单晶材料的制备

单晶材料的制备

High Pressure ResearchVol. 24 No. 4 December 2004 pp. 481 – 490 PREPARATION AND SINGLE-CRYSTAL STRUCTURE OF A NEW HIGH-PRESSURE MODIFICATION OF BaAl2Si2 SHOJI YAMANAKA MASUO KAJIYAMA SADASIVAN N. SIVAKUMAR and HIROSHI FUKUOKA Department of Applied Chemistry Graduate School of Engineering Hiroshima University Higashi-Hiroshima

739-8527 JapanA ternary element mixture of Ba Al and Si in a molar ratio of 1 : 2 : 2 was arc-melted and treated under a high-pressure and high-temperature condition of 5 GPa at 1200 8C. X-ray structural analysis was performed on thesingle crystal obtained by this treatment. The crystal was found to be a new high-pressure modication ofBaAl2Si2 and isotypic with layer structured ThCr2Si2 in the space group I4/mmm. The crystal obtained by thesimple arc-melting also had the same structure high-pressure phase. The low-pressure phase a-BaAl2Si2 wasprepared by annealing the arc-melted sample at 1200– 1000 8C. The single crystals of the a-phase werealso obtained which crystallized in the space group Cmcm. This structure was closely related to the structure ofa-BaAl2Ge2 space group Pnma. It is interesting to note that BaAl2Si2 has a pressure induced polymorphwhereas BaAl2Ge2 has a temperature dependent dimorphism.Keywords: Silicide High pressure Synthesis Clathrate

BaAl2Si2 Phase transitionINTRODUCTIONIn a series of studies on the synthesis of new silicon clathrate compounds containing bariumand

iodine we found that the use of high-pressure and high-temperature HPHT conditionshas been effective and essential. The type I clathrate

Ba82dSi46 can be prepared under apressure .3 GPa at 800 8C 1 – 3. A new binary clathrate Ba24Si100 was obtained under apressure 1.5 GPa 4.

A type I clathrate containing iodine I8Si44I2 was prepared for therst time under a pressure of 5 GPa at temperatures 800 – 900 8C 5. The Ba containingtype I silicon clathrate compounds became superconductors with a transition temperatureTc 4– 8 K 3 6. This is the rst superconductor having a Si-sp3 3D three-dimensionalnetwork. Note that the corresponding germanium analogs are known such as Ba8Ge437 8 and Ba24Ge100 9 – 11 which can be prepared under the ambient pressure by asimple melting. The iodine containing germanium clathrate Ge462x I8x x 8/3 was pre-pared by the decomposition of GeI2 in Xe atmosphere 12. It is also well known thatalkaline-earth metal containing Zintl disilicides MSi2 M Ca Sr and Ba have pressure Corresponding author. Tel./Fax:

81-82-424-7740 E-mail: syamanahiroshima-u.ac.jpISSN 0895-7959 print ISSN 1477-2299 online 2004 Taylor amp Francis LtdDOI:

10.1080/08957950412331331763482 S. YAMANAKA et al.sensitive silicon networks and undergo pressure induced structural phase transitions 13 –15.Recently it was shown that CaSi2 was transformed into MgB2 type layered structure andshowed a superconducting transition of as high as 14 K under a pressure of 16 GPa 16. Inthis study we have