Molecular dynamics simulation of high frequency (10¹⁰ to 10¹² Hz) dielectric absorption in the hollandite Na_x(Ti_8-xCr_x)O₁₆

The charge-compensating sodium ions that reside interstitially in the one-dimensional tunnels of the hollandite Na_x(Ti_8-xCr_x)O₁₆ are used as a simple model for a fluid. Molecular dynamics are used to calculate the motions of the ions at a range of temperatures between 200 K and 373 K. The polarization response of the system to a step-up electric field is calculated for field strengths between 7.43 MV/m and 74.3 GV/m, and converted to an ac susceptibility. Resonance absorption is found, peaking at frequencies between 4.5×10¹⁰ and 8.8×10¹⁰ Hz at 297 K. The origin of the response is shown to be the anharmonically coupled ion vibrations damped by ion hopping to neighbouring sites. The relationship of the result to the experimentally observed Poley absorption is explored, and a brief comparison of the calculated dynamics to previous theoretical models is made.