NaCu₆Se₄: A Layered Compound with Mixed Valency and Metallic Properties

Abstract

A new ternary compound NaCu6Se4 was synthesized from the reaction of Cu in a molten sodium polyselenide flux. The compound crystallizes in trigonal space group R3̅m with a = 4.0465(3) Å and c = 41.493(5) Å. The crystal structure contains flat two-dimensional slabs of (1)/∞[Cu6Se4] with a unique structural arrangement, separated by Na cations. The compound contains mixed valency and has a high conductivity of ∼3 × 10(3) S cm(-1) at room temperature, and exhibits increasing conductivity with decreasing temperature, indicating metallic behavior. A small positive thermopower (4-11 μV K(-1) from 300 to 500 K) and Hall effect measurements indicate p-type transport with a carrier concentration of ∼2.8(3) × 10(21) cm(-3) and a hole mobility of ∼8.75 cm(2) V(-1) s(-1) at 300 K. NaCu6Se4 exhibits temperature-independent Pauli paramagnetism.

High Hole Mobility and Nonsaturating Giant Magnetoresistance in the New 2D Metal NaCu4Se4 Synthesized by a Unique Pathway

“…[57][58] This mobility value is the highest among all metallic copper selenides. For example, at 2 K the mobilities are ~25 cm 2 V -1 s -1 for CuSe 59 , ~6.5 cm 2 V -1 s -1 for NaCu4S4 13 , ~1.9 cm 2 V -1 s -1 for NaCu4Se3 12 and ~22 cm 2 V -1 s -1 for NaCu6Se4 15 , comparison in Figure 4d.…”

“…As depicted in Figure 4b, n at room temperature is ~5.39 × 10 21 cm -3 which is of the same order found for NaCu4Se3 (~6.12 × 10 21 cm -3 ) and NaCu6Se4 (~2.83 × 10 21 cm -3 ). 12,15 This value decreases slightly to 2.95 × 10 21 cm -3 at 2 K. The , estimated by the formula  = 1/(ne), is ~11 cm 2 V -1 s -1 at room temperature, and remarkably increases by nearly two orders of magnitude to ~808 cm 2 V -1 s -1 at 2 K (Figure 4c). In the range of 15  300 K, the  displays a T 3/2 dependence behavior, suggesting acoustic phonon scattering is occurring.…”

“…As for the reaction of Na2Se/Cu/Se, it generated NaCu6Se4 and NaCuSe as the products without any sign of NaCu4Se4 formation. 15 Using the stoichiometry determined by EDS the X-ray single crystal diffraction analysis, we targeted NaCu4Se4 by direct combination of elemental Na, Cu and Se in a 1:4:4 ratio, and heating this mixture to 873 K for 10 hr.…”

“…The chemical formula of NaCu4Se4, however, can be better represented either by (Na + )(Cu + )3(Cu 2+ )(Se 2 )2(Se2) 2 or (Na + )(Cu + )4(Se 2 )(Se  )(Se2) 2 with mixed valent (Cu + /Cu 2+ ) 52-54 or (Se 2 /Se  ). 12,15 To interrogate valence states of the atoms in NaCu4Se4, we used XPS. In the case of Na, only one binding energy of 1072.1 eV corresponding to Na + was observed (Figure 3a).…”

“…The low-melting flux synthesis method, which gives access to intermediate temperatures, has been proven a powerful approach to discover new phases in the A/Cu/Q system. [18][19] Despite the great success of the polychalcogenide flux method in enabling productive synthesis routes to new materials, e.g., KCu3-xSe2, 5 NaCu4Se3, 12 and NaCu6Se4, 15 in rare instances we also discover that the nature of the precursors can also be critical in forming new phases. For example, the use of Cu2O was shown to be unique in obtaining the mixed-anion oxychalcogenide compound Na1.9Cu2Se2•Cu2O which features metallic behavior with mixed valency.…”