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Luminescence characteristics of undoped and Eu-doped GdCOB single crystals and nanopowders

Piotr Solarz ¹, Martin Nikl ², Andrzej Kłos ³, Radosław Lisiecki ², Witold Ryba-Romanowski ¹, Agnieszka Rzepka ³, Anna Pajaczkowska ³

1. Polish Academy of Sciences, Institute of Low Temperature and Structure Research (INTiBS), Okólna 2, Wrocław 50-422, Poland
2. Czech Academy of Sciences, Institute of Physics, Cukrovarnicka 10, Prague 16253, Czech Republic
3. Institute of Electronic Materials Technology (ITME), Wólczyńska 133, Warszawa 01-919, Poland

Abstract

Single crystals GdCa₄O(BO₃)₃ (GdCOB) pure and doped Eu with concentration of 1 at% and 4 at% were grown by Czochralski method. The distribution of Eu ions in GdCOB crystals was uniform. The substitution of Eu³⁺ in Gd, Ca(1) and Ca(2) cation sites and eventually formation Eu²⁺ have been investigated. The spectroscopic properties of crystals are compared with properties of nanopowders obtained by sol-gel method.

The excitation spectra recorded in the 125 - 330 nm spectral region consist of two broad bands with maxima at 182 and 250 nm and considerable less intense typical gadolinium narrow lines at about 312 nm. The broad bands can be assigned as Eu³⁺ - O^2- (250 nm) and Gd³⁺ - O^2- (182 nm). The origin of the Eu³⁺ - O^2- band was confirmed by absorption spectrum of undoped GdCOB crystal.

Radioluminescence spectra of undoped GdCOB crystal show the characteristic emission of Gd at about 312 nm, whereas this emission dramatically decreases in Eu-doped crystals upon X-ray excitation, as well as in Eu-doped nanopowders excited in VUV region. The VUV excitation in the range from 125 to 333 nm (Eu-doped samples) leads to strong emission in red coming from the ⁵D₀ multiplet of Eu³⁺, only. Even trace concentration of Eu²⁺ seems to be excluded due to complete absence of any emission within 360-500 nm [1]. It is interesting to note that absolute radioluminescence intensity in GdCOB:Eu 4at% crystal is somewhat weaker than in GdCOB:Eu 1 at% one. When the crystals were excited at 275 nm the emission of GdCOB:Eu 4 at% is stronger than GdCOB:Eu1at% one (in opposite to X-ray excitation). Probably, GdCOB:Eu 4 at% crystal is of lower quality, which can induce higher nonradiative losses in radioluminescence processes, in which the energy of incoming X-rays is absorbed in the GdCOB host. In the photoluminescence decay kinetics of 312 nm emisssion it is clearly observed substantial shortening and departure for single exponential decay in Eu-doped samples. Higher Eu doping results in further acceleration of the decay. In undoped GdCOB crystal, the lifetime of ⁶P_7/2 is 3 ms. The ⁵D₀ decay kinetic monitored at 613 nm is rather constant. Numerical fitting, of fully exponential curves, returns 2.7 ms for nanopowder and 2.5 ms for single crystal. The presented data suggest this material maybe used as a phosphor in plasma display panels.

This work was supported by Ministry of Education and Science under the research project no. 3T11B00430.

[1] P. Dorenbos, J. Phys. Cond. Matter 15 (2003) 575

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