Vapour growth, morphology, absolute structure and pyroelectric coefficient of meta-nitroaniline single crystals

Abstract

The growth speed of (hkl) faces in the vapour phase, the absolute structure obtained by X-ray crystallography, and the value and the sign of the pyroelectric coefficient of meta-nitroaniline (mNA) were analysed in detail. The in situ observation of morphologically well developed faces of several mNA crystals growing in evacuated ampoules reveals no pronounced growth speed anisotropy for polar faces defining the unique axis 2 of the mm2 group. Scanning pyroelectric microscopy confirms mono-domain mNA crystals. X-ray measurements in the space group Pca21 show that the molecular planes coincide with the {\bar 211} and {\bar 2\bar 1\bar 1} faces, and the nitro groups cover the {201} face in the opposite direction to the crystal tip, characterizing the polar habitus studied here. At room temperature, the sign of the pyroelectric coefficient is positive for a measured effective value of 6.3 µC m−2 K−1, in good agreement with values reported by other authors. From previous elastic and piezoelectric published data, the secondary pyroelectric effect was calculated to be positive and far greater than the effective one, yielding a negative value for the primary pyroelectric coefficient.