Ab initio study of the microscopic mechanism leading to antiferroelectricity in NH4H2PO4 (ADP)

J. LASAVE; S. F. KOVAL; N. S. DALAL; R. L. MIGONI

FERROELECTRICS

Taylor & Francis Inc

Año: 2007

0015-0193

The microscopic mechanim leading to a low-temperature antiferroelectric (AFE) phase in H-bonded NH4H2PO4 is studied by means of first-principles calculations in the framework of the Density Functional Theory. We compute the total energy for atomic displacements following the ferroelectric (FE) and AFE mode patterns and also constraining the atoms in different cases. We calculate Mulliken populations for atoms and orbitals to show the important charge transfers and instabilities of the system. The AFE phase appears as a consequence of the H ordering in the O-H-O bridges leading to polarizations of the H2PO4 perpendicular to the tetragonal crystal axis and alternating in chains. It is found that the formation of N-H-O bridges with correlated charge transfers is essential in stabilizing this configuration against a FE phase polarized along the tetragonal axis, as e.g. in KH2PO4(KDP). We also study de closeness of FE with AFE configuration energies and other hypothetical phases, which may explain the experimental detection of signals arising from small FE defects in the PE phase near the transition.