Temperature- and Gaseous Phase-Mediated Reorganization and Paramagnetic Doping of Solid Aluminum Fluorides: ESR andab InitioQuantum Chemical Studies

Abstract

Based on cw-X-band ESR spectroscopic measurements of Mn2+doped AlF3powder samples and DFT(B3LYP)/6-31+G*quantum chemical calculations it is shown that structural reorganization of AlF3(AlF3(amorphous)→AlF3(crystalline)) are necessarily assisted by chemical reactions with the participation of water.It could be unambiguously demonstrated that Mn2+ions are suitable spin probes for reorganization processes from amorphous to local crystalline regions in fluoride matrices. The resolution of the55Mn–19F– superhyperfine structure (both the formation of regular MnF4−6– species as well as the reduction of strain effects by transformation of the amorphous parts) is a sensitive indicator of the formation of local crystalline regions.DFT(B3LYP)/6-31+G*calculations of (AlF3)n(H2O)mcomplexes (n: 1,2;m: 1–3) resulted in first and acceptable ideas of structures, energetical stabilities, and vibrational frequencies of hydrated AlF3. The calculated strength of the Al–O bond, resulting in the stable {AlF3–OH2} subunit, and the favored splitting of Al–F–Al bonds by H2O molecules, are the main reasons for the immediate and spontaneous hydration of freshly prepared amorphous AlF3. Independent of the size of the model complexes, stable substructures like {AlF3–H2O} and {F3AlFAlF2–OH2} can be recognized in all optimized structure models.