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arXiv 1111.1737cond-mat.mtrl-sci
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碳化硼取向相变的预测

Prediction of Orientational Phase Transition in Boron Carbide

Michael Widom, William Paul Huhn

摘要:

所评估的硼-碳二元相图显示出被称为“B4 C”的具有菱形对称性的单一合金相,其占据了刚好低于20%的标称碳含量的宽的组成范围。由于该组成范围几乎与温度无关,相图表明违反了热力学第三定律,该定律通常要求化合物在低温下达到确定的化学计量。通过第一性原理总能量计算,我们预测在T= 0 K时存在两个化学计量相:一个是具有单斜对称性的B4 C组合物;另一个是具有菱面体对称性的B13 C2组合物。利用统计力学扩展到有限的温度,我们证明,单斜相回复到菱面体相以上T= 600 K,随着碳含量略有减少。

英文摘要:

The assessed binary phase diagram of boron-carbon exhibits a single alloy phase designated "B4C" with rhombohedral symmetry occupying a broad composition range that falls just short of the nominal carbon content of 20%. As this composition range is nearly temperature independent, the phase diagram suggests a violation of the third law of thermodynamics, which typically requires compounds to achieve a definite stoichiometry at low temperatures. By means of first principles total energy calculations we predict the existence of two stoichiometric phases at T=0K: one of composition B4C with monoclinic symmetry; the other of composition B13C2 with rhombohedral symmetry. Using statistical mechanics to extend to finite temperatures, we demonstrate that the monoclinic phase reverts to the rhombohedral phase above T=600K, along with a slight reduction on carbon content.

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