THE HIGH-RESOLUTION CRYSTAL-STRUCTURE OF POTASSIUM AMMONIUM DIHYDROGEN PHOSPHATE MIXED-CRYSTALS - K0.11(NH4)0.89H2PO4 AT 154-K, 164-K, 192-K AND 295-K

The x-ray crystal structure of a mixed crystal of potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (ADP), K1-x(NH4)XH2PO4 (KADPX) With X = 0.89, has been refined in the paraelectric phase at 154, 164, 192 and 295 K, against high-resolution data (((sin theta)/lambda)max congruent-to 1.2 angstrom-1) with multipolar pseudoatom form factors. The phase transition temperature is observed to lie at T(c) = 132 +/- 1 K; a linear temperature dependence of the atomic vibration amplitudes of P and O is observed; extrapolation of U33 curves to T = 0 K suggests a distribution of P between two sites separated by 0.2 angstrom. The structure of KADP0.89 is compared with ADP and KDP structural data.