Influence of ionic radii on the conduction mechanism in lanthanum silicate oxyapatite

Lanthanum Silicate oxyapatite has attracted researchers due to its interesting ion channels and conduction pathways. In the present work, the average ionic radii of La site are altered via substitution of La3+ with Ba2+ and Ca2+ to form (La1-xAx)9.67(SiO4)6O2+δ (x = 0.0, 0.05, 0.10 and 0.15) where A = Ca and Ba. Further, the ionic radii of substituents will alter the average area of critical triangle (saddle point). Therefore, it is expected to affect the ion mobility. The single-phase Ca and Ba substituted samples are observed and Rietveld refinement suggests no phase change with the ionic radii. The elemental content obtained from Rietveld refinement, Energy dispersion spectra, X-ray photoelectron spectroscopy and thermogravimetric analysis suggests La deficiency in Ca substituted sample and O deficiency in Ba substituted sample. Temperature and frequency dependent ac conductivity in the temperature range (548 K–973 K) suggests that the conductivity is higher at x = 0.05 than x = 0.1 in Ba substituted sample. On a comparative note, the conductivity is higher in Ca substituted sample than that of the Ba substituted sample for x = 0.1.