From glissile to sessile: Effect of temperature on 〈110〉 dislocations in perovskite materials

Abstract

In perovskite-type strontium titanate (SrTiO 3 ) 〈110〉 dislocations, which are the main carriers of plastic flow at low temperature, lose their mobility as temperature increases, leading to brittle failure above 1050 K. We present theoretical evidence for a change in their core structure into a sessile, climb-dissociated configuration at high temperature. This mechanism is shown to operate in both SrTiO 3 and MgSiO 3 , indicating that it may be a general feature of perovskite-type materials. It follows that the activity of the 〈110〉 slip system depends critically on the strain rate-temperature couple, ð _ ε; TÞ.The discovery of the atypical mechanical behavior of strontium titanate (SrTiO 3 or STO) and its double ductile-brittle-ductile transition [1,2] has challenged our understanding of the plastic behavior of perovskite-type materials, and has since motivated several studies. A series of experiments [3,4] and simulations [5,6] have provided a good understanding of the ductile behavior of STO at low temperature and demonstrated its direct link with the glide of 〈110〉 dislocations. However it is still unclear why this mechanism becomes inactive at high temperature, leading to brittle failure above 1050 K. It was initially proposed that 〈110〉 dislocations become sessile by dissociating into two 〈100〉 dislocations, or because of a change of preferential slip plane [2]. Later it was proposed that 〈110〉 dislocations dissociate by climb at high temperature, into a configuration similar to the one observed in low-angle tilt grain boundaries [7,3]. Despite arguments in favor of the latter [8], there has been no direct evidence for this change of core structure, the mechanisms by which it operates are still unknown, and whether this transition is specific to STO or a general feature of perovskite-type materials remains an open question.In this work we investigate the effect of temperature on the core structure of individual [110] pc edge dislocations in two perovskitetype materials: the cubic strontium titanate, and the orthorhombic, high-pressure magnesium silicate MgSiO 3 which is the pure Mg endmember of bridgmanite [9], the main constituent of the Earth's lower mantle. In the following we consider MgSiO 3 under the hydrostatic pressure of 30 GPa, corresponding to a depth of ca. 700 km i.e. the uppermost lower mantle. Throughout this article crystal directions are given in the (pseudo-)cubic reference, noted with the subscript "pc", for easier comparison between the two materials (in MgSiO 3[110] pc = [100]; ½110 pc ¼ ½010 ; 2[001] pc = [001], see e.g. Ref.[10]). The 〈110〉 pc dislocations belong to the easiest slip system in both materials [11], [6]. Atomic systems were constructed with Atomsk [12], and classical molecular statics and dynamics simulations were performed with LAMMPS [13]. Interatomic interactions in SrTiO 3 were modeled with the rigid-ion potential proposed by Thomas et al. [14], and in MgSiO 3 with the one from Alfredsson et al. [15].The glide-dissociated core structures of [110] pc edge dislocation...

Influence of Dislocations in Transition Metal Oxides on Selected Physical and Chemical Properties

“…The simulations indicate that within the glide plane the dislocations tend to dissociate in two partial dislocations with distances of 4-5 nm separated by a strongly distorted antiphase boundary stacking fault [216]. The gliding of dislocations is also strongly temperature-dependent since it was calculated that at high temperatures the dislocations lose their mobility leading to brittle behavior of the crystal [105]. By employing a concurrent atomistic continuum simulation Yang et al [95] also confirmed the importance of easy glide planes in SrTiO 3 resulting in a preference of the <110> directions for dislocation nucleation and revealed that the presence of an agglomeration of dislocations in form of bicrystalline boundaries in SrTiO 3 has a significant effect on the mechanical properties and the crack evolution [97].…”

“…Because the diffusion of oxygen vacancies is easier than that of cation vacancies, this process cannot be realized in only one step. The detailed scenario of all steps of the climbing of the dislocations in this electrical complicated surrounding has been described by Hirel et al [105]. e.g., [10,42,[99][100][101][102][103][104]).…”

“…This process causes a shifting of the half plane to the surface and thus generates an additional step on the surface. Therefore, for the verification of the often-postulated extremal annihilation of the dislocations (that means a high reduction of the density of dislocations) in TiO2 or SrTiO3 (via thermal treatment at high temperature around 1000 °C [105,112,113,115]), the difference in the morphology of the surface before and after the thermal treatment should be checked. Only in the case of the unequivocal increase of the step concentration in the plane of the surface one would be able to postulate that the reduction of dislocations density via thermal treatment has actually occurred.…”

3D Dislocation structure evolution in strontium titanate: Spherical indentation experiments and MD simulations

“…The atomic interaction is modeled using the rigid-ion potential of Thomas et al, 38 which consists of a long-range Coulomb and a short-range cation-oxygen interaction term of Born-Mayer type. This interatomic potential has been used before by several groups to model dislocations 39,40 and plastic deformation of STO. 41 It was found to be more suitable for studying extended defects than more elaborate shell-model potentials 42 and predicts mechanical properties of STO and defect properties sufficiently well for the type of large-scale simulation conducted here.…”

The role of diffusion-driven pure climb creep on the rheology of bridgmanite under lower mantle conditions

“…In parallel, recent theoretical calculations related to the physics of dislocations in MgSiO 3 bridgmanite confirm that the lattice friction remains very high, even at high temperature 42 , which account very well for the high stress levels observed in experiments 36 . Indeed, insights into slip systems and plastic anisotropy of MgSiO 3 bridgmanite were first gained from modelling dislocation cores and lattice friction at the atomic scale 43–47 . More recently, high temperature dislocation glide has been characterized from the modelling of thermally activated processes underlying dislocation glide at finite temperature 42 .…”