{ "id": "ep7qrgjd4x", "slug": "layered-sodium-cobaltate", "title": "Layered sodium cobaltate", "updated": "2026-10-15", "topic_path": [ { "id": "physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/topics/physical" }, { "id": "physical.chemistry", "label": "Chemistry", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.chemistry" }, { "id": "physical.chemistry.elements", "label": "Elements and inorganic substances", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.chemistry.elements" }, { "id": "physical.chemistry.elements.elements.oxides", "label": "Oxides and oxygen compounds", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.chemistry.elements.elements.oxides" }, { "id": "physical.chemistry.elements.elements.oxides.metal", "label": "Metal oxides and hydroxides", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.chemistry.elements.elements.oxides.metal" }, { "id": "physical.chemistry.elements.elements.oxides.metal.transition", "label": "Transition-metal oxides", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.chemistry.elements.elements.oxides.metal.transition" } ], "geo": [ { "id": "geo.us.t2001.physical.chemistry.elements", "label": "United States · 2001 to 2020: Elements and inorganic substances", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t2001.physical.chemistry.elements", "path": [ { "id": "geo.us", "label": "United States", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us" }, { "id": "geo.us.t2001", "label": "United States · 2001 to 2020", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t2001" }, { "id": "geo.us.t2001.physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t2001.physical" }, { "id": "geo.us.t2001.physical.chemistry", "label": "Chemistry", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t2001.physical.chemistry" }, { "id": "geo.us.t2001.physical.chemistry.elements", "label": "Elements and inorganic substances", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t2001.physical.chemistry.elements" } ] }, { "id": "geo.asia.t1946.physical.chemistry.elements", "label": "Asia (other) · 1946 to 2000: Elements and inorganic substances", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.asia.t1946.physical.chemistry.elements", "path": [ { "id": "geo.asia", "label": "Asia (other)", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.asia" }, { "id": "geo.asia.t1946", "label": "Asia (other) · 1946 to 2000", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.asia.t1946" }, { "id": "geo.asia.t1946.physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.asia.t1946.physical" }, { "id": "geo.asia.t1946.physical.chemistry", "label": "Chemistry", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.asia.t1946.physical.chemistry" }, { "id": "geo.asia.t1946.physical.chemistry.elements", "label": "Elements and inorganic substances", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.asia.t1946.physical.chemistry.elements" } ] }, { "id": "geo.asia.t2001.physical.chemistry", "label": "Asia (other) · 2001 to 2020: Chemistry", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.asia.t2001.physical.chemistry", "path": [ { "id": "geo.asia", "label": "Asia (other)", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.asia" }, { "id": "geo.asia.t2001", "label": "Asia (other) · 2001 to 2020", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.asia.t2001" }, { "id": "geo.asia.t2001.physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.asia.t2001.physical" }, { "id": "geo.asia.t2001.physical.chemistry", "label": "Chemistry", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.asia.t2001.physical.chemistry" } ] } ], "excerpt": "Layered sodium cobaltate (NaxCoO2) is a layered cobalt oxide whose sodium content controls its properties; it shows a large thermoelectric power, and its hydrate superconducts near 5 K.", "snippet": "Layered sodium cobaltate (NaxCoO2) is a layered cobalt oxide whose sodium content controls its properties; it shows a large thermoelectric power, and its hydrate superconducts near 5 K.", "node": "physical.chemistry.elements.elements.oxides.metal.transition", "markdown": "# Layered sodium cobaltate\n\n**Layered sodium cobaltate** (NaxCoO2, also written NaCo2O4 for x ≈ 0.75) is a family of layered transition-metal oxides made of stacked sodium layers and two-dimensional CoO2 sheets, in which the sodium content x can be varied continuously and controls the electronic properties. It is a strongly correlated oxide that shows an unusually large thermoelectric power, a spin-entropy-driven [Seebeck effect](https://www.edgepedia.org/seebeck-effect), and, when hydrated near x ≈ 0.35, superconductivity at about 5 K.[1](https://www.nature.com/articles/nature01450) • [2](https://doi.org/10.1103/PHYSREVB.56.R12685) • [3](https://link.springer.com/article/10.1007/s40243-020-00175-5)\n\n| Key fact | Detail |\n|---|---|\n| Structure | Hexagonal lattice (space group P63/mmc) of edge-sharing CoO6 octahedral layers alternating with Na layers along the c axis; four packing variants (O3, O′3, P2, P3) occur depending on sodium content[4](https://tuprints.ulb.tu-darmstadt.de/776/2/Thesis_Part_B.pdf) • [3](https://link.springer.com/article/10.1007/s40243-020-00175-5) • [5](https://www.dora.lib4ri.ch/psi/islandora/object/psi%3A72412/datastream/PDF/Forslund-2025-Revisiting_NaxCoO2-%28published_version%29.pdf) |\n| Thermopower | In-plane thermoelectric power about 100 μV/K at 300 K in single crystals, nearly ten times that of typical metals, with in-plane resistivity as low as 200 μΩ cm[2](https://doi.org/10.1103/PHYSREVB.56.R12685) |\n| Thermoelectric performance | Power factor of 5 mW m−1 K−2 in-plane at room temperature; ZT of about 1.2 at 800 K reported for single crystals[6](https://www.nature.com/articles/s41427-023-00520-w) |\n| Carrier concentration | On the order of 1021 to 1022 cm−3, p-type, with hole density inversely proportional to x[3](https://link.springer.com/article/10.1007/s40243-020-00175-5) |\n| Superconductivity | Hydrated NaxCoO2·yH2O (x ≈ 0.35, y ≈ 1.3) superconducts at Tc of about 5 K, the first cobalt oxide superconductor[1](https://www.nature.com/articles/nature01450) • [7](https://archive.nims.go.jp/eng/publicity/nimsnow/2003/hdfqf10000009kz3-att/NIMSNOWVol1No3.pdf) |\n| Insulating phase | Na0.5CoO2 is a charge-ordered insulator with orthorhombic Pnmm symmetry[8](https://journals.aps.org/prb/abstract/10.1103/PhysRevB.73.134401) |\n\n## Structure and composition\n\nNaxCoO2 consists of rigid CoO2 layers and mobile Na+ layers stacked alternately along the c axis. Co and O occupy the Wyckoff 2a and 4f sites of the hexagonal lattice, and the unit cell contains two Na layers and two CoO2 layers.[3](https://link.springer.com/article/10.1007/s40243-020-00175-5) The sodium content x is the main control parameter: each Na vacancy creates a hole that converts a Co3+ ion into Co4+, so the material conducts p-type with a carrier concentration inversely proportional to x.[3](https://link.springer.com/article/10.1007/s40243-020-00175-5) Four distinct layered packings, O3, O′3, P2, and P3, appear across the composition range, distinguished by the number of CoO2 slabs per unit cell.[5](https://www.dora.lib4ri.ch/psi/islandora/object/psi%3A72412/datastream/PDF/Forslund-2025-Revisiting_NaxCoO2-%28published_version%29.pdf)\n\nThe sodium ions are not randomly distributed. An electron diffraction study over 0.15 < x < 0.75 found an extensive series of ordered Na ion–Na vacancy superlattices beyond the simple hexagonal average structure, with the most strongly developed superlattice at Na0.5CoO2; near x = 0.5 the ordering takes the surprising form of lines of Na ions and vacancies rather than simply maximized Na–Na separations.[9](https://arxiv.org/abs/cond-mat/0403206) This ordering matters physically: later work showed that not only the Na content but also the Coulomb potential caused by Na ordering can influence the low-temperature ground state.[5](https://www.dora.lib4ri.ch/psi/islandora/object/psi%3A72412/datastream/PDF/Forslund-2025-Revisiting_NaxCoO2-%28published_version%29.pdf)\n\n## Thermoelectric behavior\n\n**Discovery of the large thermopower.** In 1997, I. Terasaki, professor of physics known for work on oxide thermoelectrics then at the Superconductivity Research Laboratory in Tokyo, and colleagues measured single-crystal NaCo2O4 and found an in-plane thermoelectric power of about 100 μV/K at 300 K, nearly ten times that of typical metals, together with a low in-plane resistivity of 200 μΩ cm; the large thermopower and low resistivity suggested a potential thermoelectric material.[2](https://doi.org/10.1103/PHYSREVB.56.R12685) The same single crystal showed an in-plane power factor S2·σ of 5 mW m−1 K−2 at room temperature, comparable in magnitude to the alloy Bi2Te3.[6](https://www.nature.com/articles/s41427-023-00520-w) • [10](https://pmc.ncbi.nlm.nih.gov/articles/PMC10421396/) In 2001, Fujita and coworkers reported ZT values of about 1.2 at 800 K for NaxCoO2 single crystals.[6](https://www.nature.com/articles/s41427-023-00520-w)\n\n**Spin entropy.** The origin of the large thermopower lies in the spin entropy carried by the charge carriers. In an electric field, electrons in a solid produce an entropy current in addition to the charge current, the Peltier effect that underlies thermoelectric refrigeration; theory predicted that the electronic spin entropy should dominate this entropy current in transition-metal oxides.[11](https://pubmed.ncbi.nlm.nih.gov/12761545/) In 2003, Yayu Wang, Nyrissa S. Rogado, R. J. Cava, and N. P. Ong of Princeton University reported the crucial test: in NaxCo2O4 the thermopower is strongly suppressed by a longitudinal magnetic field, and at T = 2 K a magnetic field completely suppresses the spin-entropy contribution to zero.[11](https://pubmed.ncbi.nlm.nih.gov/12761545/) • [3](https://link.springer.com/article/10.1007/s40243-020-00175-5) When an external field aligns all the spins, the disappearance of this contribution implies that the Seebeck effect originates from spin entropy flow carried by the holes.[3](https://link.springer.com/article/10.1007/s40243-020-00175-5)\n\nThe material behaves as a Curie–Weiss metal: its spin population is approximately equal to the hole population and stays constant as temperature falls, giving metallic conductivity with the magnetic behavior of a frustrated insulator.[3](https://link.springer.com/article/10.1007/s40243-020-00175-5) The conduction path lies in the CoO2 layers, where the low-spin Co3+ state and the hopping conduction between Co3+ and Co4+ generate the spin entropy flow; the high-temperature [Seebeck coefficient](https://www.edgepedia.org/seebeck-coefficient) follows the Koshibae equation, which depends on the spin and orbital degeneracies and concentrations of the two cobalt valence states.[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC10421396/) Reducing the Co4+ concentration raises the spin entropy, so doping at the A site is a way to tune the thermopower.[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC10421396/)\n\n**Lattice part.** The mobile Na+ ions diffusively scatter phonons, reducing the lattice thermal conductivity to about 0.01 W cm−1 K−1 near 1000 K, roughly 100 times smaller than that of ZnO at the same temperature.[3](https://link.springer.com/article/10.1007/s40243-020-00175-5) Because the layered structure makes the electrical resistivity sensitive to microstructure, good texturation is needed, and polycrystalline material made by conventional solid-state reaction reaches only about zT of 0.11 at 973 K, while the polymerized complex process gives 0.8 and single crystals prepared by a flux technique reach zT of 1.0 at 800 K.[12](https://onlinelibrary.wiley.com/doi/10.1002/sstr.202400136) Sodium volatility and degradation above 1173 K limit high-temperature use and motivated the study of related misfit cobaltites such as Ca3Co4O9.[12](https://onlinelibrary.wiley.com/doi/10.1002/sstr.202400136)\n\n## Superconductivity in the hydrate\n\nIn 2003, Kazunori Takada of the National Institute for Materials Science (NIMS) in Japan and colleagues reported that NaxCoO2·yH2O with x ≈ 0.35 and y ≈ 1.3 is a superconductor with Tc of about 5 K, the first superconducting cobalt oxide.[1](https://www.nature.com/articles/nature01450) • [7](https://archive.nims.go.jp/eng/publicity/nimsnow/2003/hdfqf10000009kz3-att/NIMSNOWVol1No3.pdf) Before this, searches for superconductivity in layered oxides of cobalt and nickel had failed, and the copper oxide layer was thought to be essential for superconductivity.[1](https://www.nature.com/articles/nature01450) The hydrate is made by soft chemistry from the thermodynamically stable γ-Na0.7CoO2: sodium is removed by Br2 in acetonitrile, water is intercalated, and the resulting material is superconducting.[13](https://www.sciencedirect.com/science/article/pii/S0921453415000374/pdfft?md5=475302e8abe2b78053be3d5584853e1c&pid=1-s2.0-S0921453415000374-main.pdf) • [14](https://www.nims.go.jp/eng/publicity/publication/nims-accomplishments/hdfqf1000000betd-att/NIMS21_en.pdf) NIMS work later clarified that hydration is not a simple water insertion: Na+ ions are substituted by oxonium ions (H3O+) during hydration, giving the composition Nax(H3O)zCoO2·yH2O with typical values x ≈ 0.35, z ≈ 0.16, and y ≈ 1.14.[14](https://www.nims.go.jp/eng/publicity/publication/nims-accomplishments/hdfqf1000000betd-att/NIMS21_en.pdf)\n\nThe superconducting compound has a highly two-dimensional triangular lattice of Co ions and a transition at TC = 4.7 K; a 2015 review classifies it as an extreme type-II unconventional superconductor whose superconducting gap has a line-nodal structure, while noting that contradictory experimental results and interpretations remain to be resolved.[13](https://www.sciencedirect.com/science/article/pii/S0921453415000374/pdfft?md5=475302e8abe2b78053be3d5584853e1c&pid=1-s2.0-S0921453415000374-main.pdf) Interest in the hydrate comes from the analogy with the cuprates: the Co4+ ions are in the low-spin state and carry S = 1/2 on a triangular rather than square lattice, so pairing might have triplet or d-wave symmetry, though conventional electron-phonon s-wave pairing with a role for anharmonic hydrogen and oxygen motion has also been considered.[15](https://www.nist.gov/ncnr/structure-and-dynamics-superconducting-naxcoo2-hydrate-and-its-unhydrated-analog) Neutron studies found that most of the water in the hydrate is static on an energy scale of 1 meV.[15](https://www.nist.gov/ncnr/structure-and-dynamics-superconducting-naxcoo2-hydrate-and-its-unhydrated-analog)\n\n## Phase diagram of the unhydrated compound\n\nThe sodium content x divides the unhydrated phase diagram into distinct regimes. At large x (0.75 < x < 0.95) the material is antiferromagnetic; at x = 0.5 it is a charge-ordered semiconducting phase; and intercalating water promotes superconductivity at x = 0.3.[5](https://www.dora.lib4ri.ch/psi/islandora/object/psi%3A72412/datastream/PDF/Forslund-2025-Revisiting_NaxCoO2-%28published_version%29.pdf) High-resolution neutron diffraction confirmed that Na0.5CoO2, the low-temperature insulator separating the antiferromagnetic and normal metals, has orthorhombic Pnmm symmetry with layers of edge-sharing CoO6 octahedra and ordered Na positions; the Na ordering creates two distinct Co sites in parallel chains, although the differences in Co–O bond distances and derived bond valence sums, which reflect the degree of [charge ordering](https://www.edgepedia.org/charge-ordering), are very small.[8](https://journals.aps.org/prb/abstract/10.1103/PhysRevB.73.134401) Pure density functional calculations fail to predict an insulating ground state for the sodium-ordered phase, indicating that Na ordering alone cannot produce the accompanying Co charge ordering without additional correlation effects.[16](https://journals.aps.org/prb/abstract/10.1103/PhysRevB.71.024502) Reported transport trends also depend on synthesis route: chemical de-intercalation from Na0.77CoO2 revealed a metal–insulator transition of the charge ordering in Na0.5CoO2 single crystals, an effect not detected in polycrystalline samples made by solid-state reaction.[3](https://link.springer.com/article/10.1007/s40243-020-00175-5)\n\n## Related layered cobaltates\n\nThe A-site cation can be exchanged. Using NaxCoO2 (0.6 ≤ x ≤ 1.0) as a precursor, multivalent ion-exchange reactions produce CaxCoO2, SrxCoO2, and BaxCoO2 with topotactic phase transitions.[6](https://www.nature.com/articles/s41427-023-00520-w) • [10](https://pmc.ncbi.nlm.nih.gov/articles/PMC10421396/) Among epitaxial films of these cobaltates, Ba1/3CoO2 is the most thermally robust, keeping a stable phase to 600 °C and reaching ZT of about 0.55 at 600 °C in air, a value comparable to p-type PbTe and p-type SiGe.[6](https://www.nature.com/articles/s41427-023-00520-w) Na3/4CoO2 itself has a practical weakness: it decomposes into insulating Co(OH)2 under high-humidity conditions (80 °C, about 80% humidity) because Na+ dissolves in water.[6](https://www.nature.com/articles/s41427-023-00520-w)\n\n## References\n\n1. [Kazunori Takada et al. (2003). Superconductivity in two-dimensional CoO2 layers. Nature 422, 53–55.](https://www.nature.com/articles/nature01450)\n2. [I. Terasaki, Y. Sasago, K. Uchinokura (1997). Large thermoelectric power in NaCo2O4 single crystals. Phys. Rev. B 56, R12685.](https://doi.org/10.1103/PHYSREVB.56.R12685)\n3. [A review of recent progress in thermoelectric materials through computational methods. Materials for Renewable and Sustainable Energy (2020).](https://link.springer.com/article/10.1007/s40243-020-00175-5)\n4. [Sodium cobaltate superconductors, TU Darmstadt thesis.](https://tuprints.ulb.tu-darmstadt.de/776/2/Thesis_Part_B.pdf)\n5. [Ola Kenji Forslund et al. (2025). Revisiting Na_xCoO_2: A renewed magnetic phase diagram based on electrochemical reaction synthesis. Phys. Rev. Research 7, 023138.](https://www.dora.lib4ri.ch/psi/islandora/object/psi%3A72412/datastream/PDF/Forslund-2025-Revisiting_NaxCoO2-%28published_version%29.pdf)\n6. [Recent progress in thermoelectric layered cobalt oxide thin films. NPG Asia Materials (2023).](https://www.nature.com/articles/s41427-023-00520-w)\n7. [NIMS NOW, Vol. 1 No. 3 (2003), announcement of the first cobalt oxide superconductor.](https://archive.nims.go.jp/eng/publicity/nimsnow/2003/hdfqf10000009kz3-att/NIMSNOWVol1No3.pdf)\n8. [A. J. Williams et al. (2006). High-resolution neutron diffraction study of possible charge ordering in Na0.5CoO2. Phys. Rev. B 73, 134401.](https://journals.aps.org/prb/abstract/10.1103/PhysRevB.73.134401)\n9. [H. W. Zandbergen et al. (2004). Sodium ion ordering in NaxCoO2: Electron diffraction study. Phys. Rev. B 70, 024101.](https://arxiv.org/abs/cond-mat/0403206)\n10. [Oxide Materials for Thermoelectric Conversion. PMC (2023).](https://pmc.ncbi.nlm.nih.gov/articles/PMC10421396/)\n11. [Yayu Wang, Nyrissa S. Rogado, R. J. Cava, N. P. Ong (2003). Spin entropy as the likely source of enhanced thermopower in Na_xCo_2O_4. Nature 423, 425–428.](https://pubmed.ncbi.nlm.nih.gov/12761545/)\n12. [M. González-Barrios et al. (2024). Perspective on Crystal Structures, Synthetic Methods, and New Directions in Thermoelectric Materials. Small Structures.](https://onlinelibrary.wiley.com/doi/10.1002/sstr.202400136)\n13. [Hiroya Sakurai, Yoshihiko Ihara, Kazunori Takada (2015). Superconductivity of cobalt oxide hydrate, Na_x(H_3O)_zCoO_2·yH_2O. Physica C 514, 378–387.](https://www.sciencedirect.com/science/article/pii/S0921453415000374/pdfft?md5=475302e8abe2b78053be3d5584853e1c&pid=1-s2.0-S0921453415000374-main.pdf)\n14. [NIMS, Synthesis and control of novel nano materials (2005).](https://www.nims.go.jp/eng/publicity/publication/nims-accomplishments/hdfqf1000000betd-att/NIMS21_en.pdf)\n15. [Structure and Dynamics of Superconducting NaxCoO2 Hydrate and Its Unhydrated Analog. NIST Center for Neutron Research.](https://www.nist.gov/ncnr/structure-and-dynamics-superconducting-naxcoo2-hydrate-and-its-unhydrated-analog)\n16. [Zhenyu Li, Jinlong Yang, J. G. Hou, Qingshi Zhu (2005). Geometrical, electronic, and magnetic properties of Na0.5CoO2 from first principles. Phys. Rev. B 71, 024502.](https://journals.aps.org/prb/abstract/10.1103/PhysRevB.71.024502)\n\n---\n*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Elements and inorganic substances › Oxides and oxygen compounds › Metal oxides and hydroxides › Transition-metal oxides*\n\n*Initially written Oct 15, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n", "same_as": [], "url": "https://www.edgepedia.org/layered-sodium-cobaltate", "markdown_url": "https://www.edgepedia.org/layered-sodium-cobaltate.md", "license": { "name": "Edgepedia Community License 1.0", "url": "https://www.edgepedia.org/edgepedia/license", "summary": "Free with credit, commercial use included. AI training is open to everyone. For other uses, organizations over USD 100M in revenue or 100M monthly users license separately.", "spdx": "LicenseRef-Edgepedia-Community-1.0" }, "credit": "\"Layered sodium cobaltate\", Edgepedia (EdgeChat), https://www.edgepedia.org/layered-sodium-cobaltate. Edgepedia Community License 1.0.", "credit_md": "\"[Layered sodium cobaltate](https://www.edgepedia.org/layered-sodium-cobaltate)\", Edgepedia (EdgeChat), [https://www.edgepedia.org/layered-sodium-cobaltate](https://www.edgepedia.org/layered-sodium-cobaltate). [Edgepedia Community License 1.0](https://www.edgepedia.org/edgepedia/license).", "credit_html": "\"Layered sodium cobaltate\", Edgepedia (EdgeChat), https://www.edgepedia.org/layered-sodium-cobaltate. Edgepedia Community License 1.0.", "speakable": "Layered sodium cobaltate is a layered cobalt oxide whose sodium content controls its properties; it shows a large thermoelectric power, and its hydrate superconducts near 5 K." }