Dependence of the Physical Properties on the Cation Ordering in the New Ternary Phosphide ZnCu₂P₈

Abstract

: The air‐stable phosphide, ZnCu2P8, was synthesized from its elements in gram amounts. It crystallizes in the Cu2.5In0.5P8 structure type [C2/c, Z = 4, a = 1110.8(2) pm, b = 964.7(2) pm, c = 752.7(1) pm, β = 110.027(3)°], and is the first ternary phosphide in the system Zn/Cu/P. Its crystal structure consists of two‐dimensional, layered polyphosphide‐anions, separated by tetrahedrally‐coordinated cations. 135 symmetry independent configurations, each with a different and specific Zn2+ and Cu+ arrangement, where explored with density functional methods. Whereby, each of the 135 cation ordering pattern has a diverse bandgap, and the total energies vary in the range of 1 eV. As several configurations with low energy are close in energy, an intrinsic disorder is discussed as origin of the promising low lattice thermal conductivity, observed.

Thermoelectric Performance Enhancement of the Cost-Effective Phosphide ZnCu2P8

“…For ZnCu 2 P 8 , there are two distinct metal sites, but due to the similar X-ray scattering factors of Zn and Cu, the initial structure refinement could not distinguish specific elemental sites; however, applying mixed occupied sites resulted in the most favorable structure solution. 16 Using the Zintl−Klemm formalism in combination with the reported crystal structure of ZnCu 2 P 8 , one can deduce an electron-balanced (Zn 2+ )-(Cu + ) 2 (P 0 ) 4 (P 2− ) 4 formula.…”

“…While an in-depth computational study on relative structure energies and the effects of cation ordering on the band gap was previously performed, suggesting that cation influence on the polyphosphide anion electronic structure dictates the band gap, there were no mentions of any attempts to experimentally confirm the computed band gaps. 16 Thus, we performed solidstate diffuse reflectance on the synthesized compositions to supplement known theoretical work. Figure 2 shows that the direct band gap fittings of the Zn 1.25 Cu 1.75 P 8 , ZnCu 2 P 8 , Zn 0.9 Cu 2.1 P 8 , and Zn 0.75 Cu 2.25 P 8 compositions were found to be 1.11(1), 1.03(1), 1.00(2), and 0.89(2) eV, respectively.…”

“…15−18 Contrary to this misconception, there have been a number of reports on phosphides with high phosphorus content, exhibiting low thermal conductivity, for example, Ba 8 Au 16 P 30 , Ba 2 Si 3 P 6 , ZnCu 2 P 8 , and NiP 2 . 12,15,16,19 Especially of note are the ultralow thermal conductivities of 0.62 and 0.56 W m −1 K −1 observed at 300 K for Ba 8 Au 16 P 30 and Ba 2 Si 3 P 6 , respectively. 12,19 Such low thermal conductivities are comparable to state-of-the-art thermoelectric materials such as Yb 14 MnSb 11 and SnSe single crystals.…”

“…15,20 Furthermore, experimentally, there have been promising reports on the phosphides YbCuZnP 2 and Ag 6 Ge 10 P 12 achieving thermoelectric figures of merit, zT, as high as 0.6 and 0.7, respectively. 18,22 Recently discovered ZnCu 2 P 8 is reported to be air and acid stable, as well as possesses high thermal stability up to 960 K. 16 Even with such high stability, the characterization of ZnCu 1.2 W m −1 K −1 at 390 K. 16 However, the previous crystallographic and computational work by Nuss et al suggest that there is significant mixing of Zn and Cu in the metal crystallographic positions and that the cation ordering can significantly influence the physical properties in this system, resulting in band gaps in the range of 0.62−1.29 eV. 16 This indicates that ZnCu 2 P 8 could be a cheap, nontoxic, highly tunable chemical template composed of earth-abundant elements for thermoelectric applications, contrary to many widely studied thermoelectrics.…”

“…18,22 Recently discovered ZnCu 2 P 8 is reported to be air and acid stable, as well as possesses high thermal stability up to 960 K. 16 Even with such high stability, the characterization of ZnCu 1.2 W m −1 K −1 at 390 K. 16 However, the previous crystallographic and computational work by Nuss et al suggest that there is significant mixing of Zn and Cu in the metal crystallographic positions and that the cation ordering can significantly influence the physical properties in this system, resulting in band gaps in the range of 0.62−1.29 eV. 16 This indicates that ZnCu 2 P 8 could be a cheap, nontoxic, highly tunable chemical template composed of earth-abundant elements for thermoelectric applications, contrary to many widely studied thermoelectrics. Given the appeal of costeffective thermoelectric materials, coupled with the high stability of this phosphide, we seek to further study ZnCu 2 P 8 , at higher temperatures and different cation compositions to further explore its thermoelectric potential, expanding known works on thermoelectric phosphides in the process.…”