Unexpected wide tuning of ferroelectric properties by varying the Er concentration in La2-xErx(MoO4)3 (x = 0.75, 1, 1.25) solid solutions

Powder samples of the series La2-xErx(MoO4)3 (x ​= ​0.75, 1, 1.25) were prepared by solid-state reaction from the La and Er trimolybdates. The correlation of the ferroelectric properties with the crystal structure, as a function of the x concentration, was studied by X-ray diffraction (XRD), dielectric and optical spectroscopy, and the ferroelectric hysteresis loop measurements. The β′-Gd2(MoO4)3 structure type was identified by a non-conventional Rietveld method, including a symmetry analysis. Contrary to the expectations, the ferroelectric properties were intensified by the Er3+ concentration increase: the maximum polarization for x ​= ​1.25 reaches one order of magnitude larger than that of x ​= ​0.75, and the Curie temperature increase was around 200 ​K. This unexpected behavior is opposite of that observed for the improper ferroelectrics RE2(MoO4)3, where the ferroelectric properties are enhanced with the RE ionic radius increase. Hence, the series La2-xErx(MoO4)3 show an "wider tuning range" for ferroelectric properties opposite to the rare earth trimolybdate family.