Radiation damage effects in the perovskite CaTiO 3 and resistance of materials to amorphization Kostya Trachenko, Miguel Pruneda, Emilio Artacho, and Martin T. Dove Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom (Received 15 January 2004; revised manuscript received 26 May 2004; published 29 October 2004) We combine classical and quantum-mechanical simulations to study the structural changes in CaTiO 3 under irradiation. We identify common defects, and suggest that their stability is related to the covalent character of Ti-O bonding. We address the issue of resistance to amorphization by radiation damage, and propose that a complex material is amorphizable by radiation damage if it is able to form a covalent network. On a more detailed level, we suggest that resistance to amorphization is defined by the competition between the short- range covalent and long-range ionic forces. DOI: 10.1103/PhysRevB.70.134112 PACS number(s): 61.80.-x, 61.82.-d, 62.20.-x I. INTRODUCTION The future of nuclear power is often linked to our ability to effectively manage nuclear waste. This ability is also im- portant for immobilizing the current stockpiles of highly ra- dioactive waste and weapons-grade plutonium. Vitrification, or immobilization of nuclear waste in glass, has been and remains a popular way of its handling. The effective alterna- tive to vitrification has been immobilization in crystalline solids. Several potential waste forms have been proposed and studied, including the composite titanate ceramic SYNROC. 1 Under irradiation from immobilized elements, SYNROC phases amorphize. Radiation-induced amorphization can af- fect several physical properties of a material, including its ability to remain an effective immobilization barrier. It has been shown recently that when the damaged structure perco- lates, transport phenomena can experience typical percolation-type increases, affecting the stability of a waste form. 2 CaTiO 3 is a much studied perovskite, and is a major phase in SYNROC, in which it carries the function of cap- turing strontium, rare earths, and actinide radioactive ele- ments. Most of the damage comes from the large energetic recoiling nuclei during alpha decay. Under irradiation, per- ovskite loses long-range order (becomes “x-ray amor- phous”), as is witnessed from irradiation by energetic ions and actinide doping, 3 with a large accompanying increase in volume. The nature of the damaged state, the structural changes under irradiation, what drives amorphization in this and other titanate materials, etc., remain unknown. To an- swer these and other important questions, we combine clas- sical and quantum-mechanical calculations of radiation dam- age effects in perovskite. We show that the damage is stabilized by alternative Ti-O-Ti bridges with substantial co- valent contribution to bonding. Finally, we discuss a long-standing problem of resistance of materials to amorphization by radiation damage. We pro- pose that resistance to amorphization of a complex non- metallic compound is defined by the competition between the short-range covalent and long-range ionic forces. II. DAMAGED PEROVSKITE STRUCTURE AND THE NATURE OF BONDING We begin by simulating a 30-keV U recoil in perovskite structure using molecular dynamics (MD) simulation. First, since it has been shown that the short-range repulsion poten- tial is improved if calculated from first principles (relative to the empirical potential), 4 we compute short-range forces be- tween all pairs of U, Ca, Ti, and O atoms. The ab initio energy calculation was performed using the self-consistent SIESTA program. 5 This code is an implementation of density functional theory 6 combined with the pseudopotential ap- proximation to remove the core electrons from the calcula- tions. Due to the large overlap between the semicore and the valence states, the 3s and 3p electrons of Ti and Ca were included in the calculation. The electronic density is obtained using the exchange-correlation potential of Ceperley and Al- der in the Perdew-Zunger parametrization, 7 and norm- conserving pseudopotentials in the Kleinman-Bylander form. 8 The core radiis used for the pseudopotential genera- tion were 1.15, 1.30, and 1.40 bohr for O, Ti, and Ca, respec- tively. The Kohn-Sham eigenstates were expanded in a local- ized basis set of numerical orbitals. We used a single- basis set for the semicore states of Ti and Ca, and a double- plus polarization for the valence states of all the atoms. Periodic boundary conditions require the definition of a supercell large enough for the interaction between replicas of the at- oms to be cancelled L  20 Å. A uniform space grid with an equivalent cutoff energy of 350 Ry was used to project the charge density and calculate the exchange-correlation and Hartree potentials. We have shown elsewhere 9 that the short-range potentials calculated using pseudopotentials in the described way are in a good agreement with those ob- tained from the all-electron treatment, with neither core nor basis approximations. The short-range potentials were joined to the perovskite equilibrium interatomic potential 10 to be used in the MD simulations. We have used the DL_POLY MD package. 11 The system contained 500 940–832 000 atoms, and we have em- ployed constant energy ensemble. The damage propagation was simulated for about 20 ps at room temperature, and the result is shown in Fig. 1. What are the key features of the damaged structure? Mak- ing definite conclusions about the structure of the damage is challenging, because in MD simulations of radiation damage, the empirical potential is fitted to the properties at equilib- rium, and generally we cannot expect it to give correct re- sults in the highly disordered state. In some cases, the simu- PHYSICAL REVIEW B 70, 134112 (2004) 1098-0121/2004/70(13)/134112(6)/$22.50 ©2004 The American Physical Society 70 134112-1