Quaternary selenostannates Na2−xGa2−xSn1+xSe6 and AGaSnSe4 (A=K, Rb, and Cs) through rapid cooling of melts. Kinetics versus thermodynamics in the polymorphism of AGaSnSe4

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Abstract

The quaternary alkali-metal gallium selenostannates, Na2−xGa2−xSn1+xSe6 and AGaSnSe4 (A=K, Rb, and Cs), were synthesized by reacting alkali-metal selenide, Ga, Sn, and Se with a flame melting–rapid cooling method. Na2−xGa2−xSn1+xSe6 crystallizes in the non-centrosymmetric space group C2 with cell constants a=13.308(3) Å, b=7.594(2) Å, c=13.842(3) Å, β=118.730(4)°, V=1226.7(5) Å3. α-KGaSnSe4 crystallizes in the tetragonal space group I4/mcm with a=8.186(5) Å and c=6.403(5) Å, V=429.1(5) Å3. β-KGaSnSe4 crystallizes in the space group P21/c with cell constants a=7.490(2) Å, b=12.578(3) Å, c=18.306(5) Å, β=98.653(5)°, V=1705.0(8) Å3. The unit cell of isostructural RbGaSnSe4 is a=7.567(2) Å, b=12.656(3) Å, c=18.277(4) Å, β=95.924(4)°, V=1741.1(7) Å3. CsGaSnSe4 crystallizes in the orthorhombic space group Pmcn with a=7.679(2) Å, b=12.655(3) Å, c=18.278(5) Å, V=1776.1(8) Å3. The structure of Na2−xGa2−xSn1+xSe6 consists of a polar three-dimensional network of trimeric (Sn,Ga)3Se9 units with Na atoms located in tunnels. The AGaSnSe4 possess layered structures. The compounds show nearly the same Raman spectral features, except for Na2−xGa2−xSn1+xSe6. Optical band gaps, determined from UV–Vis spectroscopy, range from 1.50 eV in Na2−xGa2−xSn1+xSe6 to 1.97 eV in CsGaSnSe4. Cooling of the melts of KGaSnSe4 and RbGaSnSe4 produces only kinetically stable products. The thermodynamically stable product is accessible under extended annealing, which leads to the so-called γ-form (BaGa2S4-type) of these compounds.

Introduction

The structural chemistry of complex chalcogenides containing tetravalent group 14 elements (i.e., Si, Ge, Sn) has been relatively well investigated [1], [2], [3]. Due to the strong preference of these elements for tetrahedral geometry, most compounds possess crystal structures consisting of a variety of edge- or corner-shared MQ4 (M=Si, Ge, Sn; Q=S, Se, Te) tetrahedra. These include (i) [MQ4]4− tetrahedra [4], [5], (ii) the [M2Q7]6− unit formed by the corner-sharing of two tetrahedra [6], (iii) the adamantane-like [M4Q10]4− unit formed by the sharing of three corners of each tetrahedron [7], and (iv) the [M2Q6]4− dimers formed by the edge-sharing of two tetrahedra [8]. Several quaternary selenostannates are known with transition metals such as Cu, Ag, Au, and Mn [9], [10].

Fewer reports exist with group 13 elements (e.g., Al, Ga, and In). From the viewpoint of synthetic methodology, selenostannates have been generally prepared in flux medium at intermediate temperature or by direct combination reactions followed by slow cooling to achieve crystallization. The latter technique results in thermodynamically stable phases in the range of crystallization temperature. In an attempt to avoid thermodynamically stable phases and favor kinetically stable or even metastable phases we opted to take the exact opposite approach. That is a fast reaction, followed by rapid quenching to induce rapid crystallization. In this regard, we have explored a flame melting–rapid cooling method in which the reactants are thoroughly melted by a torch flame and then rapidly quenched into room temperature or even liquid N2 temperature. This approach can produce cooling rates in the order of 200°C/min and can be useful (although it does not guarantee) in accessing kinetically stable phases that cannot be prepared by conventional slow cooling processes. Such a tactic has been little exploited. Recently, we reported some interesting metastable compounds (e.g., KBiP2Se6 and KSbP2Se6) [11], KSb5S8[12], and β-KInSnSe4[13] that have been trapped through rapid melt cooling. These materials exhibit phase transitions to more thermodynamically stable configurations simply by annealing below the melting point.

Here we report several quaternary alkali-metal gallium selenostannates synthesized through this flame melting–rapid cooling method, and also describe their crystal structures and optical properties. Na2−xGa2−xSn1+xSe6 (I) has a three-dimensional framework structure consisting of trimeric (Sn,Ga)3Se9 units and tunnels where the Na atoms reside. The structures of KGaSnSe4 (II), RbGaSnSe4 (III), and CsGaSnSe4 (IV) are layered in which one-dimensional chains consisting of edge-shared (Sn,Ga)2Se6 dimers and (Sn,Ga)Se4 tetrahedra are linked side-by-side. We show that in KGaSnSe4, RbGaSnSe4, cooling of the melts does not produce the thermodynamically stable product. In fact the latter is inaccessible under melt-cooling conditions; instead a proper solid-state annealing protocol is necessary to obtain the so-called γ-form of these compounds. The implications of these results are discussed.

Section snippets

Experimental

Chemicals were used as obtained without further purification: (i) Ga metal (3 mm shots 99.999%, CERAC, Inc.) (ii) Sn metal (99.8%, −325 mesh, CERAC, Inc.), and (iii) Se (99.99%, Noranda Advanced Materials, Quebec, Canada) A2Se (A=Na, K, Rb, and Cs) were prepared by reaction of stoichiometric amounts of alkali-metal and selenium in liquid ammonia [14]. All manipulations were carried out under a dry nitrogen atmosphere inside a glovebox.

Red crystals of Na2−xGa2−xSn1+xSe6 (I), yellowish-brown

Structure of Na0.9Ga0.9Sn0.6Se3

The Ga and Sn sites in the lattice are mixed occupancy sites and compared to the ideal composition of “NaGaSn0.5Se3” there exist a slight excess of tin atoms on metal sites M(1) and M(2). As a result, the concentration of Na and Ga is slightly decreased from the ideal concentration to maintain charge neutrality. As illustrated in Fig. 1, the structure of I is non-centrosymmetric and polar and contains a three-dimensional network of corner-shared (Sn,Ga)Se4 tetrahedra. The network consists of

Conclusion

The quaternary selenostannates, Na2−xGa2−xSn1+xSe6 and AGaSnSe4 (A=K, Rb, and Cs), were prepared with a flame melting–rapid cooling method. The structure of Na2−xGa2−xSn1+xSe6 can be described as three-dimensional polar network of trimeric (Sn,Ga)3Se9 units with Na atoms in one-dimensional tunnel. This phase is found to require a fast crystallization condition to form and it is regarded to be only kinetically stable. In light of this, it is evident that the flame melting–rapid cooling method is

Acknowledgements

Financial support from the National Science Foundation (DMR-0127644) is gratefully acknowledged. This work was supported in part by Korea Research Foundation Grant (KRF-2003-042-C00065).

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Present address: Department of Applied Chemistry, Center for Optoelectronics and Microwave Thin-Film Devices, College of Natural Sciences, Konkuk University Chungju Campus, Chungju, Chungbuk 380-701, South Korea.

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