Original Research PaperCrystallization kinetics and structural properties of nanocrystalline europium-yttrium-titanate (Eu0.5Y0.5)2Ti2O7
Graphical abstract
Introduction
Lanthanide titanium oxides, which crystallize in a face-centered cubic (FCC) pyrochlore structure with the general formula A2B2O7 [1], have attracted significant attention in recent materials research because of their distinctive magnetic and optical properties. The spin arrangement of rare earth (RE) elements in the pyrochlore structure exhibits short-range ordering [2], allowing the synthesis of spin-glass, spin-liquid, and spin-ice compounds [3], [4]. In the yttrium-co-doped pyrochlore structure, the sites of A-cations are occupied by RE ions or yttrium ions, sites of B-cations are occupied by titanium ions [2], and general formula can be expressed as follows: (RExY1 - x)2Ti2O7 (RE = rare earth element, x=<0, 1 > ). Pure RE2Ti2O7 pyrochlores are optically inactive. However, the yttrium ions in the pyrochlore structure break the spin interactions and prevent the non-radiative transitions between RE ions [5], [6]. Consequently, the yttrium-co-doped pyrochlores exhibit highly effective luminescence properties [5]. This discovery has boosted research on novel (RExY1 - x)2Ti2O7 luminophores with various RE ions incorporated in the pyrochlore structure, for example Er3+ [5], [6], Eu3+ [7], and Ho3+ [8]. The pyrochlores’ magnetic and optical properties strongly depend on the local arrangement of RE ions in the crystal lattice. Therefore, the synthesis and thermal processing must be precisely tailored to maximize the regularity of the crystal structure. The non-uniform distribution of RE ions, increased defects, and lattice frustration can significantly modify the final material properties [9], [10]. Inappropriate heat treatment can result in an undesirable transformation of the pyrochlore structure, such as that of perovskite [11].
In addition to the common solid-state ceramic approaches [3], [4], “bottom-up” methods have been used for the synthesis of pyrochlores [5], [12], [13]. Bottom-up methods usually involve the preparation of a solid amorphous powder followed by heat treatment to induce crystallization [8], [12], [13]. In this method, the final material properties are susceptible to the heat-treatment conditions, and the crystallization kinetics represent a critical factor that affects the structural properties of the formed nanocrystals [12], [13], [14], [15], [16]. Johnson, Mehl, and Avrami (JMA) proposed a general crystallization kinetics model [17], [18], that has been successfully applied to evaluate the crystallization properties of RE2Ti2O7 pyrochlores [12], [13], [14]. Two types of nucleation mechanisms were considered in the JMA model: site-saturated nucleation and homogenous nucleation with a constant nucleation rate. Site-saturated nucleation is characterized by the simultaneous formation of nuclei in the entire volume of an amorphous material. Once the formation of nuclei is complete, recrystallization occurs. This mechanism leads to highly uniform materials with a narrow nanocrystal size distribution. Homogeneous nucleation at a constant nucleation rate is characterized by the simultaneous formation of nuclei and crystal growth. These materials exhibit significantly lower uniformity and broader nanocrystal size distribution than those formed via site-saturated nucleation [19]. The crystallization kinetic parameters have been reported for many RE2Ti2O7 pyrochlores [12], [13], [14], [16]. Despite the considerable application potential of these materials in photonics [6], [20], [21], [22], the crystallization properties of optically active pyrochlores (RExY1 - x)2Ti2O7 require additional investigation. Eliminating this knowledge gap is necessary to prepare transparent coatings and upscale the synthesis of (RExY1 - x)2Ti2O7 luminophores with tailored properties.
This study presents a generic sol–gel approach to prepare nanocrystalline (Eu0.5Y0.5)2Ti2O7 powders. We studied both the kinetic parameters of the nucleation process and crystallization mechanism of (Eu0.5Y0.5)2Ti2O7 from an amorphous xerogel. We evaluated the crystal structure of (Eu0.5Y0.5)2Ti2O7, and compared the results the data reported for the isostructural end-members Eu2Ti2O7 and Y2Ti2O7. These results provide information on the crystallization properties and crystal structure of nanocrystalline (Eu0.5Y0.5)2Ti2O7. This knowledge provides data for the upscale synthesis of pure nanocrystalline powders with tailored structural properties and transparent coatings suitable for photonic applications.
Section snippets
Materials and sample preparation
The methodical approach is summarized in Fig. 1. The samples were prepared by a sol–gel method followed by the thermal treatment of a xerogel. To prepare a sol, a total of 10 g titanium(IV)butoxide (Fluka, Purum) was dissolved in 500 ml of anhydrous ethanol (Sigma–Aldrich, Spectranal grade), after which a total of 5.64 g of yttrium(III) nitrate hexahydrate (Aldrich, 99.8%) and 6.3 g of europium(III) nitrate pentahydrate (Aldrich, 99.9%) were dispersed in the solution. All the precursors were
Thermal evolution of xerogels
The preparation of materials with tailored structural properties using the sol–gel approach requires in-depth knowledge of their thermal behavior. The general DTA curve, in Fig. 2a, shows several endothermic peaks below 600 K, a main exothermic peak around 653 K, and an exothermic peak at 1097 K. The endothermic peaks below 600 K and first exothermic peak at approximately 650 K were accompanied by gradual weight loss. The minor weight loss accompanied the second exothermic peak at 1097 K, which
Conclusions
We demonstrated a versatile sol–gel approach to prepare nanocrystalline (Eu0.5Y0.5)2Ti2O7. The thermal analyses showed that the crystallization temperature of (Eu0.5Y0.5)2Ti2O7 was 1050.1 ± 0.8 K, and the activation energy of the crystallization was 605 kJ mol−1. The application of the non–isothermal JMA model revealed that the crystallization of (Eu0.5Y0.5)2Ti2O7 was initiated by homogenous nucleation with a constant nucleation rate. Nanocrystal growth was limited by mass transfer through the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge the financial support of the Czech Science Foundation under contract N°21-45431L. This research was partially supported by the Institute of Geology of the CAS Research Plan RVO67985831.
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