Overlapping large polaron tunnelling in lanthanum silicate oxyapatite

Amongst the various fast ion conductors, lanthanum excess lanthanum silicate oxyapatite (La10-α(SiO₄)₆O2+δ) has shown higher oxide ion conductivity with lower activation energy. On the other hand, the activation energy increases with La vacancies (La at 4f site). In the present work, La site is altered with Ca to form (La1-xCa)_9.67(SiO₄)₆O2+δ(x=0.0,0.05,0.10and 0.15) with minimum oxygen non-stoichiometry and studied the hopping/tunnelling mechanism with the Ca substitution. The elemental content obtained from Rietveld refinement of the x-ray diffractograms suggests La deficiency with minimum oxygen deficiency. Further, XPS and TGA studies confirm the formation of La deficient samples. Temperature and frequency dependent ac conductivity in the temperature range (548-973 K) suggests that the conduction takes place via overlapping large polaron tunnelling. Further, the tunnelling distance and polaron radii as a function of temperature and frequency are observed to be altered with Ca and affecting the ion conducting channel through the elongation of La(6 h) triangles. Our study suggests the phononic contribution play a pivotal role in ionic transport.