A phase transition in the novel three-dimensional compound [Eu2(mal)3(H2O)6] (H2mal = malonic acid)

Abstract

Slow diffusion of aqueous solutions of europium(III) chloride into gel of sodium metasilicate containing malonic acid (H₂mal) yields single crystals of the three-dimensional compound of formula [Eu₂(mal)₃(H₂O)₆] whose structure was determined by X-ray diffraction methods at 293 and 173 K. It crystallizes in the monoclinic system but the spatial group changes from I2/a in the high temperature range (293 ≥ T ≥ 236 K) to Ia in the low temperature range (T < 236 K). In both cases, nine oxygen atoms forming a distorted monocapped square antiprism surround the Eu³⁺ ions. The structure at 293 K consists of a three-dimensional arrangement of triaquaeuropium(III) units bridged by malonate groups which result from cross-linking of the single chains running parallel to the c axis and the double zig-zag chains which grow in the ab plane. At low temperature the structure of the compound can be visualised as chains of europium(III) ions linked through two of the three crystallographically independent malonate ligands, whose chains run parallel to the b axis and a second family of chains (along the c axis) through the third independent malonate ligand forming a three-dimensional network. In both the crystal structure is stabilised through extensive hydrogen bonding involving carboxylate and water molecules. Studies of the magnetic behaviour, spectroscopic, thermogravimetric and calorimetric characteristics of [Eu₂(mal)₃(H₂O)₆] are reported. Laser-excited site selective spectroscopy shows a unique crystal-field site for Eu^III ions in the crystal at room temperature and down to 236 K. However, below this temperature, two different sites are clearly identified, in agreement with a change in the crystal structure.