Anomalous Vegard law in mixed cryocrystals N2O–CO2 as a phase transition in molecular orientational subsystem

In the paper, we discuss the effects of the molecular rotation degrees of freedom on the anomalous concentration and temperature dependencies of the lattice parameter (the so-called Vegard law), which are experimentally observed in the molecular cryosolution N2O–CO2 within the all concentration region of the fcc solid mixtures disordered on the rotation subsystem. The anomalies of the concentration dependencies of the fcc lattice parameter in the solid solution along the isotherms have evident peculiarities of the orientation phase transition in a quadrupole glass (the structure which is characteristic for the majority of molecular cryomixtures composed of diatomic and triatomic molecules) at the presence of essential contributions from dipole-dipole, quadrupole-quadrupole, and dipole-quadrupole electrostatic interactions caused by the specific electronic distribution in the asymmetric rotator N≡N–O (the carbon dioxide molecule O=C=O is a symmetric rotator with only quadrupole momentum).