Alloys” of the isostructural porphyrinic molecular conductors (phthalocyaninato)cobalt iodide, Co(pc)I, and (phthalocyaninato)nickel iodide, Ni(pc)I, of composition CoxNi1-(pc)I, x = 0.02, 0.10, 0.15, 0.20, 0.33, 0.50, 0.75, 0.90, and 0.95, have been prepared and shown to be homogeneous solid solutions by EPR and energy-dispersive X-ray microprobe analysis. The high (σRT ~ 500 Ω−1cm−1) metallic conductivity of Ni(pc)I (x = 0) is associated with holes (positive thermopower) created by partial oxidation from the valence π-band of the pc macrocycle, whereas the lower (σRT ~ 50 Ω-1cm−1) nonmetallic conductivity of Co(pc)I (x = 1) is associated with electron carriers (negative thermopower) in the partially oxidized dz2band of the metal-ion spine. For x≲ 0.75 EPR and static susceptibility measurements show the existence of Co2+(S = 1/2) local moments, although collective effects progressively modify both the EPR intensity and Curie constant as x increases. For 0 ≤x ≲0.2 the thermopower and magnetic data indicate that the band filling on the macrocycle is constant and the cobalt sites retain their Co2+valency, although the room-temperature conductivity falls ~ 3-fold and a maximum appears in the conductivity between 170 and 260 K. This change is attributed to paramagnetic scattering by the Co2+local moments. With further increase in x the conductivity continues to change smoothly toward the nonmetallic behavior of Co(pc)I. Surprisingly, the thermopower does not smoothly tend toward the large negative values for Co(pc)I but becomes increasingly positive as x is increased from 0.2 to 0.75 and begins to decrease only by x ~ 0.90. These results are interpreted to mean that alloys with 0.2 ≲ x ≲ 0.75-0.9 exhibit two independent conduction channels, hole carriers on the pc macrocycles and electron carriers on the metal-ion spine, and that the site of oxidation shifts progressively from the macrocycle to the metal ion as x is increased. Only by x ~ 0.95 (and T ~ 240 K) does the thermopower become negative, which signifies the crossover point beyond which all charge-transport behavior is determined by the electron carriers on the metal spine rather than the hole carriers on the macrocycle.
ASJC Scopus subject areas
- Colloid and Surface Chemistry