Structural and luminescent properties of Mg
4
Nb
2
O
9
nanocrystals
Te-Hua Fang
a,c,, Yu-Jen Hsiao
b, Liang-Wen Ji
c, Yee-Shin Chang
c, Sung-Shui Chi
aa
Institute of Mechanical and Electromechanical Engineering, National Formosa University, Yunlin 632, Taiwan
b
National Nano Device Laboratories Tainan 744, Taiwan
cInstitute of Electro-optical and Materials Science, National Formosa University, Yunlin 632, Taiwan
a r t i c l e
i n f o
Article history: Received 3 January 2008 Received in revised form 1 April 2008
Accepted 3 April 2008 Communicated by M. Schieber Available online 12 April 2008 PACS: 61.10.Nz 71.35.Cc 68.37.Lp 68.55.Ln 78.55.-m Keywords: A1. Nanostructures B1. Nanomaterials B1. Niobates B2. Dielectric materials B2. Nonlinear optic materials B2. Phosphors
a b s t r a c t
Synthesis and luminescence properties of Mg4Nb2O9 nanocrystals by citric gel process were investigated in this work. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL). The excitation band at about 241 and 341 nm originates from the regular and defect octahedra group [NbO6]7. The room-temperature PL spectra under 241 and 341 nm excitations show a broad violet and a very strong emission peaks located at 390 and 402 nm, respectively.
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1. Introduction
Fundamental studies concerning the phase diagram and the characterization of the MgO–Nb2O5 system have been studied
since 1970s. Four possible oxides have been identified: MgNb2O6,
Mg4Nb2O9, Mg5Nb4O15, and Mg2/3Nb11(1/3)O29in the past[1]. You
et al.[2]reported that MgNb2O6and Mg4Nb2O9are the only stable
phases that can be found at room temperature. High Q value and high dielectric constant of Mg4Nb2O9 with corundum-like
structure have attracted interest in microwave dielectric applica-tions [3,4]. Mg4Nb2O9 is also regarded as a room-temperature
photoluminescent material. It can be seen that the materials emit luminescence in the blue–green region, and it will be a potential material for high-definition television and field emission display (FED) applications[5].
It is well known that the properties of materials depend on their synthesis processes. In general, there were some methods to prepare Mg4Nb2O9 powder, including conventional solid-state
reaction[6], rapid vibro-milling[7], and coprecipitation technique
[8]. Recently, a few niobate-based complexes were synthesized by the citric gel method. Low-temperature processing is the major advantage of sol–gel process. Chemically synthesized ceramic powders often show better chemical homogeneity and finer particles together with a better control of particle morphology than those produced by the mixed oxide route, and these features will result in improved sinterability[9]. In this work, we use a sol–gel method to prepare Mg4Nb2O9 powders and discuss the
structure and luminescence characterization of the samples.
2. Experiments
The Mg4Nb2O9powders were prepared by the sol–gel method
using magnesium nitrate [Mg(NO3)2d6H2O], niobium chloride (NbCl5), ethylene glycol (EG), and citric acid (CA) anhydrous. First,
the stoichiometric amount, of magnesium nitrate and niobium ethoxide, was dissolved in distilled water. Niobium ethoxide,
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Corresponding author at: Institute of Mechanical and Electromechanical Engineering, National Formosa University, Yunlin 632, Taiwan.
Tel.: +886 5 631 5395; fax: +886 5 631 5397.
E-mail address:[email protected] (T.-H. Fang).
niobate center can be viewed as a partial disorder of the Mg2+and
Nb5+ions on the cation sites such that there exist Mg2+–O–Nb5+
pairs instead of Nb5+–O–Nb5+ pairs [5]; the formation of the
defect can be found inFig. 4c. The luminescence properties of the pervoskite-like compounds are also determined mainly by the extent of delocalization of the excited state [15]. This effect depends on the structure, and in particular on the M–O–M angle (M ¼ Nb5+, Ta5+, Ti4+,y) between the corner-sharing octahedra
[16]. In this study, the delocalization is large, and the regular [NbO6]7is observed at high sintering temperature. Therefore, the
excitation band of the luminescence of Mg4Nb2O9shifts to lower
energy than that of other samples with low sintering tempera-tures. Macke[13]found only one broad peak by the conventional ceramic technique and measured the excitation spectra of Mg4Nb2O9 with 4.96 eV (250 nm) at room temperature. Similar
peak was observed at about 248 nm inFig. 5a. However, there are two excitation peaks by sol–gel process and another excitation peak was firstly found at 341 nm. The emission peak of 402 nm was due to the disorder defect structure as mentioned above by TEM analysis. The main difference of convention and sol–gel process was added one excitation peak, when we synthesized the nanoparticles.
The PL emission spectra of Mg4Nb2O9 phosphors under 241
and 341 nm excitation at room temperature are shown inFig. 5b. The result shows a broad violet and a strong emission peak at 390 and 402 nm, respectively. This luminescence effect depends on the Nb–O–Nb bonding, that the conduction band is composed of Nb5+4d orbitals, and the valence band of O22p orbitals between
the corner-sharing octahedra[17]. In other word, this lumines-cence originated from the nanocrystals of absorbing groups of regular or defect niobate octahedra group [NbO6]7.
4. Conclusions
In summary, Mg4Nb2O9nanocrystals were prepared by a
sol-gel method, the well-crystalline hexagonal Mg4Nb2O9 can be
obtained by heat-treatment at 700 1C. TEM analysis of the nanocrystal provides further insight into the microstructure of Mg4Nb2O9. The diameters of the nanocrystals are in the range of
50–100 nm at 700 1C. The Mg4Nb2O9 nanocrystals exhibit
red-shift excitation spectra when the calcining temperature increased from 700 to 800 1C. The excitation band at about 241 and 341 nm originates from the regular and defect octahedra group. The PL spectra show a broad violet and a strong emission peaks at about 390 and 402 nm, respectively.
Acknowledgment
The authors would like to thank the Cooperating Teacher Project in National Formosa University for supporting this research.
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Fig. 5. (a) Excitation (lem¼390 nm) spectra of Mg4Nb2O9 powder samples
annealed at various temperatures. (b) Emission (lex¼241 and 341 nm) spectra
of Mg4Nb2O9powder samples annealed at 700 1C.
T.-H. Fang et al. / Journal of Crystal Growth 310 (2008) 3331–3334 3334
