KUDOU Kunio

OKADA Shigeru

SHISHIDO Toetsu

OKADA, Shigeru

7197

KUDOU, Kunio

16947

SHISHIDO, Toetsu

7170

Kokushikan University Academic Information Repository / 国士舘大学　学術情報リポジトリ

21Original PaperCrystal structure of the GdCo2B2Cx (*=0- 1)Shigeru OKADA**1, Kunio KUDOU*2, Toetsu SHISHIDO*3Abstract: GdCo^Cy (x=0 — 1) has been synthesized to study the role of C in the formation of new quaternary compounds RT2B2C (R = rare earth, T = transition metal); RT2B2C belongs to a tetragonal system withThCr2Si2 derivative type. Samples of GdCo^C* U=0, 0.25, 0.50, 0.75 and 1.00) were prepared by the arc-melting synthetic method. The lattice parameter a slightly decreased, while the lattice parameter c and the unitcell volume increased with increasing x in GdCo2B2Cx. This result shows that C element serves to expand the entire unit cell in the shape of a square column composed of R atoms (eight at the corners) especially along the indirection, and permit T atoms, which are too large for the normal lattice, to join in combination. The role of Cto form the quaternary borocarbides RT2B2C has been discussed.Keywords: GdCo2B2Cx (#=0-1), Quaternary borocarbides, Crystal structure1. IntroductionThe crystal chemistry of R-T-B and R-T-B-C (R =rare earth element, T = transition element) systems hasreceived considerable attention from many researchers inthe fields of crystallography, magnetism, superconductivity, heavy-electron behaviour, valence fluctuations andcatalytic properties[1"4]. To study the crystallographicrelationship between two compounds of RT2B2(tetragonal system; ThCr2Si2 type structure) and RT2B2C(tetragonal system; modified model of ThCr2Si2 typestructure), in which a rare-earth element R and a transition element T are specified, is considered important inconsidering what role is being played by the element Cduring the formation of the latter borocarbide compound,in particular. However, in case some of these are present,the others are not likely to be present. Within the range ofthe investigation carried out up to now by the authors, it isonly in a combination of GdCo2B2 and GdCo2B2C that bo-ride and borocarbide are present at the same time. Inregard to GdCo2B2, a group including one of the presentauthors reported[5]. On the other hand, in respect toGdCo2B2C2, Mulder et al. have recently given a report[6].In this study, compounds were prepared by varying thequantity x of C in GdCo2B2Cx between 0 and 1, and an investigation was carried out into how the crystal structurechanges depending on the amount of C.Faulty of Science and Engineering, Professor, Dr. of Engineeringt Corresponding author, Tel/fax: +81-3-5481-3292E-mail address: sokada@kokushikan.ac.jpFaulty of Engineering, Kanagawa University, AssociateProfessor, Dr. of Engineering*3 Mit±¥&m#nmftm mmn, i^i#±Institute for Materials Research, Tohoku University, Associate Professor, Dr. of Engineering2. Experimental Details2.1 Synthesis of CompoundsFor raw materials, 99.9% Gd (block) by Nippon Yttrium, 99.96% cobalt (powder) by Hirano, 99.86% crystalline boron (aggregate) by Hirano, and 99.999% carbon(block) by Kojundo Kagaku Kenkyusho were used. Theblocks were cut into small pieces, while the aggregate waspulverized and made into powder. An attempt was madeto synthesize compounds by an arc melting method. Compounds were synthesized by varying the amount of x inGdCo2B2CAr continuously. In other words, the raw materials were mixed so as to have an atomic ratio of Gd : Co :B : C= 1 : 2 : 2 : 0, 1 : 2 : 2 : 0.25, 1:2:2: 0.50, 1:2:2:0.75, and 1:2:2:1.00. A total of about 2.5 g of the rawmaterials were placed inside a copper health, and the vacuum level inside the chamber was reduced to about 7 x 10 "3N/m2. Then argon gas was injected in the oven to a pressure of 1 atm. By generating an argon arc plasma flame,titanium for the oxygen getter was first heated and meltedto remove traces of oxygen remaining inside the chamber.Next, by focusing the flame on the raw materials, theywere melted for 3 min with AC 20 V x 100 A to cause reactions. The samples were then turned over and melted threetimes under the same conditions.2.2 AnalysisThe chemical compositions of the three elements Gd,Co, and B were investigated by using an inductively coupled plasma atomic emission spectrometry (ICP-AES).The samples were dissolved in a mixed acid of nitric acid,hydrochloric acid, and phosphoric acid, and were measured by the ICP method. The element C was measured bya combustion-infrared absorption method. Substances obtained by adding high-purity Fe, W and Sn to the sampleswere placed inside a mortar and were heated at a high-frequency wave while flowing oxygen. Fe, W and Sn actedfirst as activators for introducing the high-frequency wave22 Transactions of the Kokushikan Univ. Science and Engineering. No. 2 (2009)into the contents of the mortar, and then worked as solvent (fluxing agent) for the samples after the entire contents had been heated to a high temperature. C inside thesamples is burned to become CO2, which is then transported by oxygen. CO2 was measured by an infrared absorption method. By determining the entire amount of CO2 byintegration and converting this into the amount of C, thisamount was assumed to be the amount of C in the originalsamples. The identification of the compounds and the determination of the lattice constants were carried out by apowder XRD method using copper as its beam source. Byprofile-fitting the 20 values of the peaks, the lattice constants were measured.3. Results and Discussion3.1 Chemical CompositionsThe ingot obtained by completing the arc-melting synthesis has a silvery luster. The results of the chemical analyses are listed in Table 1. A comparison of the chemicalcompositions of the samples after the synthesis with thestarting compositions indicates that Gd is shifted slightlyto the larger-value side and B to the smaller-value side,while Co and C are stationary.3.2 Crystal Structure of CompoundsFig. 1 presents the XRD profiles of the samples obtained. The compositions of x=0, 0.25, 0.50, 0.75 and1.00 in GdCo2B2CA: correspond to the profiles (a) through(e), respectively. Each spot with a circle (O) indicated atetragonal system compound. In terms of their correspondence with the concentration x of C, it is suggested that asx increases from x=Q, the group of peaks with O shiftsrelatively toward the lower-angle side and the unit latticevolume increases gradually. Throughout the entire synthesized samples the main impurity phase is the GdCo3B2phase indicated by V in the profile. The GdCo3B2 phasegradually decreases as the concentration of C increases.The crystal structures of the two compounds are illustrated in Fig. 2 (a) and (b). Boride GdCo2B2 is a tetragonalsystem compound of the ThCr2Si2 type. On the otherhand, borocarbon GdCo2B2C is a modified model basedTable 1 Results of the chemical analyses on the synthesized samples for the GdCo2B2-GdCo2B2C systemNominal compositionGdCo2B2C0.ooGdCo2B2C0.25GdCo2B2C0.5oGdCo2B2C0.75GdCo2B2Ci.ooResultsmass%Gd54.1(52.99)53.4(52.46)52.3(51.94)52.0(51.43)52.0(50.93)of the chemical analyses(Nominal composition)Co39.7(39.72)39.2(39.32)38.6(38.93)38.2(38.55)38.1(38.17)B5.62(7.29)6.25(7.21)6.34(7.14)6.22(7.07)6.13(7.00)C(0.00)0.99(1.00)2.00(1.99)2.93(2.95)3.59(3.89)on the ThCr2Si2 type, and is a more complex compound tothe extent of containing C. Each of these has the elementGd occupying the eight corner positions of the frame. Aninvestigation was carried out into the relationship betweenthe contraction of C and lattice constants. With an increase in the level of the C content as indicated in Fig. 3(a), the value of the lattice constant a lowers (even thoughno change occurs monotonously). However, a comparison of the difference in C amount between jc=O and x= 1indicates that the change in the lattice constant a is asextremely small as 0.003 nm. The element C is introducedto the base center of Fig. 2(b), but depending on theamount introduced, the a-axis is considered to contract asthe interface bond between the rare-earth element Gd andC gets stronger. This situation is illustrated by horizontalI20 40 60 80 100Fig. 1(a)Diffraction angle, 20/°XRD profiles of GdCo2B2Cx (O) with x=0 (a), 0.25(b), 0.50 (c), 0.75 (d) and 1.00 (e). Peaks from impurity phase of GdCo3B2 (V) are also indicated.Fig. 2 Crystal structure of GdCo2B2 (a) and GdCo2B2C (b).Crystal structure of the GdCo2B2Cx 0t=0- 1) 23(a) (b) (c)0 0.2 0.4 0.6 0.8 1Carbon content x in GdCo2B2Cx1.060 0.2 0.4 0.6 0.8 1Carbon content x in GdCo2B2Cx0 0.2 0.4 0.6 0.8 1Carbon content x in GdCo2B2CxFig. 3 The lattice constant a (a), lattice constant c (b) and unit cell volume (c) as a function of the carbon content x inGdCo2B2Q.arrows in Fig. 3 (b). On the other hand, if the C content inthe compound increases as indicated in Fig. 3 (b), then thevalue of the lattice constant c increases almost linearly. Inthis case, a comparison of the difference in the C contentbetween x—0 and x— 1 shows that the change in the latticeconstant c is extremely large at 0.10 nm. A possibility isconsidered to exist that a repulsive force works on thebond between C, introduced to the bottom center of Fig. 3(b), and B that already exists right above it. A strongbond exists between C and Gd or between B and Co; it istherefore considered that in proportion to an increase inthe amount of C introduced, repulsion to the bond between C and B takes place, thereby causing the frame toexpand considerably in a vertical direction. This situationin indicated by vertical arrows in Fig. 3 (b). The size of theunit cell volume as a whole body increases almost linearlywith an increase in the C content, as indicated in Fig. 3(c). A comparison in C content between x=0 and x= 1 indicated that the unit cell volume in the latter increases byabout 8.5%. Nevertheless, the vacant areas inside the lattice for the latter are also considered to increase, so thevolume decreases by 4.3% on the contrary, in accordancewith a comparison in X-ray density. Table 2 presents crys-tallographic data on GdCo2B2 and GdCo2B2C. The latticeconstants of GdCo2B2 are a = 0.35747 (8) nm and c =0.9537(2) nm. The crystal structure of GdCo2B2C is basically the same as those of GdRh2B2C and ErRh2B2C, onwhich the authors have reported previously. Here, if acomparison is made between the values of the lattice constants obtained by the authors and by Mulder et al., then a= 0.35492(7) nm and c= 1.0491 (5) nm vs. tf = 0.3548 nmand c= 1.0271 nm.The results of the present experiment may be regardedas providing a suggestion in considering the role of the element C in the formation of 4f-element system borocarbideRT2B2C^7-16^.4. ConclusionBy synthesizing compounds by arc melting while changing the amount of C continuously to set a: = 0, 0.25, 0.50,0.75 and 1.00 in GdCo2B2Cx, an investigation was con-Table 2 Crystallographic data of the GdCo2B2 and GdCo2B2C.Chemical formulaCrystal systemStructure typeNumber of molecules perunit cell ZLattice parametera (nm)c (nm)Unit cell volume, V(xl0"28m3)X-ray density, g/cm3GdCo2B2TetragonalThCr2Si2 type20.35747 (8)0.9537 (2)1.2187(8)8.09GdCo2B2CTetragonalderivative ofThCr2Si2 type20.35492 (7)1.0491 (5)1.322(1)7.76ducted into how the structures and lattice constants of thesamples changed. The chemical compositions of the samples after the synthesis shift slightly, relative to the compositions of the raw materials, toward the larger-value side inthe case of Gd and toward the smaller-value side for B, butno shift is seen in the case of Co and C. According to theresults of a powder XRD analysis, with an increase in theamount of C, the value of the lattice constant a declinessomewhat, the value of the lattice constant c increases considerably, and the unit cell volume increases greatly. It isthought that the lattice constant a contracts slightly because it is subjected to a strong bond between C, introduced into the base center of the tetragonal crystalsystem, and Gd, which receives it, and that the lattice constant c expanded greatly as a repulsion to the bondbetween C and B present right above it due to the introduction of C.AcknowledgmentThe authors would like to express thanks to Dr. J. Ye ofNIMS for her fruitful discussions. The authors are also indebted to Miss A. Nomura, Mrs. T. Sugawara, K. Obara,S. Tozawa, Y. Murakami and S. Ito of IMR in TohokuUniv., for their technical contributions.24 Transactions of the Kokushikan Univ. Science and Engineering. No. 2 (2009)References[ 1 ] L. D. Woolf, D. C. Johnston, H. B. MacKay, R. W.McCallum, M. B. Maple, J. Low Temp. Phys., 35, 651(1979).[ 2 ] B. K. Cho, M. Xu, P. C. Canfield, L. L. Miller, D. C.Johnston, Phys. Rev., B52, 3767-3772 (1995).[ 3 ] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani,J. Akimitsu, Nature, 410, 63-66 (2001).[ 4 ] T. Shishido, J. Ye, K. Kudou, S. Okada, K. Iizumi et al.,J. Alloys Compds., 375, 217-220 (2004).[ 5 ] K. Niihara, T. Shishido and S. Yajima, Bull. Chem. Soc.Jpn., 46, 1137-1140 (1973).[ 6 ] F. M. Mulder, J. H. V. J. Brabers, R. Coehoorn, R. C.Thiel, K. H. J. Buschow and F. R. de Boer, J. Alloys Compounds, 217, 118-122 (1995).[ 7 ] C. Mazumundar, R. Nagarajan, C. Godarat, L. G. Gupta,M. Latroche, S. K. Dhar, C. Levy-Clement, B. D. Padaliaand R. Vijayaraghavan, Solid State Commun., 87, 413-416(1993).[ 8 ] R. J. Cava, H. Takagi, B. Batlogg, H. W. Zandbergen, J.J. Krajewski, W. F. Peck, Jr., R. B. van Dover, R. J.Felder, T. Siegrist, K. Mizuhashi, J. O. Lee, H. Eisaki, S.A. Canter and S. Uchida, Nature, 367, 146-148 (1994).[9] R. J. Cava, H. Takagi, H. W. Zandbergen, J. J.Krajewski, W. F. Peck, Jr., T. Siegrist, B. Batlogg, R. B.van Dover, R. J. Felder, K. Mizuhashi, J. O. Lee, H.Eisaki and S Uchida, Nature, 367, 252-253 (1994).[10] R. Nagarajan, C. Mazumundar, Z. Hossain, S. K. Dhar,K. V. Golpakrishnan, L. C. Gupta, C. Godart, B. D.Padalia and R. Vijayaraghavan, Phys., Rev. Lett., 72,274-277 (1994).[11] T. Shishido, J. Ye, T. Sasaki, T. Matsumoto and T.Fukuda, J. Ceram. Soc. Jpn., 104, 1117-1120 (1996).[12] J. Ye, T. Shishido, T. Matsumoto and T. Fukuda, ActaCryst. C, 54, 1211-1214 (1998).[13] J. Ye, T. Shishido, T. Matsumoto and T. Fukuda, ActaCryst. C, 52, 2652-2655 (1996).[14] T. Shishido, J. Ye, T. Sasaki, R. Note, K. Obara, T.Takahashi, T. Matsumoto and T. Fukuda, J. Solid StateChem., 133, 82-87 (1997).[15] J. Ye, T. Shishido, T. Sasaki, T. Takahashi, K. Obara. R.Note, T. Matsumoto and T. Fukuda, J. Solid State Chem.,133,77-81 (1997).[16] T. Shishido, J. Ye, K. 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Crystal structure of the GdCo2B2Cx (x = 0－1)

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Crystal structure of the GdCo2B2Cx (x = 0－1)

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Crystal structure of the GdCo2B2Cx (x = 0－1)

Abstract

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Kokushikan University Academic Information Repository / 国士舘大学　学術情報リポジトリ