Praseodymium induced symmetry switching and electrochemical characteristics of LaCoO3 nanostructures

Abstract

A comparative study of the crystal structures and electrochemical characteristics of bulk and nanoscale La_1-xPr_xCoO₃ (x = 0, 0.3 and 0.6) perovskites is made using synchrotron x-ray diffraction, cyclic voltammetry, and galvanostatic charge/discharge methods. It is shown that the sol-gel synthesized nano structures and bulk LaCoO₃ exhibit different crystal structures, viz., rhombohedral [a = b = 5.4401 Å, c = 13.134 Å (on hexagonal axes), Z = 6, R $\bar{3}$ c] and monoclinic [a_m = 5.3865 Å, b_m = 5.4482 Å, c_m = 7.6365 Å, β_m = 89.010°, Z = 4, I2/a], respectively. The evidence for CoO₆ octahedra distortion in bulk LaCoO₃ is also gathered from the three distinct Raman active modes at 518, 646, and 688 cm⁻¹ emerging due to the Jahn-Teller effect, which, in-turn, reduces the crystal symmetry for achieving structure stabilization. This amounts to changes in Co–O bond lengths and Co–O–Co bond angles with promotion of t_2g electron to e_g level simultaneously for Co³⁺(3d⁶) to attain an intermediate spin state (S = 1) or higher. However, Pr-insertion induces phase transition to orthorhombic in both but with space group Pnma in nanostructures and equivalent Pbnm in bulk. The substitution effect on the specific capacitance (C) is opposite in nature, i.e., while ‘C’ decreases from 149 to 12 F/g in nano structures, it increases from 0.4 to 4 F/g in bulk with increase in Pr-content from x=0 to 0.6. A galvanostatic charge-discharge test of pristine nano LaCoO₃ performed at a constant scan rate of 50 mVs⁻¹ reveals electrode electrochemical stability by retaining 96 % of specific capacitance ∼82.5 F/g for 2000 cycles at least. The variation in the crystal structure and bond length and/or angle plays a key role in controlling the electrochemical performance of Pr-substituted LaCoO₃ perovskites.