Thermochemistry, Morphology, and Optical Characterization of Germanium Allotropes Julia V. Zaikina, †,‡ Elayaraja Muthuswamy, † Kristina I. Lilova, ‡ Zachary M. Gibbs, ∥ Michael Zeilinger, § G. Jeffrey Snyder, ⊥ Thomas F. Fa ̈ ssler, § Alexandra Navrotsky,* ,‡ and Susan M. Kauzlarich* ,† † Department of Chemistry and ‡ Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California, One Shields Avenue, Davis, California 95616, United States § Department of Chemistry, Technische Universitä t Mü nchen, Lichtenbergstrasse 4, 85747 Garching, Germany ∥ Division of Chemistry and Chemical Engineering, California Institute of Techology, 1200 East California Boulevard, Pasadena, California 91125, United States ⊥ Department of Materials Science, California Institute of Technology, Pasadena, California 91125, United States * S Supporting Information ABSTRACT: A thermochemical study of three germanium allotropes by differential scanning calorimetry (DSC) and oxidative high-temperature drop solution calorimetry with sodium molybdate as the solvent is described. Two allotropes, microcrystalline allo- Ge (m-allo-Ge) and 4H-Ge, have been prepared by topotactic deintercalation of Li 7 Ge 12 with methanol (m-allo-Ge) and subsequent annealing at 250 °C (4H-Ge). Transition enthalpies determined by differential scanning calorimetry amount to 4.96(5) ± 0.59 kJ/ mol (m-allo-Ge) and 1.46 ± 0.55 kJ/mol (4H-Ge). From high-temperature drop solution calorimetry, they are energetically less stable by 2.71 ± 2.79 kJ/mol (m-allo-Ge) and 5.76 ± 5.12 kJ/mol (4H-Ge) than α-Ge, which is the stable form of germanium under ambient conditions. These data are in agreement with DSC, as well as with the previous quantum chemical calculations. The morphology of the m-allo-Ge and 4H-Ge crystallites was investigated by a combination of scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. Even though the crystal structures of m-allo-Ge and 4H-Ge cannot be considered as truly layered, these phases retain the crystalline morphology of the layered precursor Li 7 Ge 12 . Investigation by diffuse reflectance infrared Fourier transform spectroscopy and UV−vis diffuse reflectance measurements reveal band gaps in agreement with quantum chemical calculations. ■ INTRODUCTION The allotropy (polymorphism) of the Group 14 elements provides fascinating examples of structure−bonding−property relationships within a single element. The textbook example is carbon: superhard transparent insulating diamond and soft black conducting graphite. The remarkable discoveries of fullerenes, 1,2 carbon nanotubes, 3,4 fullerides, 5,6 and, recently, single-atom-thick graphene 7,8 show the significance of different bonding and structural arrangements and are of great technological and scientific importance. The heavier carbon analogue, germanium, also exhibits diverse crystal chemistry and forms several allotropic modifications. Under ambient conditions, diamond structure α-Ge is the most stable form. Additional germanium allotropes can be stabilized by high pressure, 9−11 while others can be prepared by mild oxidation routes. 12−15 For instance, guest-free clathrate-II structure □ 24 Ge 136 can be prepared by oxidation of the Zintl phase Na 12 Ge 17 with an ionic liquid. 13 Surfactant-directed oxidative polymerization of K 4 Ge 9 , KGe, and Mg 2 Ge leads to different forms of mesoporous Ge. 16−18 Another example is an allotropic modification called allo-Ge, which was identified by von Schnering and Nesper ∼30 years ago. 12 Recently, its crystal structure was reinvestigated, and an optimized synthesis method for producing microcrystalline allo-Ge (m-allo-Ge) was suggested. 15 m-allo-Ge can be obtained through topotactic reaction at room temperature of the Zintl phase Li 7 Ge 12 , combining deintercalation of Li + ions with mild oxidation of the two-dimensional [Ge 12 ] 7− slabs. The structure of the resulting m-allo-Ge bears a close resemblance to that of the starting precursor Li 7 Ge 12 (Figure 1). The crystal structure of Li 7 Ge 12 features homoatomic layers of Ge formed by distorted Ge pentagons, leading to small and large channels. Li + ions reside between the layers, and additional Li + ions are located in the large channels. Upon deintercalation, the basic structural motif is retained, but the structure transforms from two-dimensional to three-dimensional because of the formation of covalent Ge− Ge bonds, which bring the Ge slabs together. Systematic investigation by Conesa 19 of different m-allo-Ge models revealed that there are two representative ways to form interlayer bonds between Ge slabs when such slabs alternate in a manner similar to that of the parent Li 7 Ge 12 (Figure 2). By a Received: March 24, 2014 Article pubs.acs.org/cm © XXXX American Chemical Society A dx.doi.org/10.1021/cm5010467 | Chem. Mater. XXXX, XXX, XXX−XXX