Double Perovskite Structure: A Vibrational and Luminescence Investigation Providing a Perspective on Crystal Field Strength Wenyu Li, † Lixin Ning, ‡ and Peter A. Tanner* ,§ † Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong S.A.R., P. R. China ‡ Department of Physics, Anhui Normal University, Wuhu, Anhui 241000, P. R. China § Department of Science and Environmental Studies, Faculty of Liberal Arts and Social Sciences, The Hong Kong Institute of Education, Tai Po, Hong Kong S.A.R., P. R. China ABSTRACT: The luminescence spectra of Eu 3+ doped in a series of double perovskite lattices Ba 2 LnMO 6 (Ln = Y, Gd; M = Nb, Ta) have been recorded at room temperature and 10 K. Together with FT-IR and FT-Raman spectra and aided by DFT vibrational energy calculations, assignments have been made for the crystal field levels of the 5 D J (J = 0,1) and 7 F J (J =0-2) multiplets. The luminescence spectra are consistent with monoclinic symmetry of these systems. The crystal field parameters from the fitting of the energy level data set of Ba 2 YNbO 6 :Eu 3+ enable the crystal field strength to be calculated, and the order of magnitude is Cl - <O 2- <F - for the EuX 6 n- (n = 6 for halogen, 9 for oxide) moieties. For these systems, an empirical linear relationship between crystal field strength and electronegativity of ligand X has been found. By contrast, the nephelauxetic series from the depression of the Slater parameter F 2 is Cl - ≈ O 2- >F - > free ion for these systems. ■ INTRODUCTION The double perovskite structures of Ba 2 Ln(III)M(V)O 6 comprise the large alkaline earth cation Ba 2+ in the 12- coordinated A-site and the Ln 3+ and M 5+ cations located at octahedral sites in the lattice. Figure 1 shows the structure of Ba 2 YNbO 6 plotted from the X-ray data from Fu and IJdo. 1 The NbO 6 and YO 6 octahedra link at corners in a three-dimensional framework. The Goldschmidt tolerance factor, t =(r A + r O )/ [√2(r B + r O )], of simple cubic perovskites ABO 3 is close to 1.0, whereas values between 0.75 and 1.0 are associated with distortion to tetragonal or lower symmetries. For double perovskites, the mean radius of Ln(III) and M(V) replaces r B in the expression for the tolerance factor. The t values for eight Ba 2 Ln(III)M(V)O 6 systems investigated by Lavat and Baran 2 were between 0.91 and 0.99, and they were assumed to be cubic ordered perovskites (space group Fm3̅m, Z = 4) in that 2 and some other studies. 1,3-7 However, lower symmetry space groups have been proposed for these materials from other vibrational and X-ray diffraction studies. This is not unexpected, given the large difference in ionic radii between, for example, Nb 5+ (VI) 0.64 Å and Y 3+ (VI) 0.9 Å. The difficulties in establishing the structures of double perovskites have been discussed by Kennedy et al. 8 The double perovskites have found various important applications, besides phosphors, in materials science. Nanorods of Ba 2 YNbO 6 have been embedded in YBa 2 Cu 3 O 7 films in order to improve flux pinning in magnetic fields. 9 Other applications include a lead-free ultrasonic transducer for microelectronics, solid oxide fuel cells, and microwave tunable systems. 10-12 The band gaps of Ba 2 YTaO 6 and Ba 2 YNbO 6 are 4.6 and 3.8 eV, respectively. 13 The valence band is mainly composed of O(2p) orbitals, whereas the conduction band arises from the interaction between oxygen and transition metal t 2g orbitals for which the overlap increases when replacing a 4d ion with a more electropositive 5d ion. 14 The luminescence of Ba 2 GdNbO 6 doped with Eu 3+ has been reported as a stick diagram by Blasse et al. 4 The local site symmetry of Eu 3+ was assumed to be O h , and the strong self- quenching of emission was attributed to exchange coupling via the NbO 6 moiety since the Eu-Eu separation is quite large (6 Å). An alternative explanation was offered by Qi et al. 15 in their study of the optical spectra of Ba 2 NdNbO 6 . A crystal disorder of ∼1% would be scarcely detectable by X-ray diffraction. In such a case, the separation of some Eu 3+ ions would, however, be much smaller (3.6 Å) when occupying both of the Gd 3+ and Nb 3+ sites in Ba 2 GdNbO 6 , providing a self-quenching pathway. The luminescence of several double perovskites doped with the probe ion Eu 3+ has been investigated in the present study for a dual purpose. First, the interpretation of the low- temperature spectra of the Eu 3+ ion in crystals is well- documented and can provide an indication of the site symmetry (or symmetries) of this impurity ion. This is simple because the electronic ground state ( 7 F 0 ) and the luminescent state ( 5 D 0 ) are both nondegenerate, and the observation of multiple 5 D 0 Received: April 15, 2012 Revised: June 10, 2012 Published: June 17, 2012 Article pubs.acs.org/JPCA © 2012 American Chemical Society 7337 dx.doi.org/10.1021/jp303626v | J. Phys. Chem. A 2012, 116, 7337-7344