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.

Solution-combustion Synthesized Nano-pellet α-Al2O3 and Catalytic Oxidation of Cyclohexane by Its Supported Cobalt Acetate

“…This phenomenon is consistent with the amorphous γ-Al 2 O 3 of the alumina samples added with cerium ions, demonstrated by the XRD characterization analysis. It was also reported that rare earth elements could stabilize the amorphous form of alumina by inhibiting the phase transition of alumina [45][46][47][48] . The action of the cobalt ions on the formation of the alumina phase was similar to that of cerium ions after the addition to the precursor solution.…”

Photoluminescence of rare‐earth/transition metal‐doped transparent/translucent polycrystalline Al2O3 ceramics: A review

“…However, this problem can be partially resolved by producing ceramics with relatively fine average grain size of several hundreds of nanometers 8,9 . The second crucial obstacle is very low RE ions equilibrium solubility in α‐Al 2 O 3 (in range between ∼10 −3 and 10 −2 at.%) 12–14 arising from ionic radius mismatch between the dopants (Ln 3+ ) and Al 3+ ions (0.0540 nm for Al 3+ and 0.1030 nm → 0.0861 nm for Ln 3+ series) 15 . As a consequence, the equilibrium dopant concentration in bulk is too low for efficient luminescence‐based light production; the RE doping level typically ranges from 10th up to several atomic percent 16–20 .…”

“…In order to achieve a higher functional level of RE‐dopant concentration, a processing technique such as spark plasma sintering (SPS), which can operate out of thermodynamic equilibrium conditions, was used to produce Al 2 O 3 :RE transparent ceramic with a higher RE 3+ doping level 21 . When traditional sintering techniques, operating at nearly thermodynamic equilibrium conditions, were used to prepare polycrystalline Al 2 O 3 :RE, detailed analysis of microstructure revealed the dopant segregation at the grain boundaries sometimes accompanied by the formation of secondary phases 13–15,22 . Considering the size of alumina grains of the order of several tens up to hundreds of nanometers, the fine microstructure provides a large grain boundary area capable of absorbing a relatively high amount of dopants 22–24 .…”

“…8,9 The second crucial obstacle is very low RE ions equilibrium solubility in α-Al 2 O 3 (in range between ∼10 −3 and 10 −2 at.%) [12][13][14] arising from ionic radius mismatch between the dopants (Ln 3+ ) and Al 3+ ions (0.0540 nm for Al 3+ and 0.1030 nm → 0.0861 nm for Ln 3+ series). 15 As a consequence, the equilibrium dopant concentration in bulk is too low for efficient luminescencebased light production; the RE doping level typically ranges from 10th up to several atomic percent. [16][17][18][19][20] This indicates that at least a 100-fold higher than equilibrium dopant concentration is required to produce Al 2 O 3 :RE ceramics suitable for photonics applications (e.g., lasers, solid-state lighting).…”

Electronic and optical behaviour of lanthanum doped CaTiO3 perovskite

“…First of all, it has been observed experimentally that the solubility of Lanthanum in perovskite materials such as lead titanate (PbTiO 3 ), lead zirconate (PbZrO 3 ), lithium tantalate (LiTaO 3 ), barium titanate (BaTiO) and niobate (LiNbO 3 ) [26] is critically lower [27,28], which limits higher dopant concentration in the host lattices. Secondly La prone to form secondary phases with oxygen upon higher doping concentration (even 5%-10%), thereby adding further un-clarity in understanding the role of Lanthanum on structural and optical modifications of perovskites and oxides [29][30][31].…”