New insights into structural, optical, electrical and thermoelectric behavior of Na0.5Bi0.5TiO3 single crystals

The single crystals of lead-free Na0.5Bi0.5TiO3 were grown using the Czochralski method. The energy gaps determined from X-ray photoelectron spectroscopy (XPS) and optical measurements were approximately 2.92 eV. The current-voltage characteristics, depolarization current, dc (σdc) and ac (σac) electrical conductivity, and Seebeck coefficient of the crystals were investigated. The frequency/temperature-dependent electrical properties were also measured and analyzed through complex impedance spectroscopy. An overlapping reversible insulator-metal transition (resistive switching) on nanoscales, caused by the electric field, was detected. Most of these properties were measured for the first time. The activation energy values determined from the conductivity data, the imaginary part of the electric impedance and the modulus indicate that the relaxation process in the high-temperature range is attributable to both single and double ionized oxygen vacancies, in combination with the hopping of electrons between Ti4+ and Ti3+. P-type electrical conductivity was also found. These discoveries create new possibilities of reducing the electrical conductivity of NBT and improving the process of effectively poling this material. Our results indicate the possibility of tuning the material properties by intentionally creating non-stoichiometry/structural defects (oxygen vacancies, cation excess and cation deficiency).

Improved Crystallization Quality of FAPbBr3 Single Crystals by a Seeded Solution Method

Solution-grown hybrid perovskite, FAPbBr₃, has attracted great attentions recently due to its inspiring optoelectronic properties and low-cost preparation method. However, challenges of solution growth for FAPbBr₃ bulk crystals remain yet, such as uncontrollable crystalline morphologies, irregular shapes, and limited crystal sizes, which are attributed to the dense crystallization nucleus. In this work, we investigate the effects of growth conditions and seed behaviors on the crystallization quality and the yield of FAPbBr₃ single crystals. First, the spontaneous nucleation is tailored by optimizing the precursor concentration and heating rate. Furthermore, the seeded crystals are introduced to solve the issues related to the morphology and the yield of single crystals. Based on the above-mentioned investigations, an optimized growth method, a seeded solution method, under a heating rate of 0.1 °C/h is proposed, and centimeter-scale FAPbBr₃ single crystals with a very narrow FWHM of high-resolution X-ray diffraction rocking curves and a high yield of ∼100% of single crystals are obtained. The resulting FAPbBr₃ single crystal exhibits a bulk resistivity of 3.42 × 10⁹ Ω·cm and a superior I_ON/I_OFF ratio over 10⁴ under 405 nm light at a bias of 10 V. Finally, the pulse height spectra with an energy resolution of ∼21.4% are also achieved based on an AZO/FAPbBr₃/Au detector, illuminated using an uncollimated ²⁴¹Am@5.49 MeV α-particle source at room temperature. This modified seeded solution method shows great potential in preparing high-quality and high-yield perovskite single crystals.