Research ArticleLuminescence of Mn4+ in the orthorhombic perovskites, AZrO3 (A=Ca, Sr)
Abstract
The spectroscopic properties of the Mn4+ (3d3) ion in the orthorhombic perovskites ABO3 (A = Ca, Sr; B
Zr4+) is the subject of this investigation. The optical properties of the Mn4+ ion in CaZrO3 is sensitive to oxygen stoichiometry. Comparison with the reported optical properties of oxygen-stoichiometric CaZrO3:Mn4+ obtained by synthesizing in oxygen atmosphere indicates that the air sintered sample represents Mn4+ luminescence in an oxygen deficient perovskite lattice. The luminescence of SrZrO3:Mn4+ suggests that this perovskite is less prone to the formation of oxygen vacancies. The difference between the energies of the zero-phonon line (2E→4A2 transition; R-line) in CaZrO3 and SrZrO3 is related to the deviation of the octahedral O–Mn–O bond angle from the ideal 90° value. Comparison with isostructural perovskites ASnO3 (A = Ca, Sr) shows that the energy of the 2E level depends on the covalent character of B–O bonding. The stronger covalence of Zr4+ 4d0- O2− 2p bonding compared to that of Sn4+ 4d10 - O2− 2p bonding is responsible for the higher energy position of the Mn4+ 2E level in the zirconate perovskite. Mulliken population analysis and bond order calculations are made to understand the nature of chemical bonding in the perovskites ABO3 (A = Ca, Sr; B= Zr4+, Sn4+).
Introduction
Information on the factors that control the luminescence of Mn4+ (3d3) in solids can be obtained by systematically studying the spectroscopic properties of the ion in an isostructural series of compounds with varying chemical composition. In the spirit of this approach, we have examined the spectroscopic properties of Mn4+ in a series of orthorhombic perovskite ABO3 compounds which crystallize with the Pbnm space group [[1], [2], [3], [4]]. The structure consists of corner sharing BO6/2 octahedra with the A cations in an eight-fold coordinated of oxygen anions. The Mn4+ ion substitute at the B-site of the perovskite structure because of the strong preference of d3 ions for octahedral coordination.
As a part of these investigations, in this article we report on the synthesis and optical properties of the Mn4+ ion in the zirconate perovskites AZrO3 (A = Ca, Sr) while providing a comparative study of the optical properties with those reported in other orthorhombic perovskite compounds. Specifically, attention is focused on the factors responsible for the energy of the 2E level in the perovskites AZrO3 (A = Ca, Sr). According to the Tanabe-Sugano diagram for d3 ions, the R-line energy is independent of 10Dq and dependent on the covalence of “Mn4+-Ligand” bonding [[5], [6], [7]]. A systematic examination of the Mn4+optical properties in 10 oxo-perovskite compounds showed that the deviation from 90° of the O–Mn–O angle (Δθ), which provides a measure of the octahedral distortion, is correlated with the R-line energy [3,4]. Increasing Δθ increases the R-line energy by decreasing the covalence of Mn–O bonding. The energies of the 2E level of Mn4+ in CaZrO3 and SrZrO3 follow this trend. However, argument based on the angular distortion of the octahedron (Δθ) fails when comparing the energy of the 2E level in perovskites ASnO3 and AZrO3. We show that this can be rationalized based on the covalent character of B–O bonding. In the current study we also calculate the Mulliken charge and bond population values to correlate the energies of 2E level to chemical bonding effects in these perovskites. We also show that the atmosphere employed in the solid-state synthesis process has a profound influence on the luminescence of CaZrO3:Mn4+.
Section snippets
Experimental
The syntheses of perovskites AZr0.99Mn0.01O3 (A = Ca, Sr) was accomplished by blending and air sintering the required amounts of high purity starting materials, CaCO3, SrCO3, ZrO2 and MnCO3 at 1000 °C for a period of 10 h in covered alumina crucibles. The resulting powders were homogenized and fired at 1400 °C for a period of 10 h. The X-ray diffraction pattern indicated the formation of pure phases.
Luminescence measurements at low temperatures were accomplished as described previously [8].
Luminescence of AZrO3:Mn4+(A = Ca, Sr)
The luminescence of CaZrO3:0.1% Mn4+ have been reported by Blasse and Kort [9]. The electronic energy levels of Mn4+ in CaZrO3 that are important for the interpretation of the spectra have been calculated by Brik and Srivastava using the exchange charge model of the crystal field [10]. Fig. 1, Fig. 2 compares the excitation and emission spectra of Mn4+ in our and their CaZrO3 samples. It should be pointed out that the spectra of reference [9] were acquired at 77 K whereas our spectra were
Conclusions
This work presents a comparative study of the optical properties of Mn4+ in orthorhombic perovskites AZrO3 (A = Ca, Sr). The CaZrO3 phase obtained by high-temperature solid-state synthesis is oxygen deficient. The luminescence is attributed to Mn4+ ions associated with oxygen vacancies. The study provides further evidence for the increase in the energy of R-line with increasing deviation of the octahedral bond angle from the ideal 90° value. However, this relationship can be altered by chemical
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.
Acknowledgement
M.G. Brik thanks the supports from the Chongqing Recruitment Program for 100 Overseas Innovative Talents (Grant No. 2015013), the Program for the Foreign Experts (Grant No. W2017011) and Wenfeng High-end Talents Project (Grant No. W2016-01) offered by Chongqing University of Posts and Telecommunications (CQUPT), Estonian Research Council grant PUT PRG111, European Regional Development Fund (TK141) and NCN project 2018/31/B/ST4/00924. Dr G. A. Kumar, University of Texas, San Antonio, is thanked
References (17)
- et al.
J. Lumin.
(2017) - et al.
Opt. Mater.
(2017) - et al.
J. Lumin.
(2019) - et al.
J. Lumin.
(2013) - et al.
J. Inorg. Nucl. Chem.
(1981) - et al.
Solid State Ionics
(1999) - et al.
J. Alloys Compd.
(2013) - et al.
Phys. B Condens. Matter
(2018)