Displacive disorder and spin frustration hosted multiferroic orders in pyrochlore–spinel composites

Abstract

Samples with nominal composition BiNbMn₂O_7−δ (BNMO), BiTaMn₂O_7−δ (BTMO) and BiSbMn₂O_7−δ (BSMO) have been synthesized via a mixed oxide route. Powder X-ray diffraction (PXRD) patterns of the investigated samples reveal a major cubic pyrochlore phase along with additional weak reflections due to a tetragonal Mn₃O₄ spinel phase (hausmannite). Phase analysis using Rietveld refinement shows the presence of significant 442 reflection in the powder X-ray diffraction (PXRD) pattern corresponding to displacive disorder of the pyrochlore. Room temperature Raman spectra also reveal the displacive disorder of the A site cations from the ideal pyrochlore (A₂B₂O₆O′) Wyckoff positions. Microstructure images captured using a high-resolution scanning electron microscope (HRSEM) illustrate exaggeratedly grown grains with a euhedral twinned like morphology. Further examination of the microstructure and composition using an electron probe microanalyzer (EPMA) shows the hausmannite phase dispersed in the pyrochlore matrix. Twinning like characteristics in BNMO grains were readily observed using a high-resolution transmission electron microscope (HRTEM). Divergence of the zero field cooled (ZFC) and field cooled (FC) magnetic moments at lower temperatures and unsaturation of the M(H) loop recorded at 5 K upon a maximum applied field of 5 Tesla imply features identical to the spin glass in all three compositions. X-Ray photoelectron spectroscopy (XPS) spectra of the representative BNMO pellet surface show Mn in 2+, 3+, Nb in 4+, 5+ and Bi in 3+ oxidation states. The variation of the dielectric constant with temperature from 297 K to 463 K measured at various frequencies in the case of BNMO and BTMO exhibits characteristics of a typical relaxor ferroelectric. The ferroelectric behavior is further verified by recording P–E hysteresis loops at room temperature, which displayed closed hysteresis loops for BNMO and BTMO with lossy behavior and no polarization hysteresis loop for BSMO. These findings affirm the presence of multiferroic order in the synthesized BNMO and BTMO composites and further exploration is expected to provide interesting insight into these materials.