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The morphology of nanoscopic Ag grains significantly affects the phonons.Atomistic simulations show that realistic nanograin models display complexvibrational properties. (1) Single-crystalline grains. Nearly-pure torsionaland radial phonons appear at low frequencies. For low-energy, faceted models,the breathing mode and acoustic gap (lowest frequency) are about 10% lower thanpredicted by elasticity theory (ET) for a continuum sphere of the same volume.The sharp edges and the atomic lattice split the ET-acoustic-gap quintet into adoublet and triplet. The surface protrusions associated with nearly spherical,high-energy models produce a smaller acoustic gap and a higher vibrationaldensity of states (DOS) at frequencies \nu<2 THz. (2) Twined icosahedra. Incontrast to the single-crystal case, the inherent strain produce a largeracoustic gap, while the core atoms yield a DOS tail extending beyond thehighest frequency of single-crystalline grains. (3) Mark's decahedra, incontrast to (1) and (2), do not have a breathing mode; although twined andstrained, do not exhibit a high-frequency tail in the DOS. (4) Irregularnanograins. Grain boundaries and surface disorder yield non-degenerate phononfrequencies, and significantly smaller acoustic gap. Only these nanograinsexhibit a low-frequency \nu^2 DOS in the interval 1-2 THz.Comment: Version published in Phys. Rev.

Effects of morphology on phonons in nanoscopic silver grains