Elastic properties of degenerate f.c.c. crystal of polydisperse soft dimers at zero temperature

a r t i c l e i n f o Elastic properties of soft, three-dimensional dimers, interacting through site-site n-inverse-power potential, are determined by computer simulations at zero temperature. The degenerate crystal of dimers exhibiting (Gaussian) size distribution of atomic diameters – i.e. size polydispersity – is studied at the molecular number density 1= ffiffiffi 2 p ; the distance between centers of atoms forming dimers is considered as a length unit. It is shown that, at the fixed number density of the dimers, increasing polydispersity causes, typically, an increase in pressure, elastic constants and Poisson's ratio; the latter is positive in most directions. A direction is found, however, in which the size polydispersity causes a substantial decrease in Poisson's ratio, down to negative values for large n. Thus, the system is partially auxetic for large polydispersity and large n. Elastic properties are those among the characteristics of real materials that are crucial from the point of view of various applications. Recently, increasing interest has been observed in the area of materials with unusual elastic properties—an example are materials with negative Poisson's ratio [1–7]. When stretched (or compressed) such materials increase (decrease) their size not only in the direction of the stretch (compression) but also in one or more directions transverse to it [7]. Materials exhibiting such counterintu-itive behavior are known as anti-rubber [8], dilational materials [9] or auxetics [2]. In this paper, the last name will be used for its brevity. As for isotropic systems, Poisson's ratio is direction independent, any isotropic material can be either auxetic or not. For anisotropic systems, which are the subject of the present paper, the situation is more complex. Apart from the alternative that Poisson's ratio is negative or not in all directions, a third possibility exists: in some directions it is negative and in other—it is not. In the literature related to the field, various names have been proposed to describe different situations [4,10–12]. Here we will adopt the scheme introduced in Ref. [12], in which – as in the isotropic case – the names auxetics and nonauxetics correspond, respectively, to negative or non-negative Poisson's ratio in all directions. Materials with Poisson's ratios negative in some directions and nonnegative in the other ones will be referred to as partially auxetic materials or just partial auxetics. The very existence of auxetics and partial auxetics encourages one to study the influence of various …