Layered Iron Oxyhydroxides (LIOX)

• Layered iron oxyhydroxides (LIOX) are iron-containing oxyhydroxide materials with anisotropic Fe–O/OH frameworks formed by specific octahedral connectivity and hydrogen bonding.
• They exhibit diverse crystal chemistries across polymorphs such as goethite, lepidocrocite, and novel monoclinic structures, underpinning unique electrocatalytic and ferroic properties.
• Synthesis strategies including controlled precursor chemistry, urea hydrolysis, and size-tuning treatments optimize their performance in oxygen evolution, magnetic response, and optical applications.

Layered iron oxyhydroxides (LIOX) comprise iron-containing oxyhydroxide materials with layered or strongly anisotropic Fe–O/OH frameworks whose structure is governed by octahedral connectivity, hydrogen bonding, intercalation chemistry, or related stacking motifs. In the cited literature, the term encompasses classical FeOOH polymorphs such as goethite, lepidocrocite, and $\epsilon$-FeOOH; NiFe layered double hydroxides treated as part of a broader layered iron (oxy)hydroxide class for oxygen-evolution electrocatalysis; and a recently reported FeOOH polymorph with monoclinic $Pn$ symmetry, dimerized FeO$_6$ octahedra, and zig-zag stacking (Guo et al., 2011, Tyndall et al., 2022, Wu et al., 2018, Park et al., 17 Aug 2025).

1. Scope and nomenclature

Taken together, these sources suggest that “LIOX” is used in two related senses. One is a family-level designation for layered iron (oxy)hydroxide materials, including FeOOH polymorphs and iron-containing layered hydroxides. The other is a material-specific shorthand for a new FeOOH polymorph whose layered topology, magnetic behavior, and ligand intercalation chemistry were examined in detail (Park et al., 17 Aug 2025).

At the family level, the defining feature is not a single space group or stoichiometry but a recurrent combination of Fe-centered octahedra, anisotropic stacking, and chemically consequential OH-containing motifs. In lepidocrocite, double chains of Fe(O,OH)$_6$ octahedra form extended sheets held together by weak hydrogen bonds; in goethite, double chains of edge- and corner-sharing FeO$_6$ octahedra define the structure; in distorted-rutile oxyhydroxides such as $\epsilon$-FeOOH, asymmetric O–H···O bonds become central to ferroic behavior; and in NiFe LDH, carbonate-intercalated hydroxide layers provide a catalytically active layered host (Guo et al., 2011, Wu et al., 2018, Tyndall et al., 2022).

This breadth of usage is important because structure–property relations differ across the family. Some LIOX systems are discussed primarily as electrocatalysts for the oxygen evolution reaction (OER), some as hydrogen-bond-driven ferroelectrics or multiferroics, some as environmentally responsive interfacial phases, and some as quantum magnetic or chiroptical materials (Tyndall et al., 2022, Wu et al., 2018, Kraushofer et al., 2019, Park et al., 17 Aug 2025).

2. Crystal chemistry and structural motifs

The crystallographic diversity of LIOX is large, but a limited set of structural motifs recurs.

Material	Reported structure	Distinctive structural feature
Goethite, $\alpha$-FeOOH	Orthorhombic $Pnma$	Double chains of edge- and corner-sharing FeO$_6$ octahedra
Lepidocrocite, $\gamma$-FeOOH	Orthorhombic $Pn$0	Extended sheets held together by weak hydrogen bonds
$Pn$1-FeOOH	Distorted rutile, $Pn$2	O–H···O bonds associated with proton-transfer ferroelectricity
NiFe LDH	Hexagonal platelet phase	$Pn$3 Å, $Pn$4 Å, $Pn$5 Å, consistent with $Pn$6-intercalated LDH
Monoclinic LIOX polymorph	$Pn$7	Dimerized FeO$Pn$8 octahedra, zig-zag chains, basal spacing $Pn$9 nm

The layered nature of these materials arises through different chemical mechanisms. In lepidocrocite, weak hydrogen bonding between sheets creates a delicate interlayer direction whose description is computationally difficult and physically important (Guo et al., 2011). In NiFe LDH, interlayer carbonate and water stabilize a brucite-derived layered arrangement; one synthesized composition was approximated as $_6$0, and atomic-resolution HAADF-STEM and SAED showed highly crystalline hexagonal platelets with a defect-free basal plane (Tyndall et al., 2022). In the monoclinic FeOOH polymorph, the basal spacing exceeds the interatomic distances required for magnetic exchange, suppressing interlayer coupling and favoring two-dimensional magnetic behavior (Park et al., 17 Aug 2025).

A recurring structural theme is the functional asymmetry between basal planes and edges. In NiFe LDH, the basal plane was reported to be defect-free, whereas catalytic activity was assigned primarily to edge sites; this distinction later becomes central to morphology–activity correlations in OER (Tyndall et al., 2022). In the chiral FeOOH polymorph, by contrast, the decisive structural motif is the zig-zag ladder of dimerized FeO$_6$1 units, whose geometry can be distorted by bidentate chiral ligands to generate screw-axis-related spin helicity (Park et al., 17 Aug 2025).

3. Formation pathways, synthesis, and morphology control

A low-temperature synthetic route to crystalline NiFe LDH relies on controlled precursor chemistry rather than direct precipitation. At room temperature and $_6$2 after TEA addition, Fe$_6$3 forms a thermodynamically metastable iron oxyhydroxide, likely ferrihydrite, as a gel-like brown precipitate, while Ni$_6$4 remains in solution as a Ni–TEA complex. Upon heating to $_6$5C, urea hydrolyzes, raising pH and supplying carbonate, the ferrihydrite decomposes, Ni–TEA begins to release Ni$_6$6, and the ions recombine into highly crystalline Ni–Fe LDH hexagonal platelets. The urea-hydrolysis step was written as

$_6$7

In this route, the oxyhydroxide intermediate acts as a transient storage and controlled-release reservoir for Fe$_6$8 (Jaskaniec et al., 2017).

The resulting LDH flakes were reported with diameters in the range $_6$9–$_6$0 and thicknesses between $_6$1 and $_6$2 nm in one study, and as highly regular hexagonal platelets with lateral sizes $_6$3–$_6$4 and average $_6$5 in a later study using homogeneous precipitation followed by reflux at $_6$6C for $_6$7 h (Jaskaniec et al., 2017, Tyndall et al., 2022).

Direct statistical size control during NiFe LDH synthesis was reported as not achievable, attributed to the non-amphoteric Fe$_6$8 intermediate chemistry. Post-synthetic treatments were therefore used instead. Tip-sonication mechanically fragmented platelets to lateral sizes as low as $_6$9–$_6$0, and centrifugation-driven size selection separated flakes by sedimentation rate. Both methods retained crystallinity and oxidation state while increasing the edge-to-area ratio (Tyndall et al., 2022).

Related hydrothermal work on two-dimensional layered transition metal hydroxides proposed a general growth picture based on edge-on condensation of $_6$1-type precursors, slow OH$_6$2 generation from urea, and careful tuning of temperature, cooling rate, and initial pH. For $_6$3-Ni(OH)$_6$4, the reported optimum was $_6$5C, $_6$6 h, cooling at $_6$7, and initial $_6$8 without added base. The same study states that these considerations can inform LIOX synthesis, while also cautioning that Fe coordination and redox chemistry may make direct transfer nontrivial (Ping et al., 2022). This suggests that precursor geometry and controlled hydroxyl supply are likely to remain important variables when large-domain LIOX crystals are targeted.

4. Ferroic, magnetic, and optical properties

First-principles work on distorted-rutile oxyhydroxides identified proton-transfer ferroelectricity in $_6$9-GaOOH, InOOH, $\epsilon$0-CrOOH, and $\epsilon$1-FeOOH. These compounds were reported in the non-centrosymmetric space group $\epsilon$2, with O–H···O bonds providing the broken inversion symmetry required for polarization switching. In this mechanism, protons hop between O atoms along hydrogen bonds; Berry-phase calculations gave polarizations up to $\epsilon$3, with $\epsilon$4-FeOOH reported at $\epsilon$5 $\epsilon$6 and $\epsilon$7 $\epsilon$8 (Wu et al., 2018).

The same study reported an energy barrier of $\epsilon$9 meV for proton transfer and used

$\alpha$0

to estimate a ferroelectric transition temperature of $\alpha$1 K. In $\alpha$2-CrOOH, ferroelectricity and magnetism coexist in two distinct ways: type-I multiferroicity in the $\alpha$3-plane and type-II multiferroicity along $\alpha$4, with a calculated vertical polarization of $\alpha$5. Tensile strain $\alpha$6 was reported to drive the system ferromagnetic and raise the mean-field Curie temperature to $\alpha$7 K (Wu et al., 2018).

A separate FeOOH polymorph designated LIOX was described as a room-temperature quantum magnet built from dimerized FeO$\alpha$8 octahedra with zig-zag stacking. Chiral amino-acid intercalation replaces acetate by ion exchange, introduces a twist distortion, breaks mirror symmetry, and generates $\alpha$9 screw axes along the zig-zag topology. The resulting chirality activates Dzyaloshinskii–Moriya interactions and converts quasi-one-dimensional spin ladders into helical spin ladders with mirror-asymmetric spin alignments (Park et al., 17 Aug 2025).

Its magnetic susceptibility was summarized with

$Pnma$0

and the spin gap increased from $Pnma$1 meV in achiral material to $Pnma$2 and $Pnma$3 meV for L- and D-proline intercalates, respectively. The same system displayed mirror-image magnetic circular dichroism in the UV-visible range, with an external field of $Pnma$4 T modulating the MCD signal by up to $Pnma$5 (Park et al., 17 Aug 2025). A notable point is that the spin ladders were described as statically rather than dynamically polarized, which the study connected to robust room-temperature spin selectivity.

5. Electrocatalysis and aqueous interface chemistry

For OER in alkaline medium, the reported half-reaction is

$Pnma$6

In NiFe LDH-based LIOX electrocatalysts, the central mechanistic claim is that edge sites, rather than basal planes, are the major active centers. Accordingly, reducing platelet size increases the edge-to-area ratio and improves activity. The overpotential at $Pnma$7, $Pnma$8, decreased from approximately $Pnma$9 mV for as-synthesized NiFe LDH to $_6$0 mV for size-selected smaller platelets, all versus RHE before $_6$1-correction. Electrochemical controls showed that the improvement was not due to increased electrochemical surface area or introduced defects (Tyndall et al., 2022).

Composite formation with single-wall carbon nanotubes was then used to suppress agglomeration, increase electrical conductivity, and provide mechanical strength. The optimized size-reduced NiFe LDH/SWCNT composite, with $_6$2, $_6$3 SWCNT, and mass loading $_6$4–$_6$5, reached $_6$6 mV. The same study reported Tafel slopes of $_6$7–$_6$8, only $_6$9 increase in overpotential after $\gamma$0 cycles, $\gamma$1 mV after $\gamma$2 h at $\gamma$3, and $\gamma$4 mV after $\gamma$5 h of continuous operation. Under similar conditions, pure commercial IrO$\gamma$6 was quoted at $\gamma$7 mV (Tyndall et al., 2022).

A complementary catalytic picture emerges in the chiral FeOOH polymorph. There, large basal spacing was reported to facilitate water access to the active spin ladders, and static spin polarization was connected to OER because triplet $\gamma$8 is spin-sensitive. The reported Tafel slopes were $\gamma$9 for acetate LIOX, $Pn$00 for L-Pro LIOX, and $Pn$01 for D-Pro LIOX; an external magnetic field reduced the Tafel slope of achiral LIOX by $Pn$02 even at $Pn$03 T (Park et al., 17 Aug 2025). This indicates that in LIOX, catalytic optimization can proceed not only through morphology and conductivity engineering but also through spin-state engineering.

At aqueous iron-oxide interfaces, oxyhydroxide formation can also be self-limiting. On Fe$Pn$04O$Pn$05(001), exposure to liquid water or $Pn$06 mbar water vapor caused extensive hydroxylation, removal of the $Pn$07 reconstruction in LEED, and slow growth of an oxyhydroxide phase that saturated at approximately $Pn$08 coverage. STM revealed chain-like protrusions aligned along $Pn$09, while XPS showed a hydroxyl O 1s component at $Pn$10 eV rising to $Pn$11 of total O 1s area and no molecular-water peak at $Pn$12 eV. Growth ceased once the surface oxygen lattice was saturated with H, preventing further dissociation of water; similar chain motifs were observed by in situ EC-STM at $Pn$13 (Kraushofer et al., 2019). This is a kinetic-passivation picture rather than a simple adsorption equilibrium.

6. Computation, phase stability, and high-pressure analogs

Theoretical treatment of LIOX is demanding because competing iron oxide and oxyhydroxide phases can differ in formation energies by only several kJ/mol, may undergo magnetization transitions with temperature, may contain partially occupied sites or vacancy ordering, and can depend sensitively on hydrogen bonding and dispersive forces. Comparative density-functional studies therefore used consistent settings across phases, with VASP plane-wave calculations, SIESTA locally confined atomic orbitals, GGA–PBE, and GGA+$Pn$14 with $Pn$15 eV chosen to improve agreement with band gaps and structures (Guo et al., 2011).

Within that framework, goethite and lepidocrocite were treated as model LIOX systems. Goethite was predicted to be more stable than lepidocrocite and hematite under wet conditions, while lepidocrocite was always metastable with respect to goethite. The same work emphasized that lepidocrocite’s interlayer direction is especially delicate because weak hydrogen bonding can be poorly described and may collapse during optimization if weak interactions are not handled adequately (Guo et al., 2011). The broader implication is that environmental phase selection in LIOX is controlled by small thermodynamic differences coupled to substantial kinetic persistence.

A pressure-extreme extension of layered iron chemistry appears in iron-rich Fe–O compounds synthesized at $Pn$16–$Pn$17 GPa and $Pn$18–$Pn$19 K. These $Pn$20 phases, including $Pn$21, $Pn$22, $Pn$23, $Pn$24, and experimentally resolved layered supercells such as $Pn$25 and $Pn$26, consist of oxygen-only close-packed monolayers distributed between iron-only layers. They are not oxyhydroxides, but the study explicitly identified them as anhydrous, metallic analogs of low-pressure LIOX motifs, with pure O layers replacing hydroxide layers (Liu et al., 2021).

All of these high-pressure $Pn$27 phases were reported metallic at $Pn$28 GPa and $Pn$29 K, and the density of states at the Fermi level increased systematically with oxygen content. This suggests that the layered arrangement of iron and anions remains energetically viable even when hydrogen is excluded and pressures reach inner-core conditions (Liu et al., 2021). In that sense, high-pressure Fe–O phases define a structural boundary condition for LIOX-like layering: the layered topology persists, while hydroxyl chemistry is replaced by metallic close-packed oxygen planes.