SYMMETRY OF THE NORMAL AND LOW-TEMPERATURE PHASE OF BETAINE CALCIUM-CHLORIDE DIHYDRATE AS REVEALED BY CL-35 NMR

Quadrupolar perturbed Cl-35 NMR spectra are measured in the high-tempe rature normal (N) phase and in tile low-temperature (LT) phase of beta ine calciumchloride dihydrate (BCCD) single crystals. The electric fie ld gradient (EFG) tensors at the chlorine sites are precisely determin ed for both phases. In the N phase, the symmetry of the tensors is in agreement with the space group Pnma known from X-ray studies. The symm etry of the tensors in the LT phase allows to exclude the P11a symmetr y proposed previously and confirms the space group Pn2(1)a derived the oretically on the assumption that one order parameter drives the phase transitions in BCCD.