Orbital ordering transition in La₄ Ru₂ O₁₀ probed with Mössbauer spectroscopy

The discovery of an orbital ordering transition in La₄ Ru₂ O₁₀ provided an exciting link between ruthenate physics and that of the 3d transition metals. Despite clear evidence for the La₄ Ru₂ O₁₀ orbital ordering at ∼ 160 K (phase transition from a high temperature monoclinic structure to a low temperature triclinic structure accompanied by the opening of a spin gap), the atomic mechanism for this orbital ordering transition remains unresolved. We studied the local environment via Mössbauer effect (ME) measurements of ⁹⁹Ru (97%) enriched samples over a temperature range of 4.2-196 K. Fits to the spectra show that they arise solely from electric field gradients at the Ru sites without any hyperfine magnetic fields, ruling out the possibility of long range order. While the high temperature structure is accurately described by a single-site ME spectrum, the low temperature measurements can only be explained by a two-site model with significantly different symmetry at the two sites. At all temperatures the isomer shift is consistent with a + 4 oxidation state.