Magnesium oxide, lattice

An indirect estimate of surface tension may be obtained from the change in lattice parameters of small crystals such as magnesium oxide and sodium chloride owing to surface tensional compression [121] however, these may represent nonequilibrium surface stress rather than surface tension [68]. Surface stresses may produce wrinkling in harder materials [122]. 

Calculations of the interaction energy in very fine pores are based on one or other of the standard expressions for the pair-wise interaction between atoms, already dealt with in Chapter 1. Anderson and Horlock, for example, used the Kirkwood-Miiller formulation in their calculations for argon adsorbed in slit-shaped pores of active magnesium oxide. They found that maximum enhancement of potential occurred in a pore of width 4-4 A, where its numerical value was 3-2kcalmol , as compared with 1-12, 1-0 and 1-07 kcal mol for positions over a cation, an anion and the centre of a lattice ceil, respectively, on a freely exposed (100) surface of magnesium oxide. 

Magnesium oxide is an ionic solid that crystallizes in the sodium chloride type lattice. 

NMR properties, 33 213, 274 in NMR studies of zeolites, 33 254-264 in sheet silicate studies, 33 342-345 -magnesium oxide catalyst, lattice parameter, 35 75... 

Magnesium oxide crystals about 500 A. in diameter were prepared in vacuo by Nicolson 26). Lattice determinations by X-rays showed that the parameter of these small crystals was smaller than that of large crystals. The surface tension obtained from these experiments (- -3,020 dynes/cm.) was 46% of the theoretical value. Similar experiments were carried out with sodium chloride crystals made in vacuo (size about 2000 A), and the agreement between experiment and theory was better, the observed surface tension (- -390 dynes/cm.) being 70% of that calculated. 

Direct experimental determinations of these quantities do not exist. The nearest approach seems to be in some observations made by Nicolson (26) in his work on surface tension. He found that when he made magnesium oxide particles by burning magnesium in air, their lattice constants were the same as those of the bulk material. When the crystals were made by the decomposition of magnesium carbonate in vacuo, the expected change in lattice parameter took place due to the surface tension. These negative results obtained in the first method of preparation were attributed to the presence of gases adsorbed from the air. 

The extinction curves for magnesium oxide particles (Fig. 11.2) and aluminum particles (Fig. 11.4) show the dominance of surface modes. The strong extinction by MgO particles near 0.07 eV( - 17 ju.m) is a surface mode associated with lattice vibrations. Even more striking is the extinction feature in aluminum that dominates the ultraviolet region near 8 eV no corresponding feature exists in the bulk solid. Magnesium oxide and aluminum particles will be treated in more detail, both theoretically and experimentally, in this chapter. 

Stoichiometric titanium(II) oxide and magnesium oxide have densities of 4.93 and 3.58 g cm-3, respectively. They both have the NaCl structure, with unit cells of edge 416.2 and 421.2 pm, respectively. Show that only about 84% of the lattice sites in TiO, but essentially all in MgO, are filled. 

Explain why the lattice energy of magnesium oxide (3850 kj-mol 1) is greater than that of barium oxide (3114 kj-mol 1), given that they have similar arrangements of ions in the crystal lattice. See Appendix 2D. 

Ans. For an oxide lattice, a = 396 pm. With magnesium and calcium ions in the octahedral holes, anion-anion contact is broken and a expands to 410 pm and 478 pm, respectively. 

For ionic compounds, the formula represents a ratio rather than a discrete particle. For example, the formula for magnesium oxide, MgO, signifies that magnesium and oxygen exist in a one-to-one atomic ratio. Recall that MgO exists in a lattice structure held together by ionic bonds, as shown in Figure 3.41. The formula MgO represents the ratio in which ions are present in the compound. 

If there is so much lattice energy to be gained in going from singly charged to doubly charged ions in the case of magnesium oxide, why then does solid... 

Duffy TS, Hemley RJ, Mao HK (1995) Equation of state and shear-strength at multimegabar pressures— magnesium-oxide to 227GPa. Phys Rev Lett 74 1371-1374 Duffy TS, Shen GY, Heinz DL, Shu IF, Ma YZ, Mao HK, Hemley RJ, Singh AK (1999a) Lattice strains in gold and rhenium imder nonhydrostatic compression to 37 GPa. Phys Rev B-Condens Matter 60 15063-15073... 

An excellent example of the attribution of a chemisorption to a specific atomic point defect is afforded by the Harwell work on magnesium oxide and nickel oxide (37, 38). The work was undertaken to discover whether the specific electronic nature of an oxide was the determining factor in chemisorption, or whether more general structural features were important, and the choice of these oxides, isomorphous but different electronically, was dictated by this intention. Likewise, neutron bombardment was chosen in order to emphasize structural defects and determine whether vacancies and interstitials, which would be similar in the two oxides, would lead to similar changes in adsorption, or whether the electronic differences in the host lattices would impose differences in adsorptive behavior. 

The crystal structure of magnesium oxide is illustrated in Figure 37. This is one of the simplest structures and is very similar to that of rocksalt, NaCl. Each Mg atom is surrounded by six O atoms, and to preserve electrical neutrality, each O atom is surrounded in turn by six Mg atoms. The entire lattice can be considered as built up from cubes, with O and Mg atoms at alternate corners this is the only known crystalline form of MgO and is sometimes called periclase. 

---