Nematic fluctuations occur in a wide range of disparate physical systems from liquid crystals to biological molecules to solids such as exotic magnets, cuprates and iron-based high-T c superconductors. Spin-nematic fluctuations are thought to be closely linked to the formation of Cooper-pairs in iron-based superconductors. To date, it is unclear whether the anisotropy inherent in this nematicity arises from electronic spin or orbital degrees of freedom. We have studied the iron-based Mott insulators La 2 O 2 Fe 2 OM 2 M = (S, Se) that are structurally similar to the iron pnictide superconductor. A spin nematic precursor phase was revealed by a critical slowing down of nematic fluctuations observed in the spin-lattice relaxation rate(1/T 1 ) obtained by nuclear magnetic resonance. This is complemented by the observation of a change of electrical field gradient over a similar temperature range using Mössbauer spectroscopy. Theoretical modeling of a geometrically frustrated spin-1 Heisenberg model with biquadratic and single-ion anisotropic terms provides the interpretation of magnetic fluctuations in terms of hidden quadrupolar spin fluctuations. We find that nematicity is not due to orbital anisotropy, since the iron d xz,yz orbital degeneracy is locally broken due to an alternating orientation of the FeM 4 O 2 octahedra. Neutron diffraction indicates that global C 4 symmetry is preserved. We find that the spin nematicity is closely linked to geometrically frustrated magnetism, itself emerging from orbital selectivity. Our findings highlight the interplay between orbital order and spin fluctuations in the emergence of nematicity in strongly correlated Fe-based oxychalcogenides.

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