The continuum theory of elasticity has been used for more than a century and has applications in

We describe a detailed study of the aluminum phosphate AlPO-53 in both its as-made and calcined forms. In its as-made state, AlPO-53(A), the material is templated by methylammonium cations and contains occluded water molecules and also hydroxide ions that bridge pairs of aluminum atoms, increasing their coordination number to 5. Solid-state NMR experiments confirm the local environment of the aluminum and phosphorus atoms proposed in a previous structural model from powder X-ray diffraction. 31P NMR shows the presence of four distinct resonances with an intensity ratio of 1:1:2:2, consistent with the expected six crystallographic P sites. 27Al triple-quantum MAS NMR resolves five aluminum peaks, two with NMR parameters characteristic of four-coordinate Al and three of five-coordinate Al. One of these latter signals has a greater intensity than that of the others, consistent with the presence of two overlapping signals from two distinct crystallographic Al sites. First-principles calculations of NMR parameters provide a complete spectral assignment and confirm our interpretation of unresolved spectra. AlPO-53(A) is found to convert easily into a second crystalline phase on moderate heating (upon spinning in the NMR rotor for an extended period, for example), and variable-temperature powder X-ray experiments, together with TGA, suggest that this is a dehydration process yielding a second aluminophosphate, JDF-2. This is confirmed using both 31P and 27Al NMR, with the spectral assignment of JDF-2 supported by first-principles calculations. Calcination of AlPO-53(A) or of the dehydrated material, JDF-2, at 300 °C yields the microporous open-framework material AlPO-53(B), a tetrahedral network with three Al and three P sites, as confirmed by NMR and first-principles calculations. In addition to demonstrating the power of the combined use of NMR, first-principles calculations, and diffraction for detailed structural investigations, we show that the possibility of a reversible dehydration in as-made AlPO-53 and similar systems is an important consideration in structural studies and provides evidence that the published structural model for AlPO-53(A) may be incomplete.

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Simulations of the atomic structure and elastic constants of five zeolites with the NAT-type structure, namely, natrolite, mesolite, scolecite, metanatrolite, and ammonium-exchanged natrolite, using the CVFF force field implemented within Cerius2, are presented. The validity of the simulation method is shown by the excellent agreement between simulated and experimental crystal data, including location of extraframework ions and water molecules, and for natrolite, the only zeolite studied here for which experimental studies of elasticity have been reported, the good agreement between the simulated and experimental elastic constants. For all materials, an off-axis analysis of the elastic constants reveals that the Poisson's ratios ν xy and ν yx become negative when stress is applied at 45° to the crystallographic axes. Further simulations of the elastic behavior of the materials under stress, using the molecular dynamics method of Parrinello and Rahman, reveal that the elastic behavior may be described by a “modified rotating squares” model. Here three-dimensional structural distortions are reduced to a two-dimensional model where square cross-section units of structure both rotate about their hinges and undergo change of dimension:  the balance of these competing processes dictates the resulting Poisson's ratio and is highly dependent upon the direction along which stress is applied. We discuss the effect that various concentrations of extraframework cations and water have on the elastic properties of the NAT-type zeolites.

The Brillouin light-scattering technique was used to investigate the single-crystal elastic properties of two aluminosilicate zeolites, natrolite ͑NAT͒ and analcime ͑ANA͒, at ambient conditions. An inversion of the acoustic velocity data results in the full set of elastic stiffness moduli ͑C ij 's͒ for both materials. From the single-crystal moduli the aggregate adiabatic bulk moduli ͑K s ͒, shear moduli ͑G͒, and Poisson's ratios ͑ ͒ were found to be K s = 48.5͑1.0͒ GPa, G = 31.6͑1.0͒ GPa, and = 0.232͑5͒ for NAT, and K s = 59.8͑1.2͒ GPa, G = 32.1͑1.0͒ GPa, and = 0.272͑5͒ for ANA ͑Voigt-Reuss-Hill averages͒. The bulk and shear moduli of both zeolites are relatively low compared with those of densely packed aluminosilicates, reflecting an open framework structure of ͑Al, SiO 4 ͒ tetrahedra which is easily deformed by bending the Si-O-Al angles. As expected for a less dense crystal, NAT is softer and more compressible than ANA. An evaluation of the directional Young's moduli shows that the compressibility of NAT is nearly uniform along the ͓100͔ and ͓010͔ axes, while ͓001͔ is stiffer, in agreement with previous compression studies. We do not find experimental evidence of negative Poisson's ratios for NAT zeolites as predicted by recent theoretical calculations.

A new type of magnetic nanoparticle was synthesized using mesoporous silica MCM-48 as a template. Magnetite (Fe(3)O(4)) nanocrystals were incorporated onto the MCM-48 silica structure by thermal decomposition of iron(III) acetylacetonate. The particle size of these Fe(3)O(4)-MCM-48 composite particles is around 300 nm with an iron oxide content of ca. 20% w/w. Measurements from SQUID magnetometry suggest that these nanoparticles possess superparamagnetic properties similar to those of Fe(3)O(4) nanoparticles. By coating positively charged polyethylenimine on to the surface, DNA can be bound onto the Fe(3)O(4)-MCM-48 nanoparticles. Transfection studies showed that these PEI-Fe(3)O(4)-MCM-48 particles were highly effective as a transfection reagent, and a 400% increase of transfection efficiency compared with the commercial products was recorded.

Magnetic iron metal-silica and magnetite-silica nanocomposites have been prepared via temperature-programed reduction (TPR) of an iron oxide-SBA-15 (SBA: Santa Barbara Amorphous) composite. TPR of the starting SBA-15 supported Fe(2)O(3) generated Fe(3)O(4) and FeO as stepwise intermediates in the ultimate formation of Fe-SBA-15. The composite materials have been characterized by means of x-ray diffraction, high resolution transmission electron microscopy and SQUID (superconducting quantum interference device) magnetometry. The Fe oxide and metal components form a core, as nanoscale particles, that is entrapped in the SBA-15 pore network. Fe(3)O(4)-SBA-15 and Fe-SBA-15 exhibited superparamagnetic properties with a total magnetization value of 17 emu g(-1). The magnetite-silica composite (at an Fe(3)O(4) loading of 30% w/w) delivered a magnetization that exceeded values reported in the literature or obtained with commercial samples. Due to the high pore volume of the mesoporous template, the magnetite content can be increased to 83% w/w with a further enhancement of magnetization.

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