Rb₄Sn₅P₄Se₂₀: A Semimetallic Selenophosphate

Abstract

Chalcophosphates are ternary (A/P/Q) and quaternary (A/M/ P/Q) compounds with [P y Q z ] nÀ anions in their structure, where M is a metal, A is an alkali metal, and Q is sulfur, [1] selenium, [2] or tellurium. [3] They can exhibit fundamentally and technologically important nonlinear optical [4] and ferroelectric properties, [5] reversible redox chemistry relevant to secondary batteries, [6] photoluminescence, [7] and phasechange properties. [8] However, no metallic selenophosphates have been reported to date. This lack is in striking contrast to the oxophosphate counterparts, which are a well-defined series of unusual metals, such as (PO 2 ) 4 (WO 3 ) 2m , A x (PO 2 ) 4 -(WO 3 ) 2m (A = Na, K), and A x (P 2 O 4 ) 2 (WO 3 ) 2m (A = K, Rb, Tl, Ba). [9] The metallic properties of these phosphates derive from mixed valency of the transition metals. There have been attempts to explore chalcophosphate compounds as thermoelectric materials. For example, in light of the known alkali chalcophosphate compounds, the Tl + analogues Tl 3 Ti 2 P 5 S 18 , Tl 2 CeP 2 S 7 , TlTiPS 5 , and Tl 2 BiP 2 S 7 were investigated. [10] However, they were wide-gap semiconductors and too resistive for such applications. Inclusion of a transition metal gave a better-conducting semiconductor Ni 3 Cr 2 P 2 Q 9 (Q = S, Se). [11] Herein, we report on Rb 4 Sn 5 P 4 Se 20 , the first metallic selenophosphate. Its structure belongs to a class not previously known for this family, namely, a lamellar hybrid of "conducting" [Sn 5 Se 8 ] layers and "insulating" [P 2 Se 6 ] ligands. The structure is unique in that the [P 2 Se 6 ] 4À anions coordinate solely to the outer shell of the [Sn 5 Se 8 ] backbone layers. Consequently, the core of the latter consists only of SnÀSe bonds and is free of [P y Se z ] nÀ units. Temperature-dependent electrical conductivity, thermopower, and Hall effect measurements on Rb 4 Sn 5 P 4 Se 20 show n-type metallic behavior. The

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

“…Na2Se was synthesized by reacting stoichiometric amounts of the elements in liquid ammonia. [33][34] Synthesis. All chemical handling was carried out in a dry nitrogen-filled glovebox.…”