Silver(I) selenide

Silver selenide (Ag2Se) is the reaction product formed when selenium toning analog silver gelatine photo papers in photographic print toning. The selenium toner contains sodium selenite (Na2SeO3) as one of its active ingredients, which is the source of the selenide (Se2−) anion combining with the silver in the toning process.

Silver(I) selenide
Silver(I) selenide
Names
IUPAC name
Silver(I) selenide
Other names
Naumannite
Argentous selenide
Identifiers
3D model (JSmol)
ECHA InfoCard 100.013.727
UNII
  • InChI=1S/2Ag.Se
    Key: GNWCVDGUVZRYLC-UHFFFAOYSA-N
  • [Se].[Ag].[Ag]
Properties
Ag2Se
Molar mass 294.7 g/mol
Density 8.216 g/cm3, solid
Melting point 896.85 °C (1,646.33 °F; 1,170.00 K)
insoluble
Band gap 0.15 eV [1]
Structure
orthorhombic, oP12
P212121, No. 19
Thermochemistry
Enthalpy of fusion (ΔfH⦵fus)
 56.9 J/g
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

It is found in nature as the mineral naumannite, a comparatively rare silver mineral which has nevertheless become recognized as important silver compound in some low-sulfur silver ores from mines in Nevada and Idaho.[2][3]

Structure

Silver selenide has two crystal phases on the bulk phase diagram. At lower temperatures, it has an orthorhombic structure, β-Ag2Se. This orthorhombic phase, stable at room temperature, is a narrow-gap semiconductor, with space group P212121. The exact size of the band gap has been given variously from 0.02 eV to 0.22 eV.[4]

There is also a high temperature cubic phase, α-Ag2Se.,[5] which it transforms into at temperatures above 130 °C. This high temperature phase has space group Im3m, No. 229, Pearson symbol cI20. The phase transition increases ionic conductivity by 10,000 times to about 2 S/cm.

A third metastable phase with a monoclinic structure and space group P21/n is known to form for colloidal Ag2Se nanocrystals.[6] The crystal structure of this polymorph is highly related to the acanthite phase of silver sulfide. For Ag2Se, this polymorph is increasingly unstable for larger crystallites, which explains its absence on the bulk phase diagram.[7] It is thought that the influence of the surface energy plays a role stabilizing this phase on the nanoscale, thus allowing its experimental isolation. Electronic structure calculations for this polymorph of Ag2Se suggest that it is a narrow-gap semiconductor,[6] which is supported by experimental evidence as well.[8] Prior to 2021, the crystal structure of this polymorph was unknown, and this material was informally referred to as a "tetragonal" or "pseudo-tetragonal" polymorph of Ag2Se.[9] This terminology, while not technically correct, is prevalent in the literature pertaining to this metastable phase of Ag2Se.

References
  1. O. Madelung (2004). Semiconductors: data handbook. Birkhäuser. p. 461. ISBN 978-3-540-40488-0.
  2. http://nevada-outback-gems.com/Reference_pages/sulfide_ores.htm Notes on naumannite.
  3. Guilbert, John M.; Park Jr, Charles F. (2007-02-09). The Geology of Ore Deposits. Waveland Press. p. 557. ISBN 978-1-4786-0887-5.
  4. Kirchhoff F.; Holender J.M.; Gillan M.J. (1996). "Structure, dynamics, and electronic structure of liquid Ag-Se alloys investigated by ab initio simulation". Physical Review B. 54 (1): 190–202. arXiv:mtrl-th/9602001. Bibcode:1996PhRvB..54..190K. doi:10.1103/PhysRevB.54.190. PMID 9984246. S2CID 9316728.
  5. An, Boo Hyun; Ji, Hye Min; Wu, Jun-Hua; Cho, Moon Kyu; Yang, Ki-Yeon; Lee, Heon; Kim, Young Keun (2009). "Phase changeable silver selenide thin films fabricated by pulse electrodeposition". Current Applied Physics. 9 (6): 1338–1340. Bibcode:2009CAP.....9.1338A. doi:10.1016/j.cap.2008.12.017. ISSN 1567-1739.
  6. Tappan, Bryce A.; Zhu, Bonan; Cottingham, Patrick; Mecklenburg, Matthew; Scanlon, David O.; Brutchey, Richard L. (2021). "Crystal Structure of Colloidally Prepared Metastable Ag2Se Nanocrystals". Nano Letters. 21 (13): 5881–5887. Bibcode:2021NanoL..21.5881T. doi:10.1021/acs.nanolett.1c02045. PMID 34196567. S2CID 235698416.
  7. Sahu, Ayaskanta; Braga, Daniele; Waser, Oliver; Kang, Moon Sung; Deng, Donna; Norris, David J. (2014). "Solid-Phase Flexibility in Ag2Se Semiconductor Nanocrystals". Nano Letters. 14 (1): 115–121. Bibcode:2014NanoL..14..115S. doi:10.1021/nl4041498. PMID 24295334.
  8. Sahu, Ayaskanta; Khare, Ankur; Deng, Donna D.; Norris, David J. (2012). "Quantum confinement in silver selenide semiconductor nanocrystals". Chemical Communications. 48 (44): 5458–5460. doi:10.1039/C2CC30539A. PMID 22540121.
  9. Wang, Junli; Fan, Weiling; Yang, Juan; Da, Zulin; Yang, Xiaofei; Chen, Kangmin; Yu, Huan; Cheng, Xiaonong (2014). "Tetragonal–Orthorhombic–Cubic Phase Transitions in Ag2Se Nanocrystals". Chemistry of Materials. 26 (19): 5647–5653. doi:10.1021/cm502317g.


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