Quaternary selenostannates Na2-xGa2-xSn1+xSe6 and AGaSnSe(4) (A = K, Rb, and Cs) through rapid cooling of melts. Kinetics versus thermodynamics in the polymorphism of AGaSnSe(4)

摘要

The quaternary alkali-metal gallium selenostannates, Na2-xGa2-xSn1+xSe6 and AGaSnSe(4) (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) Angstrom, b = 7.594(2) Angstrom, c = 13.842(3) Angstrom, beta = 118.730(4)degrees V = 1226.7(5) Angstrom(3). alpha-KGaSnSe4 crystallizes in the tetragonal space group I4/mcm with a =8.186(5) Angstrom and c = 6.403(5) Angstrom, V = 429.1(5) Angstrom(3). beta-KGaSnSe4 crystallizes in the space group P2(1)/c with cell constants a = 7.490(2) Angstrom, b = 12.578(3) Angstrom, c = 18.306(5) Angstrom, beta = 98.653(5)degrees, V = 1705.0(8) Angstrom(3). The unit cell of isostructural RbGaSnSe4 is a = 7.567(2) Angstrom, b = 12.656(3) Angstrom, c = 18.277(4) Angstrom, beta = 95.924(4)degrees; V = 1741.1(7) Angstrom(3). CsGaSnSe4 crystallizes in the orthorhombic space group Pmcn with a = 7.679(2) Angstrom, b = 12.655(3) Angstrom, c = 18.278(5) Angstrom, V = 1776.1(8) Angstrom(3). The structure of Na2-xGa2-xSn1+xSe6 consists of a polar three-dimensional network of trimeric (Sn,Ga)(3)Se-9 units with Na atoms located in tunnels. The AGaSnSe(4) 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 gamma-form (BaGa2S4-type) of these compounds. (C) 2004 Elsevier Inc. All rights reserved.