Pressure-induced structural phase transition of alkaline-earth
dihydrides
K. Kinoshita, M. Nishimura, Y. Akahama and H. Kawamura*
Graduate School of Material Science, University of Hyogo, 3-2-1, Kamigori, Hyogo 678-1297,
Japan
[email protected]
Summary
The powder X-ray diffraction experiments on CaH2 and SrH2 were performed at high
pressures and room temperature. CaH2 and SrH2 with the cotunnite structure at ambient
conditions transformed into a hexagonal structure with two formula units in a unit cell at 15 GPa
and 10 GPa, respectively. The c/a ratios of the hexagonal lattice of both compounds are about
4/3, which is close to those of BaF2 and YbH2 with a Ni2In structure (P63/mmc). Thus the high
pressure phases of CaH2 and SrH2 are proposed to have the Ni2In structure.
Introduction
At ambient conditions, alkaline-earth dihydrides crystallize into the cotunnite-type structure
(PbCl2-type; Pnma) (Zintl and Harder. 1935). This structure type is the example of the highest
known coordination (CN=9) in ionic AX2 compounds and is adopted by more than 400
compounds. The transformation of such a high CN compound caused by the application of
pressure is of interest. Up to now, two types of a crystal structure into which the cotunnite
structure is transformed by the application of pressure have been reported. One is a monoclinic
P21/a structure (Z=8), which is a distorted orthorhombic Co2Si structure and has the anion CN of
10. PbCl2 and SnCl2 transform from the cotunnite to this monoclinic structure at 16 GPa (Leger et
al., 1996). The other is a Ni2In type structure (P63/mmc, Z=2), in which CN is 11. This type of the
transformation takes place in the case of the ionic AX2 compounds with smaller anions. BaF2 and
YbH2 of the cotunnite phase transform into the Ni2In structure at 12 GPa (Leger et al., 1995) and
15 GPa (Staun-Olsen et al., 1984), respectively.
In this paper, we present the results of powder X-ray diffraction on CaH2 and SrH2 at high
pressures and room temperature, where pressure-induced structural phase transitions were
examined.
Experimental
Alkaline-earth dihydrides were prepared by reaction of hydrogen gas with metals. A
diamond anvil cell was used for the X-ray diffraction experiments at high pressures. The diamond
anvils had a top surface diameter of 0.6 mm. Since the compound is sensitive to water and
oxygen, it was ground into a fine powder in a dry box filled with nitrogen gas and loaded with a
ruby chip into a 0.3 mm diameter hole of a metal gasket (U-700). Pressure transmitting medium
was not used. An X-ray diffraction experiments at room temperature was carried out with a
synchrotron radiation source on the beam line BL04B2 at SPring-8. The wavelength tuned with a
Si (111) double-crystal monochrometer to 0.3293Å. Powder patterns were obtained by an angle
dispersive method with an image plate detector. Pressures were determined by the ruby
fluorescence method (Mao et al., 1978). The diffraction images obtained were analyzed using
the integration software Fit2d, and the Rietveld refinement of the powder X-ray diffraction data
was performed with the program RIETAN-2000 (Izumi and Ikeda, 2000).
Results and Discussion
Figure 1 shows the typical diffraction patterns of CaH2 at selected pressures. The sample
contains a small amount of CaO with a NaCl-type structure and Ca(OH)2 with a CdI2-type
structure if OH is treated as a single anion.
λ = 0.3293 Å
CaO
25.5 GPa
CaO
18.1 GPa
CaO
Intensity
14.9 GPa
6
111
8
10
313 410
220 122
12
400 204
013
113
203 020
302
311
213
211
0.4 GPa
CaO
002
Ca(OH)2
011
200 102
Ca(OH)2
CaO
7.3 GPa
14
2θ [deg]
Fig. 1 Typical diffraction patterns of CaH2 at various pressures.
From the refinement on the diffraction pattern obtained at 0.4 GPa, the mole fractions of CaO
and Ca(OH)2 were estimated to be 0.02 and 0.14. It has been reported that CaO maintains a
NaCl-type structure up to 60 GPa (Jeanloz, et al., 1997) and Ca(OH)2 amorphizes at pressure
around 12 GPa (Kruger, et al., 1989). All lines except ones from CaO and Ca(OH)2 are attributed
to those from the cotunnite phase of CaH2. The diffraction lines from Ca(OH)2 is certainly
disappeared in the pattern at pressure of 14.9 GPa and above this pressure, the x-ray diffraction
profiles showed changes with pressure which indicated the appearance of a new phase. In the
top panel, the shoulder appeared at the lower angle side of the strongest peak is from 111
diffraction of CaO. The pattern at 25.5 GPa is assigned to a hexagonal lattice with two formula
units in the unit cell and with lattice constants of a = 3.522 and c = 4.613 Å. The c/a ratio of 1.31
is comparable to 1.32 and 1.34 in the Ni2In-type structure of BaF2 (Leger et al., 1995) and YbH2
(STAUN-OLSEN et al. 1984), respectively.
Due to the similarity between the diffraction profiles of the high pressure phase of CaH2 and
the Ni2In-type structure of BaF2, a structure refinement with the Ni2In-type structure model
(P63/mmc, Z=2), was performed. The observed and the calculated patterns are illustrated in
Fig.2. The refinements converge with Rwp = 25.81. The reliability factor is fairly large; it is merely
due to the diffuse scattering appeared at around 8~10 degree in 2θ, which diffuse scattering
comes from an amorphous phase of Ca(OH)2. We can thus conclude that the crystal structure of
the high-pressure phase of CaH2 is a Ni2In-type one. Figure 3 shows the pressure dependence
of the cell volume. The volume in the transition from the cotunnite to the Ni2In-type structure at 15
GPa was observed to decrease by about 6.6 %.
25.5 GPa
Intensity
x
y
z
Ca ( 1/3 , 2/3 , 1/4 )
a = 3.5221(25) Å
H1 ( 0 , 0 , 0 )
c = 4.6125(8) Å
H2 ( 1/3 , 2/3 , 3/4 )
V = 49.55(10) Å3
CaH2
CaO
Fig. 2 Observed profile (dotted line) of the high pressure phase of CaH2 at 25.5 GPa and
calculated profiles (solid line) based on the Ni2In-type structure.
P63/mmc phase
Pnma phase
Cell Volume [Å3/f.u.]
35
30
ΔV ～ 6.6%
25
0
10
20
30
Pressure [GPa]
Fig. 3 Pressure dependence of cell volume of CaH2.
The synchrotron radiation experiments were performed at SPring-8 with the approval of the
Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2004A0069-ND2a-np).
This work is partly supported by a Grant-in-Aid for Scientific Research (B) (No. 16340132) from
the Japan Society for the Promotion of Science.
References
IZUMI, F., IKEDA, T., 2000. A Rietveld-analysis program RIETAN-98 and its application to
zeolites. Mat. Sci. Forum, 321-324, 198-203.
JEANLOZ, R., AHRENS, T.J., MAO, H.K., BELL P.M., 1979. B1-B2 Transition in calcium oxide
from shock-wave and diamond-cell experiments. Science, 206, 829-830,
KRUGER, M.B., WILLIAMS, Q., JEANLOZ, R., 1989. Vibrational spectra of Mg(OH)2 and
Ca(OH)2 under pressure. J. Chem. Phys., 91(10), 5910-5915.
LEGER, J.M., HAINES, J., ATOUF, A., SCHULTE, O., HULL, S., 1995. High-pressure x-ray- and
neutron-diffraction studies of BaF2: An example of a coordination number of 11 in AX2
compounds. Phys. Rev., B52, 13247-13256.
LEGER, J.M., HAINES, J., ATOUF, A., 1996. The high pressure behaviour of the cotunnite and
post-cotunnite phases of PbCl2 and SnCl2. J. Phys. Chem. Solids, 57(1), 7-16.
MAO, H.K., BELL, P.M., SHANER, J.W., STEINBERG, D.J., 1978. Specific volume
measurements of copper, molybdenum, palladium, and silver and calibration of the ruby R1
fluorescence pressure gauge from 0.06 to 1 Mbar. J. Appl. Phys., 41(6), 1276-1281.
STAUN-OLSEN, J., BURAS, B., GEWARD, L., JOHANSSON, B., LEBECH, B., SKRIVER, H.,
STEENSTRUP, S., 1984.
A new high-pressure phase and the equation of state of YbH2.
Physica Scripta, 29, 503-507.
ZINTL, E., HARDER, A., 1935. Constitution of the alkaline earth hydrides. Z. Elektrochem., 41,
33-52.