AB-INITIO STUDY OF THE RELATIONSHIP BETWEEN THE PRESSURE AND THE CHANGE IN THE DIPOLE-MOMENT OF A MOLECULE IN A FERROELECTRIC CRYSTAL

The typical properties of a ferroelectric material are piezoelectricit y and pyroelectricity. In this study we considered in detail the relat ionship between the pressure and the change in the dipole moment of a molecule in a ferroelectric crystal from the standpoint of piezoelectr icity. We applied ab initio molecular orbital calculations involving t he crystal effect to the orthorhombic thiourea ((NH2)(2)CS) crystal mo del as the ferroelectric crystal model. The molecular structure obtain ed by this calculation was in fairly good agreement with the result ob tained by X-ray analysis. In order to investigate the substituent effe cts, we calculated a hypothetical orthorhombic N, N'-difluorothiourea ((NKF)(2)CS) crystal model in a similar manner.