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Negative absolute temperatures were shown to be achievable in the nuclear spin systems in a pure LiF crystal in the 1950’s. The nuclear spin system can adequately be considered as a thermodynamic subsystem describable by a temperature, and the various nuclear spins must interact among themselves in such a way that thermodynamic equilibrium is achieved rapidly. However, these findings and interpretations, misperceptively, might lead to the generalization of the idea too, for example, in the same pure crystal, the lattice subsystem capable of lattice vibrations (phonons) with a stable equilibrium distribution of the vibrations. The lattice subsystem can be assigned a separate temperature. Only the latter system is a thermal subsystem capable of heat transfer for which achievability of negative absolute temperatures is an impossibility. This article attempts to clarify the thermodynamics of the negative absolute temperatures that may be plausible or impossible for subsystem constructs in the same material.
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