Elsevier

Journal of Crystal Growth

Volume 468, 15 June 2017, Pages 387-390
Journal of Crystal Growth

Compositional variations in optical characteristics of Mn doped spinel crystals

https://doi.org/10.1016/j.jcrysgro.2016.10.062Get rights and content

Highlights

  • Crystals were grown with various compositions and O2 concentrations.

  • Colors of Mn doped spinel vary with compositions and O2 concentrations.

  • Absorption and photoluminescence vary with composition and O2 concentrations.

Abstract

Mn doped spinel (MgAl2O4) crystals have been grown by floating zone (FZ) techniques with various conditions of O2 concentrations in the growth atmosphere from 0 to 75 vol%, with a starting composition of molar ratio x=MgO/(MgO+Al2O3) from 0.3 to 1.0 and/or Mn concentrations from 1.0 to 6.0 at%. Optical absorption spectra and photoluminescence spectra were evaluated using crystals grown under various growth conditions. The color of the crystals is found to vary with their composition and the O2 concentration in the atmosphere. Crystals grown under a 100 vol% Ar atmosphere, were a pale green color and emitted a strong green luminescence at λ=520 nm. Colors of stoichiometric crystals, x=1.0, and Mg-poor crystals grown under an oxidizing atmosphere are yellow and red, respectively. Both optical absorption spectra and photoluminescence spectra of Mg-poor, x<1.0 crystals grown under an oxidizing (O2–Ar) atmosphere are found to vary greatly from those of the stoichiometric crystals.

Introduction

Mn doped spinel (MgAl2O4) crystal is a phosphor emitting green colored luminescence [1], [2], [3] peaking at λ=520 nm. This crystal has been reported to be applicable for a green phosphor, a fluorescence thermometer [4], [5] and an X-ray phosphor [6], [7].

Spinel (MgAl2O4) crystal has a wide solid-solution region around the stoichiometric composition [8], x=MgO/(MgO+Al2O3), from 1.7 to 0.2. Composition, x=MgO/(MgO+Al2O3), of spinel crystal varies widely within the solid–solution region. The compositional variation of optical properties have been reported on optical absorption spectra from Cr doped Mg deficient MgAl2O4 crystals by Garapon [9] and Kamin´ska [10]. Compositional and thermal variation of photoluminescence from Cr doped MgAl2O4 have also been reported by Aizawa [11]. Compositional and thermal variation of photoluminescence from Mn doped MgAl2O4 crystal has also been reported by Sakuma [4]. Compositional variations of crystal field strength have been reported to be responsible for the compositional variations of optical absorption and photoluminescence.

In addition to the compositional variations, Mn ions in this crystal can be easily oxidized and/or reduced by oxygen concentration in the growth atmosphere. Effects of the other crystal growth conditions, such as oxygen (O2) concentration in the growth atmosphere and/or Mn concentrations, must, therefore, be studied for growing high quality Mn doped spinel (MgAl2O4) crystals in phosphor applications. In this paper, effects of the starting composition of x=MgO/(MgO+Al2O3) molar ratio, the oxygen (O2) concentration in the growth atmosphere and the Mn concentration on the colors of the crystals, optical absorption spectra and photoluminescence spectra of Mn doped MgAl2O4 crystals, were evaluated using Mn doped spinel (MgAl2O4) crystals grown by FZ technique under various conditions.

Section snippets

Experiments

Mn doped spinel (MgAl2O4) crystals were grown from highly pure 99.99% (4N) Al2O3, (4N) MgO, and (3N) Mn2O3 powders. Powders were mixed, pressed into a rod with 10 mm in diameter and 60 mm in length, and then sintered at 1150 °C for 3 h in air. Crystals were grown in various O2 concentrations, 0, 25 and 50 vol% O2–Ar atmosphere, using a floating zone (FZ) technique (Crystal Systems Co., Japan) [4], [5], [6]. Crystals with a nominal composition of (Mn0.03, Mg0.97)xAl2O3+x were grown with various Mg

Results and discussion

Mn doped spinel (MgAl2O4) crystals have been grown by a floating zone (FZ) technique with various conditions of O2 concentration in the growth atmosphere with a starting composition of x=MgO/(MgO+Al2O3) molar ratio and/or an Mn concentration as shown in Fig. 1. Crystals grown are found to be the spinel (MgAl2O4) structure from X-ray powder diffraction experiments. The lattice parameter is found to increase with composition x of the crystals. These results agree with those in the previous

Conclusion

Mn doped spinel (MgAl2O4) crystals have been grown with various conditions of O2 concentration in the atmosphere from 0 to 75 vol%, starting composition of molar ratio, x=MgO/(MgO+Al2O3), from 0.3 to 1.0 and/or Mn concentrations from 1.0 to 6.0 at%. Crystals grown under 100 vol% Ar atmosphere were a pale green color. The colors of stoichiometric crystals, x=1.0, and Mg-poor crystals grown under an oxidizing atmosphere were yellow and red, respectively. Optical absorption peaks at λ=425, 450, 490,

Acknowledgements

The authors thank the members of the Crystal Engineering Laboratory, Faculty of Science and Engineering, Toyo University, for their help in the crystal growth experiments.

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