Copper oxide selenite

Copper oxide selenite

Identifiers
3D model (JSmol)	Interactive image
InChI InChI=1S/2Cu.H2O3Se.O/c;;1-4(2)3;/h;;(H2,1,2,3);/q2*+1;;-2/p-2 Key: CGYPTBPQZTWRHL-UHFFFAOYSA-L
SMILES [O-][Se](=O)[O-].[Cu+].[Cu+].[O-2]
Properties
Chemical formula	Cu2OSeO3
Molar mass	270.059 g/mol
Appearance	Green dodecahedral crystals
Density	5.1 g/cm3
Band gap	2.5 eV
Thermal conductivity	400 W/(m·K) (9 K)
Refractive index (nD)	3.8 (100 K, 1 kHz)
Structure
Crystal structure	Cubic
Space group	P213, #198, cP56
Lattice constant	a = 0.8924 nm
Formula units (Z)	8
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Copper oxide selenite is an inorganic compound with the chemical formula Cu₂OSeO₃. It is an electrically insulating, piezoelectric and piezomagnetic material, which becomes a ferrimagnet upon cooling below 58 K. As of 2021, Cu₂OSeO₃ is the only insulating material that hosts magnetic skyrmions.

Synthesis

Cu₂OSeO₃ polycrystals can be grown by heating a 2:1 molar mixture of CuO and SeO₂ powders at 600 °C for 12 hours in vacuum. They can be converted into olive-green single crystals ca. 4 mm in size by chemical vapor transport. NH₄Cl is used as the transport agent; it sublimes at 340 °C, yielding NH₃ and HCl gases.

Structure

Cu₂OSeO₃ crystals have a cubic, distorted pyrochlore structure built by Cu₄O and SeO₃ units. The spins on three Cu²⁺ ions in each tetrahedron (Cu1 sites) are aligned, while the Cu2 spin is facing in the opposite direction, resulting in a ferrimagnetic order. The helical spin and skyrmion textures emerge at low magnetic fields due to the Dzyaloshinskii-Moriya interaction.

• 
  (a) Crystal structure of Cu₂OSeO₃ consisting of (b) Cu1 bipyramids and (c) Cu2 distorted square-based pyramids. Bonds to Se ions are omitted for clarity. (d) The ferrimagnetic structure of Cu₂OSeO₃ with spins (green arrows) on Cu1 site antiparallel to the spins (red arrows) on Cu2 sites.]]

Properties

Magnetic phase diagram of Cu₂OSeO₃ for H ∥ [111] crystal axis. H, C, FP and SL stand for helical, conical, field-polarized (ferrimagnetic or paramagnetic) and skyrmion lattice phases, respectively.

Cu₂OSeO₃ is a ferrimagnet, and all its properties below the Curie temperature strongly depend on magnetic field. With increasing field, its spin texture changes from helical stripes to conical stripes or skyrmion lattice, and then to a "field polarized", i.e., ferrimagnetic alignment. Thermal conductivity peaks around 9 K with a value of ca. 400 W/(m·K). The magnetization damping constant is 1×10⁻⁴ at 5 K. This value is only 4 times larger than that of yttrium iron garnet, which has the lowest magnetization damping value among all materials. This property is advantageous for high-frequency electronic applications, as it results in low current-induced heat.