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" with the metallic characteristics. The results of ab initio density functional theory (DFT) calculations using the allelectron full-potential linearized augmented plane wave (FLAPW) method [12] reveal that the metallic behavior originates from the overlap of conduction and valence bands and not any formal mixed valency.Rb 4 Sn 5 P 4 Se 20 was synthesized by the reaction of Sn/Rb 2 Se/ P 2 Se 5 /Se in a 1:1:1:5 molar ratio at 490 8C for four days. However, the complicated Lewis acid-base equilibria in the flux also yielded Rb 3 Sn(PSe 5 )(P 2 Se 6 ) [13] as by-product. On the other hand, the direct combination of the elements and reaction at 515 8C gave SnP 2 Se 6 . [14] We could obtain pure Rb 4 Sn 5 P 4 Se 20 only by direct combination reactions of Sn/ Rb 2 Se/P/Se at 850 8C. According to differential thermal analysis (DTA), Rb 4 Sn 5 P 4 Se 20 melts congruently at 517 8C, and the melt crystallizes at 494 8C. The X-ray powder diffraction patterns before and after melting and recrystallization were identical (Supporting Information Figure S1).Rb 4 Sn 5 P 4 Se 20 crystallizes in the trigonal space group P3 m1.[15] The structure features thick anionic [Sn 5 P 4 Se 20 ] n 4nÀ layers ( Figure 1 and Figure S2 in the Supporting Information). The layer is based on the SnSe 2 structure (CdI 2 type); the latter consists of planes of hexagonally packed Se atoms interleaved with planes of octahedral Sn atoms.[16] The removal of 1

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

“…The following chemicals were used as purchased: copper powders (99.9%, Sigma-Aldrich), sodium chunks (99.99%, Sigma-Aldrich), CuO powders (99.9%, Sigma-Aldrich), and selenium shots (99.999%, American Elements). Na 2 Se was synthesized by reacting stoichiometric amounts of the elements in liquid ammonia. , …”

“…Na 2 Se was synthesized by reacting stoichiometric amounts of the elements in liquid ammonia. 33,34 Synthesis. All chemical handling was carried out in a dry-nitrogenfilled glovebox.…”

Panoramic Synthesis as an Effective Materials Discovery Tool: The System Cs/Sn/P/Se as a Test Case

“…The STOE programs X-Area, X-RED, and X-SHAPE were used to conduct the data collection, integration, and numerical absorption, respectively. The atomic sites without disorder from the analogous Rb 4 Sn 5 P 4 Se 20 were used to solve the crystal structure of Cs 4 (Sn 3 Se 8 )­[Sn­(P 2 Se 6 )] 2 and SHELXT was used to solve the crystal structures of Cs 4 Sn­(P 2 Se 6 ) 2 and α-Cs 2 SnP 2 Se 6 . The structures were all refined in the SHELXTL program package .…”

New compounds and structures in the solid state

“…The valence precise compound Rb 4 Sn 5 P 4 Se 8 is the first metallic selenophosphate. 44 It has a new structure consisting of ''conducting'' [Sn 5 Se 8 ] layers and insulating P 2 Se 6 ligands. This is a new structural motif for the ternary A/M/P/Q chalcophosphates (A = alkaline, M = metal, SQS, Se, Te).…”