Structures and physical properties of epsilon-FeSi-type and CsCl-type RuSi studied by first-principles pseudopotential calculations

An investigation of the relative stability of the two known polymorphs of RuSi, having the epsilon-FeSi and CsCl structures, has been made by first-principles pseudopotential calculations. The resulting cell volumes and fractional coordinates at P = 0 are in good agreement with experiment. Application of high pressure to the epsilon-FeSi phase of RuSi is predicted to produce a structure having almost perfect sevenfold coordination. However, it appears that RuSi having the CsCl-type structure will be the thermodynamically most stable phase for pressures greater than 3.6 GPa. Fitting of the calculated internal energy versus volume to a fourth-order logarithmic equation of state led to values (at T = 0 K) for the bulk modulus, K0, of 202 and 244 GPa for the epsilon-FeSi and CsCl phases, respectively, in excellent agreement with experiment. Band-structure calculations for both phases are also presented.

Constraints on the composition of the Earth's core from ab initio calculations

Knowledge of the composition of the Earth's core is important for understanding its melting point and therefore the temperature at the inner-core boundary and the temperature profile of the core and mantle. In addition, the partitioning of light elements between solid and liquid, as the outer core freezes at the inner-core boundary, is believed to drive compositional convection, which in turn generates the Earth's magnetic field. It is generally accepted that the liquid outer core and the solid inner core consist mainly of iron. The outer core, however, is also thought to contain a significant fraction of light elements, because its density--as deduced from seismological data and other measurements--is 6-10 per cent less than that estimated for pure liquid iron. Similar evidence indicates a smaller but still appreciable fraction of light elements in the inner core. The leading candidates for the light elements present in the core are sulphur, oxygen and silicon. Here we obtain a constraint on core composition derived from ab initio calculation of the chemical potentials of light elements dissolved in solid and liquid iron. We present results for the case of sulphur, which provide strong evidence against the proposal that the outer core is close to being a binary iron-sulphur mixture.

Crystal structure, compressibility and possible phase transitions in \boldvarepsilon-FeSi studied by first-principles pseudopotential calculations.

An investigation of the relative stability of the FeSi structure and of some hypothetical polymorphs of FeSi has been made by first-principles pseudopotential calculations. It has been shown that the observed distortion from ideal sevenfold coordination is essential in stabilizing the FeSi structure relative to one of the CsCl type. Application of high pressure to FeSi is predicted to produce a structure having nearly perfect sevenfold coordination. However, it appears that FeSi having a CsCl-type structure will be the thermodynamically most stable phase for pressures greater than 13 GPa. Fitting of the calculated internal energy vs volume for the FeSi structure to a third-order Birch-Murnaghan equation of state led to values, at T = 0 K, for the bulk modulus, K(0), and for its first derivative with respect to pressure, K(0)', of 227 GPa and 3.9, respectively.

Prostaglandin-induced cervical dilatation prior to intracavitary radiotherapy for carcinoma of the cervix: a pilot study.

The efficacy of 1 mg 16,16-dimethyl-trans-Delta2 prostaglandin E1 (Gemeprost) pessaries in achieving cervical dilatation prior to intracavitary brachytherapy was investigated in 16 post-menopausal women with cervical carcinoma. All had received external beam pelvic radiotherapy in the preceding 6 weeks. Four patients were nulliparous and 12 multiparous (mean parity 1.9). FIGO stages were IB (2), IIA (4), IIB (5), IIIA (1), IIIB (3), IVB (1). The cervical os was assessed before pessary insertion and again at the time of intracavitary insertion. The os was closed in 100% (16/16) of patients before and open in 75% (12/16) of patients after pessary insertion. The maximum size of Hegar dilator passed without mechanical dila-tation was recorded. Mean cervical dilatation was 4.25 H (5.5 H in those with a clinical response). The 12 responding patients had rapid and uncomplicated procedures with no need for additional mechanical dilatation. Both patients in whom attempted mechanical dilatation failed had had previous conization of the cervix. The following mild side-effects were reported: abdominal cramps (43.8%), headache (12.5%) and fever (6.3%). These data support the use of Gemeprost pessaries to achieve cervical dilatation in post-menopausal women undergoing intracavitary brachy-therapy following external beam radiotherapy.