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Mori, H.

Satoh, T.

Yashima, H.

Kyoto University Research Information Repository

Title Thermal and Magnetic Properties of Ce-Si SystemAuthor(s)Yashima, H.; Mori, H.; Satoh, T.Citation物性研究 (1982), 37(5): 42-48Issue Date1982-02-20URL http://hdl.handle.net/2433/90471RightType Departmental Bulletin PaperTextversionpublisherKyoto UniversityThermalandMagneticPropertiesofCe-SiSystem*H. Yashima,H.MoriandT.SatohDepartment ofPhysics,Faculty ofScience,TohokuUniversity,Sendai,Japances土2andCeGe2 aremembersofα-ThS土2type lntermetall土c com-pounds. The crystalstructure isshown inFig.i. Thisstructureisknown tobe foundonly for silicidesandgermanides. Thereforeit seemsthatthe sublatt土ce structure constructedby SiorGe,which maybe looked upon a 3-dimensionalgraphite structure,isimportant in formingthiscrystalstructure.工n thisnotewewish to report tne resultsofour study inCe-SiandCe-Ge system. The latticeparametersmeasured at roomtemperature are a-b=4･184A and c-13･856A forCeS土2 and a-4･282A･b-4･199A and c-14･081A forCeGe2･ Themeasured speci-ficheatsare given in Fig.2 andFig.3. Themagneticsusceptibili-tiesare shown in Fig.4.From Fig･2and Fig･4we can seethatCeGe2undergoesamagne-tictransition atT = 7K. AssumingCurie-Weiss law forthehighCtemperature susceptib土1土ty′we obtain りeff=2･41uB Which corres-ponds approximately to themagneticmoment ofthe freeCe3+ ionリeff(+3)-2･56UB･ Although thebehaviorofmagneticsusceptibi-lity nearT suggeststhat themagnetically ordered state is ferr0-Cmagnetic,the temperature dependenceofspecificheatbelowT canCbe fittedverywellwlthで3 law asshown in the inserted figure inFig.2. The calculation oftheentropybelow T canbemadewithCFig.2 and givesthe valueofS-5.74J/mole･degwhich isnearlyequaltoRR,n2where R isthe gas constant. Neglectingthe ortho-rhombicdistortion 土n CeGe2′ the crystalfieldpotentialcanbeexpressed asvc=AoO 4 + A IJz4 + A 2J z2 (1)Zfwetake into account upto the 4th nearestneighbor atoms andassumethe charge at Ce site is+3and that attheGe site is zero,thenwehavealevelschemeinwhich theground state isadoubletV乍7百 l± 5/2 , + vr76 2手 3/2 , . Magnetizationmeasurementswereperformedto glVe aSaturationmagnetizationM 20･86uB･ There-Sfore it maybemore reasonableto assign the ground statetobe ar7doublet (/手行 い 5/2 ' 一 帯7首巨 3/2>)which gives Ms = 0･71uB･42ForCeSi2,aSisclearly seen fromFig･3andFig･4,vedonotsee any phasetransition intheinvestigatedtemperaturerange downto 100mK･ The low temperaturespec土f土Cheat canbeexpressedverywellby theequationc=YT + 6T3 (2)With Y= 104mj/mole･deg2 and 6- 0･211mj/mole･deg4which corres-ponds to theDebvetemperature0ef302K･ It isnotedthe Yvalueobtainedisalmost 26times largerthan thatofLaGe20rLaSi21)･whereas0isnearly the same･ A simple fittingofthehightempeTra-turesusceptibilitywith Curie-Weiss law glVeSUeff=3･14日BWhich ismuch largerthan リeff(+3)IThe finiteground state susceptibilityisobtainedby extrapolation asx(0)-4.33x10-3emu/mole･Thusthekratiodefinedas 心 T2kB2舟=g2UB2J (J+i)x(0)Y(3)is calculatedtogive ル =i.41. The 孔 ratiointheRondo limitforCe3' issaidtobeg - (2J.i)/2J- i.202). The low temperaturesusceptibility canbeexpressedverywellwithx(T)-x(0) (i+aT2) for T <25Ka= -0.42Ⅹ 10-3 (k-2)(4)asshown inFig.4.Acc｡rdingt｡Ourpreviousstudy ｡n La-Ge systeml三 ,α-ThSi2typecrystalstructureisstablein aviderangeofnon-stoichiometriccomposition. Referr土ngtOtheSi-sublatticestructureshown 土nFig･1(C), itwouldbeexpectedthat thedeficiencyofSicauses achangeofthenumberofelectronsinthe conductionband. The intermediatevalence stateoccursduetotheproximityofthe 4fleveltotheFermienergy level. Thereforeit isinteres-tingtoseehow themagneticbehavior foundinCeSi2ChangeswithS土defic土ency.The latticeparametersmeasured at room temperature aregiveninTab･Z･ Althoughthe lattice constants a,b and cvarywith x,theunit cellvolumedoesnotshow an appreciablechangeW土th x .Magneticsusceptibilitiesweremeasured inthetemperaturerange 4.2K to 300K. The resultsare shown inFig･5･ Specificheatmeasurementsweremade inthetemperature region from 0.lKto 70K･The resultsare given inFig.6andFig.7. From Fig･5,Fig･6 andFig.7,we can summarizethemain featuresofourresultsasfollows.45(i)工nthe composition range2.00三X ≧ i.85,CeSi doesnotshowXany phasetransition inthe investigatedtemperaturerange down toO･lK･ Thevery-low temperature specificheat canbeexpressedverywellbytheequationC - yT (5)Thevalueofy isvery largecomparedwith that ofLaGe20rLaSi2and it increaseswith decreasing x ･ TheFermitemperature isob-tained from yas7T2RTF(Y)--2Y R:gasconstant .(6)ThosevaluesofYandTF(Y)are listed inTab･==･(ii)Forthe samplesofx=2.00,i.90and i.85, the finitegroundstatesusceptibility x(0)isobtainedby extrapolation. The lowtemperature susceptibility canbeexpressedwellwith theequationx(T)-x(0)(i+aT2) (7)asshown inFig･8･ The sign OftheT2 term isnegative forallsamples･ Thosevaluesofx(0)andthe coefficient aaregiven inTab.ZZ. TheFermitemperatureestimated from x(0)as･F(x , -言 論 , (C-o･807emuK/mole, (8,are also listedinthe 畠ametable.(iii)Forthe composition x≦ i.85the system undergoesasharpmagnetictransition. Thehigh temperature susceptibility canbefittedverywellwith theCurie-We土sslawCIX=-T+0(9)whereC'-0･768emuK/molewhich correspondsto リeff=2･47uB ･(iv)Ascanbe seen from Fig.5,thehigh temperature susceptibili-ty ofCeSil.90andCeS土1.85Can alsobeexpressedwithEq･(9)･ 工nthese cases0maybe considered asa characteristictemperaturedeterminedby thestrengthofthe s-fmixing interaction3)･ Thevaluesof0are listed inTab.ZZ.(Ⅴ)Below about 50K,allsamplesshow arathersharp increaseofthemagneticsusceptibility.Abovementioned features (i)and (ii)suggestthatCe inCeSixx三 1.85is inthe intermediatevalence state and the ground stateisametallicFermi liquid. ThevaluesoftheparameterdefinedwithEq.(3)aregiven inTab.=工. Asshown inTab.=工′乱 increases44rapidly'nearthe criticalcompos土tion. From thevaluesofYandx(0)given inTab.H ,we can seethat wehave an approximatere-lation y ∝ R,nx(0)asinthe case ofastrongly exchange-enhancedparamagnon system4). Discussionsofthepresent databasedontheparamagnonmodelwillbepresented.工ftheFerm土 1土quid-likebehaviorcontinuestoexist also土nthe composition rangeofx三 1.80,it seems likelythat themagne-tically orderedphasemightbeaspin densitywavestate･ Theen-tropybelowT canbe calculated from the speclf土cheat dataandCisgiven inTab.ZZ asAS. Ascanbe seen ASismuch smallerthanRLn2･ =t isinterestingtonotethat inthe caseofCeGe2,△S isalmost equalto RR,n2,althotightheT isnearly the sameasfoundC土nthepresent Ce-Sisystem.Resultsontheternary system CeS土2-XGexwillbe91Ven andthepreliminary resultsofresistivitymeasurement willalsobepresen-ted.References★ A partofthepresent paperwillappearin SolidStateCommun土-cations. ThermalandMagneticPropertiesandCrystalStructureofCeGe2andCeSi2by H･Yashima,T･Satoh,H･Mori,D･WatanabeandT.Ohtsuka,Nonmagnetic-MagneticTransioninCe-SiSystembyH.YashimaandT.Satoh.i)T.Satoh andY.Asada,･J.Phys.Soc.Japan,27 (1969)1463.A Sawada andT.Satoh,･J.ofLowTemp･Phys･~包 (1978)455･2)D.M.NewnsandA.C.Hewson,･J.Phys.F.MetalPhys.10(1980)2429.3)C.M.VarPaandY.Yafet,･Phys.Rev･･138 (1976)1950･4)∫.R.Schr土efer;∫.Appl.Phys.39 (1968)642･45TABLE∫.×=2.00 x=1.90 ×=1.85 ･×=1.80 k=1.70b(A)C(A)a x b x ci Ll.278 Li.192 lil.170 L.170 L.09815.815TABLEII.1Y : TF(Y)(K) x(0) TF(X)(l<) Ax103=:.丸 ;o ■ T AS･-二 . (_竺聖)role (K-2) /K)I C .(K) (ny)Jle.K)x=2.()O 104JltI '390 4.2x10-3 288ー 一0.42l ll.41. ∫l Ix=1.9O I･i.151.lJ 271 7.Ox10-3 173】 ■-0.55 I ri.62 '101 vix=1.85 I234一 175 4.0.x10-2 30.1 1L-2.7t 6.04 !76i I】I ∫x=1.80 】 1;42】 9.0 3二41x=1.70 I モ25.- 10.9 4.53.46? ? ??? ????????? ???～o tn o(扇･10LU/｢)⊃??? ?? ?? ?? ??〜宍･ Lh O Lr'(y･7OUJ/r)3.景(n･uJ;/10LU)二x oe8 ●8 8 --二 ･ †lJl1,-I.I日 .lh47? ? ?????? ??????? ????? ???????? ?????? ? ?ナ 0> ○< al二:●◆+/ p 〇■● ○･-'I-I ;三も-~ -t←◆ 4&23〇･--+,◆r.1-- - 4%もqL -＼. ~ ･t･_:O 鳶 . ･･os(nu｣占/10山)L声.くつ⊂)eヽ の●●U7Ul･.○ -Q) Q}.. (..U:U○●○○ ●○○.○.a○●〇I- ○●○○ヽ■㌔う.-'.iac,p○.I○○◆■〇㌔ ち⊂> ●一∽ ~~`●も~CJ ト ､ .ら'3, U ≡ ◆'■鶏 .鑑 ○○L｡.Qd/-i.′〟I;さ･●ノ､■■ 一-●一､㌧.一O/ '-.､.㌔→J.-蛋. ､､;:㌔､ul ⊂〉 Ll⊂)⊂) Ln くつ ul～(Ⅵ･lO_Ll_/｢･) つ-一芸 g 3■_ ._ く､■･' ul Uー U1.｡ CJ dJ Q}< ｡ L U .U U〇 一< ●■○i▲○.- 'q.■1 <○● ○ ■一 〇 ■～て1 -○1'J.〇I.～㌔ ■ヽヽ I aLー S: _-oG, _ 3 三 ㌔_ヽ′ヽ 一` ●し′ー 一 馬 tU1 .. . C}'祝 砲 ＼＼ ∵1し~ q/._ UQ4,.如 ､, , 苧 萄転O. il).⊂'⊂) l∫つ｢iF(M.10山/｢うつrFIIo=ヾ n⊂)- ヽ LLl??? ????????? ?????????????????-1 ~£- 冨~ 呂~~.- 一･.- ～∽ 一uI U1I-I-cj- qJ-~Q)-U U U> X 一 〇???? ???? ????? ? ?? ? ? ?Toq>(ul(lOUJ/nuJ占)7t48

Thermal and Magnetic Properties of Ce-Si System

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Thermal and Magnetic Properties of Ce-Si System

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