The sequence of transitions between different phases of BiNbO 4 has been thoroughly investigated and clarified using thermal analysis, high-resolution neutron diffraction, and Raman spectroscopy. The theoretical optical phonon modes of the α-phase have been calculated. Based on thermoanalytical data supported by density functional theory (DFT) calculations, the βphase is proposed to be metastable, while the αand γ-phases are stable below and above 1040 °C, respectively. Accurate positional parameters for oxygen positions in the three main polymorphs (α, β, and γ) are presented and the structural relationships between these polymorphs are discussed. Even though no significant changes, only relaxation phenomena, are observed in the dielectric behavior of α-BiNbO 4 below 1000 °C, evidence of two further subtle transitions at ∼350 and 600 °C is presented through careful analysis of structural parameters from variable temperature neutron diffraction measurements. Such phase variations are also evident in the phonon modes in Raman spectra and supported by changes in the thermoanalytical data. These subtle transitions may correspond to the previously proposed antiferroelectric to ferroelectric and ferroelectric to paraelectric phase transitions, respectively.

Aurivillius phase BaBi4Ti4O15 micro-sized powders were produced by solid-state reaction and their photocatalytic properties were reported for the first time. X-ray diffraction revealed the polar orthorhombic structure. BaBi4Ti4O15 ceramics exhibited diffuse phase transition at ~ 410 °C. The freezing temperature of 274 °C was obtained by fitting the Vogel-Fulcher law. The distinct ferroelectric domain switching current peaks in current-electric field (I-E) loop and piezoelectric coefficient d33 value of 7.0 ± 0.1 pC/N at room temperature further demonstrated relaxor ferroelectric behavior of BaBi4Ti4O15. UV-vis absorption spectra indicated that BaBi4Ti4O15 had a direct band gap of 3.2 eV. The photocatalytic study showed 15 % degradation of Rhodamine B (RhB) solution by BaBi4Ti4O15 powders after 3.5 h UV-vis irradiation. The RhB degradation rate was further enhanced by depositing Ag nanoparticles on the BaBi4Ti4O15 powders surface. This work suggested that the relaxor ferroelectric BaBi4Ti4O15 is promising for photocatalytic applications.

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The magneto-optical and dielectric behavior of M-type hexaferrites as permanent magnets in the THz band is essential for potential applications like microwave absorbers and antennas, while are rarely reported in recent years. In this work, single-phase SrFe12–x Nb x O19 hexaferrite ceramics were prepared by the conventional solid-state sintering method. Temperature dependence of dielectric parameters was investigated here to determine the relationship between dielectric response and magnetic phase transition. The saturated magnetization increases by nearly 12%, while the coercive field decreases by 30% in the x = 0.03 composition compared to that of the x = 0.00 sample. Besides, the Nb substitution improves the magneto-optical behavior in the THz band by comparing the Faraday rotation parameter from 0.75 (x = 0.00) to 1.30 (x = 0.03). The changes in the magnetic properties are explained by a composition-driven increase of the net magnetic moment and enhanced ferromagnetic exchange coupling. The substitution of the donor dopant Nb on the Fe site is a feasible way to obtain multifunctional M-type hexaferrites as preferred candidates for permanent magnets, sensors, and other electronic devices.

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The Na and Ce co-doped CaBi4Ti4O15 (CBT) Aurivillius ceramics in a Ca1-x(Na0.5Ce0.5)xBi4Ti4O15 (CNCBT, x = 0, 0.03, 0.05, 0.08 and 0.12) system were synthesized by the conventional solid-state sintering method.All compositions show a single-phase orthorhombic (space group A21am) structure at room temperature. The shift of the Curie point Tc towards lower temperatures on doping results from the increased tolerance factor t.The substitution-enhanced ferroelectric performance with large maximum polarization and facilitated domain switching is evidenced by the developed P-E and I-E hysteresis loops. The piezoelectric coefficient d33 (20.5 ± 0.1 pC/N) of the x = 0.12 sample is about four times larger than that of pure CBT. The improved piezoelectric properties can be attributed to the high remnant polarization and relatively high permittivity. In addition, multisized (micron and sub-micron) domain structures were observed in the CNCBT ceramics by piezoelectric response microscopy. The multiple sized ferroelectric domain structure with smaller domains is beneficial to the easy domain switching, enhanced ferroelectric performance and improved piezoelectric properties of the CNCBT ceramics. The designed Aurivillius-phase ferroelectric ceramics with the Curie point around 765 °C and high d33 are suitable for high-temperature piezoelectric applications.

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