The absence of ferroelectric polarization in layered and rock-salt ordered NaLnMnWO6 (Ln = La, Nd, Tb) perovskites.

The ordered perovskites, NaLnMnWO6 (Ln = La, Nd, Tb), are reported to exhibit simultaneous ordering of A-site cations (Na and Ln) in layered arrangement and B-site cations (Mn and W) in rock salt structure. They have been shown to crystallize in a monoclinic structure with the polar space group P21. Based on density functional calculations and group theoretical analysis, it has recently been proposed that NaLaMnWO6 should be ferroelectric with a relatively large polarization (16 μC cm(-2)). Contrary to this prediction, our electrical measurements such as conventional P-E loop, Positive-Up and Negative-Down (PUND), piezoelectric response and Second Harmonic Generation (SHG) reveal the absence of ferroelectric polarization in NaLnMnWO6 (Ln = La, Nd, Tb). A dielectric anomaly is observed just below room temperature (∼270 K) for all the three compounds, which is related to the change in conductivity as revealed by temperature dependent ac and dc resistivity. A pyrocurrent peak is also observed at the same temperature. However, its origin cannot be attributed to a ferroelectric transition.