X-Ray Study of Thermal Parameters of NH₄H₂PO₄and ND₄D₂PO₄in the Paraelectric Phase

Abstract

Thermal parameters of P and N atoms of the antiferroelectric NH 4 H 2 PO 4 (ADP) and the deuterated isomorphous ND 4 D 2 PO 4 (DADP) have been observed as a function of temperature in the paraelectric (PE) phase. The thermal parameters U 33 P and U 11 N of both ADP an DADP show linear temperature-dependence indicating positive intercepts at T =0 K and the values of intercept for DADP are larger than those for ADP. The parameter U 11 P of DADP also has a positive intercept at 0 K in contrast with that of ADP which becomes zero as T →0 K. By the comparison with the structural data of the potassium salts (KH 2 PO 4 and KD 2 PO 4 ) reported by Nelmes et al. , it is concluded that the dipole moment of the ammonium ions polarized along the a ( b ) axis plays an essential role in the antiferroelectric phase transition.