High pressure synthesis of RE6Cu23 compounds (RE  Tb, Dy, Yb, Lu)

Journal

of the Less-Common

Metals,

99 (1984)

L9

L9

Lll

Letter

High pressure

synthesis

of RE,Cu,,

compounds

(RE e Tb, Dy, Yb, Lu)

A. V. TSVYASHCHENKO Institute Moscow (Received

of High Pressure (U.S.S.R.) January

Physics

of the U.S.S.R.

Academy

of Sciences,

142092

Troitsk,

13, 1984)

1. Introduction It is known that copper in the intermediate phases of the RE-Cu systems (RE = Tb, Dy, Yb, Lu) obtained at atmospheric pressure has the same valency as the other group Ib metals. The distinctive compounds of the RE-Cu systems are RECu, RECu, and RECu, which have CsCl-type, CeCu,type and AuBe,-type structures respectively [l]. However, the distinctive intermetallic compounds of the RE-T systems (where T q Mn, Fe) are RET;?, RE,Tz3 and RE*Ti, which have MgC+,-type (or MgZn,-type), Th,Mn23-type and Th,Ni,,-type (or Th,Zn i7-type) structures respectively [ 11. It is considered that the transition metal ions in these compounds are divalent. In this work the possibility of crystallizing new compounds of copper with structures of the types which exist in the intermetallic compounds of the RE-T systems was investigated. 2. Experimental procedure The syntheses were carried out by application of high temperatures at a constant pressure of 7.7 GPa to a stoichiometric mixture of the elements in a chamber constructed by Khvostantsev et al. [2]. The powdered metals were mixed well, pressed and then placed in a rock salt pipe ampoule. The sample was directly heated electrically to above the melting temperature of the mixture and was then rapidly quenched to room temperature at which the pressure was released. The melting temperature was controlled by using the voltage-current plot. The new compounds were detected on X-ray powder photographs obtained using Cu Kcr radiation, nickel filters and a camera of diameter 114 mm. 3. Results Runs were carried out using heavy rare earth metals and yttrium. The new intermetallic compounds Tb6Cu2s, Dy,Cu,,, Yb,Cu,, and Lu,Cu,, were obtained. The crystalline structure of the compounds was indexed on the basis of a cubic unit cell of the Th,Mn,s-type structure (space group, 0022-5088/84/$3.00

0 Elsevier

Sequoia/Printed

in The Netherlands

LlO

Fn3m). Table 1 shows the lattice parameters obtained in this study. Table 2 shows the X-ray powder data for Yb,Cu,, and Tb,Cu,,. The samples obtained using lutetium metal contain the Lu,Cuz3 phase and phases that remain unindexed. TABLE Lattice

1 parameters

Compound

Lattice 12.20 12.14 12.03 12.00

Tb6CU23 DY6CU23 Yb6CU23 Lu6cu23

TABLE X-ray

parameter

(A)

+ 0.01 4: 0.01 & 0.01 + 0.01

2 data

TbScu23

yb6cu23

hkl

111 220 311 222 331 420 422 333 440 442 533 551 731 800 733 822 751 753 844 933 951 953 880 971 866

Zntensitya

VW W W

s ms W

ms s vs ms VW ms ms W

ms S

ms ms ms s W W

mw mw

d( meas)

d(calc)

(A)

(A)

7.025 4.384 3.678 3.473 2.763 2.683 2.445 2.309 2.127 2.004 1.833 1.684 1.566 1.502 1.467 1.415 1.388 -

6.940 4.250 3.624 3.470 2.758 2.688 2.454 2.313 2.125 2.003 1.833 1.683 1.565 1.503 1.468 1.417 1.388 -

1.225 1.209 1.162 1.119 1.065 1.049 1.033

1.227 1.208 1.162 1.121 1.062 1.050 1.031

%s, very strong; s, strong; ms, medium VW, very weak; vvw, very very weak.

strong;

intensity

d( meas)

d(calc)

(A)

(A)

-

ms VW vvw m S vs vvw vvw VW VW W

m VW vvw VW VW W -

m, medium;

3.507 2.784 2.722 2.485 2.341 2.151 2.029 1.859 1.708 1.588 -

3.522 2.799 2.728 2.490 2.348 2.157 2.033 1.860 1.708 1.588 -

1.490 1.438 1.411 1.342 1.247 1.228 1.181 -

1.490 1.438 1.409 1.339 1.245 1.226 1,179 -

mw, medium

weak, w, weak;

Lll

12.4-

\

!Tb kll~k~!EdT~i~ /YdLu

P~.J.

Fig. 1. Plots of the lattice parameter of REeMn *a (0) REhCuzs (0) and RE6Fe2a (0) vs. the atomic mass of the rare earth metals (ionic radii: MnZ+, 0.91 A; Cu+, 0.98 8; Cu*+, 0.80 A; Fez+, 0.80 A). (The results are taken from refs. 1, 4 and 6 - 9 and from this work.)

4. Conclusion Ytterbium metal is known to have an anomalous valency. From the experimental data on the atomic volume and the magnetic moment, ytterbium is divalent in compounds prepared at atmospheric pressure [3] and is trivalent in compounds with manganese obtained at high pressure [ 4, 51. The filling of the f electronic shell ((4f)‘4(6s)2) of ytterbium is similar to that of the d electronic shell ((3d)“(4s)‘) of copper. Therefore, the valency of copper is also expected to change at high pressure. In Fig. 1, the plot of the lattice parameter of the RE,Cu,, compounds against the atomic mass of the rare earth metals is not very different from the plot of that of the RE,Fe,, compounds, and the divalent ionic radii are similar. Therefore, copper can be considered to be divalent in the RE,Cu,, compounds. It follows that compounds may exist with copper in an intermediate d electron valency. The change in the valency of pure copper metal could be proved by direct measurement of the X-ray absorption spectrum of copper at high pressure, as has been carried out for ytterbium by Syassen et al. [lo]. 1 K. N. R. Taylor, Adv. Phys., 20 (1971) 551. 2 L. G. Khvostantsev, L. V. Vereshchagin and A. P. Novikov, High Temp. High Pressures, 9 (1977) 637. 3 E. P. Wohlfarth (ed.), Ferromagnetic Materials, Vol. 1, North-Holland, Amsterdam, 1980, Chap. 4. 4 A. V. Tsvyashchenko and S. V. Popova, J. Less-Common Met., 90 (1983) 211. 5 A. V. Tsvyashchenko, S. V. Popova, V. E. Makhotkin, V. A. Fradkov and V. N. Zaritskii, J. Less-Common Met., 96 (1984) 99. 6 T. J. O’Keefe, G. J. Roe and W. J. James, J. Less-Common Met., 15 (1968) 357. 7 A. Meyer, J. Less-Common Met., 18 (1969) 41. 8 M. P. Dariel and G. Erez, J. Less-Common Met., 22 (1970) 360. 9 K. H. J. Buschow, J. Less-Common Met., 26 (1972) 329. 10 K. Syassen, G. Wortmann, I. Feldhaus, K. H. Frank and G. Kaindl, Phys. Rev. B, 26 (1982) 4745.