Silicon is ubiquitous in contemporary technology. The most stable form of silicon at ambient conditions takes on the structure of diamond (cF8, d-Si) and is an indirect bandgap semiconductor, which prevents it from being considered as a next-generation platform for semiconductor technologies. Here, we report the formation of a new orthorhombic allotrope of silicon, Si24, using a novel two-step synthesis methodology. First, a Na4Si24 precursor was synthesized at high pressure; second, sodium was removed from the precursor by a thermal 'degassing' process. The Cmcm structure of Si24, which has 24 Si atoms per unit cell (oC24), contains open channels along the crystallographic a-axis that are formed from six- and eight-membered sp(3) silicon rings. This new allotrope possesses a quasidirect bandgap near 1.3 eV. Our combined experimental/theoretical study expands the known allotropy for element fourteen and the unique high-pressure precursor synthesis methodology demonstrates the potential for new materials with desirable properties.

We report on a new method for the synthesis of single-crystal intermetallic clathrates. Alkali-metal is slowly removed from an alkali-silicide precursor by reaction of the vapor phase with spatially separated graphite, in a closed volume under uniaxial pressure, to form single-crystals of the binary intermetallic clathrates Na 8 Si 46 and Na 24 Si 136 . Single-crystal structure refinement for Na 8 Si 46 is reported for the first time. For both Na 8 Si 46 and Na 24 Si 136 , full occupation of all Si framework sites as well as full Na occupancy in both polyhedra was observed. In addition to comprising a simple method for selective, phase-pure crystal growth of clathrates such as Na 8 Si 46 or Na 24 Si 136 which was previously challenging to achieve, this or similar approaches are applicable in the preparation of new compositions from different alkali-metal precursors.

Na(x)Si(136) clathrate-II single crystals with x = 2.9, 5.1, 8.2, and 14.7 were prepared by a two-step process. In the first step, Na(24)Si(136) single crystals were grown from the precursor Na(4)Si(4) by reaction of the vapor phase with spatially separated graphite in a closed volume. In the second step, the Na(24)Si(136) single crystals were subjected to thermal decomposition in a nitrogen atmosphere at 10 Torr and 405 °C. The Na content was controlled by the duration of thermal decomposition. The structural properties were investigated using single-crystal X-ray diffraction and compared with those of single-crystal Na(24)Si(136). The quality of the obtained products also allowed for low-temperature transport property measurements on agglomerates of crystals allowing for an investigation into the low-temperature electrical and thermal properties as a function of Na content.

Articles you may be interested in Large magnetocaloric effects over a wide temperature range in MnCo1−xZnxGe A phenomenological fitting curve for the magnetocaloric effect of materials with a second-order phase transition We observe an enhanced magnetic entropy change (ÀDS M) at cryogenic temperatures (T < 20 K) in Eu 8 Ga 16 Ge 30 clathrate (type-I) nanocrystals prepared by a ball milling method. With reduction in the crystal size to 15 nm, ÀDS M is enhanced at low temperatures, reaching the highest value ($10 J/kg K) at 5 K for a field change of 5 T. For all samples investigated, there is a cross-over temperature ($25 K) in ÀDS M (T) above which ÀDS M decreases with crystal size, opposite to that observed at low temperatures. A careful analysis of the magnetic and magnetocaloric data reveals that as the crystal size decreases the magnetic interaction between Eu 2þ ions on the Eu2 site governing the primary ferromagnetic transition at $35 K becomes gradually weaker, in effect, altering the interaction between Eu 2þ ions occupying the Eu1 and Eu2 sites responsible for the secondary ferromagnetic transition at 15 K. As a result, we have observed a strong change in magnetization and the enhancement of ÀDS M at low temperature. V C 2015 AIP Publishing LLC.