Uncovering the crystal nucleation and growth mechanisms in NASICON materials

What is it about?

We investigate the process of crystal nucleation in a class of glass-ceramic materials that are super-ionic conductors with battery and sensor applications. We identify the nucleation sites and the mechanism for crystal growth.

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Photo by Emily Bernal on Unsplash

Photo by Emily Bernal on Unsplash

Why is it important?

Homogeneous nucleation in glass offers the means of preparing materials with a uniform distribution of crystallites of controllable size and shape. The process is crucial in the production of many glass-ceramics that include the superionic conducting materials for battery and sensor applications. It is therefore desirable to know the glass structure, and how this structure changes during thermal annealing and the early stages of crystal nucleation and growth. The latter is of general scientific interest for any glass undergoing homogeneous nucleation but has not been explored in non-siliceous systems. Here we address this issue for amorphous and crystalline materials in the sodium (Na) superionic conductor (NASICON) system Na1+xAlxGe2-x(PO4)3. Complementary information on the structure was obtained by combining neutron and x-ray powder and pair-distribution function analysis with solid-state NMR techniques. We find a glass structure that is built from super-structural units embedded in a matrix of corner-sharing tetrahedral GeO4 units. The super-structural units contain phosphate groups that provide the nucleation sites for crystal growth via a mechanism that converts GeO4 to octahedral GeO6 units. Our approach can be adapted to other glass-ceramic materials in order to help identify the mechanisms of homogeneous nucleation.

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