Orientations of the Optical Indicatrices of KH₂PO₄ and NH₄H₂PO₄ in Their Low Temperature Phases

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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, 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 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.