Positron annihilation study in binary molecular solid solutions of metal acetylacetonate complexes using positron annihilation lifetime (PAL) and Doppler broadening (DBS) spectroscopies

Abstract

PAL and DBS techniques have been used to study inhibition of positronium (Ps) formation in binary molecular solid solutions of Al(III) (tris)-acetylacetonate (acac), as matrix, and Co(III), Fe(III), Ru(III), Rh(III), and Ir(III) (tris)-acetylacetonates, as guest molecules. For all the studied systems, no Ps quenching reactions have been observed. For Al (1− x) Co ( x) (acac) 3, Al (1− x) Fe ( x) (acac) 3, and Al (1− x) Ru ( x) (acac) 3 systems, a strong inhibition of Ps formation was observed with increasing of guest molecule concentration. The obtained Ps yields measured by PAL were fitted by the Stern-Volmer equation. The DBS results confirm the inhibition constants ( k) previously determined by PAL technique, except for the Al (1− x) Ru ( x) (acac) 3 system, where the experimental and calculated Doppler broadening curves are quite different. Only in Al (1− x) Ir ( x) (acac) 3 system, the inhiition effect was not detected. The highest total inhibition constants belong to Al (1− x) Co ( x) (acac) 3 and Al (1− x) Ru ( x) (acac) 3 systems and the smallest one to Al (1− x) Rh ( x) (acac) 3 system. The magnitude of the obtained k values was associated with the ability of the guest complexes to scavenge electrons and are closely related to the redox properties of the complexes. X-ray diffraction measurements were made to confirm the hypothesis that all studied samples are single phase systems and that the introduction of the guest molecules in the matrix lattice does not result in significant crystalline changes. These results are well explained in the framework of the spur model and show that the chemical properties determine the Ps yields in molecular solids, regardless of their free volume characteristics.