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.