Structural Effects of Lanthanide Dopants on Alumina

Abstract

Lanthanide (Ln³⁺) doping in alumina has shown great promise for stabilizing and promoting desirable phase formation to achieve optimized physical and chemical properties. However, doping alumina with Ln elements is generally accompanied by formation of new phases (i.e. LnAlO₃, Ln₂O₃), and therefore inclusion of Ln-doping mechanisms for phase stabilization of the alumina lattice is indispensable. In this study, Ln-doping (400 ppm) of the alumina lattice crucially delays the onset of phase transformation and enables phase population control, which is achieved without the formation of new phases. The delay in phase transition (θ → α), and alteration of powder morphology, particle dimensions, and composition ratios between α- and θ-alumina phases are studied using a combination of solid state nuclear magnetic resonance, electron microscopy, digital scanning calorimetry, and high resolution X-ray diffraction with refinement fitting. Loading alumina with a sparse concentration of Ln-dopants suggests that the dopants reside in the vacant octahedral locations within the alumina lattice, where complete conversion into the thermodynamically stable α-domain is shown in dysprosium (Dy)- and lutetium (Lu)-doped alumina. This study opens up the potential to control the structure and phase composition of Ln-doped alumina for emerging applications.

Figure 1

(a) Schematic representation of the Ln-doped alumina sample preparation. (b) Schematic representation of the five possible locations for the Ln-dopants to reside in alumina. [S-1: - Ln-dopant intercalating in the lattice by substituting some of the aluminum cations in the octahedral sites. S-2: - Formation of LnAlO₃ or Ln₂O₃ nanoparticles, which were entirely separate from the alumina nanoparticles. S-3: - Formation of a layer or a protective shell of LnAlO₃ or Ln₂O₃ around the alumina nanoparticles. S-4: - Ln-dopants residing in the one-third vacant octahedral sites or the vacant tetrahedral sites. S-5: - Ln-dopants residing on the grain boundaries of the alumina lattice].

Figure 2

(a) Overlay of 1D ²⁷Al MAS NMR spectra for Al₂O₃, Er-doped Al₂O₃, un-doped α-Al₂O₃ phase and un-doped θ-Al₂O₃ phase. The MAS rate was equal to 10 kHz. (b) 2D ²⁷Al MQ-MAS spectra of un-doped Al₂O₃ (c) 2D ²⁷Al MQ-MAS spectra of Er-doped Al₂O₃.

Figure 3

(a) High temperature differential scanning calorimetry (HT-DSC) curves of Al₂O₃ (purple) and Er-Al₂O₃ (red). (b) SEM image of Er-doped Al₂O₃. (c) EDS elemental mapping of Er-Al₂O₃.

Figure 4. The 2D MQ-MAS spectra of Ln-doped Al₂O_3.

Figure 5

(a) Low angle HR-XRD data of the Ln-doped alumina. From the top to the bottom, Ln = Pr, La, Gd, Yb, Nd, Tm, Lu, Dy, respectively. The data were sequentially offset by a factor of 10 for clarity. The peaks denoted by “*” represented the α-phase and the peaks denoted by “o” symbols represented the θ-phase. (b) Graphical representation of the α- and θ-phase populations at ~1300 °C. The Dy- and Lu-doped alumina exhibited only α-phase. (c) Graphical representation of Ln-cationic phase fraction, octahedral coordination radius and phase diameters. The data points represented Ln-series elements in decreasing order of atomic number, moving from left to right (Lu, Yb, Tm, Er, Dy, Gd, Nd, Pr, and La). The diameter was represented by the orange data points and the phase fraction was represented by the blue data points. A refinement summary of all Ln³⁺ doped alumina was included. (d) SEM images of Dy- and Tm-doped alumina.