Structural Chemistry of Akdalaite, Al10O14(OH)2, the Isostructural Aluminum Analogue of Ferrihydrite

John B Parise,William R Woerner,Jack W Simonson,Brian L Phillips,Lars Ehm,Bingying Xia,Anna M Plonka

doi:10.3390/cryst9050246

Abstract

As part of an effort to characterize clusters and intermediate phases likely to be encountered along solution reaction pathways that produce iron and aluminum oxide-hydroxides from Fe and Al precursors, the complete structure of Al10O14(OH)2 (akdalaite) was determined from a combination of single-crystal X-ray diffraction (SC-XRD) data collected at 100 K to define the Al and O positions, and solid-state nuclear magnetic resonance (NMR) and neutron powder diffraction (NPD) data collected at room temperature (~300 K) to precisely determine the nature of hydrogen in the structure. Two different synthesis routes produced different crystal morphologies. Using an aluminum oxyhydroxide floc made from mixing AlCl3 and 0.48 M NaOH, the product had uniform needle morphology, while using nanocrystalline boehmite (Vista Chemical Company Catapal D alumina) as the starting material produced hexagonal plates. Akdalaite crystallizes in the space group P63mc with lattice parameters of a = 5.6244(3) Å and c = 8.8417(3) Å (SC-XRD) and a = 5.57610(2) Å and c = 8.77247(6) Å (NPD). The crystal structure features Al13O40 Keggin clusters. The structural chemistry of akdalaite is nonideal but broadly conforms to that of ferrihydrite, the nanomineral with which it is isostructural.

Highlights

Nanocrystalline materials are ubiquitous in modern materials science and technology [1]
The akdalaite synthesized from nanocrystalline boehmite had a minor corundum impurity but had a hexagonal plate morphology suitable for single-crystal X-ray diffraction (SC-XRD) (Supplementary Materials Figure S2)
The 1-g sample synthesized from boehmite for neutron powder diffraction (NPD) utilized a much longer capsule (5.5 cm) and was likely subject to a higher thermal gradient in the cold-seal vessel, which resulted in a higher amount of impurity corundum (Supplementary Materials Figure S3)

Summary

Introduction

Nanocrystalline materials are ubiquitous in modern materials science and technology [1]. They are prevalent throughout the natural environment, where they commonly act as substrates for the sorption, transport, and desorption of contaminants [2,3]. Nanoscale oxides and oxide-hydroxides produced via metal hydrolysis routes in solution, such as those of iron and aluminum, possess unique surface properties utilized for catalysis, separations, and other important applications [6,7,8]. Solution synthesis in general promotes reactions at lower T, where weak solvent–solute interactions and changes in surface energies contribute sufficiently to the overall energy of the system. Small changes in synthesis conditions radically alter the properties of the materials produced

Methods

Results

Conclusion