The construction of molecular dynamics model for the response of NaxCrxTi8-xO16(x=1.7) to electric fields of up to 100ps duration
The hollandite Nax(Ti8--xCrx)O 16, (x = 1.7) is a non-stoichiometric material that is based on a family of compounds of general formula AxM4--xNy O8. The chromium and sodium ions are randomly placed in unit cells according to the relative proportions of titanium and chromium ions. Charge compensating sodium ions are located in interstitial sites along tunnels. Because of the non-stoichiometric nature, there are vacant sites into which the sodium ions may move.;A molecular dynamics (MD) simulation was used to determine the motion of the sodium ions in the tunnel, when the surrounding ions were held rigid. The dielectric response was calculated from the rate of change of polarisation, dP/dt under the action of a de step-field of 7.43MV/m to 74.3GV/m, at temperature between 200K and 373K.;A resonance behaviour has been observed. The absorption peaks in chi" at the resonance frequency lie between 4.5x1010 and 8.8x10 10 Hz at 297K which matches very well with the Poley absorption which is typically observed in polar liquids in the 1.2 - 70 cm--1 (3.6x1010 -- 2.1x1012 Hz) region at room temperature. The resonance frequency and the resonance peak height are independent of temperature. The peaks show a non-linear dependence upon electric field. At the very high field of 74.3GV/m, the sodium ions move in collective modes of vibration.