The metastable garnet lattice of Gd3Al5O12 (GdAG) was effectively stabilized via doping with significantly smaller Lu3+, and based on which (Gd,Lu)AG:Yb/Ho was developed in this work as a new type of upconversion phosphor. The phosphor particles calcined from the precursors synthesized via carbonate precipitation were observed to have good dispersion and fairly uniform morphologies. Optical spectroscopy found that the [(Gd1-xLux)0.948Yb0.05Ho0.002]3Al5O12 (x=0.1-0.5) garnet powders exhibit a green emission centered at ~543 nm (the 5F4,5S25I8 transition of Ho3+) and a red emission centered at ~668 nm (the 5F55I8 transition of Ho3+) under laser excitation at 978 nm. The upconversion emission intensity was found to decrease with increasing Lu3+ doping. Meanwhile, the dependence of up-conversion emission intensity on the pumping power was measured and the up-conversion mechanism was discussed in detail. The Yb/Ho codoped (Gd,Lu)AG garnet system developed herein may potentially be used as a new type of luminescent material.

To suppress the thermal decomposition and to stabilize the crystal structure of Gd3Al5O12 (GdAG) garnet, doping GdAG with smaller Ln3+ (Ln=Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y, respectively) to form (Gd,Ln)AG solid solutions was proposed in work. Carbonate precursors of (Gd,Ln)AG with an approximate composition of (NH4)x(Gd,Ln)3Al5(OH)y(CO3)z•nH2O were synthesized via coprecipitation from a mixed solution of ammonium aluminum sulfate and rare earth nitrate, using ammonium hydrogen carbonate as the precipitant. The precursors and the calcination derived oxides were characterized using FT-IR spectroscopy, DTA/TG, XRD, BET and FE-SEM. The results showed that smaller Ln3+ doping can indeed stabilize GdAG against its thermal decomposition to a mixture of GdAlO3 (GdAP) and Al2O3 phases at elevated temperatures and at the same time effectively lowers the temperature for garnet crystallization. The carbonate precursors are loosely agglomerated and the resultant (Gd,Ln)AG powders show good dispersion and a fairly uniform particle morphology. The (Gd,Ln)AG solid solutions exhibit decreasing lattice parameters along with decreasing radius of the dopant ions at the same dopant content of 50 at%. Photoluminescence properties of some of the garnet solid solutions are also studied. The materials developed herein may potentially be used for photoluminescent and scintillation applications.

Trivalent thulium ions (Tm3+) doped GdAlO3 (Gd1-xTmxAlO3) phosphors which show a blue luminescence of high color purity have been synthesized by using solid-state reaction method starting from nanosized powders. X-ray diffraction (XRD) measurements were used to analyze the phase transformations that take place during the preparation of the phosphors. The morphologies of the powders calcined at different temperatures were studied by using scanning electron microscopy (SEM). The luminescence properties of the compounds were investigated. Pure phase of orthorhombic type GdAlO3 (GAP) was yielded by calcining the phosphors at 1200°C for 8 h. The PL spectra showed representative Tm3+ emission. The strong band centered at ~488 nm and the weak one centered at 697 nm were attributed to the 1D2-3F4 and 1G4-3F4 transitions of Tm3+, respectively. The quenching concentration of Tm3+ was estimated to be ~0.75at.% (x=0.0075), for which can be ascribed to the exchange interactions. The decay curve was fitted to be a single exponent and the estimated fluorescent lifetime of the GdAlO3:Tm3+ phosphor was 1.73±0.08 ms.

A series of layered rare-earth hydroxides (LRHs) of (Y,Ln)2(OH)5NO3•nH2O ( Ln=Tb,Eu), have been synthesized via a hydrothermal route. Crystal structures and optical properties of the materials have been investigated in detail by the combined techniques of XRD, FT-IR, FE-SEM, HR-TEM, and PLE/PL spectroscopies. It is shown that Tb3+ and Eu3+ are successfully incorporated into the Y-LRH host lattice to form solid solutions. Under UV excitation, the binary (Y0.97Tb0.03)- and (Y0.97Eu0.03)-LRHs exhibit their respective characteristic photoluminescence of the Eu3+ and Tb3+ activators. The ternary (Y0.965Eu0.005Tb0.03)-LRH simultaneously shows red and green emissions and both the intensity and emission color can be adjusted by changing the excitation wavelength. Compared with (Y0.995Eu0.005)-LRH, the ternary (Y0.965Eu0.005Tb0.03)-LRH exhibits appreciably enhanced 615nm red-emission, which indicates the existence of non- radiative energy transfer from Tb3+ to Eu3+. The ternary (Y0.965Eu0.005Tb0.03)-LRH also readily undergoes anion exchange with a series of inorganic and organic anions at room temperature.

Direct coagulation casting (DCC) is a relatively new ceramic near-net-shape forming process which can form homogeneous ceramic green body with complex shape and high density. Direct coagulation casting of aqueous alumina slurries by adding Y3+ions, which have been frequently used as additive for the sintering of alumina ceramics, were studied. Two different kinds of Y3+-ion releasing substances, Y2O3powder and Y(NO3)3, were used as coagulants and were introduced into stabilized alumina suspensions directly after dispersing. Compared with that of Y2O3powder, both the coagulation time and the added amount required for a reasonable shaping were significantly reduced for using Y(NO3)3as coagulant. The effects of the two coagulants on the properties of the consolidated green bodies and sintered ceramics were studied. The mechanisms of coagulation induced by the addition of Y3+-ions were discussed on the basis of interactions between Y3+ions and the added polyelectrolyte dispersant.