Mössbauer stuties of neptunium fluorozirconate glass

— A new neptunium fluorozirconate glass (NpBasZrnF*) has been prepared and its Mossbauer spectrum was measured at 4.2 K and 77 K. Two resonance lines were observed. One is assigned to the Np ions occupying the normal Np sites in the compound. The other is thought to arise from Np which has replaced some of the Ba ions. The isomer shift of those ions suggests a configuration near 5f 6d . The Np ions exhibit paramagnetic relaxation. The measurement of hyperfine interactions by Mossbauer spectroscopy is a sensitive tool for the determination of electronic properties of actinide materials [1]. In particular, neptunium and its compounds were studied thoroughly. From the isomer shift one deduces the charge state of the neptunium ion and other properties of the chemical bond. Magnetic hyperfine splittings have largely been interpreted in terms of the counterplay between localized and itinerant magnetism which is a unique feature of the light 5f elements. Quadrupolar interactions give information on local site symmetry (crystal structure) and also on the covalency of the chemical bond [2]. In this note we report the successful preparation together with first Mossbauer measurements of a vitreous neptunium compound. In particular isomer shift data will be discussed. Non-crystalline Np materials have previously not been investigated by Mossbauer spectroscopy except for some studies using frozen solutions of Am compounds as sources for the neptunium resonance [3]. A neptunium fluorozirconate glass was prepared on the basis of the phase diagram [4] for U and Th glasses. The pertinent parameters were extrapolated to the Np case. The reaction was performed in a Ni crucible and resulted in a glass which was hardly transparent to light and exhibited a violet to black colour. Its nominal composition is NpBa6ZrI2Fx, x = 60. X-ray measurements proved the compound to be amorphous. Transmission Mossbauer spectra of the 60keV resonance in Np were obtained using a standard electromechanical unit coupled to a fast nuclear counting system [5]. The source was 300 mCi of Am metal [6] deposited onto a Ta foil. Its temperature remained at 4.2 K while the absorber could be brought from 4.2 K to 77 K. Since the glass sample ( ~ 500 mg/cm) contains the Np in rather high dilution and since its Debye temperature is comparatively low, the strong source and the fast counting system are essential to obtain spectra of . sufficient quality at temperatures above 4.2 K. Figure 1 shows the spectrum at 4.2 K. It mainly consists of two resonances : a broad line centred around ( — 4.0 ± 1.0) mm/s and a sharp line located at ( + 41.0 ± 0.2) mm/s. All shifts are given relative to the absorption line of NpAl2 at 77 K ( + 13.8 mm/s vs. Am metal). In figure 2 isomer shifts of some neptunium compounds are depicted [1]. The