Anomalous dynamical charges, phonons, and the origin of negative thermal expansion in Y<SUB>2</SUB>W<SUB>3</SUB>O<SUB>12</SUB>

We use a combination of classical model and first-principles density functional theory calculations to study lattice dynamics of Y<SUB>2</SUB>W<SUB>3</SUB>O<SUB>12</SUB> and identify phonons responsible for its negative thermal expansion (NTE). Born dynamical charges of various atoms are found to deviate anomalously from their nominal values. We find that the phonons with energy from 4 to 10 meV are the primary contributors to its NTE. These phonons involve rotations of the YO<SUB>6</SUB> octahedra and WO<SUB>4</SUB> tetrahedra in mutually opposite sense and collective translational atomic displacements, reflecting a strong mixing between acoustic and optic modes