The equilibrium phase boundary between single-bonded, threefold-coordinated polymeric forms of nitrogen, and the observed, triple-bonded diatomic phases, is predicted to occur at relatively low (50 + 15 GPa) pressure. This conclusion is based on extensive local-density-functional total-energy calculations for polymeric structures (including that of black phosphorus, and another with all gauche dihedral angles) and diatomic structures (including that of the observed high-pressure e-N2 phase). We believe the diatomic phase of nitrogen, observed up to 180 Gpa and room temperature, to be metastable at these conditions, and that such hysteresis enhances the prospects for the existence of a metastable polymeric form of nitrogen at ambient conditions. In this regard, we show that the black-phosphorus and cubic gauche polymeric forms of nitrogen would encounter signi6cant barriers along high-symmetry paths to dimerization at atmospheric pressure.

We show that strained type II superlattices made of InAs-Ga1−xInxSb x∼0.4 have favorable optical properties for infrared detection. By adjusting the layer thicknesses and the alloy composition, a wide range of wavelengths can be reached. Optical absorption calculations for a case where λc∼10 μm show that near threshold the absorption is as good as for the HgCdTe alloy with the same band gap. The electron effective mass is nearly isotropic and equal to 0.04 m. This effective mass should give favorable electrical properties, such as small diode tunneling currents and good mobilities and diffusion lengths.

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