RE2+xI2M2+y (RE = Ce, Gd, Y; M = Al, Ga): Reduced rare earth halides with a hexagonal metal atom network

Article abstract of DOI:10.1515/znb-2007-0502

The title compounds were synthesized from RE, REI₃ (RE = Ce, Gd, Y) and Al or Ga under an Ar atmosphere at 930-950 °C. The non-stoichiometric Ce_2+xI₂Al_2+y and Ce _2+xI₂Ga_2+y compounds crystallize in the space group R3?m (No. 166) with lattice constants a = 4.3645(3), c = 35.914(2) A? for the Al and a = 4.3009(2), c = 35.680(4) A? for the Ga compound. Excess electron density found in the Wyckoff position 3a could be due to a fractional occupation by Ce or M (x = 0.06, y = 0 or x = 0, y = 0.11 in the case of the Ga_compound). The stoichiometric Gd₂I₂Ga ₂ and Y₂I₂Ga₂ compounds crystallize in the space group P3?m1 (No. 164) with lattice constants a = 4.1964(1) and 4.1786(7) A?, c = 11.4753(4) and 11.434(2) A?, respectively. Their structures feature M-centered (M = Al, Ga) RE trigonal prisms condensed via common rectangular faces. The electronic origin of the surplus of metal atoms in the octahedral voids between the I-layers of the Ce compounds was explored via extended Huckel-type calculations. Magnetic susceptibility, electrical resistivity and heat capacity measurements have also been carried out. These reveal a metal-insulator transition of Gd₂I₂Ga₂ at 40 K.

Full text of DOI:10.1515/znb-2007-0502

RE2+xI2M2+y (RE = Ce, Gd, Y; M = Al, Ga): Reduced Rare Earth Halides
with a Hexagonal Metal Atom Network
Mar’yana Lukachuka, Chong Zhengb, Hansju¨rgen Mattauscha, Michael G. Banksa,
Reinhard K. Kremera, and Arndt Simona
a Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany
b Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL 60115, USA
Reprint requests to C. Zheng and A. Simon.
E-mail: zheng@cz.chem.niu.edu and A.Simon@fkf.mpg.de
Z. Naturforsch. 2007, 62b, 633 – 641; received October 23, 2006
The title compounds were synthesized from RE, REI3 (RE = Ce, Gd, Y) and Al or Ga under an Ar
◦
atmosphere at 930 – 950 C. The non-stoichiometric Ce2+xI2Al2+y and Ce2+xI2Ga2+y compounds
¯
˚
crystallize in the space group R3m (No. 166) with lattice constants a = 4.3645(3), c = 35.914(2) A
˚
for the Al and a = 4.3009(2), c = 35.680(4) A for the Ga compound. Excess electron density found
in the Wyckoff position 3a could be due to a fractional occupation by Ce or M (x = 0.06, y = 0 or x =
0, y = 0.11 in the case of the Ga compound). The stoichiometric Gd2I2Ga2 and Y2I2Ga2 compounds
¯
˚
˚
crystallize in the space group P3m1 (No. 164) with lattice constants a = 4.1964(1) and 4.1786(7) A,
c = 11.4753(4) and 11.434(2) A, respectively. Their structures feature M-centered (M = Al, Ga) RE
trigonal prisms condensed via common rectangular faces. The electronic origin of the surplus of metal
atoms in the octahedral voids between the I-layers of the Ce compounds was explored via extended
Hu¨ckel-type calculations. Magnetic susceptibility, electrical resistivity and heat capacity measure-
ments have also been carried out. These reveal a metal-insulator transition of Gd2I2Ga2 at 40 K.
Key words: Cerium, Gadolinium, Yttrium, Aluminum, Gallium, Reduced Halide
Introduction
were used as starting materials. CeI3 was synthesized from
the reaction of Ce metal with I2. GdI3 and YI3 were pre-
pared from the reaction of the oxides (99.9 %) with HI and
NH4I. REI3 were purified twice by sublimation in a Ta tube
before being used. Due to the air and moisture sensitivity
of the starting and product materials, all handling was car-
ried out under Ar atmosphere either in a glovebox or through
standard Schlenk technique.
Reduced rare earth metal halides have demonstrated
extremely rich chemistry and structural variations
[1 – 4]. They typically have metal octahedra or trigo-
nal prisms as their building units [5 – 9]. These poly-
hedra usually contain a third element at their centers.
In the case of trigonal prism based structures, these
units as a rule condense into two-dimensional layers
by sharing their common rectangular faces. As a result,
the element that centers the trigonal prisms can form a
two-dimensional extended network of triangular, pen-
tagonal, hexagonal or even octagonal rings [2, 10, 11].
The honeycomb network is particularly interesting, as
it can be planar or puckered depending on its environ-
ment and electronic configuration [12]. In this contri-
bution, we report four new compounds of this type.
The mixtures (∼ 1 g) of the starting materials in an atomic
ratio 1 : 1 : 1 were arc-sealed in Ta tubes under an Ar atmo-
sphere. The Ta tubes were then sealed into silica glass am-
poules under a vacuum of ca. 10−2 mbar. The reaction tem-
◦
perature 930 C was kept 15 days for Ce/I/Al, 10 days for
Ce/I/Ga, and 15 days for Gd2I2Ga2. Y2I2Ga2 was prepared
at 950 ◦C (10 days). A powder sample of Tb2I2Ga2 was also
prepared at the same reaction conditions as for Gd2I2Ga2.
Its powder X-ray pattern showed that it is isostructural to
Gd2I2Ga2 with lattice constants a = 4.1807(7) and c =
˚
11.438(3) A. However, the sample contained some oxide
Experimental Section
halide according to its Guinier pattern.
After the reactions, the ampoules were opened under Ar
atmosphere. Many metallic golden thin platelets were ob-
Synthesis
RE metals (RE = Ce, Gd, Y) (sublimed, 99.99 %; Alfa- served in the products (Fig. 1). The yield, estimated from
Aesar, small pieces), REI3 and Al or Ga (99.99 %; Aldrich) Guinier measurements, was greater than 90 %. EDX analyses
0932–0776 / 07 / 0500–0633 $ 06.00 © 2007 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
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M. Lukachuk et al. · Reduced Rare Earth Halides with a Hexagonal Metal Atom Network
Table 1. Crystal data
and structure refine-
ment for Ce2.29I2Al2,
Empirical formula
Formula weight
Temperature [K]
Wavelength [A]
Crystal system
Space group
Ce2.29I2Al2
629.10
293(2)
Ce2.06I2Ga2 (CeI2Ga2.11
681.89
293(2)
)
Y2I2Ga2
571.06
293(2)
˚
0.56086
0.71073
0.71073
trigonal
Ce2+xI2Ga2+y
Y2I2Ga2.
and
rhombohedral
rhombohedral
¯
¯
¯
R3m (No. 166)
R3m (No. 166)
P3m1 (No. 164)
˚
a
Unit cell dimensions [A] a:
4.3645(3)
35.914(2)
592.46(6), 3
5.29
11.09
1.19
4.3009(2)
35.680(4)
571.57(7), 3
5.94
27.04
4.81
4.1786(7)
11.434(2)
172.91(5), 1
5.48
33.12
5.92
R1 = ΣꢀFo| − |Fcꢀ/Σ|Fo|;
wR2 = [Σ[w(Fo − Fc2)2]/
2
c:
3
˚
Σ[w(Fo2)2]]1/2
.
Volume [A ], Z
Dcalc [g cm−3
]
Absorption coefficient [mm−1
Transm. ratio (max/min)
F(000) [e]
]
795
862
246
θ range for data collection [deg]
2.69 – 23.92
0 ≤ h ≤ 6
−6 ≤ k ≤ 0
−51 ≤ l ≤ 51
994
291 / 14
1.231
0.0186 / 0.0496
0.0192 / 0.0499
1.363 / −1.044
5.50 – 29.96
−5 ≤ h ≤ 6
−6 ≤ k ≤ 6
−50 ≤ l ≤ 50
1623
246 / 14
1.195
0.0216 / 0.0646
0.0220 / 0.0648
2.463 / −1.292
3.56 – 27.44
−5 ≤ h ≤ 5
−5 ≤ k ≤ 5
−14 ≤ l ≤ 14
1473
194 / 11
1.165
0.0268 / 0.0572
0.0289 / 0.0577
1.939 / −0.960
Index ranges
Reflections collected
Independent data / parameters
Goodness-of-fit on F2
R1/wR2a [I ≥ 2σ(I)]
R1/wR2a (all data)
−3
˚
Largest diff. peak / hole [e A
]
Table 2. Crystal data and Rietveld structure refinement for
Gd2I2Ga2.
Empirical formula
Formula weight
Temperature [K]
Wavelength [A]
Crystal system
Space group
Gd2I2Ga2
353.88
293(2)
˚
0.71073
trigonal
¯
P3m1 (No. 164)
˚
Unit cell dimensions [A] a:
4.1964(1)
c:
11.4753(4)
175.00(1), 1
6.72
3
˚
Volume [A ], Z
Dcalc [g cm−3
]
Number of collected points (N)
Number of reflections
Number of refined parameters (P)
S
5800
272
14
2.51
Rp / Rwp / Rexp
0.0603 / 0.0799 / 0.0319
Fig. 1. SEM image of Ce2+xI2Ga2+y crystals.
They were first examined by the precession technique be-
fore being characterized on a Bruker CCD or a Stoe IPDSII
diffractometer. The structures were solved with Direct Meth-
ods using the SHELXS and SIR97 programs [14, 15]. Full-
matrix least-squares refinement on F2 was carried out using
SHELXTL [14]. The structure of the Gd compound was de-
termined by powder diffraction data based on the Rietveld
profile refinement method using FullProf [16].
of single crystals, using a TESCAN scanning electron micro-
scope with an Oxford EDX detector, confirmed the presence
of the component elements in the average atomic ratio of
1 : 1 : 1 with a possible experimental error of approximately
10 %.
Structure determination
The reaction products were ground to fine powders under
an Ar atmosphere and sealed in glass capillaries for phase bohedral space group R3m (No. 166) with Z = 3, whereas
Ce2+xI2Al2+y and Ce2+xI2Ga2+y crystallize in the rhom-
¯
¯
identification by a modified Guinier technique [13] (CuKα1
:
Gd2I2Ga2 and Y2I2Ga2 crystallize in P3m1 (No. 164) with
˚
˚
λ = 1.54056 A; Si as internal standard with a = 5.43035 A; Z = 1. The crystallographic information including the frac-
Fujifilm BAS-5000 image plate system). Single crystals of tional coordinates and interatomic distances of these com-
the Y and Ce compounds were transferred to glass capillaries pounds is listed in Tables 1 to 5. Further details on the crystal
under Na-dried paraffin oil and sealed under Ar atmosphere. structure investigation are available.
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M. Lukachuk et al. · Reduced Rare Earth Halides with a Hexagonal Metal Atom Network
635
Table 3. Atomic coordinates and equivalent isotropic Table 4. Atomic coordinates and equivalent isotropic dis-
2
3
2
3
˚
˚
displacement parameters (A ×10 ) for Ce2.29I2Al2, placement parameters (A ×10 ) for Gd2I2Ga2.
Ce2.06I2Ga2 (Ce2I2Ga2.11) and Y2I2Ga2. U(eq) is defined
as one third of the trace of the orthogonalized Ui j tensor.
Atom
Wyckoff
position
2c
x/a
y/b
z/c
U(eq)
Gd
I
0
1/3
2/3
0
2/3
1/3
0.1790(1)
0.3532(2)
0.0244(3)
3(1)
5(1)
10(1)
Atom
Wyckoff x/a y/b
position
z/c
U(eq)
sof
2d
2d
Ga
¯
Ce2.29I2Al2 (R3m)
Ce
I
Al
Ce1
6c
6c
6c
3a
1/3 2/3 0.10795(1) 11(1)
2/3 1/3 0.04822(1) 20(1)
1
1
1
˚
Table 5. Selected interatomic distances (A) and angles (deg)
0
0
0
0
0.16296(6) 11(1)
33(1)
for RE2+xI2M2+y (RE = Ce, Gd, Y; M = Al, Ga).
0
0.29
Ce2.29I2Al2
3.202(1)
3.373(1)
3.3092(4)
3.0576(3)
2.5339(5)
Ce2.06I2Ga2 (Ce2I2Ga2.11
3.0871(7)
3.4588(8)
3.2552(5)
3.0211(3)
2.5485(5)
115.09(3)
Gd2I2Ga2
3.003(2)
)
¯
Ce2.06I2Ga2 (Ce2I2Ga2.11) (R3m)
Ce
I
Ga
Ce–M
(×3)
(×3)
(×3)
(×6)
(×3)
6c
6c
6c
3a
1/3 2/3 0.10722(1) 10(1)
2/3 1/3 0.04823(2) 15(1)
1
1
1
Ce–M#4
Ce–I
0
0
0
0
0.15863(3) 10(1)
Ce1(M1)–I
Ce1 (Ga1)
0
30(5) 0.06 (0.11)
M–M#4
M
#4–M–M#6
(×3) 118.91(3)
¯
Y2I2Ga2(P3m1)
Y
I
2c
2d
2d
0
0
0.1771(1) 12(1)
1
1
1
Y2I2Ga2
1/3 2/3 0.3509(1) 16(1)
2/3 1/3 0.0292(1) 12(1)
RE–Ga
(×3)
(×3)
(×3)
(×3)
2.947(1)
3.374(2)
3.125(1)
2.5030(9)
Ga
3.364(3)
3.141(2)
2.487(1)
115.08(4)
RE–I
Ga–Ga
Ga–Ga–Ga
Details may be obtained from Fachinformations-
(×3) 113.18(6)
zentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen,
Germany (Fax: +49-7247-808-666; e-mail: crysdata@fiz-
karlsruhe.de, http://www.fiz-informationsdienste.de/en/DB/
icsd/depot anforderung.html) on quoting the deposi-
tion numbers CSD-417147 (Ce2.29I2Al2), CSD-417148
(Ce2.06I2Ga2), and CSD-417149 (Y2I2Ga2).
Symmetry transformations used to generate equivalent atoms:
−x+2/3, −y+1/3, −z+1/3; #6 −x−1/3, −y+1/3, −z+1/3.
#4
Table 6. Extended Hu¨ckel parameters.
a
a
Orbital
6s
Hii (eV)
−4.97
−4.97
−6.43
−18.0
−12.7
−14.58
−6.75
ζ1
1.799
2.20
ζ2
c1
c2
Ce
6p
5d
Computational study
3.90
2.10
0.7765
0.4586
I
5s
5p
4s
2.679
2.322
1.77
The density of states (DOS) and the crystal orbital overlap
population (COOP) [17] curves were computed using both
the tight-binding extended Hu¨ckel method (EH) [18, 19] and
the self-consistent linear muffin-tin orbital density functional
scheme using the local density approximation (LDA) as im-
plemented in the Stuttgart-TB-LMTO-ASA program [20]. The
electron localization function (ELF) [21 – 28] was also calcu-
lated using the LDA method. Approximately 500 k points in
the irreducible wedge of the Brillouin zone were used in the
EH computation of the DOS and COOP curves. More than
1000 k points in the whole Brillouin zone were generated in
the LDA calculations. These k points were then reduced to
the irreducible wedge of the Brillouin zone. The EH param-
eters used in the computation are listed in Table 6.
Ga
4p
1.55
a Exponents and coefficients in a double ζ expansion of the d orbital.
(Gd2I2Ga2 and Y2I2Ga2) and with the quasi-adiabatic step-
heating method (Nernst’s method) on a powder sample of
ca. 0.4 g, which was encapsulated in a glass ampoule under
1 bar He gas to ensure thermal coupling. The heat capacity
of the glass ampoule was determined in a separate run and
subtracted in the case of Ce2+xI2Ga2+y [30].
Results and Discussion
Crystal structures
Physical property measurements
Ce2+xI2Al2+y and Ce2+xI2Ga2+y are isostructural
to Y2Br2Fe2+x and feature Al and Ga centered Ce
trigonal prisms, respectively, condensed into two-
dimensional layers propagating in the a and b direc-
tions (Fig. 2) [31]. These layers stack along the c axis
with the I atoms between them. The sequence of each
layer I-Ce-(MM)-Ce-I follows a close-packing pattern
For conductivity measurements, the polycrystalline sam-
ples were pressed into pellets of 5 mm in diameter and ca.
2 mm thickness. The conventional four-point van-der-Pauw
method [29] was used to measure the resistivities. The mag-
netic susceptibilities were determined with a MPMS SQUID
magnetometer (Quantum Design) using sample quantities of
ca. 50 mg. The heat capacities were measured with a PPMS
6000 system (Quantum Design) on samples of ca. 20 mg cA(bc)Ab. In the layer below, the first I atom sheet then
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636
M. Lukachuk et al. · Reduced Rare Earth Halides with a Hexagonal Metal Atom Network
Fig. 3. The Gd2I2Ga2 structure viewed approximately
along [110]. It consists of one I-Gd-(GaGa)-Gd-I layer per
unit cell.
pound Y2Br2Fe2+x the corresponding excess scatter-
ing density has been assigned unambiguously to ad-
ditional Fe atoms by XPS measurements [31]. Refine-
ment of the Al-containing compound as Ce2I2Al2+y re-
sulted in an unreasonable value y = 1.29. With Ce in the
position 3a the structure refined as Ce2.29I2Al2. Alter-
native refinements of the Ga-containing compound as
Fig. 2. The Ce2+xI2M2+y (M = Al, Ga) structure viewed ap- either Ce2.06I2Ga2 or Ce2I2Ga2.11 converged to only
proximately along [110]. It consists of three I-Ce-(MM)-Ce-I
marginally different R values. The slightly increased
layers per unit cell. The Ce1(M1) atoms reside in the octahe-
magnetic moment (vide infra) is in fact in favor of an
dral voids between two I sheets.
excess of Ce. However, the metal-to-iodine distances
are somewhat short for Ce3+ if one compares to the
shifts to the a position, and the layer packing becomes
−
sum of ionic radii of Ce3+ (1.01 A) and I (2.20 A).
Ce4+ in an iodide can be ruled out for chemical rea-
sons. In conclusion, a definite decision about the occu-
pation of the 3a position cannot be made at present.
˚
˚
aB(ca)Bc. In the second layer below, the sequence is
bC(ab)Ca, and the pattern then repeats itself. Within
a layer, the Al or Ga atoms connect to form a two-
dimensional honeycomb net which is slightly puck-
ered with an Al–Al–Al angle of 118.9◦ and a Ga–Ga–
Ga angle of 115.1◦. The Al–Al and Ga–Ga distances
The compounds Gd2I2Ga2 and Y2I2Ga2 crystallize
¯
in the space group P3m1 (Fig. 3) and are isostruc-
˚
are 2.5339(5) and 2.5485(5) A, respectively. These are
tural to Ce2I2Si2 and La2I2Ge2 [3, 12]. We will fo-
cus our discussion on the Gd compound. The Ga-
centered Gd6 trigonal prisms are condensed into a two-
dimensional layer in the same fashion as for the pre-
viously discussed Ce compounds. The only difference
lies in the stacking of these layers. In Gd2I2Ga2 the
comparable to the sum of covalent radii of two Al
˚
˚
(2×1.25 A in Pauling scale) or Ga atoms (2×1.26 A).
˚
The shortest Ce–Ce distances are 4.217 and 4.242 A in
the Al and Ga compounds along the rectangular edge
of the Ce6 trigonal prism.
In the structure refinement significant excess elec- I-Gd-(GaGa)-Gd-I layer stacks on top of the previous
tron density is found in Wyckoff site 3a, i.e. at the cen- one without the shift of the I sheet as in the Ce com-
ters of the trigonal antiprisms formed by the I atoms. pounds. Thus the lattice type is primitive and the cell
The distances from the centers to the adjacent I atoms volume is 1/3 of that of the rhombodedral lattice of the
˚
are 3.058 and 3.021 A in the Al- and Ga-containing Ce compounds. Because of this direct stacking, the dis-
compounds, respectively. In the isostructural com- tance from the center of the void between the I layers
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M. Lukachuk et al. · Reduced Rare Earth Halides with a Hexagonal Metal Atom Network
637
Fig. 5. EH COOP curves for representative bonds for a hypo-
thetical Ce2+xI2Ga2+y calculated for a full Ga1 occupancy at
the 3a position (x = 0, y = 1). The + region is the bonding
area and the − region the antibonding area. The bond type,
its distance (d) and integrated overlap population (OP) to the
Fermi level are indicated in the panel. The dashed vertical
line is the Fermi level corresponding to y = 0.
tion at the Al or Ga node is therefore expected from an
electronic point of view as well as from the positions of
the nearest I atoms. Within this picture, all states below
the Fermi level will contribute to bonding as confirmed
by the EH COOP curves of representative atom con-
tacts calculated for the stoichiometric Ce2I2Ga2 struc-
ture shown in Fig. 4. In this figure, the features in the
Ce-I COOP curve are relatively sharp, as it is typi-
cal for an ionic interaction. The Ga–Ga states spread
over a large energy range due to the extended Ga–Ga
covalent bonding within and between cells. The fea-
tures remain essentially the same in a calculation for
nonstoichiometric Ce2+xI2Ga2. Assuming a composi-
tion Ce2I2Ga2+y (Fig. 5), the Ga1–I antibonding state
will be occupied even for y = 0 because Ga is more
electronegative than Ce. Hence the Ga1–I antibonding
states will be lower in energy than those of the Ce1–I
interaction. Consequently, the EH calculation also sup-
Fig. 4. EH COOP curves for representative bonds in
Ce2I2Ga2 calculated for a stoichiometric compound. The
+ region is the bonding area and the − region the antibond-
ing area. The bond type, its distance (d) and integrated over-
lap population (OP) to the Fermi level are indicated in the
panels. The dashed vertical line is the Fermi level.
˚
to a nearest Gd atom is 3.684 A. This distance would
likely be too short for an occupation of the void by
an additional metal atom. The Ga node in this honey-
comb net is also puckered with the Ga–Ga–Ga angle
◦
˚
equal to 115.1 . The Ga–Ga distance is 2.487 A and
ports the Ce occupancy of the 3a site. For Y2Br2Fe2+x
,
the shortest Gd–Gd distance, again along the rectan-
the d orbitals of Fe complicate the matter further and
the Fe occupancy is favored [31].
The pyramidal configuration at the Al/Ga site in the
puckered honeycomb net leaves lone pairs which are
clearly seen in the ELF plot in Fig. 6 calculated through
the LDA method for the three compounds. However,
˚
gular edge of the Gd6 trigonal prism, is 4.108 A. The
shorter Gd–Gd distance compared to that of Ce–Ce is
as expected considering the atomic radii.
Electronic structure
For the stoichiometric compounds a formal electron there is a back donation of electrons especially from
partition as (RE3+)2(I−)2(M2−)2 implies that they are the lone pairs to the rare earth metal framework. The
electron-precise leaving no valence electrons in the RE EH estimate (usually an overestimate) is 1.3 electrons
framework. The M2− species of the honeycomb net is donated to Ce3+ per Ga2− species in the idealized
valence isoelectronic to N, and a pyramidal configura- Ce2I2Ga2 composition.
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638
M. Lukachuk et al. · Reduced Rare Earth Halides with a Hexagonal Metal Atom Network
Fig. 6. ELF surface at an isovalue of 0.75, computed by the
LDA method for Ce2I2Al2, Ce2I2Ga2(i. e., x = 0, y = 0) and
Gd2I2Ga2. For clarity, only the region of one honeycomb
layer is shown.
The question of why Gd2I2Ga2 is a stoichiometric
compound can not be answered by purely electronic
reasons. An occupation of the voids between I atoms
results in unreasonably short Gd–Gd contacts, men-
tioned above as a structural argument. Experiments
with La2I2Ge could serve as a test case, because it crys-
¯
¯
tallizes in both the R3m and P3m1 space groups at dif-
◦
ferent reaction temperatures, 1050 C for the former
and 1175 ◦C for the latter [12].
Fig. 7. (a) Electrical resistivity of Gd2I2Ga2 (◦) and Y2I2Ga2
(ꢀ). (b) Heat capacities of Gd2I2Ga2 (◦) and Y2I2Ga2 (solid
line) normalized to REIGa (RE = Ce, Y). The inset shows the
Magnetic and electrical properties
Fig. 7 summarizes the magnetic and electrical prop- magnetic contribution to the heat capacity of Gd2I2Ga2. The
erties of Gd2I2Ga2 and the nonmagnetic isotypic sys-
tem Y2I2Ga2.
solid line indicates a power law behavior Cmag ∝ T2. (c) Field
cooled (fc) and zero-field cooled (zfc) magnetic susceptibil-
ity of Gd2I2Ga2 (per unit GdIGa) determined in a magnetic
Y2I2Ga2 shows the typical decrease of the resistiv-
field of 0.1 T. The arrows show the direction of the temper-
ity with decreasing temperature characteristic of metal-
lic behavior (Fig. 7a). The r. t. resistivity of Y2I2Ga2
amounts to about 1 mΩcm and decreases by about one
order of magnitude on cooling to liquid-He tempera-
ture. The r. t. resistivity of Gd2I2Ga2 is about an order
of magnitude larger and remains constant down to a
temperature of about 100 K. Below this temperature
the resistivity starts to grow and undergoes a rapid in-
crease by about two orders of magnitude in a rather
narrow temperature interval around 40 K. Below this
regime the resistivity starts to level off and saturates
at a value of 100 Ωcm at lowest temperatures. We as-
cribe the rapid growth of the resistivity by two orders
of magnitude to a metal-insulator transition. The heat
capacity data (Fig. 7b) and the magnetic susceptibility
(Fig. 7c) indicate a strong correlation of this transition
with magnetic anomalies. The magnetic susceptibility
ature change. The inset displays the reciprocal susceptibility
measured in a field of 1 T. The solid line represents a Curie-
Weiss law with an effective magnetic moment µeff = 8.04 µB
and a paramagnetic Curie temperature θ = −36 K. For details
see text.
(Fig. 7c) shows a Curie-Weiss behavior (solid line, in-
set Fig. 7c) with a slope corresponding to an effective
magnetic moment of 8.04(3) µB consistent with the
expected effective moment of 7.93 µB for Gd3+ with
8
a 4 f7 electronic configuration and a spin-only S7/2
ground state. The excess effective moment of 0.1 µB
corresponds to an excess of about 3 % Gd atoms. The
paramagnetic Curie temperature is negative indicating
predominant antiferromagnetic exchange interactions
and amounts to −36(2) K. Below about 100 K the
magnetic susceptibility deviates from the Curie-Weiss
behavior due to beginning antiferromagnetic correla-
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M. Lukachuk et al. · Reduced Rare Earth Halides with a Hexagonal Metal Atom Network
639
tions. The low-field magnetic susceptibility (measur-
ing field 0.1 T) reveals a thermal hysteresis character-
ized by a splitting of the field cooled (fc) and zero-field
cooled (zfc) susceptibility branch below about 25 K.
The electrical and magnetic anomalies around 100 K
are also visible in the magnetic contribution to the heat
capacity. Gd2I2Ga2 shows an excess heat capacity of
magnetic origin which starts to grow below ∼ 100 K
and becomes clearly visible if we subtract from the
heat capacity of Gd2I2Ga2 that of Y2I2Ga2. The lat-
ter can be considered to represent the lattice contribu-
tions to the heat capacities, neglecting the 10 % differ-
ence due to the different masses of Y and Gd. The thus
obtained magnetic contribution to the heat capacity of
Gd2I2Ga2 is spread over the whole temperature regime
below 100 K. There is no sharp (λ-type) anomaly in
the heat capacity indicating a transition to a long-range
ordered magnetic phase. Rather, a broad maximum is
found between 30 K and 10 K, exactly in the tem-
perature regime where the metal-insulator transition
is seen in the electrical resistivity. Below ∼ 10 K the
Fig. 8. (a) Heat capacities of Ce2+xI2Ga2+y (◦) and Y2I2Ga2
(solid line) normalized to REIGa (RE = Ce, Y). The dashed
line represents the heat capacity expected for a magnetic two-
level system (Schottky anomaly) given by eq. (1). (b) Molar
magnetic susceptibilities of Ce2+xI2Ga2+y measured with in-
creasing magnetic fields (ꢀ 0.01 T, ◦ 0.1 T, ꢁ 1 T and ꢂ 5 T).
The upper inset displays the reciprocal magnetic susceptibil-
ity of Ce2+xI2Ga2+y measured in a magnetic field of 5 T.
The solid line represents a Curie Weiss line as described in
the text. The slope of the Curie Weiss law corresponds to
an effective magnetic moment of 2.61 µB. The lower inset
shows the magnetization of Ce2+xI2Ga2+y at 1.88 K. The
small spontaneous component is readily saturated and para-
magnetic behavior is recovered. A noticeable hysteresis is
not observed. All data are referred to one half of the formula
unit Ce2+xI2Ga2+y with either x = 0.06 and y = 0 or x = 0
and y = 0.11.
magnetic heat capacity decreases again according to a
power law Cmag ∝ T2 common for spin wave contri-
butions of a two-dimensional antiferromagnet. If we
integrate Cmag/T to obtain the magnetic entropy re-
moved below 100 K we find a value consistent with
R ln(2 × 7/2 + 1) = 17.3 J/molK as expected for an
S = 7/2 spin system.
The magnetic properties of Ce2+xI2Ga2+y (x = 0.06,
y = 0 or x = 0, y = 0.11) (see Fig. 8) show some similar-
ities with that of Gd2I2Ga2. Above ∼ 150 K the mag-
netic susceptibility follows a modified Curie-Weiss be-
havior, χ = C/(T − θCW) + χ0 (solid line, upper in-
set Fig. 8b) with a negative paramagnetic Curie tem-
perature of −20(1) K and a Curie constant C corre-
sponding to an effective magnetic moment of 2.61(2)
µB which is in fair agreement with the value of 2.54
µB expected for a free Ce3+ ion with a 4 f1 elec-
tronic configuration [32] in a 2F5/2 ground state [33].
The excess magnetic moment of 0.07 µB corresponds
to an excess of about 6 %. The diamagnetic contri-
bution γ0 was estimated from Pascal’s increments to
−80×10−6 cm3/mol (Ce3+: −20×10−6 cm3/mol; I−:
−52×10−6 cm3/mol; Ga3+: −8×10−6 cm3/mol). Be-
low about 30 K deviations from the Curie-Weiss law
ture, which is reminiscent of an onset of long-range
ferromagnetic or ferrimagnetic ordering with the build-
up of a spontaneous magnetization, is readily sup-
pressed by increasing the external field (see lower in-
set in Fig. 8b) and a normal paramagnetic behavior is
restored. In contrast to the low temperature magnetic
behavior of Gd2I2Ga2, the field cooled and zero-field
cooled susceptibilities of Ce2+xI2Ga2+y show almost
no thermal hysteresis.
The heat capacity of Ce2+xI2Ga2+y at ∼ 10 K shows
are seen which arise either from crystal field splitting no sharp feature indicative of a transition to a long-
effects or from exchange interactions. Below ∼ 10 K range ordered magnetic state. Instead, a broad anomaly
a sharp increase of the susceptibility is observed when develops below ∼ 10 K. This can be described rather
a small external field of 0.01 T is applied. This fea- well by a Schottky anomaly expected for thermal exci-
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640
M. Lukachuk et al. · Reduced Rare Earth Halides with a Hexagonal Metal Atom Network
tations across a two-level system. A Schottky anomaly essentially single ion behavior is realized. In view of
for a two-level system is given by
the findings for Gd2I2Ga2, where apparent exchange
interactions lead to a broadened magnetic contribution
to the heat capacity, one could also attribute the Schot-
tky anomaly to an exchange split ground state doublet
assuming that the exchange fields from neighboring
Ce3+ions are frustrated and long range ordering is sup-
pressed as has been observed for some geometrically
frustrated systems [34].
ꢂ
ꢂ
ꢃ
∆
ꢀ
ꢁ
2 ꢂ
exp
Cmag
R
k T
∆
kBT
B
=
(1)
ꢃꢃ2
∆
1 +exp
k T
B
where R is the molar gas constant and ∆ the energy dif-
ference between the ground state and the excited state.
In Fig. 8a we have plotted the heat capacity according
to eq. (1) in comparison with the experimental data as-
suming an energy separation between the ground and
the excited state of approx. 11 K.
The origin of such a Schottky anomaly is not fully
clear at present. It could be due to an excitation from
the ground state doublet to the first excited crystal
field doublet. Such an explanation implies that any ex-
change interaction between the Ce ions is small and
Acknowledgements
We thank Dr. C. Hoch for the intensity data collection,
C. Kamella for the EDX measurements, E. Bru¨cher for the
magnetic susceptibility measurements, and G. Siegle for the
conductivity measurements. C. Z. acknowledges the support
by NSF and by the Max-Planck-Gesellschaft which made his
visit at MPI possible.
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