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

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Transition-metal phosphates are versatile materials used in catalysis, batteries, and fuel cells.
• Their diverse compositions and structures offer tunable properties for various applications.

Purpose of the Study:

• To determine the crystal structures of two novel titanium pyrophosphates, Ti(III)p and Ti(IV)p, using ab initio methods.
• To investigate the structural transformation from Ti(III)p to Ti(IV)p.
• To evaluate the proton conductivity of Ti(III)p for potential use in electrochemical devices.

Main Methods:

• Ab initio structure determination from powder X-ray diffraction (PXRD) data.
• Combination of spectroscopic and diffraction techniques for low-symmetry structures.
• In situ PXRD to study the transformation mechanism between Ti(III)p and Ti(IV)p.

Main Results:

• Successfully determined the low-symmetry crystal structures of Ti(III)p (space group P2₁/c) and Ti(IV)p (space group P1̅).
• Ti(III)p features 1D channels stabilized by NH₄⁺ ions, while Ti(IV)p has a similar framework with empty channels.
• The transformation from Ti(III)p to Ti(IV)p occurs via a two-step mechanism involving NH₄⁺ decomposition and structure relaxation.
• Ti(III)p demonstrates high proton conductivity and low activation energy, attributed to the Grotthus mechanism.

Conclusions:

• The novel titanium pyrophosphates Ti(III)p and Ti(IV)p have been successfully synthesized and structurally characterized.
• Ti(III)p exhibits excellent proton conductivity, making it a promising candidate for intermediate-temperature fuel cells and water electrolyzers.
• The findings provide insights into the structure-property relationships of transition-metal phosphates for energy applications.