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Highlights: [•] The energy level of Mn4+ in LaAlO3 is calculated using the exchange charge model of crystal field theory. [•] The calculated energy levels match the experimental optical data. [•] A comparative study pertaining to variations in crystal field splitting and Racah parameter (B) is presented. [•] A dimensionless parameter is introduced to quantify the nephelauxetic effect. [Copyright &y& Elsevier]

Abstract: The electronic energy levels of the six-coordinated Cr4+, Cr3+, and Mn4+ ions in the pyrochlore, Y2Sn2O7, were calculated using the exchange charge model of crystal field theory. For these transition metal ions, the calculated energy levels and their trigonal splitting are in good agreement with experimental data. The calculations of the crystal field parameters show that the increased crystal field strength is related to the increased values of the moments of the 3d electron density in the sequence Mn4+ →Cr4+ →Cr3+. The nephelauxetic effect (degree of covalency) in Y2Sn2O7 was deduced by comparing the energies of the first spin-forbidden transitions of the free Cr4+, Cr3+, and Mn4+ ions with the experimentally determined values and with the estimation of the Racah parameter ratio in the crystal and in the free state. It was observed that the covalency increases in the sequence Mn4+ →Cr4+ →Cr3+, which is not easy to understand. [Copyright &y& Elsevier]

Abstract: This paper offers a study that compares the nature of Pr3+ luminescence in crystalline and glass modifications of LaB3O6, LiLaP4O12 and SrB4O7. In the crystalline derivative of these materials, the crystal-field split Pr3+ 4f15d1 state is higher in energy than the 1S0 state. The resulting energy level structure permits the observation of the Pr3+ 1S0 → 1I6 emission transition and the production of two photons for every absorbed UV photon, a process which is referred to as Photon Cascade Emission. In the corresponding glass derivatives, the Pr3+ 4f15d1 lies below the 1S0 state with the consequence that the emission is dominated by the interconfigurational (broad-band) Pr3+ 4f15d1 →4f2 transition. The reason for this is traced to increased strength of the crystalline field at the Pr3+ site in the glass phase. The increased crystal-field strength is traceable to decreased local coordination number and to shorter Pr3+–O2− bond distances. In this paper we also analyze the results of the Pr3+ ion luminescence in fluoride, oxyfluoride, borate and phosphate glasses. With the exception of one pure fluoride glass (ZBLAN), the Pr3+ luminescence in all these glasses is dominated by the interconfigurational Pr3+ 4f15d1 →4f2 emission transition. It is pointed out that under broad-band excitation, emission from the Pr3+ 1S0 state has yet to be observed in an oxide based glass (phosphates and borates). [Copyright &y& Elsevier]

Abstract: A systematic study of the excitation spectrum of TbX3 (X=Cl−, Br−, I−) is presented in this work. In general, the excitation spectra of TbX3 can be divided into three major regions: (1) the short-wave host lattice absorption region, (2) the intermediate absorption region where the Tb3+ 4f8 →4f75d1 interconfigurational excitation transition are located, and (3) the long-wave excitation region where the Tb3+ 4f8 →4f8 intraconfigurational excitation transition are located. The high spin and the low spin components of the Tb3+ interconfigurational excitation transition are clearly identified in the case of TbCl3. The luminescence of TbX3 (X=Cl−, Br−, I−) is dominated by emission transitions emanating from the Tb3+ 5D4 state. A comparative study of the optical properties of TbX3 (X=Cl−, Br−, I−) with the properties of the Tb3+ ion in several halide host lattices is presented. Further, a comparative study of the fundamental host lattice optical transitions in terbium halides and other halide materials is also presented. [Copyright &y& Elsevier]

Abstract: The electronic energy levels of the six-coordinated Mn4+ ion in the pyrochlores Y2B2O7 (B =Sn4+, Ti4+) have been computed using the exchange charge model of crystal field theory. The calculated Mn4+ energy levels and their trigonal splitting are in good agreement with the experimental spectra. The calculated crystal field parameters show that the higher crystal field strength in Y2Sn2O7 arises from an increased orbital overlap effect between the Mn4+ ion and the nearest oxygen ions, which are located at the 48f crystallographic position of the pyrochlore lattice. This increased overlap in Y2Sn2O7 occurs despite the fact that the Mn4+–O2− bond distance in Y2Sn2O7 is longer than in Y2Ti2O7 and is attributed to a lack of hybridization (covalent bonding) between the filled 2p orbital of oxygen ion occupying the 48f site of the pyrochlore lattice and the filled Sn4+ 4d 10 orbital. The low temperature emission spectrum of Mn4+ activated Y2Sn2O7 is analyzed in terms of a weak zero phonon line (R-line) with accompanying vibrational side bands. [Copyright &y& Elsevier]

The Ruddlesden-Popper (RP) phases of SrLaAlO 4 and Sr 2 TiO 4 crystallize in the tetragonal A 2 BX 4 structure that consists of an intergrowth of rocksalt (AX) and perovskite (ABX 3) layers. In this article, the spectroscopic properties of Mn4+ embedded in the perovskite layer of SrLaAlO 4 and Sr 2 TiO 4 are compared with those in the perovskites, LaAlO 3 and SrTiO 3. For both RP phases, we find that energy of the zero-phonon line (ZPL; R-line) is essentially the same as in the corresponding perovskites. Despite the presence of inversion symmetry about the Mn4+ ion, the R-line is intense in the RP compounds while they appear with weak intensity in the emission spectrum of the corresponding perovskites. In SrLaAlO 4 this is attributed to the removal of inversion symmetry by Al3+ vacancy and to the random distribution of Sr2+ and La3+ cations. In Sr 2 TiO 4 , the removal of inversion symmetry is hypothesized to occur via the off-center displacement of the Mn4+ ion. • Luminescence of Mn4+ in Ruddlesden-Popper (RP) phases of SrLaAlO 4 and Sr 2 TiO 4 are discussed. • Comparison with corresponding luminescence in the perovskites, LaAlO 3 and SrTiO 3 , is provided. • The Mn4+2E→4A 2 (R-line) emission energy is similar in RP and perovskite phases. • The reasons why R-line is intense in the RP compounds and weak in the perovskites is explored. [ABSTRACT FROM AUTHOR]