Preparation and spectroscopic properties of rare earth-doped bismuth sodium titanate-based ferroelectric ceramics

PolyU Library Call No.: [THS] LG51 .H577M AP 2015 Lauxiv, 109 pages :illustrations (some color)The photoluminescence (PL) properties of various rare-earth (RE)-doped materials have been investigated extensively. The up-conversion (UC) and down-conversion (DC) luminescent emissions have attracted considerable interest due to the wide variety of applications such as biological imaging, spectral convertor, optical telecommunication and military use. Er³⁺ and Pr³⁺are the most common lanthanide ions and efficient activators for UC and DC PL. Various multifunctional materials have been developed by combining two or more materials via different fabrication methods or special structure designs. In this work, novel lead-free RE3³⁺-doped PL ferroelectric ceramics have been fabricated and their PL, dielectric, piezoelectric and ferroelectric properties have been investigated. The corresponding PL mechanisms and energy transfer processes have also been deduced. 0.93(Bi₀.₅Na₀.₅TiO₃)-0.07(BaTiO₃) (abbreviated as BNBT) ferroelectric ceramic with good ferroelectric and ferroelectric properties is chosen as the host material for the new PL materials. BNBT ceramics doped with 0.01 mole Er³⁺ at various sites have been fabricated by the solid state reaction (SSR) method and the effects of the resulting oxygen and cation vacancies have been investigated. The samples are abbreviated as BNBT-Er(y), where y is Bi, Ti, Ba or Na for denoting the ions replaced by the RE dopants. The XRD results reveal that Er³⁺ has diffused into and entered the corresponding sites successfully, and no secondary phase is observed. Under an excitation of 980 nm, the ceramics exhibit strong green and red UC emissions and near-infrared (NIR) and mid-infrared (MIR) DC emissions. Our results also reveal that the vacancies arisen from charge imbalance between the dopants and the replaced ions could increase the PL intensity of the NIR and MIR emission bands at the expense of the visible emissions. For the BNBT-Er(Ba) and BNBT-Er(Na) ceramics containing cation vacancies (Vc), a looping mechanism is established such that their PL intensities of NIR emission increase by more than 50 % and 70 % respectively. In addition to the good dielectric, piezoelectric and ferroelectric properties, the BNBT-Er(y) ceramics should have great potential for multifunctional applications. On the basis of the results, BNBT ceramics doped with various contents of Er3+ at the Bi-site have then been prepared for studying the resulting effects of Er³⁺. The samples have a formula of 0.93(Bi₀.₅-x/0.93Erx/0.93Na₀.₅TiO₃)-0.07(BaTiO₃) and are abbreviated as BNBT-xEr, with x varying from 0 to 0.07. Owing to the higher probability of cross relaxation (CR) and efficient multi-phonon relaxation (MPR) at higher Er³⁺concentrations, the PL intensity of the red emission (660 nm) increases significantly by more than 47 times at x = 0.06. As a result, the Commission Internationale de L’Eclairage (CIE) chromaticity coordinates shift from (0.29, 0.69) at x = 0.005 to (0.49, 0.50) at x = 0.07, and the observed emission color changes from green to yellowish green. Moreover, due to the establishment of a dynamic circulatory energy process at high Er³⁺ concentration (x = 0.06), the PL intensity of the MIR emission (2.62 μm 2.84 μm) increases by more than 4 times at the expense of NIR emission. The BNBT-xEr ceramics also exhibit good dielectric, piezoelectric and ferroelectric properties, and thus suggesting that they should be promising candidates for multifunctional optoelectronic applications.For comparing the effects of various RE ions, Pr³⁺-doped 0.93(Bi₀.₅-x/0.93Erx/0.93Na₀.₅TiO₃)-0.07(BaTiO₃) ceramics (abbreviated as BNBT-xPr, where x = 0 0.02) have been fabricated. The DC PL mechanism and properties have been systemically investigated. The BNBT-xPr ceramics exhibit a low quenching concentration of x = 0.005. Owing to the enhancement of MPR and CR, the PL intensity of most of the emissions decreases by 50% to 75% as the Pr³⁺concentration increases from 0.0025 to 0.02. On the other hand, the PL intensities of the emissions at 654 nm and 741 nm increase by more than 2 times at x = 0.02. The BNBT-xPr ceramics with x ≤ 0.005 also exhibit good dielectric, piezoelectric and ferroelectric properties, showing great potential for multifunctional optoelectronic applications. The electric field (E-field)-dependent PL properties of the BNBT ceramics doped with 0.005 mole Er³⁺at the Bi-site (i.e. BNBT-0.005Er) have been studied and the corresponding mechanism has been investigated. Our results reveal that the E-field does not affect the splitting of energy levels of Er³⁺ in BNBT ceramics. However, it can decrease the PL intensity effectively. Under a static E-field of 3.6 kV/mm, the decrease can be up to 22%. This should be partly attributed to the increase in structural symmetry arisen from dipoles alignment, and partly attributed to the increase in crystal symmetry arisen from E-field induced transformation. Although the change is not entirely reversible in the first E-field cycle (i.e. the E-field is increased stepwise from 0 to 3.6 kV/mm and then decreased back to 0 kV/mm), the reversibility is improved in the consecutive cycles. The difference between the observed PL intensity at the minimum and maximum E-field (0 and 3.6 kV/mm, respectively) remains almost the same for different E-field cycles as well as different samples, suggesting that the ceramics should be potential candidates for electric controlled multifunctional optoelectronic applications.Department of Applied PhysicsM.Phil., Department of Applied Physics, The Hong Kong Polytechnic University, 2015MPhi