Linear structural evolution induced tunable photoluminescence in clinopyroxene solid-solution phosphors

Abstract

Clinopyroxenes along the Jervisite (NaScSi2O6)-Diopside (CaMgSi2O6) join have been studied, and a solid-solution of the type (Na(1-x)Ca(x))(Sc(1-x)Mg(x))Si2O6 has been identified in the full range of 0 ≤ x ≤ 1. The powder X-ray patterns of all the phases indicate a structural similarity to the end compounds and show smooth variation of structural parameters with composition. The linear structural evolution of iso-structural (Na(1-x)Ca(x))(Sc(1-x)Mg(x))Si2O6 solid-solutions obeying Vegard's rule has also been examined and verified by high resolution transmission electron microscopy (HRTEM). The continuous solid-solutions show the same structural type, therefore the photoluminescence spectra of Eu(2+) doped samples possess the superposition of spectral features from blue-emitting component (CaMgSi2O6:Eu(2+)) and yellow-emitting one (NaScSi2O6:Eu(2+)). This indicates that the spectroscopic properties of (Na(1-x)Ca(x))(Sc(1-x)Mg(x))Si2O6 clinopyroxene solid-solutions are in direct relations with structural parameters, and it is helpful for designing color-tunable photoluminescence with predetermined parameters.

Figure 1. Variation of unit cell parameters (a, b, c and V) of Na_1−xCa_x)(Sc_1−xMg_x)Si₂O₆:0.03Eu²⁺ solid-solution series dependent on x values.

Figure 2. Powder XRD patterns for Rietveld structure analysis of the selected (Na_1−xCa_x)(Sc_1−xMg_x)Si₂O₆:0.03Eu²⁺ (x = 0.3) sample.

The inset gives the polyhedral representation of the structure of solid-solution sample, and the (Na, Ca, Eu) position and the co-existing [(Sc/Mg)O₆] octahedra and [SiO₄] tetrahedra are shown in it.

Figure 3. (a) TEM image of (Na_1−xCa_x)(Sc_1−xMg_x)Si₂O₆:0.03Eu²⁺ (x = 0.4) sample, (b) corresponding EDS analysis on a single crystal, and HRTEM images of the selected area for different samples of (Na_1−xCa_x)(Sc_1−xMg_x)Si₂O₆:0.03Eu²⁺ solid-solution series with different x values, x = 0 (c), x = 0.4 (d), x = 0.8 (d) and x = 1 (f).

The corresponding enlarged lattice fringes and their FFT patterns of the HRTEM images are given in the respective inset.

Figure 4

(a) PL spectra (λ_ex = 365 nm) of (Na_1−xCa_x)(Sc_1−xMg_x)Si₂O₆:0.03Eu²⁺ solid-solution phosphors with different x values, and the right inset shows the PLE and PL spectra of the selected (Na_0.7Ca_0.3)(Sc_0.7Mg_0.3)Si₂O₆:0.03Eu²⁺ sample, and the upper inset gives the digital images of some selected phosphors under 365 nm UV lamp. (b) the calculated spectra of (Na_1−xCa_x)(Sc_1−xMg_x)Si₂O₆:0.03Eu²⁺ phosphors by using functions F_x(λ) based on the spectra of NaScSi₂O₆:0.03Eu²⁺ (x = 0) and CaMgSi₂O₆:0.03Eu²⁺ (x = 1) in the linear superposition of functions F₀(λ) and F₁(λ). The inset gives the EL spectrum of a fabricated w-LED lamp by using (Na_0.7Ca_0.3)(Sc_0.7Mg_0.3)Si₂O₆:0.03Eu²⁺ sample and the photographs of the w-LED lamp package.