168 U Ul lt tr ra ah hi ig gh h- -p pr re es ss su ur re e S Sy yn nc ch hr ro ot tr ro on n R Ra ad di ia at ti io on n 5 57 7 F Fe e- -M Mö ös ss sb ba au ue er r S Sp pe ec ct tr ro os sc co op py y u us si in ng g S Si in ng gl le e- -l li in ne e P Pu ur re e N Nu uc cl le ea ar r B Br ra ag gg g R Re ef fl le ec ct ti io on n Pure nuclear Bragg reflection of a 57 FeBO 3 single crystal at the Néel temperature can select single-line 57 Fe Mössbauer radiation from synchrotron radiation (SR) [1,2]. This fully recoilless single-line is a very attractive probe beam for various applied researches using energy-domain SR Mössbauer spectroscopy because it has high directivity, small beam size, and pure linear polarization. In particular, SR Mössbauer spectroscopy using single-line pure nuclear Bragg reflection combined with a focusing X-ray optics has enabled us to easily achieve micron-scale small-target researches. As a typical example of such experiment, the Mössbauer transmission spectra were observed for polycrystalline iron metal and hematite under multimegabar pressures (> 200 GPa) in a diamond anvil cell (DAC) for the first time [3]. The nuclear diffraction optics for ultrahigh-pressure SR Mössbauer spectroscopy is shown in Fig. 1. The experiments were performed at beamline BL11XU (JAEA). A σ -polarized X-rays with an energy width of 2.5 meV at 14.4 keV nuclear resonance of 57 Fe were produced by a liquid-nitrogen-cooled Si (111) double- crystal pre-monochromator and a high-resolution monochromator (HRM). A bent elliptical multilayer mirror focused the incidence X-rays were horizontally with a focal size of 400 × 36 μ m 2 at the sample position. Downstream of a Doppler vibrating sample mounted on a velocity transducer, the transmission X- rays were ultimately monochromatized into a 15.4 neV bandwidth by the electronically forbidden (333) pure nuclear Bragg reflection of a 57 FeBO 3 single crystal near the Néel temperature (75.8°C) in a 150 Oe external magnetic field [2]. Behind a slit, the nuclear Bragg diffracted X-rays were detected by a NaI(Tl) scintillation detector. The peak photon counting rate of 57 FeBO 3 (333) reflection was 2.4 × 10 3 cps, and the noise level was below 8.0%. In this optics, the Mössbauer spectrum was measured by counting the intensity of single-line nuclear Bragg reflection as a function of velocity. The measurement scheme is shown in Fig. 2. As the first demonstration of ultrahigh pressure SR Mössbauer spectroscopy, a micron-sized polycrystalline iron metal ( 57 Fe 95%) placed in a DAC was measured in the multimegabar range. The specimen and small platinum (Pt) chip were enclosed in a hole of 18 μ m diameter in a rhenium (Re) gasket between the flat parallel faces of two oppositely facing diamond anvils, without a pressure medium. The pressure was estimated at 252 GPa from the X-ray diffraction profile of Pt. The Mössbauer spectrum was observed using a vibrating DAC mounted on a velocity transducer at sample position of Fig. 1. The result is shown in Fig. 3. The statistically sufficient spectrum of a polycrystalline iron metal under the ultrahigh pressure of 252 GPa was obtained in a short measurement time of 2.0 h. It clearly showed a single-line absorption profile, which was a typical Mössbauer spectrum of paramagnetic α -Fe. As for the quadrupole interaction of α -Fe, no significant change was observed in the spectrum. However, the isomer Δ E = 2.5 meV at 14.4 keV BL11XU HRM Si (111) Sample V Si (111) Si (511) × Si (975) X-ray mirror h σ 57 FeBO 3 (333) H ex = 150 Oe Slit NaI detector NAC Fig .1. Optics for the energy domain SR Mössbauer spectroscopy using single-line pure nuclear Bragg scattering. (HRM: high-energy- resolution monochromator; NAC: nuclear analyzer crystal). Instrumentation & Methodology 169 shift showed a marked energy shift (-0.85 mm/s) at 252 GPa. This is attributed to the considerable increase in the s-electron charge density at the 57 Fe nucleus owing to the decrease in atomic volume at 252 GPa. Figure 4 shows the high-pressure Mössbauer spectra of a hematite ( 57 Fe 95%) polycrystal measured in the multimegabar range. The measured spectra show the clear dependence of hyperfine magnetic field and electric quadrupole splitting under pressure. This method will become a powerful tool for high-pressure science and geophysics. Takaya Mitsui SPring-8 / JAEA E-mail: taka@spring8.or.jp References [1] G.V. Smirnov et al. : Phys. Rev. B 55 (1997) 5811. [2] T. Mitsui et al. : Jpn. J. Appl. Phys. 46 (2007) 821. [3] T. Mitsui, M. Seto, N. Hirao, Y. Ohishi, Y. Kobayashi, S. Higashitaniguchi and R. Masuda: Jpn. J. Appl. Phys. 46 (2007) L382. I Doppler energy shift of Mössbauer absorption Single-line nuclear Bragg reflected X-ray E E 0 : Resonance energy of NAC Fig. 2. Conceptual diagram for the energy domain SR Mössbauer spectroscopy with single-line pure nuclear Bragg scattering. 252.0(9) GPa Velocity (mm/s) –10 Relative Transmission –5 0 5 10 0.8 0.9 1 Fig. 3. SR Mössbauer spectrum of a polycrystalline iron metal ( 57 Fe 95%) in DAC at ultrahigh pressure of 252 GPa. The solid line is fit with Lorenzian line. Velocity (mm/s) –10 –5 0 5 10 Relative Transmission 0.7 (a) 0.8 0.9 1 0.8 0.9 1 0.8 0.9 1 0.9 1 0.8 0.9 1 (b) (c) (d) (e) Fig. 4. SR Mössbauer spectra of 57 Fe 2 O 3 at different pressure conditions. (a) 0 GPa, (b) 43 GPa, (c) 91 GPa, (d) 121 GPa and (e) 204 GPa. The solid lines are fits with Lorenzian lines.
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