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CHEMICAL PHYSICS

18 NIST CENTER FOR NEUTRON RESEARCH

Probing structure-dependent 

reorientational behavior of SiH3

-

 anions in alkalimetal silanides by quasielastic neutron scattering

M. Dimitrievska,1,2 J.-N. Chotard,3 R. Janot,3 A. Faraone,1 W. S. Tang,1,4,5 A. V. Skripov,6

and T. J. Udovic1

1 NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899

2 National Renewable Energy Laboratory, Golden, CO 80401

3 Laboratoire de Réactivité et Chimie des Solides, UMR 7314 CNRS, Université de Picardie Jules Verne, 80039 Amiens Cedex, France

4 Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742

5 Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015

6 Institute of Metal Physics, Ural Branch of the Russian Academy of Sciences, Ekaterinburg, 620108, Russia

Alkali-metal silanides (MSiH3, M = K, Rb, and Cs) have recently 

gained attention as potential hydrogen-storage compounds due 

to their reversible dehydrogenation/ hydrogenation properties. 

Typically, a silanide↔silicide equilibrium (i.e., MSiH3 ↔ MSi + 

1.5H2) exists, providing favorable hydrogen vapor pressures 

of 0.1 MPa near 410 K for all three alkali-metal compounds 

due to an enthalpy−entropy compensation effect, i.e., as the 

cation size increases, there are simultaneous linear increases 

in the enthalpy (more stable silanide) and entropy changes 

(more localized H atoms in the crystal structure). Hence, 

the dynamical nature of the H atoms in the various MSiH3

compounds is of interest from an entropic viewpoint. All MSiH3

compounds undergo hysteretic phase transitions upon heating 

and cooling between ordered monoclinic or orthorhombic 

β-phase structures to disordered face-centered-cubic α-phase 

structures (see Figure 1). In contrast to crystallographically 

ordered β-phase SiH3

-

 anions, the α-phase anions exhibit threedimensional orientational disorder. Previous quasielastic neutron 

scattering (QENS) results for all three α-phase compounds 

indicated decreasing SiH3

-

 reorientational mobility (increasing 

order) with increasing cation size [1,2], which is qualitatively 

consistent with the observed compensation effect. Here, QENS 

measurements on the DCS, HFBS, and NSE spectrometers over 

a wide range of resolutions were used to fully characterize the 

alkali-metal-dependent anion reorientational dynamics in the 

ordered β-MSiH3 compounds as well as the unusual nature 

of the rotator-phase transition between the β-phases and 

α-phases for CsSiH3 [3]. 

The QENS data in Figure 2 show that the anion reorientational 

mobility in the β-phases trends dramatically higher with 

increasing cation size, which is opposite to what was observed 

in the α-phases, but makes sense from a purely steric 

perspective (i.e., lattice volume to accommodate the SiH3

-

anion increases with increasing cation size). Maximum jump 

frequencies of the β-phase anions near the β-α transitions 

range from around 109 s-1 for β-KSiH3 to 1010 s-1 and higher 

for β-RbSiH3 and β-CsSiH3. Due to the ordered nature of the 

β-phase lattice, the anions are effectively restricted to undergo 

only uniaxial three-fold rotational jumps around the anion 

quasi-C3 symmetry axis, which is consistent with the observed 

behaviors of the β-phase elastic incoherent structure factors 

(EISFs) in Figure 3. 

CsSiH3 was the focus of further studies to map out the 

evolving anion dynamical behavior at temperatures above 

the β-phase region. As in α-KSiH3 and α-RbSiH3, the highly 

mobile anions (with reorientational jump frequencies 

approaching and exceeding 1012 s-1, see Figure 2) in the 

disordered α-CsSiH3 are all adequately modeled by H jumps 

between 24 different locations distributed radially around 

the anion center of gravity (see Figure 3), although even 

higher anion reorientational disorder cannot be ruled out. 

QENS data for CsSiH3 in the transition region between the 

α- and β-phases corroborated the appearance of dynamically 

distinct intermediate (i-) phases. The associated SiH3

-

 anions 

possess orientational mobilities that are an order-ofmagnitude lower than those for α-phase anions but also an 

FIGURE 1: Structural differences between the ordered β-MSiH3 and 

disordered α-MSiH3 (M = K, Rb, Cs) phases showing the respective nearneighbor cation environments surrounding the pyramidal SiH3

-

 anions. The 

multiple H atoms distributed radially around the Si atom represent all of the 

possible H positions associated with the orientationally disordered SiH3

-

 anions 

in this structure. i-MSiH3 denotes the possible formation of ‘intermediate” 

phases in the transition regions.
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CHEMICAL PHYSICS

2019 ACCOMPLISHMENTS AND OPPORTUNITIES 19

order-of-magnitude higher than those for β-phase anions 

(see Figure 2). These anions appear to undergo uniaxial 

small-angular-jump reorientations that are more akin to the 

lower-dimensional β-phase anion motions rather than to 

the multi-dimensional α-phase anion motions (see Figure 

3). Combined QENS and neutron powder diffraction results 

strongly suggest that this i-phase is associated chiefly with 

the more short-range-ordered, nanocrystalline portions 

(invisible to diffraction) that appear to dominate the CsSiH3. 

Figure 2 depicts the hysteretic CsSiH3 phase diagrams 

in heating and cooling regimens derived solely from the 

temperature-dependent QENS measurements for the various 

component phases.

These QENS results for CsSiH3 bolster the previous observation 

for KSiH3 and RbSiH3 [4] that distinct nanosized intermediate 

phases exist and exhibit anion orientational mobilities 

intermediate between those for the α- and β-phases. 

Determining the exact structural nature of these i-phase 

nanodomains will require further investigation.

References

[1] C. Österberg, H. Fahlquist, U. Häussermann, C. M. Brown, 

T. J. Udovic, M. Karlsson, J. Phys. Chem. C 120, 6369 

(2016).

[2] W. S. Tang, M. Dimitrievska, J.-N. Chotard, W. Zhou, R. 

Janot, A. V. Skripov, T. J. Udovic, J. Phys. Chem. C 120, 

21218 (2016).

 [3] M. Dimitrievska, J.-N. Chotard, R. Janot, A. Faraone, W. 

S. Tang, A. V. Skripov, T. J. Udovic, J. Phys. Chem. C 122, 

23985 (2018).

[4] R. Nedumkandathil, A. Jaworski, A. Fischer, C. Österberg, 

Y.-C. Lin, M. Karlsson, J. Grins, A. J. Pell, M. Edén, U. 

Häussermann, J. Phys. Chem. C 121, 5241 (2017).

3 4 5 6 7

108

109

1010

1011

1012 -CsSiH3

i-CsSiH3

Jump correlation frequency (s-1)

1000/T (1/K)

-CsSiH3 -RbSiH3

-KSiH3

T400 (K)350300 250 200 150

T (K)

400 300

0.0

0.2

0.4

0.6

0.8

1.0

(heating)

i-CsSiH3

Phase fraction

-CsSiH3 -CsSiH3

150 200 250 300 350

0.0

0.2

0.4

0.6

0.8

1.0 i-CsSiH3

Phase fraction

T (K)

-CsSiH3

-CsSiH3

(cooling)

FIGURE 2: (left) Anion jump correlation frequencies τ1

-1 vs. 1/T for various 

MSiH3 phases as determined from measurements on DCS (circles, diamonds, 

triangles, and pentagons), HFBS (squares), and NSE (stars) spectrometers. Filled 

and open symbols indicate measurements during cooling and heating regimens, 

respectively. (right) Phase diagrams for CsSiH3 derived from anion dynamical 

behavior with temperature upon (a) cooling from 350 K and (b) heating from 

25 K. The symbols present the calculation of the phase fractions obtained from 

QENS measurements.

FIGURE 3: Elastic incoherent structure factor (EISF) data vs. Q (using various 

spectrometers) for α-CsSiH3 (270 K), i-CsSiH3 (cooling (c)/heating (h), 240 K), 

β-CsSiH3 (green, 200 K; red, 140 K), β-RbSiH3 (247 K), and β-KSiH3 (292 K). EISF 

model curves for various reorientational mechanisms are shown for comparison.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

24 sites on sphere

 C3 (uniaxial

rotational diffusion)

C3 (3-fold jumps)

-CsSiH3 (DCS)

i-CsSiH3 (c) (DCS)

i-CsSiH3 (h) (DCS)

-CsSiH3 (NSE)

-CsSiH3 (HFBS)

-RbSiH3 (HFBS)

-KSiH3 (HFBS)

Elastic Incoherent Structure Factor

Q (Å-1)