Light-Emission Properties and Internal Energy Transfer Phenomenon of Calcium Zirconate Phosphor Doped with Mn

Zirconates (XYO3; X = Sr, Ba, Ca; Y = Zr) have high resistance to corrosion, high chemical stability and high melting points, making them popular materials in the nuclear industry and metallurgy. In addition, zirconates doped with acceptor ions, such as Lu, Y, Gd, Ga, Sc and In, allow proton conduction at high temperatures. This attribute has wide industrial applications in hydrogen sensors, fuel cells, solid electrolytes, electronic ceramics and refractory materials. Previously, calcium zirconate powder (CaZrO3) was obtained using a conventional solid state reaction in which calcium carbonate and zirconium dioxide (ZrO2) are ground, mixed and heated to 1850 °C. However, the calcium zirconate powder derived through this method is prone to inconsistencies in particle size and clusters. Accordingly, researchers developed the cellulose-citric acid method for the synthesis of powder. Previous studies have established that adding cellulose can enhance the uniformity of positive ions mixed in solutions. Moreover, the resulting powder particles are smaller and spread more evenly. Thus, we believe that the cellulose-citric acid method could be used to resolve the issue of uneven particles and clusters in the synthesis of CaZrO3:Mn phosphor. However, the light-emission and energy-transfer properties of CaZrO3 phosphor doped with varying quantities of Light-Emission Properties and Internal Energy Transfer Phenomenon of Calcium Zirconate Phosphor Doped with Mn

Mn 2+ have not been previously investigated. This study successfully synthesized CaZrO3:Mn 2+ phosphor in a method using cellulose-citric acid to examine the light-emission properties of this material, as well as the distance and mechanism of energy transfer between Mn 2+ ions. The crystal structure of CaZrO 3 :Mn 2+ phosphor was analyzed using X-ray powder diffraction (XRD, X' Pert PRO, λCuK α = 1.5406 Å, scanning rate = 4° min -1 , scanning range = 20° ≤ 2θ ≤ 80°) and the appearance and element composition of the particles were observed using a field emission scanning electron microscopy with energy-dispersive X-ray spectroscopy (FE-SEM/EDX, S-4700). The excitation and emission spectra of the phosphor were recorded using a fluorescence spectrophotometer (FS, Hitachi F-4500).

RESULTS AND DISCUSSION
Crystal structure and particle appearance of CaZrO 3 : Mn 2+ phosphor: Fig. 1 presents the XRD images of CaZrO 3 phosphor with varying quantities of Mn 2+ calcined at 1200 °C for 6 h. As shown in the figure, CaZrO 3 :Mn 2+ phosphor displays a single crystalline phase. Compared with standard powder diffraction cards, the intensity of the diffraction peak was consistent with PDF card number 35-0645 11,12 . The XRD diffraction peaks for various quantities of doped Mn 2+ were not significantly different, indicating that the doping of different Mn 2+ contents did not influence the crystal structure of CaZrO 3 :Mn 2+ phosphor. Using XRD data and XRD comparison software, CaZrO 3 :Mn 2+ phosphor was found to exhibit an orthorhombic structure belonging to the Pnma{62} space group. The lattice parameters are a = 5.762 Å, b = 8.017 Å, c = 5.591 Å and the unit cell volume is V = 258.27 Å 3 .  [24][25][26][27][28] , an excitation wavelength of 457 nm corresponds to the 6 A 1 ( 6 S) → 4 T 2 ( 4 G) transition 29,30 and an emission wavelength of 541 nm corresponds to the 4 T 1 ( 4 G) → 6 A 1 ( 6 S) transition 29,30 .
Energy-transfer properties of CaZrO 3 :Mn 2+ phosphor: To understand the energy transfer mechanism in CaZrO 3 :Mn 2+ phosphor, the theories of Blasse and Dexter were employed to calculate the energy-transfer distance and verify the energy transfer mechanism between Mn 2+ ions.
An increase in doping content causes the Mn 2+ ions to become more densely packed, resulting in the transfer of energy between them. The close distance between the ions under such conditions is referred to as the energy transfer distance; the distance at which the maximum intensity of light emission peaks in the phosphor is defined as the maximum transfer distance. The content that contributes to this maximum value is the quench threshold. In Fig. 4, emission intensity peaks at a Mn 2+ doping level of 0.001 mole, which represents the quench threshold. The maximum value can be calculated using Blasse's formula 31,32 : where xc is the critical content of the activator; N is the number of sites per unit cell that the Mn 2+ ion can occupy and V is the unit cell volume. By substituting the known values into the equation (xc = 0.001 mol, N = 2, V = 258.27 Å 3 ), the maximum distance for the transfer of energy (RM) between Mn 2+ ions was calculated to be 62.72 Å.
Dexter's theory states that if energy transfer occurs between the same type of activator ions, the energy transfer mechanism can be determined by the intensity of the emission spectra. Dexter's equation is as follows 33,34 : where I is the luminous intensity of the phosphorus material, x is the content of the activator ion and ω is the evaluation parameter of the energy-transfer mechanism between the activators. An ω value equal to 6 indicates electric dipole interaction, an ω value equal to 8 indicates electric dipole and electric quadrupole interaction and an ω value equal to 10 indicates electric quadrupole interaction. The parameters ξ and µ are constants in identical host lattice structures under identical excitation conditions. When, ( ) ( ) Fig. 4 shows that the critical content of Mn 2+ in CaZrO3: Mn 2+ phosphor is 0.001 mole. Fig. 5 is the slope map of log (I/xMn 2+ ) and log (xMn 2+ ) for a Mn 2+ content greater than 0.001 mole in Fig. 4, displaying a straight line with a slope of -ω/3. From Fig. 5, the ratio of log (I/xMn 2+ ) to log(xMn 2+ ) is constant with a slope of -3.636. Therefore, ω = 10.908, which is close to 10. This shows that the energy transfer mechanism between Mn 2+ ions in CaZrO3:Mn 2+ phosphor is electric quadrupole interaction.

Conclusion
This study successfully synthesized CaZrO3 phosphor doped with Mn 2+ in a method that employed cellulose-citric acid. XRD analysis confirmed the crystal structure of CaZrO3: Mn 2+ phosphor as orthorhombic structure, belonging to the Pnma{62} space group. FE-SEM/EDX analysis revealed the appearance and element composition of CaZrO3:Mn 2+ particles. From a structural perspective, the light emitted by CaZrO3: Mn 2+ phosphor can be attributed to replacement between Mn 2+ ions and Zr 4+ ions, forming the emission center. The excitation spectrum presented an excitation peak at a wavelength of 457 nm, corresponding to the 6 A1( 6 S) → 4 T2( 4 G) transition in Mn 2+ ions. The emission spectrum exhibits a peak emission at 541 nm, corresponding to the 4 T1( 4 G) → 6 A1( 6 S) transition in Mn 2+ ions. The maximum distance for energy transfer between Mn 2+ ions was estimated to be 62.72 Å. Finally, energy transfer between Mn 2+ ions was found to result from electric quadrupole interactions.