X-ray powder diffraction structure reinvestigation of the alpha and beta forms of cobalt phthalocyanine and kinetics of the alpha ->beta phase transition

The structures of the acid-paste-prepared a-form of cobalt phthalocyanine ( alpha-PcCo) and of its corresponding beta-form (beta-PcCo) have been refine d through X-ray powder diffraction using the Rietveld method. The ct-polymo rph is triclinic, space group P (1) over bar, cell parameters a = 12.090(8) Angstrom, b = 3.754(2) Angstrom, c=12.800(9) Angstrom alpha = 88.96(6)degr ees, beta = 90.97(6)degrees, gamma = 95.09(7)degrees, and Z = 1; the beta-f orm is monoclinic, space group P2(1)/a, cell parameters a = 14.5982(9) Angs trom, b = 4.7937(3) Angstrom, c = 19.4348(11) Angstrom, beta = 120.782(3)de grees, and Z = 2. alpha-PcCo consists of columnarly arranged molecules, tho se of adjacent columns being aligned parallel to each other, whereas a near ly perpendicular arrangement is present in beta-PcCo. alpha-PcCo also diffe rs from the alpha-forms of PcH2 and PcPt in which an angle of ca. 125 degre es is observed between molecules of closely contacting chains. From the ref ined peak-profile parameters of the powder pattern a crystallite size of si milar to 150 A has been estimated for alpha-PcCo, a value approaching that of nanocompounds. The isothermal alpha --> beta phase transition has been f ollowed in-situ real-time by means of EDXD at two different temperatures, 1 52 and 250 degrees C. Calorimetric data indicate two different behaviors of the rate constant in the temperature ranges 182.5-220 and 220-270 degrees C leading to values of the empirical activation energy, E-a, of 36.6(16) an d 14.8(9) kcal/mol, respectively. However the various DSC runs lead to valu es of the phenomenological n parameter in the range 2.34-2.68 (indicating a n isokinetic two-dimensional growth). According to the combined EDXD and DS C data a three-step model for the alpha --> beta PcCo phase transition may be proposed: (1) disordering of adjacent layers of PcCo molecules from thei r original alpha-type arrangement; (2) crystallization of the beta-form fro m the disordered phase through rearrangement of the layers of the phthalocy anine units; (3) crystal growth of the beta-phase from the nuclei to an ave rage particle size of similar to 2500 Angstrom, as indicated by the fitted peak-profile parameters. No evidence of the occurrence of intermediate orde red phases was observed.