Phase transitions and electrical properties of NH4H2(PO4)(0.52)(AsO4)(0.48) mixed crystals

A new compound NH4H2(PO4)(0.52)(AsO4)(0.48) (ADAP) of the NH4H2PO4-NH4H2AsO 4 system has been prepared by slow evaporation of an aqueous solution at ro om temperature. This mixed compound is isostructural of the tetragonal phas es NH4H2PO4 (ADP) and NH4H2AsO4 (ADA) with the following parameters: a = 7. 5920(13) Angstrom, c = 7.6260(2) Angstrom (Z = 4). Two kinds of disorder ca n be expected in this structure: one, statistical or dynamical, in the O-H- O hydrogen bond and the other related to the reorientational motion of NH4 ions. Two-phase transitions at T-1 = 193 and T-2 = 418 K were detected and studied by differential scanning calorimetry, dielectric measurements and impedance spectroscopy. The evolution of the dielectric constant as a funct ion of frequency and temperature revealed the first transition to be of fir st order and antiferro-paraelectric. It is assumed that the reorientation o f NH4+ ions is more responsible for this phase transition than the hydrogen dynamics of the H2P(As)O-4(-) groups. The high-temperature phase transitio n at 418 K leads to a superionic-protonic phase. A jump in the conductivity is associated with this transition at an unusually high value of conductiv ity (4 X 10(-3) Omega (-1) cm(-1)) and low activation energy (E-a congruent to 0.3 eV) above 418 K. (C) 2001 Elsevier Science B.V. All rights reserved .