Crystal structure and energy gap of Ba1−xSrxSi2 (x = 0.20 and 0.41)

The crystal structure and energy gap of Ba1−xSrxSi2 (x = 0.20 and 0.41), a promising material for solar cells, have been investigated. Ba1−xSrxSi2 has the BaSi2-type structure (orthorhombic, Pnma, Z = 8, a = 8.8258(7), b = 6.6736(4), c = 11.4364(8) Å for x = 0.20, a = 8.7466(4), b = 6.6278(3), c = 11.3553(5) Å for x = 0.41). The unit cell volume decreases with respect to the Sr content x. Ba atoms at a specific crystallographic site, A1 site, are substituted by Sr atoms preferentially. As a result, The Sr substitution deforms BaSi2 inhomogeneously; the Sr substitution of 41% reduces the volume of coordination polyhedra of A1 and A2 atoms by 5.9 and 4.4%, respectively, while it reduces the volume of Si tetrahedron by a small amount, less than 1%. Diffuse reflectance spectroscopy has demonstrated that the energy gap increases with Sr content x, which is qualitatively consistent with the previous results obtained using Ba1−xSrxSi2 epitaxal thin films.