Structure of cathodically deposited nickel hexacyanoferrate thin films using XRD and EXAFS

X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) data were used to explore the structure of cathodically deposited thin films of nickel hexacyanoferrate (NiHCF). Thin films were electrodeposited on Pt substrates and analyzed in both the reduced and oxidized states and when intercalated with Cs⁺, K⁺, and mixtures of Cs⁺/K⁺. All experiments were performed at room temperature. XRD data provided lattice parameters and confirmed the cubic nature of this polycrystalline material, but it could not establish the exact space group from the many known to exist for this class of materials. The lattice parameter monotonically increased from ca. 10.15 to 10.21 Å as the fraction of intercalated cation went from 100% K⁺ to 100% Cs⁺. EXAFS data permitted the determination of average nickel coordination, a property directly related to the material's refined structure. Fe and Ni K-edge data were fit using feffit2.32 and FEFF7; the data were fit simultaneously in order to break the correlation between coordination number and the Debye-Waller factor. This resulted in mean nickel coordination values between 4.4 and 5.1 and estimated lattice parameters which are ca. 0.1 Å greater than those found from XRD, a trend indicative of local disorder within the sample.