Raman scattering study of crystal dynamics and order-disorder transitions in alkali hydrosulfides

Raman spectra have been measured for NaSH, CsSH and RbSH over a range of temperatures from 85 to 400°K. The results show that the order-disorder phase transitions in these solids are associated with significant changes in the details of their crystal dynamics. In addition the Raman spectra provide clear evidence for the existence of previously unobserved low-temperature phases in each compound, which apparently involve total ordering of the SH^? ions.     

Mössbauer study of a single crystal of RbFeF4

Mössbauer measurements taken between 1.4 and 300 K on a single crystal of RbFeF₄ show that below the Néel temperature of 133.8 K the hyperfine field is at an angle of 16.5 ± 0.5° to the principal axis of the electric field gradient tensor. The hyperfine field is seen to be along the [001] axis allowing the identification of the principal axis of the EFG with the shortest ligand of the tilted FeF₆ octahedra that are constituents of the crystal structure. At 300 K the ratio of recoil-free fractions (?_⊥/?_:) is measured as 0.96 ± 0.05 but at 77 K the measured ratio is 1.7 ± 0.2.     

Detection of dynamical transitions in hydrogenous materials using transmission measurements with very cold neutrons

We have tested the transmission of very cold neutrons as a method to measure dynamical transitions in hydrogenous materials. Transmitted intensities vs. temperature at 30 Å neutron wavelength were measured for four materials that undergo phase transformations associated with changes in dynamics: ammonium iodide, sodium borohydride, hexamethylbenzene, and dicesium dodecahydro-closo-dodecaborate. In some cases, neutron vibrational spectra above and below the transformation temperatures are compared to the transmission results. The measurements show changes in transmission at or near the transition for all these compounds, reflecting dynamical changes. The results demonstrate that the transmission method is sensitive to motional changes due to a wide range of structural transitions, from first-order to much more subtle order–disorder effects and for both small molecular species and larger molecules. The technique is valuable for rapid (ca. hours) scans of new materials to guide neutron inelastic scattering experiments or to complement the results of other techniques.     

Sodium magnesium amidoborane: the first mixed-metal amidoborane

The first example of a mixed-metal amidoborane Na(2)Mg(NH(2)BH(3))(4) has been successfully synthesized. It forms an ordered arrangement in cation coordinations, i.e., Mg(2+) bonds solely to N(-) and Na(+) coordinates only with BH(3). Compared to ammonia borane and monometallic amidoboranes, Na(2)Mg(NH(2)BH(3))(4) can release 8.4 wt% pure hydrogen with significantly less toxic gases.