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-Hiroshima739-8527 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 ClathrateBaAl2Si2 Phase transitionINTRODUCTIONIn a series of studies on the synthesis of new silicon clathrate compounds containing bariumandiodine we found that the use of high-pressure and high-temperature HPHT conditionshas been effective and essential. The type I clathrateBa82dSi46 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 havedeveloped a new ternary clathrate system Ba – Al– Si and found a newhigh-pressure modication of BaAl2Si2. The structural transition will be discussed incomparison with the germanium analog BaAl2Ge2 which shows a temperature-dependentstructural transition17.EXPERIMENTALMaterialsBaAl2Si2 was rst prepared from the stoichiometric element mixture of Ba99 KatayamaChemicals Al99.999 Katayama Chemicals and Si99.999 Furu-uchi Chemicals byarc-melting in a water-cooled Cu crucible in Ar atmosphere. The oxidized layer of Ba metalwas removed in an Ar-lled glove box mBraun prior to thearc-melting. The as-meltedsample was ground in the glove box and treated under HPHT conditions using a cubicmultianvil type press Riken model CP-10. The sample was contained in an h-BN cell5 mm in inner diameter and 5 mm in depth which was placed in a carbon tube heater.The sample assembly was set in a pyrophyllite cube 20 20 20 mm3 as a pressuremedium. The reaction temperature was monitored by a thermo-couple placed under theh-BN cell. The details of the sample assembly were shown elsewhere 2. The typical press-ing and heating modes were the following: the sample was rst compressed to 5 GPa andheated to 1200 8C in 1 h and then kept for 1 h at the temperature followed by cooling to700 8C in 2 h. The sample was further cooled down to room temperature in a fewminutes. The applied pressure was gradually released overnight. The separate arc-meltedsamples wereannealed in an h-BN cell vacuum-sealed in a quartz glass tube. The tubewas kept in a furnace at 1200 8C for 1 day and cooled down to 800 8C for 2 days followedby cooling to room temperature for 2days.CharacterizationPowder X-ray diffraction XRD patterns were measured using graphite-monochromated CuKa radiation. X-ray single crystal structural analysis was carried out using a Rigaku RAXISdiffractometer with an imaging plate IP area detector and graphite-monochromated Mo Karadiation. The structure was solved by direct methods and rened with the program SHELX97 18 and WinGX software package 19. Further crystallographic details are given inTable I. The positional the equivalent isotropic and anisotroic displacement parametersare summarized in Table II. Differential thermal analysis DTA was made on the powdersample using an h-BN cell which was sealed in a thin quartz glass tube to protect thesample from oxidation in air. The sealed sample was heated and then cooled down at arate of 10 8C/min using a Shimazu DTA apparatus TA-50WS.RESULTS AND DISCUSSIONArc-MeltingThe powder XRD data of the arc-melted sample of the ternary mixture of BaAl2Si2 is shownin Figure 1a. The pattern could be indexed on a tetragonal unit cell of a 4.1992 andHIGH-PRESSURE MODIFICATION OF BaAl2Si2 483 TABLE I Crystallographic data for a- and b-BaAl2Si2. a-Phase low-pressureb-Phase high-pressureFormula BaAl2Si2 BaAl2Si2Formula weight247.46 247.46Crystal size mm 0.047 0.068 0.13 0.16 0.14 0.025Space group Cmcm no. 63 I4/mmm no. 139a A 4.2385 4.2312b A 10.89054.2312c A 10.1065 12.6019V A3 Z 466.46 4 225.64 2dcalcd g/cm3 3.524 3.644lMo Ka A 0.71069 0.71069m mm21 9.200 9.5122umax8 54.8 54.8Total reections 3315 2739Unique reections 319 101Reections with I 2sI 283 100Number of variables 18 9GOF on F2 0 1.169 1.127R1/wR2I 2sI 0.0266/0.0463 0.0154/0.0367R1/wR2 all data 0.0305/0.04670.0155/0.0367c 12.8736 A. This crystal was different from the BaAl2Si2 prepared by Carrillo-Cabreraet al. 20 using a glassy carbon crucible. This crystal is presumably isotypic with theThCr2Si2 structure 21 a new modication of BaAl2Si2.HPHT TreatmentThe arc-melted sample was treated in the h-BN cell under HPHT conditions. Figure 1c showsthe XRD pattern of the sample obtained by the treatment at 5 GPa and 1200 8C. The detailedcondition was described in Experimental section. The XRD pattern of the HPHT-treatedsample could be indexed on a similar tetragonal cell with a 4.2315 andc 12.6015 A. Although the lattice constant c was slightly decreased from 12.873 A ofthe as-arc-melted sample it was apparent that the structure was essentially unchanged bythe HPHT treatment.Annealing the CrystalsThe HPHT-treated samples were annealed in h-BN cells vacuum-sealed in quartz glass tubeat 800 – 1200 8C. Figure 1b shows the XRD pattern of the sample obtained after annealing.TABLE II Atomic coordinates equivalent isotropic andanisotropic temperature factors A2 for a- andb-BaAl2Si2. Site x y z U11 U22 U33 U23 U13 U12 Ueqa-BaAl2Si2 Low-pressure phase space group CmcmBa 4c 0 0.07766 0.2500 0.01283 0.00973 0.01593 0 0 0 0.01282Al 8f 0 0.38883 0.03262 0.00739 0.0592 0.019311 20.023911 0 0 0.02857Si 8f 0 0.79421 0.12443 0.013710 0.010611 0.078619 20.010111 0 00.03436b-BaAl2Si2 High-pressure phase space group I4/mmmBa 2a 0 0 0 0.02163 0.02163 0.02865 0 0 0 0.02393Al 4d 1/2 0 1/4 0.015860.01586 0.039113 0 0 0 0.02365Si 4e 1/2 1/2 0.13612 0.01606 0.01606 0.042413 0 0 0 0.02485Note: Ueq is dened as one-third of the trace of the orthogonalized U tensor.484 S. YAMANAKA et al.FIGURE 1 Powder XRD patterns of BaAl2Si2 prepared in different conditions: a arc-melted b annealed at1200 8C and c HPHT treated.The pattern was changed to that of an orthorhombic symmetry of a 4.2385b 10.8905 and c 10.1065 A. Single crystals were obtained by annealing from atemperature of 1200 8C and were used for the structural analysis.DTA AnalysisFigure 2 shows the DTA curves measured on the crystals obtained by the HPHT treatmentand the annealing at 1200 8C. The annealed sample shows a sharp endothermic peak due tothe congruent melting of the sample at 1027 8C and an exothermic peak at 1017 8C due to thecrystallization on the cooling process. The HPHT sample also shows similar sharp endother-mic and exothermic peaks coupled with a broad endothermic peak starting from about500 8C. After melting the HPHT-treated sampledid not show the broad endothermicpeak in the second measurement.Single Crystal AnalysisThe two types of single crystals the HPHT-treated sample high-pressure phase orb-BaAl2Si2 and the annealed sample low-pressure phase or a-BaAl2Si2 were performedand the results are shown in Tables I and II. The high-pressure phase crystallized in the spacegroup I4/mmm isotypic with the popular crystal system of the ThCr2Si2 structure 21. Thisis isotypic with thehigh-temperture phase of b-BaAl2Ge2 reported by Leoni et al. 17. Theannealed or low-pressure phase crystallized in the space group Cmcm. This is isotypic withthe low-temperature phase of a-BaAl2Ge2 space group Pnma. The space group Cmcmfor a-BaAl2Si2 is a minimalnon-isomorphic supergroup of Pnma for a-BaAl2Ge2 22.The Si and Al sites cannot be distinguished by x-ray. The Si and Al sites of BaAl2Si2 HIGH-PRESSURE MODIFICATION OF BaAl2Si2 485 FIGURE 2 DTA curves of a annealed and b HPHT-treated samples.were tentatively assigned to the Ge and Al sites of BaAl2Ge2 respectively. Some typicalinteratomic lengths and bond angles are given in TableIII.Structural Details and TransitionsThe germanide analog BaAl2Ge2 has been studied by Leoni et al. 17. They found thatthe germanide shows a temperature dependent dimorphism the 3D network ofthelow-temperature a-phase the a-BaCu2Si2 structure 23 space group Pnma transformsinto the layer structured high-temperature b-phase theThCr2Si2 structure space groupI4/mmm. The transformation occurs reversibly at 1100 K and the crystal meltsincongruently at 1310 K. Similar dimorphisms were found in BaCu2S2 and BaCu2Se223 24. It is interesting to note that the b-phase of BaAl2Si2 in this study was obtainedas a high-pressure phase which is transformed into the a-phase by annealing at elevated486 S. YAMANAKA et al. TABLE III Main interatomic distances d and bond angles for a- and b-BaAl2Si2.a-BaAl2Si2 b-BaAl2Si2 Distances d A Distances d A Ba222Si 3.3182 Ba228Si 3.4483 Ba224Si 3.4303 Ba228Al 3.7951 Ba224Si 4.0402Ba222Si 4.5911 Ba224Al 3.5773 Ba224Al 3.6692 Ba222Al 4.0562 2Si Si2 2.5384 2Si Si2 3.4333 2Al Al2 2.5103 2Al Al2 2.9911 2Al Si22.5322 2Al Si2 2.5571 2.5483 Angles 8 Angles 8 2Si2 Al2 2Al 113.6110 2Si2 Al2 2Al 111.679 95.159 71.624 2Al2 Si2 2Si 113.6110 2Al2 Si22Si 111.676 84.859 108.383 119.247 126.2811temperatures. It is reasonable to consider that the a-phase of BaAl2Si2 should becharacterized as a low-pressure stable phase i.e. the dimorphism of BaAl2Si2 is pressuredependent. Note that the high-pressure phase of BaAl2Si2 was also obtained by arc-melting in the ambient pressure. This can be interpreted as follows: in the arc-melting themelted sample was rapidly cooled and quenched to room temperature and the surface ofthe melt was rst solidied into a hard crust. The melt inside the crust will bethencooled. If the sample volume of the melt increases upon the crystallization.。