Growth And Luminescence Properties Of Tm₂O₃,Yb₂O₃ And Ho₂O₃ Doped Yttria Stabilized Zirconia ?YSZ? Single Crystals

Zirconia?ZrO₂?has been widely used in optical devices,oxygen sensors,high temperature sensor,solid fuel cell electrolyte and aero-engine blade coating,because of its excellent physical and chemical properties,such as high melting point?2700??,wide band gap?5.0 eV?and low phonon energy(470 cm^-1)etc.Zirconia crystals stabilized with 6 mol%,8 mol%,10 mol%and 12 mol%Y₂O₃?abbreviated to 6YSZ,8YSZ,10YSZ and 12YSZ?were grown by the optical floating zone method,were typically 7 mm in diameter and 70 mm in length.The same method then used to grow single crystals of 8YSZ doped with varisous concentrations of Tm₂O₃?abbreviated to x mol%Tm₂O₃:YSZ?,8YSZ co-doped with 0.5 mol%Tm₂O₃ and various concentrations of Yb₂O₃?abbreviated to x mol%Yb/Tm:YSZ?and 8YSZ co-doped with 0.5 mol%Tm₂O₃,2.0mol%Yb₂O₃ and various concentrations of Ho₂O₃?abbreviated to x mol%Ho/Yb/Tm:YSZ?.The samples were characterized by XRD,TG-DSC,Raman spectroscopy and positron annihilation techniques,and their optical properties measured by UV-Vis-NIR absorption,PL and PLE spectroscopy,the color coordinates and purity were also calculated,and information on oscillator intensity,luminescence and electronic structures of the doped YSZ crystals was derived using the Judd-Ofelt theory and first principle calculations.By analyzing the luminescence characteristics of different activated ions in 8YSZ crystal,the physical mechanism of the optical and magnetic properties of the crystal affected by such deep-seated factors as electronic structure,rare earth ion 4f electron and microscopic defects was explored.The 6YSZ crystal was a mixture of tetragonal and cubic phases,resulting in an opalescent appearance,whereas 8YSZ,10YSZ and 12YSZ crystals were entirely in the cubic phase.The lattice parameter and the average positron lifetime,?_m increased monotonously wuth increasing Y₂O₃ content,whereas the optical bandwidth decreased.Fluorescence spectra showed a single emission peak centred at 416 nm when the crystals were excited by light with250 nm wavelength.The excitation spectra were also composed of a single peak at 250 nm.In addition,the crystals showed weak ferromagnetism in a small magnetic field,and saturation magnetization increased with the increase of Y₂O₃ content.Oxygen vacancies,F⁺centres,AOD⁺centers and a samall amount of Zr vacancies were centers in the Y₂O₃ stabilized ZrO₂crystals.In addition,the top of the valence band?VB?consisted mainly of the occupied 2p energy band of O atoms and the bottom of the conduction band?CB?was composed of unoccupied 4d energy levels of Zr.When Zr⁴⁺was substituted by Y³⁺,the bottom of the valence band was broadened and the band gap width narrowed because of an increase in the number of oxygen vacancies.Moreover,the length of the Zr-O bond shortened and the bonding strength increased,the average coordination number of Zr was reduced from eight in the perfect fluorite to a value closer to seven,and the coupling between electrons in the valence and conduction bands wans enhanced.Therefore,8YSZ crystal can be used as a host crystal of luminescent materials due to its good optical transmittance and fewer defects.The Tm₂O₃:YSZ single crystals were in the cubic phase,and the lattice parameters first increased and then decreased with increasing Tm₂O₃ content.The absorption spectra showed four peaks at around 356 nm?³H₆?¹D₂?,460.5 nm?³H₆?¹G₄?,678.5 nm(³H₆?³F_2,3)and784 nm?³H₆?³H₄?in the visible region,and the optical bandgap energy increased with increasing Tm₂O₃ content,as a result of the Moss-Burstein effect.Measurements of PL spectra indicated a strong blue emission peak at 458 nm?¹D₂?³F₄?,and three weak emission peaks at 487 nm?¹G₄?³H₆?,497 nm?¹D₂?³H₅?,and 656.5 nm?¹G₄?³F₄?when the crystals were excited with?_ex=356 nm.The intensities of the emission peaks were strongly affected by the Tm₂O₃ content of the YSZ single crystals;the intensity increased with Tm₂O₃ content at low doping levels,reached the maximum at 0.5 mol%Tm₂O₃,then decreased with further increase in Tm₂O₃ content due to the concentration quenching effect.Additionally,the luminescent intensity of 0.5mol%Tm₂O₃:YSZ crystal was 1.37 times and 1.42 times of0.5mol%Tm₂O₃:YSZ ceramics and 0.5mol%Tm₂O₃:YAG crystal,respectively.And the color changed from blue to cyan as the Tm₂O₃ content was increased.Overall,this work demonstrated that the cubic YSZ single crystal is a suitable host material for solid state luminescence.The Yb/Tm:YSZ single crystals were also in the cubic phase,and the Yb³⁺preferentially substituted for the Y³⁺.Seven absorption peaks were observed in the range of 200-1800 nm around 358 nm,460 nm,679 nm,783.3 nm,850-1000 nm 1200 nm and 1721.5 nm.The first four transitions correspond to those in Tm₂O₃:YSZ crystals and the next three were assigned to the ²F_7/2?²F_5/2(Yb³⁺),³H₆?³H₅(Tm³⁺)and ³H₆?³H₄(Tm³⁺)transitions,respectively.And the absorption cross section of Yb³⁺was about 104.09×10^-2020 cm^-2.Three up-conversion emission peaks around 488 nm,658nm and 800 nm were observed when Yb/Tm single crystal was excited with a 980 nm laser.The intensities of the emission peaks reached maximum at2.0 mol%Yb₂O₃,and a two-photon process should be involved for populating the ³H₄ NIR level of Tm³⁺,whereas three-photon processes are involved in the generation of blue emissions.Based on the absorption spectrum of the 2.0Yb/Tm:YSZ crystal,the?₂??₄ and?₆oscillator strength parameters calculated by the Judd-Ofelt theory were 0.41×10^-20 cm²,0.12×10^-20 cm² and 0.15×10^-20 cm²,respectively.The observation of?₂>?₄ indicates that Tm³⁺preferentially substituted for Zr⁴⁺with seven coordination.In addition,the fluorescence lifetime of the ¹G₄ level was measured to be 7.72 ms for 2.0Yb/Tm:YSZ crystal excited with a 980 nm laser.Therefore,Yb/Tm:YSZ crystals had considerable potential for laser and luminescence applications.The Ho/Yb/Tm:YSZ single crystals were in the tetragonal phase.In addition to the four absorption peaks of Tm³⁺,six additional absorption peaks around 383 nm?417 nm?447 nm?485 nm?537 nm and 636 nm were observed in the visible region,and assigned to the Ho³⁺⁵I₈?⁵G₄+³K₇?⁵I₈?⁵G₅?⁵I₈?⁵F₁+⁵G₆+³K₈+⁵F₂?⁵I₈?⁵F₃?⁵I₈?⁵F₄+⁵S₂ and ⁵I₈?⁵F₅ transitions,respectively.Moreover,the optical bandgap energy values for the x=0.01,0.05,0.2,0.5,0.75and 1.0 samples were 4.77 eV,4.78 eV,4.23 eV,4.21 eV,4.25 eV and 4.25 eV,respectively.Four down-conversion emission peaks centred at 461 nm,551 nm,658 nm and 760 nm were obtained under excitation with?_ex=356 nm,and the intensity of blue peak decreased with increasing of Ho₂O₃ content.Three emission peaks were observed at around 551 nm,656 nm and 757 nm when the crystals were excited by light with a wavelength of 448 nm.The intensity of the green emission peak was strongest for Yb/Tm:YSZ co-doped 0.75 mol%Ho₂O₃ single crystals,but decreased with further increase in Ho₂O₃ content,probably as a consequence of non-radiative relaxation increased.In addition,four up-conversion emission peaks centered at about 488 nm,539.5 nm,670.5 nm and 800 nm were observed under excitation by a 980 nm laser and the intensity of the green and red emission peak were strongest for the Yb/Tm:YSZ co-doped 0.5 mol%Ho₂O₃ single crystals,and decreased with higher Ho₂O₃ contents as a result of the concentration quenching effect.The intensity of blue emission peak was decreased by Ho₂O₃ doping because of more efficient energy transfer between Ho³⁺and Yb³⁺.The values of the critical distances for Ho³⁺up-conversion and down-conversion luminescence were 0.82 nm and 1.097 nm,respectively.So the concentration of Ho³⁺in up-conversion luminescence was lower when the phenomenon of concentration quenching occurred.Moreover,compared with Yb/Tm:YSZ and Yb/Ho:YSZ crystals,the number of photons involved in the up-conversion luminescence were lower for the Ho/Yb/Tm:YSZ crystals,and indicate that the luminescence process is simplified when Ho₂O₃ is doped into the Yb/Tm:YSZ crystals and its efficiency is improved.Overall these results indicate that the Ho/Yb/Tm:YSZ crystals could be used as a potentially efficient up-conversion fluorescent crystal.