Mermin, N. D. Solid State Physics; Saunders College
Publishing: Philadelphia, 1976.
(33) Ziebarth, R. P.; Hwu, S.-J.; Corbett J. D. J. Am. Chem. Soc. 1986,
108, 2594.
1989.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10731

Ryazanov et al.
Figure 6. Fermi surface of a single La2TeI2 slab calculated in the
hexagonal unit cell. The first Brillouin zone with characteristic high-
symmetry points [Γ ) (0,0,0), K ) (2π/3a,2π/3a,0), M ) (π/a,0,0), A )
(0,0, π/c), H ) (2π/3a,2π/3a, π/c), and L ) (π/a,0, π/c)] is outlined.
3
, 2b*/3) points, respectively, and six cylinderlike electron
pockets along the c* direction, which lie between the Γ and
K points. The electron FSs are strongly anisotropic in the
c* direction and squeezed along [-1 1 0]*, yielding flat
regions with a nesting vector parallel to this direction.
However, nesting is much weaker than in the case of layered
transition metal compounds.
According to band structure calculations performed with
the structural parameters of La TeI determined at 120 K,
2 2
there are no significant changes neither in the electronic band
structure nor in the Fermi surface except that the density of
Figure 5. Crystal orbital Hamilton population (COHP) diagram of the
La-La, La-I, and La-Te bonds in La2TeI2. The negative COHP values
correspond to bonding interactions and the positive ones to antibonding
interactions. The dashed line displays the position of the Fermi level.
states at E decreases slightly from 2.37 to 2.35 states/(eV‚
f.u.) when compared with the room-temperature value.
F
relative to the Fermi level, E
tellurium 5p states lie at around -5 eV. The conduction band
E > -2 eV) is primarily comprised of the La 5d and 4f
states (above E ) with the predominant contribution of the
orbitals to its bottom part. The Fermi level
F
. The bands derived from
(
Concluding Remarks
F
d
xy, dyz, and d 2
z
La TeI can be prepared in high yields by a solid-state
2
2
crosses the conduction band close to Γ, just below a high-
density spike in the DOS which results from the degeneracy
of the metal d bands at Γ and their weak dispersion, if any,
along the Γ-Z direction. The DOS per formula unit at the
reaction of LaI , La, and Te in welded Ta ampoules filled
with argon. Similar to other interstitial derivatives of the rare
3
earth metal halides M ZX , the main structural feature of
2
2
La TeI is the presence of the metal atom double layers.
2
2
Fermi level is N(E
To analyze the chemical bonding, the COHP curves have
been calculated for several representative bonds in La TeI
Figure 5). These show that the La-La interactions, both
F
) = 2.37 states/(eV·f.u.).
Although the La-La distances (4.41-4.51 Å) are substan-
tially larger than those found in binary layered metal-rich
34
35
2
2
halides (3.93 Å for LaI, 3.92 Å for LaI ), electronic band
2
(
structure calculations and analysis of the COHPs indicate
intralayer and interlayer, are bonding in nature. However,
the bonding states are not completely filled up to the Fermi
weak bonding within the metal atom framework, so that the
system is expected to exhibit metallic yet strongly anisotropic
transport properties. Indeed, La TeI is metallic at room
level E
of optimized La-La bonding. In this regard, the optimal
overlap would be reached at E - E
F
, indicating that the electron count is far from that
2
2
temperature, but exhibits a resistivity anomaly around 140
K. Such an electrical anomaly is frequently observed in
layered transition metal chalcogenides with nested Fermi
surfaces and associated with the formation of a charge density
wave instability. Interestingly, the calculated Fermi surface
for the title compound does contain small flat regions
perpendicular to the [-1 1 0]* direction, which are however
far from optimal nesting. It is striking that the a lattice
parameter shows the same feature with temperature as seen
in F(T), indicating a close correlation between both. More
F
≈ 1.0-1.5 eV, e.g., by
introducing extra valence electrons through cation intercala-
tion between the halide layers. It is interesting to note that
there are minor contributions of the La-La and La-I
bonding components in the “tellurium” band. For comparison
of bond strengths, we have analyzed integrated COHP values
(ICOHP) of the intralayer and the interlayer La-La interac-
tions. The shorter interlayer La-La distances of 4.41 Å have
an ICOHP value of - 0.11 eV/bond, compared with one of
-
0.08 eV/bond for the longer in-plane La-La distances of
detailed studies on single crystals of La TeI2 will be
2
4.51 Å. This relation indicates stronger interactions between
necessary to better understand the origin of the unusual
the metal atoms along the c axis, being in line with the
Pauling bond order parameters, see above.
electrical properties found for this compound with a strongly
anisotropic electronic structure.
Figure 6 displays the Fermi surface (FS) of La
2 2
TeI
calculated for a single slab 1T-La TeI structure within the
2
2
(34) Martin, J. D.; Corbett, J. D. Angew. Chem. 1995, 107, 234-236;
Angew. Chem., Int. Ed. Engl. 1995, 34, 233.
hexagonal unit cell. It consists of two parts: hexagonal/
(35) Burrow, J. H.; Maule, C. H.; Strange, P.; Tothill, J. N.; Wilson, J. A.
triangular hole cylinders around the Γ ) (0,0) and K ) (2a*/
J. Phys. C: Solid State Phys. 1987, 20, 4115.
10732 Inorganic Chemistry, Vol. 45, No. 26, 2006

2 2
Unusual Electrical Properties of La TeI
Acknowledgment. We gratefully thank G. Siegle for the
electrical resistivity measurements and E. Br u¨ cher for the
magnetic susceptibility measurements.
2 2
tions, and LMTO band structure of La TeI in the fat band
representation showing contributions of selected La (s, p, and 5d)
atomic orbitals. This material is available free of charge via the
Internet at http://pubs.acs.org.
Supporting Information Available: Temperature-dependent
magnetic susceptibility data, parameters of the TB-LMTO calcula-
IC061675R
Inorganic Chemistry, Vol. 45, No. 26, 2006 10733