Structural, vibrational and calorimetric study of a new ammonium dihydrogen phosphate-arsenate: NH4H2(PO4)(0.52)(AsO4)(0.48)

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 evapor ation 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 isostrucrural with the tetragonal phases of ADP and ADA. This mixed compound crystallizes in space group I (4) over bar 2d and has lattice parameters a = 7.5920(13) Angstrom, c = 7.6260(2) Angstrom with four formula units in the unit cell. Two kinds of disorder can be exp ected in this structure: a statical or dynamical disorder of the acidic pro ton in the O-H...O hydrogen bond and another one which is connected with a reorientational motion of NH; ions. Broader peaks in the IR spectrum confir m a structural disorder in this material. Crystals of ADAP undergo two phas e transitions at T-1 = 193 K and T-2 = 423 K which are detected by differen tial scanning calorimetry (DSC). (C) 2001 Published by Elsevier Science Ltd .