Copper selenide (Cu2Se and CuSe) Cu2Se

spore: profile
id: copper-selenide
spore_version: 0.2
family: copper-selenides
name: Copper selenide (Cu2Se and CuSe)
abbreviation: Cu2Se
role: Superionic thermoelectric (Cu2Se); low-band-gap semiconductor studied for solar-driven photocatalysis (CuSe)
category: energy
type: thermoelectrics
summary: Copper(I) selenide (Cu2Se) is a superionic thermoelectric material. Nieroda et al. (2021) report a ZT of about 1.75 at 970 K for an undoped sample densified by spark plasma sintering at 720 °C. Copper(II) selenide (CuSe) is a hexagonal semiconductor with an estimated optical band gap of 1.67 to 1.74 eV in Mn-doped nanosheets (2025). Both compounds are toxic if swallowed or inhaled according to their supplier and PubChem hazard data (H301 and H331). The main stability problem is that copper ions move under current, which the 2021 and 2023 papers both report.
best_for: Laboratory studies of thermoelectric copper selenide, where a ZT of about 1.75 has been measured in a sintered sample
weakest_at: 'Stability: copper ions migrate under current load, and the 2023 Mn-doping study says this makes Cu2Se unsuitable for use in applications'
identity:
  formula: Cu2Se (copper(I) selenide); CuSe (copper(II) selenide) is covered in the same profile
  cas: 20405-64-5 (Cu2Se); 1317-41-5 (CuSe)
  crystal_system: cubic after hydrothermal synthesis (Nieroda et al. 2021); the paper also names a monoclinic alpha phase after sintering
  source: nieroda2021
  where: Section 3.1 (Structural and Microstructural Analysis); CAS numbers from the two supplier SDS documents and PubChem CID 73980
tags:
  environments: []
  stresses: []
  applications:
    - thermal-control
properties:
  - benchmark: figure_of_merit_zt
    label: Thermoelectric figure of merit ZT (undoped Cu2Se)
    value: 1.75
    unit: '-'
    scope: material
    conditions:
      material: undoped Cu2Se, hydrothermal nanopowder densified by spark plasma sintering (SPS) at 720 °C
      temperature: 970 K (text of Section 3.2); the abstract gives 965 K for the same ZT of about 1.75
    method: Maximum ZT printed in the text of Section 3.2 for the sample sintered at 720 °C
    status: ground
    note: The abstract gives ZT ~1.75 at T = 965 K and Section 3.2 gives about 1.75 at 970 K. Both printings are kept as one value.
    source: nieroda2021
    where: Section 3.2 (Thermoelectric properties, text beside Figure 7); Abstract
  - benchmark: density
    label: Density of Cu2Se densified at 823 K (550 °C)
    value: 4.84
    unit: g/cm3
    scope: material
    conditions:
      material: Cu2Se nanopowder densified by SPS at T = 823 K
      measurement: hydrostatic method
    method: Hydrostatic density printed in Section 2 (Materials and Methods); sintering temperature in the Figure 1 caption (550 °C)
    status: ground
    source: nieroda2021
    where: Section 2 (Materials and Methods), densification paragraph; Figure 1 caption
  - benchmark: density
    label: Density of Cu2Se densified at 923 K (650 °C)
    value: 5.58
    unit: g/cm3
    scope: material
    conditions:
      material: Cu2Se nanopowder densified by SPS at T = 923 K
      measurement: hydrostatic method
    method: Hydrostatic density printed in Section 2 (Materials and Methods); sintering temperature in the Figure 1 caption (650 °C)
    status: ground
    source: nieroda2021
    where: Section 2 (Materials and Methods), densification paragraph; Figure 1 caption
  - benchmark: density
    label: Density of Cu2Se densified at 993 K (720 °C)
    value: 6.02
    unit: g/cm3
    scope: material
    conditions:
      material: Cu2Se nanopowder densified by SPS at T = 993 K
      measurement: hydrostatic method
    method: Hydrostatic density printed in Section 2 (Materials and Methods); sintering temperature in the Figure 1 caption (720 °C)
    status: ground
    source: nieroda2021
    where: Section 2 (Materials and Methods), densification paragraph; Figure 1 caption
  - benchmark: density
    label: Relative density, Cu2Se (supplier SDS)
    value: 6.84
    unit: g/mL
    scope: material
    conditions:
      temperature: 25 °C
      form: powder, Cu2Se, CAS 20405-64-5
    method: Printed as Relative density in Section 9 of the supplier SDS; the SDS does not say whether the figure is measured or theoretical
    status: ground
    note: Labelled relative density in the SDS. Numerically it is a density in g/mL, which equals g/cm3.
    source: sam-cu2se-sds
    where: Section 9 (Physical and chemical properties), Relative density row
  - benchmark: phase_transition_temperature
    label: Alpha-to-beta phase transition of Cu2Se (inflection point)
    value: 396
    unit: K
    scope: material
    conditions:
      material: sintered Cu2Se samples in the paper
      basis: inflection point in the temperature dependence of the alpha(T) curve, as printed
    method: Read from the paper's temperature-dependence curves as printed in the text ('around 396 K')
    status: ground
    note: The paper names this as the transition between alpha- and beta-Cu2Se. It also refers once to Cu2S phases, which looks like a typo for Cu2Se.
    source: nieroda2021
    where: Section 3.2 (Thermoelectric properties), paragraph on the Seebeck coefficient
  - benchmark: phase_transition_temperature
    label: Phase transition temperature, undoped Cu2Se (DSC)
    value: 400.1
    unit: K
    scope: material
    conditions:
      composition: Cu2-xMnxSe at x = 0 (undoped Cu2Se, the start of the Mn series)
      method_detail: differential scanning calorimetry, heat-absorption peak
    method: Read from the DSC map; the paper says the phase transition temperature 'decreases from 400.1 K to 390.3 K' as Mn content rises
    status: ground
    note: Differs from the 396 K inflection point in Nieroda et al. (2021). The two studies used different samples and methods, so both values are kept with their sources.
    source: mn2023
    where: Results, DSC paragraph (Figure 3)
  - benchmark: phase_transition_temperature
    label: Phase transition temperature, Cu1.94Mn0.06Se (DSC)
    value: 390.3
    unit: K
    scope: material
    conditions:
      composition: Cu2-xMnxSe at the highest Mn content in the series (x = 0.06)
      method_detail: differential scanning calorimetry, heat-absorption peak
    method: End value of the printed range (400.1 K to 390.3 K). The paper gives the range across the series; x = 0.06 is the series end point and is assigned here by Space Mushroom
    status: ground
    source: mn2023
    where: Results, DSC paragraph (Figure 3)
  - benchmark: power_factor
    label: Power factor, Cu1.94Mn0.06Se at 873 K
    value: 1.62
    unit: mW m-1 K-2
    scope: material
    conditions:
      composition: Cu1.94Mn0.06Se (Mn-doped, not undoped Cu2Se)
      temperature: 873 K
    method: Value printed in the Results text for the power factor at 873 K
    status: ground
    note: Mn-doped composition, not undoped Cu2Se. The undoped power factor is not printed in the text read.
    source: mn2023
    where: Results, electrical transport paragraph (power factor at 873 K)
  - benchmark: molar_mass
    label: Molar mass, Cu2Se (supplier SDS)
    value: 206.05
    unit: g/mol
    scope: material
    conditions:
      formula: Cu2Se
      cas: 20405-64-5
    method: Printed as Molecular weight in Section 3 of the supplier SDS
    status: ground
    source: sam-cu2se-sds
    where: Section 3 (Composition/information on ingredients)
  - benchmark: molar_mass
    label: Molar mass, CuSe (PubChem)
    value: 142.52
    unit: g/mol
    scope: material
    conditions:
      formula: CuSe
      cas: 1317-41-5
    method: MolecularWeight field in the PubChem property table for CID 73980; computed by PubChem from the formula
    status: calculated
    source: pubchem-cuse-props
    where: PropertyTable, MolecularWeight field (CID 73980)
  - benchmark: density
    label: Density, CuSe (supplier MSDS)
    value: 5.99
    unit: g/cm3
    scope: material
    conditions:
      temperature: 20 °C
      form: black powder, as printed in the sheet
    method: Printed as Density at 20 °C in Section III of the Stanford Advanced Materials MSDS (revision 2.0)
    status: ground
    note: The 2012 sheet's fire-hazard text names aluminum fluoride, which does not fit CuSe, so that text was not used.
    source: sam-cuse-msds
    where: Section III (Physical/chemical characteristics), Density row
  - benchmark: melting_point
    label: Melting point, CuSe (supplier MSDS)
    value: 550
    unit: °C
    scope: material
    conditions:
      basis: value as printed; the sheet does not say whether the compound decomposes at this point
    method: Printed as Melting Point 550 °C (1022 °F) in Section III of the supplier MSDS
    status: ground
    source: sam-cuse-msds
    where: Section III (Physical/chemical characteristics), Melting Point row
  - benchmark: band_gap
    label: Optical band gap, Mn-doped hexagonal CuSe nanosheets
    value: 1.67
    value_high: 1.74
    bound: range
    unit: eV
    scope: material
    conditions:
      phase: hexagonal CuSe nanosheets, undoped and Mn2+-doped
      estimate: Tauc plot, linear for n = 2 (indirect band-to-band transition)
    method: Estimated by the authors from UV-visible absorption spectra; the paper gives the range 1.67 eV to 1.74 eV across its photocatalyst series
    status: ground
    note: An estimate from optical absorption, not a bulk electrical measurement. Other CuSe and Cu-Se film values were not read in a fetched page and are not recorded.
    source: cuse-bandgap
    where: Results, Tauc analysis (Figure 4(b) and its inset)
corrosion:
  - environment: Electric current or temperature gradient across a Cu2Se sample (operating conditions)
    headline: Copper ions migrate under current load, and the paper calls this the cause of Cu2Se's lack of stability.
    effect: The paper says the mobility of Cu+ ions gives Cu2Se very good thermoelectric transport properties, but is also responsible for the lack of stability of these materials under current load.
    rating: moderate
    conditions:
      basis: statement in the introduction; no test duration or current density given in the text
    source: nieroda2021
    where: Section 1 (Introduction), paragraph on Cu+ ions
  - environment: Electric field or temperature gradient, cycled at high temperature (undoped and Mn-doped Cu2Se)
    headline: The Mn-doped sample varied less over four cycles than undoped Cu2Se.
    effect: The 2023 study cycled Cu2Se and Cu1.94Mn0.06Se four times. It reports that the variation of the Seebeck coefficient, electrical conductivity and power factor after cycling is significantly smaller for the doped sample than for pure Cu2Se, which it attributes to Mn reducing Cu+ migration. The same study says that under an electric field or temperature gradient copper ions can precipitate out, making Cu2Se unsuitable for use in applications.
    rating: moderate
    conditions:
      cycles: four cycles
      composition: Cu2Se and Cu1.94Mn0.06Se
      temperature: high-temperature cycling (range not quoted in the text read)
    source: mn2023
    where: Section 3.4 (Cyclic Test of Thermoelectric Properties), Figure 12; Section 1
  - environment: Oxidation during hydrothermal preparation and sintering
    headline: Small amounts of copper oxide form and can hinder sintering.
    effect: TEM and XRD show small amounts of copper oxide phases in the sintered samples. The paper says the material may have been oxidized during its preparation, and that the copper oxide phases can hinder sintering.
    rating: low
    conditions:
      basis: hydrothermal synthesis followed by SPS at 550 to 720 °C; no storage test
    source: nieroda2021
    where: Section 3.1 (Structural and Microstructural Analysis); Conclusions
  - environment: Storage in a sealed container, away from strong oxidizing agents
    headline: Stable under recommended storage; incompatible with strong oxidizing agents.
    effect: The Cu2Se supplier SDS says the product is stable under recommended storage conditions, lists strong oxidizing agents as incompatible, and gives selenium oxides and copper oxides as hazardous decomposition products in a fire.
    rating: low
    conditions:
      basis: supplier SDS statements; no test duration or temperature is given
    source: sam-cu2se-sds
    where: Section 10 (Stability and reactivity)
risks:
  hazards:
    - text: 'Cu2Se supplier SDS (Stanford Advanced Materials): signal word Danger. H301 + H331 toxic if swallowed or inhaled; H373 may cause damage to organs through prolonged or repeated exposure; H410 very toxic to aquatic life with long lasting effects.'
      ghs:
        - H301
        - H331
        - H373
        - H410
      source: sam-cu2se-sds
      where: Section 2 (Hazards identification), GHS classification and label elements
    - text: 'PubChem GHS summary for CuSe (CID 73980), aggregated from 175 company reports: H301 toxic if swallowed; H331 toxic if inhaled; H373 damage to organs through prolonged or repeated exposure; H400 and H410 very toxic to aquatic life; signal word Danger.'
      ghs:
        - H301
        - H331
        - H373
        - H400
        - H410
      source: pubchem-cuse-ghs
      where: Safety and Hazards, GHS Classification (CID 73980)
    - text: 'CuSe supplier MSDS: irritant to skin and mucous membranes, with an irritating effect on the eyes. The sheet states that the acute and chronic toxicity of the substance is not fully known.'
      ghs: []
      source: sam-cuse-msds
      where: Section VI (Toxicological), Acute toxicity and Additional toxicological information
  failure_risks:
    - mode: Copper-ion migration under current load (Cu2Se)
      trigger: Electric current through the material, or a temperature gradient across it
      consequence: The paper says Cu+ mobility gives good transport properties but also the lack of stability under current load, so the material changes in use.
      severity: high
      source: nieroda2021
      where: Section 1 (Introduction)
    - mode: Change in transport behaviour at the Cu2Se phase transition
      trigger: Heating through about 396 K
      consequence: The paper reports inflection points in the temperature dependence of alpha(T) at around 396 K, and variations in how the Seebeck coefficient changes with temperature near that point.
      severity: moderate
      source: nieroda2021
      where: Section 3.2 (Thermoelectric properties)
  side_effects: []
  handling:
    - text: 'Cu2Se SDS: store in a well-ventilated place and keep the container tightly closed (P403 + P233); store locked up (P405).'
      source: sam-cu2se-sds
      where: Section 2 (precautionary statements) and Section 7 (Handling and storage)
    - text: 'Cu2Se SDS: avoid formation of dust and aerosols, provide appropriate exhaust ventilation where dust is formed, and do not breathe dust (P260).'
      source: sam-cu2se-sds
      where: Section 2 (precautionary statement P260) and Section 7 (Handling and storage)
    - text: 'Cu2Se SDS: wear respiratory protection during a release, and avoid dust formation.'
      source: sam-cu2se-sds
      where: Section 6 (Accidental release measures)
    - text: 'CuSe MSDS: use a suitable respirator when high concentrations are present; wear safety glasses and protective work clothing.'
      source: sam-cuse-msds
      where: General protective and hygienic measures paragraph (before Section V)
  mitigations:
    - text: Mn doping at x = 0.06 reduced the variation in Seebeck coefficient, electrical conductivity and power factor across four cycles compared with undoped Cu2Se, which the paper credits to reduced Cu+ migration.
      source: mn2023
      where: Section 3.4 (Cyclic Test of Thermoelectric Properties), Figure 12
uses:
  - application: thermal-control
    device: Thermoelectric generator, medium-temperature range
    example: Nieroda et al. say Cu2Se is 'suitable for the construction of thermoelectric generators that work in a medium temperature range (600-950 K)'.
    source: nieroda2021
    where: Section 1 (Introduction), first paragraph
  - application: thermal-control
    device: Thermoelectric heat pump
    example: The same paper says the ZT value is directly related to the efficiency of thermoelectric devices such as thermoelectric generators and thermoelectric heat pumps.
    source: nieroda2021
    where: Section 1 (Introduction), paragraph on the ZT parameter
supply:
  - subject: Copper
    producers:
      - country: China
        share_percent: 48.3
        year: 2025
      - country: Chile
        share_percent: 5.9
        year: 2025
      - country: Japan
        share_percent: 4.8
        year: 2025
    critical_lists:
      - list: USGS 2025 Final List of Critical Minerals (copper added, per the copper chapter)
        listed: true
    recycling: 'USGS copper chapter: old (post-consumer) scrap is converted to refined metal, alloys and other forms each year.'
    substitutes: []
    source: usgs-mcs2026-copper
    where: World Mine and Refinery Production and Reserves table, refinery production 2025e (world total 29,000 thousand t; shares calculated by Space Mushroom from China 14,000, Chile 1,700 and Japan 1,400 thousand t); critical-list paragraph
  - subject: Selenium
    producers:
      - country: China
        share_percent: 53
        year: 2025
    critical_lists: []
    recycling: 'USGS selenium chapter: recycling is insignificant. Most scrap from electronic materials was exported for recovery of contained selenium.'
    substitutes:
      - 'USGS selenium chapter: amorphous silicon and cadmium telluride are the two principal competitors with CIGS in thin-film photovoltaic cells.'
    source: usgs-mcs2026-selenium
    where: Domestic Production and Use (selenium is recovered principally as a byproduct of copper refining); world refinery production table (2025, world total 3,800 t excluding the United States); Recycling; Substitutes
documents:
  - title: 'Stanford Advanced Materials SDS: Copper(I) Selenide (CAS 20405-64-5)'
    type: sds
    url: https://samaterials.com/tds/1697119477-1117-copper-i-selenide-powder-sds.pdf
    year: 2023
  - title: 'Stanford Advanced Materials MSDS: Copper (II) Selenide (CAS 1317-41-5)'
    type: sds
    url: https://www.sputtertargets.net/Public/pdf/copper-selenide-msds.pdf
    year: 2012
  - title: 'PubChem: Copper selenide (CuSe), CID 73980, GHS classification summary'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/73980
    year: 2026
  - title: Thermoelectric Properties of Cu2Se Synthesized by Hydrothermal Method and Densified by SPS Technique (open access)
    type: paper
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC8269654/
    year: 2021
  - title: Study on Enhancing the Thermoelectric Stability of the beta-Cu2Se Phase by Mn Doping (open access)
    type: paper
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC10383636/
    year: 2023
  - title: Investigation of Mn2+ dopant-induced crystal defects in photoactive CuSe nanosheets (open access)
    type: paper
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC12352628/
    year: 2025
  - title: 'USGS Mineral Commodity Summaries 2026: Copper'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-copper.pdf
    year: 2026
  - title: 'USGS Mineral Commodity Summaries 2026: Selenium'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-selenium.pdf
    year: 2026
gaps:
  - No flight, vacuum, radiation or space-environment result for Cu2Se or CuSe was found in any source read in this run.
  - Nieroda et al. (2021) give the Seebeck coefficient, electrical conductivity and thermal conductivity only in figures, so no numbers for these are recorded.
  - 'Two phase-transition values are printed for Cu2Se: 396 K (Nieroda et al. 2021, inflection point) and 400.1 K (Mn-doping study, DSC, x = 0). Both are kept with their sources and not reconciled.'
  - The ZT of about 1.75 is printed as 965 K in the abstract and 970 K in the text of Section 3.2; both are kept.
  - The band gap for CuSe comes from one paper on Mn-doped hexagonal nanosheets (estimated by Tauc analysis). Other CuSe and Cu-Se film band-gap values were not read in a fetched page and are not recorded.
  - The undoped Cu2Se cycling data (Figure 12 of the 2023 study) are shown only in figures; the text gives the comparison in words, so no undoped cycling number is recorded.
  - No atomic weights were taken from a fetched source, so story.recipe gives no atomic_weight values.
  - Crustal abundance of copper and selenium was not found in a fetched source.
  - The USGS selenium chapter read here does not state whether selenium is on the critical-minerals list, so no list status is recorded for selenium. EU critical raw materials status was not checked.
  - No UNS, EN, DIN or other grade designation is used for these compounds in any source read in this run. CAS numbers only.
  - Hazard codes differ between suppliers (search snippets showed H302 and H315 for some Cu2Se sheets). Only the Cu2Se SDS and the PubChem record fetched in this run are used.
  - The CuSe MSDS is from 2012 and its fire-hazard section names aluminum fluoride and fluorine, which do not fit CuSe, so that section is not used.
  - No LD50, occupational exposure limit or long-term oxidation data were found in the sources read. The Cu2Se SDS gives 'No data available' for most toxicology and for melting point.
  - The CuSe supplier sheet gives a melting point of 550 °C but does not say whether CuSe decomposes there. The Wikipedia page was not used for values.
  - Use as a solar absorber in a device, and any thin-film photovoltaic performance, were not found in a fetched open source. The CuSe use recorded here is a laboratory photocatalysis study.
  - The property names ZT, band_gap, power_factor, molar_mass and phase_transition_temperature are not in spores/property-canon.yaml and are used as short snake_case names.
sources:
  nieroda2021:
    title: Thermoelectric Properties of Cu2Se Synthesized by Hydrothermal Method and Densified by SPS Technique
    id: doi:10.3390/ma14133650 (Materials 14(13), 3650, PMC8269654)
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC8269654/
    year: 2021
    kind: paper
    license: CC BY (Creative Commons Attribution, as shown on the PMC page)
  mn2023:
    title: Study on Enhancing the Thermoelectric Stability of the beta-Cu2Se Phase by Mn Doping
    id: Materials (MDPI) 16(14), 5204; PMC10383636
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC10383636/
    year: 2023
    kind: paper
    license: license as listed on the PMC page (not checked in this run)
  cuse-bandgap:
    title: Investigation of Mn2+ dopant-induced crystal defects in photoactive CuSe nanosheets for enhanced visible-NIR range absorption and natural solar-driven photocatalysis
    id: PMC12352628; PMID 40822875
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC12352628/
    year: 2025
    kind: paper
    license: license as listed on the PMC page (not checked in this run)
  sam-cu2se-sds:
    title: 'Safety Data Sheet: Copper(I) Selenide (Stanford Advanced Materials, CAS 20405-64-5)'
    id: CAS 20405-64-5; revision date 27/01/2023
    url: https://samaterials.com/tds/1697119477-1117-copper-i-selenide-powder-sds.pdf
    year: 2023
    kind: datasheet
    license: supplier document, linked not reproduced
  sam-cuse-msds:
    title: 'Material Safety Data Sheet: Copper (II) Selenide (Stanford Advanced Materials, CAS 1317-41-5)'
    id: CAS 1317-41-5; version 2.0, revision date 2012-09-05
    url: https://www.sputtertargets.net/Public/pdf/copper-selenide-msds.pdf
    year: 2012
    kind: datasheet
    license: supplier document, linked not reproduced
  pubchem-cuse-props:
    title: 'PubChem compound record, Copper selenide (CuSe), CID 73980: molecular weight'
    id: CID 73980; CAS 1317-41-5
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/73980/JSON?heading=Molecular+Weight
    year: 2026
    kind: database
    license: public domain (NCBI)
  pubchem-cuse-ghs:
    title: 'PubChem compound record, Copper selenide (CuSe), CID 73980: GHS classification'
    id: CID 73980
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/73980/JSON?heading=GHS+Classification
    year: 2026
    kind: database
    license: public domain (NCBI)
  usgs-mcs2026-copper:
    title: Mineral Commodity Summaries 2026, Copper
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-copper.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  usgs-mcs2026-selenium:
    title: Mineral Commodity Summaries 2026, Selenium
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-selenium.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
license: CC-BY-4.0
updated: '2026-11-01'
review:
  status: draft
  method: assistant
  checked_values: 14
  unreachable: 2
  caveats: 2
  note: Written 2026-11-01 by an AI assistant from pages read in this run. The RSC and American Elements pages returned a bot check and were not used.
story:
  recipe:
    line: Copper(I) selenide (Cu2Se) is made by hydrothermal synthesis, which gives a black nanopowder, and is then densified by spark plasma sintering (SPS). Copper(II) selenide (CuSe) has one copper atom for each selenium atom and is a separate phase, studied here for its optical absorption.
    parts:
      - name: Copper
        symbol: Cu
        role: Mobile copper ions move through the selenium framework. This mobility gives Cu2Se its thermoelectric transport and also its instability under current load.
      - name: Selenium
        symbol: Se
        role: Forms an anionic framework through which electron carriers move, in the paper's PLEC concept (an extension of the phonon-glass electron-crystal idea).
  risk_groups:
    - title: For people
      level: 0
      verdict: Toxic if swallowed or inhaled (H301, H331)
      lead: Both compounds carry supplier or PubChem hazard codes for acute toxicity and for organ damage after repeated exposure.
      points:
        - label: Cu2Se
          line: The Cu2Se supplier sheet lists H301 + H331, H373 and H410 with the signal word Danger.
        - label: CuSe
          line: PubChem's aggregated GHS data for CuSe, from 175 company reports, list H301, H331, H373, H400 and H410.
        - label: Dust
          line: Avoid dust, and provide exhaust ventilation where dust forms (Cu2Se sheet).
    - title: In use
      level: 1
      verdict: Unstable under current; phase change near 396 K
      lead: Cu2Se changes in use, which limits how the material can be operated.
      points:
        - label: Current
          line: Copper ions migrate under current load. The 2023 study says they can precipitate out, making Cu2Se unsuitable for use in applications.
        - label: Heating
          line: Inflection points at about 396 K mark the phase transition between alpha- and beta-Cu2Se.
  why: 'Copper selenide in this profile is two compounds. Copper(I) selenide (Cu2Se) is a thermoelectric material, one that converts a temperature difference into electricity or pumps heat. In one undoped sample densified by spark plasma sintering at 720 °C, Nieroda et al. (2021) report a ZT of about 1.75 at 970 K. ZT is the dimensionless figure of merit that sets thermoelectric efficiency. Copper(II) selenide (CuSe) is a hexagonal semiconductor with an estimated optical band gap of 1.67 to 1.74 eV in Mn-doped nanosheets, which is why it has been studied for solar-driven photocatalysis. Both compounds share one weakness: copper ions move, so the material changes under use.'
  uses:
    - where: Thermoelectric generators (Cu2Se)
      line: Nieroda et al. (2021) say Cu2Se is suited to thermoelectric generators in the 600 to 950 K range. The paper calls it one of the most intensively studied thermoelectric materials in recent years.
    - where: Thermoelectric heat pumps (Cu2Se)
      line: The same paper names thermoelectric heat pumps alongside generators as devices whose efficiency rises with the ZT value.
    - where: Solar-driven photocatalysis (CuSe, laboratory)
      line: A 2025 study of Mn-doped hexagonal CuSe nanosheets reports improved visible and near-infrared absorption and a test of methylene blue degradation under natural solar-driven photocatalysis. It is a laboratory study, not a space application.
  can_do:
    - label: Thermoelectric figure of merit
      line: Give a ZT of about 1.75 at 970 K in one undoped sample, the figure reported by Nieroda et al. (2021).
    - label: Phase change
      line: Show a phase change near 396 K, where the thermoelectric transport behaviour shifts.
    - label: Density
      line: Reach a density of 6.02 g/cm3 when densified by SPS at 720 °C, per the same paper.
    - label: Photocatalysis
      line: Serve as a photocatalyst under sunlight in the CuSe form, with an estimated band gap of 1.67 to 1.74 eV.
  compare: The two compounds differ in density and role. Cu2Se is the denser of the two, at 6.84 g/mL at 25 °C per its supplier, against 5.99 g/cm3 at 20 °C for CuSe. CuSe melts at 550 °C according to its supplier sheet, and its estimated band gap is 1.67 to 1.74 eV. Cu2Se is the thermoelectric member of the pair, with a ZT of about 1.75 at 970 K in one sample. Both compounds are unstable under current because copper ions move. The compare rows below place the printed values side by side.
  compare_rows:
    - label: Density
      benchmark: density
    - label: Melting point
      benchmark: melting_point
    - label: Band gap
      benchmark: band_gap
  handling:
    - Keep the container tightly closed in a well-ventilated place, and store it locked up (Cu2Se SDS, P403 + P233 and P405).
    - Avoid dust. Provide exhaust ventilation where dust forms, and wear respiratory protection (Cu2Se SDS).
    - Use a suitable respirator at high concentrations, and wear safety glasses and protective work clothing (CuSe MSDS).
    - Keep the material away from strong oxidizing agents (Cu2Se SDS).
  good_for:
    - Laboratory thermoelectric research on Cu2Se, where a ZT of about 1.75 at 970 K has been measured in a sintered sample.
    - Optical absorption studies of hexagonal CuSe, with an estimated band gap of 1.67 to 1.74 eV.
  watch_out:
    - Copper-ion migration under current load and temperature gradient, which the 2021 and 2023 papers both report as the main stability problem.
    - A phase change near 396 K, where transport properties shift.
    - 'Toxicity: both compounds carry H301 and H331 (toxic if swallowed or inhaled) and H373 (organ damage after repeated exposure) in their hazard data.'
    - No space, radiation or flight data were found for either compound.
  verdict: Cu2Se is a high-ZT laboratory thermoelectric whose copper-ion instability limits use. CuSe is a low-band-gap semiconductor studied for photocatalysis. Both carry toxicity hazards.