Work directed toward understanding the high pressure properties of molecular solids and molecular surfaces deposited on hetrogeneous substrates is reported. The motivation, apart from expanding our basic knowledge about these systems, was to understand and predict the properties of new materials synthesized at high pressure, including pressure induced metallic and superconducting states. As a consequence, information about the states of matter of the Jovian planets and their satellites, which are natural high pressure laboratories was also provided. The work on molecular surfaces and finite two and three dimensional clusters of atoms and molecules was connected with the composition and behavior of planetary atmospheres and on the processes involved in forming surface layers, which is vital to the development of composite materials and microcircuitry

Etters, R. D.

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Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19850008340 2020-03-20T20:27:22+00:00Z
Final Report on NASA grant no. NAs 2-171
Richard D. Etters
Physics Department
Colorado State University
Fort Collins, Colorado 80523
(BiASA-CH-174262) [UNDERSTANDING ThE HIGH
	
Ne5-1(649
PRESSURE EROPERTIIS CI MCLECUII6 SOLIDS AND
BULECOLAR SURFACES DEFGSITFD Ch HETROGENECUS
SUBSTRATES] Final Report (Ccictado State
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Univ.) 6 p HC AO2/hF A01	 CSCL 20L G3/76 13372
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1Abstract
The work associated with NASA grant no. NAG 2-171 has been airected
toward understanding thJJ igh pressure properties of molecular solids
r
and molecular surfaces de}osited on htteroSenepuS substrates. f The moti-
vation for this work, apart fror: expanding our basic knowledge about
these systems, has been to understand and predict the properties of
new materials synthesized at high pressure, including pressure induced
metallic and superconducting states. As a consequence, it has been
our intention to also provide information about the states of matter
of the Jovi<<- planets and their satellites, which are natural high
pressure laboratories.
The work on molecular surfaces and finite two and three dimensional
clusters of atoms and ualecules is connected with the composition and
behavior of planetary atmospheres and on the processes involved in form-
ing surface layers, which is vital to the development of composite
materials and microcircuitry.
2Progress Report:
During the period of this grant eight articles were published in
international refereed journals. Copies of these articles are included
in this final report and in this section a brief summary of the major
accomplishments achieved in those works will be presented.
A. "Prediction of structures and magnetic orientations in Solid a
and 5-02 ," Phys. Rev. B 28, 2166 (1983); "Ground State Structural and
Magnetic Properties of Solid a and 5--0 2 ," Int. J. Quantum Chem. 17,
491 (1983).
In this work we established that the Heisenberg magnetic inter-
action is responsible for stabilizing the a phase over 5-0 2 at low
temperatures and pressures, as had earlier been speculated but hereto-
fore unproven. The predicted zero pressure sublimation energy,, 1
lattice parameters, 2 and molar volume  were found to be ir good agree-
ment with experiment as was the second virial coefficients. 3 On the
basis of energetic and structural results it was concluded that the
quasihelical structure with long range .,rder is the most likely magnetic
arrangement for S-0 9 . Recent neutron scattering experiments  support
this conclusion. Also calculated were the zero pressure bond lengths,
libron frequencies and the dispersion curves.
B. "Pressure dependence of intramolecular mode frequencies in solid
N2, 02, and C01," Phys. Rev. B 2 7, 6439 (1983); "Pressure dependence
of intramolecular vibrational mode frequencies in simple molecular
crystals," Proceedings of the eighth symposium on Them-agi ►ysical
Properties, edited by J. V. Sengers, Vol. II, the American Society
of Mechanical Engineers, N.B.S., Gaithersburg. MD, June 15-18 (1981).
In this work a simple classical perturbation theory was formulated
to calculate the frequency change with pressure of the internal vibra-
tional modes (vibrons) in solids composed of linear molecules. It was
J. 	I
3
argued that the pressure dependent crystal field of the solid, embodied
in the total intermolecular potential energy per molecule, slig:ltly
perturbs the intramolecular potential of the molecule from its gas
phase value, changing the inteamolecular force constants and thus the
vibron frequency. It was found that the pressure dependence of the
vibron frequencies is sensitive to the short ranged, repulsive part
of the intermolecular potential and thus provides a sensitive test of
the insitu intermolecular pair potential. It was also found that
intermolecular potentials that best reproduce existing pressure-volume
relations also give the best predictions for the pressure dependence
of the vibron frequencies, which were found t^ be in good Agreement
with experimental data, mostly determined from diamond cell Raman
and Infrared absorption measurements. 5
 Results were shown for CO2,
02 , and N2.
C. "Calculation of high pressure phase transitions in solid N2 and
the pressure dependence of inteamolecular mode frequencies,"
Phys. Rev. B 28, 1095 (1983); "Prediction of a new phase in solid
N2
 at high pressure," Phys. Rev. B 26, 5996 (1982).
In our first paper on the high pressure properties of solid N2
at low temperature we proposed the existence of a phase transiticn
from the y phase into an R_m structure, similar to 6-02 , at P = 40 kbar.
This was determined by comparing the free energy of these two structures.
Subsequent Raman scattering measurements  showed that there is in fact
a lower energy structure for which a phase transition occurs at P =
20 kbar. The existence of two vibron branches in this new phase showed
that the structure is neither Ram or Y-N 2 . In fact, the two branches
gave an indication that the structure was similar to the cubic Pm3n
phase with 8 molecules per unit cell, known to exist at high pressures
4near room temperature. 6
 However, the observed phonon peaks indicated
that the structure was not cubic. Consequently, we considered a
rhombohedral distortion of the Pm3n structure and minimized the lattice
energy with respect to the distorted lattice parameters and the posi-
tions and orientations of the molecules in the unit cell. The results
showed that this rhombohedral structure is preferred over the Y-N2
structure above 19.2 kbar, in good agreement with experiment. 2 The
calculated structure is in fact Ric, which has recently been confirmed
by recent x-ray diffraction measurements. 7
 We also calculated the
pressure dependence of the two vibron branches, which were in qualita-
tive agreement with experiment, but the calculated frequencies were
considerably higher than experiment at high pressures. Since the cal-
culated pressure-volume relation was also higher than experiment,8
this lack of quantitative agreement was attributed to the repulsive
part of the intermolecular pair potential used, which we believe is
too stiff. Nevertheless, the overall agreement with experiment is
quite satisfactory and establishes the character of this low temperature,
high pressure phase of N2.
D. "Lattice dynamics cf solid 12 under pressure," J. Chem. Phys.
79, 3018 (1983).
The structure, phonon, libron, and vibron frequencies of solid
I2
 at low temperature under pressures 0 < P < 210 kbar has been experi-
mentally determined. 9
 These results are exciting in that it is the
first clear evidence of a pressure induced transition from a molecular
insulator to an atomic conductor, which takes place at 210 kbar. Such
transitions are predicted in all diatomic insulators at sufficiently
high pressure. Calculations are difficult because it is well known
5that pressure induced charge rearrangement in the molecule is signifi-
cant, so that the determination of the effective intermolecular pair
potential in the solid is very complicated, especially since it is
quite different from that determined from gas phase properties. In
this work we developed a multi-parameter potential, including charge
transfer terms, and fitted these parameters so that a best fit to
experimental structural and dynamic data could be obtained at all pres-
sures. The results are in good agreement with experiment but they are
not completely satisfying because of the empirical nature if the poten-
tial.
E. "Predictions for partial and monolayer coverages of 02 on graphite,"
J. Chem. Phys. 77, 1035 (1982).
In this work the equilibrium structures and the energy per molecule
were calculated as a function of surface density, using a pattern recog-
nition optimization scheme. It was found that the lowest energy con-
figuration forms a centered rectangular structure with all molecular
axes collinear and lying in the plane of the substrate. This result
was contrary to previous neutron scattering interpretations 10 but
subsequent measurements 11 proved it to be correct. At higher surface
densities another minimum in the energy occurs in which the lattice is
triangular and the molecular axes are all collinear and point normal
to the substrate plane. These results are consistent with the observed
e and 9 phase, found at higher surface densities. Thus, theory and
experiment are in good qualitative and quantitative agreement.
6Bibliography:
1. M. P. Orlova, Sov. J. Phys. Chem. 12, 1603 (1966); W. F. Giaugne
and H. L. Johnston, J. Am. Chem. Soc. 51, 2300 (1929).
2. I. N. Krupskii, A. I. Prokhvatilov, Y. A. Freiman, and A. I.
Erenburg, Fiz. Nizk. Temp. 5, 271 (1979) [Sov. J. Low Temp. Phys.
5, 130 (1979)].
3. J. H. Dymond and E. B. Smith, The Viri.al Coefficient of Gases
(Clarendon, Oxford, 1969).
4. P. W. F:ephens, R. J. Birgeneau, C. F. Maikrzak, and G. Shirane,
Phys. Rev. B 28, 452 (1983); R. J. Meier and R. B. Helmholdt,
ibid. (in press).
5. R. C. Hanson and K. Bachman, Chem. Phys. Lett. 73, 338 (1980);
R. C. Hanson and L. H. Jones, J. Chem. Phys. 75 1102 (1981).
M. Nicol, K. R. Hirsch, an,i W. B. Holzapfel, Chem. Phys. Lett.
68, 49 (1979); M. Nicol, in Proceedings of the 8th International
High Pressure Conference, AIRAPT, Uppsala, Sweden, 1981, edited
by T. Johannisson and L. Tegner (in press), p. 527.
R. LeSar, S. A. Ekberg, L. H. Jones, R. L. Mills, L. A. Schwalbe,
and D. Schiferl, Solid State Commun. 32, 131 (1979); D. T. Cromer,
R. L. Mills, D. Schiferl, and L. A. Schwalbe, Acta. Crystallogr.
B 37, 8 (1981).
I. F. Silvera and R. J. Wijngaarden, Phys. Rev. Lett. 47, 39 (1981).
S. K. Sharma, H. K. Mao, and P. M. Fell, Phys. Rev. Lett. 44, 886
(1980).
A. Coulon and M. Jean-Louis, in High Pressure Science and Technology,
edited by B. Vodar and Ph. Marteau (Pergamon, New York, 1980), Vol. 2,
p. 598.
K. Takemura, S. Minomura, 0. Shimomura, and Y. Fujii, Phys. Rev.
Lett. 45, 1881 (1980).
6. R. LeSar, S. A. Ekberg, L. H. Jones, R. L. Mills, L. A. Schwalbe,
and D. Shiferl, Solid State Commun. 32, 131 (1979); D. T. Cromer,
R. L. Mills, D. Schiferl, and L. A. Schwalbe, Acta Crystallogr.
B37, 8 (1981).
7. B. Olinger (private communication).
8. C. A. Swenson, J. Chem. Phys. 23, 1963 (1955).
T. A. Scott, Phys. Rep. 27, 89 (1976).
B. Olinger (private communication).
7Bibliography (continued)
9. K. Takemura, Y. Fujii, S. Minomura, and 0. Shimomura, Solid State
Commun. 30, 137 (1979).
K. Takemura, S. Minomura, 0. Shimomurc, and Y. Fujii, Phys. Rev.
Lett. 45, 1881 (1980).
K. Syassen, K. Takemura, H. Tups, and A. Otto, Physics of Solids
Under High Pressure, edited by J. S. Schilling and R. N. Shelton
(North-Holland, Amsterdam, 1981), p. 125.
K. Takemura, S. Minomura, 0. Shimomura, Y. Fujii, and J. D. Axe,
Phys. Rev. B 26, 998 (1982).
K. Takemura, Ph.D. thesis, University of Tokyo, 1979.
10. M. Nielsen and J. P. McTague, Phys. Rev. B 19, 3096 (1979).
11. P. W. Stephens, P. A. Heiney, R. J. Birgeneau, P. M. Horn, J.
Stoltenberg, and 0. E. Vilches, Phys. Rev. Lett. 45, 1959 (1980).
0


Understanding the high pressure properties of molecular solids and molecular surfaces deposited on hetrogeneous substrates

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