Oxyselenide

Figure 1: Crystal structure of the BiCuSeO with a layered ZrSiCuAs structure

Oxyselenides are a group of chemical compounds that contain oxygen and selenium atoms (Figure 1). Oxyselenides can form a wide range of structures in compounds containing various transition metals, and thus can exhibit a wide range of properties. Most importantly, oxyselenides have a wide range of thermal conductivity, which can be controlled with changes in temperature in order to adjust their thermoelectric performance. Current research on oxyselenides indicates their potential for significant application in electronic materials.^[1]

Synthesis

The first oxyselenide to be crystallized was manganese oxyselenide in 1900.^[2] In 1910, oxyselenides containing phosphate were created by treating P₂Se₅ with metal hydroxides.^[3] Uranium oxyselenide was formed next by treating H₂Se with uranium dioxides at 1000 °C.^[4] This technique was also utilized in synthesizing oxyselenides of rare earth elements in the mid-1900s.^[5] Synthesis of oxyselenide compounds currently involves treating oxides with aluminum powder and selenium at high temperatures.^[6]

Figure 2: Some recently discovered oxyselenide structures crystallize in such a way that the metal oxide layers (a) and the metal selenide layers (b) form an alternating pattern (c). Color code: Yellow − Strontium; Pink − Cobalt; Blue − Oxygen; Green − Selenium; Orange − Copper

Recent discoveries in iron oxyarsenides and their superconductivity have highlighted the importance of mixed anion systems.^[7] Mixed copper oxychalcogenides came about when the electronic properties of both chalcogenides and oxides were taken into account. Chemists began pursuing the synthesis of a compound with metallic and charge density wave properties as well as high temperature superconductivity. Upon synthesizing the copper oxyselenide Na_1.9Cu₂Se₂*Cu₂O by reacting Na2Se3.6 with Cu₂O,^[8] they concluded that a new type of oxychalcogenides could be synthesized by reacting metal oxides with polychalcogenide fluxes.

Derivatives

Figure 3: Orthorhombic structure of β-La₂O₂MnSe₂ at low temperatures.

New oxyselenides of the formula Sr₂AO₂M₂Se₂ (A=Co, Mn; M=Cu, Ag) have been synthesized. They crystallize into structures consisting of alternating perovskite-like (metal oxide) and antifluorite (metal selenide) layers (Figure 2). The optical band gap of each oxyselenide is very narrow, indicating semiconductivity.^[9]

Another derivative that reveals oxyselenide properties is β-La₂O₂MSe₂ (M= Fe, Mn). This molecule possesses an orthorhombic structure (Figure 3), opening up the possibilities for different packing arrangements of oxyselenides. They are ferromagnetic at low temperatures (~27 K) and show high resistivity at room temperature. The Mn analogue, diluted in NaCl solution, suggests an optical band gap of 1.6 eV at room temperature, making it an insulator. Meanwhile, the band gap for the Fe analogue is approximately 0.7 eV at 150 K-300 K, making it a semiconductor.^[7] In contrast, cobalt oxyselenide La₂Co₂O₃Se₂ is antiferromagnetically ordered, suggesting that although the different transition metals are responsible for the changes in an oxyselenide's magnetic property, the molecule's overall lattice structure may also influence its conductivity.^[10]

Figure 4: Comparison of figure-of-merit ZT compounds Bi1-xMxCuSeO. Higher ZTs indicate more efficient energy conversions.

The magnetic and conducting properties of different metal compounds coordinated with oxyselenide are not only affected by the transition metal used, but also by the synthesis conditions. For example, the percentage of aluminum used during the synthesis of Ce₂O₂ZnSe₂ as an oxygen retriever affected the band gaps, indicated by the varying product colors.^[6] Various structures allow for many potential configurations. For example, as observed before in La₂Co₂O₃Se₂, Sr₂F₂Mn₂Se₂O exhibits a frustrated magnetic correlation in the structure resulting in an antiferromagnetic lattice.^[11]

In 2010, p-type polycrystalline BiCuSeO oxyselenides were reported as possible thermoelectric materials.^[12] The weak bonds between the (Cu₂Se₂)-2 conducting and (Bi₂O₂)+2 insulating layer, as well as the anharmonic crystal lattice structure, may account for the substance's low thermal conductivity and high thermoelectric performance. Recently, BiCuSeO's ZT value, a dimensionless figure-of-merit indicating thermoelectric performance, has been increased from 0.5 to 1.4. Experiment has shown that Ca doping can improve electrical conductivity, thereby increasing the ZT value.^[1] Additionally, replacing 15% of the Bi3+ atoms with group 2 metals, Ca2+, Sr2+, or Ba2+ (Figure 4), also optimizes the charge carrier concentration.^[12]

References

1. Pei, Yan-Ling; He, Jiaqing; Li, Jing-Feng; Li, Fu; Liu, Qijun; Pan, Wei; Barreteau, Celine; Berardan, David; Dragoe, Nita; Zhao, Li-Dong (2013). "High thermoelectric performance of oxyselenides: intrinsically low thermal conductivity of Ca-doped BiCuSeO". NPG Asia Materials 5 (5): e47. doi:10.1038/am.2013.15. ISSN 1884-4057.
2. Fonzes-Diacon, H. (1990). "A synthesized selenide and an oxyselenide of manganese." Competes Rendus Hebdomadaires des Seances de l'Academie des Sciences 130: 1025: 1025-6.
3. Ephraim, F., and E. Majler. (1910). "Selenophosphates." Berichte der Deutschen Chemischen Gesellschaft 43: 277-285.
4. Khodadad, P. (1957) "Uranium Oxyselenide. UOSe." Compt. Rend. 245: 226: 2286-8.
5. Guittard, M., Flahaut, J., and Domange, L. "The oxyselenide of Yttrium and all the rare earths". Acta Crystallographica 21 (5).
6. Ainsworth, Chris M.; Wang, Chun-Hai; Tucker, Matthew G.; Evans, John S. O. (2015). "Synthesis, Structural Characterization, and Physical Properties of the New Transition Metal Oxyselenide Ce₂O₂ZnSe₂". Inorganic Chemistry 54 (4): 1563–1571. doi:10.1021/ic502551n. ISSN 0020-1669.
7. McCabe, Emma E.; Free, David G.; Mendis, Budhika G.; Higgins, Joshua S.; Evans, John S. O. (2010). "Preparation, Characterization, and Structural Phase Transitions in a New Family of Semiconducting Transition Metal Oxychalcogenides β-La₂O₂MSe₂ (M= Mn, Fe)". Chemistry of Materials 22 (22): 6171–6182. doi:10.1021/cm1023103. ISSN 0897-4756.
8. Park, Younbong; DeGroot, Don C.; Schindler, Jon L.; Kannewurf, Carl R.; Kanatzidis, Mercouri G. (1993). "Intergrowth of two different layered networks in the metallic copper oxyselenide Na_1.9Cu₂Se₂⋅Cu₂O". Chemistry of Materials 5 (1): 8–10. doi:10.1021/cm00025a004. ISSN 0897-4756.
9. Jin, Shifeng; Chen, Xiaolong; Guo, Jiangang; Lei, Ming; Lin, Jingjing; Xi, Jianguo; Wang, Wenjun; Wang, Wanyan (2012). "Sr2Mn3Sb2O2Type Oxyselenides: Structures, Magnetism, and Electronic Properties of Sr2AO2M2Se2(A=Co, Mn;M=Cu, Ag)". Inorganic Chemistry 51 (19): 10185–10192. doi:10.1021/ic301022g. ISSN 0020-1669.
10. Fuwa, Yayoi; Endo, Takashi; Wakeshima, Makoto; Hinatsu, Yukio; Ohoyama, Kenji (2010). "Orthogonal Spin Arrangement in Quasi-Two-Dimensional La₂Co₂O₃Se₂". Journal of the American Chemical Society 132 (51): 18020–18022. doi:10.1021/ja109007g. ISSN 0002-7863.
11. Liu, Y.; Zhang, S.B.; Li, L.J.; Lu, W.J.; Zhao, B.C.; Tong, P.; Song, W.H.; Lin, S.; Huang, Y.N.; Huang, Z.H.; Tan, S.G.; Sun, Y.P. (2013). "Synthesis, structure and properties of the new layered manganese oxyselenide Sr₂F₂Mn₂Se₂O". Journal of Alloys and Compounds 580: 211–216. doi:10.1016/j.jallcom.2013.05.048. ISSN 0925-8388.
12. Zhao, Li-Dong; He, Jiaqing; Berardan, David; et al. (2014). "BiCuSeO oxyselenides: new promising thermoelectric materials". Energy & Environmental Science 7 (9): 2900. doi:10.1039/C4EE00997E. ISSN 1754-5692.