Temperature dependence of electric field gradient in LaCoO₃ perovskite investigated by perturbed angular correlation spectroscopy

The time differential perturbed angular correlation (TDPAC) technique was used to study the temperature dependence of electric field gradient (EFG) in LaCoO₃ perovskite using and nuclear probes. The radioactive parent nuclei ¹¹¹In and ¹⁸¹Hf were introduced into the oxide lattice through a chemical process during sample preparation and were found to occupy only the Co sites in LaCoO₃. The PAC measurements with ¹¹¹Cd and ¹⁸¹Ta probes were made in the temperature range of 4.2-1146 K and 4.2-1004 K, respectively. No long-range magnetic order was observed up to 4.2 K. The EFGs at ¹¹¹Cd and ¹⁸¹Ta show very similar temperature dependences. They increase slowly between 4.2 and about 77 K and then decrease almost linearly with increasing temperature until about 500-600 K, where a broad peak-like structure is observed, followed by linear decrease at still higher temperatures. These discontinuities at about 77 K and 500-600 K have been interpreted as thermally activated spin state transitions from the low-spin (t_2g⁶e_g ⁰) ground state configuration to the intermediate-spin (t _2g⁵e_g¹) state and from the intermediate-spin to the high-spin (t_2g⁴e_g ²) state of the Co³⁺ ion, confirming previous observation in other recent studies. An indication of a Jahn-Teller distortion, which stabilizes the intermediate-spin state with orbital ordering, is also pointed out.