EVOLUTION OF THE STRUCTURAL AND MEAN-SQUARE DISPLACEMENT PARAMETERS IN (NH4)(X)K1-XH2PO4 SOLID-SOLUTIONS VERSUS CONCENTRATION AND TEMPERATURE

Accurate x-ray data sets were collected at different temperatures for mixed crystals of (NH4)(x)K1-xH2PO4 (KADP(x)), where x = 0.9, 0.8, 0.5 , 0.4 and 0.0. The phase transition temperatures of KADP(0.8) and KADP (0.9) were observed at T-c = 109 K and T-c = 132 K respectively. The v ibration amplitudes ate found to be larger for concentrations correspo nding to the glassy phases. These large vibrations result from the sta tic part of the mean square displacement which may be due to a static disorder due to a large difference between cell parameters of pure pha ses, or to fluctuations of the concentration throughout the crystal. I n the range 0.23 < x < 0.75, competition between the two H2PO4 configu rations occurs (competition between the ferroelectric and antiferroele ctric states): this leads to the observed static disorder of H2PO4 gro ups which prevents the phase transition from occurring.