X-ray Diffraction and Fluorescence Spectroscopy Analysis of Sm3+ in Lithium Calcium Silicate

Sep 01, 2014

Trivalent samarium–doped lithium calcium silicate phosphors with the nominal formulas of Li 2 Ca 1-x Sm x SiO 4 were prepared with a conventional solid-state reaction technique and the dopant concentration was varied in the range of 0.01 ≤ x ≤ 0.10. The phase and structure of the as-prepared samples were characterized using X-ray diffraction. The excitation and emission spectra were measured to characterize the luminescent properties of the Li 2 CaSiO 4 :Sm 3+ phosphors. The excitation and emission spectra at room temperature show the typical f–f transitions of Sm 3+ . The dominant excitation line is around 408 nm because of the 6 H 5/2 4 K 11/2 transition. The emission spectrum of all phosphors exhibited four sharp emission peaks corresponding to the 4 G 5/2 6 H 5/2 (565 nm), 4 G 5/2 6 H 7/2 (601 nm), 4 G 5/2 6 H 9/2 (654 nm), and 4 G 5/2 6 H 11/2 (708 nm) transitions of Sm 3+ . The emission intensity depended on the Sm 3+ concentration and it was observed that the optimum concentration of Sm 3+ in phosphor was 4 mol% for the highest emission intensity at 601 nm.

Luminescence materials are important for modern technology because of their ability to convert electromagnetic radiation (that is, ultraviolet [UV] and infrared [IR] light) into visible light (1). Recently, the development of rare earth, ion-activated, novel luminescent materials have attracted more attention because of their potential applications in optical display systems (2–9). Because the 4f electrons in rare earth ions are shielded by the outer 5s and 5p electrons, the intra-4f emission spectra of rare earth ions are characterized by narrow lines with high color purity (10).

The Sm3+ ion is widely used as an activator of reddish orange emission because of its 4 G5/26 HJ (J = 5/2, 7/2, 9/2, and 11/2) transitions (11–16), which is the most suitable source for lighting and display, from a practical viewpoint. Moreover, Sm3+ ions act as a spectroscopic probe of local structure around rare-earth ions in condensed matter. This is because of the electric-dipole (ED) character of the 4 G5/26 H9/2 hypersensitive transition, whose intensity increases as the environmental symmetry of the luminescent site decreases (17). Those investigations of Sm3+ -doped materials mainly focused on the host matrixes, which included borates (18), silicates (19), and vanadates (20).

In this article, Li2CaSiO4:Sm3+ reddish orange phosphors were synthesized by a solid-state reaction method. The photoluminescence of the phosphors was systematically investigated using a spectrofluorometer at room temperature. Also, the dependence of the emission intensity on the Sm3+ concentration for the Li2Ca1-xSmxSiO4 (0.01 ≤ x ≤ 0.10) was studied in detail.

Experimental


Table I: Sm3+ concentration (x) for Li2Ca1-xSmxSiO4
Li2Ca1-xSmxSiO4 (x = 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, and 0.10) phosphors were prepared according to a standard solid-state technique (Table I). The starting materials were Li2CO3, CaCO3, SiO2, and Sm2O3 (all materials were analytical grade). The raw materials were weighed according to the stoichiometric proportions. The chemicals were ground in an agate mortar at room temperature and later were collected into an alumina crucible for heating. First, the powder mixtures were calcined at 450 °C for 1 h. After grinding and the homogenization of preannealed mixtures, the mixtures were heat treated at 600 °C for 1 h and 850 °C for 2 h.

The structural characterization of the fine powders was performed by X-ray diffraction (XRD) analysis using a Bruker AXS D8 Advance diffractometer. The system was run at 20–60 kV and 6–80 mA, 2θ = 10–90° and a step of 0.002° using the CuKα line of 1.5406 A° (quartz [PDF 46-1045] was used as reference material for XRD). Scanning electron microscopy (SEM) images were taken with a FEI Quanta 250 FEG model scanning electron microscope using an accelerating voltage of 20 kV.

All the luminescence characteristics of these phosphors were investigated at room temperature using a Thermo Scientific Lumina fluorescence spectrometer equipped with a 150-W xenon lamp.