Chemical reactions of Zintl phases: Mild oxidation of germanides and stannides

Theoretical investigations have predicted that Ge-Sn alloys exhibit important optoelectronic properties for a variety of applications. These have been confirmed by the preparation of metastable thin-film of Ge 1-xSnx alloys, with limited compositional range, from MBE and CVD techniques. The bulk forms of metastable Ge-Sn alloys having width significant solubility have yet to be synthesized. Bulk preparation under thermal equilibrium condition only has the maximum solubility of Sn in Ge of 1% (x ≤ 0.01). Using the mild oxidation of the Zintl phases in DTAC/AlCl 3 ionic liquid at the temperatures range of 200--300°C, we report the successful synthesis of Ge1-xSnx alloys with the diamond structures. The unit cell of the Ge1-xSn x with the cubic alpha-Ge structure (space group Fd3¯m) ranges from a = 5.686(1)--5.706(5) A, as compared to that of a-Ge (a = 5.654(1) A). The optical band gaps of the alloys were determined to be narrower than that of Ge (Eg = 0.67 eV). The Eg of the prepared alloys were decreased to 0.32(3) eV and determined to be indirect gaps by using Tauc plots. Raman scattering results indicate the vibrational modes of Ge-Ge, Ge-Sn and Sn-Sn. The Ge-Ge vibrational mode is the most intense peak since the alloys is Ge-rich phases. The Ge-Sn alloys decompose to alpha-Ge and beta-Sn upon heating under inert atmosphere. This thermal decomposition confirms the metastable behavior of the alloys. The novel synthesis and results of the chemical and physical characterization of the unprecedented Ge-Sn alloys will be reported.