Evaluation of the solid state dipole moment and pyroelectric coefficient of phosphangulene by multipolar modeling of X-ray structure factors

The electron density distribution of the molecular pyroelectric material ph osphangulene has been studied by multipolar modeling of X-ray diffraction d ata. The "in-crystal" molecular dipole moment has been evaluated to 4.7 D c orresponding to a 42% dipole moment enhancement compared with the dipole mo ment measured in a chloroform solution. It is substantiated that the estima ted standard deviation of the dipole moment is about 0.8 D. The standard un certainty (s.u.) of the derived dipole moment has been derived by splitting the dataset into three independent datasets. A novel method for obtaining pyroelectric coefficients has been introduced by combining the derived dipo le moment with temperature-dependent measurements of the unit cell volume. The derived pyroelectric coefficient of 3.8(7)x 10(-6) Cm-2K-1 is in very g ood agreement with the measured pyroelectric coefficient of p = 3 +/- 1 x 1 0(-6) Cm-2 K-1. This method for obtaining the pyroelectric coefficient uses information from the X-ray diffraction experiment alone and can be applied to much smaller crystals than traditional methods.