Introduction

Calcium (mono)silicide (CaSi) is a Zintl-phase with semimetallic and semiconducting properties and a potential for future use in thermoelectric applications ^[1]. It has a melting point of 1320°C and may be successfully grown as single crystals. Its structure consists of twofold bonded Si^2- 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 ^[2] giving this material a weak ability to conduct electricity^[3]. 

Figure 1. Structure of CaSi created in VESTA with Si pictured in blue and Ca in teal color (Figure: Marcus Olsio)

Synthesis

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 ^[1]. 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 ^[3]. 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. ^[1]

Crystal structure

Table 1. 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.

Physical properties

Table 2. General physical properties of CaSi.

Bond Type

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 ^[2]. 

Applications

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. ^[5]

Figure 2. Proposed pathway featuring CaSi as an intermediate formed in reaction 3 (License: CC BY 4.0)^[5]

References