X‐ray diffraction studies have been carried out on carbon monoxide in the temperature range of 8 to 63 °K. The analysis of the Bragg reflection intensity has shown that the α‐CO structure belongs to the space group Pa3. From the data of the temperature dependence of the lattice parameter the molar volume thermal expansion coefficient has been calculated and the heat capacity Cv and Grüneisen coefficient γ were found. The heat capacity component Cvlib due to angular vibrations of the molecules has been separated. The temperature dependence found for Cvlib and γ is in good agreement with that calculated in the molecular field approximation for a libron subsystem of the crystal, where anharmonic librations and lattice thermal expansion are taken into account.

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The thermal expansion coefficients of pure fullerite are determined on the basis of powder x-ray studies in the temperature range 30–293 K. The obtained results are in good agreement with dilatometric and neutron-scattering data. The data on thermal expansion are used to analyze the heat capacity at constant volume CV. The intramolecular component of CV is analyzed consistently and accurately taking into account the complete set of temperature-dependent intramolecular eigenfrequencies. The rotational component of heat capacity is obtained by subtracting the intramolecular and phonon contributions from the total CV. The phonon component is evaluated on the basis of the Debye model using the Debye temperature (ΘD(0)=55.4 K) calculated from the known sound velocities. The general and partial Grüneisen parameters are calculated as functions of temperature. The results obtained for the high-temperature phase indicate that rotations of C60 molecules are strongly hindered and intercorrelated.

Powder x-ray diffractometry was used to study the effect of intercalating C60 fullerite with helium and argon atoms on the fullerite structure. The samples were saturated at room temperature and normal pressure. The dependences obtained for the lattice parameter and half-width of certain reflections on the intercalation time with helium atoms showed that the voids in the C60 lattice were filled in two stages. Helium filled the octahedral voids relatively rapidly first and then the tetrahedral subsystem, but much more slowly. Both intercalants affected the half-width and intensity of the reflections, the matrix lattice parameters, the phase transition temperature, the volume jump at the transition, and the thermal expansion coefficients of C60 fullerite.

The issue concerning the nature and the role of microstructural inhomogeneities in iron chalcogenide superconducting crystals of FeTe0.65Se0.35 and their correlation with transport properties of this system was addressed. The presented data demonstrate that chemical disorder originating from the kinetics of the crystal growth process significantly influences the superconducting properties of an Fe–Te–Se system. Transport measurements of the transition temperature and critical current density performed for microscopic bridges allow us to deduce the local properties of a superconductor with microstructural inhomogeneities, and significant differences were noted. The variances observed in the local properties were explained as a consequence of weak superconducting links existing in the studied crystals. The results confirm that the inhomogeneous spatial distribution of ions and small hexagonal symmetry nanoscale regions with nanoscale phase separation also seem to enhance the superconductivity in this system with respect to the values of the critical current density. Magnetic measurements performed in order to determine, in an alternative way, the values of the critical current density, as well as to find the relaxation rate and to check the scaling of the pinning force, confirm the conclusions drawn from the transport measurements.