Structure refinement, microstrains and crystallite sizes of Mg-Ni-phyllosilicate nanoscroll powders

Abstract

The morphology and structure of (Ni x Mg1−x )3Si2O5(OH)4 synthetic phyllosilicate nanoscrolls have been studied by means of electron microscopy and X-ray powder diffraction. Scrolling of phyllosilicate layers originates from size differences between octahedral and tetrahedral sheets. This strain-energy-driven process raises a number of questions, including the preferred direction of scrolling (along the a or b axis) and the presence of residual microstrain. In order to clarify these points, the structure of (Ni x Mg1−x )3Si2O5(OH)4 phyllosilicates (x = 0, 0.33, 0.5, 0.67, 1) was first described by a monoclinic Cc (9) unit cell, whose parameters decrease with increasing Ni concentration. The Williamson–Hall plots constructed for x = 0 and 0.67 reveal the absence of microstrain, which suggests that scrolling is an effective means of stress relaxation. The sizes of the crystallites were determined by using Rietveld refinement with predefined needle-like models and fundamental parameter fitting with crystallites of arbitrary form. Both approaches show qualitative and quantitative correlation, in terms of aspect ratio, with electron microscopy data. At the same time, the phyllosilicates studied do not demonstrate one preferred direction of scrolling: instead, there might be a mixture of chirality vectors codirected with the a or b axis, with the proportion altering with Ni concentration.