Effect of Eu3+ Doping and Self-generated Pressure on the Crystallographic/morphological Features and Thermal Stability of LaPO4∙nH2O Single-crystal Nanorods obtained by Microwave-assisted Hydrothermal Synthesis

Rhabdophane-type single-crystal La_1-xEu_xPO₄∙nH₂O (x=0‒0.10) nanorods with high-quality (that is, well-shaped and linear/planar defect free) were prepared by a simple and fast microwave-assisted hydrothermal route, being the optimum autogeneous pressure of 2.3±0.1 bar during their synthesis. Higher values of pressure lead to the loss of the nanorod shape. Eu³⁺ doping does not affect the thermal stability of the nanorods, indicating that Eu³⁺ cations, consistently with their smaller size can occupy substitutional positions in the lattice, are not involved in the rate-limiting mechanism of ion diffusion/mobility in the La_1-xEu_xPO₄∙nH₂O lattice. Monazite-type single-crystal La_1-xEu_xPO₄ nanorods were obtained by calcining their rhabdophane-type La_1-xEu_xPO₄∙nH₂O counterparts at 700 ºC in air.

In addiditon, rhabdophane-type Eu³⁺,Tb³⁺-codoped LaPO₄∙nH₂O single-crystal nanorods with the compositions La_0.99999-xEu_xTb_0.00001PO₄∙nH₂O (x=0‒0.03), La_0.99999-yTb_yEu_0.00001PO₄∙n‘H₂O (y=0‒0.010), and La_0.99999-zTb_zEu_0.000007PO₄∙n»H₂O (z=0‒0.012) were hydrothermally synthesized with microwaves. The Eu³⁺,Tb³⁺ codoping did not affect the thermal stability of these nanorods, which is due to the formation of substitutional solid solutions with both Eu³⁺ and Tb³⁺ replacing La³⁺ in the crystal lattice. Monazite-type Eu³⁺,Tb³⁺-codoped LaPO₄ single-crystal nanorods can be obtained by calcining their rhabdophane-type Eu³⁺,Tb³⁺-codoped LaPO₄∙(n,n‘,n»)H₂O counterparts at moderate temperature in air, and that they are thermally stable. It is also observed that, for the same Eu³⁺,Tb³⁺-codoping content, the monazite-type Eu³⁺,Tb³⁺-codoped LaPO₄ nanorods exhibit higher photoluminescent efficiency than the rhabdophane-type Eu³⁺,Tb³⁺-codoped LaPO₄∙nH₂O nanorods, as it was expected. Moreover, it was found that the highest photoluminescence emission corresponds to the monazite-type La_0.96999Eu_0.02Tb_0.00001PO₄ nanorods for the La_0.99999-xEu_xTb_0.00001PO₄ system. However, for those compositions energy transfer from Tb³⁺ to Eu³⁺ did not occur. In addition, for an efficient energy transfer to occur, a content of at least 1 mol.% Tb³⁺ is needed in all the studied materials