Articles written in Bulletin of Materials Science

    • Characterization of the structure and photoluminescence properties of tetragonal structure La$_{0.995}$Pr$_{0.005}$VO$_4$ phosphor via a post heat treatment


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      A hydrothermal method is used to prepare La$_{0.995}$Pr$_{0.005}$VO$_4$ nano-crystal phosphors, which are then post heat treated at 500–800$^{\circ}$C for 4 h. The X-ray diffraction patterns show that the La$_{0.995}$Pr$_{0.005}$VO$_4$ phosphors retain a tetragonalstructure at a lower temperature (500 and 600$^{\circ}$C). When the temperature is greater than 600$^{\circ}$C, the structure ofLa$_{0.995}$Pr$_{0.005}$VO$_4$ changes from the t-zircon-type to the m-monazite-type lanthanum orthovanadates (LaVO$_4$) structure. The temperature at which the structure changes ($T_{\rm st}$) is measured using differential scanning calorimetry as 625$^{\circ}$C. The scanning electron microscopy results show that the surface morphology of the phosphor particles is granular and has a uniform distribution. The particle size increases from 0.1 to 2.5 $\mu$m as the temperature is increased. The excitation spectra show that the absorption behaviour for the t-zircon-type and the m-monazite-type La$_{0.995}$Pr$_{0.005}$VO$_4$ phosphor is not significantly different, but there is a little red shift due to host absorption for La$_{0.995}$Pr$_{0.005}$VO$_4$ with an m-monazite-typestructure. Under excitation at 315 nm, the main emission band retains the characteristics of Pr$^{3+}$ ion-doped LaVO$_4$ phosphor, which is attributed to the host luminescent and the ${}^1$D$_2\to {}^3$H$_4$, ${}^3$P$_0\to {}^3$H$_6$ electron transition of the Pr$^{3+}$ ion. As the temperature for the heat treatment increases, the intensity of the excitation and emission peaks has a maximum value for a temperature of 600$^{\circ}$C and the intensity decreases as the temperature increases further, because the structure ofLa$_{0.995}$Pr$_{0.005}$VO$_4$ phosphors changes from tetragonal to monoclinic when the heat-treatment temperature exceeds 600$^{\circ}$C. This demonstrates that the tetragonal structure is better than the monoclinic structure for LaVO$_4$ if it is used as a host material for phosphor applications. The colour tones are initially in the white light region for La$_{0.995}$Pr$_{0.005}$VO$_4$ phosphors heat treatment at 500 and 600$^{\circ}$C. When the heat-treatment temperature is greater than 600$^{\circ}$C, the colour tones shift to the orange light region.

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