Effect of Some Isovalent and Aliovalent Substitutions on Structure and Ferroelectric Properties of BaBi4Ti4O15

Abstract

Bismuth layer-structured ferroelectrics are commercially attractive for applications such as non-volatile random access memory and high temperature piezoelectric devices due to their excellent fatigue endurance, fast switching speed, good polarization retention and high Curie temperature. BaBi4Ti4O15 is a four layered member of this family and is relaxor ferroelectrics.The aim of the present study is to investigate the effect of differentsubstitution on the properties of BaBi4TiO15 ceramics.The effect of some isovalent substitutions such as La3+, Zr4+ and aliovalent substitutions such as Na+, Al3+, and Nb5+ on the structural, ferroelectric and piezoelectric properties of BaBi4Ti4O15 were studied. The pure and substituted BaBi4Ti4O15 powders were synthesized by a modified chemical route. The precursor powders were prepared by precipitating bismuth and barium oxalate in the TiO2 powder suspension. Exothermic decomposition of oxalates facilitated the formation of BaBi4Ti4O15phase at 600oC onwards. Pure phase was formed after calcinations of precursor powder at 1000oC. The material was sintered at ~1100oC. Pure BaBi4Ti4O15 showed relaxor behaviour with a critical exponent γ ~ 1.88. Its d33 coefficient was ~11 pC/N.La3+ was substituted for Bi3+in the range x≤ 1.0 in BaBi4-xLaxTi4O15.La3+ ions was found to occupy the Bi2O2 layer for compositions with x ≥ 0.5. A significant increase in the diffuseness and relaxor behaviour were observed with increasing La3+. The increase in the relaxor behaviour can probably be attributed to the compositional fluctuation induced by La3+,partly in perovskite and partly in Bi2O2 layers. The substitution also resulted in a significant reduction in the dc conductivity for x=0.3 composition.In BaBi4Ti(4-x)ZrxO15 ceramics, the solid solubility limit for Zr4+ was up to x ~0.2. However, the lattice parameters increased beyond the solid solubility limit. Dielectric studies revealed the relaxor nature of the ceramics decreased with the substitution and vanished at x =0.5 composition. The dielectric loss decreased for x = 0.2 composition. The permittivitytemperature peak broadened significantly with increase in Zr4+ concentration.Na+ and La3+ cations were substituted for Ba2+ in BaBi4Ti4O15.The lattice parameters decreased with increase in substituent’s concentration accompanied with a structural change from orthorhombic to tetragonal. The grain sizes of the ceramics increased with increase in substations. The substitutions suppressed the relaxor behaviour and decreased the loss. BaBi4Ti4-x(Nbx/2Alx/2)O15 ceramics showed the solid solubility limit of (Nbx/2Alx/2) up to x~0.2. The permittivity increased significantly accompanied by adecrease in dielectric loss of the ceramics. In addition to these, the polarization and the piezoelectric coefficients also enhanced.In Ba(1-x)NaxBi4Ti(4-x)NbxO15ceramics the solid solubility was in the composition range 0.1 ≤x≤ 0.5, while the composition with x = 0.6, showed the presence of secondary phases like Bi2O3 and Na2Ti3O7. The lattice parameters decreased with an increase in orthorhombicity with the substitution. The ceramics with x = 0.2 showed highest resistivity.In BaBi3.8M0.2Ti3.8Nb0.2O15 (M = Mg, Ca, Sr, Ba) ceramics, the crystal structure was orthorhombic in case of Mg-based compound and was tetragonal for Ca, Sr and Ba-based compounds. The detailed Rietveld analysis revealed that Mg2+ ions seemed to occupy the Bi2O2 layer compared to the other cations occupying A-site of perovskite block. Maximum relatively permittivity and d33 piezoelectric coefficient increased in Mg-based compound, while the relaxor behaviour and remnant polarization enhanced the Ba-based compound.