NASICON Framework: Structure & Ionic Transport

• NASICON is a robust three-dimensional polyanionic network known for its high sodium-ion conductivity and interconnected open-framework channels.
• Its structure is defined by corner-sharing MO6 octahedra and PO4 tetrahedra that ensure both compositional flexibility and structural stability.
• Ionic transport in NASICON is modulated by Na ordering and doping strategies, making it crucial for sodium-ion battery and solid electrolyte applications.

The NASICON (Sodium [Na] SuperIonic CONductor) framework is a robust three-dimensional polyanionic network widely studied for its high ionic conductivity, open-framework transport pathways, and structural versatility. The archetypal NASICON structure accommodates diverse transition metals and polyanion chemistries, making it foundational in sodium-ion battery cathode research, solid electrolytes, and ionic conductors. The canonical formula is $\mathrm{Na}_x M_2(\mathrm{PO}_4)_3$ ($M=$ transition metal), but significant compositional and topological flexibility exists through aliovalent doping and polyanion substitution. The defining motif is the three-dimensional linkage of $M$O$_6$ octahedra and PO$_4$ tetrahedra via corner sharing, producing large, interconnected cages and quasi-hexagonal channels that host mobile Na ions (Sharma et al., 11 Jan 2026, Sapra et al., 2024, Sharma et al., 14 Apr 2026).

1. Crystallographic Characteristics and Symmetry

NASICON frameworks typically crystallize in rhombohedral $R\bar{3}c$ (No. 167) symmetry, expressed in the hexagonal setting as $a \simeq 8.6$–$8.9$ Å, $c \simeq 20.9$–$22$ Å, and $M=$0. The cell accommodates $M=$1 formula units. The basic Wyckoff sites are as follows:

• 6b—Na(1): (0,0,0) or (0,0,¼)
• 18e—Na(2): ($M=$2, 0, ¼), typically partially occupied
• 12c/12e—Transition metal $M=$3: (0,0,$M=$4)
• 18e—P, S, or other polyanion centers: ($M=$5, 0, 0.25)
• 36f—O: general sites

Atomic arrangements are highly symmetric and maintain three-fold rotational and inversion symmetries, yielding degenerate Na migration pathways and minimizing channel bottlenecks (Sharma et al., 11 Jan 2026, Singh et al., 2024).

2. Polyhedral Connectivity and Framework Motifs

The structure is defined by corner-sharing $M=$6O$M=$7 octahedra and PO$M=$8 tetrahedra—alternatively, in mixed-polyanion systems, SO$M=$9 and PO$M$0 alternate at 18e positions (Singh et al., 2024). Octahedra and tetrahedra build “lantern units” (paired $M$1O$M$2 bridged by three PO$M$3). Each $M$4O$M$5 shares its six vertices with adjacent tetrahedra, and PO$M$6 units link three $M$7O$M$8 and one other tetrahedron (Figure 1(g) in (Sapra et al., 2024)). This generates an "open" three-dimensional network.

Table 1. Representative Bond Distances and Coordination Environments for Typical R$M$9c NASICONs

Cation	Environment	Bond Lengths (Å)
M (V, Ni)	MO$_6$0 octahedron	1.95–2.15
Na(1)	NaO$_6$1 octahedron	2.36–2.81
Na(2)	NaO$_6$2 dodecahedron	2.36–2.80
P	PO$_6$3 tetrahedron	1.49–1.60

The rigid polyanionic backbone minimizes lattice deformation during Na insertion/extraction and supports high framework stability under deep cycling (Sharma et al., 11 Jan 2026, Sharma et al., 13 May 2025).

3. Na Sublattice, Order-Disorder Phenomena, and Mobile Ion Pathways

The conducting Na sublattice is distributed over two primary sites:

• Na(1) [6b]: Octahedral void, typically fully or nearly filled in fully sodiated states.
• Na(2) [18e]: Dodecahedral/bicapped-prism sites, partially filled; site occupancy modulates with overall Na content.

Na(1) and Na(2) are linked via continuous three-dimensional channels running parallel to $_6$4 and within the ab plane. BVEL mapping quantifies migration energy barriers (e.g., $_6$5 eV for Na$_6$6NiCr(PO$_6$7)$_6$8 (Sharma et al., 11 Jan 2026); $_6$9 eV for Na$_4$0Mn$_4$1Ti$_4$2Mo$_4$3[PO$_4$4]$_4$5 (Sharma et al., 13 May 2025)). These values are compatible with measured and calculated Na$_4$6 diffusivities in the $_4$7–$_4$8 cm$_4$9 s$R\bar{3}c$0 range, with the entire structure supporting high Na$R\bar{3}c$1 mobility in the rhombohedral phase (Sapra et al., 2024, Sharma et al., 14 Apr 2026, Wang et al., 2021).

Site-specific and global Na ordering phenomena coexist:

• Order–disorder transitions: Temperature-dependent symmetry-lowering transitions frequently occur (e.g., monoclinic $R\bar{3}c$2 with ordered Na at low T; disordered rhombohedral $R\bar{3}c$3 at high T (Sharma et al., 14 Apr 2026)).
• Coupled Na and charge order: Partial occupancies lead to specific Na/vacancy arrangements, often stabilized by charge order on $R\bar{3}c$4 sites (e.g., V$R\bar{3}c$5/V$R\bar{3}c$6 ordering in Na$R\bar{3}c$7V$R\bar{3}c$8(PO$R\bar{3}c$9)$a \simeq 8.6$0 (Wang et al., 2021)).

4. Compositional Flexibility: Mixed Metals and Polyanions

NASICON accommodates substantial M-site substitution (Ni, Cr, V, Fe, Mn, Co, Ti, Mo), polyanion mixing (PO$a \simeq 8.6$1, SO$a \simeq 8.6$2), and aliovalent doping:

• Transition-metal mixing: E.g., Na$a \simeq 8.6$3NiCr(PO$a \simeq 8.6$4)$a \simeq 8.6$5 with mixed Ni/Cr on the 12e site (Sharma et al., 11 Jan 2026); Na$a \simeq 8.6$6FeCr(PO$a \simeq 8.6$7)$a \simeq 8.6$8 with Fe/Cr (Sharma et al., 14 Apr 2026).
• Polyanion mixing: NaFe$a \simeq 8.6$9PO$8.9$0(SO$8.9$1)$8.9$2 alternates PO$8.9$3 and SO$8.9$4 on 18e sites (Singh et al., 2024).
• Effect of substitution: Cation doping can modulate cell size, Na-site preference, charge states, electronic/ionic conductivities, and structural stability. For example, low-level Co doping in Na$8.9$5V$8.9$6Co$8.9$7(PO$8.9$8)$8.9$9 ($c \simeq 20.9$0) slightly contracts the $c \simeq 20.9$1-axis without symmetry lowering; small Mo doping in Na$c \simeq 20.9$2Mn$c \simeq 20.9$3Ti$c \simeq 20.9$4Mo$c \simeq 20.9$5[PO$c \simeq 20.9$6]$c \simeq 20.9$7 suppresses Jahn–Teller distortions (Sapra et al., 2024, Sharma et al., 13 May 2025).

5. Lattice Dynamics, Stability, and Structural Response

NASICON frameworks exhibit exceptionally low volume change ($c \simeq 20.9$8\% across full desodiation), preserving structural integrity upon deep cycling (Wang et al., 2021). Thermal order-disorder transitions are governed by configurational entropy of Na/vacancy distributions, and are often first order with significant lattice strain at the symmetry-breaking transition. Calorimetric signatures (e.g., $c \simeq 20.9$9 kJ/mol for $22$0 in Na$22$1FeCr(PO$22$2)$22$3) quantitatively differentiate substantial configurational rearrangement intervals from minor transitions (Sharma et al., 14 Apr 2026). Distortions in $22$4O$22$5 octahedra and Na$22$6 environments are typically modest (octahedral $22$7 Å$22$8; cation shifts $22$9 Å), with high-symmetry $M=$00 frameworks suppressing significant polyhedral tilting.

6. Functional Implications: Ionic and Electronic Transport, Applications

The open 3D channel system and moderate Na$M=$01 migration barriers underwrite high ionic conductivity, cyclic stability, and tolerance to repeated Na extraction/insertion—essential for sodium-ion batteries (Sharma et al., 11 Jan 2026, Sapra et al., 2024). Typical limitations are poor electronic conductivity and, in some high-voltage variants, redox irreversibility (e.g., negligible discharge capacity in Na$M=$02NiCr(PO$M=$03)$M=$04 despite facile Na$M=$05 transport (Sharma et al., 11 Jan 2026)). Doping, carbon-coating, and structural optimization are routine strategies to overcome these bottlenecks.

NASICON-type frameworks are also relevant in solid electrolytes and biomimetics of biological ion conductors, owing to their tunable conduction pathways and chemical robustness.

7. Comparison with Other Framework Topologies and Outlook

Compared with low-symmetry (monoclinic $M=$06) NASICON polymorphs, the rhombohedral $M=$07 structure supports more symmetric and contiguous Na$M=$08 channels and single A-cation and M-cation sublattices. Monoclinic variants exhibit split sublattice occupation and “zig-zag” migration paths, reducing ionic mobility and introducing more pronounced distortion fields (Singh et al., 2024, Sharma et al., 14 Apr 2026).

Structural rigidity, compositional versatility, and robust Na$M=$09 transport distinguish NASICON from Prussian blue analogues, layer-structured sodium hosts, and other polyanionic or oxide frameworks central to energy storage research.

The principal challenges remain systematic control of Na ordering, optimization of the electronic conduction network, and understanding the interplay between Na/vacancy configurational entropy and framework energetics. The application space for NASICON frameworks is broadening with the emergence of multi-polyanion chemistries, multi-electron transition metal redox, and integration into all-solid-state battery architectures (Sharma et al., 11 Jan 2026, Sharma et al., 14 Apr 2026, Sharma et al., 13 May 2025).