Interpretation of the Spectroscopic Properties of α-LiAlO₂: Mn⁴⁺

Abstract

The energy position of the 2 E g → 4 A 2g emission transition of Mn 4+ in α-LiAlO 2 is identified and the electronic energy levels calculated using the exchange charge model of crystal field theory. The results of our calculations yield the crystal field splitting and Racah parameters of Dq = 2351 cm −1 , B = 650 cm −1 and C = 3469 cm −1 , with C/B = 5.3. The calculated Mn 4+ energy levels are in good agreement with the experimental spectra that have been presented in the literature. A study that develops the relationship between the energy of the 2 anions. The Mn 4+ with the 3d 3 electronic configuration prefers to occupy an octahedral site due to the attainment of large crystal field stabilization energy when each of the 3d electron occupy singly each t 2g orbital. The Mn 4+ will occupy the Al 3+ site in α-LiAlO 2 due to closeness in the electronic charges and ionic radii; the Shannon and Prewitt ionic radii for Al 3+ = 67 pm, Mn 4+ = 68 pm and Li + = 88 pm, respectively.4 Charge compensation could be attained via cation vacancies. The hexagonal α-LiAlO 2 transformed to the tetragonal (γ) modification beyond 700 C. 3 The latter structure which was fully formed at 900• C does not support Mn 4+ luminescence because in this structure the Al 3+ ions occur in tetrahedral coordination. Although the local symmetry differs from the ideal O h , in what follows we shall use the "cubic" crystal field notation t 2g and e g , and we shall denote by those symbols the groups of levels arising from the t 2g and e g states after they are split by the low symmetry component of crystal field. We shall also keep the cubic crystal field notations for the groups of levels arising from the split orbital triplets and doublets of the Mn 4+ ions.The energy of the Mn 4+ 2 E g → 4 A 2g emission transition in α-LiAlO 2 is not properly identified in the work of Aoyama et al. 3 The purpose of this paper is to identify the energy position of this transition and to calculate the electronic energy levels using the exchange charge model of crystal field theory. The calculated energy levels are compared with the experimental data. The calculations yield the extraction of three principal physical parameters of interest (1) the crystal field splitting (10Dq) and, (2) the Racah parameters, B and C. In this paper, we also develop relationship between the energy of the 2 E g → 4 A 2g emission transition with the connectivity of the octahedral moieties and the deviation of O-Mn-O bond angle in the equatorial plane from the ideal value of 90• . * Electrochemical Society Member. z E-mail: srivastava@ge.com Crystal Structure of α-LiAlO 2The structural modification of LiAlO 2 studied in the present work is described by the R-3m space group (No. 166), 2 with the lattice constants (in Å) a = b = 2.8003, c = 14.216 and three formula units per unit cell. One unit cell of α-LiAlO 2 is depicted in Fig. 1. Both Li and Al ions are six-fold coordinated by the oxygen ions forming trigonally distorted octahedra. The Li-O bond distances are 2.11 Å and the Al-O bond ...

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“…It has been established that Mn 4+ dopants preferably occupy the octahedral sites in most host lattices due to the attainment of large crystal field stabilization energy [27,28] . The α-LiAlO 2 synthesized via sol-gel technique has been reported by Iwamoto et al [29] and Mn 4+ ion exhibits luminescence in the spectral regions around 620-700 nm originating from 2 E g → 4 A 2g transition of Mn 4+ and the associated vibronic bands [25] . The PL spectrum of α-LiAlO 2 :Mn 4+ prepared by SSR (700 •C) exhibits emission peaks around 674 nm (in Figure 3), similar to literature reported elsewhere [25,29] .…”

“…The α-LiAlO 2 synthesized via sol-gel technique has been reported by Iwamoto et al [29] and Mn 4+ ion exhibits luminescence in the spectral regions around 620-700 nm originating from 2 E g → 4 A 2g transition of Mn 4+ and the associated vibronic bands [25] . The PL spectrum of α-LiAlO 2 :Mn 4+ prepared by SSR (700 •C) exhibits emission peaks around 674 nm (in Figure 3), similar to literature reported elsewhere [25,29] . The PLE spectrum is typical for Mn 4+ ions, consisting of two broad bands from the 4 A 2g → 4 T 2g and 4 A 2g → 4 T 1g transitions of Mn 4+ and the charge transfer of Mn 4+ -O 2- [30] .…”

“…With increasing calcination temperatures (above 800 •C), the α phase transforms to the γ phase. No traces of the α phase of It has been reported that the α phase structure of LiAlO 2 consists of both [LiO 6 ] and [AlO 6 ] units and the two units are ordered into alternate planes [25] . Mn 4+ , having a 3d 3 electronic configuration prefer to occupy Al 3+ site with six-fold coordination due to similarity in ionic radius and valence state (in Table 1 [26] ).…”

Spectroscopy of Mn4+in Double Perovskites, La2LiSbO6and La2MgTiO6: Deep Red Photon Generators for Agriculture LEDs

“…24 We examined cases where the octahedral groups share corners (as in the ABO 3 and A 2 B ' B '' O 6 perovskites) or edges and faces as in α-LiAlO 2 , Sr 4 Al 14 O 25 and CaAl 12 O 19 . 24 It was concluded that a highly symmetrical octahedral moiety increases "Mn 4+ -ligand" bonding covalence. The resulting emission spectrum is in the deep red because of the lowering of the 2 E g → 4 A 2g emission transition energy.…”

“…24 We examined cases where the octahedral groups share corners (as in the ABO 3 In this paper, we report the synthesis and spectroscopic properties of the double perovskite La 2 LiSbO 6 activated with Mn 4+ . For comparative purposes, we also evaluate the spectroscopic data of the double perovskite, La 2 MgTiO 6 :Mn 4+ , which has been reported in the literature.…”