Lanthanum hafnate
{{Short description|Ceramic metal compound}}
{{Chembox
| ImageFile =
| IUPACName =
| Section1 = {{Chembox Identifiers
| CASNo =
| PubChem =
| SMILES = [La+3].[La+3].[Hf+4].[Hf+4].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2]
| StdInChI=1S/2Hf.2La.7O/q2*+4;2*+3;7*-2
| StdInChIKey = AUXGONVMFPDWKD-UHFFFAOYSA-N
}}
| Section2 = {{Chembox Properties
| Formula = {{chem2|La2Hf2O7}}
| MolarMass = 746.79 g/mol
| Appearance = colorless solid
| Density = 7.86 g/cm3{{cite journal |title=Materials Data on La2Hf2O7 by Materials Project |url=https://materialsproject.org/materials/mp-12533/ |publisher=Materials Project |access-date=30 April 2021 |doi=10.17188/1188964 |date=2020|author1=None Available }}
| MeltingPt =
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| Section3 = {{Chembox Structure
| CrystalStruct = Cubic
| SpaceGroup = Fm3m (< 1000 °C)
Fd3m (> 1000 °C)
}}
| Section8 = {{Chembox Related
| OtherCations = Lanthanum zirconate
}}
}}
Lanthanum hafnate ({{chem2|La2Hf2O7|auto=yes}}) or lanthanum hafnium oxide is a mixed oxide of lanthanum and hafnium.
Properties
Lanthanum hafnate is a colorless ceramic material{{cite journal |last1=Ji |first1=Yaming |last2=Jiang |first2=Danyu |last3=Fen |first3=Tao |last4=Shi |first4=Jianlin |title=Fabrication of transparent La2Hf2O7 ceramics from combustion synthesized powders |journal=Materials Research Bulletin |date=March 2005 |volume=40 |issue=3 |pages=553–559 |doi=10.1016/j.materresbull.2004.10.010 }} with the La and Hf atoms arranged in a cubic lattice. The arrangement is a disordered fluorite-like structure below {{convert|1000|C|K F}}, above which it transitions to a pyrochlore phase; an amorphous phase also exists below {{convert|800|C|K F}}.{{cite journal |last1=Popov |first1=V V |last2=Menushenkov |first2=A P |last3=Yastrebtsev |first3=A A |last4=Zubavichus |first4=Ya V |title=La 2 Hf 2 O 7 crystal and local structure changes on the fluorite - pyrochlore phase transition |journal=Journal of Physics: Conference Series |date=September 2016 |volume=747 |issue=1 |pages=012043 |doi=10.1088/1742-6596/747/1/012043|bibcode=2016JPhCS.747a2043P |doi-access=free }}{{cite journal |last1=Blanchard |first1=Peter E. R. |last2=Liu |first2=Sam |last3=Kennedy |first3=Brendan J. |last4=Ling |first4=Chris D. |last5=Avdeev |first5=Max |last6=Aitken |first6=Jade B. |last7=Cowie |first7=Bruce C. C. |last8=Tadich |first8=Anton |title=Investigating the Local Structure of Lanthanoid Hafnates Ln 2 Hf 2 O 7 via Diffraction and Spectroscopy |journal=The Journal of Physical Chemistry C |date=7 February 2013 |volume=117 |issue=5 |pages=2266–2273 |doi=10.1021/jp311329q}}
The compound decomposes into its constituent oxides at 18 GPa.{{cite journal |last1=Garg |first1=Nandini |last2=Pandey |first2=K. K. |last3=Murli |first3=Chitra |last4=Shanavas |first4=K. V. |last5=Mandal |first5=Balaji P. |last6=Tyagi |first6=A. K. |last7=Sharma |first7=Surinder M. |title=Decomposition of lanthanum hafnate at high pressures |journal=Physical Review B |date=13 June 2008 |volume=77 |issue=21 |pages=214105 |doi=10.1103/PhysRevB.77.214105|bibcode=2008PhRvB..77u4105G }}
=Luminescence=
Oxygen vacancies in the base material give luminescence spanning across the visible light spectrum, with a peak near 460 nm.{{cite journal |last1=Eagleman |first1=Yetta |last2=Weber |first2=Marvin |last3=Derenzo |first3=Stephen |title=Luminescence study of oxygen vacancies in lanthanum hafnium oxide, La2Hf2O7 |journal=Journal of Luminescence |date=May 2013 |volume=137 |pages=93–97 |doi=10.1016/j.jlumin.2012.10.034 |bibcode=2013JLum..137...93E }} The luminescent properties can be fine-tuned by doping with various rare earth and group 4 metals;{{cite journal |last1=Gupta |first1=Santosh K. |last2=Zuniga |first2=Jose P. |last3=Abdou |first3=Maya |last4=Thomas |first4=Melonie P. |last5=De Alwis Goonatilleke |first5=Manisha |last6=Guiton |first6=Beth S. |last7=Mao |first7=Yuanbing |title=Lanthanide-doped lanthanum hafnate nanoparticles as multicolor phosphors for warm white lighting and scintillators |journal=Chemical Engineering Journal |date=January 2020 |volume=379 |pages=122314 |doi=10.1016/j.cej.2019.122314 |s2cid=199650816 |url=https://scholarworks.utrgv.edu/chem_fac/137 |doi-access=free |bibcode=2020ChEnJ.37922314G }}{{cite journal |last1=Trojan-Piegza |first1=Joanna |last2=Zych |first2=Eugeniusz |title=White persistent luminescence of La2Hf2O7:Ti,Pr |journal=Optical Materials |date=March 2021 |volume=113 |pages=110896 |doi=10.1016/j.optmat.2021.110896 |bibcode=2021OptMa.11310896T |s2cid=233842128 }} for example, {{chem2|La2Hf2O7:Eu(3+)}} nanoparticles exhibit a red photoluminescence or radioluminescence near 612 nm when exposed to ultraviolet or X-ray radiation.{{cite journal |last1=Wahid |first1=Kareem |last2=Pokhrel |first2=Madhab |last3=Mao |first3=Yuanbing |title=Structural, photoluminescence and radioluminescence properties of Eu3+ doped La2Hf2O7 nanoparticles |journal=Journal of Solid State Chemistry |date=January 2017 |volume=245 |pages=89–97 |doi=10.1016/j.jssc.2016.10.004|bibcode=2017JSSCh.245...89W |doi-access=free }}
Synthesis
Bulk ceramics can obtained by combusting the elements in powder form, and then pressing and sintering the powder at 180 MPa and {{convert|1850|C|K F}} for 6 hours:
:{{chem2|4 La + 4 Hf + 7 O2 -> 2 La2Hf2O7}}
It may also be made by precipitating hafnium and lanthanum hydroxides from solution and then calcinating in air at {{convert|600–1400|C|K F}} for 3 hours:
:2 {{chem2|La(OH)3}} + 2 {{chem2|Hf(OH)4}} → {{chem2|La2Hf2O7}} + 7 {{chem2|H2O}}.