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

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 d_max 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 Ti^IV+O₆ octahedra and an empty one-dimensional channel with a diameter d_max 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.

Figure 1

Examples of the different classes of condensed phosphates: (a) orthophosphate, (b) pyrophosphate, (c) ring, and (d) chain metaphosphates. The classification is according to Popović et al.

Figure 2

Measured Raman spectra of (a) Ti(III)p and (b) Ti(IV)p, together with their theoretical Raman shifts (Ti(III)p) or spectra (Ti(IV)p) obtained from DFT modeling, as well as (c) the Raman spectrum of TiP₂O₇.

Figure 3

Experimental and fitted PDF data of (a and b) Ti(III)p, (c and d) Ti(IV)p, and (e and f) TiP₂O₇ and the respective difference curves (gray). In parts b, d, and f, the corresponding atom pairs are marked with lines.

Figure 4

Rietveld refinement plots for (a) Ti(III)p, (b) Ti(IV)p, and (c) TiP₂O₇. The measured PXRD data are displayed as solid lines, the calculated PXRD patterns from the refined models are shown as dotted lines, and the difference curves are shown as solid gray lines.

Figure 5

(a) Crystal structure of Ti(III)p viewed along the one-dimensional channel with incorporated NH₄⁺ ions. (b) Crystal structure of Ti(IV)p displaying the empty one-dimensional channels. (c) [TiP₄O₁₂] layers of the Ti(III)p structure in the ab plane, For the sake of clarity, the NH₄⁺ ions are not included. (d) [TiP₄O₁₂] layers in the Ti(IV)p structure displayed in the ab plane.

Figure 6

³¹P MAS NMR spectrum of the same Ti(IV)p sample that was used for the PXRD measurement (ν_MAS = 10 kHz). The dashed blue line depicts the same spectrum magnified by a factor of 8. The width of both resonance lines (fwhh) is about 140 Hz. The assignment is based on the results of DFT calculations.

Figure 7

³¹P MAS NMR spectra of Ti(IV)p measured at different spinning speeds. The data were shifted for the sake of clarity. The experimental spectra (black curves) are shown in comparison with those calculated using the parameters given in Table 2 for ν_MAS = 5 kHz (red curves).

Figure 8

Arrhenius plots of the conductivities of Ti(III)p (blue) and Ti(IV)p (red) as a function of the temperature under fully hydrated (under DI water) conditions.

Figure 9

PXRD data obtained during heating of Ti(III)p in an inert atmosphere.

Figure 10

(a) Evolution of the unit cell parameters of Ti(III)p with temperature: the lattice parameters a, b, and c and the angle β. (b) Plus the unit cell parameters of Ti(IV)p: the lattice parameters a, b, and c and the angles α, β, and γ.

Figure 11

Second coordination spheres of titanium cations within (a) Ti(III)p, (b) Ti(IV)p, and (c) TiP₂O₇. The [P₂O₇]^4– group, which is connected via two coordination sites, is marked in green.