2020同步年報

068 ACTIVITY REPORT 2020 X-ray absorption near edge structure (XANES) spectrum of PbZr(PO 4 ) 2 was compared with the standard references (BaZrO 3 , m-ZrO 2 and ZrSiO 4 ) to fingerprint the Zr coordination, showing that the Zr K-edge XANES spectrum of PbZr(PO 4 ) 2 closely resembles that of BaZrO 3 and Zr is strongly consid- ered to be six-fold coordinated with oxygen in the target structure shown in Fig. 1 . The peak intensity from PbZr(PO 4 ) 2 was then compared with those of the standard referenc- es—PbO and PbO 2 , which corresponded to PbO 4 and PbO 6 coordination, respectively. These results indicated that Zr ions were located in octahedral and pentagonal bipyrami- dal sites; Pb ions were found to be in tetrahedral sites in the local structural environment, indicating the stability of the coordination polyhedra in the target structure. Based on these findings, Shih et al . suggested that Zr ions locate in octahedral and pentagonal bipyramidal sites and Pb ions in tetrahedral sites in the local structural environment. A comparison of the evaluation of the effect of phosphate treatment on Pb stabilization for PbZrO 3 and PbZr(PO 4 ) 2 was performed after a long contact time; a continuous increase of Pb leachability was observed from the PbZrO 3 sample. Higher Pb concentrations and pH variations revealed sub- stantial cation dissolution more from PbZrO 3 than from PbZr(PO 4 ) 2 , as they were more vulnerable to proton-medi- ated dissolution; the pH values slightly increased from the original 2.85 to 3.03 by the end of the leaching period ( Fig. 2 ). After a long-term durability test, Shih and his collaborators suggested that PbZr(PO 4 ) 2 possessed a strong structur- al stability of the target phase and still maintained high crystallinity, as no new XRD diffraction signal was observed. Furthermore, the Pb stabilization mechanism was sug- gested as the oxygens play a crucial role in the first coor- dination shell for the PbZr(PO 4 ) 2 sample and the reference compounds herein, whereas the second coordination shell varied among the standards shown in Fig. 3(a) . The second shell of Zr in the PbZr(PO 4 ) 2 sample was strongly related to the Zr−P bonding environment because of the intensity and position of signals of zircon from PbZr(PO 4 ) 2 in the range 2.5−3.2 Å of R space. Consequently, a Zr−O−P bond was highly possibly formed in the crystalline phase; the structural alteration might improve the product durability. The Pb−O bond length in PbZr(PO 4 ) 2 was near that of a free PbO molecule based on the extended X-ray absorption fine structure (EXAFS) data, indicating that oxygens be consid- ered as a hard-sphere atom showing a strong electronega- tivity so that only a slight difference was induced in the first coordination shell. The O signal was assigned to the lattice oxygen and surface oxygen; a decreased intensity was ob- served for the O lattice after leaching, indicating depletion of the mineral phase by the acidic attack shown in Fig. 3(b) . The variation of surface element composition indicated that an amorphous Zr(OH) 4 layer might have been formed on the outer surface of PbZr(PO 4 ) 2 , as crystalline Zr(OH) 4 was easily transformed into an amorphous structure on incorpo- rating water molecules into the polymerized structure. Materials with perovskite structures, of general formula of ABX 3 , have been widely used in the field of piezoelectric ceramics and optoelectronic devices, but increasing envi- ronmental awareness and more restrictive regulations have been made for environmental protection. Accordingly, the decreased Pb leachabilities in PbZr(PO 4 ) 2 solid solutions show the effectiveness of a phosphate treatment in stabi- lizing Pb from all forms of waste PZT ceramics and provide insight into waste form design for environmental remedi- ation. Through alteration of the local coordination in the matrix and elemental doping in the solid solutions, heavy metals could be effectively stabilized in a robust structure. In the real wastes, however, a challenge will be to prevent the competitive effect of the heteroatom from entering the coordination sites designed for the target hazardous met- als. Pb zero-emission and Pb-free piezo material systems are thus definitely the next generations of piezo material; further efforts should be made to investigate the metal-in- corporation affinities in a complicated system. (Reported by Yao-Chang Lee) Fig. 2 : Normalized lead concentrations (a) and pH variations (b) of PbZrO 3 leachates and PbZr(PO 4 ) 2 leachates. [Reproduced from Ref. 1]