Modulated crystal structure of chimney-ladder higher manganese silicides MnSiγ (γ∼1.74)

The crystal structure of a polycrystalline sample of higher manganese silicide (HMS) has been determined by means of the (3+1) -dimensional superspace group approach. The structural parameters were refined with a superspace group of I 41 /amd (00γ) 00ss using powder neutron-diffraction data collected at 295 K. The compound belongs to a composite crystal family consisting of [Mn] and [Si] subsystems, with an irrational c -axis ratio (misfit parameter) of γ= cMn / cSi ∼1.74. Significant in-plane rotational modulation was revealed in the "chimney"-[Si] subsystem, while positional modulation in the "ladder"-[Mn] subsystem was only realized along cMn. The electronic structure of the sample was calculated on the basis of a commensurate approximation of the modulated structure using the full potential linearized augmented plane-wave method. The obtained band gap of Eg ∼0.6 eV agreed well with the experimentally observed one. It appears that the band gap and density of states of the HMS samples depend on the positional modulation of the Si atoms. The various controversial formulas (for example, Mn4 Si7, Mn11 Si19, and so on) of the HMS phases reported thus far can be regarded as commensurate cases of a series of incommensurate MnSiγ phases in which the γ value ranges from ∼1.70 to 1.75.