The crystal and magnetic structures of Mn2GeO4

Solid State Communications,

Vol. 8, PP. 1183—1188, 1970.

Pergamon Press.

THE CRYSTAL AND MAGNETIC STRUCTURES OF Mn

Printed in Great Britain

2GeO4

J.G. Creer and G.J.F. Troup Physics Department, Monash University, Clayton, Victoria, 3168, Australia. (Received 22 May 1970 by E.F. Bertaut)

The crystal and magnetic structures of Mn2GeO4, an isomorph of 5~85I~B and coefficients 0 = 162°K. for Neutron olivine, are reported. Curie—Weiss law the paramagnetic region diffraction experiments are Peffindicate = a complex spin structure at 4.2°K.

MANGANESE orthogermanate, (Mn isomorphus with olivine, the hexagonal 2 Ge04), close is packed analogue to spinel. Compounds related to Mn 2GeO4 include Mn2GeS4, Fe2SiO4, Mn2SiS4 Co2 Si04 and Cr2BeO4. The results of susceptibility, X-ray and neutron diffraction experiments on a polycrystalline sample of manganese germanate are reported in this communication,

3 This film was indexing indexed of a on powder an orthorhombic diffraction unit film. cell, with basis vectors of length a = 10,70 A, b = 6.26 A and c = 5.04 A. X-ray diffraction spectra recorded o’i a Phillips diffractometer with filtered CuKa radiation confirmed these cell dimensions. However, when intensities were compared with the earlier work, there was considerable disagreement.

SAMPLE PREPARATION AND ANALYSIS The samples used in this work were synthesized from intimate stoichiometric mixtures of MnCO3 Ge02. The mixture was compressed into pellets and heated under vacuum to guard against 2~ions. At about 550°C oxidization of the Mn the carbonate decomposes and the temperature was then raised to 1100°Cwhere it was held for 48 hr. When ground, the end product of the reaction was a pale grey-brown colour. The manganese content of the compound was determined by complexometric titration and the germanium content by atomic absorption spectroscopy. The results were within 0.5 per cent of the theoretical values for Mn 2GeO4. CRYSTAL STRUCTURE Unlike Mn2 GeS4, the crystal and magnetic 2 the structures of which have only data published on Mnbeen reported,” 2 Ge04 has been the measurement of unit cell parameters and the

In the olivine structure, space group Pnma, and in the previously mentioned compounds in particular, the transition metal ions occupy distorted octahedral sites. Half are located on inversion sites and theposition other sites mirror plane symmetry. The of thehave inversion sites are 1

1

4(a) 0, 0, 0; 0, ~, 0; ~, ~, ~ ~, 0, ~. The mirror sites are specified by two parameters x and z as 1

4(c) ±(x,

~

1 ,

1

~

+

x, ~

—

z)

The germanium and two sets, each of four oxygen .

anions, also occupy 4(c) sites. The remaining eight oxygen ions are at 8(d) sites, given by the co-ordinates 8(d) ±(x,y, z;

+

x,

—

y;

—

z; .1

—

X, 2 — )‘, Z, 2 + X, Y, 2 Z In all, eleven positional parameters are needed

1183

THE CRYSTAL AND MAGNETIC STRUCTURES OF Mn2 Ge04

1184

Vol. 8, No. 15

Table 1. Calculated and observed intensities for X-ray, neutron—nuclear and neutron—magnetic diffraction spectra. Magnetic intensities calculated assuming spin values as in text. no = not observed. X-ray

Neutron 20 deg.

Nuclear hkl

d

!/I~

1/10

1/10

(A)

(calc)

(obs)

(calc)

(obs)

0 0 0 13 0 3 44 15

no no no 15 no 2 42 12 17

0 0 0 5 0 54 49~ 125

no no no 3 no 46

100 010 110 200 001 101 210 011

10.700 6.260 5.403 5.350 5.040

111

3.686

61

201 300

3.669 3.567

0

211

3.165

ii

no 19

020 310 120 301

3.130 3.099 3.004 2.911

195 0 0 56

no no 60

0 0 10 0 0 0

220

2.702

12

13

55

400 021 311 121

2.675 2.659 2.640 2.580

0 0 94 101

no no 94 100

51 0 2 01

002 410 102 221 401 320 012 112 202 411 212 500 321 030 302

2.520 2.460 2.453 2.381 2.363 2.353 2.338 2.284 2.280 2.211 2.142 2.140 2.132 2.087 2.058

17 171 65 5 3 0 0 15~

20 27

311 01

130 420 510

2.048 2.034 2.025

0 7 0

501 022

1.970 1.963

111

312

1.955

4.560 4.067 3.926

71

oj 3 2 0

2 0 4

1/1~

62

311 35

0 0

14 3 2

18 0 2 2

no no no

0 0 0

1.07 A

1/1~

1/1~

(calc)

(obs)

5.7

33

29

9.8 11.3 11.4 12.1 13.4 15.0 15.6 16.6

0 19 0 85 0 0 0 0

no 118 18 no no no no 16

3

17.2 19.4 19.6 19.8 20.4 21.1

17 3 0 10 1 0

22.7

0

23.0 23.1 23.3 23.8

0 0 0 0

24.4 25.0 25.1 25.8 26.0

0 0 2 15 14

no no

26.2 26.3

0 0

17

27.0 27.0 27.9

11 0 4

28.8 28.8 28.9 29.6

0 0 0

8

—

—

—

—

no no no

100 40

12

54~ 261

no no

=

16.7 no

71 6 5

A

Magnetic

no no no no no

4

3

no 7 no

0 2 0

30.0

no

30.1 30.4 30.5

35

12

6 1

11

31.4 31.5

1

no

2

5

31.6

14

no

no no no no

28

•

11

—

—

—

—

—

—

—

—

—

—

Vol. 8, No. 15

CRYSTAL AND MAGNETIC STRUCTURES OF Mn

2GeO4

X-ray

Neutron Nuclear

hkl

230 122 031 131 421 511 222 402

ci (A)

1/1~ (caic)

1.944 1.931 1.928 1.897 1.886 1.879 1.843 1.834

0 1 0 2 0 3 31\ 115

1185

(obs)

(caic)

no no no no no 2 40

0 0 2 4 0 0 0 0

20 deg.

1/1~ (obs) no no no

A = 1.07

A

31.8 32.0 32.1 32.6 32.8 32.9 33.6 33.8

13 no no

Magnetic 1/1~ (caic)

1/1~ (obs)

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

Table 2. Positional parameters for three members of the olivine series. Atom

Site

Parameter

Mn Fe

4(c)

Ge Si

4(c)

x z x z

O 5 O S

4 (c) 4f ~

0, 5

8(d)

Mn2GeO4

Mn2GeS4

Fe2SiO4

00.275(±2) —O.002(±2) 00.095(±1) 00.438(±4)

00.266 00.004 00.098 00.411

00.280 —0.013 00.098 00.433

z x x z

00.088(±5) 00.770(±5) 00.420(±5) 00.214(±2)

00.768 00.095 00.434 00.243

00.092 00.769 00.455 00.209

x

00.156(±5)

00.178

00.1645

y z

00.046(±4)

00.014

00.038

00.291(±5)

00.257

00.287

to specify the positions of the four formula weights per unit cell,

for X-ray and neutron nuclear diffraction are 0.11 and 0.10 respectively.

From the diffraction data, part of which is shown in Table 1, the 42 non-forbidden reflections with d-values greater than 1.690 A were 4used intoa Due modified least-squares fitting the relatively small amount of procedure. experimental data, (as compared with single crystal structure refinements), no temperature factors were included in the fitting procedure. Neutron nuclear diffraction data (Table 1) was also fitted using the same modified technique. The results from both types of scattering were combined and the final values of the positional parameters are shown in Table 2, together with those of Mn 2GeS4 and Fe2SiO4. Probable errors are shown in brackets. The residual, defined as

SUSCEPTIBILITY MEASUREMENTS

V

F

0

F —

F0

The magnetic susceptibility Mn2GeO4 was measured with a vibrating sampleofmagnetometer from liquid helium temperature to room temperature, in a field of 12 Kg. When plotted as a function of temperature, the reciprocal susceptibility, Fig. 1, shows a local minima at (24 ±2)°K. For temperatures above 60°K,the susceptibility shows typical Curie—Weiss law behaviour, with Peff (5.S5±O.O6)/i~ and 0 = (162 ±5)°K. Assuming g values of 2.0 for the d5 Mn2~ions, the value of atomic spin is 2.48 ±0.03, in good agreement with the spin only value of 2.5. Between 24°Kand about 60°K,there is a significant deviation from the expected behaviour. For most antiferromagnets, as the Néel point is approached from above, the values of the measured susceptibility are less than that

1186

THE CRYSTAL AND MAGNETIC STRUCTURES OF Mn

2GeO4

Vol. 8, No. 15

1,x,,~-’ g/emu 14

1•

08

06

04 I—I

I

~

I

I

I

I

I (K)

FIG. 1. The reciprocal magnetic susceptibility of Mn 2GeO4 plotted as a function of temperature. expected from a Curie—Weiss law fitted at high temperatures. This is due to the onset of short range ordering. In Mn2GeO4 this variation is in the opposite direction, and may be due to a different type of long range order to that occuring below 24°K. This behaviour is similar to 5 that where which has been found to exist in Mn2 Si04, three distinct magnetic regions were established by neutron diffraction experiments.

— .

To confirm these observations, the E.P.R. spectra of function of the temperature. Mn2~ions The was intensity studied asof athe signal decreased in the range 70°K— 30°K, falling to half power at approximately 55°K. There was a small signal remaining at 20°K, possibly due to superparamagnetic effects in the fine powder sample. Below the Néel point, 24°K, the susceptibility is not that typical of an ideal antiferromagnet. No large field dependence of magnetization was observed below the Néel point, MAGNETIC NEUTRON SCATTERING Neutron 4.2°K diffraction recorded at 77°Kand with spectra neutronswere of wavelength 1.083 A. The magnetic difference pattern is shown in Fig. 2. The peaks marked by Miller indices are indexed on the chemical unit cell.

S AI~4.E 28

FIG. 2. Neutron diffraction difference pattern for Mn 2GeO4. Notation given in text. The remaining small peaks, marked by the letter S, could not be indexed on this unit cell, nor could they be fitted when the magnetic cell was increased in all directions by factors of two and three. Returning to the indexed peaks, all possible theoretical intensities were calculated 6 the aid of Bertaut’s with a propagation vector representation of k = (0, 0, 0).theory From the neutron diffraction investigations of Mn 2SiO4, Mn2GeS4 and other similar olivines, it

Vol. 8, No. 15.

THE CRYSTAL AND MAGNETIC STRUCTURES OF Mn

2GeO4

seems that the assumption of coupling between the two types (mirror and inversion) of manganese ion sites is reasonable. The irreducable representations of the space group2’6 Pnma for orthok = (0, 0, 0) In the are given in the literature. thiogermanate and orthosilicate of manganese the representation is of the F, 2’5 For the 0 on type. type, the only mode allowed the second sublattice is ferromagnetic. This representation is rules out because of the susceptibility results showing no ferromagnetic effects below the ordering temperature. Intensities for the three remaining representations were calculated, varying the relative strengths of the constituent modes. The F~ representation predicts a large (111)

1187

not be completed. By trial and error, assuming that there are two propagation ±q,which describe the spiral, nine sets vectors, of q vectors were found which predict peaks in the observed positions. These vectors, ±q,are the Fourier components of the spiral defined by S,~%. = ~ A (q) exp (iq.R ,~,) where S~,is the spin of the v th atom in the n th unit cell, position vector R ,,~,. Sets of A(q), of magnitude 4’0/~B’ which describe a spiral in the a—c plane and also alying fan arrangement with the varying components in the a—c plane

reflection and the F 20 representation predicts no intensity for the (100) reflection. These are contrary to the observed spectrum. The best fit was calculated for the F,,, representation with the

were tried with each of the nine sets of q vectors.

magnitude of the spin of the first and second sublattices being 1.50 ±0.05 and 2.45 ±0.05 respectively. The vector components belonging to F,~,which have these magnitudes are both C0 modes. This is similar to Mn2GeS4, but in this compound the spin of the manganese ions on both sublattices is 2.36. The reduction from the spin only value is attributed to covalency effects. A reduction of 3.3 case of MnO.7 For per Mn cent was reported in the 2GeO4 the large (40%) reduction for the first sublattice cannot be attributed to this cause. The decrease is probably due to some spiralling component of spin of the first sublattice. The small non-indexed peaks in Fig. 2. would then be due to this spiral. Also, some of the disagreement in the magnetic scattering in Table 1 may be credited to the spiral. Because of the loss of information about scattering vectors in reciprocal space when polycrystelline samples are investigated, interpretation of satellite peaks is difficult, and in this work could

CONCLUSION

None of the intensities calculated for these spirals agreed with the observed intensities.

The results reported above indicate a complicated spin arrangement in Mn2 GeO4 at low temperatures. Because of the lack of a single crystal and a variable temperature neutron diffraction crystat this ordering could not be investigated further. Spin arrangements in olivine~ 8 Mn 2), range from simple collinear (Co2SiO4, 2GeS4, canted (Fe2SiO4, Mn2SiO4 5) to complicated 9,10 spirals in Mn2GeO4 and Cr2BeO4. Susceptibility measurements indicate a Néel temperature of (24 ±2)°K, with the possibility of another ordering temperature at approximately 60°X. Acknowledgements — We would like to thank Professor R. Street of this Department for helpful advice. The project was supported by a grant from the Australian Institute for Nuclear Science and Engineering. One of us (J.G.C.) acknowledges a Commonwealth Postgraduate Award.

REFERENCES

J., Bull.

1.

HARDY A., PEREZ G. and SERMENT

Soc. Chim. France 2638 (1965).

2.

TRANQUI DUC, VINCENT H., BERTAUT E.F. and VU VAN QUI, Solid State Commun. 7, 641 (1969).

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DURIF-VARAMBON A., Thesis Grenoble (1958).

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PRICE D.C., to be published.

5.

SANTORO R.P., NEWNHAM R.E. and NOMURA S., J. Phys. Chem. Solids 27, 655 (1966).

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THE CRYSTAL AND MAGNETIC STRUCTURES OF Mn

2GeO4

Vol. 8, No. 15

6.

BERTAUT E.F., Magnetism (edited by RADO and SUHL) Vol.111 p. 150 Academic Press (1963).

7.

NATHANS R., WILL G., and COX D.E., Proc.

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NOMURA S., SANTORO R., FANG

9.

CQX D.E., FRAZER B.C., NEWNHAM R.E. and SANTORO R.P.J., J. app!. Phys. 40, 1124 (1969).

J.

mt.

Conf. Magnetism Nottingham p. 327 (1964). and NEWNHAM R., J. Phys. Chem. Solids 25, 901 (1964).

10. ELLISTON P.R. and TROUP G.J., Proc. Phys. Soc. (Lond.) 92, 1040 (1967).

Les structures cristalline et magnétique de Mn2GeO4, isomorphe de l’olivine, sont rapportées. Les coefficients585/JR de la loi et de 0 =Curie—Weiss 162°K. Diffraction neutronique est ~ 4.2°K, indicative cPune structure pour la region paramagnétique sont Pe(f =

magnétique complexe.