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Title: Synthesis and single-crystal X-ray diffraction studies of new framework substituted type II clathrates, Cs{sub 8}Na{sub 16}Ag{sub x}Ge{sub 136-x} (x<7)

Abstract

New inorganic type II clathrates with Ag atoms substituting for framework Ge atoms, Cs{sub 8}Na{sub 16}Ag{sub x}Ge{sub 136-x} (x=0, 5.9, and 6.7), have been synthesized by reaction of the pure elements at high temperature. Structural refinements have been performed using single crystal X-ray diffraction. The materials crystallize with the cubic type II clathrate crystal structure (space group Fd3-barm) with a=15.49262(9)A, 15.51605(6)A, and 15.51618(9) for x=0, 5.9, and 6.7, respectively, and Z=1. The structure is formed by a covalently bonded Ag-Ge framework, in which the Cs and Na atoms are found inside two types of polyhedral cages. Ag substitutes for Ge in the tetrahedrally bonded framework positions, and was found to preferentially occupy the most asymmetric 96g site. The proven ability to substitute atoms for the germanium framework should offer a route to the synthesis of new compositions of type II clathrates, materials that are of interest for potential thermoelectrics applications.

Authors:
 [1];  [2];  [3];  [2];  [4]
  1. Department of Physics, University of South Florida, 4202 East Fowler Ave., PHY 114, Tampa, FL 33620 (United States)
  2. Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899 (United States)
  3. INEOS Technologies, Naperville, IL 60566 (United States)
  4. Department of Physics, University of South Florida, 4202 East Fowler Ave., PHY 114, Tampa, FL 33620 (United States), E-mail: gnolas@cas.usf.edu
Publication Date:
OSTI Identifier:
21015749
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Solid State Chemistry; Journal Volume: 180; Journal Issue: 3; Other Information: DOI: 10.1016/j.jssc.2007.01.003; PII: S0022-4596(07)00020-5; Copyright (c) 2007 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; CESIUM COMPOUNDS; CLATHRATES; CUBIC LATTICES; GERMANIUM COMPOUNDS; MONOCRYSTALS; SILVER COMPOUNDS; SODIUM COMPOUNDS; SPACE GROUPS; SYNTHESIS; X-RAY DIFFRACTION

Citation Formats

Beekman, M., Wong-Ng, W., Kaduk, J.A., Shapiro, A., and Nolas, G.S. Synthesis and single-crystal X-ray diffraction studies of new framework substituted type II clathrates, Cs{sub 8}Na{sub 16}Ag{sub x}Ge{sub 136-x} (x<7). United States: N. p., 2007. Web. doi:10.1016/j.jssc.2007.01.003.
Beekman, M., Wong-Ng, W., Kaduk, J.A., Shapiro, A., & Nolas, G.S. Synthesis and single-crystal X-ray diffraction studies of new framework substituted type II clathrates, Cs{sub 8}Na{sub 16}Ag{sub x}Ge{sub 136-x} (x<7). United States. doi:10.1016/j.jssc.2007.01.003.
Beekman, M., Wong-Ng, W., Kaduk, J.A., Shapiro, A., and Nolas, G.S. Thu . "Synthesis and single-crystal X-ray diffraction studies of new framework substituted type II clathrates, Cs{sub 8}Na{sub 16}Ag{sub x}Ge{sub 136-x} (x<7)". United States. doi:10.1016/j.jssc.2007.01.003.
@article{osti_21015749,
title = {Synthesis and single-crystal X-ray diffraction studies of new framework substituted type II clathrates, Cs{sub 8}Na{sub 16}Ag{sub x}Ge{sub 136-x} (x<7)},
author = {Beekman, M. and Wong-Ng, W. and Kaduk, J.A. and Shapiro, A. and Nolas, G.S.},
abstractNote = {New inorganic type II clathrates with Ag atoms substituting for framework Ge atoms, Cs{sub 8}Na{sub 16}Ag{sub x}Ge{sub 136-x} (x=0, 5.9, and 6.7), have been synthesized by reaction of the pure elements at high temperature. Structural refinements have been performed using single crystal X-ray diffraction. The materials crystallize with the cubic type II clathrate crystal structure (space group Fd3-barm) with a=15.49262(9)A, 15.51605(6)A, and 15.51618(9) for x=0, 5.9, and 6.7, respectively, and Z=1. The structure is formed by a covalently bonded Ag-Ge framework, in which the Cs and Na atoms are found inside two types of polyhedral cages. Ag substitutes for Ge in the tetrahedrally bonded framework positions, and was found to preferentially occupy the most asymmetric 96g site. The proven ability to substitute atoms for the germanium framework should offer a route to the synthesis of new compositions of type II clathrates, materials that are of interest for potential thermoelectrics applications.},
doi = {10.1016/j.jssc.2007.01.003},
journal = {Journal of Solid State Chemistry},
number = 3,
volume = 180,
place = {United States},
year = {Thu Mar 15 00:00:00 EDT 2007},
month = {Thu Mar 15 00:00:00 EDT 2007}
}
  • The substitution of cations in Rb{sub x}Cs{sub 8-x}Sn{sub 44}square{sub 2}(1.3<=x<=2.1) is reported. The compounds crystallize at room temperature in the space group la3-bard adopting the type-I clathrate 2x2x2 superstructure with partly ordered framework vacancies (square), whereas at higher temperatures they transform to the primitive, more disordered modification (space group Pm3-barn). The guest atom distributions in the Sn cages on the Rb: Cs ratios is studied by means of single-crystal X-ray diffraction for Rb{sub 2.1(1)}Cs{sub 5.8(1)}Sn{sub 44} at T=293 K (1), Rb{sub 1.42(8)}Cs{sub 6.58(8)}Sn{sub 44} at T=293 K (2a), Rb{sub 1.46(5)}Cs{sub 6.54(5)}Sn{sub 44} at T=373 K (2b) and Rb{sub 1.32(8)}Cs{sub 6.68(8)}Sn{submore » 44} at T=293 K (3). The structural order-disorder phase transition influences the electrical resistivity. The hysteresis observed for the electrical resistivity in combination with the symmetric shape of the specific heat anomaly suggests that the transformation is of first-order type and is characterized by an entropy change of about 2.5 J mol{sup -1} K{sup -1}. The Raman spectrum for the low-temperature modification of 2 is also reported. - Graphical Abstract: The effects of substitution of cations in the type-I clathrates Rb{sub x}Cs{sub 8-x}Sn{sub 44} (1.3<=x<=2.1) are reported. The distribution of the guests in the Sn cages under different reaction stoichiometries and annealing times is studied by X-ray diffraction. A structural phase transition in Rb{sub 1.4}Cs{sub 6.6}Sn{sub 44} at 333-363 K affects significantly the electrical resistivity and heat capacity.« less
  • The search for new and better thermoelectric materials has recently brought to light a somewhat forgotten class of compounds, the clathrates of group 14. This and some purely theoretical interest in these and similar structural types have instigated much research on their electronic, magnetic, spectral, and conducting properties. Furthermore, renewed interest in these systems was sparked by the recent discovery of superconductivity in a Na-Ba-Si compound with the clathrate-I structure. Last, the clathrates are a step closer to porous silicon and the valuable optical properties that come with it. All of this interest has led to the search for reliablemore » and reproducible ways for the synthesis of such silicon and germanium clathrates in high yields and with defined stoichiometry.« less
  • Temperature dependent synchrotron powder diffraction and single crystal neutron diffraction data are used for probing the vibrational states and disorder in type I clathrates Ba{sub 8}Ga{sub 16}Si{sub 30}, Ba{sub 8}Ga{sub 16}Ge{sub 30}, Ba{sub 8}In{sub 16}Ge{sub 30}, and Sr{sub 8}Ga{sub 16}Ge{sub 30}. If an empirical disorder term is included, the temperature dependence of the atomic displacement factors (ADPs) of the framework and guest atoms can be described by a Debye and Einstein model, respectively. None of the guest atoms in the large cages are located in the center and the vibrational frequencies ({theta}{sub E}) are of the order 80 K ormore » larger for all structures, in good agreement with theoretical predictions. Even though the Sr ADPs are larger than the Ba ADPs in all the clathrates, the data show that {theta}{sub E} of Sr in Sr{sub 8}Ga{sub 16}Ge{sub 30} is larger than for the Ba atoms. This is due to stronger guest-host chemical bonding in Sr{sub 8}Ga{sub 16}Ge{sub 30}. Since {theta}{sub E} of Sr has been reported to be much smaller in the literature we have also measured the specific heat of Sr{sub 8}Ga{sub 16}Ge{sub 30} with Ba{sub 8}Ga{sub 16}Ge{sub 30} as a reference. It is found that localized excitations with a characteristic energy of approximately 35 K exist in both compounds, however, the total number of states is too low to be associated with either tunneling states or localized vibration of each of the guest atoms.« less
  • This paper presents results from our exploratory work in the systems K-Cd-Ge, Rb-Cd-Ge, and Cs-Cd-Ge, which yielded the novel type-I clathrates with refined compositions K 8Cd 3.77(7)Ge 42.23, Rb 8Cd 3.65(7)Ge 42.35, and Cs 7.80(1)Cd 3.65(6)Ge 42.35. The three compounds represent rare examples of clathrates of germanium with the alkali metals, where a d 10 element substitutes a group 14 element. The three structures, established by single-crystal X-ray diffraction, indicate that the framework-building Ge atoms are randomly substituted by Cd atoms on only one of the three possible crystallographic sites. Furthermore, this and several other details of the crystal chemistrymore » are elaborated.« less