Synthesis, crystal structure, optical, color, thermal, magnetic, and spectroscopic properties of the substituted honeycomb-lattice BaNi2-xCox(PO4)2 (x=0, 0.5 and 1)

The ceramic phosphate materials BaNi2-xCox(PO4)2 (x = 0, 0.5 and 1) have been elaborated for the first time by the hydrothermal process at T = 180 degrees C and characterized by various techniques. The structure was determined from X-ray powder diffraction using Fullprof software based on Rietveld refinement. All investigated materials crystallize in trigonal system with R3 (no.148) space group. The BaNi2-xCox(PO4)2 (x = 0, 0.5 and 1) structure can be described as a three-dimensional framework, where two layers including edge-sharing XO6 (X = Ni2-xCox) octahedra and PO4 tetrahedra are connected by Ba2+ cations possessing twelve-fold coordination. The octahedral environment of both Ni2+ and Co2+ ions was confirmed through optical analysis. Numerous bands between 400 and 760 nm were assigned to allowed and forbidden d-d transitions of Ni2+ and Co2+ ions. CIE L*a*b* and RGB parameters were measured in order to analyse the color properties of the prepared materials. DTA/TGA analysis showed excellent thermal stability of the substituted honeycomb-lattices. Magnetic measurements revealed the presence of an antiferromagnetic order in the studied systems; for x = 0, one transition at TN = 24 K was observed. For x = 0.5 and 1, two transitions at TN and Tsr were detected pointing out the spin reorientation caused by the ferromagnetic (FM) and antiferromagnetic (AFM) competition occurred when substituting between Ni2+ and Co2+ ions. FTIR spectroscopy revealed the existence of vibration modes of PO4 tetrahedra.