ɛ-δ phase transition of nitrogen and the orientational behavior of the second-order transition within the δ phase: A Monte Carlo study at 7.0 GPa

Abstract

Link to publication Citation for published version (APA):Mulder, A., Michels, J. P. J., & Schouten, J. A. (1998). epsilon-delta phase transition of nitrogen and the orientational behavior of the second-order transition within the delta phase: A Monte Carlo study at 7.0 GPa. Physical Review B, 57, 7571-7580. DOI: 10.1103/PhysRevB.57.7571 General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Monte Carlo simulations have been performed with an improved nitrogen-nitrogen potential at 7.0 GPa, to study the behavior near the -␦ phase transition, to calculate the vibrational frequencies in the ␦ phase, and to examine the second-order phase transition within the ␦ phase. The -phase structure transformed to the ␦-phase structure at 140 K, whereas the ␦-phase structure is metastable below this temperature. The second-order transition could be detected in several ways. At high temperature the orientational behavior of all molecules is highly dynamical with weakly preferred orientations. At the second-order transition the molecules at the disk sites become localized with different orientations, whereas the orientational behavior of the molecules at the sphere sites does not change. The localization causes the frequency of the disk sites to split with an intensity of 1:2, and influences the frequency of the sphere sites. The frequency behavior calculated with this model system resembles the experimental behavior. Additional calculations revealed that the anisotropic energy term as well as the Coulomb term of the intermolecular potential influence not only the stability of the -phase structure, but also the orientational behavior, and thus the nature, of the second-order phase transition.

Raman spectra and phase behavior of the mixed solidN2−Arat high pressure

“…The break in the frequencies as a function of temperature is caused by a freezing of the orientations of the molecules on the c sites, as in simulations on pure nitrogen. 6 The resemblance with the pure system and the agreement between experiments and computer simulations on the mixture, suggests that a similar transition to the second order transition in pure nitrogen is present in the mixture. A small discontinuity of the Raman frequencies, disregarding a continuous transition, cannot be excluded.…”

“…4 The behavior of the calculated frequencies resembles the experimental behavior. 6 Although, in the model hardly any change in the orientational behavior of the molecules at the a sites has been observed, the calculated vibrational frequency behavior of these sites changes at the second order transition, due to the change in orientational behavior of the twelve molecules at the nearest neighbor ͑disklike disordered͒ c sites. The anomaly in the linewidth of the 1 vibron near the second order transition, as reported by Westerhoff et al, 5 could also be caused by a change in the orientational behavior of the neighboring disklike disordered c sites.…”

“…This second order transition has also been detected using Monte Carlo simulations, 6 by a change in the thermal expansion, the orientational order parameters, and the frequency. In this model the transition is caused by a localization of the orientations at low temperatures in combination with a change of preferential orientation of the disklike disordered c sites.…”

Solid Nitrogen at Extreme Conditions of High Pressure and Temperature

“…Higher P-T data are available from shock-wave CARS study [14], which refer to a liquid state. Solid nitrogen has a wealth of molecular phases that differ in the types of orientational ordering and crystal structures formed [9,10,12,13,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. The ordering of the low pressuretemperature α and γ phases is controlled by quadrupole-quadrupole interactions, whereas at higher pressures a class of molecular structures (δ, δ loc , ε, ζ) stabilized by additional anisotropic intermolecular interactions is found [10].…”

“…With increasing pressure and/or decreasing temperature, nitrogen molecules exhibit orientational order through a sequence of the phase transformations (to δ loc and then ε) as determined by vibrational spectroscopy and x-ray diffraction [27][28][29][30][31][32][33][34][35][36].…”