super(57)Fe Mossbauer spectroscopy and magnetic study of Al sub(13)Fe sub(4)

The results of ab initio electronic structure and electric field gradient (EFG) calculations, and of X-ray diffraction, super(57)Fe Mossbauer spectroscopy, and magnetic studies of Al sub(13)Fe sub(4) are reported. It is shown that Al sub(13)Fe sub(4) crystallizes in the monoclinic space group C2/m, in which Fe atoms are located at five inequivalent crystallographic sites, with the lattice parameters a = 15.503(2) A, b = 8.063(2) A, c = 12.464(2) A, and [beta] = 107.71(2)[degrees]. We demonstrate that zero-field Mossbauer spectra can be decomposed into three quadrupole doublets. With the aid of the calculated EFG parameters we show that the first doublet results from one Fe site, the second doublet is due to two other Fe sites, and the third doublet originates from the last two Fe sites. We find that the shape of the Mossbauer spectrum of Al sub(13)Fe sub(4) measured in an external magnetic field of 90 kOe can be accounted for with five component subspectra generated using the calculated EFG parameters at five inequivalent Fe sites. The quadrupole splittings corresponding to three component doublets are shown to increase with decreasing temperature and are well described by a T super(3/2) power-law relation. The Debye temperature of Al sub(13)Fe sub(4) is found to be 383(3) K. We find a pseudogap in the density of states (DOS), with a width of ~ 0.2 eV, that is centered 0.1 eV above the Fermi energy. The finite DOS at the Fermi energy confirms good metallicity of Al sub(13)Fe sub(4). The 1/T-like dependence of the magnetic susceptibility shows that Al sub(13)Fe sub(4) is a paramagnet.