Sulfate-Type Layered Rare-Earth Hydroxide:Controlled Hydrothermal Synthesis,Structure Characterization,and Application In Oxyslufate And Oxysulfide Phosphors

This Doctoral Dissertation focuses on sulfate-type layered rare-earth hydroxide,a recent kind of layered inorganic compounds,with comprehensive investigations into its controlled hydrothermal synthesis and crystal structure and also its application in the green fabrication of rare-earth oxysulfate and oxysulfide up-/down-conversion phosphors.Through optimizing hydrothermal parameters,extension of the RE₂?OH?₄SO₄·nH₂O layered hydroxyl sulfate from the currently available RE of Ce-Tb to the wider range of La-Dy has been successfully achieved.Meanwhile,the anhydrous hydroxyl sulfate of RE₂?OH?₄SO₄?RE=Eu-Lu,including Y?was synthesized for the first time as a new type of layered hydroxide with novel structural features.The crystal structures of both RE₂?OH?₄SO₄ and RE₂?OH?₄SO₄·nH₂O were deciphered and refined with the Rietveld technique.The thermal decomposition processes of the two kinds of layered compounds in the air were systematically and comparatively investigated via DTA/TG to reveal the effects of RE type.More importantly,controlled synthesis of RE₂O₂SO₄ for the full spectrum of lanthanide elements was attained in this work for the first time,and the relationship between the thermal stability of RE₂O₂SO₄ and RE³⁺ radius was established.Taking advantages of the fact that the layered hydroxyl compounds have exactly the same RE:S molar ratio of RE₂O₂S,a series of RE₂O₂S was facilely obtained via calcination under reducing atmosphere for a wide range of RE?RE=La-Dy,except for Ce?.RE³⁺-doped La₂O₂S,La₂O₂SO₄,Gd₂O₂S,and Gd₂O₂SO₄ phosphors have also been derived as pure phases from their respective hydroxyl sulfate precursors,and their down-/up-conversion photoluminescence and cathodoluminescence properties were systematically investigated.The main achievements are summarized as follows.?1?Phase structure and crystallite morphology of the product obtained via hydrothermal reaction at 100 ? and pH=9 were found to be closely related to the type of RE,and the nominal composition of the product transforms from RE₂?OH?₄S0₄SO₄·nH₂O?RE=La-Gd?to RE2?OH?5?SO4?0.5·nH2O?RE=Tb-Lu?with decreasing ionic size of RE.Through tuning the pH of the reaction system,the synthesis of phase-pure RE₂?OH?₄SO₄·nH₂O was successfully extended from the currently RE=Ce-Tb to the wider range of La-Dy.Rietveld refinements were carried out with the X-ray diffraction data for the RE₂?OH?₄SO₄·nH₂O series?RE=La-Dy?,and the structural parameters of La₂?OH?₄SO₄·nH₂O and Dy₂?OH?₄SO₄·nH₂O were first reported.The refinements indicate that the newly obtained La and Dy members are isostructural with the reported RE=Ce-Tb analogues and are crystallized in the monoclinic system?space group:C2/m?with an axis angle of approximately 90°.The deduced lattice parameters?a,b,c?,cell volume,and axis angle ? across the series show monotonous decrease with decreasing size of RE³⁺.The synthesis of La₂?OH?₄SO₄.nH₂O,a compound has important applications in the filed of optically functional materials,was optimized,and it was found that well dispersed nanoplates can be obtained at 100 ? and pH=9.?2?The anhydrous hydroxyl sulfate of RE₂?OH?₄SO₄?RE=Eu-Lu,including Y?has been hydrothermally synthesized for the first time in this work as a new type of layered rare-earth compounds,and the influence of reaction temperature and pH on the phase formation was extensively investigated.Crystal structure refinement with the Rietveld technique found that this group of layered compounds crystallize in the monoclinic system?space group:C2/m?,with their structure built up via alternative stacking of the interlayer sulfate ions and the two-dimensional host layers,composed of[REO₉]polyhedra,along the a-axis.The lattice parameters?a,b,c?and cell volume decrease towards a smaller RE³⁺ across the series while an opposite trend was found in axis angle ?.The compounds show smaller gallery height and cell volume than RE₂?OH?₄SO₄·nH₂O,due to the absence of crystal water.The synthesis were optimized for Gd₂?OH?₄SO₄ and its formation window was defined.Well-dispersed quasi-hexagonal microplates have been obtained at 100 ? and pH=10.?3?The thermal decomposition of RE₂?OH?₄SO₄·nH₂O?RE=La-Dy,except Ce?and RE2?OH?4S04?RE=Eu-Lu?has been comparatively investigated via DTA/TG in simulated air.RE₂?OH?₄SO₄·nH₂O shows three steps of weight loss upon heating,which can be ascribed to dehydration,dehydroxylation,and desulfurization.The first two steps transform RE₂?OH?₄SO₄·nH₂O to RE₂O₂SO₄ while the last one converts RE₂O₂SO₄ into RE2O3 in the air.RE₂?OH?₄SO₄ shows the two-step weight loss of dehydroxylation and desulfurization,with RE₂O₂SO₄ and RE₂O₃ as the products,respectively.The decomposition temperature of each stage was found to show clear dependence on the type of RE.That is,dehydroxylation occurs at a higher temperature for a smaller RE³⁺ while desulfruration behaves oppositely.The existence temperature range of RE₂O₂SO₄ thus narrows towards a smaller RE³⁺,which makes La₂O₂SO4 the most stable and Lu₂O₂SO₄ the least stable amongst the RE₂O₂SO₄ family.By calcining the two kinds of layered compounds,RE₂O₂SO₄ was obtained for the first time for the full spectrum of lanthanides,and a series of RE₂O₂S were also derived for the RE of La-Dy?except for Ce?in reducing atmosphere.The deduced lattice parameters?a,b,and c?and cell volume show monotonous decreases with decreasing radius of RE³⁺ across the RE₂O₂SO₄ series,while the axis angle ? shows an opposite trend.For the small RE³⁺ ions of RE=Ho-Lu and Y,calcining the RE₂?OH?₄SO₄ precursor in hydrogen only produces a mixture of RE₂O₂S and RE₂O₃,with the portion of RE₂O₃ increasing towards a smaller RE₃.The phenomenon has been tentatively ascribed to the result of interplay between the reducing power of H₂ and the thermal stability of RE₂O₂SO₄.The phase and morphology evolutions of La₂?OH?₄SO₄·nH₂O upon calcination in H₂ and ambient air were systematically studied.?4?The up-/down-conversion?UC/DC?photoluminescence and cathodeluminescence of RE³⁺ activators in La₂O₂S,La₂O₂SO-_-4,Gd₂O₂S and Gd₂O₂SO₄ were thoroughly investigated,and red,orange red,yellow,green,and blue emissions were obtained upon exciting the phosphors with theirs most effective excitation wavelengths.The influence of calcination and Eu_<sup>3+ concentration on the DC photoluminescence of?La,Eu?₂O₂SO₄ was studied in detail,and the optimal Eu_<sup>3+ content was experimentally determined to be 5 at%.Concentration quenching of luminescence was analyzed to be due to exchange interaction among the Eu³⁺activators.Efficient energy transfer from GU³⁺ to Dy³⁺ was found in the Gb₂O₂SO₄ host to improve Dy³⁺ emission,and the possible energy transfer processes were proposed.Under laser excitation at 978 nm,the La₂O₂SO₄:Yb³⁺/RE³⁺ and La₂O₂S:Yb³⁺/RE³⁺?RE=Ho,Er,and Tm?phosphors display promising UC luminescence,which can be explained with multi-photon?2-4?processes.Cathodoluminescence study found that the Gd₂O₂S:Tb and Gd₂O₂S:Pr green phosphors are structurally stable in the range of this study and exhibits successively higher emission brightness by increasing either the acceleration voltage?up to 7 kV?or beam current?up to 50 ?A?.