Cation distribution in MgxMn1−x(HCOO)2·2H2O mixed crystals. X-ray diffraction and double matrix infrared spectroscopy

The isomorphic title series is investigated using single crystal X-ray diffraction and double matrix infrared spectroscopic techniques. The metal ion distribution at the two different metal sites (hexaformate-coordinated Me1 sites and mixed-coordinated Me2 sites) is found to exhibit a slight preference of Mn2+ ions to Me1 sites and Mg2+ ions to Me2 ones.The spectral regions at 2400–2500 cm−1 and 1300–1400 cm−1, corresponding to νOD of matrix-isolated HDO molecules and to symmetric COO stretching (ν2) and bending CH (ν5) modes, respectively, are mostly sensitive to the environment of the metal ion. When Mg2+ and Mn2+ are included in the structures of isotopically dilute Mn(HCOO)2·2H2O and Mg(HCOO)2·2H2O new infrared bands appear, corresponding to νOD of HDO molecules linked to the incorporated ions, and implying that new Mg−OH2···OCHO−Mn and Mn−OH2···OCHO−Mg interactions build up in the mixed formates. These bands appear at small concentrations of included metal ions (about 13 mol% of magnesium ions and 8 mol% of manganese ions, x = 0.13 and x = 0.92, respectively). The ν2 and ν5 modes caused by the incorporated cations bonded to formate ions are detected at x = 0.92 and x = 0.23 (Mn2+ ions in Mg(HCOO)2·2H2O and Mg2+ ions in Mn(HCOO)2·2H2O, respectively). Thus, the analysis of the infrared spectra (positions and intensities of the infrared bands) confirms the single crystal X-ray measurements that the metal ions are localized at the two metal positions. A slight preference of the Mg2+ ions to Me2 sites is owing to the stronger affinity of these ions to water molecules.