The Superstructure of Semiconducting SmTe2-x

Article abstract of DOI:10.1006/jssc.1998.7891

SmTe_1.84was synthesized and the crystal structure was studied by the single-crystal technique. The substructure was isostructural with LaTe_2-x, where corrugated rock salt LaTe slabs alternate with planar tellurium square lattices. The substructure of SmTe_1.84is tetragonal anti-Cu₂Sb type and the superstructure is √5×√5 of the tetragonal subcell. The superstructure is tetragonal, withP4₂/nsymmetry,a=9.709(1) A andc=18.007(7) A. There are both ordered and disordered defects in the Te sheet. The superstructure obtained consists of the three possible stable solutions suggested by Lee and Foran, and all three solutions were found in a single crystal. The resistivity dependence on temperature indicates that SmTe_1.84is semiconducting, which seems due to structural modulation. The structural stability of the other phases of SmTe_n(n=1-2) is discussed in terms of temperature and ionic radius ratio.

Full text of DOI:10.1006/jssc.1998.7891

JOURNAL OF SOLID STATE CHEMISTRY 140, 300—306 (1998)
ARTICLE NO. SC987891
The Superstructure of Semiconducting SmTe2؊x
Seon-Mi Park, So-Jeong Park, and Sung-Jin Kimꢀ
Department of Chemistry, Ewha Womans University, C120-750, Seoul, Korea
Received February 16, 1998; in revised form May 20, 1998; accepted May 18, 1998
Se, or Te; n"2, 2.5, or 3) have a common structural motif;
SmTe1.84 was synthesized and the crystal structure was studied that is, distorted rock salt RX slabs are separated by single
by the single-crystal technique. The substructure was isostructur- or double layers of chalcogen X (5—7). Another interesting
al with LaTe2؊x, where corrugated rock salt LaTe slabs alternate
with planar tellurium square lattices. The substructure of
fact is that commensurate CDWs have been observed in
several rare earth diselenides, in which Se—Se dimerization
occurred (8). Similar dimerization and ordered vacancies on
SmTe1.84 is tetragonal anti-Cu Sb type and the superstructure is
2
(
5؋(5 of the tetragonal subcell. The superstructure is tetra-
Se sheet were observed in RSe (R"Ce, Pr, La) (9, 10).
gonal, with P4 /n symmetry, a ؍ 9.709(1) A and c ؍ 18.007(7) A
s
s
.
ꢀꢂꢃ
2
Due to the Se vacancies, the reduction of coordination
There are both ordered and disordered defects in the Te sheet.
The superstructure obtained consists of the three possible stable
solutions suggested by Lee and Foran, and all three solutions
were found in a single crystal. The resistivity dependence on
number of rare earth metal from 9 to 8 going from PrSe
ꢁ
ꢂꢄ
to PrSe and PrSe was observed (9). In the mean time,
ꢂꢃ ꢀꢂꢅ
incommensurate ordering was observed in PrSe
ꢀ
and
ꢀ
ꢂꢃ
temperature indicates that SmTe1.84 is semiconducting, which DySe
seems due to structural modulation. The structural stability of
(9—11).
ꢀ
ꢂꢅꢆ
A structure with undistorted square coordinate tellurium
the other phases of SmTe (n ؍ 1–2) is discussed in terms of sheets has been reported for two polytellurides, SmTe and
n
ꢇ
temperature and ionic radius ratio. ( 1998 Academic Press
Sm Te (12). The ditellurides containing the undistorted
ꢁ
ꢈ
telluride sheets make these materials 2-dimensional metals.
Recently, in RTe , incommensurate CDWs have been ob-
served and the important role of Fermi-surface nesting was
ꢇ
INTRODUCTION
implied (13). Recognition of the charge density wave as
a driving force for different modulation structures has al-
lowed researchers to understand the structure of these ma-
terials (13). Even though resolving the superstructure is
essential for understanding the properties of given materials,
it is difficult to do that because of weak superstructure
reflections and twinning in crystals. Previously, DiMasi
et al. reported that SmTe is a semiconductor; however,
Early transition metal dichalcogenides MX (M"early
ꢁ
transition metals such as Ta and Nb; X"S, Se, Te) have
been extensively studied for their interesting low dimen-
sionality and resulting anisotropic properties (1, 2). In these
materials, transition metals are commonly sandwiched in
octahedral or trigonal prismatic coordinates in alternating
chalcogen layers. Especially, compounds with dꢀ electrons
such as TaS and NbSe have attracted much attention due
ꢀ
ꢂꢃ
no evidence of CDW or ordered defect in the crystal struc-
ture was found (14).
ꢁ
ꢁ
to their charge density wave (CDW) instability. In these
materials, one electron in the metal d band coupled with the
layered structure is known to be the driving force for CDW
In this paper, we report the superstructure of SmTe
determined from an X-ray single-crystal study. The struc-
as
ꢀ
ꢂꢅꢆ
(
3, 4).
tural diagram of binary rare earth tellurides in the RX
L
However, rare earth dichalcogenides such as DySe are
(
n"1—2) series with respect to ionic radius ratio is drawn.
ꢁ
inverse analogies of dꢀ compounds, since there is one hole in
an otherwise filled chalcogen p band. It is known that there
are complex structural modulations in such rare earth metal
dichalcogenides, where structural modulations are asso-
ciated with square chalcogen layers. A series of rare earth
EXPERIMENTAL
During an attempt to prepare ternary telluride, SmTe
polychalcogenides RX (R denotes rare earth; X denotes S, was obtained from mixtures of Sm chips (Aldrich, 99.9%),
ꢁ
\V
L
Te powder (Aldrich, 99.997%), and Ni powder (Aldrich,
9
(
9.99%) in a molar ratio of 2 : 3 : 1 with an excess of LiCl
Janssen, 99%) and RbCl (Strem, 99%) as a flux. The reac-
tion mixture was double-sealed in an evacuated quartz tube
ꢀ
Author to whom correspondence should be addressed. E-mail:
sjkim@mm.ewha.ac.kr.
3
00
0
022-4596/98 $25.00
Copyright ( 1998 by Academic Press
All rights of reproduction in any form reserved.

SUPERSTRUCTURE OF SEMICONDUCTING SmTe
301
ꢁ\V
and heated at 680°C for 10 days. The heated product was
then slowly cooled to room temperature. The product was
washed with water to remove the excess flux.
TABLE 1
X-ray Structure Refinement for SmTe1.84
Empirical formula
SmTe
ꢀꢂꢅꢆ
Preliminary examination and data collection were per-
formed with MoKa radiation (j"0.71073 A
s
) on an Enraf-
Formula weight (g ) mol\ꢀ)
Temperature (K)
385.14
293(2)
ꢀ
Nonius diffractometer equipped with an incident beam
monochromator graphite crystal. The unit cell parameters Wavelength (A
s
)
0.71073
Tetragonal
Crystal system
Space group
and orientation matrix for data collection were obtained
from the least-squares refinement, using the setting angles of
Cꢆ -P4 /n
ꢆF
ꢁ
Unit cell dimensions (A
Volume (Aꢇ)
s
)
a"9.709(1) c"18.008(7)
2
5 reflections in the range 22°(2h(MoK)(28°. Intensity
s
1697.5(7)
data were collected with the u—2h scan technique. The
Z
20
intensities of three standard reflections measured every hour Density (calculated) (g ) cm\ꢇ)
7.53
1.650
160.80
Absorption coefficient (mm\ꢀ)
during the data collection showed no significant deviations
during the data collection. The initial positions of all atoms
were obtained from the direct methods of the SHELXS-86
program (15). The structure was refined by full-matrix least-
squares techniques with the use of the SHELXL-93 pro-
gram (16).
Chemical compositions of the crystals were confirmed by
an energy-dispersive X-ray (EDX) spectrometer equipped
with a scanning electron microscope (SEM; Philips XL20,
EDX-PV9900).
F(000)
Crystal size (mmꢇ)
h range for data collection (deg)
Index ranges
0.01(1);0.02(1);0.24(1)
2.26—24.97
04h411
04k411
0
1501
4l421
Reflections collected
[R
*ꢄꢅ"0.0385]
Data/restraints/parameters
Goodness-of-fit on F
605/0/80
605/0/80
5.620%
11.42%
R
(Fꢁ'2p(Fꢁ))
ꢀ
ꢆ ꢆ
wR (Fꢁ'0)
Electrical resistivities of the single crystal were measured
using the four-probe method. For resistivity measurement
at low temperature, four gold wires were connected to the
crystal with silver paste.
ꢁ
ꢆ
abnormally large thermal parameters of Te atoms in the Te
square sheet were observed. Therefore, the occupancies of
Te sites were allowed to vary throughout the balance of the
RESULTS AND DISCUSSION
Single crystals of SmTe
were coffin-lid-like shaped, refinement. The occupancy factors for Te(4), Te(5), and Te(6)
ꢁ
\V
and platelet-shaped NiTe crystals were detected as a side were 85, 52, and 62%, respectively. The distance between
product. EDX analysis of the coffin-lid-shaped crystals gave Te(4) and Te(5) was 2.821(8) A
s
, which indicates a dimeriz-
a stoichiometry of SmTe
, and incorporation of RbCl ation. Three large peaks remained in the Fourier difference
ꢀ
ꢂꢅꢄ
and LiCl from the flux and Si from the quartz tube were not map and two of them were very close to the dimer of
detected. Te(4)—Te(5), e.g., 0.67(1) A from Te(4) and 0.84(2) A from
A black SmTe crystal was selected and mounted along Te(5), and one of them was on the site isolated from other
s
s
ꢁ\V
the c-axis for X-ray analysis. The tetragonal cell parameters Te’s. Te(7) and Te(8) were assigned to the first two peaks and
were related to the tetragonal subcell by a
"
the occupancies were refined to be 11.9 and 32%, respective-
ly. The isolated peak was assigned as Te(9) and the occu-
ꢀ
ꢁꢂ%ꢃ
"2c
ꢀꢁꢂ%ꢃ
(a#2b)ꢀ
,
b
"!(2a#b)
,
and
c
.
ꢁ"
ꢀꢁꢂ%ꢃ ꢀꢁ"
ꢀꢁ"
Twenty five randomly chosen reflections consistently gave pancy factor for Te(9) was refined to be 14%, which is empty
the same superstructure lattice on about 10 crystals. Weis- in the PrSe structure. No evidence of defect of Te in the
ꢀ
ꢂꢃ
senberg photographs showed consistent superlattice diffrac- rock salt slabs (Te(1), Te(2), and Te(3)) was found. The
tions. The intensities of the superstructure cell were strong overall structure and the defect square lattice of Te are
enough for the refinement. The observed Laue symmetry shown in Figs. 1 and 2. Atomic coordinates and equivalent
and systematic extinctions (hk0: h#k"2n#1, h00: isotropic displacement factors are shown in Table 2.
h"2n#1, 00l: l"2n#1) were indicative of the space
s
The distance between Te(8) and Te(7) was 2.23(2) A,
group Cꢆ -P4 /n. This space group and the cell dimensions which is too short for any Te—Te bond. A dimerization does
ꢆF
ꢁ
are very similar to those of PrSe . Therefore, this structure not likely occur between them; therefore, simultaneous oc-
ꢀ
ꢂꢃ
was used as the starting model for our refinement. Details of cupation of both sites must be precluded. However, the
the X-ray data collections and information about the struc- distance between Te(4) and Te(7) was 2.80(2) A
s
and the bond
, which were in the range
of bond distance of Te dimers. The relative orientations of
tural determination are given in Table 1.
Once all atoms were located based on the PrSe
distance Te(5)—Te(8) was 2.88(2) A
s
ꢀ
ꢂꢃ
structure, reasonable isotropic thermal parameters for Sm Te(4)—Te(7) and Te(5)—Te(8) dimers were similar; e.g., if
and Te atoms in the rock salt slabs were obtained, but Te(4)—Te(7) dimer was rotated 162.8(5)° around the origin of

3
02
PARK, PARK, AND KIM
FIG. 1. (a) Structure of SmTe
. Thermal ellipsoids (90% probability ellipsoids) of all Te and Sm atoms in rock salt and Te sheets are drawn. In (b)
ꢀꢂꢅꢆ
the Te atoms in a single Te layer are drawn with thermal ellipsoids (100% probability ellipsoids). For clarity, Te(7), Te(8), and Te(9) are omitted.
the unit cell, it was overlapped onto Te(5)—Te(8) as shown in DySe
(11). The defective Te layers were built up by
ꢀ
ꢂꢅꢆ
Fig. 2. Since the occupancies of Te(4), Te(5), and Te(7) were isolated Te and dimerizied Te—Te and their oxidation states
quite different, the possibility of forming trimers was ex- could be assigned as Teꢁ\ and (Te—Te)ꢁ\, respectively.
cluded. For clarity, only the relative orientations of When Te(6) and Te(9) were assigned to be Teꢁ\ and Te(4),
Te(4)—Te(5) and Te(4)—Te(7) dimers are presented in Figs. 2b Te(5), Te(7), and Te(8) to be Teꢀ\, the best charge balance
and 2c.
was obtained. Since overall composition and charge balance
The average Te—Te bond distance in the Te sheet was corresponded to Sm Te (Te )ꢁ\(Te )ꢀ\, there were
ꢀ
ꢄ
ꢀꢄ ꢀꢂꢁ ꢉꢂꢁ
3
.02 A
s
, which is in the range of metallic Te—Te bond dis- approximately three dimers for every monomer. Consider-
tance and comparable with those of SmTe and Sm Te . ing the occupancy factors, the coordination numbers of Sm
s
and the were between 7 and 9; e.g., the coordination numbers for
distance between Te dimers and monomers Teꢁ\—Teꢁ\ was Sm(1), Sm(2), and Sm(3) were 8.18, 7.73, and 8.38, respective-
.10 A
s
, which were shorter than those in the stoichiometric ly. In the selenium analog PrSe , the coordination num-
SmTe (n"2, 2.5, 3). The occupancy of Te(6) was refined to bers for Pr(1) and Pr(2) are 9 and the coordination number
ꢇ
ꢁ
ꢈ
The average distance of Te—Te dimers was 2.83 A
ꢁ
3
ꢀ
ꢂꢃ
L
be 62%, which is quite different from the corresponding for Pr(3) is 8.
selenium position in PrSe that is fully occupied. There-
With the composition established, data were corrected for
fore, there are disordered vacancies on Te sheets. Finally, absorption using the analytical t-scan method. Anisotropic
ꢀ
ꢂꢃ
refined occupancies gave the stoichiometry of SmTe
thermal motions were included (Table 3); however, Te(7),
.
ꢀ
ꢂꢅꢆ
Atomic coordinates, thermal factors, occupation factors, Te(8), and Te(9) were refined only isotropically because of
and the important distances are reported in Tables 2, 3, their low occupancies. Some features in thermal ellipsoids
and 4.
were observed (Fig. 1). The thermal ellipsoids of Sm(1),
Assuming Smꢇ> and Teꢁ\ to be the atoms in the rock salt Sm(2), Te(1), and Te(2) were elongated along the c-axis.
layers, the average charge on each of Te atoms in the However, Sm(3)-bounded Te(3) have pancake-shaped ther-
defective square lattice was !10/8.4"!1.19. This aver- mal ellipsoids in the ab plane direction (Fig. 1b). Telluriums
age charge was the same as that of the selenide analog in the Te sheet, Te(4), Te(5), and Te(6), commonly have

SUPERSTRUCTURE OF SEMICONDUCTING SmTe
303
ꢁ\V
FIG. 2. (a) Experimentally determined features of the defective square lattice of SmTe
illustrate Te(4)—Te(5), Te(4)—Te(7), and Te(5)—Te(8) dimers, respectively. (b) Only the orientation of Te(4)—Te(5) dimers with the (5;(5 unit cell is
presented. (c) Only the orientation of Te(4)—Te(7) dimers with the (5;(5 unit cell is presented.
. The thick solid lines, thin solid lines, and dotted lines
ꢂꢅꢆ
ꢀ

3
04
PARK, PARK, AND KIM
TABLE 2
Atomic Coordinates and Equivalent Isotropic Displacement
TABLE 4
Interatomic Distances (A
s
) for SmTe1.84
2
Factors U(eq) (A
s
) for SmTe1.84
Sm(1)—Te(1)
—Te(2)
3.213(3), 3.232(3)
3.218(3), 3.253(3)
3.206(2)
3.177(3), 3.295(3)
3.470(3)
3.226(2)
3.28(2)
3.17(2)
Occupancy
Atom Position
x
y
z
(%)
º(%ꢇ)
?
—Te(3)
—
Te(4)
Sm1
Sm2
Sm3
Te1
Te2
Te3
Te4
Te5
Te6
Te7
Te8
Te9
8g
8g
4e
8g
8g
4e
8g
8g
2b
8g
8g
8g
0.9524(4) 0.6474(3) 0.1189(1)
0.1490(3) 0.0495(4) 0.1107(1)
100
100
100
100
100
100
85(2)
52(1)
62(1)
0.0159(7)
0.0149(7)
0.005(1)
0.0101(8)
0.0137(8)
0.007(2)
0.014(1)
0.018(1)
0.028(3)
—Te(5)
—Te(6)
—Te(7)
—Te(8)
0.7500
0.2500
0.1148(2)
0.5493(5) 0.8500(4) 0.4319(1)
0.1504(4) 0.0503(6) 0.4385(1)
Sm(2)—Te(1)
3.150(3), 3.292(3)
3.189(3), 3.201(3)
3.176(2)
0.7500
0.2500
0.4337(2)
—
—
—
—
—
—
—
Te(2)
Te(3)
Te(4)
Te(5)
Te(7)
Te(8)
Te(9)
0.0559(4) 0.8350(6) 0.2503(3)
0.3200(8) 0.9672(7) 0.2509(5)
3.388(4)
0.2500
0.2500
0.7500
3.150(3), 3.292(3)
2.95(2), 3.26(2)
3.02(2)
1.118(2) 0.467(2) 0.242(1)
1.076(1) 0.906(2) 0.254(1)
1.243(3) 0.349(2) 0.255(1)
11.9(9) 0.0157(1)@
32(2)
14(1)
0.039(6)@
0.039(1)@
3.01(2), 3.02(2)
?
@
º
is defined as one third of the orthogonalized º*ꢈ tensor.
Sm(3)—Te(1)
—Te(2)
3.182(4) (2;)
3.226(6) (2;)
3.248(4)
3.165(3) (2;)
3.235(2) (2;)
3.30(4) (2;)
(
%ꢇ)
º
.
(
*ꢀꢆ)
—
—
—
—
Te(3)
Te(4)
Te(5)
Te(8)
pancake-shaped thermal ellipsoids in the ab plane. These
abnormal ellipsoids are found in other series of non-
stoichiometric compounds such as DySe and YSe
ꢀꢂꢅꢇ
11, 17) and these seem to be real features, not artificial ones
due to a fault in the absorption correction. The final cycle of
refinement was performed on Fꢁ with 1501 unique reflec-
Te(4)— Te(5)
—Te(6)
2.821(8), 3.171(2)
3.098(3)
2.80(2)
ꢀ
ꢂꢅꢆ
(
—
Te(7)
Te(5)— Te(7)
—Te(8)
3.17(2)
2.88(2)
3.12(2)
ꢆ
tions, and it gave residuals of wR (Fꢁ'0)"11.42%; the
—
Te(9)
Te(7)—Te(8)
Te(9)
Te(8)—Te(9)
ꢁ
ꢆ
conventional R index based on the reflections having
Fꢁ'2p(Fꢁ) was 5.62%. The difference Fourier synthesis
3.20(3)
2.97(3)
—
ꢆ
ꢆ
calculated with the phases based on the final parameters
showed no peak greater than 0.6% of the height of a Te
3.00(4)
atom. Low-symmetry space groups such as P4 , P4ꢀ and
ꢁ
P2/n were tested, but their R values were significantly un-
favorable.
In selenium compounds such as LaSe and CeSe , initial
ꢁ
ꢁ
square sheets were distorted to generate Seꢁ\ dimers form-
TABLE 3
ꢁ
ing a 1;2 superstructure (17). Previously, the energetic
stability of Seꢁ\ dimers and their relative orientation of
herringbone pattern were rationalized by Lee and Foran,
2
Anisotropic Displacement Parameters (A
s
) for SmTe1.84
?
ꢁ
Atom
ºꢀꢀ
ºꢁꢁ
ºꢇꢇ
ºꢁꢇ
ºꢀꢇ ºꢀꢁ
based on the second-moment scaled Hu
¨
ckel theory (18). The
Sm(1) 0.009(1) 0.013(1) 0.025(1)
Sm(2) 0.012(1) 0.009(1) 0.023(1)
Sm(3) 0.009(2) 0.007(2) 0.000(1)
0.004(1) !0.001(1) !0.001(1)
result of similar calculations for the LaSe -type structure
gave three possible superstructures with (5;(5 of subcell
in the a, b direction. In this model, the Se square consists of
ꢀ
ꢂꢃ
0.0006(1)
0
0.001(1)
0
0.001(0)
!0.003(1)
Te(1)
Te(2)
Te(3)
Te(4)
Te(5)
Te(6)
Te(7)
Te(8)
Te(9)
0.005(1) 0.009(1) 0.016(1) !0.0003(1) !0.0024(1) !0.0009(1)
four Seꢁ\ dimers per Seꢁ\ monomer as a result of displacive
0.010(1) 0.001(1) 0.022(1) !0.0031(1) !0.0011(1)
0.002(1)
0.002(1)
ꢁ
0.013(2) 0.009(3) 0.000(2)
0.015(1) 0.018(3) 0.011(1)
0.038(1) 0.007(3) 0.011(3) !0.0007(2) !0.016(3)
0.042(5) 0.042(5) 0.000(3)
0.0157(1)
0.039(5)
0.039(1)
0
0
ordering of Se and defects of 1/10 selenium atom per unit
0.0007(1)
0.0003(2) !0.015(2)
cell, and this agreed with the experimentally observed super-
0.010(2)
0
structure of PrSe (9).
0
0
ꢀꢂꢃ
A structure with undistorted square coordinate chalcogen
sheets in rare earth tellurides has been reported (5,6,12).
However, in this research, dimerization of telluriums and
complicated defects in the Te sheet were observed. The
lattice parameters a and c of the title compound decreased
relative to those of stoichiometric ditellurides, as observed
?
The anisotropic displacement factor exponent takes the form
2nꢁ(hꢁa*ꢁºꢀꢀ#2#2hka*b*ºꢀꢁ).
!

SUPERSTRUCTURE OF SEMICONDUCTING SmTe
305
ꢁ\V
in NdTe . As in other polychalcogenides such as SmTe
ꢂꢅ
and Sm Te , each Sm and Te in the rock salt slabs had five
ꢀ
ꢇ
ꢁ
ꢇ
corresponding Te and Sm neighbors in SmTe
. Accord-
ꢀ
ꢂꢅꢆ
ꢀ
ing to a previous calculation performed on La Se (18),
ꢄ ꢀꢃ
the relative orientation of Te(4)—Te(5)-type dimers is the
most stable solution and Te(4)—Te(7)-type dimerization is
the second most stable solution in terms of covalent and
total energy. The main difference between Te(4)—Te(5) and
Te(4)—Te(7) dimer orientation is the ionic energy caused by
repulsion between Teꢁ\ dimer and Teꢁ\ monomer. If Te(6)
ꢁ
is fully occupied, then the repulsion interaction between
Teꢁ\(Te(6)) and Teꢁ\(Te(4)—Te(7)) would destabilize Te(4)—
Te(7)-type dimerization. However, since Te(6) is only par- angstroms are estimated each structure type.
tially occupied, the stability of Te(4)—Te(7)-type dimeriz-
FIG. 4. Field map of R Te structure type and Rꢇ>. Radii of cations in
ꢁ
ꢁ
ꢇ
ation is comparable with Te(4)—Te(5)-type dimerization.
tration in the p band of the Te sheets with successive
This is the first case in RTe -type (n"2, 2.5, 3) tellurides
L
reduction of these compounds. Some of the factors affecting
the CDW instability are known to be the electron concen-
tration on Te, the relative size of the cation and anion, and
where the suggested superstructure model expected from the
calculation was actually observed. Furthermore, the two
most stable models were observed to coexist in a single
crystalline.
the polarizability of the chalcogen. Since SmTe
tained only in the limit of maximum x in SmTe
is ob-
, the
ꢀꢂꢅꢆ
ꢁ\V
The resistivity measured from a single crystal increased as
temperature decreased, showing that this crystal is semi-
conducting (Fig. 3). The band gap calculated in the range
observed lattice distortions in SmTe
semiconducting behavior seemed mainly due to their high
electron concentration in the Te sheet.
and induced
ꢀꢂꢅꢆ
1
00—250 K was 0.04 eV. Previously, metallic properties were
In an attempt to prepare SmTe , Sm Te with Sb S
found in SmTe and Sm Te (12). However, a semiconduct-
ꢁ\V
ꢁ
ꢇ
ꢁ ꢇ
ꢇ
ꢁ
ꢂꢃ
ꢈ
type was obtained. It is known that rare earth chalcogenides
ing property in SmTe
with large anisotropic transport
ꢀ
R Te have one of three structure types, Th P , Sb S , and
properties was observed by DiMasi et al. (14). The result of
our resistivity measurement was similar to that of DiMasi
et al. (14). Based on our structural investigation, bond-
breaking distortion in the Te square sheets seemed to lead
the semiconducting state.
ꢁ
ꢇ
ꢇ ꢆ
ꢁ ꢇ
Sc S . The limits of the stability may be understood quali-
ꢁ ꢇ
tatively in terms of the size of Rꢇ> and the synthetic temper-
ature. Figure 4 shows structural types depending on the
Rꢇ> radius (19a—h). We found that in the composition limit
of SmTe , Sm Te could be formed with two structures.
Going from SmTe to Sm Te , SmTe , SmTe , and
ꢁ\V
ꢁ
ꢇ
ꢇ
ꢁ
ꢈ
ꢁ
ꢀꢂꢃ
At low temperature (lower than 700°C), it crystallized with
the Sb S -type structure and at high temperature (higher
than 1000°C), it crystallized with the Th P -type structure.
SmTe , there was a successive reduction in the com-
ꢀꢂꢅ
pounds. In this series of compounds, there was an electron
transfer from Sm in the rock salt layer to the square Te
layers. Therefore, there is an increase in the electron concen-
ꢁ ꢇ
ꢇ ꢆ
FIG. 5. Sb S -type structure. Solid circles are metals and open circles
ꢁ
ꢇ
are nonmetals. Bond distances are illustrated by thick solid lines (d(
3.0 A), thin solid lines (3.0(d(3.2 A), and dotted lines (3.2(d(3.4 A
according to the strength of the interactions.
FIG. 3. Logarithmic electrical resistivity vs inverse temperature for
s
s
s
)
SmTe
.
ꢀ
ꢂꢅꢆ

3
06
PARK, PARK, AND KIM
The structure of Th P is a three-dimensionally conden-
ꢇ ꢆ
sed CsCl-type structure, where each metal is coordinated by
2. M. S. Whittingham, A. J. Jacobson, and S. Mahajan, ‘‘Intercalation
Chemistry.’’ Academic, New York, 1982.
3
. J. A. Wilson, F. J. DiSalvo, and S. Mahajan, Adv. Phys. 24, 117 (1975)
8
nonmetals. The structure of Sb S reveals that the chem-
ꢁ
ꢇ
4. M.-H. Whang and E. J. Canadell, J. Am. Chem. Soc. 114, 9587 (1992).
s
ical bonding can be either 3.0 or 3.4 A, where the metals are
coordinated by 3 or 5 and 5 or 6 sulfurs, respectively.
Considering three sulfur coordinates, the structure of Sb S
5
6
. R. Wang, H. Steinfink, and W. F. Bradley, Inorg. Chem. 5, 142 (1966).
. R. Wang and H. Steinfink, Inorg. Chem. 6, 1685 (1967).
7. M.-P. Pardo and J. Flahaut, Bull. Soc. Chim. Fr. 10, 3658 (1967).
ꢁ
ꢇ
8
. S. Benazeth, D. Carre, and P. Laruelle, Acta Crystallogr., Sect. B 38, 33
1982).
. P. Plambeck-Fischer, W. Abriel, and W. Urland, J. Solid State Chem.
8, 164 (1989).
0. M. Grupe and W. Urland, J. ¸ess-Common Met. 170, 271 (1991).
11. B. Foran, S. Lee, and M. C. Aronson, Chem. Mater. 5, 974 (1993).
consists of chains; however, there were strong interactions
&3.4 A) between two chains to form layers (Fig. 5). Since
Te has a strong covalency, Sm Te can be described as 2-
(
(
s
9
7
ꢁ
ꢇ
dimensional layers basically consisting of the rock salt unit.
1
In terms of continuous reduction from SmTe to SmTe
coordination numbers of Sm were reduced from 8 and 9 to
,
ꢁ
ꢁ\V
1
2. E. DiMasi, M. C. Aronson, B. Foran, S. Lee, Chem. Mater. 6, 1867
1994).
(
7
and 8 due to the defect of Te of the Te sheet. When the
1
3. (a) E. DiMasi, M. C. Aronson, J. F. Mansfield, B. Foran, and S. Lee,
Phys. Rev. B 52, 14516 (1995); (b) S. Lee and B. Foran, J. Am. Chem.
Soc. 118, 154 (1996).
structure of titled SmTe
is compared with that of Sb S -
ꢁ ꢇ
are totally
ꢁ
\V
type Sm Te , square tellurium layers of SmTe
ꢁ
ꢇ
ꢁ\V
missing in Sm Te and only the rock salt fragments remain. 14. E. DiMasi, M. C. Aronson, B. Foran, and S. Lee, Physica B 206&207,
ꢁ
ꢇ
3
86 (1995).
In the Sb S structure, the reduction of SmTe seemed to be
ꢁ
ꢇ
ꢁ
1
1
5. G. M. Sheldrick, Acta Crystallogr., Sect. A 46, 467 (1990).
6. G. M. Sheldrick, SHELXL 93, Program for the Refinement of Crystal
Structure, University of Gottingen, Gottingen, Germany, 1993.
7. S.-J. Kim and H.-J. Oh, Bull. Korean Chem. Soc. 16, 515 (1995).
complete.
1
ACKNOWLEDGMENTS
18. S. Lee and B. Foran, J. Am. Chem. Soc. 116, 154 (1994).
9. (a) W. L. Cox, H. Steinfink, and W. F. Bradley, Inorg. Chem. 5, 318
1
(
(
(
(
1966); (b) W. Lin, H. Steinfink, and E. J. Weiss, Inorg. Chem. 4, 877
1965); (c) J. P. Dismukes and J. G. White, Inorg. Chem. 4, 970 (1965);
d) D. J. Haase, H. Steinfink, and E. J. Weiss, Inorg. Chem. 4, 541 (1965);
e) J. Flahaut, L. Domange, M. Guittard, and M. P. Pardo, Bull. Soc.
This work was supported by the fund BSRI-96-3413 and partly by
Advanced Material 1996 from the Korean Ministry of Education.
Chim. Fr. 326 (1965); (f ) J. Flahaut, P. Laruelle, M. P. Pardo, and M.
Guittard, Bull. Soc. Chim. Fr. 1399 (1965); (g) J. S. Swinnea, H. Stein-
fink, and L. R. Danielson, J. Appl. Crystallogr. 20, 102 (1987); (h) V. K.
Slovyanskikh, N. T. Kuznetsov, and N. V. Gracheva, Russ. J. Inorg.
Chem., 30, 1077 (1985).
REFERENCES
1
. J. E. Rouxel, ‘‘Crystal Chemistry and Properties of Materials with
Quasi-One-Dimensional Structure.’’ Reidel, Dordrecht, Holland, 1986.