Isovalent Ca and Ba substitutions in thermoelectric layer-structured oxyselenide Sr₂CoO₂Cu₂Se₂

Abstract

Multilayered compounds typically present exotic functionalities, and some of them have been suggested as potential materials for thermoelectric conversion owing to their unique capability to decouple electronic and heat transport. Here we report new [CoO2] and [Cu2Se2] layered A2CoO2Cu2Se2 compounds in which Sr at the intervening alkaline-earth A site is partially replaced with Ca or Ba. The parent Sr2CoO2Cu2Se2 phase is a direct gap p-type semiconductor, and density functional theory (DFT) calculations indicate its topmost valence band consists of Cu 3d-Se 4p states. Upon the isovalent cation substitution the lattice modification in the ab plane is constrained by the stiff [CoO2] layer such that the lattice shrinkage/expansion mainly happens along the c axis. Substitution of Sr with the heavier and larger Ba significantly enhances the thermopower but more hole states would be required to optimize the thermoelectric performance. Thermal stability is related to the inter-oxide-selenide-layer interaction, and our thermogravimetric measurement data reveal that the A2CoO2Cu2Se2 materials could operate in the intermediate temperature region.

Synthesis, Structure, and Properties of the Layered Oxyselenide Ba2CuO2Cu2Se2

“…Taking Bi 2 YO 4 Cu 2 Se 2 as an example, it is metallic with a ρ r value of about 0.59 mΩ·cm because charge transfer happens between the [Cu 2 Se 2 ] and [Bi 2 YO 4 ] layers . In addition, for these oxychalcogenides, the valence band or the band near the Fermi surface, determining the conduction behavior, is mainly derived from antifluorite layers. ,, Hereafter, we will also illustrate that it is the same for Ba 2 CuO 2 Cu 2 Se 2 .…”

“…On the basis of the antifluorite layer of [Cu 2 S 2 ], oxysulfides with the formula (Sr n +1 M n O 3 n –1 )­Cu 2 S 2 ( n = 1–3; M = Cr, Mn, Fe, Co, Zn, and Cu) have been synthesized and systematically studied. − The intriguing oxysulfide is Sr 2 CuO 2 Cu 2 S 2 because it consists of [CuO 2 ] planes and is considered to be a candidate for the superconductor . For oxyselenides, the compounds of Sr 2 MO 2 Cu 2 Se 2 (M = Co and Mn) are currently reported, and Sr 2 CoO 2 Cu 2 Se 2 was studied as a potential thermoelectric material . Another interesting material is Na 1.9 Cu 2 Se 2 Cu 2 O, which features alternately stackable layers of [Cu 2 Se 2 ] and [Cu 2 O] and presents a metallic state.…”

Tuning the S=1/2 square-lattice antiferromagnet Sr2Cu(TMustonen1, Vasala2, Schmidt3 et al. 2018Phys. Rev. B462550View full textAdd to dashboardCite

“…Electron correlation effects of the 3d copper orbitals were included with a DFT+U approach using the fullylocalized limit double counting correction 47 . We used a Hubbard U value of U = 8 eV typical of copper [47][48][49] (with Hund's rule coupling J = 0.9 eV corresponding to Ueff = 7.1 eV), which was also the optimal value for Sr2CuWO6 23 . The calculations were carried out using 2 x 2 x 1 supercells with ferromagnetic, Néel or columnar antiferromagnetic order 23 .…”

Spin wave excitations in the tetragonal double perovskiteSr2CuWO6

“…For this reason, we have calculated the exchange constants using a range of U values typical for Cu 3d with two different double counting corrections. With the FLL correction a Hubbard U of ∼ 8 − 9 eV has been widely used for Cu in oxides, 24,[26][27][28] whereas a slightly lower U of ∼ 6 − 7 eV is typical when using the AMF correction 25,27 . The intraatomic exchange parameter I was chosen to be 0.9 eV, which is a common value in 3d transition metal oxides 24 .…”