Abstract
Mixed crystals NH4H2(PO4)1−x(AsO4)x (ADAP) of the two antiferroelectric NH4H2PO4 (ADP) and NH4H2AsO4 (ADA) were grown with x=0.48 by slow evaporation from aqueous solution at room temperature. The structural properties of the crystals were analyzed by means of X-ray diffraction, which revealed that the title compound is isostructural with the tetragonal phases of ADP and ADA. This mixed compound crystallizes in space group I4̄2d and has lattice parameters a=7.5920(13) Å, c=7.6260(2) Å with four formula units in the unit cell. Two kinds of disorder can be expected in this structure: a statical or dynamical disorder of the acidic proton in the O–H…O hydrogen bond and another one which is connected with a reorientational motion of NH4+ ions. Broader peaks in the IR spectrum confirm a structural disorder in this material. Crystals of ADAP undergo two phase transitions at T1=193 K and T2=423 K which are detected by differential scanning calorimetry (DSC).
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