Pressure measurement, powder diffraction

Table 2.1 shows the crystal structure data of the phases existing in the Mg-H system. Pnre Mg has a hexagonal crystal structure and its hydride has a tetragonal lattice nnit cell (rutile type). The low-pressure MgH is commonly designated as P-MgH in order to differentiate it from its high-pressure polymorph, which will be discussed later. Figure 2.2 shows the crystal structure of p-MgH where the positions of Mg and H atoms are clearly discerned. Precise measurements of the lattice parameters of p-MgH by synchrotron X-ray diffraction yielded a = 0.45180(6) mn and c = 0.30211(4) nm [2]. The powder diffraction file JCPDS 12-0697 lists a = 0.4517 nm and c = 0.30205 nm. The density of MgH is 1.45 g/cm [3]. 

Thermal decomposition of samples has been made by the method of prolong exposure at constant temperature on installation of high gas pressure. Specific surface of the samples has been measured by the BET method with accuracy 10% on devise Autosorb-1 (Quantchrome, USA). X-ray powder diffraction data have been obtained with Cu Ka radiation. Samples treatment under high quasihydrostatic pressures (3 GPa, 800°C, 1 h) has been done in the chamber of a high pressure. 

Dilute fluorine gas (0-20%) can be used to treat zeolites at near-ambient temperature and pressure. Most of the resulting materials retain very high crystallinity even after 600°C postcalcination for two hours. Both framework infrared spectra and X-ray powder diffraction patterns clearly show structural dealumination and stabilization. The hydrophobic nature of the fluorine-treated and 600sC-calcined material is shown by a low water adsorption capacity and selective adsorption of n-butanol from a 1 vol.% n-butanol-water solution. Fluorination also changes the catalytic activity of the zeolite as measured by an n-butane cracking method. 

Currently, techniques for analyzing powder diffraction that involve the analytical dissection of measured Bragg reflection shapes " " can be used to derive a data set similar to that for a single crystal. As a result, crystal structures may be determined, and in some cases, refined anisotropically using powder diffraction data for X rays or neutrons. Powder diffraction methods can be used at a wide variety of temperatures and pressures and are well suited for studies of phase transformations. 

Cook RK (1957) Variation of elastic constants and static strains with hydrostatic pressure a method for calculation from ultrasonic measurements. J Acoust Soc Am 29 445-449 David WIF (1992) Transformations in neutron powder diffraction. Physica B 180 181 567-574 Decker DL (1971) High-pressure equations of state forNaCl, KCl, and CsCl. J ApplPhys 42 3239-3244 Decker DL, Petersen S, Debray D, Lambert M (1979) Pressure-induced ferroelastic phase transition in Pb3(P04)2 A neutron diffraction study. Phys Rev B 19 3552-3555 Eggert JH, Xu L-W, Che R-Z, Chen L-C, Wang J-F (1992) High-pressure refractive index measurements of 4 1 methanol ethanol. J Appl Phys 72 2453-2461... 

Powder neutron-diffraction data for BiF3 show eight-fold co-ordination for bismuth (Bi—F 2.217—2.502 A), but from a comparison with the isostructural YF3 it is clear that the lone pair is stereochemically active and occupies a ninth co-ordination position.778 New vapour-pressure measurements are now available for BiCl3 over the temperature range 151— 437 °C.779... 

Lu2Fe3Si5 is isotypic with the crystal structure of Sc2Fc3Si5 P4/mnc, a = 10.34(1), c = 5.375(8) (Braun, 1980 X-ray powder diffraction data). For sample preparation, see Dy2Fe3Sij. Mossbauer data confirm the absence of a magnetic moment oh the iron site (Braun et al., 1981). Lu2Fe3Si5 is superconducting below 7 = 6.1-5.8 K. The pressure dependence was measured by Segre (1981) as dT dp... 

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