Silver sulfide

Silver sulfide is an inorganic compound with the formula Ag
₂S. A dense black solid, it is the only sulfide of silver. It is useful as a photosensitizer in photography. It constitutes the tarnish that forms over time on silverware and other silver objects. Silver sulfide is insoluble in most solvents, but is degraded by strong acids. Silver sulfide is a network solid made up of silver (electronegativity of 1.98) and sulfur (electronegativity of 2.58) where the bonds have low ionic character (approximately 10%).

Silver sulfide

Names
IUPAC name Silver(I) sulfide
Other names Silver sulfide Argentous sulfide
Identifiers
CAS Number	21548-73-2 Y
3D model (JSmol)	Interactive image
ChemSpider	145878 N
ECHA InfoCard	100.040.384
EC Number	244-438-2
PubChem CID	166738
UNII	9ZB10YHC1C N
CompTox Dashboard (EPA)	DTXSID60925902 DTXSID80893679, DTXSID60925902
InChI InChI=1S/2Ag.S/q2*+1;-2 N Key: XUARKZBEFFVFRG-UHFFFAOYSA-N N
SMILES S(Ag)Ag
Properties
Chemical formula	Ag2S
Molar mass	247.80 g·mol−1
Appearance	Grayish-blackish crystal
Odor	Odorless
Density	7.234 g/cm3 (25 °C)[1][2] 7.12 g/cm3 (117 °C)[3]
Melting point	836 °C (1,537 °F; 1,109 K)[1]
Solubility in water	6.21·10−15 g/L (25 °C)
Solubility product (Ksp)	6.31·10−50
Solubility	Soluble in aq. HCN, aq. citric acid with KNO3 Insoluble in acids, alkalies, aqueous ammoniums[4]
Structure
Crystal structure	Cubic, cI8 (α-form) Monoclinic, mP12 (β-form) Cubic, cF12 (γ-form)[3][5]
Space group	Im3m, No. 229 (α-form)[5] P21/n, No. 14 (β-form) Fm3m, No. 225 (γ-form)[3]
Point group	2/m (α-form)[5] 4/m 3 2/m (β-form, γ-form)[3]
Lattice constant	a = 4.23 Å, b = 6.91 Å, c = 7.87 Å (α-form)[5] α = 90°, β = 99.583°, γ = 90°
Thermochemistry
Heat capacity (C)	76.57 J/mol·K[6]
Std molar entropy (S⦵298)	143.93 J/mol·K[6]
Std enthalpy of formation (ΔfH⦵298)	−32.59 kJ/mol[6]
Gibbs free energy (ΔfG⦵)	−40.71 kJ/mol[6]
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	May cause irritation
GHS labelling:
Pictograms	[2]
Signal word	Warning
Hazard statements	H315, H319, H335[2]
Precautionary statements	P261, P305+P351+P338[2]
NFPA 704 (fire diamond)	[7] 0 0 0
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Formation

Silver sulfide naturally occurs as the tarnish on silverware. When combined with silver, hydrogen sulfide gas creates a layer of black silver sulfide patina on the silver, protecting the inner silver from further conversion to silver sulfide.^[8] Silver whiskers can form when silver sulfide forms on the surface of silver electrical contacts operating in an atmosphere rich in hydrogen sulfide and high humidity.^[9] Such atmospheres can exist in sewage treatment and paper mills.^[10][11]

Structure and properties

Three forms are known: monoclinic acanthite (α-form), stable below 179 °C, body centered cubic so-called argentite (β-form), stable above 180 °C, and a high temperature face-centred cubic (γ-form) stable above 586 °C.^[5] The higher temperature forms are electrical conductors. It is found in nature as relatively low temperature mineral acanthite. Acanthite is an important ore of silver. The acanthite, monoclinic, form features two kinds of silver centers, one with two and the other with three near neighbour sulfur atoms.^[12] Argentite refers to a cubic form, which, due to instability in "normal" temperatures, is found in form of the pseudomorphosis of acanthite after argentite.

Exceptional ductility of α-Ag2S

Relative to most inorganic materials, α-Ag₂S displays exceptional ductility at room temperature.^[13][14] This material can undergo extensive deformation, akin to metals, without fracturing. Such behavior is evident in various mechanical tests; for instance, α-Ag2S can be easily machined into cylindrical or bar shapes and can withstand substantial deformation under compression, three-point bending, and tensile stresses. The material sustains over 50% engineering strain in compression tests and up to 20% or more in bending tests.^[13]

The intrinsic ductility of alpha-phase silver sulfide (α-Ag₂S) is underpinned by its unique structural and chemical bonding characteristics. At the atomic level, its monoclinic crystal structure, which remains stable up to 451 K, enables the movement of atoms and dislocations along well-defined crystallographic planes known as slip planes. Additionally, the dynamic bonding within the crystal structure supports both the sliding of atomic layers and the maintenance of material integrity during deformation. The interatomic forces within the slip planes are sufficiently strong to prevent the material from cleaving while still allowing for considerable flexibility.^[13] Further insights into α-Ag₂S's ductility come from density functional theory calculations, which reveal that the primary slip planes align with the [100] direction and slipping occurs along the [001] direction. This arrangement permits atoms to glide over each other under stress through minute adjustments in the interlayer distances, which are energetically favorable as indicated by low slipping energy barriers (ΔE_B) and high cleavage energies (ΔE_C). These properties ensure significant deformation capability without fracture. Silver and sulfur atoms in α-Ag₂S form transient, yet robust interactions that enable the material to retain its integrity while deforming. This behavior is akin to that of metals, where dislocations move with relative ease, providing α-Ag₂S with a unique combination of flexibility and strength, making it exceptionally resistant to cracking under mechanical stress.^[13]

History

In 1833 Michael Faraday noticed that the resistance of silver sulfide decreased dramatically as temperature increased. This constituted the first report of a semiconducting material.^[15]

Silver sulfide is a component of classical qualitative inorganic analysis.^[16]

References

1. Lide, David R., ed. (2009). CRC Handbook of Chemistry and Physics (90th ed.). Boca Raton, Florida: CRC Press. ISBN 978-1-4200-9084-0.
2. Sigma-Aldrich Co., Silver sulfide. Retrieved on 2014-07-13.
3. Tonkov, E. Yu (1992). High Pressure Phase Transformations: A Handbook. Vol. 1. Gordon and Breach Science Publishers. p. 13. ISBN 978-2-88124-761-3.
4. Comey, Arthur Messinger; Hahn, Dorothy A. (February 1921). A Dictionary of Chemical Solubilities: Inorganic (2nd ed.). New York: The MacMillan Company. p. 835.
5. "Silver sulfide (Ag2S) crystal structure". Non-Tetrahedrally Bonded Elements and Binary Compounds I. Landolt-Börnstein - Group III Condensed Matter. Vol. 41C. Springer Berlin Heidelberg. 1998. pp. 1–4. doi:10.1007/10681727_86. ISBN 978-3-540-31360-1.
6. Pradyot, Patnaik (2003). Handbook of Inorganic Chemicals. The McGraw-Hill Companies, Inc. p. 845. ISBN 978-0-07-049439-8.
7. "MSDS of Silver Sulfide". saltlakemetals.com. Utah, USA: Salt Lake Metals. Archived from the original on 2014-08-10. Retrieved 2014-07-13.
8. Zumdahl, Steven S.; DeCoste, Donald J. (2013). Chemical Principles (7th ed.). Cengage Learning. p. 505. ISBN 978-1-111-58065-0.
9. "Degradation of Power Contacts in Industrial Atmosphere: Silver Corrosion and Whiskers" (PDF). 2002.
10. Dutta, Paritam K.; Rabaey, Korneel; Yuan, Zhiguo; Rozendal, René A.; Keller, Jürg (2010). "Electrochemical sulfide removal and recovery from paper mill anaerobic treatment effluent". Water Research. 44 (8): 2563–2571. Bibcode:2010WatRe..44.2563D. doi:10.1016/j.watres.2010.01.008. ISSN 0043-1354. PMID 20163816.
11. "Control of Hydrogen Sulfide Generation | Water & Wastes Digest". www.wwdmag.com. 5 March 2012. Retrieved 2018-07-05.
12. Frueh, A. J. (1958). The crystallography of silver sulfide, Ag2S. Zeitschrift für Kristallographie-Crystalline Materials, 110(1-6), 136-144.
13. Chen, Lidong (2018). "Room-temperature ductile inorganic semiconductor". Nature Materials. 17: 421–426.
14. Chen, Lidong. "Flexible thermoelectrics based on ductile semiconductors". Science. 377 (6608): 854–858.
15. "1833 - First Semiconductor Effect is Recorded". Computer History Museum. Retrieved 24 June 2014.
16. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.

External links

• Tarnishing of Silver: A Short Review V&A Conservation Journal
• Images of silver whiskers NASA