Infrared spectroscopy has been used to study a series of synthetic agardite minerals. Four OH stretching bands are observed at around 3568, 3482, 3362, and 3296 cm<sup>-1</sup>. The first band is assigned to zeolitic, non-hydrogen-bonded water. The band at 3296 cm<sup>-1</sup> is assigned to strongly hydrogen-bonded water with an H bond distance of 2.72 Å. The water in agardites is better described as structured water and not as zeolitic water. Two bands at around 999 and 975 cm<sup>-1</sup> are assigned to OH deformation modes. Two sets of AsO symmetric stretching vibrations were found and assigned to the vibrational modes of AsO<sub>4</sub> and HAsO<sub>4</sub> units. Linear relationships between positions of infrared bands associated with bonding to the OH units and the electronegativity of the rare earth elements were derived, with correlation coefficients >0.92. These linear functions were then used to calculate the electronegativity of Eu, for which a value of 1.1808 on the Pauling scale was found.

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