Hysteresis-like behavior in MBANP crystals

In a previous study of (-)-2-(alpha-methylbenzylamino)-5-nitropyridine (MBANP) [Avanci, L.H. et. al., Phys. Rev. B 2000, 61, 65071, X-ray multiple diffraction assessments of the small changes in the unit cell parameters due to a strain produced by (E) over right arrow, together with the resulting variations in the (hkl) peak position and the observed strain, were used to calculate the piezoelectric coefficients of this material. In the present work, we report the extension of these measurements to the determination of the variation of the unit cell parameters of this material under the influence of an externally applied dc electric field. Rocking curves were measured using a SIEMENS P4 single-crystal diffractometer equipped with encoders to ensure omega axis step sizes smaller than 1 mdeg. At the beginning of the experiments, the electric field was increased from zero to 3.2x10(5) V/m and then decreased to zero. It was then reversed in polarity from zero to -3.2x10(5) V/m and returned to zero again to complete the E-cycle. The measured strain versus E-cycle showed an interesting butterfly wing shape hysteresis behavior. Quantum mechanical calculations on isolated MBANP molecules show that the main features of the hysteresis shape can be explained in terms of field-induced changes in the charge profiles and geometry of isolated MBANP molecules.