Structure, conductivity, and ionic motion in Na_1+xZr₂Si_xP_3-xO₁₂: a simulation study

Abstract

Constant-pressure, constant-temperature variable-shape simulation cell Monte Carlo and microcanonical ensemble molecular dynamics simulation of superionic conducting rhombohedral phase of Nasicon, Na_1+xZr₂Si_xP_3-xO₁₂, 0 ≤ x ≤ 3, at a temperature of 600 K is reported. Changes in structure, conductivity, hop path, site occupancies, bond lengths of framework atoms with composition are discussed. Average Na(1)-O distance shows a peak at x = 2, while Na(2)-O distance shows a monotonic increase. Sum of the sodium occupancies at Na(1) and mid-Na sites adds up to a constant value of one which supports the conclusion of Boilot et al.1 based on X-ray diffraction. Occupancy of Na(1) site attains a minimum at x = 2. The predominant conduction channel (which carries more than 90% of the sodium ions) is found to be the one connecting Na(1)-mid-Na-Na(2). Density contours for sodium, depicting this conduction channel, are reported. Free energy profile along the conduction channel suggests that entropy contribution cannot be neglected. The mid-Na site is not associated with a free energy minimum.