Molecular dynamics study of aging effects in supercooled Al 2 O 3 Vo Van Hoang 1, * and Suhk Kun Oh 2 1 Department of Physics, College of Natural Sciences, HochiMinh City National University, 227 Nguyen Van Cu Str., Distr. 5, HochiMinh City, Vietnam 2 Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea (Received 19 August 2004; published 16 December 2004) Using molecular dynamics simulations we investigated the effect of aging on static and dynamic properties of supercooled Al 2 O 3 models at the temperatures of 2100 K and 5600 K. Simulations were done in the basic cube under periodic boundary conditions containing 3000 ions with Born-Mayer type pair potentials. We obtain the changes as a function of time for the total energy and density of the system. The aging time dependence of partial radial distribution functions (PRDFs), coordination number distributions and bond-angle distributions has been studied. We compare the PRDFs for the 10% most mobile Al or O atoms with the corresponding mean PRDFs. We find the effect of aging on the dynamical heterogeneities in the system. Al and O atoms show similar dynamical heterogeneities but with a differing rate of changes during aging. Further- more, we also find significant aging effects in mean-squared atomic displacement and diffusion constant. DOI: 10.1103/PhysRevE.70.061203 PACS number(s): 61.20.Ja, 61.43.Fs, 78.55.Qr, 61.43.Bn I. INTRODUCTION Aging effects in supercooled glasses have been under in- tensive investigation for a long time. Miyagawa and Hiwatari carried out molecular dynamics simulations on a soft-sphere model for binary alloys quenched into glassy states at differ- ent rates [1]. The main purpose of their work was to inves- tigate slow relaxation phenomena of the quenched glassy states close to the glass transition by calculating both the static and dynamical structural changes upon aging (anneal- ing) of them. It was shown that the static properties exhibit neither significant change during annealing of samples, nor changes for different samples. However, the dynamic prop- erties show remarkable aging effects as well as sample- dependent behavior, meaning that the quenched glassy state cannot attain an equilibrium state for the time scale of simu- lations, due to dynamical slowing-down phenomena [1]. The quenched sample was shown to tend to a seemingly quasi- equilibrium state after a sufficient annealing. Detailed dis- cussions on both aging effects and sample-dependent behav- iors of quenched glassy states was made, by paying attention to the behavior of the single-atom motion, mean-squared dis- placement, and self-diffusion constant of the quenched glassy states [1]. The effects of aging and drying on the structure of V 2 O 5 gels have been obtained experimentally [2]. Semiconducting V 2 O 5 gels with and without addition of GeO 2 were prepared by hydrolysis of metal alkoxides in al- coholic solutions. They found that crystalline vanadium ox- ides were not observed in xerogels, but V 3 O 7 diffraction peak was found for the aerogels kept at 250 °C and 210 atm for 2 h or longer before drying. However, the aging effects can be studied in more details only by the computer simula- tion, and historically, computer simulations have perhaps had their largest impact on the fundamentals of materials science in the study of amorphous systems. We can find numerous simulation works concerning on the aging effects in different systems. Using molecular dynamics simulations, Kob et al. studied the out of equilibrium dynamic correlations in a model glass- forming liquid [3]. The system was quenched from a high temperature to a temperature below its glass transition tem- perature, and the decay of the two-time intermediate scatter- ing function Ct w , t + t w  was monitored for several values of the waiting time t w after the quench. They found that Ct w , t + t w  showed a strong dependence on the waiting time, i.e., aging, depended on the temperature before the quench, and similar to the case of spin glasses, it could be scaled onto a master curve. In Ref. [4], Poliwa and Heuer presented simulations of a hard disk system and analyzed the time evolution of the dynamical heterogeneities, and they charac- terized the time evolution of slow regions and slow particles individually. Also, using molecular dynamics simulations Kerrache et al. [5] investigated the presence of dynamical heterogeneities in supercooled silica, a strong glass former. They also studied the changes as a function of time for these dynamical heterogeneities during aging. They compared the radial distribution function for the 10% most mobile Si or O atoms with the corresponding mean radial distribution func- tions. They also measured changes in these radial distribu- tion functions and the changes in the non-Gaussian param- eter with time after a quench, and they found that dynamical heterogeneities increased during the aging process. Recently, the cooling rate, heating rate, and aging effects in glassy water have been comprehensively investigated [6]. In this work, the authors reported a molecular dynamics simulation of the properties of the potential energy landscape sampled by a system of water molecules during the process of gener- ating a glass by cooling, and during the process of regener- ating the equilibrium liquid by heating the glass. They have studied the dependence of these processes on the cooling/ heating rates as well as on the role of aging (the time elapsed in the glass state). However, the aging effects in the super- cooled Al 2 O 3 have not been investigated yet. And therefore, *Email address: vvhoang2002@yahoo.com PHYSICAL REVIEW E 70, 061203 (2004) 1539-3755/2004/70(6)/061203(8)/$22.50 ©2004 The American Physical Society 061203-1