Characterization of the structure and photoluminescence properties of tetragonal structure La 0.995 Pr 0.005 VO 4 phosphor via a post heat treatment

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°C for 4 h. The X-ray diffraction patterns show that the La 0.995 Pr 0.005 VO 4 phosphors retain a tetragonal structure at a lower temperature (500 and 600°C). When the temperature is greater than 600°C, the structure of La 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 st ) is measured using differential scanning calorimetry as 625°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 μm 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-type structure. 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 → 3 H 4 , 3 P 0 → 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°C and the intensity decreases as the temperature increases further, because the structure of La 0.995 Pr 0.005 VO 4 phosphors changes from tetragonal to monoclinic when the heat-treatment temperature exceeds 600°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°C. When the heat-treatment temperature is greater than 600°C, the colour tones shift to the orange light region.