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Suboxides are a class of oxides wherein the electropositive element is in excess relative to the “normal” oxides.^[1] When the electropositive element is a metal, the compounds are sometimes referred to as “metal-rich”. Thus the normal oxide of caesium is Cs₂O, which is described as a Cs⁺ salt of O²⁻. A suboxide of caesium is Cs₁₁O₃, where the charge on Cs is clearly less than 1+, but the oxide is still described as O²⁻. Suboxides typically feature extensive bonding between the electropositive element, often leading to clusters.

Examples of suboxides include

Carbon suboxide, C₃O₂;
Boron suboxide, B₆O;
Rubidium suboxide, Rb₉O₂;
Silicon suboxide, SiO_x (x < 2)

Metal-containing suboxides[edit source | edit]

Suboxides are intermediates along the pathway that forms the normal oxide. Suboxides are sometimes visible when certain metals are exposed to small amounts of O₂:

22 Cs + 3 O₂ → 2 Cs₁₁O₃

4 Cs₁₁O₃ + 5 O₂ → 22 Cs₂O

Several suboxides of caesium and rubidium have been characterized by X-ray crystallography. As of 1997, the inventory includes the following Rb₉O₂, Rb₆O, Cs₁₁O₃, Cs₄O, Cs₇O, Cs₁₁O₃Rb, Cs₁₁O₃Rb₂, and Cs₁₁O₃Rb₃.^[1]

They are generally colored compounds indicating a degree of electron delocalisation. Cs₇O, has a unit cell containing a Cs₁₀O₃ cluster and 10 Cs atoms. The cluster can be visualised as being composed of three face-sharing octahedra. In the picture below the caesium atoms are purple and the oxygen atoms are red. The Cs-Cs distance in the cluster is 376 pm, which is less than the Cs-Cs distance in the metal of 576 pm. Rb₉O₂ and Rb₆O both contain the Rb₉O₂ cluster, which can be visualised as two face-sharing octahedra. Rb₆O can be formulated as (Rb₉O₂)Rb₃. The Rb-Rb distance in the cluster is 352 pm which is shorter than the Rb-Rb in the metal of 485 pm. It is suggested that caesium suboxides play a role in the Ag-O-Cs (S1) and multialkali Na-K-Sb-Cs photocathodes.^[2]


Rb₉O₂ cluster	Cs₁₁O₃ cluster

Carbon suboxide[edit source | edit]

The suboxide of carbon adopts an unremarkable structure. As for related organic cumulenes (e.g. ketene), C₃O₂ obeys the octet rule.

Related compounds[edit source | edit]

Subnitrides are also known, e.g. Na₁₆Ba₆N, which features an nitride-centered octahedral cluster of six barium atoms embedded in a matrix of sodium.^[1]

References[edit source | edit]

^ ^a ^b ^c Simon, A. ”Group 1 and 2 Suboxides and Subnitrides — Metals with Atomic Size Holes and Tunnels” Coordination Chemistry Reviews 1997, volume 163, Pages 253–270.doi:10.1016/S0010-8545(97)00013-1
^ Oxides: solid state chemistry, WH McCarrroll Encyclopedia of Inorganic chemistry. Editor R Bruce King, John Wiley and Sons. (1994) ISBN 0-471-93620-0

English Journal

Nanotwinned Boron Suboxide (B6O): New Ground State of B6O.

An Q1, Reddy KM2, Dong H3, Chen MW2, Oganov AR3,4,5,6, Goddard WA 3rd1.
Nano letters.Nano Lett.2016 Jun 8. [Epub ahead of print]
Nanotwinned structures in superhard ceramics rhombohedral boron suboxide (R-B6O) have been examined using a combination of transmission electron microscopy (TEM) and quantum mechanics (QM). QM predicts negative relative energies to R-B6O for various twinned R-B6O (denoted as τ-B6O, 2τ-B6O, and 4τ
PMID 27253270

Phosphorene: Enhanced Photoresponse from Phosphorene-Phosphorene-Suboxide Junction Fashioned by Focused Laser Micromachining (Adv. Mater. 21/2016).

Lu J1,2, Carvalho A2, Wu J1, Liu H3, Tok ES1, Neto AH1,2, Özyilmaz B1,2, Sow CH1,2.
Advanced materials (Deerfield Beach, Fla.).Adv Mater.2016 Jun;28(21):4164. doi: 10.1002/adma.201670146.
On page 4090, B. Özyilmaz, C. H. Sow, and co-workers use a focused laser beam to modify the surface of a phosphorene device. With a simple focused laser beam, a part of the phosphorene can be scanned and converted into phosphorene-suboxide species, leaving behind a functional and active phosphorene
PMID 27246921

Enhanced Photoresponse from Phosphorene-Phosphorene-Suboxide Junction Fashioned by Focused Laser Micromachining.

Lu J1,2, Carvalho A2, Wu J1, Liu H3, Tok ES1, Neto AH1,2, Özyilmaz B1,2, Sow CH1,2.
Advanced materials (Deerfield Beach, Fla.).Adv Mater.2016 Jun;28(21):4090-6. doi: 10.1002/adma.201506201. Epub 2016 Mar 29.
Enhanced photoresponse is obtained from phosphorene-phosphorene-suboxide. A scanning focused laser beam is employed as a straightforward approach to convert part of a phosphorene film into phosphorene suboxide, creating a functional junction in situ on an optoelectronic device based on phosphorene.
PMID 27028659