Molecular dynamics studies of melting and solid-state transitions of ammonium nitrate

Abstract

Molecular dynamics simulations are used to calculate the melting point and some aspects of high-temperature solid-state phase transitions of ammonium nitrate (AN). The force field used in the simulations is that developed by Sorescu and Thompson [J. Phys. Chem. A 105, 720 (2001)] to describe the solid-state properties of the low-temperature phase-V AN. Simulations at various temperatures were performed with this force field for a 4 x 4 x 5 supercell of phase-II AN. The melting point of AN was determined from calculations on this supercell with voids introduced in the solid structure to eliminate superheating effects. The melting temperature was determined by calculating the density and the nitrogen-nitrogen radial distribution functions as functions of temperature. The melting point was predicted to be in the range 445 +/- 10 K, in excellent agreement with the experimental value of 442 K. The computed temperature dependences of the density, diffusion, and viscosity coefficient for the liquid are in good agreement with experiment. Structural changes in the perfect crystal at various temperatures were also investigated. The ammonium ions in the phase-II structure are rotationally disordered at 400 K. At higher temperatures, beginning at 530 K, the nitrate ions are essentially rotationally unhindered. The density and radial distribution functions in this temperature range show that the AN solid is superheated. The rotational disorder is qualitatively similar to that observed in the experimental phase-II to phase-I solid-state transition.

Effect of method of crystallization on the IV–III and IV–II polymorphic transitions of ammonium nitrate

“…Some of the earlier papers state that the IV ↔ III transition is influenced by the amount of moisture content present in the crystals [7,22,23]. But in the present study, a few samples were analyzed for their moisture content by Karl Fisher method before doing the DTA analysis.…”

Void-induced dissolution in molecular dynamics simulations of NaCl and water

“…Recent MD studies show that supercells of this size are sufficient to reproduce solid properties. [40][41][42][43] The NPT ensemble simulates physical conditions at which dissolution might be observed. The temperature and pressure were held constant using the Nosé-Hoover algorithm, 44,45 as implemented in the DLគPOLY program.…”

A comparison of methods for melting point calculation using molecular dynamics simulations

“…These methods can be categorized into two groups. The first group, which will be referred to as "direct" methods, include the hysteresis method, 3,4,7,8 the voids method, [9][10][11][12][13][14][15] and solid-liquid interface-based methods. [16][17][18][19][20][21][22] These approaches all involve the direct simulation of the melting process in a dynamical manner.…”