Structural transition of hexagonal tube to rocksalt for (MgO)3n, 2≤n≤10

Abstract

The structures of (MgO)(3n) (2<or=n<or=10) clusters are studied using density functional theory (DFT). The starting structures are generated from empirical genetic algorithm simulations. The lowest-energy structures of (MgO)(3n) are then obtained from a number of structural isomers by using DFT optimization. It is found that when n<or=5 hexagonal tube is the most stable structure, and when n>or=6 (except 7) the rocksaltlike structure is favored, which is the same as that of the bulk. The n=7 is an interesting case, where the structure again is the hexagonal tube as the most stable structure. However, from the second order difference of the average atomization energy, we find that the n=7 case is thermodynamically unstable with respect to disproportionation to the smaller and larger clusters. The result may be the reason that it is not observed in the experiment. Therefore, we can conclude that the geometry transition really takes place at n=6. The rocksalt is the most stable structure for a large range of n numbers, from the (MgO)(3x6) cluster to bulk magnesium oxide. The result is different from Wilson's previous prediction because of the use of the ionic potential.

Synthesis Target Structures for Alkaline Earth Oxide Clusters

“…As a check that using only the top twenty was sufficient, we expanded our data mined set to 58 different initial configurations for one of the largest sized systems, n = 17, which for barium oxide also had a large density of LM near its GM. Furthermore, as the largest disparity is expected to be found when comparing (MgO) n and (BaO) n clusters (since they have the largest ratio of cationic radii), we also checked that the GM and LM previously reported for (MgO) n [11][12][13][14][15][16][17]20,21,24,25] were not missing from our results. Four of these previous studies [12,16,17,21] used genetic algorithms to predict their tentative GM.…”

“…In the case of n = 15, again all except magnesium oxide adopt the rocksalt cut structure, with (MgO) 15 forming a long hexagonal barrel. In n = 17 clusters, a structure that is partially composed of a 8-membered ring barrel (17-ACXX) is found to be the GM for (MgO) 17 and also the third-lowest energy structure of the calcium oxide cluster.…”

“…The local minimum of ∆E at n = 16 rather than 15 is perhaps more to do with the relatively low stability of n = 17, a size where no perfect barrel nor cuboid cut is possible, and the perfect bubbles of this size have low symmetry. Note that the GM for (MgO) 17 is a defective (along one edge) 3 × 3 × 3 bulk cut, whereas the GM for (MgO) 14 is a defective (along one of the shortest edges) 2 × 3 × 4 bulk cut.…”

“…Thus, we expect that clusters of (MgO) 10 , (MgO) 13 and (MgO) 17 are less likely (anticipate a smaller peak in mass spectra for each of these sizes). These sizes match the positive (disfavourable) second-order energies, whereas we find a local minimum in these at n = 9, 12 and 16.…”

“…Comparison with previous computational results for magnesium oxide and calcium oxide clusters [10][11][12][13][14][15][16][17][18][19], as well as a comparison of theory with infrared data [20,21], will also be discussed below. A comparison with results for alkali halides is also offered.…”