Synthesis of new Ln4(Al2O6F2)O2 (Ln = Sm, Eu, Gd) phases with a cuspidine-related structure.

Aroa Morán-Ruiz,María Luisa Sanjuán,Aritza Wain-Martin,Peter R Slater,Maribel Arriortua,Aitor Larrañaga,Alodia Orera

doi:10.1107/s205225251801744x

Abstract

The first fluorination of the cuspidine-related phases of Ln4(Al2O7□)O2 (where Ln = Sm, Eu, Gd) is reported. A low-temperature reaction with poly(vinyl-idene difluoride) lead to the fluorine being substituted in place of oxygen and inserted into the vacant position between the dialuminate groups. X-ray photoelectron spectroscopy shows the presence of the F 1s photoelectron together with an increase in Al 2p and rare-earth 4d binding energies supporting F incorporation. Energy-dispersive X-ray spectroscopy analyses are consistent with the formula Ln4(Al2O6F2)O2, confirming that substitution of one oxygen by two fluoride atoms has been achieved. Rietveld refinements show an expansion in the cell upon fluorination and confirm that the incorporation of fluoride in the Ln4(Al2O7□)O2 structure results in changes in Al coordination from four to five. Thus, the isolated tetrahedral dialuminate Al2O7 groups are converted to chains of distorted square-based pyramids. These structural results are also discussed based on Raman spectra.

Highlights

Minerals belonging to the cuspidine group have the general stoichiometry M4(Si2O7)X2 (M = divalent cation; X = OH, F, O), with Ca4(Si2O7)(OH,F)2 being the archetype compound
A wide range of perovskite and related phases have been successfully fluorinated using this polymer route (Clemens et al, 2014; Hancock et al, 2012; Berry et al, 2008; Heap et al, 2007), and the method has been shown to be applicable to the fluorination of thin films (Kawahara et al, 2017; Katayama et al, 2016; Moon et al, 2015). This earlier research has mainly focused on the fluorination of transition-metal containing materials, and so here we investigate the potential use for the fluorination of oxide systems that do not contain transition metals
We investigate the success and effects of fluorination on the starting structure by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), 27Al solid-state nuclear magnetic resonance (NMR), Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX)

Summary

Introduction

A wide range of perovskite and related phases have been successfully fluorinated using this polymer route (Clemens et al, 2014; Hancock et al, 2012; Berry et al, 2008; Heap et al, 2007), and the method has been shown to be applicable to the fluorination of thin films (Kawahara et al, 2017; Katayama et al, 2016; Moon et al, 2015) This earlier research has mainly focused on the fluorination of transition-metal containing materials, and so here we investigate the potential use for the fluorination of oxide systems that do not contain transition metals. The thermal stability of these samples after fluorination was evaluated in air through thermogravimetric analysis (TGA)

Powder preparation

Characterization techniques

Results and discussion

Conclusions

Funding information