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The pressure versus temperature phase diagram of the mixed-valence Eu 4 As 3 compound was determined by means of x-ray diffraction measurements using synchrotron radiation. The structural phase transition occurs at 343 K under ambient pressure with volume reduction and the transition temperature increases linearly with pressure up to about 5 GPa. The entropy change at this transition is estimated to be 41 J mol −1 K −1 from the Clausius-Clapeyron relation. The value is twice larger than the configuration entropy which is expected from a random arrangement of Eu 3+ and Eu 2+ ions in the thermally driven valence fluctuating state of Eu 4 As 3 . Moreover, a new structural phase was observed above 20 GPa at room temperature.

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We have carried out high-pressure x-ray diffraction experiments on Eu4As3 at room temperature, and individual atomic coordinates as well as lattice parameters were derived precisely from the diffraction data. We have confirmed that a trigonal structure with the R3c space group does not change up to 19 GPa. The results show that the volume of the EuAs6 octahedron and the valence state of the Eu ion are closely correlated, and pure trivalent and divalent Eu ions occupy two nonequivalent Eu sites, respectively, in the charge-ordered state at ambient pressure. We present clear evidence on the basis of the pressure dependence of these volumes at two nonequivalent Eu sites that a melting of the static charge order occurs at about 10 GPa and Eu4As3 is in a mixed-valence state above 10 GPa.

Cyclopropane derivatives Q 0021Regioselective Ring Opening of Alkylidenecyclopropanone Silyl Acetals. -Alkylidenecyclopropane silyl acetals such as (III) are directly prepared from alkylidenemethyliodonium salts and ketene silyl acetals. The C-C bonds of the cyclopropane ring can be selectively cleaved by different reagents. -(FUJITA*, M.; FUJIWARA, K.; MOURI, H.; KAZEKAMI, Y.; OKUYAMA*, T.; Tetrahedron Lett. 45 (2004) 43, 8023-8026; Grad. Sch. Sci., Himeji Inst. Technol., Hyogo 678-12, Japan; Eng.) -Mais 04-076