Charge transfer transitions in the transition metal oxides ABO₄:Ln³⁺ and APO₄:ln³⁺ (A=La, Gd, Y, Lu, Sc; B=V, Nb, Ta; Ln=lanthanide)

Abstract

We have compiled and analyzed optical and structural properties of lanthanide doped non-metal oxides of the form APO₄:Ln³⁺ with A a rare earth and of transition metal oxides with formula ABO₄:Ln³⁺ with B a transition metal. The main objective is to understand better the interrelationships between the band gap energy, the O²⁻→Ln³⁺ charge transfer energy, and the Ln³⁺→B⁵⁺ inter-valence charge transfer energy. Various models exist for each of these three types of electron transitions in inorganic compounds that appear highly related to each other. When properly interpreted, these optically excited transitions provide the locations of the lanthanide electron donating and electron accepting states relative to the conduction band and the valence band of the hosting compound. These locations in turn determine the luminescent properties and charge carrier trapping properties of that host. Hence, understanding the relationship between the different types of charge transfer processes and its implication for lanthanide level location in the band gap is of technological interest.

Introduction

Many lanthanide (Ln) doped transition metal (d-block) oxides of the form ABO₄:Ln³⁺ (A=La, Gd, Y, Lu; B=V, Nb, Ta; Ln=La, Ce, …, Lu) are well known for many different applications as laser host crystals [1], [2], solar cells [3] or phosphor materials [4]. Their luminescence quantum efficiency depends on the location of the 4f and 5d energy levels of the Ln dopants relative to the valence band (VB) and the conduction band (CB) of the host. The energies of charge transfer (CT) that can be identified in photoluminescence (PL) excitation spectra can be used in order to establish those locations. In the ABO₄ transition metal oxides, a CT can be observed from the Ln dopant to a host ion or between the host ions themselves. Fig. 1 exemplifies the three different types of CT that are of main interest throughout this paper: (i) fundamental host transition due to electron transfer from the oxygen VB to the CB which defines the band gap; (ii) inter-valence charge (electron) transfer (IVCT) from a Ln³⁺ ion to the CB leaving Ln⁴⁺; and (iii) electron transfer from an O²⁻ ion to the incompletely filled 4f shell of a Ln³⁺ dopant creating Ln²⁺.

At the beginning of this article, in Section 2, an overview of the different methods of locating Ln 4f energy levels relative to the host bands by means of observed CT energies will be given. In particular, the method of Boutinaud based on the IVCT energies and the method of Dorenbos based on CT energies will be discussed. One main objective is comparing the different approaches with each other. Another objective is finding host-related parameters which help to predict the absolute location of the 4f energy levels in any compound. Section 3 tabulates first the crystallographic and electronic properties of the group 5 transition metal oxides ABO₄:Ln³⁺ (B=V, Nb, Ta) and the non-metal oxides APO₄:Ln³⁺. Next, the observed Eu³⁺ CT and the Pr³⁺ IVCT energies in these compounds are presented and discussed. It will be shown that both the O²⁻→Eu³⁺ CT and the Pr³⁺→B⁵⁺ IVCT energies for one specific type or compound like the orthovanadates, AVO₄:Ln³⁺ depend on the electronegativities as well as on the inter-atomic distances of the ions that are involved in the CT process. An expression for the dependence of the Pr³⁺→B⁵⁺ IVCT energy on these two parameters for different transition metal oxides has been found earlier by Boutinaud et al. [8]. His model and our attempt in this work to find a related expression for the O²⁻→Eu³⁺ CT energies are an extension of the optical electronegativity model introduced by Jørgensen [9] four decades ago. The photoluminescence (PL) data that are used in this work were obtained by own measurements or from the literature. The oxygen 2p states form mainly the top of the VB and the d states of the B⁵⁺ ions form the bottom of the CB [5], [6], [7].