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Clathrate Compounds and Processes for Production Thereof

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Clathrate Compounds and Processes for Production Thereof ~™ llll III II II I II I III 1 1 II 1 1 (19) J European Patent Office Office europeen des brevets (1 1 ) EP 0 781 727 A1 (12) EUROPEAN PATENT APPLICATION (43) Date of publication: (51) Int. CI.6: C01 B 33/06, C22C 28/00 02.07.1997 Bulletin 1997/27 // |— |q 1 L39/1 2, H01 L29/24, (21) Application number: 96120943.4 H01B1/06 (22) Date of filing: 27.12.1996 (84) Designated Contracting States: • Kuroshima, Sadanori DE FR Minato-ku, Tokyo (JP) • Otto, Zhou (30) Priority: 28.12.1995 JP 342101/95 Tsukuba-shi, Ibaraki (JP) (71) Applicant: NEC CORPORATION (74) Representative: Glawe, Delfs, Moll & Partner Tokyo (JP) Patentanwalte Postfach 26 01 62 (72) Inventors: 80058 Munchen (DE) • Tanigaki, Katsumi Minato-ku, Tokyo (JP) (54) Clathrate compounds and processes for production thereof (57) Group IV metal clathrate compounds compris- ing, as the structural unit of crystals, Me46 clusters (Me F- 1 is Si or Ge) with each consisting of Me2o and Me24 clus- ters having cage structures, where the Me2o cluster encapsulate alkali metal (A) atoms and the Me24 cluster encapsulates alkaline earth metal (Ae) atoms, and has ,.--Ae=Sr, Ca a composition represented by the following formula: AxAe6Me46 wherein A is Li4, Na, K, Rb r Cs; Ae is Ba, Sr or ca; Me is Si or Ge; x is the number ratio of A relative to the other ele- ments and is 0 x 2; when Me is Si and A is Na, K, Rb or Cs, Ae is Sr or Ca; when Me is Si and A is Li4, Ae is Ba, Sr or Ca; and When Me is Ge, A is Na, K, Rb or Cs and Ae is Ba, Sr or Ca. These clathrate compounds can show the proper- ties ranging from an insulator, a semiconductor, a con- ductor to a superconductor, by appropriately selecting the type of Ae as well as A. Si20 *\ Csl A=Na, K, Rb, Cs CO o Q_ LU Printed by Rank Xerox (UK) Business Services 2.14.8/3.4 1 EP 0 781 727 A1 Description Si24 cluster (a cage structure formed by 24 silicon atoms). The electronic properties of this silicon clathrate BACKGROUND OF THE INVENTION compound are mostly determined by the framework structure of the clathrate compound and have no varia- 1 . Field of the Invention: 5 tion to be widely used in many electronic fields; there- fore, the silicon clathrate compound found no wide The present invention relates to clathrate com- usage. For more than 20 years after the report of the pounds leading to semiconductors, metals, or insulators above clathrate compound has not been applied in fab- used in production of high-performance devices, as well ricating electronic devices. as to the processes for production of such clathrate 10 Recently, however, it was reported that Na2Ba6Si46 compounds. comprising Si20 clusters each with an alkali metal atom (e.g. Na or K) encapsulated therein and Si24 clusters 2. Description of the Related Art: each with a Ba (alkaline earth metal) atom encapsu- lated therein; hence the application of Na2Ba6Si46 in the Currently, semiconductor devices (e.g. arithmetic 15 electronic field is expected [S. Yamanaka et al., Fuller- and logic circuit devices, data storage devices and pho- ene Science & Technology, 3(1), 21-28 (1995)]. toelectric conversion devices) for computer technology The reason for the above expectation is that the sil- and lasers for optical communication technology are icon clathrate compound encapsulating an alkaline produced from silicon, germanium, a group lll-V ele- earth metal (Ba) atom in each of the Si24 clusters can ment compound semiconductor (e.g. GaAs), or a group 20 have greatly different electronic states by hybridization ll-VI element compound semiconductor (e.g. zinc of the d-orbital of the alkaline earth metal atom with the sulfide). levels for the valence band and conduction band of the The performances of these elements have hereto- Si46 clathrate compound per se. In fact, while silicon cla- fore been improved mainly by the advances of LSI fabri- thrate compounds encapsulating no alkaline earth cation techniques. In the future, however, the 25 metal atom show no superconductivity, the silicon clath- improvement of such device performance by litho- rate compound encapsulating Ba shows metallic prop- graphic techniques has a limitation and its further erties and superconductivity. Superconductivity is not improvement is expected to be achieved mainly by the seen in conventional doped silicon crystals, and is properties of the materials using in fabricating devices. unique to this particular clathrate compound. This is Accordingly, for further progress of electronics, it is 30 understood by the change in band structure, caused by warranted to develop a new material having properties the encapsulation of an alkaline earth motel atom inside greatly different from those of the materials convention- the Si24 cluster cage. ally used in electronic devices. As an approach to seek a breakthrough by such development of new materials, SUMMARY OF THE INVENTION it is considered to use a clathrate compound having 35 cluster features as the structural units of crystals. As is appreciated from the above-mentioned past In the cluster/clathrate materials, the bond between studies on clathrate compounds, in order to develop a atoms constituting the substances is greatly different group IV metal clathrate compound with various elec- from that of conventional compounds, and it is possible tronic properties, widely usable in electronics, it was to control the superconductivity and magnetism via 40 necessary that not only Ba but also other alkaline earth resulting high-frequency phonon or the unique Fermi metals are encapsulated in a group IV metal clathrate surfaces and also possible to alleviate the number of compound in order to lead the resulting clathrate com- defects by making the cluster/clathrate structure as per- pounds to the materials with various intended proper- fect as possible; as a result, the fundamental properties ties. can ultimately be utilized. 45 As such a clathrate compound encapsulating alka- As a cluster/clathrate material was first reported the line earth metals, only known is a Ba-encapsulated sili- clathrate compound having silicon or germanium (group con clathrate compound, Na6Ba2Si46, and there is no IV element) atoms as the constituent of the framework report on the successful synthesis of any silicon clath- structure; and it was found that, by appropriately select- rate compounds encapsulating the other alkaline earth ing the preparation condition of such a substance, there so metals. It has hitherto been thought impossible to can be obtained a silicon or germanium clathrate com- develop a silicon or germanium clathrate compound pound having cage-like structural units of unique shape encapsulating an alkaline earth metal, having various with each encapsulating alkali metal atoms [for exam- intended properties, or to appropriately select the start- ple, C. Cross et al., Journal of Solid State Chemistry 2, ing materials for reactions so as to develop such clath- 570-581 (1970)]. 55 rate compounds, using the conventional techniques. In the silicon clathrate compound at the above Objects of the present invention are to provide sili- stage, only an alkali metal atoms as either an element con or germanium clathrate compounds with each com- or a cluster are encapsulated in each Si2o cluster (a prising Si or Ge clusters encapsulating an alkaline earth cage structure formed by 20 silicon atoms) and each metal other than Ba and alkali metals in case of need. 2 3 EP 0 781 727 A1 4 Other object of the present invention is to provide Each of the silicon or germanium clathrate com- novel electronic materials with different electrical prop- pounds of the present invention has no crystal structure erties, by encapsulating appropriately selected metals of the conventional sp3 bond but has a crystal structure inside the cluster cage of the above clathrate com- of the particular bond which is intermediate between pounds. 5 sp3 bond and sp2 bond, because it has, as the minimum The silicon or germanium clathrate compounds of structural units of crystals, clusters with each having 20 the present invention capable of achieving the above silicon or germanium atoms and clusters with each hav- objects include the followings: ing 24 silicon or germanium atoms. The silicon clathrate compound AxAe6Si46 of the a silicon clathrate compound of the following com- 10 present invention differs from the conventional silicon position: clathrate compound encapsulating a Ba atom in each AxAe6Si46 Si24 cluster unit, because the present compound encap- [wherein A is an alkali metal element selected from sulates an alkaline earth metal atom other than Ba in the group consisting of Na, K, Rb and Cs; Ae is an each Si24 cluster unit. alkaline earth metal element selected from the is The present silicon clathrate compound group consisting of Sr and Ca; and x is the number (Li4)2Ae6Si46 encapsulating a lithium cluster consisting ratio of the alkali metal element to other elements of four lithium atoms in each Si20 cluster unit and encap- and 0 < x < 2], which comprises, as a structural unit sulating an alkaline earth metal atom in each Si24 clus- of crystals, Si46 cluster with each consisting of Si2o ter unit, is different from the conventional silicon clusters and Si24 clusters having cage structures 20 clathrate compound, because the present compound formed by silicon atoms of the framework of the encapsulates a lithium cluster in each Si20 cluster unit. crystal, The present germanium clathrate compound at least a part of said Si24 clusters encapsulating AxAe6Ge46 having germanium as the framework of the alkaline earth metal atoms inside the cage, and crystal and is different from the conventional silicon cla- optionally at least a part of Si20 clusters encapsulat- 25 thrate compound.
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