Temperature dependence of the silver distribution in Ag2MnP2S6 by single crystal X-ray diffraction

Abstract

Abstract The Ag distribution in the layered compound Ag2MnP2S6 [non-conventional space group C2/n, a = 6.339(1) Å, b = 10.933(1) Å, c = 13.893(2) Å, β = 108.29(1)°, Z = 4, T = 300 K] is investigated as a function of temperature with the aid of single crystal X-ray data. Ag2MnP2S6 is built of S/(Ag2)1/3Mn1/3(P2)1/3/S sandwiches, in which Mn and P2-pairs are distributed over the octahedral voids in an ordered triangular way; at room temperature the Ag+ cations are located within the two sulfur layers of a sandwich, at the centers of the opposite triangular faces of sulfur octahedra. The Gram-Charlier expansion of the structure factor up to the fourth order is used to analyze the development of the Ag distribution with temperature. Joint probability density function maps calculated from the refined tensor elements of the atomic displacement parameter of Ag indicate that at higher temperatures the neighbouring tetrahedral sites within the gap between the sandwiches also become partly occupied. In addition they also show that the centers of the sulfur octahedra do not become occupied. The temperature dependence of the effective one-particle potential shows that the disorder in the Ag distribution is primarily static in nature. The development of the Ag distribution leads to a strengthening of the inter-sandwich bonding and to stronger distortions in the sulfur sheets. The anharmonic model is also applied to the parent phase Ag2ZnP2S6 with improved result as compared to the split structure model.