The range of mechanical, and related, properties computed by GULP has been significantly extended for the present version. Since no article on simulations of ionic materials would be complete without a mention of the ubiquitous and evergreen perennial MgO, we choose to take this well-studied system as an example.
Magnesium oxide adopted the cubic rock salt structure and possesses the well-known characteristic of exhibiting a Cauchy violation in the elastic constants (i.e. ). No simple two-body forcefield is capable of reproducing this many body effect. Consequently, it is necessary to use a breathing shell model to describe this material accurately. While there have been previous breathing shell models for MgO , we choose to fit a new set of potentials here that reproduce the structure, elastic constants, and high and low frequency dielectric constants under ambient conditions. The resulting potential model is described in Table 2.1, while the calculated properties are given in Table 2.2.
The calculated properties for magnesium oxide can be seen to be in
excellent agreement with experiment under ambient conditions, with
the exception of the Poisson ratio. Of course, this agreement is a
consequence of fitting a model with the correct essential physics
to a subset of the experimental data. The disagreement in the Poisson
ratios is because the values are calculated using different expressions.
If the Poisson ratio is evaluated based on the sound velocities according
then our calculated value becomes 0.182 in good agreement with the determination of Zha et al .
To provide a test of the model, it is possible to calculate the variation of the elastic properties of magnesium oxide as a function of applied pressure. The variation of the elastic constants up to 60 GPa is shown in Figure 2.1.
When compared to the experimental results of Zha et al, the calculated trend in the value of is in good agreement in that it consistently increases under pressure and approximately doubles in magnitude by the time that 60 GPa is reached. Unfortunately, the trends for the other elastic constants are not so good, since the curve for flattens with increasing pressure, rather than becoming steeper, and the curve for passes through a maximum which is not observed in the experimental data from the aforementioned group. However, the calculated trends do match the extrapolated behaviour based upon the results of ultrasonic measurements .