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## Fluid–fluid–solid triple point on melting curves at high temperatures

An analysis is presented of experimental data where fluid–fluid phase transitions are observed for different substances at high temperatures with triple points on melting curves. Viscosity drops point to the structural character of the transition, whereas conductivity jumps remind of both semiconductor-to-metal and plasma nature. The slope of the phase equilibrium dependencies of pressure on temperature and the consequent change of the specific volume, which follows from the Clapeyron–Clausius equation, are discussed. P(V, T ) surfaces are presented and discussed for the phase transitions considered in the vicinity of the triple points. The cases of abnormal P(T ) dependencies on curves of phase equilibrium are in the focus of discussion. In particular, a P(V, T ) surface is presented when both fluid–fluid and melting P(T ) curves are abnormal. Particular attention is paid to warm dense hydrogen and deuterium, where remarkable contradictions exist between data of different authors. The possible connection of the P(V, T ) surface peculiarities with the experimental data uncertainties is outlined.