Abstract
The addition of a small amount of dopant impurities to crystals is a common method to tune the properties of materials. Usually the doping grade is restricted by the low solubility of the dopants; increasing the doping concentration beyond this solubility limit leads to supersaturated solutions in which dopant clusters dominate the material properties, often leading to deterioration of strength and performance. Descriptions of doped solids often assume that thermal excitations of the on average perfect matrix are small. However, especially for bcc crystals close to their melting point it has recently become clear that the effects of thermal disorder are strong. Here we study the doping of weak bcc crystals of charged colloids via Brownian dynamics simulations. We find a complex phase diagram upon varying the dopant concentration. At low dopant concentrations we find an interstitial solid solution. As we increase the amount of dopants a complex meta-stable liquid-in-solid cluster phase emerges. Ultimately this phase becomes meta-stable with respect to macroscopic crystal-crystal coexistence. These results illustrate the complex behaviour that emerges when thermal excitations of the matrix drive impure crystals to a weak state.
Highlights
A common approach to tune the properties of crystalline solids is the process of doping; the introduction of interstitial atoms in a crystalline matrix with the purpose of changing the materials properties
Strong thermal disorder and large correlated fluctuations govern the crystal physics. This has some unusual effects on the properties of the crystalline solid that, while ordered on average, displays some features typically associated with disordered solids, such as a elasticity that is governed by non-affine fluctuations, leading to a breakdown of the Born-Huang lattice dynamics[19,20,21,22]
We showed that dopant dynamics in soft crystals approaching their melting point, φ ~ φmelt, can not be adequately described by theories based on the assumptions
Summary
A common approach to tune the properties of crystalline solids is the process of doping; the introduction of interstitial atoms in a crystalline matrix with the purpose of changing the materials properties. Due to the small length scales inherent in the study of atomic materials, the process of crossing the solution limit; going from a low concentration of dopant to a supersaturated solid solution, has not been investigated on a single particle level. Due to the inherently short time and length scales involved in atomic materials, investigation into supersaturated solid solutions on a single particle level is very difficult To overcome this limitation we explore the use of colloids as a model system for impure crystals; these systems are observed with microscopy techniques and show behaviour that in some cases is analogous to their atomic counterparts[9]. Only low doping levels have been investigated; this raises the inevitable question of what happens when doping concentrations reach supersaturated levels, where for atomic solids it is known that material properties rapidly change — such as a notably lower rate of diffusion similar to what we observed in colloidal systems[24]
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