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Among the vast pyrochlore group, oxide bismuth pyrochlores are quite interesting owing to the presence of stereochemically active lone pair electrons of bismuth and prominent Jahn-Teller interactions of the pyrochlore constituent cations leading to interesting structural modifications as well as emerging physical phenomenon of fundamental and technological significance. Particularly the bismuth pyrochlores nominally denoted by (Bi,M)MNO7-δ (M = transition metal, N= Nb, Ta, Sb) crystallizing with cubic symmetry at ambient conditions are intriguing due to the presence of ionic displacements and mixed cation occupancy especially at Bi site leading to non-stoichiometry and tunability of the physical properties with composition. Magnetic properties of the pyrochlore lattice has also garnered much attention as spin polarization on the frustrated square lattice indicated distinctive magnetic ground states such as spin glass, spin ice and spin liquid. In these cubic pyrochlores large cationic displacements coupled with uncontrolled random hopping, mixed cation crystallographic site occupancy and compositional variability gave rise to tunable dielectric relaxation. The cubic pyrochlore have been well known for showing low temperature as well as high temperature dielectric relaxation. Although long-range magnetic order was not supported on a bismuth pyrochlore lattice, recent findings reflected polar nature at room temperature in some of these disordered cubic pyrochlores comprising of highly polarizable cations. Herein we summarize the state of art in the exploration of cubic pyrochlores of Bi, and Nb/Ta/Sb in combination with various transition metals occupying multiple crystallographic sites. Their interesting structural, dielectric, magnetic properties and device applications are highlighted. Current challenges in the understanding of these materials are discussed and conclusions are put forth.

Lead-free sodium bismuth titanate-based (NBT) non-MPB solid solutions for different substitutions were synthesized by solid-state reaction method and their structural relation with ferroelectric and piezoelectric properties were analyzed. Extrinsic contribution to piezoelectric coefficient more particularly non-180° domain reorientation was facilitated by enhanced mobility. Minimum strain and low rhombohedral lattice distortion improves mobility and increases the piezoelectric coefficient which in turn enhances remnant polarization and reduces coercive field. Regarding substitution in NBT, as a rule of thumb, substitution of high polarisable ion in the A-site and low polarisable ion in the B-site enhances the piezoelectric coefficient in non-MPB NBT-based systems.

Sodium bismuth titanate–bismuth magnesium zirconate–bismuth magnesium titanate (NBT–BMZ–BMT) lead-free ternary ceramics are prepared by solid-state reaction method. Consequence on structural, dielectric, ferroelectric properties and piezoelectric coefficient is investigated. Single phase rhombohedral R3c phase is retained in all poled samples. Coercive field decreases due to minimum homogeneous strain (δ) which facilitates domain reorientation and domain switching. Reduced homogeneous strain is the key factor which lowers the coercive field and enhances the dielectric constant. NBT–BMZ–BMT ternary ceramics reduces the coercive field and possesses high remnant polarization, dielectric constant and piezoelectric coefficient—the important requirement in functional devices.

Multiferroic ceramics with switchable magnetization as well as high polarization at room temperature have remarkable technological impact. Herein, we report that a small concentration of Fe doping in the B-site of sodium bismuth titanate (NBT) lead to room temperature multiferroic nature with fascinating properties. The key factor responsible for the embellished multiferroic behavior at room temperature was electric field induced non-180° domain alignment bridged with dynamic exchange interaction of trapped electrons in ordered polarons. The optimum doping of 0.02 mole fractions Fe in NBT favored ferromagnetic loop with highest remanence of 198 memu/g and coercive field 133 Oe along with improved ferroelectric polarization of 31.8 μC/cm2 and a strong self-biased direct magnetoelectric coupling coefficient of 7.38 mV/cmOe. Strong coupling at low ac field, high reliability, easy control by an electric signal and fast switching behavior add importance to this lead-free Fe-doped sodium bismuth titanate ceramic as potential material, applicable in biocompatible magnetic sensors.

The search for multiferroic order in a single phase of bismuth pyrochlore has been unsuccessful so far. In this direction, our study unveiled the capability of a biphase of bismuth pyrochlore and spinel in hosting a multiferroic order at room temperature. A complex oxide biphase of cubic pyrochlore and cubic spinel crystals acquired in the Bi2O3-Nb2O5-2MnCO3-Fe2O3 system revealed the crystals of a spinel phase (Fe1.59(3)Mn1.39(3)O4.26(7)) intergrown in the dense pyrochlore (Bi1.35(1)Fe0.64(1)Nb1.26(1)Mn0.75(1)O6.39(5)) matrix. The average composition of the components of the investigated biphase was determined using an electron probe microanalyzer (EPMA). The structural features indicated the presence of large ionic displacements within the cubic pyrochlore phase as seen from the appearance of 442 reflection in the X-ray diffraction pattern and infrared active mode at ∼64 cm−1 in the Raman spectrum recorded at room temperature. The pyrochlore single-phase composition (Bi1.35(1)Fe0.64(1)Nb1.26(1)Mn0.75(1)O6.39(5)), as suggested from a thorough EPMA microstructural analysis, exhibited broad dielectric relaxation and an overall paramagnetic behavior. The observation of disordered superparamagnetism as well as dielectric relaxation in the biphase conformed to that of a relaxorlike multiferroic behavior at room temperature. Moreover, self-biased magnetoelectric voltage coefficients of 0.60 mV/cm Oe at 100 Hz and 5.54 mV/cm Oe at 1 kHz were detected between magnetization and electric polarization at room temperature. Therefore, the composite of such a pyrochlore and spinel with an inherent property of strong spin–orbit and spin–lattice coupling will be interesting from theoretical and experimental point of view in the arena of magnetoelectrics.The search for multiferroic order in a single phase of bismuth pyrochlore has been unsuccessful so far. In this direction, our study unveiled the capability of a biphase of bismuth pyrochlore and spinel in hosting a multiferroic order at room temperature. A complex oxide biphase of cubic pyrochlore and cubic spinel crystals acquired in the Bi2O3-Nb2O5-2MnCO3-Fe2O3 system revealed the crystals of a spinel phase (Fe1.59(3)Mn1.39(3)O4.26(7)) intergrown in the dense pyrochlore (Bi1.35(1)Fe0.64(1)Nb1.26(1)Mn0.75(1)O6.39(5)) matrix. The average composition of the components of the investigated biphase was determined using an electron probe microanalyzer (EPMA). The structural features indicated the presence of large ionic displacements within the cubic pyrochlore phase as seen from the appearance of 442 reflection in the X-ray diffraction pattern and infrared active mode at ∼64 cm−1 in the Raman spectrum recorded at room temperature. The pyrochlore single-phase composition (Bi1.35(1)Fe0.64(1)Nb1.26(1)Mn0.75(1)O...

Herein, we report a Na deficient single phase sodium bismuth titanate perovskite exhibiting ferromagnetism with reduced leakage conductivity and a self-biased high magnetoelectric (ME) coupling coefficient at room temperature (27 °C). Observed ferromagnetism is induced by sodium cation vacancy dominated by spin polarization of holes in the valence band. An unexpected high ME coupling coefficient of about 4.18 mV/cm Oe, observed at zero DC magnetic bias field, is relatively large and juxtapose to other single phase multiferroics including BiFeO3. This study opens up an altered strategy in designing multiferroic materials with reinforced ME coupling at room temperature.