Ferroelectric relaxor behaviour in Bi and K/Na-substituted solid solutions of tetragonal tungsten bronze PbNb2O6

Abstract

Lead metaniobate (PbNb2O6 or PNO) belongs to the tetragonal tungsten bronze (TTB) family. There are three polymorphic forms of PNO: tetragonal (stable at high temperature), rhombohedral (stable at room temperature), and the metastable orthorhombic phase, which has ferroelectric properties. The metastable orthorhombic phase is formed by quenching from the high temperature tetragonal phase due to the reconstructive/sluggish tetragonal to rhombohedral phase transition. A suitable ionic substitution can also lead to the stable ferroelectric orthorhombic phase. Stabilization of the ferroelectric orthorhombic phase by ionic substitution, and the formation of ferroelectric relaxors with increased amounts of dopants is hereby reported for the compositions Pb(1-x)Bi(x/2K(x/2)Nb2O6 and Pb(1-x)Bi(x/2)Na(x/2)Nb2O6. The solid solutions of PNO showed evidence of linked dielectric relaxation and ferroelectricity, validating the distinctive ferroelectric relaxor behaviour for the compositions Pb0.6Bi0.2K0.2Nb2O6 and Pb0.6Bi0.2Na0.2Nb2O6. As the concentrations of bismuth (Bi) and potassium/sodium (K/Na) dopants increased, the Curie temperature (Tc), or the temperature corresponding to the maximum dielectric permittivity (Tm), dropped. By using the Uchino-Nomura relation as a modification of the Curie-Weiss law, it was established that the Pb0.6Bi0.2K0.2Nb2O6 solid solution exhibit higher degree of diffuseness than Pb0.6Bi0.2Na0.2Nb2O6 at the transitions. The width of the ferroelectric hysteresis loop increased as the percentage of dopants increased, from x = 0.1 to x = 0.3. At x = 0.4, a narrow ferroelectric hysteresis loop and a frequency-dependent Tm abruptly formed, which is indicative of a ferroelectric relaxor.