Copper(I) oxide or cuprous oxide (Cu₂O) is an oxide of copper. It occurs naturally as the red mineral cuprite. It has a melting point of 1230 °C. Cu₂O is insoluble in water and organic solvents, but it does dissolve in conc. ammonia solution and in excess hydrochloric acid. Dilute sulfuric acid or nitric acid give copper metal and copper(II) sulfate or copper(II) nitrate respectively.

Formation of copper(I) oxide is the basis of the sensitive Fehling's test for sugars. In the presence of a reducing sugar, a alkaline solution of a copper(II) salt in potassium sodium tartrate (known as Fehling's solution) will be reduced and give a precipitate of Cu₂O.

Some attention to this substance is drawn by semiconductor physics. It has been the first substance to demonstrate semiconductivity and diodes have been built from it long before silicon became the industrial standard.

Nowadays, it is of great interest in fundamental semiconductor physics. It shows four well understood series of excitons with resonance widths in the range of neV. The associated polaritons are also well understood and their group velocity effects have almost been measured down to the speed of sound, i.e. light may be almost slowed down to the speed of sound in this medium. This yields to high densities of polaritons and effects like Bose-Einstein Condensation, dynamical Stark effect or phonoritons have been demonstrated.

Another extraordinary feature of the ground state excitons is that all primary scattering mechanisms are known and may be quantified. This yielded Cu₂O as the first substance where an entirely parameter-free model of absorption linewidth broadening by temperature could be established. Also, Beer's law and the corresponding absorption coefficient could be deduced. Finally, it could be shown that the Kramers Kronig Relations do not apply to polaritons.

References

• P.W. Baumeister: Optical Absorption of Cuprous Oxide, Phys.Rev. 121 (1961), 359.

• L. Brillouin: Wave Propagation and Group Velocity, Academic Press, New York, 1960.

• D. Fröhlich, A. Kulik, B. Uebbing, and A. Mysyrovicz: Coherent Propagation and Quantum Beats of Quadrupole Polaritons in Cu₂O, Phys.Rev.Lett. 67 (1991), 2343.

• L. Hanke: Transformation von Licht in Wärme in Kristallen - Lineare Absorption in Cu₂O, ISBN 3-8265-7269-6, Shaker, Aachen, 2000; (Transformation of light into heat in crystals - Linear absorption in Cu₂O).

• L. Hanke, D. Fröhlich, A.L. Ivanov, P.B. Littlewood, and H. Stolz: LA-Phonoritons in Cu₂O, Phys.Rev.Lett. 83 (1999), 4365.

• L. Hanke, D. Fröhlich, and H. Stolz: Direct observation of longitudinal acoustic phonon absorption to the 1S-exciton in Cu₂O, Sol.Stat.Comm. 112 (1999), 455.

• J.J. Hopfield, Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals, Phys.Rev. 112 (1958), 1555.

• J.P. Wolfe and A. Mysyrowicz: Excitonic Matter, Sci.Am. 250 (1984), No. 3, 98.

• Chemistry of the Elements, NN Greenwood & A Earnshaw, Pergamon Press, Oxford, 1984.

• The Merck Index, 7th edition, Merck & Co., Rahway, New Jersey, 1960.