Elsevier

Optical Materials

Volume 37, November 2014, Pages 302-305
Optical Materials

Compositional variation of photoluminescence from Mn doped MgAl2O4 spinel

https://doi.org/10.1016/j.optmat.2014.06.014Get rights and content

Highlights

  • 1% Mn doped MgAl2O4 are grown from various compositions in 50% O2–Ar gas.

  • PL peaking at 520 nm and 750 nm are observed from crystals.

  • Peak wavelength of PL increases linearly from 512 to 520 nm with composition.

  • Intensities and thermal activation energy of PL vary with crystal composition.

Abstract

Spinel (MgAl2O4) crystals doped with 1.0% Mn have been grown by floating zone (FZ) technique with various Mg compositions, x = MgO/Al2O3, from 0.2 to 1.0. Compositional variations of photoluminescence are evaluated for a fluorescence thermometer application using crystals grown. Strong photoluminescence (PL) peak is observed at λ from 512 to 520 nm from the crystals grown from compositions, x, from 0.3 to 1.0. Peak wavelength of PL increases linearly from 512 to 520 nm with x. Weak PL peaking at λ = 750 nm is also observed from the specimens. Compositional variations of PL are considered to be due to the variation of crystal field surrounding the Mn2+ ions. The variation of crystal field strength agrees with the compositional variation of lattice constant.

Introduction

Various fluorescence thermo-sensors have been reported in the fiber-optic thermometer using PL lifetime and PL intensity and PL intensity ratio. In the fluorescence thermometers luminescence properties of sensor materials are extremely important [1], [2], [3], [4]. Among many phosphors, spinel (MgAl2O4) crystals doped with Cr3+, Mn2+ and Ti3+, which emit red, green and blue colored PL, respectively, are expected to be useful fluorescence thermo-sensors [4], [5], [6], [7]. Spinel crystal has wide solid-solution region [8] with composition, (x = MgO/Al2O3), from 0.2 to 1.7. Composition, (MgO/Al2O3), of spinel crystal varies widely within the solid-solution region.

Compositional variation of optical properties from Cr doped MgAl2O4 crystals have been reported by Garapon et al. [9] and Kamińska et al. [10] on optical absorption spectra from Cr doped Mg deficient MgAl2O4 crystals. Crystal field strength of octahedral oxygen coordinated Cr3+ site varies with crystal composition. Effects of composition have been reported on the optical absorption and photoluminescence from Cr doped MgAl2O4 crystals based on the compositional variation of crystal field strength [9], [10]. Compositional variation of fluorescence thermo-sensor characteristics of Cr doped MgAl2O4 have also been reported by Aizawa et al. [4]. PL from Mn doped MgAl2O4 crystal is also considered to vary with composition. However, little has been studied on the effects of crystal composition on the optical properties of Mn doped MgAl2O4 crystals [5], [6], [7]. As was reported on Cr doped non-stoichiometric MgAl2O4 crystals [9], [10], compositional variations of optical properties must be observed in Mn doped non-stoichiometric MgAl2O4 crystals. Investigations of compositional variations in PL properties are, therefore, important for optical application of Mn doped MgAl2O4 crystals such as a fluorescence thermometer.

In this paper, Mn doped spinel crystals have been grown from various starting compositions, x = MgO/Al2O3, from 0.2 to 1.0. Effects of crystal composition on PL spectrum, temperature dependence and thermal activation energy of PL are evaluated for the fluorescence thermometer applications.

Section snippets

Experiments

Specimens were prepared from highly pure 99.99% (4 N) Al2O3, (4 N) MgO and (3 N) Mn2O3 powders. Powders were mixed and subsequently sintered at 1150 °C for 3 h in air. Crystals were grown in 50% O2–Ar atmosphere using a floating zone (FZ) technique, (Crystal Systems Co., Japan). Crystals with nominal composition of 1.0% Mn doped MgxAl2O3+x were grown with various Mg compositions, x = MgO/Al2O3, from 0.2 to 1.0, with Mn concentration of 1.0% for evaluating compositional variations of optical

Results and discussion

1.0% Mn doped MgAl2O4 crystals were successfully grown by FZ technique from various compositions, x = MgO/Al2O3, from 0.2 to 1.0. Crystals in pale red color were grown from Mg deficient compositions, x, from 0.2 to 0.9. Crystals in pale yellow color were grown from stoichiometric composition, x = 1.0. Green colored PL is observed from all crystals with UV light (λ = 365 nm) excitation. Variation of a-axis lattice constant of Mn doped spinel crystals grown from various compositions, x, from 0.2 to 1.0

Conclusions

1.0% Mn doped spinel crystal (MgAl2O4) with various compositions x = MgO/Al2O3, were grown using a floating zone technique (FZ). The PL peaking at λ from 512 to 520 nm and 750 nm is observed from the Mn doped spinel crystals. Lattice constant, PL peak wavelength, peak intensity, temperature dependences of peak intensity and peak intensity ratio are found to vary with composition of Mn doped MgAl2O4 crystals. Compositional variations of PL are considered to be due to the variation of crystal field

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