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We explore a generalised unified dark energy model that incorporates anon-minimal interaction between a tachyonic fluid and an additional scalarfield. Specifically, we require that the second field possesses a vacuumenergy, introducing an ineliminable offset due to a symmetry-breakingmechanism. After the transition (occurring as due to the symmetry-breakingmechanism of the second field), the corresponding equation of state (EoS) takesthe form of a combination between a generalised Chaplygin gas (GCG) componentand a cosmological constant contribution. We reinterpret this outcome bydrawing parallels to the so-called Murnaghan EoS, widely-employed in the realmof solid-state physics to characterise fluids that, under external pressure,counteract the pressure's effect. We examine the dynamic behaviour of thismodel and highlight its key distinctions compared to the GCG model. Weestablish parameter bounds that clarifies the model's evolution across cosmicexpansion history, showing that it, precisely, exhibits behaviour akin to alogotropic fluid that eventually converges to the $\Lambda$CDM model in theearly universe, while behaving as a logotropic or Chaplygin gas at intermediateand late times respectively. We explain our findings from a thermodynamicperspective, and determine the small perturbations in the linear regime. Atvery early times, the growth factor flattens as expected while the maindepartures occur at late times, where the Murnagham EoS results in a moreefficient growth of perturbations. We discuss this deviation in view of currentobservations and conclude that our model is a suitable alternative to thestandard cosmological paradigm, introducing the concept of a matter-like fieldwith non-zero pressure.

Unifying the dark sector through a single matter fluid with non-zero pressure