ORIENTATIONS OF THE OPTICAL INDICATRICES OF KH2PO4 AND NH4H2PO4 IN THEIR LOW-TEMPERATURE PHASES

For KH2PO4 (KDP) and NH4H2PO4 (ADP), the orientation of the optical indicatrix for NaD light (589 nm) has been determined in the low temperature orthorhombic phase, through the optical and X-ray measurements. For both KDP and ADP, the principal refractive indices, na, nb and nc have been found to satisfy na>nb>nc for Fdd2 cell in KDP, and for P212121 cell in ADP. Both crystals are optically negative and the c axes are the acute bisectrices with the optical axial angle 2Ω=61° in KDP at 90 K and 2Ω=20° in ADP at 122 K. The {110} twin of ADP observed in the low temperature phase has been found to form the rotation twin with the two fold axis normal to the {110} twin boundary. © 1990, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.