The temperature of shock compressed iron has been measured to 340 GPa, using well characterized iron films sputtered on transparent diamond substrates and a 1 ns time-resolved optical method. We find a knee on the (P, T) iron Hugoniot indicating melting at 6350 K and 235 GPa and at 6720 K and 300GPa. An extrapolation yields an iron melting temperature of 6830 (+ 500) K at 330 GPa, the pressure of the Earth inner-outer core boundary. Implication of the melting data for the iron phase diagram is also discussed. PACS numbers: 62.50.+p, 64.70.Dv The phase diagram of iron, in addition to being of intrinsic scientific interest, provides a critical constraint for modeling the chemical composition and energy balance of the Earth's core. The Earth's core contains mostly iron distributed in two layers: the solid inner layer of nearly pure iron and the liquid outer layer of iron alloys with lighter elements like S, 0, H, Si, Mg, etc. Thus, the iron melting temperature at the pressure of the Earth innercore and outer-core boundary (IOB), 330 GPa, may provide an upper bound for the temperature. The recrystallization of iron occurring at the boundary releases latent heat and gravitational energy which provide the heat necessary for convection in the outer core and produce Earth's magnetic field [1]. Current Earth core models rely strongly on extrapolations of the melting data of iron from below 100 GPa.Ho~ever, these extrapolations not only give a large uncertainty in the IOB temperature ranging from 4000 to 9000 K [1], but also yield phase diagrams that are qualitatively diA'erent from one another at the IOB conditions [2,3]. For example, the melting temperature reported by Williams and co-workers [2] increases rapidly with pressure, the extrapolation of which results in a e-y-liquid iron triple point at the IOB pressure 330 GPa and 7600 K. On the other hand, Boehler, von Bargen, and Chopelas [3] present the e-y-liquid triple point at the substantially lower pressure of 100 GPa and 2800 K and suggest an IOB temperature near 4200 K. Recently, the situation has become confused even further by the findings of Boehler [4] and Saxena, Shen, and Lazor [5] of a new solid phase of unknown structure in what has been believed to be the stability field of the e phase. Brown and McQueen [6] have observed two discontinuous changes of the iron sound velocity at shock pressures of 200 and 240 GPa, which are attributed to phase transitions of iron and the latter to melting. However, the temperatures were not measured, but were estimated from the shock energy to be 5800~500 K at the IOB pressure.A direct method for obtaining melting temperatures above a megabar and several thousand degrees is by measuring shock temperatures [7]. This is typically done by optical pyrometry, which measures the thermal radiation of shocked materials at several discrete wavelengths. However, di%culties are introduced in the case of nontransparent materials like metals, because of a thin optical penetration depth -20 nm and a short shock wave transit time over ...

Rare-earth transition metal compounds Yb 14 MnSb 11 and Yb 14 MnBi 11 have been prepared by heating stoichiometric amounts of the elements at 1000-1200 °C. These compounds are isostructural with the Zintl compound Ca 14 AlSb 11 and crystallize in the tetragonal space group I4 1 /acd (Z ) 8). Single-crystal X-ray data (143 K) were refined for Yb 14 MnSb 11 [a ) 16.615(2) Å, c ) 21.948(4) Å, V ) 6059(2) Å 3 , and R1/wR2 (0.0299/0.0479)] and Yb 14 MnBi 11 [a ) 17.000(3) Å, c ) 22.259(6) Å, V ) 6433(2) Å 3 , R1/wR2 (0.0631/0.133)]. Structural analysis is consistent with Yb 2+ . Temperature-dependent magnetic susceptibility data show that Yb 14 MnSb 11 orders ferromagnetically at 56 K and Yb 14 MnBi 11 has a ferromagnetic transition at 58 K and another transition at 28 K. High-temperature magnetic susceptibility data can be fit with a modified Curie-Weiss law and give µ eff ) 4.92(2)µ B and µ eff ) 4.9(1)µ B for the Sb and Bi compounds, respectively. This result is consistent with the assignment of Mn 3+ (d 4 ) and Yb 2+ (f 14 ). Single-crystal magnetic data provide additional evidence for the magnetic transitions and show that the compounds are magnetically anisotropic.

The compounds AuMnPnn (A = Ca, Sr, Ba; Pn = As, Sb), have been synthesized by reacting the elements in stoichiometric amounts in welded Nb tubes sealed in quartz ampules at high temperature (1250 °C). Single-crystal X-ray data (130 K, tetragonal, Iijacd (142), Z = 8) were

No abstract

No abstract