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

Abstract

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.

Spin ordering and physical properties of NaPrFeWO6 and NaSmFeWO6 with polar double perovskite structure

“…In this way, studies based on first principles and symmetry analysis [15] predicted a ferroelectric polarization of ~16 µC cm -2 for NaLaMnWO6. However, experimental measurements performed on this sample [12] failed to find both second-harmonic generation (SHG) signal and spontaneous polarization using the PUND method, casting doubts about the actual symmetry of NaLaMnWO6. However, a clear SHG signal was obtained for NaLaFeWO6 though no ferroelectric loop was found using the PUND method again [18].…”

“…Moreover, layered ordering is preferred in A'AB2O6 compounds for an even mixture of A and A' cations [8,9]. However, the simultaneous ordering in both sites to give AA'BB'O6 compounds is rare and, although it has been little explored, it seems to be observed in a limited combination of atoms [10][11][12][13][14]. Recently, interest in these compounds has increased because they usually adopt a noncentrosymmetric structure with space group P21 (No.…”

Tuning the ferroelectric polarization in AA′MnWO6double perovskites through A cation substitution

“…44 A recent experimental study, however, found zero spontaneous polarization in samples of NaLaMnWO 6 and NaNdMnWO 6 . 45 This discrepancy could be attributed to the polycrystalline samples and difficulties in poling sintered ceramic pellets.…”

Understanding ferroelectricity in layered perovskites: new ideas and insights from theory and experiments

“…51 Recent experimental study, however, found no spontaneous electric polarizations in samples of NaLaMnWO 6 and NaNdMnWO 6 , which was attributed to difficulty in making dense ceramics required for electrical poling measurements. 99 The design of new multiferroics based on this cation-ordering concept was extended to double perovskites with the same cation order on the A and B sites, but included magnetic Ni and Mn cations on the B-site to stabilize ferromagnetic order. 100 (Recall that the double order and a − a − c + tilt is sufficient to provide for an electric polarization, yet if only B-site order is present then the structure is non-polar P2 1 /n, consistent with the guidelines of Rondinelli and Fennie).…”