Evaluation of rare-earth sesquioxides nanoparticles as a bottom-up strategy toward the formation of functional structures

Background: The strategy to form functional structures based on powder technology relies on the concept of nanoparticles characteristics. Rare-earth sesquioxides (RE2O3; RE as Y, Tm, Eu) exhibit remarkable properties, and their fields of application cover energy, astronomy, environmental, medical, information technology, industry, and materials science. The purpose of this paper is to evaluate the RE2O3 nanoparticles characteristics as a bottom-up strategy to form functional materials for radiation dosimetry. Methods: The RE2O3 nanoparticles were characterized by the following techniques: XRD, SEM, PCS, FTIR, ICP, EPR, and zeta potential. Results: All RE2O3 samples exhibited cubic C-type structure in accordance with the sesquioxide diagram, chemical composition over 99.9%, monomodal mean particle size distribution, in which (d50) was inferior than 130nm. Among all samples, only yttrium oxide exhibited EPR signal, in which the most intense peak was recorded at 358mT and g 1.9701. Conclusion: The evaluation of nanoparticle characteristics is extremely important taking into account a bottom-up strategy to form functional materials. The RE2O3 nanoparticles exhibited promising characteristics for application in radiation dosimetry.

Heat-Induced Transformation of Luminescent, Size Tuneable, Anisotropic Eu:Lu(OH)2Cl Microparticles to Micro-Structurally Controlled Eu:Lu2O3 Microplatelets

Synthetic procedures to obtain size and shape-controlled microparticles hold great promise to achieve structural control on the microscale of macroscopic ceramic- or composite-materials. Lutetium oxide is a material relevant for scintillation due to its high density and the possibility to dope with rare earth emitter ions. However, rare earth sesquioxides are challenging to synthesise using bottom-up methods. Therefore, calcination represents an interesting approach to transform lutetium-based particles to corresponding sesquioxides. Here, the controlled solvothermal synthesis of size-tuneable europium doped Lu(OH)2Cl microplatelets and their heat-induced transformation to Eu:Lu2O3 above 800 °C are described. The particles obtained in microwave solvothermal conditions, and their thermal evolution were studied using powder X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), optical microscopy, thermogravimetric analysis (TGA), luminescence spectroscopy (PL/PLE) and infrared spectroscopy (ATR-IR). The successful transformation of Eu:Lu(OH)2Cl particles into polycrystalline Eu:Lu2O3 microparticles is reported, together with the detailed analysis of their initial and final morphology.

Thermal Analysis of High Entropy Rare Earth Oxides

Phase transformations in multicomponent rare earth sesquioxides were studied by splat quenching from the melt, high temperature differential thermal analysis and synchrotron X-ray diffraction on laser-heated samples. Three compositions were prepared by the solution combustion method: (La,Sm,Dy,Er,RE)2O3, where all oxides are in equimolar ratios and RE is Nd or Gd or Y. After annealing at 800 °C, all powders contained mainly a phase of C-type bixbyite structure. After laser melting, all samples were quenched in a single-phase monoclinic B-type structure. Thermal analysis indicated three reversible phase transitions in the range 1900–2400 °C, assigned as transformations into A, H, and X rare earth sesquioxides structure types. Unit cell volumes and volume changes on C-B, B-A, and H-X transformations were measured by X-ray diffraction and consistent with the trend in pure rare earth sesquioxides. The formation of single-phase solid solutions was predicted by Calphad calculations. The melting point was determined for the (La,Sm,Dy,Er,Nd)2O3 sample as 2456 ± 12 °C, which is higher than for any of constituent oxides. An increase in melting temperature is probably related to nonideal mixing in the solid and/or the melt and prompts future investigation of the liquidus surface in Sm2O3-Dy2O3, Sm2O3-Er2O3, and Dy2O3-Er2O3 systems.

Revised phase stability diagram of rare earth sesquioxides

Below 2000°C rare earth sesquioxides (RESOX) have three crystal structures: hexagonal, cubic and monoclinic, designated as A, C and B respectively [1-3]. Early studies, based on low temperature (LT) synthesis, suggested that RESOX phase stability versus temperature is a function of the metallic ion radii (MIR). La2 O3 Ce2 O3 and Nd2 O3 with the highest MIR are A-type, while for Sm2 O3 , Eu2 O3 and Gd2 O3 with intermediate MIR the structure is C-type at LT and B-type at high temperature (HT) [1-3]. All other RESOX including Y2 O3 and Sc2 O3 were assumed to be cubic (C-type) at all temperatures below 2000°C. The transformation from LT cubic to high temperature (HT) monoclinic structure in Sm2 O3 , Eu2 O3 and Gd2 O3 is unusual and therefore Brauer [4] and Yokogawa et al. [5] suggested that the stable phase is monoclinic at all temperatures below 2000°C. To resolve the controversy, we have demonstrated that slowing down grain growth of Sm2 O3 and Gd2 O3 [9] prevented transition from C to B-types in the expected temperatures (1100 and 1300°C respectively). Hence, we suggest that the surface energy plays an important role in determining the structure of nanomaterials [6,7]. The monoclinic Sm2 O3 , Eu2 O3 and Gd2 O3 is the stable structure at all temperatures below 2000 °C when the grain size is large in the nanoscale. However, for smaller nanocrystals the stable structure is cubic since it has a lower surface energy than the monoclinic phase. In addition, Kimmel et al. [9] suggested that for all RESOX with MIR lower than Gd3+ (except Sc2 O3 ) obtained by HT synthesis [10-17] or under high pressure [18-20] the monoclinic phase is the stable phase also at LT. Figure 1 shows the transition from LT monoclinic to the HT cubic phase according to Sato et al. [17]. Figue 2 shows the suggested RESOX stability diagram as function of temperature versus MIR. In sol-gel production the formation of C-type structures is due to the formation of nano-crystals. Subsequent firing at high temperatures yields the HT cubic phase. Thus. the assumption of a continuous cubic structure at all temperatures is wrong. As seen in Figure 1, in Sc2 O3 the monoclinic to cubic transition is below room temperature, in agreement with the fact that HT synthesis yields cubic Sc2 O3 [17]. (The ion radius of Sc3+ is 0.087 nm [21,22])

Microstrain-assisted polymorphic phase transitions in (Eu1−x La x )2O3

Solid solutions, (Eu1−x La x )2O3 (0 ≤ x ≤ 1), of the rare earth sesquioxides Eu2O3 and La2O3 have been prepared by a simple soft chemistry approach. The composition and morphology of the as-synthesized oxides have been characterized using energy-dispersive spectroscopy and scanning electron microscopy. The particles are of irregular shape and submicrometre size. In order to understand the structural evolution as a function of composition, angle-dispersive X-ray diffraction measurements have been carried out and the structural parameters have been obtained through Rietveld refinement. A structural phase transition from the cubic (C-type) to the monoclinic (B-type) structure and subsequently to the hexagonal (A-type) structure was observed with an increasing substitution of La. A detailed analysis of the transition boundaries in terms of the average cationic radius, R RE, shows that the onset of the C → B transition is at R RE = 0.980 Å, whereas the B → A transition occurs at R RE = 1.025 Å. A biphasic region of cubic and monoclinic structures is observed for 0.2 ≤ x ≤ 0.4 and one of monoclinic and hexagonal structures is observed for 0.5 ≤ x ≤ 0.6. The microstrain induced by the difference in size of the rare earth cations introduces a substitutional disorder in the crystal structure, which is a plausible cause of the observed phase transitions in these oxides.