Structural Analysis of Molten NaNO₃by Molecular Dynamics Simulation

Abstract

#$"$ MD simulation for molten NaNO 3 has been performed by using the Born-Mayer-Huggins-type potentials. The new structural features of molten NaNO 3 are investigated by several analytical methods. The coordination-number and bond-angle distributions are similar to those of simple molten salts such as NaCl except for the variation caused by the different size of the anion and cation. Na + ions are attracted toward O -ions, and get separated from N + ions by Coulomb interactions. The distribution of the dihedral angle between NO 3 -plannar ionic molecules has also been investigated. $" %$NaNO 3 consists of Na + ions and NO 3 -ionic molecules, where three O -ions in a NO 3 -molecule form an equilateral triangle around the central N + ion with a covalent bond between N + and O -ions. NaNO 3 is a well-known component of explosives. Recently, the molten state of NaNO 3 has also been used for fabricating heat-storage materials for application in solar power generation [1][2][3][4][5]. Adya et al. for the first time investigated the microscopic atomic configuration and diffusion in molten NaNO 3 by means of X-ray diffraction measurement and molecular dynamics (MD) simulation, and compared its partial structure factors and partial pair distribution functions with those of molten NaNO 2 and the eutectic mixture of the NaNO 3 -NaNO 2 system showed good additivity for the intermolecular structure because the ionic radius of NO 3 and NO 2 ions are similar owing to the presence of a lone pair on the nitrite ion [6]. After Adya et al. [6], several researchers have reported the MD simulation results of molten alkali nitrates [7][8][9][10][11][12]. For example, Kato et al. have reported cation dependence of the detailed structural features and ionic dynamics in molten alkali nitrates [11]. They have pointed out that the diffusion constant of small cation is larger than that of large anion. In this study, we have performed MD simulations for molten NaNO 3 , and obtained new structural information by using several analytical methods.

Effect of molten sodium nitrate on the decomposition pathways of hydrated magnesium hydroxycarbonate to magnesium oxide probed byin situtotal scattering

“…2a) . 45 No correlations above 5 Å were present due to the lack of longrange order in molten salts. 46,47 Structural evolution of HM and HM-20NaNO 3 .…”

Probing the Local Structure of Na in NaNO3-Promoted, MgO-Based CO2 Sorbents via X-ray Absorption Spectroscopy

“…Although our simplified analysis does not consider the dynamics of the local Na structures, these observations are qualitatively in line with previous total scattering and molecular dynamics simulations studies of molten NaNO 3 structures, which show that the most intense interatomic correlations of molten NaNO 3 are below ca. 8 Å (i.e., local ordering), while there is no medium and long-range order. ,, These studies have shown that the first Na–O distance in molten NaNO 3 is positioned at 2.5–2.7 Å compared with ca. 2.4 Å in crystalline NaNO 3 , ,, and a coordination number of 5.6 was reported, compared to 6 in crystalline NaNO 3 …”

“…8 Å (i.e., local ordering), while there is no medium and long-range order. 18,25,48 These studies have shown that the first Na−O distance in molten NaNO 3 is positioned at 2.5−2.7 Å compared with ca. 2.4 Å in crystalline NaNO 3 , 18,25,48 and a coordination number of 5.6 was reported, compared to 6 in crystalline NaNO 3 .…”

“…18,25,48 These studies have shown that the first Na−O distance in molten NaNO 3 is positioned at 2.5−2.7 Å compared with ca. 2.4 Å in crystalline NaNO 3 , 18,25,48 and a coordination number of 5.6 was reported, compared to 6 in crystalline NaNO 3 . 25 Next, the sorbent was exposed to 0.4 bar of CO 2 to carbonate the sample (III in Figure 3d) for about 60 min.…”

“…This lack of knowledge is because the promoter is molten under reaction conditions, and only a few experimental techniques can probe the local structure of the molten salt under reactive conditions. Previous related studies have investigated the structure of (bare) molten alkaline nitrates via X-ray total scattering in combination with molecular dynamics simulations. ,, However, due to the presence of multiple phases in the CO 2 sorbents under reaction conditions (i.e., MgO, MgCO 3 , and the molten nitrate), X-ray total scattering studies are not well-suited to study the local structure of molten promoters such as NaNO 3 under CO 2 capture conditions. In view of these challenges, element-selective techniques such as X-ray absorption spectroscopy (XAS) are highly suitable to probe the local structure around a specific element (such as Na) in multiphase systems.…”