Studies on morpho-structure and ionic conductivity of apatite-type lanthanum silicate doped with transitional metal cations

Abstract

Oxy-apatite lanthanum silicates with general formula La₁₀Si_5.9M_0.1O_27±δ (M = Cr³⁺, Mn⁴⁺, Fe³⁺, Mo⁶⁺, W⁶⁺; 0.05 ≤ δ ≤ 0.1, coded LaSiMO) were obtained by gel-combustion and they were characterized through electrochemical impedance spectroscopy. The XRD patterns show that all samples adopt apatite structure and crystallize in the hexagonal space group P-3(147) with crystallite sizes ranging from 86 nm to 103 nm. The dopant cations were accommodated at Si position with the highest occupancy factor at O6 sites (responsible in conduction) for LaSiCrO and LaSiFeO apatites. Fe and Cr doped samples contain pure apatite phase while those doped with Mn, Mo, W contain La₂SiO₅ as secondary phase which act as insulator in conduction process. Based on EPR and ICP-OES investigations we found that Mn, Mo, W cations present multiple oxidation states in the apatite lattice and a lower incorporation degree in comparison with Fe and Cr cations. Local distortions are present in Fe and Cr doped apatite lattices which explains the higher ionic conductivity of these samples. At 500^oC, the ionic conductivities, range between 1.15·10^− 4 and 3.56·10^− 4 S·cm^− 1 for samples sintered at 1400^oC, values that are higher than un-doped apatite (6.46·10^− 5 S·cm^− 1). Due to the highest ionic conductivity among the samples, LaSiCrO was additionally sintered at 1500^oC and 1600^oC in order to investigate the role of temperature on ionic conduction process. The relative density of LaSiCrO was improved from 89.03 to 93.06% and the ionic conductivity reach 0.28·10^− 2 S·cm^− 1, at 600^oC. The results of this study show that apatites doped with Cr and Fe may be used as electrolytes in SOFCs.