SPECTRAL INVESTIGATIONS ON RARE-EARTH/TRANSITION METAL ACTIVATED PHOSPHATE PHOSPHOR FOR APPLICATION IN LIGHT EMITTING DEVICES

Abstract

The present thesis focuses on the systematic development, structural optimization, photoluminescent enhancement, and thermal performance evaluation of rare-earth/ transistion metal activated BaZr(PO₄)₂ (BZP) based phosphate phosphors synthesized through a simple and cost-effective high-temperature solid-state reaction route. Phosphate materials were selected due to their excellent thermal and chemical stability, low phonon energy, high band gap, and structural flexibility to accommodate various rare-earth ions and transition metal ions. The work investigates the role of Dy³⁺, Eu³⁺, Tb³⁺, Bi3+ dopants, co-dopants, concentration tuning, and flux-assisted growth on the luminescence, structural purity, and color tunability of the host phosphor for advanced applications in solid-state lighting (SSL), white LEDs (w LEDs), and photonic devices. Chapter 1 provides an in-depth overview for the need of the efficient methods of energy generation and usage are discussed. It presents a thorough exploration of luminescence, covering its core principles and different forms. The main focus is on the process of photoluminescence, with special attention given to downconversion and upconversion photoluminescence. The chapter explores the spectroscopic properties of activator ions and presents the theoretical frameworks employed for analyzing spectral data, including ionic interactions and energy transfer mechanisms between rare earth ions and transition metal ions. SHWETA YADAV vi In addition to that, role of the phosphors in solid state lighting have been elucidated. This chapter reviews recent advancements in phosphor lattice doped with various dopants. It also examines the role of the host lattice, its characteristics, and the behavior of various rare earth ion dopants. This chapter also discusses the significance of selected host material BaZr(PO4)2 as well as the objectives of the research work. Additionally, the chapter details the methodology for evaluating critical colorimetric parameters including CIE chromaticity coordinates, color purity, and correlated color temperature (CCT). Chapter 2 describes the synthesis methodology and characterization techniques used throughout the thesis. BZP phosphors were prepared using high-purity oxides under controlled sintering conditions, ensuring monoclinic phase formation. Systematic grinding, calcination, sintering, and flux addition steps ensured homogeneity and crystallinity. Comprehensive characterization tools—XRD for structural analysis, SEM/EDS for morphology and elemental composition, UV–Vis diffuse reflectance for band gap evaluation, FTIR for vibrational modes, PL/PLE for optical transitions, TRPL for decay dynamics, and TD-PL for thermal quenching— were employed to correlate structure with luminescent behavior. Chapter 3 presents a detailed investigation of Eu³⁺ doped and flux-assisted Eu³⁺ activated BZP phosphors, targeting efficient red and reddish-orange emission for indoor lighting. The JO analysis clearly reveals enhanced hypersensitive transitions, indicating improved asymmetry around Eu³⁺ ions. Fluxes such as NH₄Cl, NaCl, KCl, and LiCl played a critical role in improving crystallinity, increasing grain size, reducing defects, and enhancing emission intensity. The optimum Eu³⁺ concentration (3 mol%) showed the strongest red emission corresponding to the ⁵D₀→⁷F₂ transition, making it favorable for combining with blue/yellow vii SHWETA YADAV phosphors for warm-white generation. Temperature-dependent PL showed excellent thermal stability, supporting device-level applications. Chapter 4 focuses on the development of Dy³⁺ doped and Dy³⁺/Eu³⁺ co-doped BZP phosphors. XRD confirmed pure monoclinic BZP structure (C2/m). PL studies demonstrated that Dy³⁺ activated samples exhibited strong blue (⁴F₉/₂→⁶H₁₅/₂) and yellow (⁴F₉/₂→⁶H₁₃/₂) emissions, producing near-white light. Co-doping with Eu³⁺ enabled efficient energy transfer from Dy³⁺ to Eu³⁺, resulting in tunable emission colors ranging from cool white to warm yellowish-orange. The optimized Dy³⁺/Eu³⁺ compositions showed superior thermal stability with activation energies up to ~0.267 eV and reduced quenching. The decay lifetimes decreased with increased dopant concentration, confirming concentration-dependent non-radiative pathways. These features make the phosphors suitable for tunable w-LED applications. Chapter 5 conatins investigations on BZP phosphors activated singly with Bi3+ and codoped with Bi3+ /Tb3+ synthesized by utilizing solid state reaction synthesis. The PL studies of singly Bi3+ activated phosphors demonstarted blue emission band around 468 nm when 313 nm wavelength source of excitation is used. The transfer of energy to Tb3+ from Bi3+ in the codoped samples was confirmed from the PL spectra and the decay kinetics. The pathway responsible for transfer of energy from Bi3+ to Tb3+ was identified as dipole dipole interactions and critical distance between Bi3+ and Tb3+ was found to be 21.85 Å. The color tunable emissios from blue to green region were realised in codoped phosphors by varying relative doping concentrations and excitation wavelengths, which illustrates potential use of BaZr(PO4)2: Bi3+, Bi3+/Tb3+ phosphors with tunable blue green emission in field of solid-state lighting. viii SHWETA YADAV Chapter 6 This chapter conclude the obtained results from the work carried out to accomplish this thesis. The futuristic scope of the thesis work is also elaborated with respect to the expected applications of these phosphors in versatile fields. The combined outcomes demonstrate that BZP is a robust host lattice for various dopant configurations, capable of producing blue yellow-white, orange-red, and warm-white emission profiles. The promising thermal stability and optical performance make the synthesized phosphors suitable for next-generation w-LEDs, display devices, and indoor illumination systems. This chapter highlights how the present research contributes to energy-efficient lighting solutions, promotes environmental sustainability, and drives further technological advancements. It emphasizes the role of the developed phosphors in delivering social benefits by lowering energy consumption, supporting rural lighting initiatives, and strengthening national self-reliance. Overall, the work successfully establishes BaZr(PO₄)₂ as a versatile and efficient host for rare earth-activated tunable phosphors. The thesis contributes new experimental insights into dopant–host interactions, JO analysis, flux-assisted luminescence enhancement, and thermal performance for sustainability-driven lighting technologies.