Study of lanthanum substitution by strontium in lanthanum ferrites for gas-sensitive sensors by total reflection X-ray fluorescence

An approach for the simultaneous quantification of La, Fe, and Sr in solutions and suspensions of (La, Sr)FeO₃ composites by the TXRF method has been developed to study the effect of lanthanum substitution with strontium on the sensory properties of lanthanum ferrites. A gallium solution with a concentration of 50 mg/liter was used as an internal standard for the determination of the elements. The validity of the obtained results was confirmed for sample solutions by inductively coupled plasma atomic emission spectrometry (ICP AES). No interelement effects were observed for La, Fe, and Sr when present together in solutions. The precision of the results of the determination (S_r) by the TXRF was calculated as 0.04, 0.05, 0.06 for La, Fe, and Sr, respectively. The results obtained for suspensions are caused to the small particle size (15 – 17 nm) and the uniform distribution of the internal standard in the aliquot with the chosen sample preparation method. It is shown that all samples have a single phase corresponding to pure lanthanum ferrite with an orthorhombic crystal lattice (ICDD 37-1493). The absence of extraneous reflexes on the diffractogram indicates the successful incorporation of Sr²⁺ cations into the La³⁺ positions. It is shown that the doping of strontium into lanthanum ferrite made it possible to increase the sensitivity of materials to CO, NH₃, methanol and acetone, as well as to reduce the optimal operating temperature of the sensor by 50 – 150°C. At the same time, the best sensory properties were found for lanthanum ferrite nanofibers with a minimum concentration of introduced Sr (0.01 % at.). The absence of sensitivity of materials to methane and benzene is due to the greater stability of these molecules with respect to the acid-base active surface centers of La_{1 – x}Me_xFeO₃.

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