The freezing tendency towards 4-coordinated amorphous networks causes an increase in the heat capacity of supercooled Stillinger–Weber silicon

Abstract

The supercooled liquid silicon, modeled by Stillinger-Weber potential, shows anomalous increase in heat capacity C p , with a maximum C p value close to 1060 K at zero pressure. We study equilibration and relaxation of the supercooled SW Si, in the temperature range of 1060 K-1070 K at zero pressure. We find that as the relaxation of the metastable supercooled liquid phase initiates, a straight line region (SLR) is formed in cumulative potential energy distributions. The configurational temperature corresponding to the SLR is close to 1060 K, which was earlier identified as the freezing temperature of 4-coordinated amorphous network. The SLR is found to be tangential to the distribution of the metastable liquid phase and thus influences the broadness of the distribution. As the bath temperature is reduced from 1070 K to 1060 K, the effective temperature approaches the bath temperature which results in broadening of the metastable phase distribution.This, in turn, causes an increase in overall fluctuations of potential energy and hence an increase of heat capacity. We also find that during initial stages of relaxation, 4-coordinated atoms form 6membered rings with a chair-like structure and other structural units that indicate crystallization.Simultaneously a strong correlation is established between the number of chair-shaped 6-membered rings and the number of 4-coordinated atoms in the system. This shows that all properties related to 4-coordinated particles are highly correlated as the SLR is formed in potential energy distributions and this can be interpreted as a consequence of 'freezing' of amorphous network formed by 4-coordinated particles.

The interactions between thermodynamic anomalies

“…and we can obtain the final expression for the gradient by inserting Eqs. (27) and (25) into the expression for the gradient of the compressibility anomaly,…”

Dynamical Instability Causes the Demise of a Supercooled Tetrahedral Liquid

“…The relaxation mechanism is qualitatively similar to that found recently for SW-Si liquid at its limit of stability 1060 K and zero pressure. [2] Further the threshold value of per particle 3-body energy of the network at the crossover φ 3B 4c b ≈ 0.22 (see Figs. 5, S6, and S12) is comparable to the value of 0.21 found in case of SW-Si at the instability point.…”

“…Following the method of the earlier work on SW-Si, [8,2] we studied the relaxation mechanism of supercooled mW liquid at 205 K (which is a more accurate estimate of the limit of stability [11]) and zero pressure in isothermal isobaric (N P T ) Monte Carlo (MC) simulations. Each MC simulation step, on average, consisted of N particle displacement attempts and 2 volume change attempts.…”

“…The properties (the per particle potential energy, the density, and the fraction of the 4-coordinated particles) at the crossover point are found to be unique (independent of the system size) and agree with the earlier studies. [10,3,11,5] Following the method of the earlier work on SW-Si, [8,2] we studied the relaxation mechanism of supercooled mW liquid at 205 K (which is a more accurate estimate of the limit of stability [11]) and zero pressure in isothermal isobaric (N P T ) Monte Carlo (MC) simulations. Each MC simulation step, on average, consisted of N particle displacement attempts and 2 volume change attempts.…”

“…[4] In the case of Stillinger-Weber [16] silicon, the liquid state properties at the lower limit of stability of 1060 K [7,8] have been precisely estimated based on free energy computations. [1,2] Here we attempt to study the relaxation at the stability limit of supercooled water modeled by mW potential of Molinero and Moore. [9] mW is a coarse-grained model of water in which water is represented by a single atom with 3-body (tetrahedral) interactions to mimic the effect of hydrogen bonds of real water.…”