Delafossite is a

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

oxide mineral The oxide mineral class includes those minerals in which the oxide anion (O2−) is bonded to one or more metal alloys. The hydroxide-bearing minerals are typically included in the oxide class. The minerals with complex anion groups such as the si ...

with formula CuFeO₂ or Cu¹⁺Fe³⁺O₂. It is a member of the delafossite mineral group, which has the general formula ABO₂, a group characterized by sheets of linearly coordinated A cations stacked between edge-shared octahedral layers (BO₆). Delafossite, along with other minerals of the ABO₂ group, is known for its wide range of electrical properties, its conductivity varying from insulating to metallic. Delafossite is usually a secondary mineral that crystallizes in association with

oxidized Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

copper and rarely occurs as a primary mineral.

Composition

The

chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

for delafossite is CuFeO₂, which was first determined through chemical analysis of the pure mineral by G. S. Bohart. The ratio he determined was very close to Cu:Fe:O=1:1:2, with slightly more iron than copper. Rogers. attributed this fact to a small amount of

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

in the sample. In order to determine the composition of delafossite Rogers used the Ziervogel process. The Ziervogel process is used to test for the presence of cuprous oxides by looking for the "spangle reaction" which produces thin flakes of metallic silver when cuprous oxide is mixed with silver sulfate. When Rogers heated powdered delafossite with silver sulfate solution, the spangle reaction occurred. The only oxides possibilities to consider for delafossite are cuprous copper and ferrous iron. Rogers concluded that the iron was combining with the oxygen as a radical and that it only acted as a radical. This indicated that the copper in delafossite was in the cuprous rather than the cupric form. Hence he concluded that the composition of delafossite was probably cuprous metaferrite, CuFe³⁺O₂. This composition was later confirmed by Pabst by the determination of interionic distances in the

crystal lattice In geometry and crystallography, a Bravais lattice, named after , is an infinite array of discrete points generated by a set of discrete translation operations described in three dimensional space by : \mathbf = n_1 \mathbf_1 + n_2 \mathbf_2 + n ...

Structure

The atomic structure of delafossite and the delafossite group consists of a sheet of linearly coordinated A cations stacked between edge-shared octahedral layers (BO₆). In the delafossite atomic structure there are two alternating planar layers. The two layers consist of one layer triangular-patterned A cations and a layer of edge-sharing BO₆ octahedra compacted with respect to the c axis. The delafossite structure can have two

polytypes In materials science, polymorphism describes the existence of a solid material in more than one form or crystal structure. Polymorphism is a form of isomerism. Any crystalline material can exhibit the phenomenon. Allotropy refers to polymorphism ...

according to the orientation of the planar layer stacking. Hexagonal 2H types that have a space group of P6₃/mmc are formed when two A layers are stacked with each layer rotated 180° in relation to one another. Alternatively, when the layers are stacked each layer in the same direction in relation to one another, it makes a rhombohedral 3R type with a

space group In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it unchan ...

of Rm.

Physical properties

The color of delafossite is black, with a hardness of 5.5, and imperfect cleavage in the direction. Pabst calculated the density of delafossite to be 5.52. Contact twinning has been observed in the direction. The

unit cell In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessaril ...

parameters were calculated to be a = 3.0351 Å, c = 17.166 Å, V = 136.94 Å³. Delafossite is tabular to equidimensional in habit and has a black streak and a metallic luster. Delafossite has hexagonal symmetry that can have the

space groups In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it unchan ...

Rm or P6₃/mmc depending on the stacking of A cation layers. Delafossite compounds can have magnetic properties when magnetic ions are in the B cation position. Delafossite compounds also have properties dealing with electric conductivity such as insulation and/or metallic conduction. Delafossite compounds can exhibit p- or n-type conductivity based on their composition. Rhombohedral (3R), CuFeO₂ properties: *P-type semiconductor, bandgap 1.47 eV *High light absorption coefficient of 7.5 cm⁻¹ near the band gap edge at 700 nm. *High hole mobility of 34 cm² v⁻¹ s⁻¹ even at doping levels as high as 1.8 × 10¹⁹ cm⁻³. *Good stability in aqueous environments. Hexagonal (2H), CuFeO₂ properties: *unknown as pure 2H CuFeO₂ is very difficult to synthesize.

Synthesis

3R CuFeO₂ is often synthesized by solid state reactions, sol gel methods, vapor deposition, and hydrothermal synthesis. Pure 2H CuFeO₂ and other 2H delafossite-type oxides are difficult to synthesize. The only pure 2H CuFeO₂ crystals were pure 2H CuFeO₂ nanoplates with a thickness of about 100 nm which were synthesized at temperatures as low as 100 °C from CuI and FeCl₃·6H₂O.

Application

Solar cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.

s: 2H CuFeO₂ has a

band gap In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in ...

of 1.33 eV and a high

absorption coefficient The linear attenuation coefficient, attenuation coefficient, or narrow-beam attenuation coefficient characterizes how easily a volume of material can be penetrated by a beam of light, sound, particles, or other energy or matter. A coefficient valu ...

of 3.8 cm⁻¹ near the band gap edge at 700 nm. It demonstrated a

photovoltaic effect The photovoltaic effect is the generation of voltage and electric current in a material upon exposure to light. It is a physical property, physical and chemical phenomenon. The photovoltaic effect is closely related to the photoelectric effect. F ...

when placed into

thin film A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ap ...

structures composed of

ITO Ito may refer to: Places * Ito Island, an island of Milne Bay Province, Papua New Guinea * Ito Airport, an airport in the Democratic Republic of the Congo * Ito District, Wakayama, a district located in Wakayama Prefecture, Japan * Itō, Shizuo ...

/ ZnO/2H CuFeO₂/graphite/carbon black. Other applications: CuFeO₂ is made of earth abundant elements and has good stability in

aqueous An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be rep ...

environments, and as such was investigated as

photocathode A photocathode is a surface engineered to convert light (photons) into electrons using the photoelectric effect. Photocathodes are important in accelerator physics where they are utilised in a photoinjector to generate high brightness electron be ...

s for photoelectrochemical reduction of CO₂, solar water reduction, and as a cathode material in

lithium batteries Lithium battery may refer to: * Lithium metal battery, a non-rechargeable battery with lithium as an anode ** Rechargeable lithium metal battery, a rechargeable counterpart to the lithium metal battery * Lithium-ion battery, a rechargeable batte ...

. Whereas the 3R phase was somewhat characterized, only

X-ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

and theoretical calculation of e_g and t_2g occupancies of the Fe³⁺ are available for 2H CuFeO₂.

Geologic occurrence

In 1873, delafossite was discovered by

Charles Friedel Charles Friedel (; 12 March 1832 – 20 April 1899) was a French chemist and Mineralogy, mineralogist. Life A native of Strasbourg, France, he was a student of Louis Pasteur at the University of Paris, Sorbonne. In 1876, he became a professor of ...

in a region of Ekaterinbug,

Siberia Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a part of ...

. Since its discovery it has been identified as a fairly common mineral found in such places as the copper mines of

Bisbee, Arizona Bisbee is a city in and the county seat of Cochise County in southeastern Arizona, United States. It is southeast of Tucson and north of the Mexican border. According to the 2020 census, the population of the town was 4,923, down from 5,575 i ...

. Delafossite is usually a secondary mineral often found in oxidized areas of copper deposits although it can be a primary mineral as well. Delafossite can be found as massive, relatively distinct crystals on hematite. Delafossite has since been found in mines around the world from Germany to Chile.

Origin of the name

Delafossite was first noted by

in 1873 and given the composition Cu₂O·Fe₂O₃. The mineral was given the name delafossite in honor of the French mineralogist and crystallographer

Gabriel Delafosse Gabriel Delafosse (16 April 1796 – 13 October 1878) was a French mineralogist who worked at the Natural History Museum in Paris and for sometime at the University of Paris. He contributed to development of the idea of unit cells in crystallogra ...

(1796–1878). Delafosse is known for noting the close relationship between crystal symmetry and physical properties.1839: Gabriel Delafosse et Alfred Des Cloizeaux
. Institut de minéralogie et de physique des milieus condensés, 19 March 2009.

References

{{reflist Oxide minerals Trigonal minerals Minerals in space group 166