Pyrochlore Rare-Earth Hafnate RE₂Hf₂O₇ (RE = La and Pr) Nanoparticles Stabilized by Molten-Salt Synthesis at Low Temperature

Abstract

Complex oxides of the RE2Hf2O7 series are functional materials that exist in the fluorite or pyrochlore phase depending on synthesis method and calcination temperature. In this study, we investigate the process of synthesis, crystal structure stabilization, and phase transition in a series of RE hafnate compounds, synthesized by the coprecipitation process of a single-source complex hydroxide precursor followed with direct calcination or molten-salt synthesis (MSS) method. Phase pure RE2Hf2O7 (RE = Y, La, Pr, Gd, Er, and Lu) ultrafine nanostructured powders were obtained after calcinating the precursor in a molten salt at 650 °C for 6 h. Moreover, we demonstrate that the MSS method can successfully stabilize ideal pyrochlore structures for La2Hf2O7 and Pr2Hf2O7 in the nanodomain, which is not possible to achieve by direct calcination of the coprecipitated precursor at 650 °C. We propose mechanisms to elucidate the differences in these two synthesis methods and highlight the superiority of the MSS method for the production of RE hafnate nanoparticles.

Structural Evolution and Magnetic Properties of Gd2Hf2O7 Nanocrystals: Computational and Experimental Investigations

“…This kind of high-temperature synthesis method doesn’t allow for the study of the WO-OP crystalline phase evolution of the GHO nanocrystal structure thermodynamically, especially when samples are calcined from lower to higher temperatures. Although the exact lower boundary temperature for the WO-OP crystalline phase evolution is still under debate, it has been reported that GHO stabilizes into fully ordered GHO at 1300 °C [ 26 , 32 ]. The ordering of anions, cations, and vacancies on a single site occurs simultaneously in order to achieve the OP-GHO structural transformation.…”

“…We recently showed how GHO and other ABO (A = Y, La, Pr, Er, and Lu, B = Hf, and Zr) compounds can be synthesized in the nanodomain, using the molten salt method. The functionality of such nanocrystals depends on the crystalline phase and local structure [ 32 , 36 , 37 ]. It should be noted that the GHO morphology and crystalline size can be altered by varying the synthesis condition and annealing temperature [ 32 ].…”

“…The functionality of such nanocrystals depends on the crystalline phase and local structure [ 32 , 36 , 37 ]. It should be noted that the GHO morphology and crystalline size can be altered by varying the synthesis condition and annealing temperature [ 32 ]. In recent years, we have investigated the molten-salt synthesis of the A 2 B 2 O 7 (A = Y, La, Pr, Gd, Er, and Lu, B = Hf, and Zr) nanocrystals and explored their unique set of optical properties when doped with trivalent rare-earth ions and actinides, such as uranium [ 11 , 32 , 38 , 39 ].…”

“…It should be noted that the GHO morphology and crystalline size can be altered by varying the synthesis condition and annealing temperature [ 32 ]. In recent years, we have investigated the molten-salt synthesis of the A 2 B 2 O 7 (A = Y, La, Pr, Gd, Er, and Lu, B = Hf, and Zr) nanocrystals and explored their unique set of optical properties when doped with trivalent rare-earth ions and actinides, such as uranium [ 11 , 32 , 38 , 39 ]. The molten-salt synthesis has the advantage of homogenizing the binary cations within the complex precursor.…”

“…With the molten salt synthesis method, we have previously shown that homogeneous ABO (A = Y, La, Pr, Gd, Er, and Lu; B = Hf, Zr) powders can be synthesized with a crystalline size <30 nm and that the diffraction patterns are similar to those observed in the DF and OP phases depending on the ionic radius ratio rules. Thus, we found the molten-salt synthesis method to be appropriate for producing homogeneous GHO nanocrystals and probing the evolution of their crystalline phases [ 32 , 37 ]. In addition, an understanding of the origin of the crystalline phase evolution is missing thermodynamically, especially when the low-temperature-stabilized GHO nanocrystals are calcined to higher temperatures compared to that of the order-disorder (O-D) transition under high-energy radiation exposure.…”