Journal of Alloys and Compounds 325 (2001) L13–L16
L
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Letter
Structure data for K 2 MgH 4 and Rb 2 CaH 4 and comparison with hydride
and fluoride analogues
b
¨
B. Bertheville a , T. Herrmannsdorfer
, K. Yvon a , *
a
` , 24 Quai Ernest Ansermet, CH-1211 Geneve
` , Switzerland
Laboratoire de Cristallographie, Universite´ de Geneve
b
¨ Neutronenstreuung, PSI and ETHZ, CH-5303 Villigen, Switzerland
Laboratorium f ur
Received 22 March 2001; accepted 30 March 2001
Abstract
Neutron powder diffraction data on the deuterides of the title compounds confirm their tetragonal K 2 NiF 4 type structures (space group
˚ c513.5921(5) A,
˚ c /a53.37; Rb 2 CaD 4 : a54.4976(2) A,
˚ c514.8294(7) A,
˚ c /a53.30; T5293 K).
I4 /mmm; K 2 MgD 4 : a54.0361(1) A,
˚
The alkaline earth centred deuterium octahedra in the magnesium compound are compressed along the tetragonal axis (Mg–D52.00 A
˚ basal), whereas those in the calcium compound are elongated (Ca–D52.30 A
˚ axial, 2.25 A
˚ basal). A survey of isostructural
axial, 2.02 A
hydride and fluoride analogues shows that big alkali cations tend to flatten the anion octahedra and to increase the cell parameter ratios
c /a, whereas big alkaline earth cations tend to elongate the anion octahedra and to decrease c /a.  2001 Elsevier Science B.V. All rights
reserved.
Keywords: Metal hydrides; Solid state reactions; Crystal structure; Neutron diffraction
1. Introduction
Alkali and alkaline earth metals form saline hydrides of
21
composition M 1
H 4 (M 1 5K 1 , Rb 1 , Cs 1 ; M 21 5
2 M
21
21
Mg , Ca ). Some (Rb 2 MgH 4 [1], Cs 2 MgH 4 [2]) crystallize with the orthorhombic b-K 2 SO 4 type structure
while others (K 2 MgH 4 [3], high-pressure Cs 2 MgH 4 [4],
Rb 2 CaH 4 [5], Cs 2 CaH 4 [6]) adopt the tetragonal K 2 NiF 4 type structure. The orthorhombic hydrides contain quasi
isolated, alkaline earth centred hydrogen tetrahedra M 21 H 4
and the tetragonal compounds quasi infinite, two-dimensional slabs of corner sharing hydrogen octahedra M 21 H 6 .
Exact hydrogen atom distributions for the tetragonal series
have only been reported for high-pressure Cs 2 MgH 4 and
Cs 2 CaH 4 . The results show that the M 21 H 6 octahedra are
not regular but more-or-less deformed along the 4-fold
axis. In order to study a possible influence of matrix effects
on these deformations it was of interest to complete the
inventory of metal–hydrogen bond distances. In this work
neutron diffraction data on K 2 MgD 4 and Rb 2 CaD 4 are
presented and compared with those of hydride and fluoride
analogues. It will be shown that the alkali and alkaline
*Corresponding author.
E-mail address: [email protected] (K. Yvon).
earth cations determine the shape of the anion octahedra
and have a counteracting influence on the cell parameter
ratio c /a.
2. Experimental
Deuteride samples of nominal composition K 2 MgD 4
and Rb 2 CaD 4 were prepared by reacting appropriate
amounts of binary deuterides (KD, RbD, a-MgD 2 and
CaD 2 ) in an autoclave at 673 K and 85 bar deuterium
pressure during 4 days. The latter were prepared by
deuteration of the metals (K: ingot, Alfa, 98%; Rb: ingot,
Aldrich, 99.6%; Mg: powder, Cerac, 99.6%, 2400 mesh;
Ca: dendritic pieces, Aldrich, 99.5%; deuterium purity 2.8
N). As the reaction products were sensitive to air and
moisture, they were handled in an argon-filled glove box.
Neutron powder diffraction data were collected on HRPT
[7] at the spallation neutron source SINQ of PSI (Villigen,
˚ 2u range 4.95–164.858; 2u step 0.058; T5293
l 51.886 A;
K). Structure refinements by the Rietveld method were
performed with FULLPROF [8] by taking as starting
parameters for the atomic coordinates those of K 2 NiF 4 .
For the K 2 MgD 4 sample three additional phases were
0925-8388 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved.
PII: S0925-8388( 01 )01368-8
B. Bertheville et al. / Journal of Alloys and Compounds 325 (2001) L13 –L16
L14
Fig. 1. Observed (top), difference (middle) and calculated (bottom) neutron diffraction patterns of the K 2 MgD 4 sample (left) and the Rb 2 CaD 4 sample
˚
(right); l 51.886 A.
included in the refinement: KD, MgO and V (sample
holder), leading to a total of 31 parameters: one zero
correction, four scale factors, 12 profile and five cell
parameters (two for K 2 MgD 4 , three for the secondary
phases), two positional parameters and seven isotropic
temperature factors (four for K 2 MgD 4 , three for the other
phases). For the Rb 2 CaD 4 sample the additional phases
Table 1
Refinement results on neutron powder diffraction data for K 2 MgD 4 and
Rb 2 CaD 4 ; T5293 K, space group I4 /mmm, Z52; e.s.d. values in
parentheses
Atom
Site
x
y
z
˚ 2)
Biso (A
˚ c513.5921(5) A,
˚ V5221.4
K 2 MgD 4 : cell parameters a54.0361(1) A,
˚3
A
K
4e
0
0
0.3513(4)
1.4(9)
Mg
2a
0
0
0
1.1(6)
D1
4e
0
0
0.1472(2)
2.6(6)
D2
4c
0
1/2
0
2.0(5)
R B 57.4%, R p 53.7%, R wp 54.8%, S52.2
˚ c514.8294(7) A,
˚ V5300.0
Rb 2 CaD 4 : cell parameters a54.4976(2) A,
3
Å
Rb
4e
0
0
0.3496(3)
2.1(1)
Ca
2a
0
0
0
2.1(1)
D1
4e
0
0
0.1548(4)
4.0(2)
D2
4c
0
1/2
0
3.7(1)
˚
R B 57.7% (Rb 2 CaD 4 ), 6.6% (RbCaD 3 : a54.5297(1) A);
R p 52.7%,
R wp 53.6%, S52.9
included in the refinement were RbCaD 3 (cubic perovskite
structure previously not yet reported), RbD and CaD 2 . A
total of 33 parameters were allowed to vary: one zero
correction, four scale factors, 12 profile and seven cell
parameters (two for Rb 2 CaD 4 , one for RbCaD 3 , three for
CaD 2 , one for RbD), two positional parameters
(Rb 2 CaD 4 ) and seven isotropic temperature factors (four
for Rb 2 CaD 4 , three for RbCaD 3 ). The observed, calculated
and difference patterns are shown in Fig. 1. Refinement
Table 2
Metal–deuterium bond distances and shortest deuterium–deuterium con˚ e.s.d. values in parentheses
tact distances (A);
K 2 MgD 4
Rb 2 CaD 4
K–D1
4D1
4D2
2.775(6)
2.8540(1)
2.856(4)
D1–Mg
K
4D2
4K
2.001(3)
2.775(6)
2.842(2)
2.8540(1)
Mg–2D1
4D2
2.001(3)
2.0180(1)
D2–2Mg
4D1
4K
2.0180(1)
2.842(2)
2.856(4)
Rb–D1
4D2
2.889(7)
3.167(3)
D1–Ca
Rb
4Rb
2.296(6)
2.889(7)
3.1810(2)
Ca–4D2
2D1
2.2488(1)
2.296(6)
D2–2Ca
4Rb
4D2
2.2488(1)
3.167(3)
3.1803(1)
B. Bertheville et al. / Journal of Alloys and Compounds 325 (2001) L13 –L16
L15
results are summarized in Table 1 and interatomic distances in Table 2. A structural drawing of the K 2 NiF 4 type
structure is given in Fig. 2.
3. Results and discussion
21
Fig. 2. K 2 NiF 4 -type structure of the ternary metal deuterides M 1
D4
2 M
(M 1 5K 1 , Rb 1 , Cs 1 ; M 21 5Mg 21 , Ca 21 ).
The present data confirm the structural assignments for
tetragonal K 2 MgH 4 and Rb 2 CaH 4 and for cubic RbCaH 3 .
The metal–deuterium bond distances in the tetragonal
compounds are consistent with those in the binary
deuterides except that K 2 MgD 4 shows a significant contraction (210%) and Rb 2 CaD 4 an expansion (13.4%) of
the molar volume compared to the weighted sum of the
molar volumes of the corresponding binary deuterides. Of
particular interest for the metal–hydrogen interactions is
the size and the deformation of the alkaline earth centred
deuterium octahedra (see axial d a versus basal d b in Fig. 2).
Clearly, the octahedra (point group symmetry 4 /mmm) are
compressed along the tetragonal axis in the magnesium
compound (d a /d b 50.99) and elongated in the calcium
compound (d a /d b 51.02). As can be seen in Table 3
similar ‘matrix effects’ also occur in other deuterides M 1
2
M 21 D 4 . In the caesium compounds Cs 2 MgD 4 and
Cs 2 CaD 4 the substitution of the smaller Mg 21 by the
bigger Ca 21 cations leads to an expansion of the octahedra
mainly along the tetragonal axis (d a /d b 50.93 (Mg 21 )
versus 1.00 (Ca 21 )) while the parameter ratio c /a decreases (3.41 vs. 3.38). On the other hand, in the magnesium compounds K 2 MgD 4 and Cs 2 MgD 4 (or in the
calcium compounds Rb 2 CaD 4 and Cs 2 CaD 4 ) the substitution of the smaller K 1 (or Rb 1 ) by the bigger Cs 1 cations
leads to an expansion of the octahedra mainly in the basal
plane (Mg: d a /d b 50.99 (K 1 ) versus 0.93 (Cs 1 ); Ca:
Table 3
Structure data of K 2 NiF 4 type deuterides and of fluoride and oxide analogues (d a , axial; d b , basal M 21 –X bond distance, X5D,F,O)
Compound
c /a
3
˚ )
V (A
˚
d(M 21 –X) (A)
d a /d b
da
db
Deuterides
K 2 MgD 4
Cs 2 MgD 4 [4]
Rb 2 CaD 4
Cs 2 CaD 4 [6]
3.37
3.41
3.30
3.38
221.4
274.0
300.0
328.2
2.00
2.01 b
2.30
2.31
2.02
2.16
2.25
2.30
0.99
0.93
1.02
1.00
Fluorides a
K 2 MgF 4
Rb 2 MgF 4
K 2 NiF 4
Rb 2 NiF 4
Rb 2 HgF 4
Cs 2 HgF 4
K 2 CuF 4
3.31
3.40
3.26
3.35
3.02
3.14
3.07
208.8
226.7
209.8
228.9
285.6
310.5
218.9
2.00 c
1.99 b 1.97
2.01
2.22
2.16
1.94
1.99
2.03
2.00
2.04
2.28
2.31
2.07
1.01
0.98
0.98
0.98
0.97
0.94
0.94
Oxide
La 1.85 Sr 0.15 CuO 4 [10]
3.50
188.8
2.41
1.89
1.28
a
Data calculated from the hydride fluoride crystal structure database, HFD [9].
b
e.s.d.50.02.
c
e.s.d50.03; all other e.s.d’s,0.01.
L16
B. Bertheville et al. / Journal of Alloys and Compounds 325 (2001) L13 –L16
d a /d b 51.02 (Rb 1 ) versus 1.00 (Cs 1 ) while c /a increases
(Mg: 3.37 vs. 3.41; Ca: 3.30 vs. 3.38). Thus the alkali and
alkaline earth elements counteract in their influence on the
octahedra dimensions and cell parameter ratios c /a.
Heavier (bigger) alkali cations tend to flatten the M 21 D 6
octahedra and to decrease the cell parameter ratio c /a,
while heavier (bigger) alkaline earth cations tend to
elongate the octahedra and to increase c /a. Note that
within a homologous pair the flatter M 21 D 6 octahedron is
always associated with the bigger c /a ratio and inversely.
This suggests that the shape of the M 21 D 6 octahedra and
the cell parameter ratios in this structural series depend
critically on the interplay between alkali–hydrogen and
alkaline earth–hydrogen interactions. Repulsive hydrogen–
hydrogen interactions do not appear to play a major role
˚ (see
because all D–D distances are greater than 2.80 A
Table 2). A comparison with isostructural fluorides (see
Table 3) confirms these trends, i.e., within each homologous pair of magnesium, nickel and mercury compounds
d a /d b increases and c /a decreases as one substitutes
smaller by bigger alkali cations (such as K 1 by Rb 1 , or
Rb 1 by Cs 1 ), and in the homologous pair of rubidium
compounds Rb 2 NiF 4 and Rb 2 HgF 4 d a /d b decreases and
c /a increases as the smaller Ni 21 is substituted by the
bigger Hg 21 . Finally, matrix effects are also likely to
contribute to the anomalous anion octahedra shapes and
c /a ratios in Jahn–Teller systems such as K 2 CuF 4 and
La 1.85 Sr 0.15 CuO 4 .
Acknowledgements
The authors thank J.–L. Lorenzoni for technical assistance. This work was supported by the Swiss National
Science Foundation and the Swiss Federal Office of
Energy.
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