Lanthanum hafnate is a colorless ceramic material[2] with the La and Hf atoms arranged in a cubic lattice. The arrangement is a disordered fluorite-like structure below 1,000 °C (1,270 K; 1,830 °F), above which it transitions to a pyrochlore phase; an amorphous phase also exists below 800 °C (1,070 K; 1,470 °F).[3][4]
The compound decomposes into its constituent oxides at 18 GPa.[5]
Luminescence
Oxygen vacancies in the base material give luminescence spanning across the visible light spectrum, with a peak near 460 nm.[6] The luminescent properties can be fine-tuned by doping with various rare earth and group 4 metals;[7][8] for example, La2Hf2O7:Eu3+ nanoparticles exhibit a red photoluminescence or radioluminescence near 612 nm when exposed to ultraviolet or X-ray radiation.[9]
Synthesis
Bulk ceramics can obtained by combusting the elements in powder form, and then pressing and sintering the powder at 180 MPa and 1,850 °C (2,120 K; 3,360 °F) for 6 hours:[2]
4 La + 4 Hf + 7 O 2 → 2 La 2Hf 2O 7.
It may also be made by precipitating hafnium and lanthanum hydroxides from solution and then calcinating in air at 600–1,400 °C (873–1,673 K; 1,112–2,552 °F) for 3 hours:[3]
^Blanchard, Peter E. R.; Liu, Sam; Kennedy, Brendan J.; Ling, Chris D.; Avdeev, Max; Aitken, Jade B.; Cowie, Bruce C. C.; Tadich, Anton (7 February 2013). "Investigating the Local Structure of Lanthanoid Hafnates Ln 2 Hf 2 O 7 via Diffraction and Spectroscopy". The Journal of Physical Chemistry C. 117 (5): 2266–2273. doi:10.1021/jp311329q.
^Garg, Nandini; Pandey, K. K.; Murli, Chitra; Shanavas, K. V.; Mandal, Balaji P.; Tyagi, A. K.; Sharma, Surinder M. (13 June 2008). "Decomposition of lanthanum hafnate at high pressures". Physical Review B. 77 (21): 214105. Bibcode:2008PhRvB..77u4105G. doi:10.1103/PhysRevB.77.214105.
^Eagleman, Yetta; Weber, Marvin; Derenzo, Stephen (May 2013). "Luminescence study of oxygen vacancies in lanthanum hafnium oxide, La2Hf2O7". Journal of Luminescence. 137: 93–97. Bibcode:2013JLum..137...93E. doi:10.1016/j.jlumin.2012.10.034.