Structure characterization of the hydrogenous Zintl
phases Ba3Si4Hx and Eu3Si4Hx
J. Nylén, J. Grins, and U.Häussermann
Department of Materials and Environmental Chemistry, Stockholm University,Svante Arrhenius väg 16C, SE- 10691
Stockholm, Sweden
Like many intermetallic compounds, Zintl phases constituting of an active metal (alkali, alkaline
earth, or rare earth) and a more electronegative p-block metal or semimetal can react with hydrogen
to form hydrides. However, the rather high ionicity of Zintl phases makes such hydrides peculiar
[1]. As a characteristic feature of Zintl phases, atoms of the electronegative component appear
reduced and may from polyanionic structures to achieve an octet. Hydrogen takes an ambivalent
role and can be incorporated in Zintl phases in two principal ways: either hydridic where H is
exclusively coordinated by active metals (interstitial hydrides), or as part of the polyanion where it
acts as a covalently bonded ligand (polyanionic hydrides). The H content of hydrogenous Zintl
phases is comparatively low, however, chemical structures and physical properties of Zintl phases
can change profoundly upon H incorporation. Considering their wide range of chemical
compositions, H induced structure and property changes in Zintl phases provide interesting
prospects for fundamental inorganic chemistry and materials science.
This study is concerned with Ba3Si4Hx and Eu3Si4Hx (Figure 1) which were obtained by reacting
Ba3Si4 and Eu3Si4 with pressurized hydrogen (20 bar) at temperatures around 300 oC. As a first step
the the location of the metal atoms in the hydride structures has to be established. In house powder
X-ray diffraction (PXRD) data were difficult to evaluate because of a combination of absorption
and structural disorder. Powder samples of Ba3Si4Hx and Eu3Si4Hx were loaded in glass capillaries
of 0.5 mm diameter and high-energy X-ray diffraction experiments were carried out at the beamline
P02.1 at PETRAIII, which operates at a fixed energy of approximately 60 keV. The wavelength was
determined to be 0.20707(6) Å. 30 2D diffraction images, each obtained through the accumulation
of 20 frames with an exposure time of one second per frame, were collected with a Perkin Elmer
amorphous silicon area detector (XRD1621) placed at a distance of 329 mm and 2799 mm from the
sample, to obtain data for PDF and high resolution structural analysis, respectively. The 2D
diffraction images were then integrated into a linear scattering signal with the software Fit2D and
averaged [2]. The PDFs were extracted by using the PDFgetX3 software [3]. The different steps of
the PDF extraction include the correction for Compton scattering, absorption effects, multiple
scattering, and normalization to obtain the total structure factor function, S(Q). Finally the pair
distribution function, G(r), is obtained by sine
Fourier transform of the normalized scattering
intensity, F(Q) = Q[S(Q)-1], to a maximum Q-value
of 17 Å–1. The software PDFGui [4] was used for
PDF refinements.
!
!
!
! Figure 1. Crystal structures of M Si and
H (M = Ba, Eu). Centers of the
! MgreySipolyhedra
are most likely locations
of the H atoms. M = grey, Si = red
! circles.
3
3
4
x
4
Evaluation of the synchrotron data of Ba3Si4Hx (Figure 2) revealed that the sample actually
represents a mixture of three phases that are closely structurally related, with phase fractions of 75,
7 and 17 wt%. The metal arrangement of the major phase is shown in Figure 1. Ba atoms and
tetrahedral Si4 clusters form a perovskite-like arrangement. Most likely the Ba6 octahedra are
centered by H atoms. This, however, has to be confirmed from neutron diffraction experiments. The
evaluation of Eu3Si4Hx diffraction data revealed that the metal atom arrangement in the hydride is
only slightly different from the one of the parent Zintl phase. Most likely H atoms are located in the
center of Eu4 tetrahedra. Most noticeable is an increase in the unit cell c parameter and a decrease
of the interatomic Si-Si distance in the hydride structure.
Figure 2. Synchrotron powder X-ray diffraction pattern (left) and pair distribution function (right) for
Ba3Si4Hx. The maxima at around 2.5 and 4.5 Å in the PDF correspond to the shortest Si-Si and Ba-Si
distances in the Ba3Si4Hx phases.
Figure 3. Rietveld refinement of Eu3Si4H2 from data collected at PETRA-III.
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References
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1. U. Häussermann, Z. Kristallogr. 223, 628-635 (2008).
2. A. P. Hammersley, S. O. Svensson, M. Hanfland, A. N. Fitch and D. Häusermann, J. High Pressure
Res. 14, 235–248 (1996).
3. PDFgetX3: P. Juhás, T. Davis, C. L. Farrow and S. J. L. Billinge, J. Appl. Cryst. 46, 560-566 (2013).
4. C. L. Farrow, P. Juhas, J. W. Liu, D. Bryndin, E. S. Bozin, J. Bloch, T. Proffen and S. J. T. Billinge,
J. Phys.: Condens. Matter. 19, 335219 (2007).