X-Ray Study of Thermal Parameters of NH₄H₂PO₄ and ND₄D₂PO₄ in the Paraelectric Phase

Abstract

Thermal parameters of P and N atoms of the antiferroelectric NH₄H₂PO₄ (ADP) and the deuterated isomorphous ND₄D₂PO₄ (DADP) have been observed as a function of temperature in the paraelectric (PE) phase. The thermal parameters U₃₃^P and U₁₁^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 U₁₁^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 (KH₂PO₄ and KD₂PO₄) 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.