Cation distribution in MgxZn1 − x(HCOO)2·2H2O mixed crystals

The analysis of the infrared spectra reveals that the spectral regions 2300-2500 cm−1 (νOD of matrix-isolated HDO molecules) and 1300-1400 cm−1 (symmetric COO stretching (ν2) and bending CH (ν5) modes) are mostly sensitive to the metal ion environment. The inclusion of Mg2+ and Zn2+ in the structures of Zn(HCOO)2·2H2O and Mg(HCOO)2·2H2O, respectively, leads to an appearance of new infrared bands corresponding to νOD of HDO molecules bonded to the incorporated ions (i.e. new hydrogen bonding systems MgOH2⋯OCHOZn and ZnOH2⋯OCHOMg are formed in the mixed formates). The respective new bands are observed at small concentrations of included Mg2+ ions (about 5 mol%, x = 0.05) and at considerably higher concentrations of included Zn2+ ions (about 30 mol%, x = 0.7). Contrarily, the ν2 and ν5 modes caused by the incorporated cations bonded to formate ions occur at x ≥ 0.3 and x ≤ 0.85 (Mg2+ ions in Zn(HCOO)2·2H2O and Zn2+ ions in Mg(HCOO)2·2H2O, respectively). Thus, the infrared spectroscopy experiments confirm the single crystal X-ray measurements that the Mg2+ ions are localized predominantly at Me2 sites and the Zn2+ ions at Me1 sites in the title mixed crystals. The pronounced preference of the Mg2+ ions to Me2 sites is owing to the strong affinity of these ions to water molecules.