Orientations of the Optical Indicatrices of KH₂PO₄and NH₄H₂PO₄in Their Low Temperature Phases

Abstract

For KH 2 PO 4 (KDP) and NH 4 H 2 PO 4 (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, n a , n b and n c have been found to satisfy n a > n b > n c for Fdd2 cell in KDP, and for P2 1 2 1 2 1 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.