Crystal Structures of Two Titanium Phosphate-Based Proton Conductors: Ab Initio Structure Solution and Materials Properties.

Hilke Petersen,Bodo Zibrowius,Michael Fischer,Wolfgang Schmidt,Niklas Stegmann,Ivan Radev,Claudia Weidenthaler,Wladimir Philippi

doi:10.1021/acs.inorgchem.1c02613

Abstract

Transition-metal phosphates show a wide range of chemical compositions, variations of the valence states, and crystal structures. They are commercially used as solid-state catalysts, cathode materials in rechargeable batteries, or potential candidates for proton-exchange membranes in fuel cells. Here, we report on the successful ab initio structure determination of two novel titanium pyrophosphates, Ti(III)p and Ti(IV)p, from powder X-ray diffraction (PXRD) data. The low-symmetry space groups P21/c for Ti(III)p and P1̅ for Ti(IV)p required the combination of spectroscopic and diffraction techniques for structure determination. In Ti(III)p, trivalent titanium ions occupy the center of TiO6 polyhedra, coordinated by five pyrophosphate groups, one of them as a bidentate ligand. This secondary coordination causes the formation of one-dimensional six-membered ring channels with a diameter dmax of 3.93(2) Å, which is stabilized by NH4+ ions. Annealing Ti(III)p in inert atmospheres results in the formation of a new compound, denoted as Ti(IV)p. The structure of this compound shows a similar three-dimensional framework consisting of [PO4]3– tetrahedra and TiIV+O6 octahedra and an empty one-dimensional channel with a diameter dmax of 5.07(1) Å. The in situ PXRD of the transformation of Ti(III)p to Ti(IV)p reveals a two-step mechanism, i.e., the decomposition of NH4+ ions in a first step and subsequent structure relaxation. The specific proton conductivity and activation energy of the proton migration of Ti(III)p, governed by the Grotthus mechanism, belong to the highest and lowest, respectively, ever reported for this class of materials, which reveals its potential application in electrochemical devices like fuel cells and water electrolyzers in the intermediate temperature range.

Highlights

Transition-metal phosphates (TMPs) are a class of functional materials that are studied for fundamental understanding and applied in industrial applications.[1−8] The members of the TMP family show a wide range of chemical compositions and a wide variety of crystal structures with variable metal coordination and different phosphate structure units
The average local structure was determined via Raman spectroscopy as well as complementary Pair Distribution Function (PDF) data analysis
The obtained information about the polyhedral coordination was used for the ab initio crystal structure solution of both structures from powder X-ray diffraction (PXRD)

Summary

Introduction

Transition-metal phosphates (TMPs) are a class of functional materials that are studied for fundamental understanding and applied in industrial applications.[1−8] The members of the TMP family show a wide range of chemical compositions and a wide variety of crystal structures with variable metal coordination and different phosphate structure units. The phosphate units are differentiated in orthophosphates ([PO4]3−) and different condensed phosphates, pyrophosphates ([P2O7]4−) and metaphosphates, consisting either of [PO3]−n chains or [PnO3n]n− rings (Figure 1). Titanium phosphates belong to the intensively studied TMP compounds They show promising behavior as photocatalysts, solid acids, N2 absorbents in the Haber−Bosch process, and proton conductors, in particular in the mediate temperature range.[5,6,16,17] The wide application range of titanium phosphates is caused by their structural variety.

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Results

Conclusion