E.P.R. study of copper fluosilicate tetrahydrate and copper fluostannate tetrahydrate

Pal, A. K. 1968

Physica 39 387-392

E.P.R. STUDY OF COPPER FLUOSILICATE TETRAHYDRATE AND COPPER FLUOSTANNATE TETRAHYDRATE by A. K. PAL Indian Association for the Cultivation of Science, Calcutta-32, India

Synopsis The paramagnetism of single crystals of copper fluosilicate tetrahydrate and copper fluostannate tetrahydrate has been investigated using the e.p.r. method for the K band both at 300°K and at 90°K. The copper (II) complex in both crystals has approximate tetragonal symmetry as expected from X-ray structural data of isomorphous copper fluotitanate tetrahydrate. In the case of copper fluosilicate tetrahydrate elaborate measurements of the line width for different planes have been carried out. This study indicates the necessity of taking into account the exchange interaction and the finestructure interaction, having their origin in hyperfine and crystal fields, in order to interpret the observed results.

1. Introchction. Preliminary e.p.r. data on copper fluosilicate tetrahydrate for the X band at room temperature have been reported by Bleaney and Ingraml) and by Y o kozawa2), who supposed that there was only one type of paramagnetic ion in the unit cell. Their experiments, however, were not decisive enough to rule out the possibility of exchange interaction masking identical reactions of dissimilar ions, if present. Recently a preliminary study on the X-ray structure 3) of CuSiFe .4&O and CuSnFs *4HsO as well as a more extensive paper on the structure 4) of CuTiF -4HsO indicate the presence of two types of ions in the unit cell. The present e.p.r. investigations have been undertaken in the 1.2 cm microwave region in order to search for resolved signals of the two ions. Also, an exhaustive study of line width in different planes of the crystal Cu.SiFe.4HsO at 300°K and at 90°K has been carried out in order to separate the different contributions to the line width. 2. Crystallography and preparation of single crystals. Both CuSiF6‘4HsO and Cu SnFs .4HsOs) 4) are isomorphous. There are two formula units in the unit cell. It is monoclinic with the space group P2i/c. The Cu(I1) ion is octahedrally coordinated with four water molecules and two fluorine ions; the distances are: Cu-Or = 1.95 A, Cu-Orr = 1.97 A and Cu-Fr = 2.31 A in case of CuTiFs *4HsO. This implies approximate tetragonal ionic symmetry -

387 -

388 ~.

A. K. P4L

about the copper-fluorine CuSiFs.4HsO: CuSnFs.4HsO: The salts

axis. The dimensions of the unit cells are as follows :

a = 5.315 A, b = 9.640 A, c = 7.184 A, /3’= a = 5.598 A, b = 9.950 A, c = 7.638 A, p’ =

were prepared

from analytical

methodss). The first crop of crystals, solutions of the salts, was recrystallised

grade

chemicals

105”38’; 103”46’.

by standard

obtained from saturated aqueous two or three times in a dust-free

chamber. The final crystallisation was done in a dust-free and vibrationproof chamber by a moderately slow evaporation of a nearly saturated accidified solution. In the case of copper fluosilicate tetrahydrate, the temperature of crystallisation is maintained somewhat above 30°C. The best crystals were chosen using a magnifying glass and subsequently examined under a polarising microscope. Only those crystals which gave sharp extinction and were free from occlusions, flaw and twinning were accepted for the measurements. A 1.2 cm e.p.r. transmission-type spectrometer, oper3. Experimental. ating at liquid-oxygen temperature, set up in this laboratorye), has been employed for the present investigations. In a given plane g-values were measured along different directions at intervals of 10” and at intervals of 5” near gmax and gmin directions. The planes chosen for g-measurements are (i) b plane containing gi and gz, (ii) plane containing b axis (ga) and with g mBx (gr) in b plane and (iii) c plane containing gs and g,, where gi, g2 and gs are the principal crystalline g tensors. Detailed line-width studies have been restricted only in the case of CuSiFs.4HsO to two planes: b plane and c plane. (a) g-values. Only one signal could be re4. Results and Discussions. solved in a arbitrary direction in the crystal, contrary to expectations from X-ray results. For observing two resolved signals the gigs-plane seems most favourable. We therefore first determined the gi direction in the b plane

Fig.

I

E.P.R.

STUDY

OF HYDRATED

COPPER

FLUOSILICATE

AND

FLUOSTANNATE

389

and then investigated the variation of the g-value in the gigs plane, see also fig. 1. But even there no indication for two signals could be found. This cannot be due to dipolar broadening since the line is quite narrow even at room temperature, and also not to spin-lattice interaction, since the line width changed very slightly when the temperature waslowered to 90°K. Also, the instrument is quite sensitive with its phase-sensitive detection system, which enables us to find g-values along any direction correct to f 0.002. It is then possible that the two signals can not be resolved due to exchange interaction. Assuming tetragonal symmetry of the ion from the measurement in the b plane and the gigs plane, the g-values along and perpendicular to the tetragonal axis Gil and G,, respectively and the angle between the two tetragonal axes, 24 can be calculated to be:

GII

G,

24

CuSiFa .4HsO

2.403

2.082

19.6”

CuSnFs - 4HsO

2.431

2.091

18.5”

The other alternative assumption that the tetragonal axes lie in the gsgs plane (gs is the gmin direction in the b plane), so that the necessary condition G, > G/l is untenable, since on that assumption G/l is found to be less than the free-spin g-value 2 which is unreasonable from spin-lattice relaxation time considerations and the value of 24 obtained is approximately 90”, which is in direct contradiction with the calculated value of 24 from X-ray data4) of isomorphous CuTiFs *4&o ; 24 = 25.7”. The assumed tetragonal symmetry of the ion is found to be a good approximation for our present purpose, since the observed mean g-values in the c plane and the gigs plane are very close to the calculated values. The orientation of one of the tetragonal axes obtained from our measurements in the case of CUSiFe.4HzO and that calculated from X-ray data on CuTiFs. 4Hzo dso correspond fairly.

(b) Line width. The observed narrow-peaked lines indicate the presence of strong exchange interaction. The line shape is found to remain unaltered in all directions of the crystal. A change in line width with the temperature is inappreciable, so there is very little contribution to it from spin-lattice relaxation. The mean square moments due to dipolar interactions, HE, have been

4. K. PAL

390 calculated

using Van Vleck’s

where the first term

well-known

represents

formula 7)

the contribution

to the broadening

from

dipolar interactions between similar ions and the second term represents that from dissimilar ions; the primed quantities in the second term correspond to an ion dissimilar to those implied in the first term. In those calculations, ions lying at distances greater than 11 A have not been considered, since their contributions towards Hs are negligibly small. The line width estimated from dipolar mean square moments assuming a Gaussian line shape is very large compared to observed values (AH should vary from 700 gauss to 268 gauss in the b plane and 695 gauss to 175 gauss in the c plane) which reveals the presence of exchange interactions. So, Anderson and Weiss’s equations) for line width based on the assumption of isotropic exchange interaction and strictly applicable to simple cubic lattices, is employed to inspect whether calculated line widths and their variations are in agreement with the experimental results : AH

=

2.

(‘_“!_3) -_ H:

(1)

Hex

4 01

I

0" o

I

36(C)

I

68

I

(9,)

I

98

I

120°

,

interactions.

n

: theoretical

points

including

dipolar,

exchange

1

I

((32) moo

Fig. 2. AH us. orientation in b plane in CuSiF6.4HzO. points; v : theoretical points including dipolar

: Experimental

interactions;

I

I

(a) 190”

and

2oo”

exchange

and fine-structure

E.P.R.

STUDY

OF HYDRATED

COPPER FLUOSILICATE

AND

FLUOSTANNATE

391

where the exchange field He, is given by = [2.83S(S + l)]’ g,

H,,

(2)

with the usual notation. Using the experimental value of AH along g2 the value of the exchange integral J is found to be about 0.2 cm-r. Subsequent computation of AH along other directions in the b plane, see fig. 2, reveals that though, in general, there is agreement between theoretical and experimental curves as regards variation of line width, there remain two main disparities: (i) observed and calculated line widths are different in most of the directions and (ii) the sequences of two unequal minima are just reverse. This might be attributable to neglecting a fine structure interaction, originating from hyperfine and crystal fields. To the dipolar (10/3) Hi term in the Anderson and Weiss’s expression for line width, the fine structure contribution Hi has to be added where Hi is related to the angle 8, between the tetragonal axis and the magnetic field direction by the equation. H;

= k( 1 + 3 co&)

(3)

where k is a constant. This expression has been derived by Pryce empirically from results of Penroseg). This affects the ultimate results. Best fit with the experimental curve is obtained with k = 1.5 x 104.The value of J has been calculated to be 0.24 cm-r.

I

I

30°

I

900

6o”

I

I

l2O0

I

Iso

Boo

0”

Fig. 3. AH vs. orientation in c plane in CuSiFs.4HsO. o:

Experimental

points;

v:

theoretical points considering

dipolar

and exchange

interactions ; A : theoretical points including dipolar, exchange and fine-structure interactions.

392

E.P.R.

STUDY

OF HYDRATED

considering next that though the

When

inferred interaction

COPPER

the curve theoretical

FLUOSILICATE

AND

FLUOSTANNATE

for the c plane, see fig. 3, it can be line width, including fine structure

is closer to the experimental

one, than the theoretical

line width

when fine structure is not taken into account, the agreement is not so good as for the b plane (fig. 2). This could be expected, since in the b plane two ions in the unit cell are similar as regards their g-values but in the c plane two ions are dissimilar,

except

along two specific directions,

namely

the a and

b axes, and so, broadening due to exchange interactions betweenFdissimilar ions can not be ruled out la). This might be the reason that some of the experimental points lie above the theoretical points. 5. Conclusions.

(i) The

ionic

symmetry

in

both

CuSiFs -4HsO

and

CuSnF6*4HsO is tetragonal in good approximation, and G,, is greater than G, as is expected from X-ray data of CuTiFa.4HsO. (ii) The angles between two tetragonal axes in two crystals, according to e.p.r., are close to that in the case of isomorphous CuTiFs.4Hs0, calculated from X-ray structural data. (iii) Contributions towards line width due to spin-lattice interaction are inappreciable. (iv) Inclusion of exchange interaction together with unresolved fine structure satisfactorily explains the salient features of line width, its magnitude and angular dependence. Acknowledgements. The author is greatly indebted to Prof. A. Bose, D. SC., F.N.I. for his valuable advice and constant encouragement throughout this investigation. He is thankful to his colleagues Sri S. Ray, Dr. U. S. Ghosh and Dr. S. K. Du t t a for their helpful discussions. Received

8-l-68

REFERENCES

1) 2) 3) 4) 5)

Bleaney, Yokozawa, De Clan, Fischer, Mellor,

B. and Ingram,

D. J. E., Proc.

Y., Monogr.

Ser. Res. Inst. Appl.

A., Fischer,

J. and Weiss,

Phys.

Sm. 63 (1950)

Electr.

Hokkardo

Ii., Bull. Sot. Chem. France

408. Univ.

No. 4 (1954)

J., Keib, G. and Weiss, R., Acta Cryst. 22 (1967) 338. J. W., A Comprehensive Treatise on inorganic and theoretical

Chemistry

(Longmans,

Green and Co., London.) Vol. VI (1925) 950; Vol. VII (1925) 423. Ghosh, U. S., Bagchi, R. N. and Pal, A. Ii., Ind. J. Phys. 38 (1964) 361.

6) J. H., Phys. Rev. 74 (1948) 1168. 7) Van Vleck, P. W. and Weiss, P. R., Rev. mod. Phys. 25 (1953) 269. 8) Anderson, B., Penrose, R. P. and Plumpton, B. I., Proc. Roy. Sot. A198 9) Bleaney, ‘0)

Garter,

C. J. and Van

Vleck,

J. H., Phys.

Rev. 72 (1947)

1138.

95.

No. 8 (1966) 2646.

(1949) 406.