Abstract:
This article studies the influence of solid-phase (type 1 samples) and melt-quenching
(type 2 samples) technological modes of obtaining Na3Fe2
(PO4
)3 polycrystals on their structures and
ion-conducting properties. α-Na3Fe2
(PO4
)3 polycrystals of the 1st type are formed predominantly
under an isothermal firing regime, and the synthesis of the 2nd type is carried out under sharp
temperature gradient conditions, contributing to the formation of glassy precursors possessing a
reactive and deformed structure, in which the crystallization of crystallites occurs faster than in
precursors obtained under isothermal firing. The elemental composition of α-Na3Fe2
(PO4
)3
type 2
polycrystals is maintained within the normal range despite the sharp non-equilibrium thermodynamic
conditions of synthesis. The microstructure of the type 1 Na3Fe2
(PO4
)3 polycrystals is dominated
by chaotically arranged crystallites of medium (7–10 µm) and large (15–35 µm) sizes, while the
polycrystals of type 2 are characterized by the preferential formation of small (3–4 µm) and medium
(7–10 µm) crystallites, causing uniaxial deformations in their structure, which contribute to a partial
increase in their symmetry. The advantage of type 2 polycrystals is that they have higher density
and conductivity and are synthesized faster than type 1 samples by a factor of 4. The article also
considers the issues of crystallization in a solid-phase precursor from the classical point of view, i.e.,
the process of the formation of small solid-phase nuclei in the metastable phase and their growth to
large particles due to association with small crystallites using phase transitions. Possible variants
and models of crystallite growth in Na3Fe2
(PO4
)3 polycrystals, as well as distinctive features of
crystallization between two types of samples, are discussed.
