Article pubs.acs.org/cm
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. Fas̈ sler,§ 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 Universität München, 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 alloGe (m-allo-Ge) and 4H-Ge, have been prepared by topotactic deintercalation of Li7Ge12 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 Li7Ge12. Investigation by diffuse reflectance infrared Fourier transform spectroscopy and UV−vis diffuse reflectance measurements reveal band gaps in agreement with quantum chemical calculations.
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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 graphene7,8 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 □24Ge136 can be prepared by oxidation of the Zintl phase Na12Ge17 with an ionic liquid.13 Surfactant-directed oxidative polymerization of K4Ge9, KGe, and Mg2Ge 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 © 2014 American Chemical Society
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 Li7Ge12, combining deintercalation of Li+ ions with mild oxidation of the two-dimensional [Ge12]7− slabs. The structure of the resulting m-allo-Ge bears a close resemblance to that of the starting precursor Li7Ge12 (Figure 1). The crystal structure of Li7Ge12 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 Conesa19 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 Li7Ge12 (Figure 2). By a Received: March 24, 2014 Published: April 21, 2014 3263
dx.doi.org/10.1021/cm5010467 | Chem. Mater. 2014, 26, 3263−3271
Chemistry of Materials
Article
theoretical calculations are a powerful tool for the determination of relative phase stability, direct experimental determination of thermodynamic properties is important both in its own right and also for validation of the computational approaches. This study investigates the enthalpies of transition of these allotropes by two complementary methods, DSC and oxidative high-temperature drop solution calorimetry in a molten oxide solvent.26−30 The latter has been used for various oxides and recently successfully extended to non-oxide materials, such as nitrides,29,31 sulfides,32 and selenides.33 Here we have applied it to the oxidative dissolution of allotropic modifications of germanium, namely, m-allo-Ge and 4H-Ge, determining their enthalpies with respect to α-Ge. The structures were confirmed by powder XRD and Raman spectroscopy. Additional characterization of the allotropes by TEM, atomic force microscopy (AFM), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and UV− vis spectroscopy is presented.
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Figure 1. Schematic representation of the m-allo-Ge and 4H-Ge preparations from Li7Ge12 and their transformation to α-Ge. Ge atoms are colored blue and Li atoms pink.
EXPERIMENTAL METHODS
Sample Preparation. A sample of m-allo-Ge was prepared following the procedure described in ref 15. In brief, the Li7Ge12 precursor was prepared from Li metal (Alfa Aesar, 99.9%) and Ge chunks (Aldrich, 99.999%) that were arc-welded on a water-cooled copper reaction plate under an argon atmosphere. The obtained ingot was ground into a gray powder in an agate mortar under an argon atmosphere and sealed in a niobium ampule with an arc welder, followed by annealing in a furnace at 480 °C for 7 days.34 The assynthesized Li7Ge12 (0.7 g) was reacted with an excess of methanol (3 mL) in an inert atmosphere (argon) using a Schlenk line at room temperature while the sample was constantly stirred for 3 days. Rapid reaction between the precursor and methanol resulting in evolution of a gaseous product (H2) was observed.15 The remaining dark gray powder was washed with distilled water and acetone and dried under vacuum at room temperature overnight. 4H-Ge was prepared by heating a sample of m-allo-Ge in a vacuum-sealed silica tube at 250 °C for 3 days (initial heating rate of 120 °C/h), followed by furnace cooling to ambient temperature. Prior to being sealed, the silica tube was cooled with liquid nitrogen to prevent transformation of m-allo-Ge into α-Ge during sealing. A single-phase sample of hexagonal (quartz) GeO2 for the calorimetric studies was prepared by annealing of commercially available GeO2 (Sigma-Aldrich, 99.999%) in a Pt crucible at 1100 °C for 24 h followed by slow (0.75 °C/min) cooling to room temperature. Characterization. Samples were analyzed by powder XRD on a Bruker D8 Advance X-ray diffractometer in Bragg−Brentano geometry with Cu Kα radiation (λ = 1.54178 Å). Jana200035 was used for the full-profile fitting and Rietveld refinement. Raman spectra were collected for samples deposited on an Al foil using a Renishaw RM1000 laser Raman microscope (514 nm) with a motorized stage. Because the laser light transforms 4H-Ge into α-Ge,14 a laser power of 0.17 mW and an acquisition time of 1 min were used for the data collection. The Li content of the sample of m-allo-Ge was determined by ICP quadrupole mass spectrometry (Agilent Technologies 7500ce). For ICP-MS analysis, the sample was digested by being heated in aqua regia, 3 parts 36% HCl and 1 part 70% HNO3. The solution was further diluted with 3% HNO3. Elemental analysis of the m-allo-Ge sample as well as the products from furnace tests was conducted on a Hitachi S4100T scanning electron microscope with an energydispersive X-ray (EDX) microanalysis attachment (Oxford INCA Energy). TG-DSC analysis was performed on a Setaram LabSYS Evo system. Samples of Ge allotropes were packed into a standard alumina crucible with a lid and heated at 10 °C/min under an argon atmosphere. For TG-DSC analysis of the GeO2 sample, a pellet (m = 114 mg) was packed in a standard platinum crucible with a lid and heated from room temperature to 800 °C at a rate of 5 °C/min in air to determine the weight loss associated with any adsorbed water. This
Figure 2. Two variants of the stacking of Ge slabs found for the structure of m-allo-Ge. The first variant includes seven-membered rings (left), while the second variant features six- and eight-membered rings. Ge atoms are colored blue and interlayer bonds orange. Adapted from refs 15 and 19.
combination of transmission electron microscopy (TEM), powder X-ray diffraction (XRD), and quantum chemical calculations, it was shown that the structure of m-allo-Ge has extensive disorder along the c-axis associated with imperfect stacking and stacking faults between layers.15 Despite a certain degree of freedom in the connectivity between Ge slabs, simultaneous deintercalation of Li and oxidation leads to the formation of an interlayer bond with a Ge−Ge distance of
