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 comprising, as the structural unit of crystals, Me46 clusters (Me F- 1 is Si or Ge) with each consisting of Me2o and Me24 clusters 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 elements 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 properties ranging from an insulator, a semiconductor, a conductor to a superconductor, by appropriately selecting the type of Ae as well as A. Si20 *\ Csl A=Na, K, Rb, Cs

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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; therefore, 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 encapsulated 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.

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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. ing alkali metal atoms inside the cage; In the conventional techniques, it was thought that a silicon clathrate compound of the following com- Ba is the only alkaline earth metal that can be encapsu- position: lated in the Si24 clusters of silicon clathrate compound (Li4)2Ae6Si46 In fact, there is no report of any kind concerning the suc- [wherein Ae is an alkaline earth metal element 30 cessful synthesis of a silicon clathrate compound selected from the group consisting of Ba, Sr and encapsulating an alkaline earth metal (e.g. Sr or Ca) Ca], which comprises, as a structural unit of crys- other than Ba, also in the experimental level. tals, Si46 cluster with each consisting of Si20 clus- With respect to germanium clathrate compounds, ters and Si24 clusters having cage structures no attempt has been made on the synthesis of any Ba- formed by silicon atoms of the framework of the 35 encapsulating clathrate compounds and, of course, no crystal, success has been reported. at least a part of said Si24 clusters encapsulating Thus, as the clathrate compound which has the alkaline earth metal atoms inside the cage, and at bonding mode above and can actually be synthesized, least a part of Si20 clusters encapsulating clustered only known has been AxBa6Si46 using Si as the frame- Li4 par Si20 cluster; 40 work element. a germanium clathrate compound of the following In the conventional techniques, only a silicon clath- composition: rate compound encapsulating Ba as an alkaline earth AxAe6Ge46 metal was synthesized. The reason is thought to be as [wherein A is an alkali metal element selected from follows. the group consisting of Na, K, Rb and Cs; Ae is an 45 A2Ba6Si46 (A is Na or K) his been synthesized by a alkaline earth metal element selected from the conventional process by preparing a ASi-BaSi2 solid group consisting of Ba, Sr and Ca; and x is the solution (e.g. A2BaSi4) having Si4 clusters as the basic number ratio of the alkali metal element to other structure, from two types of starting materials, ASi and elements and 0 < x < 2], which comprises, as a BaSi, beng followed by an alkali-removing treatment of structural unit of crystals, Ge46 cluster with each so the solid solution with heating in vacuum to change the consisting of Ge20 clusters and Ge24 clusters hav- crystal structure into the clathrate [S. Yamanaka et al., ing cage structural formed by germanium atoms of Fullerene Science & Technology, 3 (1), 21-28 (1995)]. the framework of the crystal, The reason why, in the above, only Ba was encap- at least a part of said Ge24 clusters encapsulating sulated as an alkaline earth metal and neither Sr nor Ca alkaline earth metal atoms inside the cage, and 55 was investigated, is that only BaSi2, similarly to ASi, has optionally at least a part of Ge20 clusters encapsu- a cubic crystal structure containing tetrahedral Si4 clus- lating alkali metal atoms inside the cage, so as to ters with each having the cage structure of four silicon provide said composition; atoms and thus a solid solution (a precursor to the clathrate compound) having Si4 clusters as the important

3 5 EP 0 781 727 A1 6 structural units can be also easily obtained from the two structural unit and then being followed by an alkali- materials, BaSi2 and ASi. removing treatment. The present invention has been Meanwhile, SrSi2, although having a cubic crystal completed based on this finding. structure, contains no Si4 cluster which is considered to That is, although a binary substance AeGe2 con- be necessary for the formation of the Si46 clathrate; and s sisting of Ge and Ae (Ba, Sr or Ca) has a different struc- CaSi2 has a layered crystal structure. Thus, the structure depending upon the kind of Ae used, mixing of the ture of SrSi2 and CaSi2 are greatly different from that of AeGe2 with GeA (A is Na, K, Rb or Cs) at a particular BaSi2 containing Si4 clusters. molar ratio (AGe/AeGe2 = 2/1) can form a ternary solid As mentioned above, among the alkaline earth met- solution of A2AeGe4 having a Ge4 cluster as the basic als, only Ba was investigated and there is no report on w structural unit similarly to AGe. Since this Ge clathrate the successful synthesis of any silicon clathrate com- compound has a feature that it has a tetra hederal clus- pound encapsulating Sr or Ca. ters Ge4 structure constituted by Ge atoms, by subject- The study by the present inventors, however, newly ing the ternary solid solution compound to an alkali- found out that, contrary to the anticipation based on the removing treatment, a novel germanium clathrate com- conventional techniques, a ternary solid solution 15 pound AxAe6Ge46 of the present invention can be A2AeSi4 containing Si4 clusters can be prepared even obtained. when Ae is Sr or Ca, if the condition is well defined. There is no report, either, on any silicon Si46 clath- That is, the present inventors found out that a solid rate compound encapsulating a Li4 cluster inside the solution A2AeSi4 produced from ASi and SrSi2 or CaSi2, Si20 cluster cages. Therefore, a silicon clathrate com- although having a lattice constant different from that of 20 pound (Li4)2Ae6Si46 of the present invention is also a ASi, has structure containing the same Si4 clusters as novel compound. that possessed by ASi. Using the above silicon solid According to the present invention, Group IV metal solution A2AeSi4 (containing an alkali metal and an clathrate compounds with cage structure encapsulating alkaline earth metal) as a precursor, the present inven- at least alkali earth metal atoms inside the cages can be tors succeeded in the synthesis of a novel clathrate 25 synthesized using Si or Ge (which exists abundantly on compound AxAe6Si46 of the present invention. the earth), an alkaline earth metal and, in case of The synthesis of Ge clathrate compound was necessity, an alkali metal (these metals also exist abun- thought to be impossible as well according to the gen- dantly and pose little public hazard problem). eral knowledge on clathrate compounds, using the con- These clathrate compounds have a basic structure ventional techniques. 30 greatly different from conventional Si and Ge crystals It was thought that, in the conventional techniques, and have various properties of insulator, semiconduc- Si (a Group IV element) is relatively similar to carbon tors of widend band gaps, metals or superconductors, (C) (also a Group IV element) and may partially have a depending upon the kind(s) of the sp2 hybrid orbital (which is the typical graphite struc- metal(s) encapsulated in the cluster cages; and are very ture), in addition to the sp3 diamond structure and, 35 useful as materials for fabricating various electronic therefore, use of Si, if it being compared with use of devices. The fact that various properties can be other Group IV metal elements, may make it relatively obtained using one basic clathrate structure, enables easy to synthesize a clathrate compound for which the easy production of desired electronic devices at a low coexistence of sp3 forhybrid orbital and sp2 hybrid cost, and therefore has a big significance in industry. orbital is necessary. 40 Further, the band structure of narrow dispersion is It was also known that the sp2 hybrid orbital very unique in the present clathrate compounds, as becomes more unstable in the Group IV element from compared with that in conventional semiconductors, the direction of from C to Si and further to Ge. In fact, may show large changes in magnetism and electrical materials constituted by a single type of element and conductivity, upon external influences and, therefore, having a sp2 network as the framework only exist in the 45 may be effectively utilized under small magnetic fields case of C (benzene and graphite are examples of such much as senior, magnetic resistance devices and the a substance), but no such substance exists more in the like. case of Ge. Thus, it was thought within the general knowledge BRIEF DESCRIPTION OF THE DRAWINGS based on the conventional techniques that when Ge is so used, in place of Si, as the framework element the Fig. 1 is a schematic drawing showing the structure change from Si to Ge in clathrate compound will be very of a Si46 cluster which is the crystal structural unit of a difficult. In fact, there is no report on the successful syn- silicon clathrate compound of the present invention. thesis of any Ge clathrate compound encapsulating Fig. 2 is a schematic drawing showing the structure alkaline earth metals. 55 of a Ge46 cluster which is the crystal structural unit of a The present inventors newly found out that even germanium clathrate compound of the present inven- when Ge is used, a germanium clathrate compound tion. AxAe6Ge46 can be synthesized by synthesizing a Ge-A- Ae ternary solid solution having Ge4 cluster as the basic

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DETAILED DESCRIPTION OF THE INVENTION AND AxAe6Ge46 PREFERRED EMBODIMENTS wherein A is an alkali metal; Ae is Ba, Sr or Ca; The Group IV metal clathrate compounds of the and x is a real number of 0 to 2 and indicates the atom present invention have a basic crystal structure com- 5 number ratio of A to the other elements. This germa- posed of Me46 clusters (Me is Si or Ge) each consisting nium clathrate compound, similarly to the silicon clath- of (1) Me2o clusters of a dodecahedral cage of 20 silicon rate compound of the first embodiment, has the or germanium atoms and (2) Me24 clusters with each following cases. having a tetradecahedral cage of 24 silicon or germanium atoms. 10 I. No alkali metal atom is encapsulated in any Ge20 The first embodiment of the silicon clathrate com- cluster. pounds of the present invention has, as mentioned pre- II. There are two kinds of Ge20 clusters; one kind of viously, the composition represented by the following Ge20 cluster(s) with each encapsulating an alkali formula: metal atom(s) and the other one(s) with each is encapsulating no alkali metal atom. AxAe6Si46 III. Each of the Ge20 clusters encapsulates alkali metal atom(s). wherein A is Na, K, Rb or Cs; Ae is Sr or Ca; and x is a real number of 0 to 2 and indicates the atom In the above-mentioned clathrate compounds of the number ratio of A to other elements. 20 present invention, the alkaline earth metal is encapsu- When the above compound contains an alkali metal lated in the Me24 clusters (Me is Si or Ge). (A), the alkali metal atom is encapsulated in part or all of The clathrate compounds of the present invention the Si20 clusters. can be produced, for example, by the following proc- Therefore, the silicon clathrate compound of the esses. first embodiment has the following three cases. 25 A. Processes for production of silicon clathrate com- I. No alkali metal atom is encapsulated in any Si20 pounds cluster. II. There are two kinds of Si20 clusters, one kind of First, an alkali metal (A) and Si are mixed, and the Si20 cluster(s) with each encapsulating alkali metal 30 mixture is placed in a PBN or a tungsten container. The atoms and the other one(s) with each encapsulat- tube is sealed in a stainless steel tube and heated under ing no alkali metal atom. inert gas atomosphere lik Ar at 500-650°C to obtain an III. Each of the Si20 clusters encapsulates alkali ASi compound. The heating can preferably be con- metal atom(s). ducted by the use of, for example, an electric furnace. 35 Next, an alkaline earth metal (Ae) and Si are mixed, Incidentally, when the alkali metal is Li, part or all of and the mixture is heated to about 900-1, 200°C in an the Si20 clusters can encapsulate a cluster composed of atmosphere of an inert gas, for example, argon to syn- more than one Li atom, for example, a cluster com- thesize an AeSi2 compound. The heating can preferably posed of 4 Li atoms. When the alkali metal is other than be conducted by the use of, for example, high-frequency Li, one alkali metal atom is generally encapsulated in 40 induction heating. one Si20 cluster. The above-obtained ASi compound and AeSi2 This is presumed to be because a Li atom has a compound are mixed at a stoichiometric ratio of 2:1 and significantly small ionic radius (0.78 A) as compared heated at about 600°C to synthesize a ternary solid with those of other alkali metal atoms and each Si20 solution A2AeSi4. cluster can provide an enough space for encapsulating 45 Finally, the ternary solid solution is heat-treated by some members of Li atoms. Li4 is the most stable clus- controlling pressure with an inert gas under vacuum ter in Li. (20-200 Torr) in a heating furnace whose temperature Hence, the second embodiment of the silicon clath- can be controlled to 300-600°C very accurately, and is rate compounds of the present invention has, as men- then subjected to distillation of alkali metals, whereby a tioned previously, the composition represented by the so desired silicon clathrate compound can be formed. following formula: Resulting products during the reaction can be easily removed from the above-obtained product, because (Li4)2Ae6Si46 BaSi2, etc. produced as by-products are water-soluble and easily removable by a treatment with an acidic wherein Ae is Ba, Sr or Ca. 55 aqueous solution, and a desired silicon clathrate com- The germanium clathrate compound of the present pound can be isolated in a pure form. invention has, as mentioned previously, the composition The number ratio (x) of the alkali metal can be con- represented by the following formula: trolled by selecting an appropriate conditions. The alkali metal can be Na, K, Rb or Cs; and the

5 9 EP 0 781 727 A1 10 alkaline earth metal can be Sr or Ca. B. Process for production of germanium clathrate com- In the above production process, the ternary solid pound solution A2AeSi4 obtained as an intermediate from ASi (having Si4 clusters as the basic structural unit) and First, an alkali metal (A) and Ge are mixed, and the SrSi2 or CaSi2 (these two compounds have no Si4 clus- s mixture is placed in a PBN or a tungsten container. The ters unlike BaSi2), have Si4 clusters as the basic struc- tube is sealed in a stainless steel tube and heated under tural unit; therefore, heat treatment with accurate control inert gas atomosphere at 500-650°C to obtain an AGe of the ternary solid solution can produce an intended sil- compound. The heating can preferably be conducted by icon clathrate compound containing Sr or Ca. the use of an inert gas, for example, an electric furnace. In the course of the study of the present invention, 10 Next, an alkaline earth metal (Ae) and Ge are the present inventors found out that when Li is used as mixed.and the mixture is heated to about 900-1 ,200°C an alkali metal, a silicon clathrate compound in an atmosphere of an inert gas, for example, argon to (Li4)2Ae6Si46 (As is Ba, Sr or Ca) can be obtained, synthesize an AeGe2 compound. The heating can pref- which encapsulates Li in the form of a Li4 cluster in the erably be conducted by the use of, for example, high- Si20 clusters. The existence of such a silicon clathrate is frequency compound is possible because the ionic radius (0.78 A) induction method. of Li atom is significantly small as compared with those The above-obtained AGe compound and AeGe2 of other alkali metals and therefore each Si20 cluster compound are mixed with a stoichiometric ratio of 2:1 has an inert space sufficiently large to encapsulate a and heated at about 600°C to synthesize a ternary solid stable Li4 cluster therein. 20 solution A2AeGe4. In this case, the alkaline earth metal (Ae) used in Finallythe ternary solid solution is heat-treated in the production of the above (Li4)2Ae6Si46 can be Ba, Sr an inert gas atmosphere under vacuum (20-200 Torr) or Ca. with a control of pressure by introducing insert gasses The silicon clathrate compounds with a unique using a heating furnace whose temperature can be con- bonding nature of the present invention, unlike conven- 25 trolled to 300-600°C very accurately, and is subjected to tional Si crystals having a diamond structure, show a distillation of alkali metals, whereby a desired germa- band structure of essentially narrow dispersion owing to nium clathrate compound can be synthesized. the existing cluster units in the silicon clathrate com- Resulting products can be easily removed from the pounds. The band is further greatly changed when an above-obtained product, because BaGe2, etc. pro- alkaline earth metal is encapsulated in the silicon clath- 30 duced as by-products are water-soluble and easily rate compounds, because there takes place hybridiza- removable by a treatment with an acidic aqueous solution between the d-orbital of the alkaline earth metal tion, and a desired germanium clathrate compound can and the band of the silicon clathrate framework. There- be isolated in a pure form. The atom number ratio (x) of fore, the band of the silicon clathrate compound can be the alkali metal may be controlled with an accurate dis- greatly modified by appropriately selecting the alkaline 35 tillation conditions. earth metal used. The alkali metal can be Na, K, Rb or Cs; and the Thus, the silicon clathrate compounds of the alkaline earth metal can be Ba, Sr or Ca. present invention, unlike conventional Si crystals, can In the above production process as well, the ternary have a variety of properties ranging from insulator, sem- solid solution A2AeGe4 obtained as an intermediate iconductors of different band gaps, metal to super-con- 40 from AGe (having Si4 clusters as the basic structural ductor, by selecting the type as well as the amount of units) and AeGe2 (generally having no Si4 clusters the alkaline earth metal encapsulated. This suggests a unlike BaSi2), have Si4 clusters as the basic structural possibility that a semiconductor device with high per- unit; therefore, an intended germanium clathrate com- formance can be produced using silicon, alkali metals pound containing Ba, Sr or Ca can be produced by con- and alkaline earth metals, all of which exist in rich natu- 45 trolling heat treatment of the ternary solid solution. ral abundance and pose little public pollution problem. The germanium clathrate compounds, unlike con- Further, the silicon clathrate compounds of the ventional Ge crystals having a diamond structure, show present invention, having a band with narrow dispersion a band with essentially narrow dispersion (similar to that characteristics, show a large property change against shown by the above-mentioned silicon clathrate com- external influences; that is, they can exhibit a sensing so pounds) owing to the cluster units existing in the germa- function of magnetic field, which is not available in clath- nium clathrate compounds. The band is greatly rate compounds previously reported and an unexpect- changed when an alkaline earth metal is encapsulated edly much larger change in conductivity than in in the germanium compound, because hybridization conventional materials, for example, a giant magneto take place between the d-orbital of alkaline earth metal resistance will be achieved. ss and the band of the framework of the germanium clathrate compound. Therefore, the band of germanium clathrate compound can be greatly modified by appropriately selecting the alkaline earth metal used. Thus, the germanium clathrate compounds of the

6 11 EP 0 781 727 A1 12 present invention, unlike conventional Ge crystals, can The purified product was also subjected to X-ray have a variety of properties ranging from insulator, sem- analysis, which confirmed that the product was a clath- iconductors with different band gaps, metal to super- rate compound showing an intended crystal structure. conductor, by appropriately selecting the type and the The properties of the compound were measured and amount of the alkaline earth metal encapsulated. This 5 found to be metallic and superconducting with a critical suggests a possibility that a semiconductor device with temperature of 4K. high performance can be produced using germanium, alkali metals and alkaline earth metals, all of which exist Example 2 in rich natural abundance on the earth and pose little public hazard and pollution problem. w A 1 :1 mixture of K (696 mg) and Si (500 mg) was Further, the germanium clathrate compounds of the placed in a PBN container. The PBN was sealed in a present invention, having a band with narrow disper- stainless steel tube in an argon atmosphere and heated sion, show a large property change against external at 650°C for 5 hours to give rise to a reaction. The reac- influences; that is, they can exhibit a sensing function tion system was cooled to room temperature and the not possessed in conventional clathrate compounds is reaction product (KSi) was taken out. The crystal form reported as far and larger giant magneto resistance of the compound was examined by X-ray analysis, than in conventional materials. which showed a cubic crystal system [space group: P-4 The present invention is hereinafter described in (axis of four rotary inversions) 3n]. more detail by way of Examples. The above-obtained KSi compound and the SrSi2 20 synthesized in Example 1 were mixed at a molar ratio of Example 1 2:1 and placed in a PBN cell. The PBN was sealed in a stainless steel tube in an argon atmosphere and heated A 1 :1 mixture of Na (230 mg) and Si (281 mg) was at 650°C for 48 hours to give rise to a reaction. The placed in a PBN container. The PBN was sealed in a reaction system was cooled to room temperature and stainless steel tube in an argon atmosphere and heated 25 the reaction product was taken out. The product was a at 650°C for 5 hours to give rise to a reaction. The reac- uniform solid solution with a composition of K2SrSi4, tion system was cooled down to room temperature and having a crystal structure similar to those of NaSi and the reaction product (NaSi) was taken out. The crystal KSi. form of the compound was examined by X-ray analyses, The solid solution was heated at 370°C for 120 which showed a monoclinic crystal system (space 30 hours under vacuum of 50 Torr, whereby an intended group: C2/C). clathrate compound having a composition of K2Sr6Si46 A 1 :2 mixture of Sr (440 mg) and Si (281 mg) was was synthesized. heated in an argon atmosphere by high-frequency The reaction system was cooled down to room tem- induction heating at 30A (temperature: about 1 ,200°C) perature; and the reaction product was taken out, for 10 minutes to give rise to a reaction. The reaction 35 washed with 0.1 N hydrochloric acid and water in this system was cooled down to room temperature, and the order, and dried to obtain a purified product. The puri- reaction product (SrSi2) was yielded. The crystal form of fied product was subjected to elemental analysis, which the compound was examined by X-ray analysis, which indicated K-, 9Sr6 2Si46. showed a cubic crystal system (space group: 32P43). The purified product was also subjected to X-ray The above-obtained two compounds, NaSi and 40 analysis, which confirmed that the product was a clath- SrSi2, were mixed at a molar ratio of 2:1 and placed in a rate compound having an intended crystal structure. PBN container. The PBN was sealed in a stainless steel The properties of the compound were measured and tube in an argon atmosphere and heated at 650°C for found to be metallic and superconducting with a critical 48 hours to give rise to a reaction. The reaction system temperature of 3.5K. was cooled to room temperature and the reaction prod- 45 uct was taken out. The product was a uniform solid solu- Example 3 tion with a composition of Na2SrSi4, having a crystal structure similar to that of NaSi, i.e. Si4 clusters exist as A 1 :2 mixture of Ca (201 mg) and Si (281 mg) was the basic structural unit of crystals. heated in an argon atmosphere by high-frequency The solid solution was heated at 400°C for 150 so50 induction heating at 30A (temperature: about 1 ,200°C) hours under vacuum of 20 Torr, whereby an intended for 10 minutes to give rise to a reaction. The reaction clathrate compound with a composition of Na2Sr6Si46 system was cooled to room temperature, and the reac- was synthesized. tion product (CaSi2) was taken out. The crystal structure The reaction system was cooled to room tempera- of the compound was examined by X-ray analysis, ture; and the reaction product was taken out, washed 55 which showed a trigonal crystal system [space group: with 0.1 N hydrochloric acid and water in this order, and R-3 (axis of three rotary inversions) m]. dried to obtain a purified product. The purified product The above-obtained CaSi2 compound and the NaSi was subjected to elemental analysis, which indicated synthesized in Example 1 were mixed at a molar ratio of Na! 8Sr61Si46. 1 :2 and placed in a PBN cell. The PBN was sealed in a

7 13 EP 0 781 727 A1 14 stainless steel tube in an argon atmosphere and heated was subjected to measurement of electrical conductivity at 650°C for 48 hours to give rise to a reaction. The with an standard method. As a result, the product was a reaction system was cooled to room temperature and metal but showed no superconductivity down to 2K. the reaction product was taken out. The product was a uniform solid solution with a composition of Na2CaSi4, 5 Example 5 having a crystal structure similar to that of NaSi. This solid solution showed a crystal structure similar to that A 1 :1 mixture of Na (230 mg) and Ge (726 mg) was of NaSi (although the lattice was expanded) but greatly placed in a PBN cell. The PBN was sealed in a stainless different from that of CaSi2. The solid solution was steel tube in an argon atmosphere and heated at 650°C heated at 400°C for 1 00 hours under vacuum of 50 Torr w for 5 hours to give rise to a reaction. The reaction sys- by refilling argon, whereby an intended clathrate com- tem was cooled to room temperature and the reaction pound having a composition of Na2Ca6Si46 was synthe- product (NaGe) was taken out. The crystal structure of sized. the compound was examined by X-ray analysis, which The reaction system was cooled to room tempera- showed a crystal of space group = mP32. ture; and the reaction product was taken out, washed 15 A 1 :2 mixture of Ba (1 ,374 mg) and Ge (1 ,452 mg) with 0.1 N hydrochloric acid and water in this order, and was heated in an argon atmosphere by high-frequency dried to obtain a purified product. The purified product induction heating at 30A (temperature: about 1 ,200°C was subjected to elemental analysis, which indicated for 10 minutes to give rise to a reaction. The reaction Na! 9Ca61Si46. system was cooled to room temperature, and the reac- The purified product was examined for electrical 20 tion product (BaGe2) was taken out. The crystal form of conductivity by an ordinary method and found to be a the compound was examined by X-ray analysis, which semiconductor having a relatively small band gap showed a crystal of space group = oP12. (about 0.2 eV). The above-obtained two compounds, NaGe and BeGe2, were mixed at a molar ratio of 2:1 and placed in Example 4 25 a PBN cell. The PBN was sealed in a stainless steel tube in an argon atmosphere and heated at 650°C for A 1:1 mixture of Li (70 mg) and Si (280 mg) were 48 hours to give rise to a reaction. The reaction system placed in a PBN container. The PBN tube was sealed in was cooled to room temperature and the reaction prod- a stainless steel tube in an argon atmosphere. The uct was taken out. The product was a uniform solid solu- stainless steel tube was heated in an electric furnace at 30 tion with a composition of Na2BaGe4, having a crystal 650°C for 10 hours to give rise to a reaction to obtain a structure similar to that of NaGe. LiSi compound. The reaction system was cooled to The solid solution was heated at 400°C for 200 room temperature, and the reaction product was taken hours under vacuum of 20 Torr, whereby an intended out. The product was examined for crystal form by X-ray clathrate compound with a composition of Na2Ba6Ge46 analysis, which confirmed that it had an incomplete 35 was synthesized. crystal structure but relatively similar to that of NaSi. The reaction system was cooled to room tempera- A 1 :2 mixture of Ba (687 mg) and Si (281 mg) was ture; and the reaction product was taken out, washed heated in an argon atmosphere in a furnace of high-fre- with 0.1 N hydrochloric acid and water in this order, and quency induction heating type at 30A (temperature: dried to obtain a purified product. The purified product about 1 ,200°C, for 1 0 minutes to give rise to a reaction. 40 was subjected to elemental analysis, which indicated The reaction system was cooled to room temperature, Na-| 6Ba6 2Ge46. The purified product was also sub- and the reaction product (BaSi2) was taken out. The jected to X-ray analysis, which confirmed that the prod- crystal structure of the compound was examined by X- uct was a germanium clathrate compound having an ray analysis, which showed an orthorhombic crystal intended crystal structure. The properties of the com- system (space group: Pnma). 45 pound were measured and found to be a metal with an The above-obtained two compounds, LiSi and electrical conductivity of 550 Scm"1. BaSi2, were mixed at a molar ratio of 2:1 . Thereinto was mixed Li at a molar ratio of 3, to obtain a 2:1 :3 (molar Example 6 ratio) mixture of LiSi, BaSi2 and Li. The mixture was heated at 650°C for 48 hours in an argon atmosphere to so A 1 :1 mixture of K (391 mg) and Ge (726 mg) was synthesize a solid solution having Si4 clusters. placed in a PBN cell. The PBN was sealed in a stainless The solid solution was heated at 400°C for 48 hours steel tube in an argon atmosphere and heated at 650°C under vacuum of 20 Torr, whereby a clathrate com- for 5 hours to give rise to a reaction. The reaction sys- pound having a composition of (Li4)2Ba6Si46 was syn- tem was cooled to room temperature and the reaction thesized. 55 product (KGe) was taken out. The crystal structure of The reaction system was cooled to room tempera- the compound was examined by X-ray analysis, which ture; and the reaction product was taken out, washed showed a crystal of space group = cP64. with 0.1 N hydrochloric acid and water in this order, and The above-obtained KGe and the BaGe2 obtained dried to obtain a purified product. The purified product in Example 5 were mixed at a molar ratio of 2:1 and

8 15 EP 0 781 727 A1 16 placed in a PBN cell. The PBN was sealed in a stainless (about 0.2 eV). steel tube in an argon atmosphere and heated at 650°C for 48 hours to give rise to a reaction. The reaction sys- Example 8 tem was cooled to room temperature and the reaction product was taken out. The product was a uniform solid 5 A 1:2 mixture of Sr (876 mg) and Ge (1 ,450 mg) solution with a composition of K2BaGe4, showing a was heated in an argon atmosphere with a high-fre- crystal structure similar to that of KGe. quency induction method at 30A (temperature: about The solid solution was heated at 370°C for 200 1 ,200°C) for 10 minutes hours to give rise to a reaction. hours under vacuum of 50 Torr, whereby an intended The reaction system was cooled to room temperature, clathrate compound having a composition of 10 and the reaction product (SrGe2) was taken out. The K2Ba6Ge46 was synthesized. crystal form of the compound was examined by X-ray The reaction system was cooled to room tempera- analysis, which showed a crystal of space group = ture; and the reaction product was taken out, washed oP12. with 0.1 N hydrochloric acid and water in this order, and The above-obtained SrGe2 and the NaGe obtained dried to obtain a purified product. The purified product 15 in Example 5 were mixed at a molar ratio of 1 :2 and was subjected to elemental analysis, which indicated placed in a PBN cell. The PBN was sealed in a stainless Ki 9Ba6 2Ge46. The purified product was also subjected steel tube in an argon atmosphere and heated at 650°C to X-ray analysis, which confirmed that the product was for 48 hours to give rise to a reaction. The reaction sys- a germanium clathrate compound having an intended tem was cooled to room temperature and the reaction crystal structure. The properties of the compound were 20 product was taken out. The product was a uniform solid measured and found to be a metal with an electrical solution with a composition of Na2SrGe4, having a crys- conductivity of 500 Scm"1. tal structure similar to that of NaGe (except for the expanded lattice) but different from that of SrGe2. Example 7 The solid solution was heated at 420°C for 150 25 hours under vacuum of 50 Torr, whereby an intended A 1 :2 mixture of Ca (400 mg) and Ge (1 ,452 mg) clathrate compound having a composition of was heated in an argon atmosphere by high-frequency Na2Sr6Ge46 was synthesized. induction heating at 30A (temperature: about 1 ,200°C, The reaction system was cooled to room tempera- for 10 minutes to give rise to a reaction. The reaction ture; and the reaction product was taken out, washed system was cooled to room temperature, and the reac- 30 with 0.1 N hydrochloric acid and water in this order, and tion product (CaGe2) was taken out. The crystal form of dried to obtain a purified product. The purified product the compound was examined by X-ray analysis, which was subjected to elemental analysis, which indicated showed a crystal of space group = hR6. Na-| 5Sr6 3Ge46. The purified product was also sub- The above-obtained CaGe2 and the NaGe obtained jected to X-ray analysis, which confirmed that the prod- in Example 5 were mixed at a molar ratio of 1 :2 and 35 uct was a germanium clathrate compound having an placed in a PBN cell. The PBN was sealed in a stainless intended crystal structure. The properties of the com- steel tube in an argon atmosphere and heated at 650°C pound were measured and found to be a semi-conduc- for 48 hours to give rise to a reaction. The reaction sys- tor with a relatively small band gap (about 0.1 eV). tem was cooled to room temperature and the reaction product was taken out. The product was a uniform solid 40 Claims solution with a composition of Na2CaGe4, having a crystal structure similar to that of NaGe (except 1. A silicon clathrate compound of the following com- for the expanded lattice) but greatly different from that of position: CaGe2. AxAe6Si46 The solid solution was heated at 400°C for 150 45 [wherein A is an alkali metal element selected from hours under vacuum of 50 Torr, whereby an intended the group consisting of Na, K, Rb and Cs; Ae is an clathrate compound with a composition of Na2Ca6Ge46 alkaline earth metal element selected from the was synthesized. group consisting of Sr and Ca; and x is the number The reaction system was cooled down to room tem- ratio of the alkali metal element to the other ele- perature, and the reaction product was taken out, 50 ments and 0 < x < 2] , which comprises, as the struc- washed with 0.1 N hydrochloric acid and water in this tural unit of crystals, Si46 clusters, each of which order, and dried to obtain a purified product. The puri- consists of Si20 clusters and Si24 clusters with cage fied product was subjected to elemental analysis, which structures formed by silicon atoms as the frame- indicated Na-| 9Ca61Ge46. The purified product was work of the crystal, also subjected to X-ray analysis, which confirmed that 55 at least a part of said Si24 clusters encapsu- the product was a germanium clathrate compound hav- lating alkaline earth metal atoms inside the cage, ing an intended crystal structure. The compound was and optionally at least a part of Si20 clusters encap- measured for electrical conductivity, which showed that sulating alkali metal atoms inside the cage. it is a semiconductor with a relatively small band gap

9 17 EP 0 781 727 A1 18

2. A silicon clathrate compound of the following composition: (Li4)2Ae6Si46 [wherein Ae is an alkaline earth metal element selected from the group consisting of Ba, Sr and 5 Ca], which comprises, as the structural unit of crystals, Si46 clusters, each of which consists of Si2o clusters and Si24 clusters with cage structures formed by silicon atoms as the framework of the crystal, w at least a part of said Si24 clusters encapsulating alkaline earth metal atoms inside the cage, and at least a part of Si20 clusters encapsulating clustered Li4 per Si20 cluster. 15 3. A germanium clathrate compound of the following composition: AxAe6Ge46 [wherein A is an alkali metal element selected from the group consisting of Na, K, Rb and Cs; Ae is an 20 alkaline earth metal element selected from the group consisting of Ba, Sr and Ca; and x is the number ratio of the alkali metal element to other elements and 0 < x < 2], which comprises, as the structural unit of crystals, Ge46 clusters, each of 25 which consists of Ge20 clusters and Ge24 clusters with cage structures formed by germanium atoms as the framework of the crystal, at least a part of said Ge24 clusters encapsulating alkaline earth metal atoms inside the cage, 30 and optionally at least a part of Ge20 clusters encapsulating alkali metal atoms inside the cage, so as to provide said composition.

10 EP 0 781 727 A1 EP 0 781 727 A1

12 EP 0 781 727 A1

European Patent *HMm J EUROPEAN SEARCH REPORT Office EP 96 12 0943

DOCUMENTS CONSIDERED TO BE RELEVANT Category Citation of document with indication, where appropriate, Relevant CLASSIFICATION OF THE of relevant passages to claim APPLICATION (Int.CI.6) D,A FULLERENE SCIENCE & TECHNOLOGY, C01B33/06 vol. 3, no. 1, 1995, NEW YORK, US, C22C28/00 pages 21-28, XP0OO645540 //H01L39/12, S. YAMANAKA ET AL.: "Preparation of H01L29/24, barium-containing silicon clathrate H01B1/06 compound" * page 26, line 4 - line 10; page 27 : figure 2 *

PHYSICAL REVIEW LETTERS, 20 FEB. 1995, USA, vol. 74, no. 8, ISSN 0031-9007, pages 1427-1429, XP000646872 KAWAJI H ET AL: "Superconductivity in the silicon clathrate compound (Na,Ba)/sub x/Si/sub 46/" * abstract; page 1428 : figure 2 and right- hand column, paragraph 2 *

EUROPEAN JOURNAL OF SOLID STATE AND INORGANIC CHEMISTRY, 1995, FRANCE, TECHNICAL FIELDS SEARCHED (Int.CI.6) vol. 32, no. 7-8, ISSN 0992-4361, pages 799-807, XP000646873 C01B YAMANAKA S ET AL: "Preparation and C22C superconductivity of barium containing silicon clathrate compound, (Ba,K)/sub x/Si/sub 46/" * abstract; page 804 : figure 3 and line 2 - line 6 *

The present search report has been drawn up for all claims Place of search Date of coaa>letk» of the search Exaniaer THE HAGUE 7 April 1997 Brebion, J CATEGORY OF CITED DOCUMENTS theory or principle underlying the invention earlier patent document, but published on, or X : particularly relevant if taken alone after the filing date Y : particularly relevant if combined with another document cited in the application document of the same category document cited for other reasons A : technological background O : non-written disclosure & : member of the same patent family, corresponding P : intermediate document document