YIHn: A new family of metal-rich hydride halides

Article abstract of DOI:10.1002/zaac.200400094

The graphite-like yttrium hydride halides, YIH_n (0.8 ≤ n ≤ 1.0), have been prepared in quantitative yields by heating either YI ₃, YH₂ (1:2) or stoichiometric YI₃, YH ₂, Y mixtures in sealed Ta ampoules at 900°C. A lower limit of the homogeneity range, n ≈ 2/3, has been determined from dehydrogenation experiments. All YIH_n phases adopt the ZrBr-type heavy-atom structure. The hydrogen variation is accompanied by a change in the c lattice constant from 31.162(3) to 31.033(1) A for n = 0.61(3) to 1.02(3). The YIH_n phases reversibly react with hydrogen at 400-600°C to form the light green transparent compound YIH₂. However, increasing the reaction temperature above 700°C causes decomposition to an unidentified phase being in equilibrium with YH₂ and YI₃. The arrangement of the heavy atoms in YIH₂ (P3m1; a = 3.8579(3) A, c = 10.997(1) A) corresponds to a four-layer I-Y-Y-I slab with the stacking sequence (AbaB) as was found by x-ray powder diffraction data refinement with the Rietveld method. A miscibility gap exists between YIH and YIH₂. Samples YIH_n (n ≤ 1.0) show metallic conductivity at room temperature, which changes into semiconducting behavior with decreasing temperature as n approaches its lower value ≈ 2/3.

Full text of DOI:10.1002/zaac.200400094

YIH : A New Family of Metal-rich Hydride Halides
n
Mikhail Ryazanov, Arndt Simon*, and Hansjürgen Mattausch
Stuttgart, Max-Planck Institut für Festkörperforschung
Received April 5th, 2004.
Dedicated to Professor Joachim Sieler on the Occasion of his 65th Birthday
¯
˚
2
(P3m1; a ϭ 3.8579(3) A,
Abstract. The graphite-like yttrium hydride halides, YIH
Յ 1.0), have been prepared in quantitative yields by heating either
YI , YH (1:2) or stoichiometric YI , YH , Y mixtures in sealed
Ta ampoules at 900°C. A lower limit of the homogeneity range,
n ഠ 2/3, has been determined from dehydrogenation experiments.
n
(0.8 Յ n
arrangement of the heavy atoms in YIH
˚
c ϭ 10.997(1) A) corresponds to a four-layer I-Y-Y-I slab with the
stacking sequence (AbaB) as was found by x-ray powder diffraction
data refinement with the Rietveld method. A miscibility gap exists
3
2
3
2
between YIH and YIH
2
.
All YIH
n
phases adopt the ZrBr-type heavy-atom structure. The
Samples YIH (n Յ 1.0) show metallic conductivity at room tem-
n
hydrogen variation is accompanied by a change in the c lattice con-
stant from 31.162(3) to 31.033(1) A for n ϭ 0.61(3) to 1.02(3). The
perature, which changes into semiconducting behavior with
decreasing temperature as n approaches its lower value ഠ 2/3.
˚
YIH
the light green transparent compound YIH
the reaction temperature above 700°C causes decomposition to an
unidentified phase being in equilibrium with YH and YI . The
n
phases reversibly react with hydrogen at 400-600°C to form
2
. However, increasing
Keywords: Yttrium hydride halides; Metal-rich compounds; Phase
relation; X-ray powder diffraction; Rietveld refinement; Resistivity
measurements; Electron localization
2
3
YIH : Eine neue Familie metallreicher Hydridhalogenide
n
Inhaltsübersicht. Hydridiodide YIH
quantitativer Ausbeute als graphitfarbene Substanzen durch Erhit-
zen von YI , YH (1:2) bzw. stöchiometrischen Gemengen von YI
YH , Y bei 900°C in zugeschweißten Tantalampullen präpariert.
Bei Dehydrierungsversuchen wurde der minimale H-Gehalt von
YIH mit n ϭ 0.61(3) bestimmt. Alle YIH Verbindungen kristalli-
n
(0.8 Յ n Յ 1.0) wurden in
Temperaturen über 700°C führen zur Zersetzung von YIH
Bildung von YH , gasförmigem YI und einer schwarzen unbe-
kannten Substanz. Die Schweratomstruktur von YIH wurde
durch Rietveld-Verfeinerung aus Röntgendaten bestimmt. Es kris-
tallisiert in P3m1, a ϭ 3.8579(3) A, c ϭ 10.997(1) A. Die Schwer-
atome sind in Schichten I-Y-Y-I mit der Stapelung AbaB angeord-
net. Zwischen YIH1.0 und YIH2.0 besteht eine Mischungslücke.
2
unter
2
3
3
2
3
,
2
2
¯
˚
˚
n
n
sieren im ZrBr-Typ. Mit steigendem H-Gehalt nimmt die Länge
˚
der c-Gitterkonstante von 31.162(3) zu 31.033(1) A ab. Die Verbin-
n
YIH Proben mit n Յ 1.0 zeigen metallische Leitfähigkeit mit ei-
dungen YIH
n
können reversibel unter Bildung der hellgrünen
nem Metall-Halbleiter Übergang bei sinkenden Temperaturen und
niedrigem H-Gehalt.
durchsichtigen Verbindung YIH bei 400-600°C hydriert werden.
2
Introduction
The structure of RXH consists of close-packed metal
atom bilayers, which are sandwiched between layers of hal-
ide ions to give the stacking sequences X-R-R-X as in ZrCl
n
The hydride halides RXH of scandium [1], yttrium [2] and
n
the trivalent lanthanides [3-10] with n Յ1 constitute a
closely related family of metal-rich compounds. They all are
graphite-like with a characteristic metallic luster and have
a range of homogeneity with an upper limit n ϭ 1.0. For
most of the compounds [2-8] the lower limit has been found
to approach a value of n ϭ 0.67. These conditions imply
the existence of valence electrons available for metal-metal
[
11, 12], ZrBr [13] or related “2s”-type structures [4]. The
known types of packing these four-layer slabs are rep-
resented in Figure 1. The hydrogen atoms occupy tetra-
hedral voids between the metal atom layers [3], and filling
of all the tetrahedral voids corresponds to the upper limit
RXH1.0
Further insertion of hydrogen into the structure can be
accomplished by heating RXH in a hydrogen atmosphere
.
3ϩ
-
bonding as one can infer from the ionic description R
X
-
-
n
(
H ) (e )2-n. Up to now the following phases have been syn-
n
at 400-500°C. This leads to the formation of transparent
thesized and studied: RClH (R ϭ Sc, Y, La, Ce, Pr, Gd,
n
and insulating RXH phases, in which the additional hydro-
2
Tb), RBrH (R ϭ Y, La-Nd, Gd, Tb) and GdIH .
n
n
gen atoms are distributed in the trigonal voids of the metal
atom layers, one atom in each basis of an antiprism [5].
There exists a gap of miscibility between RXH and RXH2
Prof. Dr. Dr. h. c. mult. Arndt Simon,
Max-Planck-Institut für Festkörperforschung,
Heisenbergstr. 1,
D-70569 Stuttgart,
Fax: 0711/689/1091
[
2, 7]. All known RClH (R ϭ Y, Ce, Pr, Gd, Tb, Lu),
2
RBrH (R ϭ Ce, Pr, Nd, Gd, Tb) and GdID phases [2, 8]
2
2
are isotypic and exhibit the same stacking sequence (3R
form) of the heavy atoms as in the high-T modification of
E-mail: A.Simon@fkf.mpg.de
Z. Anorg. Allg. Chem. 2004, 630, 1401Ϫ1407
DOI: 10.1002/zaac.200400094
 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
1401

M. Ryazanov, A. Simon, H. Mattausch
Fig. 1 Projections along [110] of the different stacking variants for the heavy-atom arrangements in hydride halides RXH
b) ZrBr type, (c) “2s” type and (d) RXH type. The small and capital letters refer to relative positions of close-packed metal and halide
ion layers, respectively.
n
: (a) ZrCl type,
(
2
Ta S C [14]. The transition from RXH , to RXH is ac-
Schlenk technique or in a glove box under argon. Further prep-
aration details are summarized in Table 1.
2
2
n
2
companied by significant changes in structure. The arrange-
ment of the halogen atoms within one XRRX slab redistri-
butes from trigonal prismatic to trigonal antiprismatic to
form a new layer sequence AbaB. Additionally, the inter-
Hydrogenation of YIH was carried out in a Sieverts-type apparatus
[17, 18] supplied with a manometer Membranovac DM12/DI2000
(Leybold), which allows to detect the pressure change during a re-
action. Typically, a weighted sample of approx. 0.3-0.5 g was heated
˚
layer metal-metal distances increase from 3.58 to 3.96 A
in a molybdenum boat under 1 bar H
ume of 370 cm .
2
in a closed calibrated vol-
upon conversion from TbClD0.8 to TbBrD2 [5]. The
changes in the atomic distances and the nonconducting be-
3
havior of RXH are consistent with an ionic description as
Experiments on dehydrogenation were performed by heating
samples covered with tantalum jackets in a quartz glass tube under
dynamic vacuum (10 torr).
2
3
ϩ
-
-
R
X (H ) .
2
-
5
While a great number of hydride monochlorides and
monobromides of the rare-earth elements are known, there
is much less information about the related monoiodide
phases. The present paper reports on some metal-rich and
salt-like compounds in the system Y/I/H, their phase re-
lations, structural features and transport properties.
Characterization
a) Chemical analysis
The hydrogen content n in YIH
Weighted samples (25-50 mg) were mixed with V O
2 5
n
was determined analytically.
used as an
oxidant and burned in an oxygen flow at 950 °C. The evolved gas
o
o
o
passed CuO (850 C), Ag
[
3
VO
4
(850 C), Ag
2
OϮMnO
2
(500 C)
Experimental Section
o
19], Ag-wool (500 C), and the generated water was titrated with
a Karl-Fischer coulometric detector KF 653 (Metrohm) [20]. The
hydrogen content was calculated as a mean value from two inde-
pendent measurements. The maximal spread of n calculated for the
Materials
Yttrium metal pieces (99.99 %; Johnson Matthey, Karlsruhe) and
iodine powder (99.8 %; Merck) were used as starting materials.
Yttrium triiodide was prepared by a direct reaction of the elements
in a sealed quartz glass tube at 900 °C [15] and further purified by
composition YIH was found to be ∆n ϭ 0.03.
n
b) X-ray powder diffraction
Due to their moisture- and air-sensitivity polycrystalline samples
-
4
two subsequent sublimations at 830 °C in high vacuum (10 torr).
YH was obtained from metal pieces by hydrogenation under 1 atm
(99.999 %; Messer-Griesheim) in a molybdenum boat at 400 °C
˚
YIH
n
(mixed with Si, a ϭ 5.4303 A, as an internal standard) were
2
sealed in glass capillaries filled with Ar. X-ray diffraction patterns
H
2
were recorded in Debye-Scherrer geometry on a Stoe Stadi P dif-
following the known procedure [16]. When no more pressure
change was observed the temperature was lowered to 350 °C, and
the sample was pumped until the pressure was reduced to about
˚
fractometer (MoK
α
radiation, λ ϭ 0.7093 A, germanium mono-
chromator). For a number of samples the heavy-atom arrangement
was refined by Rietveld analysis using the FullProf program [21].
Some details of data collection and the results of refinement are
shown in Table 2. The pseudo-Voigt function was used for simu-
lation of the peak shapes. All reflections were corrected for asym-
metry (2θ < 40°) and preferred orientation effects using the March-
Dollase function. Guinier diagrams were recorded on a modified
2
Ϯ10-3 torr, which corresponds to the equilibrium H-pressure for
2
YH at the given temperature.
Syntheses
1 2
Guinier camera (CuKα radiation, SiO monochromator) [22] with
an open aperture.
Yttrium hydride halides, YIH
similar conditions either from Y(filings)/YH
n
(n Յ 1), were synthesized under
/YI or from YH /YI
2
3
2
3
mixtures in arc-welded tantalum tubes filled with argon. These
were then sealed under vacuum in fused silica jackets and heated to
the reaction temperature. All manipulations were performed with
c) Resistivity measurements
Electrical resistivity was measured on pressed samples in the tem-
perature range of 8-250 K by the van der Pauw method using the
1402
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Z. Anorg. Allg. Chem. 2004, 630, 1401Ϫ1407

n
YIH : A New Family of Metal-rich Hydride Halides
Table 1 Reaction conditions and structural data for YIH
n
˚
˚
˚
3
N
Starting materials
Reaction condition
Product
a/A
c/A
V/A
Type
1
2
3
YI
YI
Y/YI
3
/YH
/YH
/YH
3 2
2
(1:2)
(1:2)
4d, 900°C
6d, 900°C
6d, 900°C
YIH1.02
YIH0.98
YIH0.81
3.9349(3)
3.9337(7)
3.9334(2)
31.033(1)
31.047(6)
31.096(2)
416.23(6)
416.1(1)
416.58(3)
ZrBr
ZrBr
ZrBr
3
2
(
19:20:21)
-
5
4
5
6
7
YIH0.98
YIH0.81
YIH0.81
YIH0.61
20 h, 730°C, 10 torr
YIH0.79
YIH0.61
YIH0.59
3.9340(7)
3.9326(3)
3.9324(2)
31.102(6)
31.162(3)
31.158(2)
416.85(9)
417.36(5)
417.27(3)
ZrBr
ZrBr
ZrBr
-
5
10 h, 730°C, 10 torr
-
5
10 h, 800°C, 10 torr
-
5
10 h, 900°C, 10 torr
3
YI , Y
Table 2 Some details of data collection and structure refinement
for YIH
In order to determine the lower limit of the homogeneity
n
range a series of dehydrogenation experiments for YIH in
n
a dynamic vacuum was carried out. Some experimental de-
tails and product descriptions are given in Table 1. Samples
4-7 have been heated in an “open” systemϪtantalum tube
sealed at one side and clinched at the otherϪfor several
hours until the pressure of approx. 10 torr in the system
reduced to its starting value of about 10-5 torr. The heating
of YIH0.81 in the range of 730-800 °C leads to a reduction
of the H-content to n ϭ 0.61(3). Further increase of the
temperature results in decomposition of the sample into
YIH0.61
Mo Kα
PSD
2-65
0.01
100
6300
236
YIH1.02
Mo Kα
PSD
2-65
0.01
100
5430
167
YIH2.0
Mo Kα
PSD
2-65
0.01
200
6300
246
20
0.0201
0.0321
0.0162
0.104
1.97
Radiation
Detector
Profile range (°2θ)
Step size (°2θ)
-3
Step scan time (s)
Number of collected points (N)
Number of reflections
Number of refined parameters (P)
15
18
R
p
0.0257
0.0346
0.0246
0.132
1.41
0.0421
0.0535
0.0513
0.132
1.04
R
wp
R
exp
volatile YI and Y, which suggests that the value n ഠ 0.67
R
S
B
3
represents the lowest hydrogen concentration in YIH , simi-
n
1
/2
lar to that in analogous YXH
hydride halides [2].
n
(X ϭ Cl, Br; 0.7 Յ n Յ 1.0)
In all the homogeneity range, 0.6 Յ n Յ 1.0, YIH crys-
n
2
2
Note: R
p
ϭ
͚
͉y
i
Ϫyc,i͉͞
͚
y
i
, Rwp
ϭ
͚
w
i
͉y
i
Ϫyc,i͉ ͞
͚
wy
i
΅ ,
΄
iϭ1,N
iϭ1,N
/2
iϭ1,N
iϭ1,N
1
2
R
exp ϭ (NϪP)͞
͚
wy
i
B
΅ , R ϭ
͚i ͉Iobs,iϪIcalc,i͉͞ i
͚
͉Iobs,i͉, S ϭ Rwp /Rexp
.
΄
iϭ1,N
tallizes with the ZrBr-type structure. The lattice constants
as a function of n are plotted in Figure 2. There is a distinct
˚
decrease (0.13 A) in the length of the c axis, whereas the a
˚
cryostat of a PPMS magnetometer (Quantum Design). The samples
were pressed together with 4 platinum contacts.
lattice constant slightly (0.003 A) increases with larger n.
Similar variations of the lattice constants were found in
RClH (R ϭ Sc [1], Y [2]) though these are accompanied
n
by a structural change from the ZrBr- to the ZrCl-type
structure with increasing H-content. This structural re-
arrangement might be accounted for by a subtle charge re-
distribution in the metal-metal bonding. The change of the
lattice constants can be rationalized in terms of an electro-
Results and Discussion
Attempts to synthesize YI H from YI /YH (2:1) reaction
mixtures by analogy with known hydride halides RI Hn
2
n
3
2
2
(R ϭ LaϪNd, Gd) [18, 23-24] were unsuccessful. Heating
-
the reagents at 820°C revealed no reaction, while increasing
the temperature to 900°C led to the formation of YIHn
static model [18], assuming the formation of H anions by
localization of electrons from itinerant M-M bonding. In-
corporating the hydrogen ions in between double metal
atom layers results in a decrease of the c lattice parameter
(n Յ 1).
Heating YI /YH (1:2) as well as stoichiometric YI3/
3
2
due to Coulomb attraction between the M3 and H ions,
ϩ
-
YH /Y reaction mixtures in welded tantalum tubes results
2
in the formation of dark grey laminar YIH (0.8 Յ n Յ 1.0)
phases with a metallic luster. Both synthetic approaches
while the a lattice constant increases due to repulsion be-
tween the H ions in the layer.
n
-
quantitatively yield YIH single phase products as has been
The atomic distances for the phase boundaries, YIH0.61(3)
and YIH1.02(3), are shown in Table 3. The differentiation of
distances in the Y-Y sheets, 3.5 and 3.9 A, indicates strong
n
found with other RXH systems (R ϭ Sc, Y, LaϪNd, Gd,
n
¯
˚
Tb, Er, Lu; X ϭ Cl, Br) [110]. The hydrogen concentration
in halide hydrides can be varied either by using the appro-
priate amounts of starting reagents or by heating the YI3/
metal-metal bonding between adjacent metal atom layers
and a somewhat weaker bonding between atoms in a layer.
The latter is mainly dictated by the I-I distance in the close-
YH (1:2) mixture at different reaction conditions (tempera-
2
˚
ture, time, permeability of the container material to hydro-
gen). When a metal powder is included into the reaction
its possible hydrogen contamination should be considered,
particularly if it was obtained by dehydrogenation of the
metal hydride.
packed anion sheets. The Y-I distances are 3.08 A, a slightly
˚
shorter value than expected for pure ionic bonding (3.10 A)
due to the asymmetric bonding situation. The interlayer I-
˚
˚
I contacts (4.19 A) are 0.16 A less than those in crystalline
iodine which indicates a weak bonding between the halide
Z. Anorg. Allg. Chem. 2004, 630, 1401Ϫ1407
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M. Ryazanov, A. Simon, H. Mattausch
It appears that more “ionic” hydride chlorides crystallize
rather in the ZrBr structure type, whereas for more “polar-
ized” bromides and iodides there is no obvious benefit be-
tween the ZrBr- and ZrCl- type stacking as polarization
effects, although in a different form, occur in both cases.
For instance, GdIH0.8 has been found to crystallize with
both the ZrCl- and ZrBr- as well as intermediate “2s”-type
structures [4]. Calculations of the Madelung part of lattice
energy for TbClD0.8 with the ZrBr- and ZrCl-type stacking
sequences using the MAPLE program [27] yields an insig-
nificantly smaller (0.02 %) energy for the latter structure.
Phase relation between YIH and YIH2
n
Continuous heating of the YIH samples under 1 bar hydro-
gen in a closed system leads to light-green and transparent
Fig. 2 Lattice constants of YIH
n
as a function of n.
YIH . The product composition is derived from the differ-
2
ence of hydrogen pressure after cooling the sample. Some
reaction temperatures and heating rates used for hydrogen-
ation are listed in Table 4. Figure 3 shows the temperature
dependence of the H-pressure in a system with and without
the sample. A characteristic drop in pressure observed at
330 °C indicates the beginning of hydrogen absorption,
which is completed at 450 °C. The lowering of the absorp-
tion temperatures with reduced heating rates is in accord-
˚
Table 3 Selected bond distances/A in YIH
n
Compound
Y Ϫ Y
Y Ϫ I
I Ϫ I a)
4.188(7)
4.186(4)
4.198(6)
YIH0.61
YIH1.02
YIH2.0
3.933(1)
3.071(6)
3.082(4)
3.095(4)
a)
a)
a)
3
.553(8)
3.935(1)
.499(5)
3.858(1)
.851(6)
3
ance with expectations. YIH can easily be dehydrogenated
2
3
to the initial YIH phases by heating under dynamic vac-
n
a)
uum. The absorption and desorption of hydrogen occur in
the same temperature range as shown in Figure 3. Thus, the
hydrogenation of YIH to YIH is a fully reversible topoch-
interlayer distances.
n
2
emical reaction. However, heating YIH (YIH) in a hydro-
2
layers belonging to the neighboring IYYI slabs. Full occu-
pation of tetrahedral voids with hydrogen yields a 0.05 A
gen atmosphere to above 700 °C leads to decomposition
of the sample with the formation of an unidentified black
˚
decrease in the interlayer metal-metal distances and almost
no change in other bond lengths.
compound (Z) being in equilibrium with YH
2
and volatile
YI . This process is also accompanied by liberation of hy-
3
It is interesting to note that the hydride chlorides of rare
drogen as one can conclude from a distinctive pressure in-
crease in the system above 700 °C (Fig. 4). The pressure
difference after the sample has been cooled leads to the for-
mal composition “YIH1.33”. The I/Y ratio of the decompo-
sition product determined by an EDX analysis on several
laminar crystallites varies in the range 0.95-1.25, which
overlaps in composition with that of several metal-rich clus-
ter compounds [28]. The XRD pattern clearly differs from
earth metals, RClH (R ϭ La, Ce, Pr, Gd) adopt the ZrBr-
n
type stacking, while the analogous hydride bromides exhibit
the ZrCl-type stacking. The two types of stacking may arise
from a competition between electrostatic second nearest
neighbor effects versus polarization effects between adjacent
metal and halide ion layers. In the ZrBr-type stacking each
halogen atom is arranged above a second nearest metal
atom in the neighboring layer so that it allows additional
attraction between the metal ions and second nearest hal-
ogen neighbors. Such a layer arrangement also favors polar-
ization of the halide atoms by the second nearest metal
atoms along the [001] direction through interstices in the
neighboring halide layer (Fig. 1). The interlayer antipris-
matic coordination of the halogen atoms by Cl and Zr
atoms in ZrCl induces a linear arrangement of alternating
pairs of metal and halogen atoms [25] in the (1-10) and
equivalent (120) and (210) planes, which implies a predomi-
nant polarization of the halogen atoms by adjacent metal
atoms along the XϪRϪRϪX atomic strings. Similar effects
patterns of YIH
(0.6 Յ n Յ 1.0) and YIH . It exhibits two
n
2
sets of diffraction peaks, characteristic broad reflections
1)
and sharp ones .
Crystal structure of YIH2
The structural study of YIH has been performed on po-
2
lycrystalline samples since suitable singe crystals were not
available. All reflections have been indexed in a primitive
˚
trigonal unit cell with a ϭ 3.8579(3) A and c ϭ 10.997(1)
in related CdI and CdCl structures have been pointed out
by Krebs [26].
1)
˚
2
2
d values (A) of sharp reflections: 7.676, 3.842, 3.441, 2.562,
1.9924, 1.9224, 1.7702, 1.7230, 1.3834.
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Z. Anorg. Allg. Chem. 2004, 630, 1401Ϫ1407

n
YIH : A New Family of Metal-rich Hydride Halides
Table 4 Reaction conditions on (de)hydrogenation of YIH
n
˚
˚
˚
N
Reagent
Heating condition
Product
Space group
a/A
c/A
V/A3
Type
1
2
3
4
5
YIH
YIH
YIH
YIH
YIH
500°C, 2 °/min, 1bar H
550°C, 2 °/min, 1bar H
750°C, 3 °/min, 1bar H
750°C, 2 °/min, 1bar H
2
2
2
2
YIH2.0
YIH2.0
P 3¯ m1
P 3m1
3.8579(3)
3.8560(6)
10.997(1)
11.001(6)
141.74(4)
141.64(7)
1T
1T
¯
YI
YI
3
, YH
, YH
2
ϩ Z
ϩ Z
2
2
3
2
-
5
a)
¯
700°C, 1 °/min, 10 torr
YIH
n
R 3m
3.9328(6)
31.094(5)
416.50(6)
ZrBr
a)
dynamic vacuum.
Fig. 3 Left: Temperature dependence of the hydrogen pressure upon hydrogenation of YIH at a heating rate of 3°C/min (ᮀ) and 2°C/min
᭺), respectively. Points (᭡) correspond to the pressure change in an “empty” system (heating rate: 3°C/min). Right: Temperature depen-
(
dence of pressure (ϩ) in a system upon dehydrogenation of YIH
hydrogenation of YIH (heating rate: 2°C/min).
2
under dynamic vacuum (heating rate: 1°C/min) compared with (᭺)
Fig. 4 Temperature dependence of the hydrogen pressure in a sys-
2
Fig. 5 (a) The final Rietveld refinement pattern of YIH . The calcu-
lated profile is denoted by “᭺”; the solid line corresponds to the
observed pattern; vertical bars indicate the calculated reflection
tem with (᭺) and without (ᎏ) YIH
2
at a heating rate of 2°C/min.
2 2
positions for YIH (upper) and YH (bottom). The bottom trace
represents the difference between the experimental and calculated
values.
˚
A. The arrangement of the heavy atoms has been refined
from X-ray powder diffraction data by the conventional
Rietveld method [29]. The analysis of the pattern profile
reveals a small contamination (2 %) of the sample with
5. Atomic positions and selected bond distances for YIH2
are given in Table 5 and 3, respectively. The final profile
YH . The results of the final refinement are plotted in Fig.
2
Z. Anorg. Allg. Chem. 2004, 630, 1401Ϫ1407
zaac.wiley-vch.de
 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
1405

M. Ryazanov, A. Simon, H. Mattausch
Table 5 Atomic positions of the heavy-atom arrangement of
a)
YIH
2
˚
2
Atom
Site
x
y
z
Biso (A )
Y
I
2d
2d
2/3
1/3
1/3
2/3
0.3572(4)
0.1618(3)
0.8(1)
1.8(1)
a)
˚
˚
¯
a ϭ 3.8579(3) A, c ϭ 10.997(1) A; space group: P 3m1
factors, R and R , converged to 2.01 and 3.21 % close to
p
wp
an expected value Rexp ϭ 1.62 %. However, there is a misfit
in intensity for a number of diffraction peaks. The calcu-
lated intensities for the reflections 00l, hhl and 100 are lower
than the observed ones, while those with indices 10l (l ϭ 2n)
are larger. These deviations can be attributed to preferred
orientation and/or stacking defects in the “weak” direction.
Both the effects occur frequently in layered compounds.
Obviously, the preferred orientation effects dominate, as
Fig. 6 Electrical resistivity ρ against T for YIHn (n Յ 1.0). The
inset picture shows the temperature dependence of ρ for YIH0.61
on a large scale.
the pattern taken with less penetrating CuKα radiation
1
shows even increased deviations of the above mentioned
intensities. Indeed, the Guinier diagram of a sample re-
corded on a modified Guinier camera [22] with an open
aperture reveals a nonconstant intensity distribution along
the Debye rings, which is a typical feature for samples affec-
ted by preferred orientation.
electrons, suggesting Anderson-type localization of elec-
trons [33] in multi-center metal-metal bonding. The latter
can originate from an ordering of the hydrogen atoms as n
approaches the lower limit of the homogeneity range [10].
The electron localization in YIH0.61 is further confirmed by
magnetic measurements, which reveal a decrease of Pauli
paramagnetism in comparison with that for YIH0.81 [34].
The structure of YIH has the same heavy atom arrange-
2
ment as in related TbBrD [5]. It consists of close-packed
2
Acknowledgement. We are grateful to C. Kamella for EDX analyses
and G. Siegle for the resistivity measurements.
bilayers of metal atoms enclosed within two close-packed
halide sheets. However, in contrast to all known RXH2
phases, the translation period of YIH corresponds to one
2
slab IYYI with the stacking sequence AbaB (1T form),
which is the characteristic sequence for ZrXH (X ϭ Cl, Br)
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[
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Z. Anorg. Allg. Chem. 2004, 630, 1401Ϫ1407
zaac.wiley-vch.de
 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
1407