Mechanism behind the calcium aluminum silicide ternary structural preference and the origin of its semimetal behavior, The

Abstract

CaAl2Si2 is the prototype of the CaAl2Si2 class of Zintl structures established to be useful as thermoelectrics. We propose that CaAl2Si2 be interpreted as an ordinary covalently bonded,tetrahedrally coordinated quasi-semiconductor consisting of a large distortion of the wurtzite structure with the almost fully ionized Ca inserted at an interstitial site. We support this interpretation via a structural mapping and calculations for both a charged binary primitive cell and a Si4 primitive cell. Our intent is to explain the unusual structure of the CaAl2Si2 class of semiconductors, the origin of its semimetallic behavior, the basis for its stability and the effect of substituting other column II atoms for Ca on these properties. To be clear, this work does not examine the nature of the true band gap, or the transport coefficients of CaAl2Si2. GW corrections are not discussed, in view of the focus on the origins of stability of this peculiar structure.