Phase transitions in PbNb0.5B0.5O3(B-Sc, In)

When studying the compounds of the PbNb0.5B0.5O3 series it has been established that with a critical ionic radius RB = 0.85Å there occurs a transition from ferroelectric (B-Cr, Fe, Mn, Sc, In) to antiferroelectric (B-Lu, Yb, Tm). The structure of crystals and polycrystalline specimens has been studied by X-ray diffraction techniques, and also the dielectric properties of PbNb0.5Sc0.5O3 and PbNb0.5In0.5O3 located near the transition boundary from ferroelectrics to antiferroelectrics are discussed. It has been established that PbNb0.5In0.5O3 corresponds to a ferroelectric with a diffuse phase transition. The phase transition in PbNb0.5Sc0.5O3 shows a small diffuse nature. The temperature-dependences of the unit cell parameters, dielectric constants and loss tangents have been obtained. It has been established for the first time that PbNb0.5In0.5O3 at room temperature is rhombohedral (R3m) with cell parameters a = 4.1132 ± 0.0004Å, α = 89.97 ± 0.02°. A preliminary determination of the PbNb0.5Sc0.5O3 structure at room temperature has been carried out.