Structural study of N2 - CH4 alloys. Cluster model
N.S. Mysko-Krutik, A.A. Solodovnik, M.A. Strzhemechny
B.Verkin Institute for Low Temperature Physics and Engineering of NAS of Ukraine, Prospekt
Nauky, 47, Kharkiv 61103, Ukraine
e-mail: [email protected]
There are many reasons for scientific interest in CH4 – N2 alloys. Primary components
alloy, solid nitrogen and methane are of fundamental substances for solving problems of the
solid state [1]. Moreover methane is known to occur in the planetary environments [2]. Solid
N2 are formed by the linear and CH4 - tetrahedral molecules. Both the crystal have cubic (fcc)
lattice at the equilibrium vapor pressure, symmetry of α – N2 corresponds to the Pa3 space
group [3] and methane is described Fm3c [4] below 20,4 K. The nitrogen – methane solid
phase diagram has been determined using X-ray diffraction method in studies [5,6]. There are
large discrepancies between results of these structural works. According to [5] a phase
separation in CH4 – N2 system was not observed and solutions are cubic at all temperatures
below α- β transitions of α – N2 at all concentrations. While authors [6] appointed that the
mutual solubility of the components is practically absent in low temperature range. For
explanation of the concentration dependence of the lattice parameter in CH4 – N2 alloys the
cluster model was used. This semiquantitative theory takes into account not only isotropic
interaction between particles, but also the presence of anisotropic forces in the molecular
matrices. In recent study on the CO2 – Kr solutions the theoretical analysis [7] considering
three mechanisms of interaction of Kr cluster with crystalline environment of CO2 matrix was
in a good agreement with the experiment.
Structure of solid CH4 – N2 mixtures was studied with the transmission electron
diffraction techniques equipped with a helium cryostat. The samples were grown “in situ” by
condensation gaseous mixtures on Al or C substrate at T = 20 and T = 5.5 K. The deposition
regime was chosen in order to obtain random distributions of impurity. The error in the lattice
parameter measurements was usually 0,1 %.
Based on analysis of the obtained diffraction patterns and the concentration
dependences of the lattice parameter and diffraction intensity the region of existence of solid
solutions was determined. The limiting solubility of CH4 in the nitrogen matrix approaches 10
% mol. The phase separation of the solutions is studied. The effect of dilution CH4 in N2
molecular crystal matrix on the orientation order has been investigated. At low enough
impurity fractions obtained data were analyzed with using cluster model.
[1] The Physics of Cryocrystal, V.G.Mangelii, Yu.A.Freiman, M.L.Klein,
A.A.Maradudin (eds.), AIP Press, Woodbury (1997)
[2] New Horizons Update: Methane Detected; New Images of Pluto and Charon;
Sunrise/Sunset Observations, Lillian Gipson(NASA), RIA NEWS, July 31, 2015
[3] Ruheman M. Z. Phys., 76 (3), (1932)
[4] W. Press S. Chem. Phys. 56, 2597 (1972).
[5] Prokhvatilov A. I.and Yantsevich L. D. Fiz.Nizk.Temp. 9, 185 (1983).
[6] D. Connolley, T. Milbrodt, and R. Stoner J. Chem. Phys. 73(10) (1980).
[7] A.A. Solodovnik, V.V.Danchuk, N.S.Mysko, Fiz.Nizk.Temp. 39, 586 (2013).