Bauman Moscow State Technical University, 2nd Baumanskaya Street 5, Moscow 105005, Russia.
Institute for High Pressure Physics RAS, Kaluzhskoe Shosse, 14, Troitsk, Moscow 108840, Russia.
J Chem Phys. 2018 Apr 7;148(13):134508. doi: 10.1063/1.5022969.
The heat capacity of classical crystals is determined by the Dulong-Petit value C ≃ D (where D is the spatial dimension) for softly interacting particles and has the gas-like value C ≃ D/2 in the hard-sphere limit, while deviations are governed by the effects of anharmonicity. Soft- and hard-sphere interactions, which are associated with the enthalpy and entropy of crystals, are specifically anharmonic owing to violation of a linear relation between particle displacements and corresponding restoring forces. Here, we show that the interplay between these two types of anharmonicities unexpectedly induces two possible types of heat capacity anomalies. We studied thermodynamics, pair correlations, and collective excitations in 2D and 3D crystals of particles with a limited range of soft repulsions to prove the effect of interplay between the enthalpy and entropy types of anharmonicities. The observed anomalies are triggered by the density of the crystal, changing the interaction regime in the zero-temperature limit, and can provide about 10% excess of the heat capacity above the Dulong-Petit value. Our results facilitate understanding effects of complex anharmonicity in molecular and complex crystals and demonstrate the possibility of new effects due to the interplay between different types of anharmonicities.
经典晶体的热容由软相互作用粒子的杜隆-珀替值 C ≃ D（其中 D 是空间维度）决定，在硬球极限下具有类似气体的热容 C ≃ D/2，而偏离则由非谐效应控制。由于粒子位移与相应恢复力之间线性关系的破坏，软球和硬球相互作用与晶体的焓和熵有关，因此特别是非谐的。在这里，我们表明这两种类型的非谐性的相互作用出人意料地诱导出两种可能类型的热容异常。我们研究了二维和三维有限软排斥粒子晶体的热力学、对关联和集体激发，以证明焓和熵类型非谐性相互作用的效果。观察到的异常是由晶体的密度触发的，在零温极限下改变了相互作用的状态，并且可以提供大约 10%的热容超过杜隆-珀替值的过剩。我们的结果有助于理解分子和复杂晶体中复杂非谐性的影响，并展示了由于不同类型非谐性相互作用而产生新效应的可能性。
