First principles study of lead free piezoelectric AgNbO3 and (Ag1−xKx)NbO3 solid solutions

We present an ab-initio study of structural, electronic and ferroic properties of lead-free orthorhombic AgNbO3 in centrosymmetric Pbcm and recently proposed noncentrosymmetric Pmc21 space group symmetries. The calculated polarization of AgNbO3 in Pmc21 phase is ∼0.09 μC/cm2, whereas it is 55.9 μC/cm2 in the ideal cubic phase. The large polarization of AgNbO3 in the ideal cubic structure is attributed to off-centering of Ag as well as Nb ions at the A and B sites, respectively. In our calculations both phases (Pmc21 and Pbcm) of AgNbO3 show small relative energy difference (∼0.1 meV/f.u), thus suggesting the coexistence of centrosymmetric and non-centrosymmetric phases and thereby antiferroelectric and ferroelctric order in room temperature AgNbO3 polycrystals. We further study structural properties and spontaneous polarization in orthorhombic Pmc21 Ag1−xKxNbO3 solid solutions. The calculated spontaneous polarization of AgNbO3 is enhanced due to suppression of NbO6 octahedral tilt angles as K ion is substituted at Ag sites. Our study suggests that appropriate cation substitution should indeed result in the improvement of ferroelectric properties of lead-free AgNbO3.