Lanthanide level location in transition metal complex compounds P. Dorenbos a, * , A.H. Krumpel a , E. van der Kolk a , P. Boutinaud b , M. Bettinelli c , E. Cavalli d a Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, 2629 JB Delft, Netherlands b Labaratoire des Materiaux Inorganiques, UMR 6002, Universite Blaise-Pascal et ENSCCF, Aubiere, France c Laboratory of Solid State Chemistry, Universita di Verona, INSTN, UdR Verona, Verona, Italy d Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Universita di Parma, Parma, Italy article info Article history: Received 22 December 2009 Accepted 9 February 2010 Available online 3 March 2010 Keywords: Transition metal compounds Charge transfer Lanthanide levels abstract We will provide a method to place the levels of all trivalent lanthanides with respect to the top of the valence band and bottom of the conduction band in oxides containing transition metal complexes. The method will be applied to CaTiO 3 , YVO 4 , LaVO 4 , CaNb 2 O 6 , YNbO 4 , CaWO 4 , YTaO 4 , and LaTaO 4 , but in prin- ciple can be applied to any oxide containing transition metal complexes with lanthanide dopants on either rare earth or alkaline earth sites. Crucial to place the energy levels is the energy for intervalence charge transfer between a lanthanide (Pr 3þ and Tb 3þ ) and a transition metal ion (Ti 4þ ; V 5þ ; Nb 5þ ; Mo 6þ ; Ta 5þ ; W 6þ ) that can be observed in luminescence excitation spectra. The quenching of Pr 3þ emission from the 3 P 0 state and of Tb 3þ emission from the 5 D 3 and 5 D 4 states provides complementary information. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Spectroscopic properties of lanthanide (Ln) activated inorganic compounds depend not only on the energies of the excited 4f and 5d states but also on the location of those states relative to the bands of the host crystal. We can use the familiar Dieke dia- gram to obtain the energies of the excited 4f n states, and the en- ergy E fd ðLnÞ of the lowest 4f n1 5d excited state is well described by the concept of red shift E fd ðLnÞ¼ E fd ðfreeÞ DðAÞ; ð1Þ where E fd ðfreeÞ is for each lanthanide a constant and DðAÞ is the red shift belonging to compound A [1]. Values for E fd ðfreeÞ for divalent and trivalent lanthanides were presented in [2]. Our problem is how to determine the ground state energy of the lanthanides relative to the top of the valence band (VB). A conve- nient method is to measure, by photon excitation, the energy needed to transfer an electron from the valence band to a trivalent lanthanide ion. It provides us with the location of the ground state energy of the corresponding divalent lanthanide [2]. It turns out that the energy of this charge transfer (CT) changes systematically, and therefore predictably, with type of lanthanide ion, i.e. the number n of electrons in the 4f shell. We only need to know the en- ergy of the CT band for Eu 3þ and a list of energy differences DE Vf ðLn 2þ Þ between Eu 2þ and all other Ln 2þ in order to place the divalent ground states for all lanthanides with respect to the top of the valence band. The compilation of DE Vf values in [2] was slightly revised in [3]. To obtain a similar set of parameters for trivalent lanthanides appears less trivial. The first set of DE Vf ðLn 3þ Þ parameters proposed in [2] was in essence an educated guess from the better established DE Vf ðLn 2þ Þ values. A revised set of DE Vf ðLn 3þ Þ parameters was pro- posed in [3], however, direct experimental evidence on Ln 3þ ground state location for dilute lanthanide activated compounds was still not available then. Recently Boutinaud and co-workers conducted an extensive study on Pr 3þ and Tb 3þ luminescence in transition metal complex compounds, see e.g. [4–8]. They particularly studied the so-called intervalence charge transfer (IVCT), i.e. the transfer of an electron from the ground 4f state of Pr 3þ or Tb 3þ to a transition metal ion. Although the phenomenon was studied before (Reut and Ryskin [9] called the transition virtual charge exchange and Blasse and Bril [10] called it metal to metal charge transfer (MMCT)), a systematic analysis comprising many transition metal compounds was, how- ever, new. The energy E IVCT of the IVCT band somehow should reflect the location of the Pr 3þ or Tb 3þ ground state with respect to the con- duction band (CB). Note the similarity with the energy of charge transfer from the valence band to a trivalent lanthanide that lo- cates the ground state of the divalent lanthanide with respect to the valence band. With the IVCT bands, for the first time we have a spectroscopic method to obtain information on Ln 3þ ground state energy location. In this work our objectives are two-fold. With the IVCT bands of Pr and Tb in transition metal compounds an im- proved DE Vf parameter set for the trivalent lanthanides is obtained. 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.02.021 * Corresponding author. Tel.: +31 15781336; fax: +31 15786422. E-mail address: p.dorenbos@tudelft.nl (P. Dorenbos). Optical Materials 32 (2010) 1681–1685 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat