X-ray powder diffraction structure reinvestigation of the α and β forms of cobalt phthalocyanine and kinetics of the α → β phase transition

The structures of the acid-paste-prepared R-form of cobalt phthalocyanine (R-PcCo) and of its corresponding �-form (�-PcCo) have been refined through X-ray powder diffraction using the Rietveld method. The R-polymorph is triclinic, space group P1h, cell parameters a )12.090(8) Å, b )3.754(2) Å, c )12.800- (9) Å, R)88.96(6)°, � )90.97(6)°, θ )95.09(7)°, and Z )1; the �-form is monoclinic, space group P21/a, cell parameters a )14.5982(9) Å, b )4.7937(3) Å, c )19.4348(11) Å, � )120.782(3)°, and Z )2. R-PcCo consists of columnarly arranged molecules, those of adjacent columns being aligned parallel to each other, whereas a nearly perpendicular arrangement is present in �-PcCo. R-PcCo also differs from the R-forms of PcH2 and PcPt in which an angle of ca. 125° is observed between molecules of closely contacting chains. From the refined peak-profile parameters of the powder pattern a crystallite size of �150 Å has been estimated for R-PcCo, a value approaching that of nanocompounds. The isothermal R f � phase transition has been followed in-situ real-time by means of EDXD at two different temperatures, 152 and 250 °C. Calorimetric data indicate two different behaviors of the rate constant in the temperature ranges 182.5-220 and 220-270 °C leading to values of the empirical activation energy, Ea, of 36.6(16) and 14.8(9) kcal/mol, respectively. However the various DSC runs lead to values of the phenomenological n parameter in the range 2.34-2.68 (indicating an isokinetic two-dimensional growth). According to the combined EDXD and DSC data a three-step model for the Rf � PcCo phase transition may be proposed: (1) disordering of adjacent layers of PcCo molecules from their original R-type arrangement; (2) crystallization of the �-form from the disordered phase through rearrangement of the layers of the phthalocyanine units; (3) crystal growth of the �-phase from the nuclei to an average particle size of � 2500 Å, as indicated by the fitted peak-profile parameters. No evidence of the occurrence of intermediate ordered phases was observed.