The role of VEC and local atomic environment in structural transitions and stability of complex intermetallic compounds through theoretical and experimental study of γ-brasses

Eveland, Stephanie
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The fusion of computational and synthetic techniques provides a powerful investigative arsenal for a systematic consideration of the factors governing the structural makeup and observed physical properties of complex intermetallic compounds such as the non-uniform mixed sites in Mn2+xZn11-x, VEC driven structural transition in Mn5-yAl8-xZnx+y, and atypical atomic decoration of Cu9Al4 γ-brasses. Classic solid state synthetic techniques, self-flux techniques, as well as multiple structural characterization methods such as X-ray diffraction and electron microscopy were employed in the experimental portion of this work. Magnetic susceptibility measurements were utilized when appropriate in the study of the γ- brasses of Mn2+xZn11-x. To compliment the information from synthetic experiments DFT based first principles computational methods, TB-LMTO-ASA and VASP, were used to investigate the electronic structure of hypothetical colorings of unit cells with mixed site occupancy, and in order to structurally relax the unit cell near an experimentally observed structural transition.

In Mn2+xZn11-x a phase width was identified experimentally, and electronic structure calculations were used to show the role of heteroatomic bonding in promoting stability through inter- and intra-cluster bonding leading to overall stability near Mn2.2Zn10.8. Through systematic doping of Mn5Al8 with Zn a series of compounds (Mn5-yAl8-xZnx+y) was created highlighting the role of VEC in the stability of different structures without introducing size effects changing atomic radii by less than 15%. Zn does not preferentially substitute Al, it dopes into both Mn and Al rich sites and as a net process lowers the VEC. The structural transition from R3m to I-43m passes through a phase width ending in Mn2+xZn11-x.

The primitive decoration of Cu9Al4 has two geometrically and compositionally distinct 26-atom clusters which increases the number of unique connections and increases the number of heteroatomic contacts as compared to body centered decorations, by spreading out Al to two separate crystallographic sites with optimized contact distances.

gamma brass, Solid State synthesis, valence electron concentration