Angle-dispersive powder diffraction using an image-plate area detector and. synchrotron radiation have been used in conjunction with first-principles pseudopotential calculations to examine the structural, electronic, and vibrational properties of the recently discovered phase XII of silicon (the R8 phase). The R8 phase is synthesized by decompression of the high-pressure P-Sn phase and exists over a relatively wide pressure range of 2 -12 GPa. Although there are structural similarities between BC8 and A8, the latter phase exhibits substantially greater local deviations from ideal tetrahedral bonding and is the most distorted crystalline structure containing fourfold-coordinated silicon. We present a detailed investigation of the pressure response of the BC8 structure, suggest plausible atomic trajectories for the P-Sn to R8 transition, and we investigate the energy of R8 silicon relative to those of other tetrahedral forms.
Task-based methodology is particularly suited to teaching languages for specific purposes, because of its affinity to behavioural objectives. Doubts have been expressed as to whether learners actually learn language through doing tasks, and if they do, exactly what they learn. This paper reports the preliminary results of an ongoing study of the benefits of building repetition into a communicative task in an English for Specific Purposes course. We compare the performances of two learners at markedly different levels of English proficiency and find that both benefited from the opportunity to recycle communicative content as they repeated complex tasks. This suggests that task repetition of the type reported here may be a useful pedagogic procedure and that the same task can help different learners develop different areas of their interlanguage.
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