The structural change of lanthanum diiodide upon hydrogenation

Article abstract of DOI:10.1016/j.jallcom.2003.11.072

Slowly heating LaI₂ under 1atm hydrogen to 650°C leads to LaI₂H, analyzed as LaI₂H_0.95(3), accompanied by a structural change from tetragonal to hexagonal. Sharp reflections in the XRD pattern can be indexed in P6₃/mmc with a=4.2158(7)A? and c=15.508(3)A?, however, diffuse reflections indicate a polytypic intergrowth of MoS₂ and NbS₂ type structures. The earlier determined miscibility gap between LaI₂ and LaI₂H _x (x≈0.5) is closely related to the change of the structure. The Pauli paramagnetism for LaI₂, χ_P = 43(2)×10 ^-6cm³/mol, derived from measurements on LaI₂ and LaI₂H is in good agreement with the value estimated from band structure calculations.

Full text of DOI:10.1016/j.jallcom.2003.11.072

Journal of Alloys and Compounds 374 (2004) 142–145
The structural change of lanthanum diiodide upon hydrogenation
∗
M. Ryazanov, A. Simon , Hj. Mattausch, R.K. Kremer
Max-Planck Institut für Festkörperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany
Abstract
◦
Slowly heating LaI
tetragonal to hexagonal. Sharp reflections in the XRD pattern can be indexed in P6
diffuse reflections indicate a polytypic intergrowth of MoS and NbS type structures. The earlier determined miscibility gap between LaI
and LaI
(x ≈ 0.5) is closely related to the change of the structure. The Pauli paramagnetism for LaI
from measurements on LaI and LaI H is in good agreement with the value estimated from band structure calculations.
2003 Elsevier B.V. All rights reserved.
2
under 1 atm hydrogen to 650 C leads to LaI
2
H, analyzed as LaI
2
H0.95(3), accompanied by a structural change from
3
/mmc with a = 4.2158(7) Å and c = 15.508(3) Å, however,
2
2
2
−
6
3
2
H
x
2
, χ
P
= 43(2)×10 cm /mol, derived
2
2
©
Keywords: Lanthanum hydride halides; Disordered systems; X-ray diffraction; Magnetic measurements
1
. Introduction
reaction products are characterized by analytical, structural,
and magnetic studies.
Layered metallic diiodides of rare earth elements are of
considerable interest. GdI2 was found to be a giant magne-
toresistive material at room temperature [1], and the results
of band structure calculations for LaI2 indicated that it might
be modified to be a superconductor [2,3].
2. Experiment
La metal pieces (99.99%; Johnson Matthey, Karlsruhe)
and iodine powder (99.8%; Merck) were used as starting
materials. LaI was prepared from La metal pieces and I in
LaI2 crystallizes with the tetragonal Ti2Cu structure [4,5].
Square planar nets of La atoms are sandwiched by nets of
I atoms. Each La atom is surrounded by a cube of I atoms
slightly compressed along the c axis. Band structure cal-
culations [2] revealed strong nesting characteristics at the
Fermi surface of LaI2. However, no superconductivity or
charge-density wave instability was found and their absence
was attributed to insufficient electron–phonon interaction. A
more detailed study of electronic structure [3] showed the
presence of Van Hove singularities 0.1 and 0.5 eV below
the Fermi level. Moving the Fermi level closer to the saddle
points via hole doping of LaI2 could result in superconduc-
tivity. However, previous attempts to substitute La by Ba,
as well as to localize part of the conduction electrons via
hydrogen insertion into the structure failed in producing su-
perconductivity [6].
3
2
an evacuated, sealed fused silica ampoule for 2 days. A part
with the metal was heated to 900 C while the other part
containing the iodine was kept at 200 C to prevent excessive
pressure.
◦
◦
LaI was prepared by reaction of sublimed LaI with
2
3
freshly prepared La filings in a sealed tantalum tube at
◦
830 C (20 h) as a dark violet metallic powder. Weighted
samples of approximate 0.5 g were hydrogenated under 1 bar
H in a calibrated volume, using the previously described
2
◦
apparatus [7] by slowly increasing the temperature to 650 C
and keeping at that temperature for 1–5 h to yield a prod-
uct, LaI H0.95(3), as determined analytically via combustion
2
and Karl–Fischer determination of the liberated H O. At-
tempts to synthesize samples with intermediate composition
2
In this paper, we report on our renewed effort to hole-dope
LaI2 via hydrogenation. The starting material as well as the
LaI H (x = 0.1, 0.25, and 0.75) were carried out by heat-
2
x
◦
ing the pressed mixtures of LaI and LaI H at 650 C (1
2
2
day) in molybdenum jackets, that were sealed in silica tubes
under 1 atm Ar.
X-ray diffraction patterns were recorded on a Stoe
diffractometer (Mo K␣ radiation, Ge monochromator) and
∗
Corresponding author. Tel.: +49-711-689-1640;
fax: +49-711-689-1642.
E-mail address: a.simon@fkf.mpg.de (A. Simon).
0
925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2003.11.072

M. Ryazanov et al. / Journal of Alloys and Compounds 374 (2004) 142–145
143
indexed using the WinXPOW software. Magnetic suscep-
tibilities of pressed powder samples were measured with
an MPMS SQUID magnetometer between 2 and 350 K at
1
080
070
1
2
0 and 10000 Oe (silica tube, 1 atm He). The susceptibili-
1060
ties were corrected for the diamagnetic contribution of the
constituent atoms estimated from Pascal’s constants [8].
1050
1040
1030
3
. Results and discussion
1020
1
010
000
Fig. 1 shows the temperature dependence of H-pressure
1
with and without LaI2 sample. The deviation in pressure
◦
100
200
300
400
500
600
700
above 150 C indicates the beginning of hydrogen absorp-
o
◦
Temperature, C
tion, which is complete after prolonged heating at 650 C
and changes the color of the metallic dark purple LaI2 into
light gray. The composition of the final product LaI2Hx, es-
timated from the difference of H pressure after cooling was
x = 0.97 in good agreement with an analytically determined
composition, x = 0.95(3). The formulation as LaI2H is jus-
tified.
Fig. 1. Temperature dependence of the hydrogen pressure in a system
◦
with (᭡) and without (᭹) LaI2 at a heating rate of 5 C/min. Point (ꢀ)
◦
indicates the pressure drop after keeping the sample at 650 C for 5 h.
dicate the change of symmetry from tetragonal to hexagonal
The XRD pattern of LaI2H differs considerably from that
of the diiodide as shown in Fig. 2a. It exhibits essentially
two sets of diffraction peaks. Based on the sharp peaks in-
dexed as 0 0 l and h h l the structure of LaI2H is hexagonal,
P63/mmc with a = 4.2158(7) Å and c = 15.508(3) Å. The
other set of broad peaks refers to reflections hkl with h–k =
upon hydrogenation of LaI . A more definite description of
2
the structure of LaI H is hampered by the disorder expressed
2
in the diffuse peaks. However, to a first approximation the
structure is related to that of 2H-MoS and 2H-NbS (Fig. 3)
2
2
as shown by the comparison with diffraction patterns of the
latter two types in Fig. 2b. Assuming a disorder in the metal
atom positions alone as indicated in Fig. 2b does not model
3
n ± 1 for the same unit cell. The XRD patterns clearly in-
(a)
LaI2
LaI H
2
MoS2 type
Occ(a) = 1
Occ(b) = 0
Occ(c) = 1
(b)
Occ(a) = 0.75
Occ(b) = 0.25
Occ(c) = 0.75
Occ(a) = 0.5
Occ(b) = 0.5
Occ(c) = 0.5
Occ(a) = 0.25
Occ(b) = 0.75
Occ(c) = 0.25
NbS2 type
Occ(a) = 0
Occ(b) = 1
Occ(c) = 0
1
0
20
30
40
50
60
q
(deg)
2
◦
Fig. 2. (a) X-ray diffraction patterns of LaI2 and LaI2H. The hexagonally indexed reflections in the pattern of LaI2H are marked (reflection at 2θ ≈ 19
corresponds to 1 1 0). (b) Calculated diffraction patterns for LaI2H based on 2H-MoS2 and 2H-NbS2 structure types (see Fig. 3), as well as those
simulated with a disorder in the metal atom position by varying the site occupation factors (Occ) of the metal atoms.

1
44
M. Ryazanov et al. / Journal of Alloys and Compounds 374 (2004) 142–145
-
3
Hal
1.0x10
C
a
C
C
b
-4
(a)
Ln
1.2x10
(
b)
C
-4
8
.0x10
-
-
5
5
A
c
A
A
b
A
8.0x10
-
4
6.0x10
4.0x10
4.0x10
structure type :
structure type:
LaI2
2
H-MoS2
2H-NbS2
-4
LaI H
2
0
.0
0.2
0.4
0.6
0.8
-1
1.0
Fig. 3. Stacking variants of the 2H-MoS2 and 2H-NbS2 structure types
used for the simulation of the heavy-atom arrangement in LaI2H projected
along [1 0 0].
1/H (Tesla )
-
4
2.0x10
0
100
200
Temperature (K)
300
400
the characteristic features of the LaI2H diagram. In particu-
◦
◦
lar, the broadness of peaks near 2θ = 14 and 17 can only
be accounted for by the simultaneous presence of multilayer
stacking sequences (2–5 layers) and the construction of an
envelope to the resulting multipeak structure.
Fig. 4. (a) Temperature dependence of the molar magnetic susceptibility
for LaI2 and LaI2H at H = 10 kOe. (b) χM values, approximated at
T → ∞, as a function of inverse magnetic field.
The change of the layer symmetry from tetragonal
to hexagonal upon hydrogenation can be rationalized in
terms of very efficient 2-electron/4-center bonding of the
H-atom, which lies in the plane of three surrounding La
atoms [9].
shown in Fig. 4a) and 0.34(1) µB for LaI2H. In addition,
a magnetic field dependence of the susceptibility has been
seen with a saturation behavior of the susceptibility towards
higher fields as is typically observed for traces of ferromag-
netic impurities (Fig. 4b). As their source we have tentatively
identified the used lanthanum metal filings. A separate sus-
ceptibility measurement carried out on the used La filings
evidenced a similar dependence of χM on the magnetic field
as the studied compounds. We therefore conclude that the
weak temperature dependent paramagnetism in our LaI2 and
LaI2H samples originates from impurities in the starting ma-
terials. The origin of the impurities we could not unambigu-
ously identify until now. Extrapolating T and H to infinity
According to an early study of the dissociation pressure
isotherms for the LaI2-H2 system measured at 1073 K [10],
a miscibility gap exists for LaI2Hx, ranging over 0 ≤ x ≤
0
.5. Indeed, our samples of overall composition LaI2H0.1
and LaI2H0 prepared from mixtures of LaI2 and LaI2H
.25
◦
at 650 C proved to consist of unchanged LaI2 and a hy-
dride halide characterized by a = 4.2356(9) Å and c =
1
5.370(8) Å. Within three e.s.d. values the lattice constants
of the latter are identical in both samples. However, sam-
ple LaI2H0.75 was prepared as a nearly single phase (<5%
LaI2) with the lattice constants a = 4.2354(6) Å and c =
(
“Honda-Owen plot,” Fig. 4b) the temperature-independent
−6
3
paramagnetic susceptibilities χM = 69(2) × 10 cm /mol
−
6
3
and 26(1) × 10 cm /mol were derived for LaI2 and
LaI2H, respectively. Under the reasonable assumption that
all electrons are localized in LaI2H, the difference in χM
1
5.412(2) Å.
Previous measurements of the magnetic susceptibil-
ity of LaI2 had revealed a paramagnetic behavior for the
compound. Burrow et al. [2] related this behavior to a
contamination of their sample by semiconducting LaI2.42.
Correction of the susceptibility values for a Curie impu-
rity contribution resulted in the Pauli paramagnetism of
−6
3
of LaI2 and LaI2H, ꢀχM = χP = 43(2) × 10 cm /mol,
is attributed to the Pauli paramagnetism of LaI2. This
value is in good agreement with the value of 39 ×
−
6
3
1
0
cm /mol, which is estimated using the calculated band
−
6
3
structure [3].
the order of χ ≈ 100 × 10 cm /mol, a value, that ex-
1
ceeds the estimated value for a free-electron d metal by
approximately a factor of 3. In a later work [11] an effec-
tive magnetic moment µeff = 0.5 µB was found for LaI2
and attributed to an intrinsic property based on a small
4. Conclusion
4
f orbital contribution to the valence band of mainly 5d
character.
Fig. 4a shows the results of our magnetic measure-
ments on LaI2 and LaI2H at 10 kOe. Starting from room
The absorption of hydrogen into LaI2 leads to a structural
change characterized by a rearrangement from square planar
nets of La(I) into trigonal layers. This change optimizes the
La–H multicenter bonding and is the reason for a miscibility
gap between LaI2 and LaI2Hx.
Attempts to move the Fermi level in LaI2 closer to the
Van Hove singularities by localizing part of the conduction
electrons at hydride ions failed as the characteristic features
in the band structure of LaI2 are lost due to the structural
change.
−
4
3
temperature values, χM = 1.09 × 10 cm /mol and
−
4
3
1
.18 × 10 cm /mol, for LaI2 and LaI2H, respectively,
the molar magnetic susceptibility increases to the values of
−
4
3
−4
3
7
.84 × 10 cm /mol and 8.61 × 10 cm /mol at 5 K. Lin-
ear fits of χM versus 1/T in the range of 100–350 K result in
= 0.23(1) µB for LaI2, 0.39(1) µB for LaI2H0.75 (not
µ
eff

M. Ryazanov et al. / Journal of Alloys and Compounds 374 (2004) 142–145
145
5
. Note added in proof
[2] J.H. Burrow, C.H. Maule, P. Strange, J.N. Tothill, J.A. Wilson, J.
Phys. C: Solid State Phys. 20 (1987) 4115.
[
[
3] O. Jepsen, O.K. Andersen, Z. Phys. B 97 (1995) 35.
4] E. Warketin, H. Bärnighausen, Proceedings of 3rd European Crys-
tallographic Meeting, Zürich, 1976, p. 354.
Detailed studies performed in the meantime reveal a
statistical intergrowth of 70% MoS2- and 30% NbS2-type
◦
structural fragments for the sample prepared at 650 C
[5] E. Warketin, Ph.D. Thesis, Karlsruhe, 1977.
[6] C. Felser, M. Bäcker, A. Simon, 2000, unpublished.
◦
whereas samples prepared at 730–800 C are composed of
[
[
[
7] C. Michaelis, Hj. Mattausch, A. Simon, Z. Anorg. Allg. Chem. 610
1
0% MoS2- and 90% NbS2-type stacking.
(1992) 23.
8] P.W. Selwood, Magnetochemistry, 2nd Edition, Interscience, New
York, 1956.
9] Y. Tian, T. Hughbanks, Inorg. Chem. 32 (1993) 400.
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[
10] H. Imoto, J.D. Corbett, Inorg. Chem. 20 (1981) 630.
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Zimmermann, G. Meng, P. Steiner, S. Hüfner, J. Alloys Compd. 246
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1] C. Felser, K. Ahn, R.K. Kremer, R. Sheshadri, A. Simon, J. Solid
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