X-ray study of betaine arsenate and deuterated betaine arsenate

Accurate X-ray measurements were performed in betaine arsenate (BA) and str ongly deuterated betaine arsenate (DBA92) in the temperature range 300 to 8 0 K. The temperature dependence of the lattice parameters was studied and i t provides a clear evidence for the existence of two phase transitions, one at 119 K and another one at 107 K. The symmetry of the low temperature pha se of BA is P1n1. Nevertheless, our results are not conclusive about the sy mmetry of the antiferroelectric and ferroelectric phases in DBA92. The corr elation lengths of the ordered phases in BA and DBA92 involve a limited num ber of unit cells, thus providing evidence for the existence of small polar clusters in these materials below T-c2.