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    • Keywords


      Post heat treatment; t-zircon-type LaVO$_4$; structure transformation; phosphor.

    • Abstract


      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.

    • Author Affiliations



      1. Department of Mechanical Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan
      2. Department of Electronic Engineering, Cheng Shiu University, Kaohsiung 347, Taiwan
      3. Department of Digital Game and Animation Design, Tungfang Design University, Kaohsiung 821, Taiwan
      4. Department of Electronic Engineering, National Formosa University, Yunlin 632, Taiwan
    • Dates

  • Bulletin of Materials Science | News

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