Thermal expansion of N2-type crystals

Linear expansion coefficients α are determined for cryocrystals with linear molecules in the Pa3 lattice: N2 (2−21 °K), CO 2−28 °K), CO2 (2−25 °K), N2O (2−90 °K). A modification is proposed for the law of corresponding states in order to describe the translational component of thermal expansion of the substances investigated and other cryocrystals with close-packed lattices. The possibility of using such a description of α, for separating out translational and librational contributions to the thermal expansion of molecular cryocrystals, is illustrated for CO2. The thermal behavior of the Grüneisen coefficient for the solid N2O and CO is interpreted as the manifestation of partial dipole ordering in these crystals. It is shown that crystals with molecules that do not have a center of inversion (N2O and CO) have excess heat capacity in comparison with crystals that have symmetrical molecules (CO2, N2). The entropies of N2O and CO corresponding to the excess heat capacities are close to the difference between R ln2 and the experimentally determined residual entropies of these substances.

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