Understanding the structural and optical characteristics in Sr2xLa2-2xLi2xTe2-2xO6 perovskites

Abstract

Perovskites of the type Sr2xLa2-2xLi2xTe2-2xO6; x = 0.25, 0.4, 0.5, 0.6, 0.75 were analyzed for the first time for their structural and optical characteristics. All the compounds were found to be monoclinic with the P21/n structure thereby indicating a double perovskite structure formation. The x = 0.5 composition with Li+ and Te6+ was found to represent an ordered double perovskite having the least B-site disorder while x = 0.4 and x = 0.6 were found to have the highest disorder based on the refined occupancies. The tolerance factor calculations indicated that the symmetry is lowered on account of the octahedral tilting and Li-O-Te bending in making room for any divergence in symmetry that may arise due to a change to monoclinic from cubic structure. A greater tilting of the octahedral were observed in all compositions except x = 0.5 in terms of the tilting angle resulting in a drop in the bond angle of Li-O-Te. FTIR measurements indicate the existence of symmetric and antisymmetric stretching vibrations of the Li(Te)O6 octahedra. Energy band gap measurements indicated a depletion in the conduction band width and an increase in the energy gap. The ability for conducting is found to be the best in x = 0.25 and x = 0.6 as these compositions have the least value of energy band gap. This behavior can be accounted for by the possible presence of polarons in these compositions. Also, an increase in the energy band gap is observed when going from x = 0.25 to x = 0.5 due to an increase in cell volume for these compositions. SEM and EDX measurements carried out for x = 0.25, 0.5 and 0.75 indicate rough and agglomerated particles with a near uniform distribution in terms of their shape and the presence of elements as per the stoichiometric formula.