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Calcium (mono)silicide (CaSi) is a Zintl-phase with semimetallic and semiconducting properties and a potential for future use in thermoelectric applications
Galkin, N. G., et al. "Formation, structure, and optical properties of single-phase CaSi and CaSi2 films on Si substrates." (2022)
. It has a melting point of 1320°C and may be successfully grown as single crystals. Its structure consists of twofold bonded Si2- anionic chains along the c-axis (as expected for a Zintl phase), which are stacked along the a-axis and separated by isolated Ca2+ cations as shown in Figure(1). Its crystal system is Orthorombic and its space group C m c m (63). The semimetallic properties of this Zintl phase are in part caused by the delocalized π-bond electrons in the Si-Si bonds
Kurylyshyn, I.M., Fässler, T.F., Fischer, A., Hauf, C., Eickerling, G., Presnitz, M. and Scherer, W. (2014), Probing the Zintl–Klemm Concept: A Combined Experimental and Theoretical Charge Density Study of the Zintl Phase CaSi. Angew. Chem. Int. Ed., 53: 3029-3032. https://doi.org/10.1002/anie.201308888
giving this material a weak ability to conduct electricity
Galkin, N. G., Galkin, K. N., Chernev, I. M., Goroshko, D. L., Chusovitin, E. A., Shevlyagin, A. V., . . . Khovaylo, V. V. (2018). Comparison of the structural, optical and thermoelectrical properties of ca silicide films with variable composition on si substrates. Defect and Diffusion Forum, 386, 3-8. doi:https://doi.org/10.4028/www.scientific.net/DDF.386.3
Films consisting mostly of calcium monosilicide may be grown on a silicate subtrate of either (100) or (111) orientation by reactive deposition epitaxy (RDE) or molecular beam epitaxy (MBE) in an ultra-high vacuum chamber
. The RDE annealing temperature for the formation of a CaSi-film was 330°C and the temperature of the substrate was 190-320°C. the Zintl phase was deposited by using silicone and calcium blocks as particle beam sources
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. By using MBE at a higher annealing temperature of 500°C a single state layer of pure, rectangular CaSi-crystals was obtained by Galkin et. al.
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(Table1) shows relevant crystal structure information, with Ca-Si distance1 signifying the distance between Si (teal) and Ca (blue) in the horizontal (ac) plane, Ca-Si distance2 being diagonally opposed and Ca-Si3 as vertically opposed on b-axis.
|Cell parameters||4.5516(2) 10.7002(4) 3.8869(1) 90. 90. 90.|
|Cell volume||189.30 Å³|
|Space group||C m c m (63)|
|Si-Si distance||2.4515(1) Å|
|Ca-Si distance1||3.08408(2) Å (4×)|
|Ca-Si distance2||3.19286(6) Å (2×)|
(Table2) shows general physical properties of this Zintl phase
|Color||glossy metallic gray|
|Absorption coefficient µ (1/mm)||3.36|
|Molar mass (g/mol)||272.68|
The electric properties of this compound do not match that of a intermetallic, saltlike compound, which is the typical definition of a Zintl-Klemm phase. The Ca-Si interactions in calcium silicide are not ionic in character, instead they consist of a polar σ-donation from Si and a weak π-donation from Si. This compound shows mostly covalent interactions in Si-Si bonds, in line with the Zintl-Klemm model, but the surprising and part covalent interaction between Si bonds and the Ca cation shows that zintl phases may display more metallic properties than previously thought
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Calcium silicate CaSi has shown to be effective at destabilizing calcium dihydride, making it an important part of thermochemical energy storage reactions used in concentrated solar powerplants. In the proposed multiple stage reaction calcium disilicate reacts with CaH2 to form hydrogen gas and calcium monosilicate as an intermediary as per Figure(2), absorbing ca. 150kJ energy per mol of H2 gas formed by the reaction.
Griffond, A., Sofianos, M., Sheppard, D.,Humphries, T.,Sargent, A., Dornheim, A., Aguey-Zinsou, K., Buckley, C., High-temperature thermochemical energy storage using metal hydrides: Destabilisation of calcium hydride with silicon, Journal of Alloys and Compounds, Volume 858, 2021,
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Figure(1) Structure of CaSi created in Jmol with Si pictured in blue and Ca in teal color (Figure: Marcus Olsio)
Figure(2) Proposed pathway featuring CaSi as an intermediate formed in reaction 3 (Creative Commons license)
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