Department of Chemistry and Institute for Research in Materials, Dalhousie University, Halifax, Nova Scotia B3H 4R2 (Canada).
Angew Chem Int Ed Engl. 2015 Mar 16;54(12):3626-9. doi: 10.1002/anie.201409169. Epub 2015 Jan 23.
Boron is an important element, used in applications from superhard materials to superconductors. Boron exists in several forms (allotropes) and, surprisingly, it was not known which form (α or β) is stable at ambient conditions. Through experiment, we quantify the relative stability of α-boron and β-boron as a function of temperature. The ground-state energies of α-boron and β-boron are nearly identical. For all temperatures up to 2000 K, the complicated β-boron structure is more stable than the simpler α-boron structure at ambient pressure. Below 1000 K, β-boron is entropically stabilized with respect to α-boron owing to its partially occupied sites, whereas at higher temperatures β-boron is enthalpically stabilized with respect to α-boron. We show that α-boron only becomes stable on application of pressure.
硼是一种重要的元素，广泛应用于从超硬材料到超导材料等多个领域。硼有多种同素异形体（allotropes），令人惊讶的是，人们一直不知道哪种同素异形体（α 或 β）在环境条件下是稳定的。通过实验，我们定量研究了α-硼和β-硼在温度下的相对稳定性。α-硼和β-硼的基态能量几乎相同。在高达 2000 K 的所有温度下，在环境压力下，复杂的β-硼结构比简单的α-硼结构更稳定。在 1000 K 以下，由于β-硼部分占据的位置，β-硼在熵方面相对于α-硼稳定，而在较高温度下，β-硼在焓方面相对于α-硼稳定。我们表明，只有在施加压力时，α-硼才会变得稳定。
