Structural Study of Europium Doped Gadolinium Polyphosphates LiGd(PO₃)₄ and Its Effect on Their Spectroscopic, Thermal, Magnetic, and Optical Properties

Alkali metal-rare earth polyphosphates LiGd_(1-x)Eu_x(PO₃)₄ (LGP:Eu³⁺) (where x= 0, 0.02 and 0.04) were synthesized by solid-state reaction. The Rietveld refinement showed the following cell parameters: I 2/a space group, a=9.635(3) Å, b=7.035(3) Å, c=13.191(3) Å, β=90.082°, V= 894.214Å3, and Z=4. The similarity between R_F=4.21% and R_B=4.31% indicated that the realized refinement is reliable. The crystal structure consists of infinite zig-zag chains of (PO₄)^3- tetrahedra, linked by bridging oxygen. The acyclic structure of polyphosphates is confirmed by infrared and Raman (IR) spectroscopies. A good thermal stability up to 940°C and paramagnetic behavior of these compounds were also proved by thermal analyses and magnetic susceptibility measurements, respectively. Excitation spectra revealed the charge transfer phenomenon between O^2- and Eu³⁺ (CTB), the energy transfer from Gd³⁺ to Eu³⁺, and the intrinsic 4f-4f transitions of Eu³⁺ where the electronic transitions were also identified. Moreover, LGP:Eu³⁺ can emit intense reddish orange light under excitation at 394 nm. The strongest tow at 578 and 601 nm can be attributed to the transitions from excited state ⁵D₀ to ground states ⁷F₁ and ⁷F₂, respectively.