Synthesis of a Family of Solids Through the Building-Block Approach: A Case Study with Ag⁺ Substitution in the Ternary Na-Ge-Se System

Author

Amitava Choudhury, Missouri University of Science and TechnologyFollow
Sabine Strobel
Benjamin R. Martin
Angela L. Karst
Peter K. Dorhout

Abstract

A new family of Ag-substituted pseudoquaternary alkali-seleno-germanates has been synthesized by two solid-state routes: the conventional flux method and metathesis. This family includes a series of semiconductors with varying amounts of Ag⁺ substituted for Na⁺ in Na ₈Ge₄Se₁₀ to form Ag_xNa _(8-x)Ge₄Se₁₀, [x = 0.31 (I), 0.67 (II), 0.77 (III), 0.87 (IV), 1.05 (V), 1.09 (VI)] and another phase with a different composition Ag_xNa_(6-x)Ge₂Se₇ (x = 1.76), VII, related to Na₆Ge₂Se₇. In I-VI, Ge₄Se₁₀ ^8- constitutes a 6-membered chairlike unit with a Ge-Ge bond, while in VII, a corner-shared dimer of GeSe₄ tetrahedra (Ge₂Se₇ ^6-) acts as the building unit. The single-crystal structure analysis indicates that there is a phase transition from P1 to C2/c, in changing from pure Na₈Ge ₄Se₁₀ to Ag_xNa_(8-x)Ge₄ Se₁₀ (I-VI), while there is no phase transition between pure Na ₆Ge₂Se₇ and Ag_xNa _(6-x)Ge₂Se₇ (x = 1.76). The structures of I-VI may be described in terms of layers of cubic close-packed Se^2- anions. In between the Se layers, octahedral holes fully occupied by Na ⁺ and mixed Ag⁺/Na⁺ cations alternate with layers formed of octahedral holes fully occupied by Na⁺ and Ge ₂ ⁶⁺ cations. Two adjacent Ge₂ ⁶⁺ cations form a chairlike Ge₄Se₁₀ ^8- anion in which Ge-Ge bonds are oriented almost parallel to the Se layers. In contrast, VII does not have close-packed anions. Corner-shared GeSe₄ tetrahedra (Ge₂Se₇ ^6- dimer) and AgSe₄ tetrahedra form layers that are cross-linked by Na/AgSe₄ tetrahedra to form a 3-dimensional (3-D) structure. An optical property investigation indicates a red shift in the band gap of Ag_xNa_(8-x)Ge ₄Se₁₀ (x = 0.67)(II) as compared to that of pure Na ₈Ge₄Se₁₀. Raman data also indicate a red shift of the Ge-Se stretching mode in the Ag⁺-substituted phase II (x = 0.67) compared to that of Na₈Ge₄Se₁₀.

Recommended Citation

A. Choudhury et al., "Synthesis of a Family of Solids Through the Building-Block Approach: A Case Study with Ag⁺ Substitution in the Ternary Na-Ge-Se System," Inorganic Chemistry, vol. 46, no. 6, pp. 2017 - 2027, American Chemical Society (ACS), Mar 2007.

The definitive version is available at https://doi.org/10.1021/ic061064z

Department(s)

Chemistry

International Standard Serial Number (ISSN)

0020-1669

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2007 American Chemical Society (ACS), All rights reserved.

Publication Date

01 Mar 2007