Mat. Res . B u U . , Vol. 17, pp. 911-916, 1982. Printed in the USA. 0025-5408/82/070911-06503.00/0 Copyright (c) 1982 Pergamon Press Ltd.

PREPARATION AND MAGNETIC PROPERTY OF NEW RARE EARTH

COMPOUNDS R2GeBe207 (R=La,Pr,Sm,Gd,Dy,Er) AND Y2GeBe207

Y.Ochi, H.Morikawa, l.Minato and F.Marumo Research Laboratory of Engineering Materials,

Tokyo Institute of Technology 227 Yokohama, Japan

(Rece ived May 3, 1982; Refereed)

ABSTRACT New rare earth compounds which are isostructural with silicate minerals of the melilite family have been prepared. The cell dimensions vary linearly with the radii of rare earth ions, showing a typical Lanthanoid contraction. Magnetic susceptibilities of the rare earth compounds were measured in the temperature range from 77 K to 1000 K. A trend toward weak ferromagnetism with decreasing temperature was observed in the heavy rare earth compounds.

Introduction

Melilite forms a solid solution between the end members of ~kermanite, Ca2MgSi207, and gehlenite, Ca2AI2Si07 (I). Since Warren and Raaz determined the crystal structure of melilite(2,3) many natural and synthetic members of the melilite family have been reported, e.g., Ca2ZnSi207 (4), SraFeSi207 (5), CaLaAla07 (6), CaLaGaa07 (6), etc.. There are two kinds of tetrahedral sites (TI and T2) in the melilite structure, Tz being occupied by Mg 2+ and T2 by Si 4+ in the case of ~kermanite. Small Be 2+ can occupy either of the tetrahedral sites, when cations of appropri- ate sizes and charges are chosen for the remaining cation sites. Thus,substitution of Hg 2+ by Be 2÷ in Ca2MgSi207 yields Ca2BeSi207 (7). When a trivalent cation Y~+ occupies the eightfold position for Ca 2+, Y2SiBe207 is formed(8). We tried synthesis of melilite type compounds containing rare earth ions with larger radii than that of ya+ at the eightfold position from the magnetic interest. For this purpose the small Si ~+ was replaced with the larger Ge ~+ and Y~+ with rare earth ions in Y2SiBe207.

911

912 Y. OCHI, et al. Vol . 17, No. 7

Experimental

Starting materials were GeO2(Soekawa Chem. Co.Ltd.,99.999%), BeO(Wako Pure Chem. Ind. Ltd., 99%), Y203, Sm203 , Pr6011, Gd203, Dy203, Era03 and La2(C03)3 (Wake Pure Chem. Ind. Ltd., 99.9%). Weighed chemicals to the melilite compositions were mixed in an agate mortar. The mixtures were pressed to discs at 1000 kg/cm 2 and heated at 1620 K for 12 hours in an SiC furnace and quenched to room temperature in air.

Powder patterns of all the products were recorded with a Philips powder diffractometer (PW-1011) equipped with a curved graphite monochromator, using Cu K~ radiation. Calibration was carried out with an internal standard of high purity silicon(a= 5.4301 ~). The cell dimensions were computed with a least- squares program UNICS (9) from 25 20-values.

Magnetic susceptibilities of these compounds were measured on a Faraday-type magnetobalance (Shimazu MB-IA) in the tempera- ture range from 77 K to 1000 K. Calibration was carried out by using Ni (99.99%) and CuSO4,SH20 (99.95%). The necessary cor- rections were made for diamagnetism.

Results and discussion

The reaction products containing Sm, Pr and Er are respec- tively coloured in pale yellow, green and pink. Others are celourless.The powder pattern of each product indicated a single phase of the melilite family. On the basis of the melilite lattice, reflection lines were indexed and the cell dimensions were calculated. The cell dimensions of the compounds prepared in the p~esent study are listed in table I. The observed d- values (A) for the Gd compound are compared with the calculated ones in table 2 as an example.

Fig. I shows plots of cell dimensions versus radii of rare earth cations. The values used are the effective ionic radii re- ported by Shannon (10). The length of the a and c axes linearly increases with increasing ionic radii. The cell dimensions (in

TABLE I

Cell dimensions of R2GeBe207 (R=rare earth elements) crystals

Standard deviations are given in parentheses

R a(A) c(~) V(A 3 ) colour

Y 7.399(I ) 4.797(I ) 262.6(I ) white La 7.695(I ) 4.968(I ) 294.1 (I) white Pr 7.611(I) 4.919(I) 284.9(1) green Sm 7.528(I ) 4.859(I ) 275.4(I ) yellow Gd 7.480(I ) 4.842(I ) 270.9(I ) white Dy 7.428(I) 4.809(I) 265.4(I) white ~r 7.380(I ) 4.786(I ) 260.7(I ) pink

Vol . 17, No. 7 NEW RARE EARTH COMPOUNDS 913

TABLE 2

X-ray powder diffraction data for Gd2GeBe207

h k i dca I dob s I/I0 h k I dca I dob s I/Io

I I 0 5.289 5,285 2 3 2 0 2.0?5 2.075 I 0 0 I 4.842 4.844 31 2 0 2 2.032 2.031 I 2 0 0 3.740 3.740 2 2 I 2 1.961 1.960 41 I I I 3.571 3.573 54 3 2 I 1.907 1.906 I 2 I 0 3.345 3.343 50 4 0 0 1.870 1.870 5 2 0 I 2,960 2.961 54 4 I 0 1.814 1.814 12 2 I I 2.752 2.753 100 2 2 2 1,786 1,786 5 2 2 0 2.645 2,646 8 3 3 0 1.763 1.763 8 0 0 2 2.421 2.421 14 4 0 I 1.745 1.745 16 3 I 0 2.366 2,365 31 4 I I 1.699 1,699 16 2 2 I 2.321 2,321 4 3 I 2 1.692 1.692 27 I 0 2 2.303 2,305 2 4 2 0 1.673 1.673 7 3 0 I 2.217 2,216 5 3 3 I 1.657 1.657 24 3 I 1 2.126 2,124 10 0 0 3 1.614 1.614 3

7 . 7 0 0

7 . 6 0 0

7.5 O0

7.40C

7.30C

[ I I I I I I

I I 1 I 1.000 1.050 1.100 1.150 (,~)

c(~,)

5.100

500O

4,900

- z~.800

FIG. I

Variation of the cell dimensions a and c versus ionic radii.

914 Y. OCHI, et a]. Vol. 17, No. 7

unit) are approximately given by the relations,

a=1.995r + 5.374

c=1.154r + 3.623

where r is the effective ionic radius (A) of the rare earth ion. A typical Lanthanoid contraction is observed in the fig. I.

Fig. 2 shows plots of the average ionic radii (10) of the cations X with eightfold coordination versus those of the cations T with fourfold coordination in the melilite-type crystal(X2T307). Compounds I-7 were synthesized in the present study and compounds 8-15 were reported in ref. (6). Compounds 16:Ca2BeSi207, 17:Ca2Ga2Si07, 18:Ca2CoSi207, 19:Ca2ZnSi207, 20:Sr2AI2Si07, 21:Sr2CuSi207, 22:Sr2FeSi207, 23:Sr2MnSi207, 24: Y2BeSi207, 25:Sr2MgGe207, 26:Pb2ZnSi207, 27:Ba2FeSi207 were found in the ASTM cards. The plots of the present compounds locate in the region associated with the melilite structure, filling a blank area in the region. Melilite-type compounds with trivalent cations at X sites like in the present crystals are rare and Y2BeSi207 is the only known example.

The temperature dependence of the reciprocal magnetic suscep- tibility is shown in fig. 3. A field-and-gradient product of

1.40

1.30

1.20L

1.10

1.00

I I I I I I I I If I e~l I ~ ~ ~ - , ~ , , ~ , , o

Ba o o 27 28

26 Pb O

Sr O O O O O - 20 21 22 23 25

I @ Lu 8o - .oco

160 211 18 19 90 017 caPr ~rCx~ OO 14 ~I - - i ~ - . NdCo

I 0 0 15 O S r n I $ n ~ a 3@ 11 O °

5 2/. o 6 ~

_ 711 Y E r ~

I I I I I 0.25 0.30 0.35 0.40 0 4 5 (~,)

FIG. 2

Plots of ionic radii of X against T cations in melilite structure, X2T307. Black circles : present study.

Vol . 17, No. ? NEW RARE EARTH COMPOUNDS 915

TABLE 3

Effective magnetic moments of R2GeBe207

R (B.M.)

S L J G theor, obs.

Pr 1.0 5 4.0 0.8 3.58 3.9 Gd 3.5 0 3.5 2.0 7.94 7.2 Dy 2.5 5 7.5 1.33 10.65 10.4 Er 1.5 6 7.5 1.2 9.58 8.4

3 0 . 0

1/~ m

20.0.

10.0.

Gd2GeBezO ~

0.0 i , :

0 100 200 300 TEMPERATURE(,i)

I

I

FIG. 3

Temperature dependence of the reciprocal magnetic susceptibilities 1/Xm for the heavy rare earth compounds.

916 Y. OCHI, et al. Vol. 17, No. 7

3.85xi0 s Oe2/cm was applied. Sm2GeBe207, Gd2GeBe207, Dy2GeBe207, and Er2GeBe207 obey the Curie-Weiss law above room temperature. The effective magnetic moments were calculated from the Curie- Weiss equation and the theoretical ones were calculated from the following equations,

= g ~ ( B . M . )

g = 1 + J(g+l)+8(8+12j(j+1t - L ( L + l l

The results were listed in table 3. The observed moments of the heavy rare earth compounds were smaller than the theoretical values. There was a trend toward weak ferromagnetism, because I/×m seemed to cut across the positive temperature axis with decreasing temperature. It seems that the magnetic ordering occurrs at lower temperatures in the heavy rare earth compounds.

Acknowledgement

The authors wish to thank Assistant Professor T.Nakamura and Mr. Y.Takagi of Tokyo Institute of Technology for use of the magnetic balance and for discussions of the results. The com- putations were carried out on an M-180 computer at the Nagatsuta Branch of the Computer Center of Tokyo Institute of Technology.

References

I. R.W.Nurse and H.G.Midgley, J.Iron and Steel Inst., 174, 121 (1953).

2. B.E.Warren, Zeit. Krist., 74, 131(1930).

3. F.Raaz, Akad. Wiss. Wien, Ij39, 645(1930).

4. S.J.Louisnathan, Zeit. Krist., 130, 427(1969).

5. C.Brisi and F.Abbattista, Atti Accad. Sci. Torino, 95, 263 (1960).

6. A.Durifu and F.Forrat, Bull. Soc. Fran~. Miner. Crist., LXXXI, 107(1954).

7. C.J.Peng, R.L.Tsao and Z.R.Chou, Scientia Sinica, 11, 977 (1962).

8. S.F.Bartram, Acta Cryst,, B25, 791(1969).

9. T.Sakurai, Universal Program System for Crystallographic Computations, Cryst. Soc. Japan(1967).

10. R.D.Shannon, Acta Cryst., A32, 751(1976).