We measured the p−T diagrams of four odd normal alkanes including ntridecane, which have two equilibrium lines on which the orthorhombic (SII) and hexagonal (SI) solids coexist, and the SI and liquid coexist. The equilibrium lines of n-tridecane nearly overlap above 40 °C and separate from each other with temperatures decreasing less than 40 °C. We also show that the metastable area of SII of n-tridecane widely spreads below the SII−SI coexistence line. We focused on the reason why the metastable region of the SII crystal spreads, based on the transformational relationship between SI and SII and the unisotropies among the SI and SII singular interfaces. The SII crystal first appears from the liquid above 40 °C just after compression, due to the narrow stable region of the liquid−SI crystal system. When the temperature and pressure of the liquid−SII crystal system vary compared with those of the wide stable region of the liquid−SI crystal system, the filmy SI crystal can grow on its SII crystal in the liquid. Therefore, the arrangement and the wide area cause the epitaxial growth of SI filmy crystal on SII crystal.

We optically observed the transformation between the solid phases of SII and SI of n-pentadecane of which a p–T diagram has a parallel and wide gap between the coexistence lines between liquid-SI and between SI–SII. We were able to grow an SII crystal in the liquid by cooling or compression, and subsequently filled the high-pressure specimen vessel with the SII crystal. The transformation to SI crystals in the SII crystal started after successive cooling or compression. Final compression transformed again all the SI crystals to the SII crystal. At the growth of an SII crystal in the liquid, we measured the SII-liquid equilibrium pressures and temperatures, which were located below the coexisting lines of the SI and liquid phases in the p–T diagram. We found that the SII crystal of n-pentadecane is stable even below the coexisting line between the SI and liquid. We elucidate thermodynamically the stabilities of the SI and SII phases based on the variations of some thermodynamic functions calculated from the measurements of the p–T diagram and the molar volume variation. Finally, we proposed a new arrangement among the planes of the Gibbs energy of the SII, SI, and liquid phases of n-pentadecane in the G–p–T diagram.

While we optically observed the pressure-induced transformation between the solid phases of SII and SI of n-heptadecane, of which p-T diagram has a parallel and wide gap between the coexistence lines between liquid-SI and between SI-SII (the same as that of n-pentadecane), we found that the SII crystal of n-heptadecane is stable even below the coexisting line between the SI and liquid, as is that of n-pentadecane. Fortunately, we can easily distinguish the SI crystal, of which morphology shows a hexagonal thin plate with very round corners, from the SII crystal, of which morphology shows a long and narrow pillar-like form in the liquid. Here, we report on the epitaxial growth of SI filmy crystals of n-heptadecane on its SII crystal in the p-T condition of 40 MPa and 30 °C. Although we could not observe the epitaxial growth of n-pentadecane, we could observe the clear epitaxial growth of n-heptadecane in the mixed liquid of n-pentadecane and n-heptadecane. Growth suppression by n-pentadecane brings about clear crystal shapes. Moreover, the epitaxially growing SI filmy crystals of n-heptadecane form a disk-like vessel which put its pillar-like SII crystal between the filmy crystals. If several billion years ago this vessel had been formed in a mixed drop of normal alkanes drifting about above the bottom of a deep sea, where the p-T condition was 40 MPa and 30 °C, the vessel might be seen as a cradle of primitive life.