Theoretical Investigations\nand Comparisons of the\nAmorphous Structures of Adamantane-like Cluster Materials Utilizing\nMolecular Dynamics Simulations

Cluster materials of the composition AdR<sub>4</sub> (Ad\n= adamantane,\nR = organic substituent) and [(RT)<sub>4</sub>E<sub>6</sub>] (R =\norganic substituent; T = Si, Ge, Sn; and E = S, Se, Te) exhibit directional\nwhite light emission or produce second harmonics when irradiated with\na continuous wave infrared laser source. The nature of the nonlinear\noptical properties correlates with the macroscopic structures of the\ncluster materials. The desired white light emission predominantly\noccurs in amorphous materials. It is therefore crucial to understand\nthe geometric structures of the materials and the order within the\nmaterials. Here, we investigate the geometric structures of 12 different\nadamantane-like cluster materials by molecular dynamics simulations\nusing a nonperiodic particle approach. The comparison of the calculated\nstructure factors for two cluster materials with the corresponding\nexperimental data obtained from diffraction and EXAFS measurements\nshows very good agreement. Our computations revealed that, on the\none hand, larger, more flexible core structures (Ad < {Si<sub>4</sub>S<sub>6</sub>} < {Ge<sub>4</sub>S<sub>6</sub>} < {Sn<sub>4</sub>S<sub>6</sub>}) tend to lead to amorphous solids. On the other hand,\nlarger substituents (methyl < phenyl < naphthyl) lead to more\ndefined nearest neighbor interactions, with a tendency toward crystalline\nsolids. Overall, our results show that a beginning order in the material\nresults from a combination of the degree of flexibility of the core\nstructure and the variation of the nearest neighbor interaction determined\nby the substituents.