Search for CaH6 in X-ray diffraction measurements at
pressures up 200 GPa
S. Besedin1, M. Eremets1, I. Troyan1, and A. Irodova2
Max Plank Institut für Chemie, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
National Research Centre “Kurchatov Institute”, Akademika Kurchatova pl. 1, 123182, Moscow, Russia
1
2
Since Ashcroft suggested in 2004 that hydrogen-dominant metallic alloys may posses
superconductivity with high Tc [1] high-pressures studies of hydrogen rich compounds have gained
considerable interest. Besides studies of three- and tetrahydrides, which are mostly hydrogen
abundant among the saturated and stoichiometric hydrides it is suggested that even more hydrogenrich compounds can be formed at high pressures. In particular, theory predicts that above ~ 100 GPa
LiHn (n=2-8) [2] and CaHn (n=4-12) [3] can be formed. High hydrogen percentage is achieved due
to the fact that large fraction of hydrogen in these compounds appears in molecular-like form.
Indeed, it similar compound - SiH4(H2)2 was found to be formed when mixture of SiH 4 and H2 is
compressed to 16 GPa at room temperature [4]. However, SiH 4(H2)2 remains dielectric. So, search
for metallic “superstoichiometric” hydride remains a problem of current importance. Lithium –
hydrogen system was studied experimentally, but no compound other than LiH was found up to 150
GPa [5]. Among predicted calcium hydrides CaH6 should be most stable above 150 GPa. It should
be metal with sodalite - like clathrate structure and its Tc is expected to as high as 220- 235 K. Here
we present preliminary results of our experimental search for CaH n where n exceeds 2. Our
approach is based on detection of any structure changes which may be associated with formation of
new compound.
Diamond anvil cells (DAC) were used for pressure generation. Single-bevelled diamonds with the
tip diameter of 50 μm were used to work in the pressure region up ~ 200 GPa (also 300 μm flat
anvils were used to cover pressure range below 30 GPa). Gasket was made of cubic boron nitride
(cBN). A grain of CaH2 (Sigma – Aldrich, purity 99.99%) about 10 μm in size was placed into the
gasket hole (about 10 μm thick and 40 μm in diameter) rest of which was loaded with hydrogen at
room temperature and ~1500 bar pressure. Because of strong tendency of CaH 2 towards oxidation
sample loading was carried out in a glow box where concentration of both H 2O and O2 was kept at
level 0.1 ppm. The sample was probed by X-ray powder diffraction at beamline P02.2. The
monochromatic beam with energy ~ 40 KeV was focused to 1.5 – 2 μm. The diffraction patterns
Figure 1: Diffraction patterns for CaH 2 in H2 medium (black curves) and He medium (red curve). Relative
intensities are distorted because of large grain size as compared to beam dimensions and preferred
orientation. Photo of the sample in transmitted light is shown in the inset. A grain of CaH 2 is situated in
the hole with 20 μm in diameter, rest of which is filled with H 2.
were recorded with 2D detector (Perkin Elmer). Several compression runs were made at room
temperature to some 160 GPa. Pressure was determined by measuring stressed diamond edge,
from known equation of state of cBN, and from known pressure dependence of hydrogen molecular
vibron. The later also served as indication of sufficient amount of hydrogen in the chamber – see also
inset in Fig.1. A separate compression run was made with CaH2 in He medium at pressures up to 140 GPa
in order to obtain reference data about behaviour of CaH2 (pressure was determined from diamond edge).
At ambient pressure structure of CaH2 is orthorhombic with Pnma space group. Under compression
in hydrogen atmosphere a transition to the phase with hexagonal lattice with P63/mmc space group
was observed at pressures between 12 and 25 GPa – lower middle black curves in Fig.1. That is in
accord with literature data where CaH 2 was compressed with no pressure medium [Tse et al. 2007].
Under further compression no structure change was observed up to 157 GPa. The same behavior
was observed when CaH2 was compressed in He medium – red curve. We therefore conclude that
no new compounds were formed in this run. Also no hydride formation was observed when pure
calcium was compressed in hydrogen medium up 180 GPa. However the situation appears not
simple. For example, in Fig.2 the diffraction pattern from another run is shown, which was obtained
after the sample was kept during month at pressure 140 GPa. The pattern corresponds to bcc lattice
formed by Ca atoms (as predicted in [Wang et al., 2009]). Unit cell volume fit reasonably well to
the value which would correspond to composition CaH3 at this pressure. So does symmetry of the
crystal. Nevertheless no definite conclusion can be made at this stage because no data about
structure evolution are available (the sample was compressed to high pressure just when loading
DAC with hydrogen).
Figure 2: Peak indexing for CaH2 in H2 medium at 140 GPa. The plot produced by FULLPROF package.
The peaks with no indexes shown are related to cBN gasket.
It is known that kinetics of the phase transitions and/or chemical reactions in hydrides can be slow,
taking up to several months (silane is an example [6,7]). That can explain existing controversy in
the data obtained. Therefore, more studies over longer time period as well at higher temperatures
are required.
References
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