Covalent Si–H Bonds in the Zintl Phase Hydride CaSiH1+x (x ≤ 1/3)

Abstract

The crystal structure of the Zintl phase hydride CaSiH≈4/3 was discussed controversially, especially with respect to the nature of the silicon-hydrogen interaction. We have applied X-ray and neutron powder diffraction as well as total neutron scattering on a deuterated sample, CaSiD1.1. Rietveld refinement (CaSiD1.1, Pnma, a = 14.579(4) Å, b = 3.8119(4) Å, c = 11.209(2) Å) and an analysis of the neutron pair distribution function show a silicon-deuterium bond length of 1.53 Å. The Si–H bond may thus be categorized as covalent and the main structural features described by a limiting ionic formula Ca2+H−(Si−)2/3(SiH−)1/3. Hydrogen atoms decorating the ribbon-like silicon polyanion made of three connected zigzag chains are under-occupied, resulting in a composition CaSiH1.1. Hydrogen-poor Zintl phase hydrides CaSiH<1 with hydride ions in Ca4 tetrahedra only were found in an in situ neutron diffraction experiment at elevated temperature. Hydrogen (deuterium) uptake and release in CaSiDx (0.05 ≤ x ≤ 0.17) is a very fast process and takes less than 1 min to complete, which is of importance for possible hydrogen storage applications.

Synthesis and Crystal Structure of BaLaSi2H0.80

“…This expansion is caused by 0.402 ϫ 8 = 3.216 hydrogen atoms per unit cell, equaling 5.340 ϫ 10 -24 mol. The estimated volume increment of hydrogen in BaLaSiH 0.80 is thus 2.875 cm 3 •mol -1 . This value is smaller than expected values of ionic or metallic hydrides, which are in the range 5-7 cm 3 •mol -1 .…”

“…The hydride CaSiH 4/3– x , for example, exhibits condensed polyanions formed of three zig‐zag chains [Si 2– ] n with additional Si–Si bonds (Figure ). Whether there are covalent Si–H bonds or not has long been a matter of controversy and only recently been proven . SrSi and BaSi form hydrides with increased hydrogen content, SrSiH 5/3– x and BaSiH 6/3– x (= BaSiH 2– x ).…”

“…Whether there are covalent Si-H bonds or not has long been a matter of controversy and only recently been proven. [3] SrSi and BaSi form hydrides with increased hydrogen content, SrSiH 5/3-x and BaSiH 6/3-x (= BaSiH 2-x ). The stitial Zintl phase hydride like LaSiH 1-x , but unlike BaSiH 2-x does not feature any covalent Si-H bonds.…”

Synthesis of calcium monosilicide nanowires by a reactive deposition technique

“…Several teams have investigated the hydriding and dehydriding properties of CaSi both theoretically and experimentally. [7][8][9][10][11][12] It has been demonstrated that the CaSiH n phase is thermodynamically stable and bulk CaSi can reversibly absorb 1.9 wt% hydrogen as the hydrogen storage material. 8,13) Ca silicides have five stable phases, so the synthesis of any specific single phase is difficult with RDE.…”