CuSe-based layered compound ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$ as a quasi-two-dimensional metal

We have investigated the physical properties of a new layered oxyselenide ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$, which crystallizes in an unusual intergrowth structure with ${\mathrm{Cu}}_2{\mathrm{Se}}_2$ and ${\mathrm{Bi}}_2{\mathrm{YO}}_4$ layers. Electric transport measurement indicates that ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$ behaves metallic. Thermal transport and Hall measurements show that the type of the carriers is holelike and it may be a potential thermoelectric material at high temperatures. First-principles calculations are in agreement with experimental results and show that ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$ is a quasi-two-dimensional metal. Further theoretical investigation suggests the ground states of the ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$ type can be tuned by designing the blocking layers, which will enrich the physical properties of these compounds.

Figure 1

The crystal structure of ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2$ ${\mathrm{Se}}_2$.

Figure 2

(a) Result of Rietveld refinement against powder XRD diffraction data for ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$; (b) the EDX spectrum of the sample; (c) and (d) are the XPS spectra of Bi, Y, and Cu.

Figure 3

(a) The temperature dependence of magnetic susceptibility $\chi \left(T\right)$ curve from 5 to 300 K. (b) The EPR spectra of ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$ at different temperatures.

Figure 4

Electrical resistivity of ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$ is plotted as a function of temperature from 300 to 2 K, and the red line is the Fermi-liquid fitting of the resistivity at low temperature.

Figure 5

Temperature dependence of specific heat for ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$. The inset represents the low-temperature data $C_p\left(T\right)/T$ plotted as a function of $T^2$.

Figure 6

(a) The thermal conductivity $\kappa$, (b) Seebeck coefficient $S$, (c) electrical conductivity $\sigma$, and (d) figure of merit $ZT$ as a function of temperature from 330 to 5 K.

Figure 7

(a) Field dependence of ${\rho }_{xy}\left(H\right)$ at various temperatures. (b) Temperature dependence of the Hall coefficient $R_H$ of ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$. Inset: temperature dependence of the carrier density $n=1/|e{R}_H|$ calculated from $R_H$.

Figure 8

The calculated DOS of ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$.

Figure 9

(a) The band structure, (b) the Brillouin zone, and (c) Fermi surface of ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$. The red and blue lines in (a) denote the bands crossing $E{}_F$. The high-symmetry points are denoted in (b).

Figure 10

“Fat bands” of ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$, decorated with partial orbital characters of Cu and Se.

Figure 11

(a) Orbital contribution of DOS of Cu-$3d$ electrons with $U=0$ eV. (b) Orbital contribution of DOS of Cu-$3d$ electrons with $U=5$ eV. (c) DOS of ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$ with $U=$ 0, 2.5, 5, and 7.5 eV.

Figure 12

(a) The DOS of BiOCuSe and LaOCuSe. (b) The DOS of ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$, and a hypothetical ${\mathrm{La}}_3{\mathrm{O}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$.

Figure 13

The DOS of one electron doped in ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$ (left panel) and ${\mathrm{La}}_3{\mathrm{O}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$ (right panel). (a) and (b) show the doping effect simulated by simply adding an electron to the system based on RBA. (c)–(h) show the doping effect simulated by doping typical tetravalent cations Zr and Sn or a typical monovalent anion F.

Figure 14

The DOS of ${\mathrm{[Sr}}_2{\mathrm{F}}_2$(${\mathrm{SF}}_2$)${}_n{\mathrm{]Cu}}_2{\mathrm{Se}}_2$ with (a) $n=0$ and (b) $n=1$. The insets show the structures of the ${\mathrm{[Sr}}_2{\mathrm{F}}_2$(${\mathrm{SF}}_2$)${}_n$]-type blocking layers.