First Order Pre-emptive Ising-nematic Transition in K₅Fe₄Ag₆Te₁₀

Transition metal pnictides and chalcogenides can exhibit intriguing properties including unconventional superconductivity, charge density waves and other correlated states with magnetism. In each of these cases, the interplay between the various order parameters together with their concomitant fluctuations is key to their physics underlying the continued need for detailed characterization. Employing inelastic X-ray and neutron scattering techniques, we observed nematic and magnetic phase transitions with distinct characters in K₅Fe₄Ag₆Te₁₀. Upon cooling, the nematic order undergoes a first-order phase transition followed by a second-order magnetic transition at T_N ≈ 34.6 K. Our results suggest that the temperature difference between these two phase transitions is ∼ 1 K which can be attributed to a distinctive first-order pre-emptive Ising-nematic transition, a characteristic unique to a quasi-two-dimensional scenario marked by strong out-of-plane spatial anisotropy due to weak coupling. Our studies suggest K₅Fe₄Ag₆Te₁₀ as the first material in the family of iron pnictides and chalcogenides that possesses a nematic tricritical point preceding the magnetic one upon decreasing nematic coupling. This contrasts with more established systems such as BaFe₂As₂ or CaFe₂As₂ which display a pre-emptive second-order nematic transition and simultaneous first-order nematic-magnetic transitions respectively.