Data_Sheet_1_Efficient Calculation of the Negative Thermal Expansion in ZrW2O8.ZIP

<p>We present a study of the origin of the negative thermal expansion (NTE) on ZrW₂O₈ by combining an efficient approach for computing the dynamical matrix with the Lanczos algorithm for generating the phonon density of states in the quasi-harmonic approximation. The simulations show that the NTE arises primarily from the motion of the O-sublattice, and in particular, from the transverse motion of the O atoms in the W–O and W–O–Zr bonds. In the low frequency range these combine to keep the WO₄ tetrahedra rigid and induce internal distortions in the ZrO₆ octahedra. The force constants associated with these distortions become stronger with expansion, resulting in negative Grüneisen parameters and NTE from the low frequency modes that dominate the positive contributions from the high frequency modes. This leads us to propose an anharmonic, two-frequency Einstein model that quantitatively captures the NTE behavior.</p

Molecular Dynamics Simulations of Supported Pt Nanoparticles with a Hybrid Sutton–Chen Potential

Understanding the physical and chemical behavior of supported nanoscale catalysts is of fundamental and technological importance. However, their behavior remains poorly understood, in part due to their complex, dynamical structure and the nature of interactions at the nanoscale. We found previously that real-time ab initio finite temperature DFT simulations provide fundamental insights into the dynamic and electronic structure of nanoparticles. Unfortunately, such first-principles calculations are very computationally intensive. To make such simulations more feasible, we have developed a hybrid version of the classical Sutton–Chen model potential which is orders of magnitude more efficient. This potential is parametrized to previous DFT/MD simulations and accounts for many-body effects induced by the support. The model is applied to Pt_10,20 nanoparticles supported on a model γ-Al₂O₃ surface. In addition to the thermal variation of the internal structure, the model also predicts diffusion coefficients and bond-breaking rates. The simulations reveal size-dependent dynamical changes with increasing temperature, as the clusters go from a “frozen” state attached to the support, to a “liquid” state where they are free to diffuse. These changes provide a rationale for the observed negative thermal expansion. Implications for nanoscale catalysis are briefly discussed