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 In-111 -> Cd-111 and Hf-181 -> Ta-181 nuclear probes. The radioactive parent nuclei In-111 and (181H)f 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 Cd-111 and Ta-181 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 Cd-111 and Ta-181 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(2g)(6) e(g)(0)) ground state configuration to the intermediate-spin (t(2g)(5) e(g)(1)) state and from the intermediate-spin to the high-spin (t(2g)(4) e(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.