Electron diffraction study of newly discovered nickel phosphides in partially crystallized amorphous electrodeposited Ni-P thin films

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Abstract
Amorphous electrodeposited thin films of nickel phosphides, containing 20 and 22 at. % P were studied by electron microscopic methods. Upon beam heating the amorphous phase transformed into Ni12P5 and several other new crystalline phases, with or without additional metallic nickel. The following crystallizations from the electrodeposited Ni-P films were observed: (1a) Amorphous Ni-P→NixPy(Ni)+Ni. The probable values of x and y are 5 and 2, respectively. The lattice parameters are a=6.61 and c=12.31 Å. Upon further beam heating (1a) transforms to: (1b) NixPy(Ni)+Ni→Ni3P+Ni. The body-centered tetragonal unit cell dimensions of Ni3P are a=8.93 and c=4.39 Å. (2) Amorphous Ni-P→Ni12P5. The unit cell of Ni12P5 is again body-centered tetragonal, with a=8.64 and c=5.07 Å. (3) Amorphous Ni-P→Ni3P. The hexagonal unit cell parameters are a=5.00 and c=8.66 Å. (4) Amorphous Ni-P→Ni3P+Ni. The unit cell is again body-centered tetragonal, like that of (1b). However, the dimensions are a=6.10 and c=5.04 Å. (5) Amorphous Ni-P→Ni3P. The body-centered unit cell dimension is a=6.10 Å. The observation of such a number of crystalline phases implies that the crystallization of the amorphous Ni-P films is more complex than expected and it must depend on a number of factors, including rates of electron beam heating, temperature gradients, and the homogeneity of the as-deposited sample. Although the unit cell dimensions of various phases are distinct, there is an apparent geometric relationship between the magnitudes of individual unit translations. This observation indicates that the primary structures of these phases must be simple closed-packed structures derivable from hcp, fcc, and bcc structures. Interestingly, while the transformation from the amorphous Ni-P to the equilibrium two-phase mixture of (1b) occurred through the crystallization of the intermediate metastable two-phase mixture of (1a) in accordance with the Ostwald rule, the transformation from the amorphous Ni-P to the equilibrium phases of 2, 3, 4, and 5 underwent no subsequent transformation, therefore violating the Ostwald rule.