POSITRON-ANNIHILATION STUDY IN BINARY MOLECULAR-SOLID SOLUTIONS OF METAL ACETYLACETONATE COMPLEXES USING POSITRON-ANNIHILATION LIFETIME (PAL) AND DOPPLER BROADENING (DBS) SPECTROSCOPIES

PAL and DBS techniques have been used to study inhibition of positroni um (Ps) formation in binary molecular solid solutions of AI(III) (tris )-acetylacetonate (acac), as matrix, and Co(III), Fe(III), Ru(III), Rh (III), and Ir(LU) (tris)-acetylacetonates, as guest molecules. For all the studied systems, no Ps quenching reactions have been observed. Fo r Al(1-x)Co-(x)(acac)(3), Al(1-x)Fe-(x)(acac)(3), and Al(1-x)Ru-(x)(ac ac)(3) systems, a strong inhibition of Ps formation was observed with increasing of guest molecule concentration. The obtained Ps yields mea sured by PAL were fitted by the Stern-Volmer equation. The DBS results confirm the inhibition constants (k) previously determined by PAL tec hnique, except for the Al(1-x)Ru-(x)(acac)(3) system, where the experi mental and calculated Doppler broadening curves are quite different. O nly in Al(1-x)Ir-(x)(acac)(3) system, the inhibition effect was not de tected. 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 A l(1-x)Rh-(x)(acac)(3) system. The magnitude of the obtained k values w as associated with the ability of the guest complexes to scavenge elec trons and are closely related to the redox properties of the complexes . X-ray diffraction measurements were made to confirm the hypothesis t hat all studied samples are single phase systems and that the introduc tion of the guest molecules in the matrix lattice does not result in s ignificant crystalline changes. These results are well explained in th e framework of the spur model and show that the chemical properties de termine the Ps yields in molecular solids, regardless of their free vo lume characteristics. (C) 1997 Elsevier Science B.V.