Cation distribution and optical properties of Cr-doped MgGa2O4 nanocrystals

doi:10.1016/j.optmat.2014.09.029

Optical Materials

Volume 37, November 2014, Pages 854-861

https://doi.org/10.1016/j.optmat.2014.09.029 Get rights and content

Highlights

•
The distribution of cations in Cr-doped MgGa₂O₄ nanocrystals was studied.
•
All the cations occupy both the tetrahedral and octahedral sites of spinel structure.
•
The inversion parameter increases with Cr-enrichment.
•
Fraction of tetrahedral Cr³⁺ decreases with increasing temperature and Cr content.
•
The relationship between the cation distribution and optical properties was discussed.

Abstract

The distribution of cations in the spinel-type MgCr₂_yGa₂₋₂_yO₄ (y = 0–0.6) nanocrystals and their optical properties as a function of annealing temperature and chromium content were investigated by using X-ray photoelectron spectroscopy (XPS) in combination with absorption spectroscopy. The cations in MgCr₂_yGa₂₋₂_yO₄ nanocrystals are disorderly distributed with mixing of divalent and trivalent cations occupying the tetrahedral and octahedral sites. With the increase of annealing temperature, the inversion parameter (the fraction of Mg²⁺ ions in octahedral sites) decreases, which has the same varying tendency with the proportion of tetrahedral Ga³⁺ or Cr³⁺ ions. The inversion parameter increases with increasing Cr³⁺ concentration. The absorption spectra indicate that Cr³⁺ ions are located in the octahedral sites as well as in the tetrahedral sites. The fraction of tetrahedral Cr³⁺ decreases with Cr-enrichment. The optical absorption properties of Cr-doped MgGa₂O₄ nanocrystals may be tuned by varying the preparation temperature or Cr concentration.

Introduction

In the structure of spinel, there are two types of coordination sites available for A²⁺ and B³⁺ cations: octahedral and tetrahedral. Many spinels exhibit complex disordering phenomena involving the mixing of cations on these two sites, which have important consequences for their properties. The disordered cation distribution is represented as (A₁₋_yB_y)[A_yB₂₋_y]O₄, where the brackets () and [] represent the tetrahedral and octahedral sites, respectively, and the inversion parameter y corresponds to the degree of inversion. The distribution of A and B cations in these two sites depends strongly on the temperature, impurity content, and particle size. Jovic et al. [1], Sreeja et al. [2], Zhang et al. [3], and Mittal et al. [4] studies of cation distribution in spinels may allow us better understanding of the correlations between structure and properties such as color, magnetic behavior and optical properties etc., which are dependent on the occupation of these two sites by metals [5], [6], [7], [8].

Magnesium gallate (MgGa₂O₄) has partially inverse spinel structure belonging to the AB₂O₄ class of materials [9], [10], [11], [12]. Cr³⁺-doped MgGa₂O₄ exhibits broad absorption and emission bands in the visible region. The optical properties of Cr³⁺: MgGa₂O₄ spinel has been studied extensively [13], [14], [15], [16]. It is previously considered that Cr³⁺ ions are situated at the octahedral coordination sites by replacing octahedral Ga³⁺ in MgGa₂O₄ system. However, the structural and optical studies on these materials indicate that fraction of Cr³⁺ ions enter the tetrahedral sites of spinel structure [17], [18]. The change in the chemical environments of Cr³⁺ will have great influence on their optical properties.

In this study, Cr³⁺-doped MgGa₂O₄ nanocrystals have been synthesized by citrate sol–gel method. The degree of the cation distribution in Cr: MgGa₂O₄ as a function of heat-treatment temperature and Cr content has been studied by means of X-ray photoelectron spectroscopy (XPS). The UV–Vis absorption spectra of the materials were studied and the relationship between the structure and the optical properties was also discussed.

Section snippets

Experimental

Cr-doped MgGa₂O₄ nanoparticles with the composition of MgCr₂_yGa₂₋₂_yO₄ (y = 0–0.6) were synthesized by sol–gel method using citric acid as a chelating agent. Firstly, stoichiometric Cr(NO₃)₃·6H₂O (analytical reagent, A.R.), Mg(NO₃)₂·6H₂O (A.R.) and Ga(NO₃)₃·xH₂O (A.R.) were dissolved in deionized water. Then citric acid was added to the above solution with stirring. The molar ratio of metal ions to citric acid was 1:2. The mixed solution was stirred for 1 h and then heated in an 80 °C water bath

Synthesis of Cr_xMg₁₋_xGa₂O₄ nanocrystals

X-ray powder diffraction (XRD) investigations were performed on all the samples with different annealing temperatures and various compositions to identify the crystalline phase and to compare the particle sizes. Fig. 1a shows the powder XRD patterns of the MgCr_0.6Ga_1.4O₄ nanoparticles annealed at 700–1000 °C. The sample annealed at 700 °C exhibits several distinct broad diffraction peaks in the XRD pattern indicating the nanocrystalline nature. The intensity of the peaks increases with increasing

Conclusions

Cr-doped MgGa₂O₄ nanocrystals were obtained by sol–gel method using citric acid as a chelating agent. The effects of annealing temperature and composition on the coordinative environments of the cations (Ga³⁺, Mg²⁺ and Cr³⁺ ions) have been investigated by XPS. It is shown that the mixing of divalent and trivalent cations located on both the sites: tetrahedral and octahedral. The Ga³⁺ ions occupy the tetrahedral sites in large proportions for a high chromium concentration and low synthesis

Acknowledgements

This work was supported by Grants from the National Natural Science Foundation of China (Nos. 51172130 and 51372140).

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View full text

Cation distribution and optical properties of Cr-doped MgGa2O4 nanocrystals

Highlights

Abstract

Introduction

Section snippets

Experimental

Synthesis of CrxMg1−xGa2O4 nanocrystals

Conclusions

Acknowledgements

Solid State Commun.

Solid State Ionics

J. Lumin.

Opt. Mater.

J. Phys. Chem. Solids

J. Alloys Compd.

Solid State Sci.

J. Catal.

Appl. Surf. Sci.

J. Phys. Chem. C

J. Phys. Chem. C

J. Am. Chem. Soc.

J. Phys. Chem. B

Inorg. Chem.

Inorg. Chem.

Acta Crystallogr.

Cation distribution and optical properties of Cr-doped MgGa₂O₄ nanocrystals

Synthesis of Cr_xMg₁₋_xGa₂O₄ nanocrystals