X-ray and neutron powder diffraction study of the order-disorder transition in Eu2IrH5 and the mixed crystal compounds Eu2-xAxIrH5 (A = Ca, Sr; x = 1.0, 1.5)

Article abstract of DOI:10.1016/S0022-4596(03)00171-3

The title compounds and their deuterides have been prepared by solid-state and solid-gas reactions from the elements and investigated by X-ray and neutron powder diffraction as a function of temperature. At room temperature they crystallize with an anion-deficient cubic K₂PtCl₆-type structure (space group Fm3m) in which five hydrogen (deuterium) atoms surround iridium randomly on six octahedral sites with average bond distances of Ir-D=169-171pm. At low temperature they undergo a tetragonal deformation (space group I4/mmm) to the partially ordered Sr₂IrD₅ (T=4.2K)-type structure in which four hydrogen (deuterium) atoms occupy planar sites with full occupancy (Ir-D=166-170pm) and two hydrogen (deuterium) atoms axial sites (Ir-D=174-181pm) with ～50% occupancy, i.e., the data are consistent with a mixture of square-pyramidal [IrD₅]^4- complexes pointing in two opposite directions. The transitions occur at ～240K (Eu_0.5Ca_1.5IrD₅, Eu_0.5Sr_1.5IrD₅), ～210K (EuSrIrD₅), ～200K (EuCaIrD₅, Eu₂IrD₅), and are presumably of first order.

Full text of DOI:10.1016/S0022-4596(03)00171-3

ARTICLE IN PRESS
Journal of Solid State Chemistry 174 (2003) 35–43
X-ray and neutron powder diffraction study of the order–disorder
transition in Eu2IrH5 and the mixed crystal compounds Eu2ꢀxAxIrH5
(A ¼ Ca; Sr; x ¼ 1:0; 1.5)
H. Kohlmann,a,b,Ã R.O. Moyer Jr.,c T. Hansen,d and K. Yvona
a
´
`
`
Laboratoire de Cristallographie, Universite de Geneve, Quai E. Ansermet 24, 1211 Geneve 4, Switzerland
b FR 8.14 Anorganische und Analytische Chemie und Radiochemie, Universita¨t des Saarlandes, Postfach 15 11 50, 66041 Saarbru¨cken, Germany
c Department of Chemistry, Trinity College, Hartford, CT 06106-3100, USA
d Institut Laue-Langevin, Avenue des Martyrs, BP 156, 38042 Grenoble Cedex, France
Received 14 November 2002; received in revised form 6 March 2003; accepted 15 March 2003
Abstract
The title compounds and their deuterides have been prepared by solid-state and solid–gas reactions from the elements and
investigated by X-ray and neutron powder diffraction as a function of temperature. At room temperature they crystallize with an
%
anion-deficient cubic K2PtCl6-type structure (space group Fm3m) in which five hydrogen (deuterium) atoms surround iridium
randomly on six octahedral sites with average bond distances of Ir–D=169–171 pm. At low temperature they undergo a tetragonal
deformation (space group I4/mmm) to the partially ordered Sr2IrD5 (T ¼ 4:2 K)-type structure in which four hydrogen (deuterium)
atoms occupy planar sites with full occupancy (Ir–D=166–170 pm) and two hydrogen (deuterium) atoms axial sites (Ir–D=174–
181 pm) with B50% occupancy, i.e., the data are consistent with a mixture of square–pyramidal [IrD5]4ꢀ complexes pointing in two
opposite directions. The transitions occur at B240 K (Eu0.5Ca1.5IrD5, Eu0.5Sr1.5IrD5), B210 K (EuSrIrD5), B200 K (EuCaIrD5,
Eu2IrD5), and are presumably of first order.
r 2003 Elsevier Science (USA). All rights reserved.
Keywords: Complex metal hydrides; Metal–hydrogen complexes; Iridium–hydride complexes; Order–disorder transition; Neutron powder
diffraction
1. Introduction
progress in the structure determination by neutrons of
heavy absorbing europium hydrides [5–9], it seemed
worthwhile to reinvestigate Eu2IrD5 and to study
further members of the previously reported solid
solution series Eu2ꢀxCaxIrD5 and Eu2ꢀxSrxIrD5 [10].
Iridium-based compounds of composition A2IrH5
(A=alkali-earth or divalent rare-earth element) belong
to the class of the so-called ‘‘complex transition metal
hydrides’’ (for a recent reviewsee Ref. [1]). At room
temperature they crystallize with a hydrogen-deficient
%
cubic K2PtCl6-type structure (space group Fm3m) in
2. Experimental details
which five hydrogen (deuterium) atoms surround
iridium randomly on six octahedral sites. At low
temperature some members undergo a tetragonal
deformation to a partially ordered structure such as
Sr2IrD5 (space group I4/mmm) [2,3], while others appear
not to transform such as Eu2IrD5 for which no
transition has been observed down to T ¼ 4 K [4].
Given the close structural similarity between europium-
and strontium-based hydrides [5,6] and in viewof recent
2.1. Synthesis
Freshly prepared samples of composition Eu2ꢀxAx-
IrD5/H5 (A=Ca, Sr; x ¼ 0:0; 1.0, 1.5) were obtained
following the procedures described earlier [10,11]. All
metal constituents consisted of natural isotope mixtures
(Ir: Electronic Space Products International, Inc.; Ca:
K&K Laboratories; Eu, Sr: A.D. Mackay, Inc.). Care
was exercised in order to reduce sources of oxygen
contamination in the formation of these hydrides. Even
though the purity level as advertised by the vendor was
ÃCorresponding author. Fax: +1-681-302-4233.
E-mail address: h.kohlmann@mx.uni-saarland.de (H. Kohlmann).
0022-4596/03/$ - see front matter r 2003 Elsevier Science (USA). All rights reserved.
doi:10.1016/S0022-4596(03)00171-3

ARTICLE IN PRESS
H. Kohlmann et al. / Journal of Solid State Chemistry 174 (2003) 35–43
36
3 N, it was essential to vacuum distill the calcium,
strontium, and europium metals at 1.3 mPa and 1073 K.
The purified metal was deposited onto a cold finger and
the metal oxide contaminant remained in the distilling
cup and was discarded. The 3 N purity iridium metal
powder was oxide free and used as received. All of the
metal and metal hydride handling was done in an
atmosphere of argon. The 3 N pure hydrogen was
passed through a heated palladium tube filter prior to
any hydriding. CP grade (99.5% pure) deuterium was
purchased from Matheson Gas Products and used
without any further purification.
Binary metal hydrides were formed by first placing
small chips (B3–4 mm in length) of calcium, strontium,
or europium in a molybdenum boat. The boat was
inserted into a quartz reaction tube and the tube was
then attached to an all glass vacuum line. The system
was evacuated to 1.3 mPa after which either deuterium
or hydrogen was introduced to approximately 10 kPa.
The binary metal hydride was formed by heating the
metal to approximately 670–770 K in approximately
10 kPa of gas. The metal hydride was removed from the
apparatus and the quartz tube was transferred to the
glove bag. The product was subsequently crushed by
rapidly gyrating the sample in a small (B2 mL) agate
ball mill. The powders were then mixed with iridium
metal powder (ꢀ325 mesh) in the appropriate mole
ratios and the homogeneous mass was compressed at
550 MPa. The resultant pellet was placed in a molybde-
num boat that was inserted into a quartz reaction tube.
The tube was attached to a glass vacuum line and
evacuated. Hydrogen (deuterium) was introduced to
approximately 10 kPa and the sample heated to 973 K.
After completion of the hydrogenation reaction the
products were removed from the furnace and crushed
easily into a powder for subsequent structural analyses.
The mixed crystal metal hydride formation is described
according to the following equation:
these reflections are marked in Fig. 1 for EuCaIrD5.
Structure refinements were carried out with the program
DBWS-9411 [12]. The 2y scale was calibrated by fitting a
second-order polynomial against the reflections of the
internal standard silicon. For the Rietveld refinements
scale factors, lattice parameters, overall Debye–Waller
factors and peak shape parameters (pseudo-Voigt
function) were refined. Iridium was detected as a
minority phase in all samples, whereas EuO was
detected only in Eu0.5Sr1.5IrD5 and Eu2IrH5. The
contribution of hydrogen (deuterium) to the patterns
was neglected. The atomic displacement and peak shape
parameters of the minority phases were constrained to
be equal to those of the main deuteride phase. Residuals
were in the ranges 9%oRpo12%, 12%oRwpo15%,
1:3oSo1:6: Lattice parameters at room and low
(7–10 K) temperature are listed in Table 1. Fig. 2a–d
shows the temperature dependence of the lattice
parameters of: (a) EuSrIrD5, (b) Eu0.5Sr1.5IrD5,
(c) EuCaIrD5, and (d) Eu0.5Ca1.5IrD5.
2.3. Neutron powder diffraction
In order to reduce effects due to neutron absorption
by europium the samples (1.5 g Eu0.5Ca1.5IrD5, 1.2 g
ð2 ꢀ xÞEuH2ðsÞ þ ðxÞAH2ðsÞ þ IrðsÞ þ 1=2H2=D2ðgÞ
-Eu2ꢀxAxIrH5=D5ðsÞ ðA ¼ Ca; Sr; x ¼ 0:0; 1:0; 1:5Þ:
2.2. X-ray powder diffraction
X-ray powder diffraction data for Eu2IrH5,
Eu0.5Ca1.5IrD5, EuCaIrD5, EuSrIrD5, and Eu0.5Sr1.5IrD5
were taken in the temperature interval 7–298 K on a
Guinier powder diffractometer equipped with a closed
cycle He cryostat (Huber G645, CuKa1 radiation,
18ꢁp2yp100ꢁ; D2y ¼ 0:02ꢁ; 20 s data collection time
per step, sample enclosed between two PET foils,
internal standard silicon). The room temperature data
confirmed the cubic K2PtCl6-type metal atom arrange-
ment, whereas the low temperature data showed a
splitting of Bragg reflections indicative for the occur-
rence of a cubic-to-tetragonal phase transition. Some of
Fig. 1. The phase transition in EuCaIrD5 as studied by temperature-
dependent X-ray powder diffraction. The splitting of two peaks,
(022)cubic and (113)cubic, of the cubic phase into (112)tet/(200)tet and
(103)tet/(211)tet of the tetragonal phase is shown on cooling down (left)
and warming up (right).

ARTICLE IN PRESS
H. Kohlmann et al. / Journal of Solid State Chemistry 174 (2003) 35–43
37
Table 1
Lattice parameters of Eu2IrH5 and Eu2-xAxIrD5 (A=Sr, Ca; x ¼ 1:0; 1.5) as derived from Rietveld refinement on X-ray powder diffraction data
%
Cubic K2PtCl6 type, Fm 3m
Tetragonal Sr2IrD5 (T ¼ 4 K) type, I4=mmm
Eu2IrH5
a(T ¼ 298 K)=758.24(4) pm
a(T ¼ 7 K)=527.59(6) pm
c(T ¼ 7 K)=775.4(1) pm
EuSrIrD5
a(T ¼ 293 K)=760.55(3) pm
a(T ¼ 298 K)=762.10(2) pm
a(T=296 K)=741.75(2) pm
a(T=298 K)=733.42(2) pm
a(T ¼ 7 K)=528.78(7) pm
c(T ¼ 7 K)=776.33(18) pm
Eu0.5Sr1.5IrD5
EuCaIrD5
a(T=10 K)=530.15(3) pm
c(T ¼ 10 K)=778.32(6) pm
a(T ¼ 8 K)=515.80(4) pm
c(T ¼ 8 K)=757.49(11) pm
Eu0.5Ca1.5IrD5
a(T ¼ 10 K)=509.54(3) pm
c(T ¼ 10 K)=751.36(7) pm
Fig. 2. Phase transition in: (a) EuSrIrD5, (b) Eu0.5Sr1.5IrD5, (c) EuCaIrD5, and (d) Eu0.5Ca1.5IrD5 studied by temperature-dependent X-ray (filled
symbols) and neutron powder diffraction (open symbols). The error bars correspond to 7s of the lattice parameters as refined by the Rietveld
method.

ARTICLE IN PRESS
H. Kohlmann et al. / Journal of Solid State Chemistry 174 (2003) 35–43
38
EuSrIrD5, and 3.0 g Eu2IrD5) were filled in double-
walled vanadium cans (64 mm length, 9.15 mm inner
diameter of the outer cylinder, 0.6 mm annular thick-
ness, hermetically closed by an indium wire) and
neutron wavelengths close to the minimum of the
neutron absorption cross-section of natural europium
were chosen [8,13]. Except for Eu2IrD5 all data were
collected on the powder diffractometer D4b (ILL,
Grenoble, France) as equipped with two 64-cell 3He
multi-detectors covering 6.4ꢁ and 12.8ꢁ in 2y; respec-
tively. Due to a failure of the hot source it was operating
only at B30% of its original neutron flux. Wavelengths
and zero shifts were determined to 70.647(8) pm and
ꢀ0.1713(2)ꢁ, respectively, by measuring a standard
nickel sample. For Eu0.5Ca1.5IrD5 and EuSrIrD5 data
were collected as a function of temperature with a step
size of 1.9ꢁ and a resolution of 0.1ꢁ during 3.75 h. For
Eu0.5Ca1.5IrD5 four data sets were recorded at T ¼ 298;
240, 200, and 100 K and at intervals of B1–1.5 h
between each measurement. Due to technical problems
with the temperature control the EuSrIrD5 sample had
to be mounted in the cold cryostat at 123 K. During the
3.5 h scan the temperature dropped inadvertently from
123 to 116 K. The sample was then cooled to a constant
temperature of 1.6 K and diffraction data were collected
at this temperature over a period of 7 h. At this point the
cryostat was disengaged from the sample, the sample
was warmed to room temperature, and diffraction data
were then collected once again over a period of 3.5 h.
For Eu2IrD5 data were taken on D20 (ILL, Grenoble,
France) equipped with a large-area linear curved
position-sensitive detector, at constant temperatures of
298 K (1.1 h), 100 K (3.4 h), and 2 K (3.4 h) with cooling
rates of 100 K/h between data collections. Wavelength
and zero shift were determined to 79.923(8) pm and
0.776(5)ꢁ, respectively, from a measurement of a
standard silicon sample.
2.4. Structure refinements
Crystal structures were refined with the Rietveld
method (computer program FullProf [14]) by using
pseudo-Voigt functions for reflection profiles. The
background was modelled by linear interpolation
between 15 and 30 points. The neutron wavelengths
and zero points of the diffractometers were fixed at the
experimentally determined values (see above), and
secondary phases (EuO, Ir) and vanadium (sample
holder) were included in the refinements. The reflection
profiles and half-width parameters were constrained to
be equal to those of the main phase. For reasons
outlined before [15] the data were not corrected for
absorption effects (m ¼ 6:1; and 9.5 cmꢀ1 at
l ¼ 70:647 pm for Eu0.5Ca1.5IrD5 and EuSrIrD5, respec-
tively,
and
m ¼ 18:7 cmꢀ1
for
Eu2IrD5
at
l ¼ 79:923 pm). All phases were found to crystallize
with an anion-deficient cubic K2PtCl6-type structure
%
(space group Fm3m) having deuterium site occupancies
of B5=6: For the mixed crystals the assumption of
random occupancy of A sites was confirmed. On
lowering the temperature a splitting of reflections and
the occurrence of newreflections indicated the onset
of a structural phase transition. Reflection conditions
were consistent with space group I4=mmm: No reflec-
tions (hkl) with h þ k þ la2n were observed that
would indicate a further lowering of symmetry. The
Table 2
Crystal structure data of cubic Eu2IrD5 (T ¼ 298 K), EuSrIrD5 (T ¼ 298 K), and Eu0.5Ca1.5IrD5 (T ¼ 298 and 240 K) refined from neutron powder
diffraction data
%
Space group Fm3m (no. 225)
Atom
Site
Symmetry
x=a
y=b
z=c
A
Ir
D
8c
4a
ꢀ43m
1/4
0
1/4
0
1/4
0
%
m3m
4m:m
24e
x
0
0
Compound
T=K
Eu2IrD5
298
EuSrIrD5
298
Eu0.5Ca1.5IrD5
1/4Eu+3/4Ca
298
733.41(7)
240
733.1(2)
a/pm
A
756.7(2)
Eu
1.1(2)
759.5(2)
1/2Eu+1/2Sr
2.3(2)
Biso(A)/104 pm2
1.1(1)
0.68(7)
1.1(2)
0.6(1)
Biso(Ir)/104 pm2
x(D)
0.8(2)
0.223(1)
2.0(1)
2.2(2)
0.225(1)
3.0(2)
0.2309(5)
1.75(9)
0.833(11)
0.2300(9)
1.5(2)
Biso(D)/104 pm2
Occupation (D)
0.855(27)
Eu2IrD5.13(16)
0.021
0.882(22)
EuSrIrD5.29(13)
0.014
0.808(16)
Eu0.5Ca1.5IrD4.85(10)
0.013
Refined composition
Rp
Eu0.5Ca1.5IrD5.00(7)
0.014
0.016
0.070
2.33
Rwp
RBragg
S
0.026
0.108
1.23
0.017
0.100
1.71
0.016
0.074
2.23

ARTICLE IN PRESS
H. Kohlmann et al. / Journal of Solid State Chemistry 174 (2003) 35–43
39
Table 3
Crystal structure data of tetragonal Eu2IrD5 (T ¼ 100 K), EuSrIrD5 (T ¼ 120 K), and Eu0.5Ca1.5IrD5 (T ¼ 100 and 200 K) refined from neutron
powder diffraction data
Space group I4=mmm (no. 139)
Atom
Site
Symmetry
x=a
y=b
z=c
%
4m2
A
4d
2a
8h
4e
0
0
x
0
1/2
0
1/4
0
Ir
4=mmm
m:2m
4mm
D1
D2
x
0
0
z
Compound
T=K
Eu2IrD5
100
EuSrIrD5
120
Eu0.5Ca1.5IrD5
1/4Eu+3/4Ca
100
509.3(2)
751.4(3)
200
510.4(1)
750.0(3)
a=pm
c=pm
527.0(1)
773.0(4)
Eu
529.11
776.61
1/2Eu+1/2Sr
3.0(5)
A
Biso(A)/104 pm2
Biso(Ir)/104 pm2
x(D1)
0.6(2)
0.6(2)
1.2(2)
1.1(1)
1.1(2)
1.1(1)
0.1(1)
0.228(1)
0.226(4)
1.2(2)
0.222(3)
0.259(6)
2.3(4)
0.2339(8)
0.241(3)
1.8(2)
0.2317(7)
0.239(2)
2.1(2)
z(D2)
Biso(D1)/104 pm2
Biso(D2)/104 pm2
Occupation (D2)
Refined composition
Rp
2.0(9)
5(1)
1.0(6)
0.512(24)
Eu0.5Ca1.5IrD5.02(5)
1.2(6)
0.546(42)
Eu2IrD5.09(8)
0.010
0.78(9)
EuSrIrD5.5(2)
0.016
0.528(16)
Eu0.5Ca1.5IrD5.06(3)
0.013
0.014
0.018
0.086
2.42
Rwp
0.012
0.083
1.87
0.019
0.170
2.47
0.016
0.062
2.13
RBragg
S
low-temperature structures were described with the
partially ordered tetragonal Sr2IrD5-type structure [2].
The refined occupancies of the deuterium sites led to a
value not significantly different from 1.0 for D1 and to
values close to 0.5 for D2. In all cases the phase
transition was not complete and some cubic phase
remained down to the lowest investigated temperatures.
Due to the poor quality of the 120 K data of EuSrIrD5
its lattice parameter had to be held constant at the value
determined before from X-ray diffraction. Refinement
results are listed in Tables. 2 and 3 and interatomic
distances can be found in Table. 4. Graphical represen-
tations of the Rietveld refinements on the neutron data
are shown in Figs. 3–5.
Table 4
Selected interatomic distances in Eu2IrD5, Eu0.5Ca1.5IrD5, and
EuSrIrD5 in pm
Eu2IrD5 EuSrIrD5
Eu0.5Ca1.5IrD5
A
Eu
298
1=2Eu þ 1=2Sr 1=4Eu þ 3=4Ca
Cubic: T/K
A–12Da
Ir–6Da
Closest D–Da
298
298
268.3(6) 269.2(5)
168.8(9) 171.2(7)
238.7(9) 242.1(7)
259.7(3)
169.4(4)
239.5(4)
Tetragonal : T=K
A–8D1
100
120
100
269.0(5) 270.4(9)
264.2(2) 264.7(1)
169.9(9) 166(1)
260.3(3)
254.9(1)
168.6(4)
181(2)
A–4D2b
Ir–4D1
Ir–2D2b
174(3) 201(4)
Closest D–D (D1–D1) 240.2(9) 235(2)
238.4(6)
a occupancy B5=6; see Table 2.
b occupancy B1=2; see Table 3.
3. Results and discussion
3.1. Cubic room temperature modification
D/Ir=5.0. In the mixed crystals the cations A (Eu2+
,
At room temperature all members of the series
Eu2ꢀxAxIrH(D)5 (A=Ca, Sr; x ¼ 0:0; 1.0, 1.5) crystal-
lize with an anion-deficient disordered K2PtCl6-type
structure. As shown for Eu2IrD5 in Fig. 6 iridium is
surrounded by an average of B5 hydrogen (deuterium)
atoms that are randomly distributed on octahedral sites.
The refined deuterium contents for the various members
do not differ significantly from the molar ratio of
Ca2+, Sr2+) are randomly distributed, in agreement
with a previous study [10]. As expected from the ionic
radii [16], the cubic cell parameter decreases consider-
ably on replacement of europium by calcium, whereas it
increases slightly on replacement by strontium (Table 1).
The average iridium–deuterium distances are fairly con-
stant throughout the series (169–171 pm, Table 4) w hich is
accomplished by adjusting the free positional parameter x

ARTICLE IN PRESS
H. Kohlmann et al. / Journal of Solid State Chemistry 174 (2003) 35–43
40
Fig. 3. Observed (top), calculated (middle) and difference (observed—calculated, bottom) neutron powder diffraction patterns of Eu2IrD5 at
(a) T ¼ 298 K and (b) T ¼ 100 K (D20, l ¼ 79:923ð8Þ pm). All calculated patterns include the background. Intensity in total detector counts.
Fig. 4. Observed (top), calculated (middle) and difference (observed—calculated, bottom) neutron powder diffraction patterns of EuSrIrD5 at
(a) T ¼ 298 K and (b) T ¼ 120 K (D4b, l ¼ 70:647ð8Þ pm). All calculated patterns include the background. Intensity in total detector counts.

ARTICLE IN PRESS
H. Kohlmann et al. / Journal of Solid State Chemistry 174 (2003) 35–43
41
Fig. 5. Observed (top), calculated (middle) and difference (observed—calculated, bottom) neutron powder diffraction patterns of Eu0.5Ca1.5IrD5 at
(a) T ¼ 298 K and (b) T ¼ 100 K (D4b, l ¼ 70:647ð8Þ pm). All calculated patterns include the background. Intensity in total detector counts.
Fig. 6. Crystal structure of Eu2IrD5. (a) cubic modification at T ¼ 298 K, D sites showrandom occupancy of E56; (b) tetragonal modification at
T ¼ 100 K, planar D sites fully occupied, apical D sites half occupied. The broken line in (a) shows the relation to the tetragonal unit cell in (b).
of deuterium on the 24e position (x00) according to the
varying lattice parameter (Table 2). The average cation
(A)–deuterium distances, on the other hand, vary with the
lattice parameter, i.e., they are considerably shorter for
calcium containing samples (260 pm in Eu0.5Ca1.5IrD5)
than for strontium containing samples (268 pm in Eu2IrD5

ARTICLE IN PRESS
H. Kohlmann et al. / Journal of Solid State Chemistry 174 (2003) 35–43
42
and 269 pm in EuSrIrD5, see Table 4). The D–D
separations are all longer than the so-called ‘‘blocking
distance’’ of 210 pm. For cubic Eu0.5Ca1.5IrD5 no
significant change in structural parameters occurs on
cooling to 240 K (Table 2).
Fm 3m
K2PtCl6
A2IrD5 (A=Ca,Sr,Eu ; T=298K)
t3
3.2. Tetragonal low-temperature modification
½ (a-b), ½ (a+b), c
Upon cooling the members of the Eu2ꢀxAxIrH(D)5
series all transform to a tetragonal low-temperature
modification having the cell parameter relationships
ctetEacub; atetEacub=O2: These findings contrast with
earlier neutron diffraction studies on Eu2IrD5 for which
no phase transition was found down to 4 K [4]. As
shown in Fig. 1 the splitting of two typical diffraction
peaks of cubic EuCaIrD5, (022) and (113) is fully
reversible on cooling and warming up and shows a
hysteresis of about 2 K. The temperature-dependent cell
parameters for the mixed crystals as shown in Fig. 2
suggest that the transitions start at 200–240 K, depend-
ing on the degree of substitution and the nature of the
cations A, and proceed down to about 150 K with a
concomitant increase in cell distortion. A quantitative
phase analysis (data not shown) shows that the
proportion of tetragonal phase increases as the tem-
perature is lowered and that 8–13% of the samples
remain cubic even after slowcooling and prolonged (4
days) exposure at the lowest investigated temperature
(7 K). This provides evidence for the transitions to
be of first order. Clearly, the transitions in the
Eu2ꢀxAxIrH(D)5 series are due to an ordering of
deuterium atoms. However, in contrast to the cobalt
analogue Mg2CoD5 [17] in which the deuterium atoms
become fully ordered and form square–pyramidal
[CoH5]4ꢀ complexes, they are only partially ordered as
in Sr2IrD5 (T ¼ 4:2 K). Among the six available sites
only four showfull occupancy (‘‘planar sites’’ D1) and
two B50% occupancy (‘‘axial sites’’ D2), i.e., square–
pyramidal [IrD5]4ꢀ complexes are presumably pointing
in two opposite directions (Fig. 6b). As with Mg2CoD5
and Mg2IrD5 the transition metal–deuterium bond
distances are longer for the axial (IrꢀD=174–181 pm)
than the planar sites (IrꢀD=166–170 pm) and correlate
with an increase of the parameter ratio of the pseudo-
cubic cell (c=a ¼ 1:04). The only ordered, although
distorted square–pyramidal [IrD5]4ꢀ complex known so
far are that in Mg6Ir2D11 [18].
I4/mmm
A2IrD5 (A=Ca,Sr,Eu; T=100 K)
Na2PtD4
k2
(¼, ¼, ¼)
P4/nmm
Mg2CoD5
Fig. 7. Ba¨rnighausen tree of the group–subgroup relationships between
some cubic and tetragonal A2MD6, A2MD5, and A2MD4 compounds.
gonal structure of which one is fully and the other half
occupied. No indication exists for a further symmetry
reduction by a klassengleiche transition of index 2 to
space group P4=nmm such as for Mg2CoD5. The space
group relation between the high- and low-temperature
modification of the Eu2ꢀxAxIrH(D)5 series are consis-
tent with a second-order phase transition. However, the
rather abrupt changes in cell parameters as a function of
temperature (Fig. 2) together with the coexistence of
cubic and tetragonal phase down to very low-tempera-
ture point rather towards a first-order transition. The
role of oxygen impurities in these transitions cannot be
excluded. Previous studies in other metal hydride
systems have shown [19] that surface oxidation can
lead to incomplete order–disorder transitions. It is
possible that similar conditions prevailed in the present
Eu2ꢀxAxIrH(D)5 samples. As can be seen on Figs. 3–5
the amount of retained cubic phase correlates with the
oxygen content of the sample. The sample with the
highest EuO content (Eu2IrD5) also shows the largest
fraction of cubic phase at lowtemperature. The
retention of about 13% of cubic phase was also
observed in the phase transition of Sr2IrD5 [2]. On the
other hand, small deviations from the hydrogen
(deuterium) content may also play a role. However,
the methodology used in the present work does not
allowan analysis of the deuterium content to be
made at a degree of accuracy sufficient to answer this
question.
3.3. Symmetry relationships
The structural relationship between the cubic and the
tetragonal phase can be illustrated by the symmetry tree
as shown in Fig. 7. The space groups are consistent with
a translationengleiche transition of index 3 that splits the
deuterium position of the cubic structure into two
crystallographic non-equivalent positions in the tetra-

ARTICLE IN PRESS
H. Kohlmann et al. / Journal of Solid State Chemistry 174 (2003) 35–43
43
Acknowledgments
[6] K. Yvon, H. Kohlmann, B. Bertheville, Chimia 55 (2001)
505–509.
[7] H. Kohlmann, F. Gingl, T. Hansen, K. Yvon, Angew. Chem. 111
(1999) 2145–2147;
We gratefully acknowledge the help of Pierre Palleau
and Dr. Gabriel Cuello (both ILL, Grenoble, France)
with the neutron diffraction experiments on D4b. One
of us (ROM) acknowledges a Trinity College Faculty
Research Grant and thanks the University of Geneva
for hospitality during a recent leave of absence. This
work was supported by the Swiss National Science
Foundation and the Swiss Federal Office of Energy.
H. Kohlmann, F. Gingl, T. Hansen, K. Yvon, Angew. Chem. Int.
Ed. Engl. 38 (1999) 2029–2032.
[8] H. Kohlmann, Physica B 276–278 (2000) 288–289.
[9] H. Kohlmann, B. Bertheville, T. Hansen, K. Yvon, J. Alloys
Compd. 322 (2001) 59–68.
[10] R.O. Moyer Jr., B.J. Burnim, R. Lindsay, J. Solid State Chem.
121 (1996) 56–60.
[11] R.O. Moyer Jr., R. Lindsay, D.F. Storey, Z. Phys. Chem., Neue
Folge, 165 (1989) 83–94.
[12] R.A. Young, A. Sakthivel, T.S. Moss, C.O. Paive-Santos, J. Appl.
Crystallogr. 28 (1995) 366–367.
References
[13] J.E. Lynn, J. Appl. Crystallogr. 22 (1989) 476–482.
[14] J. Rodriguez-Carvajal, FullProf.’98, 1998 (LLB, unpublished).
[15] H. Kohlmann, K. Yvon, J. Alloys Compd. 299 (2000) L16–L20.
[16] R.D. Shannon, Acta Crystallogr. Sect. A 32 (1976) 751–767.
[17] P. Zolliker, K. Yvon, P. Fischer, J. Schefer, Inorg. Chem. 24
(1985) 4177–4180.
[1] K. Yvon, Chimia 52 (1998) 613–619.
[2] J. Zhuang, J.M. Hastings, L.M. Corliss, R. Bau, C.Y. Wei,
R.O. Moyer Jr., J. Solid State Chem. 40 (1981) 352–360.
[3] R.O. Moyer Jr., B. Toby, J. Alloys Compd. 2003, in press.
[4] J. Zhuang, W. Kunnmann, L.M. Corliss, J.M. Hastings,
R.O. Moyer Jr., J. Solid State Chem. 48 (1983) 117–120.
[5] H. Kohlmann, H.E. Fischer, K. Yvon, Inorg. Chem. 40 (2001)
2608–2613.
[18] R. Cerny, J.-M. Joubert, H. Kohlmann, K. Yvon, J. Alloys
Compd. 340 (2002) 180–188.
[19] I.I. Borisov, A.V. Irodova, O.A. Lavrova, G.V. Laskova,
V.A. Somenkov, Sov. Phys. Solid State 26 (1984) 2031–2033.