XRD analysis of Doped Monoclinic LaPO4 (Monazite)
Kenta Ohtaki, Neshat J. Heravi, Peter E. D. Morgan, and Martha L. Mecartney
Department of Chemical Engineering and Materials Science University of California, Irvine
Irvine, CA 92697-2575
Monazite has unique properties when doped with divalent or rare-earth elements; rare-earth
doped LaPO4 is luminescent1), Sr2+ doped LaPO4 is an intermediate temperature proton
conductor2). LaPO4 can also accommodate a wide range of nuclear fission products. Solution
routes to form monazite usually result in the hexagonal hydrate, rhabdophane (LaPO4•0.5H2O),
but we have developed a direct precipitation route to monoclinic LaPO43).
Monoclinic LaPO4 was synthesized via the direct precipitation route at different temperatures.
Change in particle morphology and size are observed as a function of lowering temperature. The
phase transition to rhabdophane was observed at 90-100 ºC. This is due to incorporation of H2O
in the system since the synthesis is adding La and Sr nitride aqueous solution drop wise into
heated phosphoric acid. In order to eliminate H2O from the system, the acid temperature needs to
be above 100 ºC. X-ray diffraction peaks of rhabdophane are observed among monazite peaks at
90-100 ºC (Fig. 1).
The direct precipitation route was found to be successful for incorporation of dopants of rareearth or Sr2+. Sr2+ or Eu3+ single doped LaPO4 were prepared with nominal dopant ratio of 10, 20
mol% for Sr2+ and 5 mol% for Eu3+. The dopant ratio were determined by Rietveld refinement.
The result of refinement for Sr2+ 10 mol% corresponded to the nominal dopant concentration of
10 mol%, whereas refinement was not at all consistent for Sr2+ 20 mol%, possible due to (OH)in the crystal. The 5 mol% for Eu3+ refinement indicated the dopant ratio of Eu3+ is 2%, which
implies Eu3+ ions in the solution were only partially incorporated. The complete solid solubility
of Sr in LaPO4 via this precipitation route was also investigated. Ideally, were Sr to have
complete solid solubility, SrHPO4 might be expected to form, however, Sr was found to
precipitate as Sr(H2PO4)2 (Fig. 2). The solid solubility limit is currently under investigation.
100-110
ºC
100-110oC
90-100
ºCoC
90-100
Sr(H
2PO4)2
Sr(H2PO4)2
Intensitya. u.
Intensitya. u.
140-150
140-150
ºCoC
20% Sr:Sr2+
LaPO
4
20%
Rhabdophane
20
30
40
2degree
50
Monazite
60
Fig.1 Phase transition with precipitation
temperature.
10% Sr:Sr2+
LaPO
4
10%
10
20
30
40
2degree
50
Fig.2 Doping LaPO4 with Sr
1) Heike Meyssamy et al. Adv. Mater., 11 (1999) 840-844
2) N. Kitamura et al. Solid State Ionics, 162 (2003) 161-165
3) Michael T. Schatzmann et al. J. Mater. Chem., 19 (2009) 5720–5722
60