Mössbauer spectra of Nd2Fe14B and related compounds

Mössbauer spectra and ac susceptibility data on Nd₂Fe_{1 4} B with substitution of Pr or Sm for Nd indicate that the spinreorientation temperature is depressed by Pr substitution but increased in alloys containing Sm so that it occurs at roomtemperature for Nd_{1 2}Sm₅Fe_{7 6}B₇. In the Nd end-member, the onset of the transition appears about 210 K although the main reorientation is at 134 K. Nickel shows no marked site preference: its effects are to increase slightly the Curie temperature and reduce the iron moment.     

57Fe Mössbauer Spectroscopy Studies of Meteorites: Implications for Weathering Rates,Meteorite Flux,and Early Solar System Processes

Ordinary chondrite finds, terrestrial age dated using ¹⁴C analyses, from different meteorite accumulation sites, have been examined by Mössbauer spectroscopy to quantitatively determine terrestrial oxidation. We observe differences in weathering rates between sites, and also between different chondrite groups. A comparison of weathering over time, and its effect in eroding meteorites, together with the number and mass distribution of meteorites in each region, enables us to derive estimates of the number of meteorite falls over a given mass per year. Studies of how the oxygen isotopic composition of samples varies with weathering indicate that incipient alteration may occur without a pronounced isotopic effect, possibly due to weathering of silicates to topotactically oriented smectite confined spaces where the water volume is limited. This finding has profound implications for the use of oxygen isotopes as a tool in understanding water–rock interaction. It also may reconcile previously contradictory data regarding the nebular or asteroidal location of pre-terrestrial aqueous alteration. Finally, Mössbauer spectroscopy is also found to be a useful tool in determining mineral abundance in carbonaceous chondrites, where a fine-grained matrix makes traditional approaches inapplicable. Again, the results have implications for the modification of chondritic materials in the early solar system.     

Generation and trapping of monomeric metaphosphoric acid esters in solution: Mechanistic investigation into the fragmentation of mixed carbonic-phosphoric acid anhydrides and the chelotropic breakdown of cyclic pyrocarbonate esters

The generation of alkyl-substituted monomeric metaphosphoric acid esters (ROPO₂) 4 in solution is described, utilizing two different methods based on the condensation of alkyl phosphorodichloridates either with anhydrous potassium hydrogen carbonate or with the novel disodium pyrocarbonate salt 5, which is formed in quantitative yield from diethyl pyrocarbonate by treatment with two equivalents of sodium trimethylsilanolate in THF at 0°C. The first method is believed to proceed via a transient mixed carbonic-phosphoric anhydride 3, which decomposes at room temperature with release of 1 mol each of hydrogen chloride and carbon dioxide to produce 4. In the other method, ³¹P NMR spectroscopy indicated the involvement of a cyclic pyrocarbonate phosphate (2-alkoxy-1,3,5-trioxaphosphorinane-4,6-dione-2-oxide) 6, which decomposed in situ with the release of 2 mol of carbon dioxide. In both cases, the metaphosphate thus formed spontaneously self-condensed to produce polymeric species with P-O-P bonds having characteristic ³¹P NMR signals clustered in the δ -12 and -24 regions. When metaphosphate 4 is generated by either process in the presence of styrene oxide, polymerization is avoided, and trapping occurs with the exclusive formation of a diastereomeric mixture of 2-alkoxy-1,3,2-dioxaphospholane-2-oxides 7 (cis, trans). In order to shed some mechanistic light on the formation of 7, ³¹P NMR studies have been carried out on the individual trapping reactions between (—)-methyl metaphosphate 4d and racemic styrene oxide, as well as its optically pure forms. Based on evidence that these transformations proceed by inversion of configuration at the phenyl-bearing carbon of styrene oxide, a mechanism is invoked whereby the dipolar metaphosphate 4d reacts both as an electrophile and as a nucleophile in a self-catalyzed process via the activated complex 9d. © 1996 John Wiley & Sons, Inc.     

166Er and 170Yb Mössbauer Studies of Magnetic Order and Valence

We present three case studies of magnetic ordering in Er and Yb intermetallic compounds (ErFe₆Sn₆, Er₃Ge₄ and YbMn₂Si_2?x Ge _x ). Mössbauer measurements of electric field gradients and hyperfine fields at the rare-earth sites provide direct, local information which yields insight beyond that which could be obtained using neutron diffraction. We argue that such Mössbauer work is an ideal complement to bulk studies.     

An Overview of 166Er, 169Tm and 170Yb Mössbauer Spectroscopy

The hyperfine splittings of the nuclear energy levels in rare-earth (R) isotopes are sensitive measures of the complex interplay between magnetic exchange and electrostatic crystal-field interactions operating at the atomic level. Mössbauer spectroscopy has been used to great effect in the on-going investigation of these fundamental interactions in R compounds and in this paper we present an overview of ¹⁶⁶Er, ¹⁶⁹Tm and ¹⁷⁰Yb Mössbauer spectroscopy. In particular, we derive expressions for the nuclear energy level splittings incorporating both magnetic and electric quadrupole interactions, using second-order perturbation theory. Such expressions provide a useful means of fitting experimental spectra and also yield criteria for determining whether or not a proposed set of energy level values is physical or not. We also present a number of useful rules of thumb for the analysis of ¹⁶⁶Er, ¹⁶⁹Tm and ¹⁷⁰Yb Mössbauer spectra and, as such, this paper is in effect a Resource Letter.