Fabrication and Phase Transition of La2-X Lu x Zr2O7 Transparent Ceramics

Abstract

Materials with A2B2O7 formula have been drawing the attention of researchers for a diversity of applications, such as fast-ion conductors, photocatalysts, thermal barrier coatings, matrices for immobilization of highly active radionuclides from nuclear wastes and host materials for scintillators. A2B2O7 materials have three crystal phases decided by the ionic radius ratios (r A/r B). According to the rule of Subramanian et al [1], pyrochlore phase was favored when r A/r B lay within the range of 1.46~1.78. Defective fluorite phase is stable for r A/r B < 1.46, whilst the monoclinic phase is stable for r A/r B > 1.78. In this study, a new series of La2-x Lu x Zr2O7 (x = 0~2.0) transparent ceramics [2] with high density and average atomic number, which are hoped to be scintillator matrix, were fabricated by vacuum sintering and an interesting phenomenon was found. As Lu content increased, the crystal phase gradually transformed from pyrochlore to defective fluorite phase with the existence of a two-phase region (x = 0.6 ~ 1.2) (Figure 1). The appearance of a two-phase region illustrated that Subramanian’s theory is not available when there is large difference in ionic radii of the elements that occupy at Ln position (r La 3+ = 1.160 Å, r Lu 3+ = 0.977 Å). In addition, the grain growth of pyrochlore phase was inhibited due to the appearance of defective fluorite phase. Grain sizes of samples dominated by pyrochlore phase were 11~14 μm. Samples with two phases (x = 0.6~1.2) have much smaller grain sizes (< 3 μm), while samples dominated by defective fluorite phase have larger grain sizes (82~60 μm). All the samples (except for x = 0) are transparent and the highest in-line transmittance is reached to 72.4% (1100 nm) at x = 1.2 though two phases coexist. To verify that the two-phase region was not an accident, we fabricated LaLuZr2O7 powder by combustion method instead of solid state reaction and sintered in the same way. From XRD results, two phases are observed in both the synthesized powder and ceramic. Also, With 2 ~ 3 μm average grain size, the LaLuZr2O7 ceramic reaches high transparency with an in-line transmittance of 73.4% at 1100 nm, which is slightly higher than that made by solid state reaction method, showing the advantage of combustion method in synthesizing good powder. Rayleigh-Gans-Debye scattering theory was used to explain the high transmittance of the ceramics with two phases. When the refractive indices of the coexisting two phases are close to each other and the average grain size is smaller than 10 μm, the two phases have little influence on transparency of the ceramics. [1] Subramanian MA, Aravamudan G, Subba Rao GV. Oxide pyrochlores-A review. Prog Solid State Chem 1983, 15: 55-143. [2] Z.J. Wang, G.H. Zhou, X.P. Qin, F. Zhang, J.P. Ai, P. Liu, S.W. Wang. Fabrication and phase transition of La2-xLuxZr2O7 transparent ceramics. J Eur Ceram soc 2014, 34: 3951–3958. Figure 1