Sol-Gel Synthesis, Magnetic and Optical Properties of Co2+:Nd3+::2:1 molar ratio doped in SiO2

Abstract

Synthesis of Co²⁺:Nd³⁺::2:1 molar ratio doped SiO₂ with Co²⁺ ions concentrations viz. 0.0004, 0.001,  0.002, 0.003, 0.004 moles in S1-S5, respectively are performed by sol-gel method. Observed values of the densities and the concentrations of the Co²⁺ ions in S1-S5 are found to be 2.26, 2.29, 2.37, 3.44, 2.49 g/cm³, and ~ 10²⁰ ions/g, respectively. Powder XRD results show the formation of amorphous SiO₂ phase in the samples. DTA-TGA/DTG traces in the range 27-700 ËšC show no characteristic event in the samples. The IR peak in the range 412-418 cm^-1 is ascribed to the asymmetric mode of the octahedral [NdO_6/2]^3- units. The peak around 440 cm^-1 in S1-S5 is due to the ν₂-symmetric bending mode of the tetrahedral [SiO_4/2]^4- units. The weak IR peak ~ 670 cm^-1 is due to the ν₁-mode of the tetrahedral [CoO_4/2]^4- units. The calculated values of the magnetic susceptibility from the observed magnetic moments data at 300 K are found to be ~10^{-6 Â} emu/gG in S1-S5 which show the weak paramagnetic nature of the materials.  The EMR lineshapes recorded at 6, 50, 77, and 300 K are very broad, isotropic and smeared out with only two detectable characteristic peaks. The g_iso-value ~ 2.3 is due to the doped Co²⁺(3d⁷) ions in tetrahedral coordinations, while the g_iso-value ~ 2.01 is due to the doped Nd³⁺(4f³) ions in distorted octahedral coordinations. The lineshapes only at 6 K are distinct, while at higher temperatures  these peaks are severely smeared out. The optical absorption broad band at ~750 nm (13262 cm^-1) is assigned to ⁴I_9/2 ® ⁴F_7/2 + ⁴S_3/2 transitions in optically active Nd³⁺(4f³) ions, and the  peak ~ 520 nm (19230 cm^-1) is attributed to ⁴A_2g ® ⁴T_1g(P) transitions in Co²⁺ (S=3/2, 3d⁷) ions in tetrahedral coordinations.