Temperature dependent luminescence Cr3+-doped GdAl3(BO3)4 and YAl3(BO3)4

doi:10.1016/j.jlumin.2015.10.042

Journal of Luminescence

Volume 171, March 2016, Pages 246-253

https://doi.org/10.1016/j.jlumin.2015.10.042 Get rights and content

Abstract

Chromium activated YAl₃(BO₃)₄ (YAB) and GdAl₃(BO₃)₄ (GAB) were synthesized and show efficient broad band near infrared emission under excitation in the UV, blue and orange spectral regions. Temperature dependent luminescence measurements for GAB:1%Cr³⁺ and YAB:1%Cr³⁺ reveal high quenching temperatures for the broad band Cr³⁺ emission, 620 K for YAB:Cr and 650 K for GAB:Cr. To gain insight in the luminescence the crystal field strength, Racah parameters, nephelauxetic effect and phonon coupling parameters have been investigated by using spectroscopic data. The crystal field splitting for Cr³⁺ is similar in the two borates (around 1670 cm⁻¹). However, the calculated B Racah parameters indicate a higher global covalency in GAB. The electron phonon-coupling parameter (S) and the effective phonon energy (ħω) were determined to be 5.9 and 263 cm⁻¹ for YAB and 5.4 and 309 cm⁻¹ for GAB, respectively. The larger electron–phonon coupling strength in YAB:Cr is in agreement with the lower quenching temperature in comparison with GAB:Cr.

Introduction

The transition metal ion Cr³⁺ is widely applied in solid state laser gain materials, as a co-dopant for persistent phosphors emitting in the NIR, and as the optical centre in thermographic phosphors for optical sensing [1], [2], [3], [4], [5], [6]. Moreover, Cr³⁺ doped luminophores are also considered as near infrared (NIR) emitting materials for in-vivo optical imaging due to the rather high penetration depth of NIR radiation into human tissue [7], [8]. Just as in the field of lighting, the availability of efficient and high power blue LEDs enables the manufacturing of energy efficient and high power NIR sources by pumping an NIR phosphor with by a blue or near ultraviolet (NUV) emitting (In,Ga)N diode. Spectral conversion from blue/NUV to NIR involves a significant energy dissipation, more than for blue/NUV to visible conversion. This means that the radiation converter should not suffer from thermal quenching up to about 450 K, a temperature that may even exceeded if the converter is in intimate contact to the die [9], [10].

The borate hosts YAl₃(BO₃)₄ and GdAl₃(BO₃)₄ are known for their good chemical and thermal stability [11], [12]. Moreover, Cr³⁺ doped YAB and GAB show efficient NIR photoluminescence upon excitation by NUV and blue light [13]. Here we will investigate the optical properties of both Cr-doped borates and evaluate their potential as NIR emitter for application in high power broad band NIR sources. To this end, both low temperature studies (down to 4 K) are performed to provide understanding of the energy level structure underlying the optical properties as well as high temperature measurements to study the thermal quenching and the temperature dependence of the photoluminescence spectra of YAB:Cr and GAB:Cr up to 800 K.

Section snippets

Synthesis

YAl₃(BO₃)₄ and GdAl₃(BO₃)₄ doped with Cr³⁺ ions were synthesized by conventional solid state reaction in air atmosphere. As starting materials Al₂O₃ (VWR-Prolabo, 99.99%), Cr₂O₃ (Alfa Aesar, 99.95%), Y₂O₃ (Treibacher, 99.95%), Gd₂O₃ (Treibacher, 99.90%), H₃BO₃ (Merck, 99.95%), were used. An excess of H₃BO₃ was added in order to compensate its evaporation during the calcination process. The starting materials were mixed and ground in an agate mortar. Then, the powders were pre-calcinated at 500

Description of the crystal structure of the GAB and YAB host materials

The crystal type RX(BO₃)₄, where R=Y³⁺ or Gd³⁺ and X=Al³⁺ belongs to a group of double borates which have a huntite [CaMg₃(CO₃)₄] structure with trigonal crystal system in the R32 space group (Fig. 1)[14], [15].

In this system, Y³⁺ and Gd³⁺ ions occupy nearly regular trigonal prismatic sites. Al³⁺ ions reside in distorted octahedral sites and boron is arranged in sheets of BO₃³⁻ triangles. Gd³⁺ (Y³⁺) ions are separated from each other by the borate groups and do not share the same oxygen ion.

Temperature dependent luminescence

The values of Dq/B for YAB and GAB are 2.40 and 2.47, respectively. YAB:Cr³⁺ and GAB:Cr³⁺ show both narrow and broad band emission at room temperature (Fig. 4) and therefore we classify the situation for these materials as intermediate crystal field strengths (Dq/B values are near the crossover point~2.2) [28]. In such a system, the energy between the two excited states ²E_g and ⁴T_2g is small and the long lived ²E_g state can be considered as an energy level that acts as a reservoir for the

Electron–phonon coupling parameters

In order to study the interaction of the ⁴T₂–⁴A₂ transition of the Cr³⁺ ions and the vibrations of the host lattice, the single-coordinate configuration model in harmonic approximation is used [48]. Two main parameters describing the electron–phonon coupling are the effective phonon energy ħω and the Huang–Rhys parameter S. Both parameters can be determined from the spectroscopic data [34] and are included in Table 4.

The Huang–Rhys parameter S is larger for YAB (S=5.9) than for GAB (S=5.4) what

Conclusions

The photoluminescence of Cr³⁺ in YAl₃(BO₃)₄ and GdAl₃(BO₃)₄ has been investigated between 4 and 800 K. At a low temperature ²E_g emission dominates in both hosts. From the excitation and emission spectra the crystal field splitting and the Racah parameters B and C have been determined. The smaller value for B in GAB:1%Cr in comparison to YAB:1%Cr indicates a higher covalency (stronger nephelauxetic effect) for Cr³⁺ in GAB. Temperature dependent studies reveal a shift from ²E_g to ⁴T_2g emission

Acknowledgment

We gratefully acknowledge financial support from the Bundesdruckerei GmbH, Berlin, Germany.

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Full Length ArticleTemperature dependent luminescence Cr3+-doped GdAl3(BO3)4 and YAl3(BO3)4

Abstract

Introduction

Section snippets

Synthesis

Description of the crystal structure of the GAB and YAB host materials

Temperature dependent luminescence

Electron–phonon coupling parameters

Conclusions

Acknowledgment

Spectrochim. Acta A

J. Cryst. Growth

Solid State Sci.

Mater .Chem. Phys.

J. Lumin.

Solid State Commun.

J. Phys. Chem. Sol.

J. Phys. Chem. Solids

J. Lumin.

J. Lumin.

J. Lumin.

Phys. Rev.

Phys .Rev. B

IEEE J. Quantum Electron

Nat. Mater.

Meas. Sci. Technol.

Phys. Rev.

J. Phys. D.: Appl. Phys.

Opt. Express

Luminescence and Display Phosphors: Phenomena and Applications

Quantum Electron

J. Electrochem. Soc.

J. Phys. Chem. C.

Full Length Article
Temperature dependent luminescence Cr³⁺-doped GdAl₃(BO₃)₄ and YAl₃(BO₃)₄