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Inorg. Chem. 2001, 40, 196-207
Articles Ni(II)-Doped CsCdBrCl2: Variation of Spectral and Structural Properties via Mixed-Halide Coordination S. R. Lu1 thi* and M. J. Riley Department of Chemistry, University of Queensland, St. Lucia, QLD 4072, Australia ReceiVed August 8, 2000
The mixed-halide compound CsCdBrCl2 is studied by X-ray diffraction and by using the Ni(II) ion as an optical probe. Low-temperature absorption, luminescence, and EXFAS spectra of the Ni(II) impurity are recorded. The structure of CsCdBrCl2 is shown to consist of corner-sharing [Cd3X12]6- trimers. Each trimer has a structure of three face-sharing octahedra Cl3CdX3CdX3CdCl3 where X has an equal probability of being a Br- or a Cl- ion. This equal occupancy on the bridging halide positions occurs over the whole crystal rather than within each [Cd3X12]6- trimer unit. Optical spectroscopy shows that the Ni(II) ion exists in all possible [NiBrxCl6-x]4- isomeric forms (x ) 0, 1, ..., 6). The energy of the 3A2g f 1A1g transition is a linear function of x, due to the change in inter-electron repulsion through the differing covalencies of the ligand compositions. The energy of this transition can be varied over 2750 cm-1. The inhomogeneous broadening that results from the halide disorder is discussed from the point of view of the variation of ligand-field strength and inter-electron repulsion. A model including a differential nephelauxetic effect is required to explain the energies of the ligand-field states.
Introduction Over recent years, Ni(II)-doped materials have received considerable attention in optical spectroscopic research because of their ability to show multiple luminescence transitions.1 There is a major interest in Ni(II) as an active center in the development of tunable lasers in the near-infrared spectral range, at about 1.5 µm. This wavelength has important applications in measuring techniques and optical data transmission in glass fibers.2,3 A broad-band emission around 1.5 µm stems from the 3T 3 2g f A2g transition from the first excited to the electronic ground state of the Ni(II) ion in octahedral coordination. The energy of this transition is sensitive to the strength of the ligand field4,5 and thus can be varied over a wide range by changing the host lattice.2,4 Studies on the influence of chemical variation on the spectroscopic properties of a material also provide insight into the mechanisms that control properties such as the radiative and nonradiative relaxation rates. This insight allows the spectroscopic properties to be improved for optical devices. To date, the main disadvantage of Ni(II)-doped compounds for use as tunable near-infrared laser sources has been the thermal quenching of the 3T2g f 3A2g luminescence at higher temper(1) Iverson, M. V.; Sibley, W. A. J. Lumiun. 1979, 20, 311-324. (2) Koetke, J. Ph.D. Thesis, University of Hamburg. (3) Alcala, R.; Gonzalez, J. C.; Villacampa, B.; Alonso, P. J. J. Lumin. 1991, 48/49, 569-573. (4) Liehr A. D.; Ballhausen, C. J. Ann. Phys. 1959, 6, 134-155. (5) De Viry, D.; Tercier, N.; Denis, J. P.; Blanzat, B.; Pelle´, F. J Chem. Phys. 1992, 97, 2263-2270. (6) Elejalde, M. J.; Balda, R.; Fernandez, J. J. Phys. IV 1994, C4, 411414. (7) Moulton P. F.; Mooradian, A. Springer Ser. Opt. Sci. 1979, 21, 584589.
atures.2,6 Lasing has been restricted to temperatures below -30 and 0 °C for continuous-wave and pulsed operations, respectively.2,7,8 Interest in the spectroscopic properties of Ni(II), however, is not restricted to the 3T2g f 3A2g luminescent transition. The Ni(II) ion is one of the rare transition-metal ions that show emission from higher excited states when doped into different inorganic host materials.1,5,9-11 Depending on the ligand-field strength of the chosen host lattice, the 1T2g f 3A2g, 1T2g f 3T , and 1T 3 2g 2g f Tlg luminescent transitions cover the green to near-infrared spectral range. This higher excited-state luminescence is of interest not only for the application of Ni(II)doped compounds as phosphors and laser materials but also for the application of transition-metal/rare-earth-metal codoped systems with Ni(II) as a sensitizer for enhanced rare-earth-metal emission.12 The energies of the Ni(II) excited states leads to a competition of different radiative and nonradiative processes. Oetliker et al.13 have shown that efficient cross-relaxation and multiion energytransfer processes in Ni(II)-doped CsCdCl3 can lead to nonlinear optical behavior called an “excitation avalanche”. This phenomenon results in up-conversion in energy and has been observed in a number of rare-earth-metal-doped compounds (8) Moulton, P. F. In Laser Handbook, 5th ed.; Bass, Stitch, Eds.; NorthHolland Publishing Co.: Amsterdam, 1979; pp 257-266. (9) May, P. S.; Gu¨del, H. U. Chem. Phys. Lett. 1989, 164, 612-616. (10) May, P. S.; Gu¨del, H. U. J Lumin. 1990, 46, 277-290. (11) May, P. S.; Gu¨del, H. U. J. Chem. Phys. 1991, 95, 6343-6354. (12) Lu¨thi, S. R.; Riley, M. J. Unpublished results. (13) Oetliker, U.; Riley, M. J.; May, P. S.; Gu¨del, H. U. J. Lumin. 1992, 53, 553-556.
10.1021/ic000899l CCC: $20.00 © 2001 American Chemical Society Published on Web 12/15/2000
Ni(II)-Doped CsCdBrCl2 (Pr3+:LaCl3, Pr3+:LaBr3, Sm3+:LaBr3, Nd3+:YLiF4),14 but Ni(II):CsCdCl3 is the only species providing an example of an excitation avalanche in a transition-metal system. The study and optimization of these processes could lead to the development of Ni(II)-based materials with nonlinear optical properties for applications in optical devices. In this work, we demonstrate the influence of chemical variation on the spectroscopic properties of Ni(II) on a finer scale than is normally possible. The synthesis of the novel mixed-halide compound CsCdBrCl2 offers the possibility to study the effect of partial exchange of halide counterions on the energy levels of the Ni(II) d-d excitations. The observed spectral properties are compared with those of the structurally related Ni(II)-doped CsCdBr3 and CsCdCl3 compounds and are simulated with calculations. Chemical substitution of one-third of the chloride ligands with bromide in CsCdBrCl2 does not result in a simple additive effect expected for chemical variation along the halide series. The structural changes have a strong impact on the spectroscopic properties. We can distinguish various effects of chemical substitution on the basis of interelectron repulsions, ligand-field strengths, bond lengths, and Ni(II) site symmetries all within the single Ni(II):CsCdBrCl2 system. Only the study of Ni(II) in glasses would offer a comparable or finer grid for varying the chemical environment; however, in glasses, the advantage of Ni(II) in a well-defined host is lost. Experimental Section Synthesis. The synthesis, crystal growth, and preparation of samples for spectroscopic measurements of the ternary metal halides CsCdX3 (X ) Cl, Br) are dictated by the hygroscopic nature of the compounds and their binary starting materials CsX and CdX2. Under ambient conditions, the binary and ternary halides react with water to form hydrated halides, resulting in crystal decomposition. For this reason, the preparations of the pure CsCdX3 and the mixed CsCdBrCl2 were carried out in a dry nitrogen atmosphere and special purification procedures for the starting materials were applied. The ternary metal halides CsCdBrCl2, CsCdBr3 and CsCdCl3 were synthesized from a congruently melting phase which forms from the reaction of the binary halides CsX and CdX2. Water-free binary halides of high purity are the key to high-quality CsCdX3 crystals for spectroscopic applications. The commercially available starting materials CsX, CdX2, and NiX2 of >99.9% purity were dried under vacuum at 250 °C for 3 h and then sublimed to remove traces of hydrocarbons and other impurities. During crystal growth, hydrocarbons decompose to form carbon particles that have been shown to adversely affect the crystal quality.15 For sublimation, the powdered starting materials were transferred into a quartz glass ampule, which was evacuated to
