Cation distribution

Adsorption Kinetics. In zeoHte adsorption processes the adsorbates migrate into the zeoHte crystals. First, transport must occur between crystals contained in a compact or peUet, and second, diffusion must occur within the crystals. Diffusion coefficients are measured by various methods, including the measurement of adsorption rates and the deterniination of jump times as derived from nmr results. Factors affecting kinetics and diffusion include channel geometry and dimensions molecular size, shape, and polarity zeoHte cation distribution and charge temperature adsorbate concentration impurity molecules and crystal-surface defects. 

Because of very high dielectric constants k > 20, 000), lead-based relaxor ferroelectrics, Pb(B, B2)02, where B is typically a low valence cation and B2 is a high valence cation, have been iavestigated for multilayer capacitor appHcations. Relaxor ferroelectrics are dielectric materials that display frequency dependent dielectric constant versus temperature behavior near the Curie transition. Dielectric properties result from the compositional disorder ia the B and B2 cation distribution and the associated dipolar and ferroelectric polarization mechanisms. Close control of the processiag conditions is requited for property optimization. Capacitor compositions are often based on lead magnesium niobate (PMN), Pb(Mg2 3Nb2 3)02, and lead ziac niobate (PZN), Pb(Zn 3Nb2 3)03. 

In the case of lithium orthoniobate, Li3Nb04, no meta-stable phase was found that had a rock-salt crystal structure with disordered cation distribution [268]. Nevertheless, solid solutions Li2+xTii-4xNb3x03, where 0 < x < 0.22, have a monoclinic structure at low temperatures and undergo transformation to a disordered NaCl type structure at high temperatures [274]. 

In all cases except Li3Nb04, phases similar to Li4Nb04F and Li4Ta04F were found. Pure binary oxides transform easily to modified structures characterized by ordered cation distribution, while oxyfluorides are more stable and require treatment at a high temperature, for an extended period of time, in order to be transformed into a state of ordered cation distribution. 

Figure 4.12 On-line coupling of the bifur-cation-distributive chip micro mixer to a Perseptive Biosystems Mariner TOF-MS [25],...

Space charge arises because the character of cation distribution differs from that of anion distribution (the signs of Zj are different). The volume charge density depends on the ion distribution,... 

An interesting study of oxidic spinel ferrites of the type CO cNi5/3 xFeSbi/304 was reported [21], where three different Mbssbauer-active probes Fe, Ni and Sb were employed on the same material. The results have been interpreted in terms of the cation distributions over spinel A- and B-lattice sites, magnetic moments and spin structure, and the magnitude of the supertransferred hyperfine... 

Cation distribution in zeolitic structures is one of the key aspects to the understanding of the adsorption mechanisms and selectivities. Many experimental and simulation methods have been used to try to localise the cations. The present work confronts the different analytical methods and gives general distribution trends in accordance with results from the literature. 

In mixed spinels, electron transfer between tetrahedral and octahedral Mn3+ ions is hampered. As the cation distribution is temperature sensitive, preparation of materials for NTC use must optimize the amount of Mn in octahedral sites. 

The spinel family of oxides with composition AB2O4 has the A and B cations distributed in octahedral and tetrahedral sites in a close-packed oxygen structure (Supplementary Material SI). Impurity doping can take place by the addition of a dopant to octahedral or tetrahedral sites. In this, the spinel family of compounds is quite different from the A2B04 perovskite-related phases of the previous section in that both cation sites are similar in size and can take the same cations. 

In reality, very few spinels have exactly the normal or inverse structure, and these are called mixed spinels. The cation distribution between the two sites is a function of a number of parameters, including temperature. This variability is described by an occupation factor, X, which gives the fraction of B3+ cations in tetrahedral... 

Similar methods have been used in other cases as well, and a recent example is an analysis of the cation distribution in the complex oxide, LaSr2 xCaxCu2Ga07. Here site preference enthalpies for La, Sr and Ca have been derived [21]. 

Johnson, D. W. (1992). Base cation distribution and cycling. In D. W. Johnson A. E.Lindberg (Eds.) Atmospheric Deposition and Forest Nutrient Cycling, Springer-Verlag, New York, pp. 275-340. 

Martin BE and Petrie A. Electrical properties of copper-manganese spinal solutions and their cation valence and cation distribution. J. Phys. Chem. Solids 2007 68 2262-2270. 

Besides the 29Si and 27 A1 NMR studies of zeolites mentioned above, other nuclei such as H, 13C, nO, 23Na, 31P, and 51V have been used to study physical chemistry properties such as solid acidity and defect sites in specific catalysts [123,124], 129Xe NMR has also been applied for the characterization of pore sizes, pore shapes, and cation distributions in zeolites [125,126], Finally, less common but also possible is the study of adsorbates with NMR. For instance, the interactions between solid acid surfaces and probe molecules such as pyridine, ammonia, and P(CH3)3 have been investigated by 13C, 15N, and 31P NMR [124], In situ 13C MAS NMR has also been adopted to follow the chemistry of reactants, intermediates, and products on solid catalysts [127,128],... 

The System Calciumhydroxyapatite - Strontiumhydroxyapatite. From a study of the cation distribution over the two cation sublattices in solid solutions of calciumhydroxyapatite and strontiumhydroxy-apatite (65) it was shown that such solid solutions are ideal. 

The crystal structures of the end-members with x = 0 are so-called y-tetrahedral structures , with distorted hexagonal close packed oxide arrays and cations distributed over various tetrahedral sites. In the solid solutions, Li ions are found, by powder neutron diffraction, to occupy partially various tetrahedral and octahedral interstitial sites, which link up to form an essentially three-dimensional conduction pathway. 

Erising, T. and Leflaive, P. (2008) Extraframework cation distributions in X and Y faujasite zeolites a review. Micropor. Mesopor. Mat., 114, 27-63. 

Akamatsu T, Kumazawa M., Aikawa N., and Takei H. (1993). Pressure effect on the divalent cation distribution in nonideal solid solution of forsterite and fayalite. Phys. Chem. Minerals, 19 431-444. 

Dal Negro A., Carbonin S., Molin G. M., Cundari A., and Piccirillo E. M. (1982). Intracrystalline cation distribution in natural clinopyroxenes of tholeiitic, transitional and alkaline basaltic rocks. In Advances in Physical Geochemistry, vol. 1, S. K. Saxena (series ed.), New York Springer-Verlag. 

Dunitz J. D. and Orgel L. E. (1957). Electronic properties of transition metal oxides, II Cation distribution amongst octahedral and tetrahedral sites. J. Phys. Chem. Solids, 3 318-323. 

Papike J. J. and Ross M. (1970). Gedrites Crystal structures and intracrystalline cation distributions. Amer. Mineral, 55 1945-1972. 

Robinson K., Gibbs G. V., Ribbe R H. and Hall M. R. (1973). Cation distribution in three hornblendes. Amer. Jour. Set, 273A 522-535. 

Ni-Co ferrites with the general formula Nii cCoxFe204 were tested for the methylation of pyridine [110]. It was observed that the systems possessing x values >0.5 are selective for 3-picoline formation, whereas the ones with x values 0 and 0.2 give a mixture of 2- and 3-picolines. Pyridine conversion increased with the progressive substitution of Ni ions by Co ions. The cation distribution in the spinel lattice influences their acidic and basic properties, and these factors have been considered as helpful to evaluate the activity of the systems. 

Papike, J., and J. R. Clark (1968). The crystal structure and cation distribution of... 

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