thermodynamically stable allotropes at standard pressure, but several other crystal structures are known at higher pressures. The general trend is one of increasing coordination number with pressure, culminating in a hexagonal close-packed allotrope at about 40 gigapascals known as Si–VII (the standard modification being Si–I). An allotrope called BC8 (or bc8), having a body-centred cubic lattice with eight atoms per primitive unit cell (space group 206), can be created at high pressure and remains metastable at low pressure. Its properties have been studied in detail. Silicon boils at 3265 °C: this, while high, is still lower than the temperature at which its lighter congener carbon sublimes (3642 °C) and silicon similarly has a lower heat of vaporisation than carbon, consistent with the fact that the Si–Si bond is weaker than the C–C bond. It is also possible to construct silicene layers analogous to graphene. Isotopes Naturally occurring silicon is composed of three stable isotopes, 28Si (92.23%), 29Si (4.67%), and 30Si (3.10%). Out of these, only 29Si is of use in NMR and EPR spectroscopy, as it is the only one with a nuclear spin (I =). All three are produced in Type Ia supernovae through the oxygen-burning process, with 28Si being made as part of the alpha process and hence the most abundant. The fusion of 28Si with alpha particles by photodisintegration rearrangement in stars is known as the silicon-burning process; it is the last stage of stellar nucleosynthesis before the rapid collapse and violent explosion of the star in question in a type II supernova. Twenty radioisotopes have been characterized, the two stablest being 32Si with a half-life of about 150 years, and 31Si with a half-life of 2.62 hours. All the remaining radioactive isotopes have half-lives that are less than seven seconds, and the majority of these have half-lives that are less than one tenth of a second. Silicon has one known nuclear isomer, 34mSi, with a half-life less than 210 nanoseconds. 32Si undergoes low-energy beta decay to 32P and then stable 32S. 31Si may be produced by the neutron activation of natural silicon and is thus useful for quantitative analysis; it can be easily detected by its characteristic beta decay to stable 31P, in which the emitted electron carries up to 1.48 MeV of energy. The known isotopes of silicon range in mass number from 22 to 44. The most common decay mode of the isotopes with