Mat. Res. Bull . , Vol. 19, pp. 1237-1243, 1984. Printed in the USA. 0025-5408/84 $3.00 + .00 Copyright (e) 1984 Pergamon Press Ltd.

A NEW FAMILY OF NIOBIUM BRONZES OF FORMULA Na. Sr NbO. : SYNTHESIS AND X-RAY CHARACTERIZATIO~ -x x J

Brian Ellis, Jean-Pierre Doumerc, Michel Pouchard and Paul Hagenmuller

Laboratoire de Chimie du Solide du C.N.R.S. Universit~ de Bordeaux I

351, cours de la Liberation, 33405 Talence Cedex, France.

(Received July 11, 1984; Communieated by P. Hagenmuller)

ABSTRACT The preparation of the niobium bronzes of formula Na. Sr NbO~ has been carried out in sealed nickel tu~e~ a~ I~00°C. Four phases, whose structures are closely related to that of perovskite, have been characterized as x increases : an orthorhombic pha- se, O, derived from NaNbO 3 (0 ~ x ~ 0.02), a mono- clinic phase, M, (0.03 ~ x ~ 0.06), a phase R, of rhombohedral symmetry (0.07 { x ~ 0.i0) and finally a tetragonal phase (0.15 ~ x ~ 0.60)

A metal-non metal transition is observed as the composition varies in certain families of transition metal oxides, such as for instance tungsten bronzes (1-2) or vanadium bronzes (3-4) in which Anderson localization seems to play an important role in the localization of carriers. As far as we know no extensive study of such a transition has been carried out in niobium bronzes.

Ward et al. (5) reported that the SrxNbO 3 compounds have a cubic perovskite type structure for 0.7 < x < 0.95. However below the lower value of x they obtained complex X-ray diffraction patterns and no phase could be precisely characteri- zed.

Since our purpose was to investigate in a ternary niobium oxide having a relatively simple structure, the evolution of the transport properties vs. the Nb-4-+/Nb ~÷ ratio, it seemed necessary to look for another system. We describe here the pre- paration and X-ray diffraction study of a new family of niobium bronzes of formula Nal_xSrxNbO 3.

1237

1238 B. ELLIS, et al. Vol. 19, No. 9

Experimental procedure

In order to avoid the difficulties generally encountered in using SrO, i.e. sensitivity to hydration and carbonation, a mixture of strontium niobates, corresponding to the global composition 5SrO:2Nb^O., was used instead as starting material.

• z D The appropriate mlxfure of strontium carbonate and niobium (V) oxide was heated in air at 1250°C for 12 hrs. Complete reaction was checked by controlling the loss of CO 2 by weighing.

The Nal_xSrxNbO 3 compounds were prepared "according either to :

+ x/5 Ms[5SrO:2Nb2051+ x/5 Nb ÷ ( l-x)NaNbO 3 Nal_xSrxNbO 3

or to :

10x Na0.90Sr0.10NbO 3 + (l-10x)NaNbO 3 ÷ Nal_xSrxNbO 3

for small x values (x < 0.i0).

Homogenized starting mixtures were pelletized and put into nickel tubes sealed under argon, which themselves were sealed in evacuated silica tubes and heated at 1300°C for 2 hours. At lower temperatures the reaction was invariably incomplete even after several repetitions of the procedure.

After quenching samples were characterized by X-ray dif- fraction using a Guinier-H~gg camera with Cu K~ 1 radiation.

Careful examination of the inner wall of the nickel tubes after each experiment showed them to remain bright which sug- gests that they are not attacked by the contents. The absence of nickel in the reaction products has been checked by X-fluo- rescence analysis .

Results

The color of the samples varied from white for the end member NaNbO 3 to light blue, blue and deep blue as x increases.

From X-ray diffraction patterns four phases can be identified and it appears clearly that their structures are derived from that of cubic perovskite with small distortions (Table I). The variation of the lattice parameters with x is given in Fig. i.

The various phases observed as x increases are the follo- wing :

a) For 0 ~ x ~ 0.02 an orthorhombic phase O isostructural with NaNbO3, whose structure was accurately determined by Sakowski- Cowley et al. (6). This structure is related to that of perovs- kite by a slight distortion of the NbO 6 octahedra and tilt an- gles between them.

The reduced lattice parameters, respectively equal to a0/ ~-~ b~/4, c0//~, have values close to the parameter a of corresponaing fdeal perovskite unit cell. They tend towards a common value which however is not attained (Fig. I).

Vol. 19, No. 9 NIOBIUM BRONZES 1239

TABLE 1

X-ray powder diffraction data of some Nal_xSrxNbO 3 phases.

x = 0.02 (grth.) x = 0.04 (monocl.) O

b = 5.561 A ; b = 15,568 ~ b = 3.919 $ ; b = 3.942 A c = 5,511 ~ c = 3.903 A ; S = 89.75 °

h k 1 dobs. dcalc" I

1 0 1 3,921 3.914 s 0 4 0 3.900 3,892 s 2 0 0 2.784 2.780 w 0 0 2 2.760 2.755 vs 2 0 2 1.956 1.957 m 0 8 0 1.947 1.946 m 2 4 2 1.750 1,749 m 1 8 1 1.741 J,743 m 3 4 1 1.601 1,602 m 2 8 0 1.595

i. 593 m 1 4 3 1,592 4 0 0 1.391 1.390 w 0 0 4 1.378 1.377 m 4 4 0 1.309 1.309 w 3 8 1 1.304 1,304 w 0 4 4 1.299 1,298 w 0 12 0 1.298 1.297 w 3 4 3 1.238 i.q37 w 1 12 1 1.232 1.231 w 4 4 2 1.!84 i~182 m

h k 1 dobs. dcalc " I

0 1 0 3,952 3.942 w 1 0 0 3,921 3~919 s 0 0 1 3~902 3,903 s 1 1 0 2.781 2,779 w/m 1 0 1 2.776 2.771 m/s 1 0 T 2.764 24759 s 0 2 0 1,973 1,971 vs 2 0 0 1,958 1 .960 m 0 0 2 1.949 1.951 m/s 1 2 0 1.760 1,761 w/m 2 0 ~ 1.749 1.748 m 1 0 ~ 1,745 1.744 m 1 1 2 1.601 1,600 m 1 1 2 1.595 1.595 m/s 2 0 2 1.395 1,386 w 2 0 2 i~380 1.380 m

x = 0,07 rhomb, cell hexag, c911

O

aR=5.528 ~, =60,26°,o aH=5.553 A,CH=I3,510 A)

x = 0.30 (tetrag.) O o

a = 3.945 A ; c = 3.960 A, c/a = ].004

h k 1 dobs. dcalc" I

0 1 2 3.922 3.917 s 1 1 0 2.781 2.777 vs 1 0 4 2.763 2,764 vs 0 2 4 1.958 1.959 m 1 2 2 i. 755 1.755 m 1 1 6 1.748 1.749 m 2 1 4 1.601 1.600 m 0 1 8 I. 594 1.594 m 2 2 0 1.388 i~388 w/m 2 0 8 1.379 1.382 w/m 0 3 6 1.304 1.306 w 1 3 4 1.238 1.240 w

h k 1 dobs. dcalc"

0 0 1 3~966 3 ~910 s 1 0 0 3.950 3.944 s 1 0 1 2,796 2.795 vs 1 1 0 2.790 2.789 vs 0 0 2 1.981 1.981 s 2 0 0 1.973 1,972 s 1 0 2 1,771 1.770 m 2 1 0 1,765 1 .765 m 1 1 2 1.615 1.615 m 2 1 1 1.512 1 .611 m 2 0 2 1.397 1.398 m/w 2 2 0 1.394 1 .395 m 2 1 2 1 .317 1 .317 w 2 2 1 1 .315 1 .315 w 1 0 3 1 ,252 1 .252 w 3 1 0 1 .248 1 .247 w

1240 B. ELLIS, et al. Vo]. 19, No. 9

lattice parameters

(i) Io M

3.q6~-

3.94

3.92 ,~

3.9C Y

3.88

(o) 90.0,~ 89.51- ," 60,5i I t 60.0! I

0

I RI i

i I i i I"

i t i I I I

I I i i i i 0.10 020 0.30 0.40 050 0~0 x"

bM

b0

\ "-.j,

P_@

FIG. 1 Variation with x of the lattice

parameters of Nal_xSrxNbO 3.

FIG. 2 Relations between the unit cell of NaNbO3(a0,b0,c0) , its mono- clinic pseudocell(aM,bM,CM, B) and the'rhombohedral c e l l ( a R , a )

b) For 0.03 ~ x ~ 0.06 a monoclinic phase.X-ray diffraction patterns at first sight are similar to those of the O phase. However a few additional lines could not be indexed using the previous orthorhombic unit cell, and only a monoclinic cell could account for their presence. Table I gives the indexation for x = 0.04 with lattice parameters which are close to that of the corresponding ideal perovskite cell (a = 3.919, b = 3.942, c = 3.903 A, B = 89.75°). Only a complete determination of the structure on a single crystal could ascertain the actual symmetry of this phase. Indeed the unit cell chosen here is derived from the monoclinic smaller pseudocell of NaNbO3, in which the corresponding pseudoparameters a M and c M are no longer equal (Fig. 2).

c) For 0.07 $ x ~ 0.i0 a rhombohedral perovskite-type phase R. X-ray powder diffraction data do not allow us to distinguish clearly between the possible space groups : R3, R3m, R3c or R3c.

Vol. 19, No. 9 NIOBIUM BRONZES 1241

d) For 0.15 ,~ x ~ 0.60 a tetragonal phase whose Guinier photo- graphs suggest a tetragonal BaTiO3-type structure. The c/a ra- tio, equal to 1.005 for x = 0.15, decreases slightly with in- creasing x (Fig. i).

However for x > 0.60 instead of the expected cubic phase, complex X-ray diffraction patterns are obtained. They correspond apparently to a mixture of different phases which have not been identified.

Discussion

The evolution vs. x of the reduced unit cell vo- lume (cell volume/Z is given in Fig. 3. The ten- dency observed for the cell volume to increase with x can be attributed to two simultaneous effects :

- ~(+i)isthe ionic radius of Sr larger than that of Na by about 10%, regard- less of coordination number (8),

- (ii) the decrease of the effective charge of niobium atoms as sodium is substi- tuted by strontium, which can be formally schematized by :

Na++Nb 5+ ÷ Sr2++Nb 4+,

should give rise to an increase of the averaged ionic radii of the niobium atoms.

R,~duc,d unil cell volume (,~3)

OIMIR L T 64.0

63.0

6 2 0

61.0

60.0

59.0

4/ I i I i i I

o.1o o.~o o.~o o.'4o o~ o~o , '

FIG. 3

Variation with x of the reduced unit cell volume (V/Z) for Nal-xSrxNbO 3

In the NaNbO, lattice, niobium atoms are shifted from the center of nearly ~egular NbO~ octahedra in the ~i0~ direction. The corner-linked octahedra ~re tilted around the [i00~ and [0101 axes by angles ¢ and ~ respectively (6) (Fig. 2).Using the description of the NaNbOq structure given in ref. 6 the following relations can be derived (7) :

a = 2r cos ~ (i) a b = 4h cos # (2)

c = 2r cos wcos ~ (3) c

where r and r are the edge length of the octahedra in directions close t~ the c a0 and C 0 axes respectively ; h is the heiqht of the octahedra in a direction near the b 0 axis.

1242 B . ELLIS, e t al . Vol . 19, No. 9

Assuming that the octahedron size does not vary greatly as x increases from x = 0 to 0.02, the variation of the a and b lattice parameters given in Fig. 1 could result, according to equations (i) and (2), from a simultaneous increase of ~ and decrease of 9.

In differentiating equations 1-3 we may write : dc/c = da/a + db/b (assuming ra, h and r c to be constant).

The observed change of c with x is in qualitative agreement with that predicted from our simple hypothesis. However the magnitude of the variation of c is only 50% of that expected, which is not surprising since we have neglected the slight variation of the octahedron shape and size which should accompa- ny the change in the ~ and ~ values.

A discontinuous increase in the reduced unit cell volume is observed between x = 0.02 and 0.03 due mainly to a significant increase between the normalized b0/4 parameter of the 0 phase and the b. parameter of the M phase. This could result from the fact thatM the tilt angle ~ vanishes in the M phase. Assuming that the NbO~ octahedron keeps the same height (h) in both 0 and M phases, th~ value ~f b. expected from the value of b 0 and ~ in NaNbO~ (b 0 = 15.520 A an~ ~ = 9.5 ° (6)) should be using equation (2) a~out 3.934 A. This result is in relatively good agreement with the value observed for b M at x = 0.03 (b M = 3.945 A).

In the rhombohedral N-NaNbO 3 phase (space group R3c) which is stable below -150°C nearly regular NbO. octahedra tilt around the ternary axis (9) giving cH/a ~ rati~ >V~. The rela- tionship between the structure of the R ~hase obtained for 0.07 ~ x ~ 0.10 and that of N-NaNbO~ is not obvious since for R we find cH/a H ~ ~. It is impossibl~ from our X-ray powder dif- fraction ~a[a to determine whether the rhombohedral distortion results only from displacements of the Nb atoms or from both Nb displacements and tilting of octahedra.

Darlington and Megaw have suggested that in alkali nio- bates ANbO 3 the shift of the Nb atoms from the center of the octahedra is rather an intrinsic property of the NbO 6 octahedra themselves, whereas the tilting is mainly related to the nature of the A atoms and decreases with increasing A size (9). This model leads us to expect a decrease of tilt angles as the subs- titution rate of sodium by strontium increases, and could ex- plain why the structure of the tetragonal phase (0.15~x~0.60) seems to be related to that of the tetragonal form of BaTiO~ in which the distortion is only due to the shift of Ti atoms @rom the center of TiO 6 octahedra.

References

i. J.P. Doumerc, P. Dordor, E. Marquestaut, M. Pouchard and P. Hagenmuller, Phil. Mag. B4_~2, 487 (1980).

2. P. Dordor, J.P. Doumerc and G. Villeneuve, Phil. Mag. B47, 315 (1983).

3. P. Dougier and A. Casalot, J. Solid State Chem., 2, 396 (1970).

NIOBIUM BRONZES 1243 Vol. 19, No. 9

4. G. Villeneuve, H. Kessler and J.P. Chaminade, Journal de Physique, Colloque C4, 37, C4-79 (1976).

5. D. Ridgley and R. Ward, J. Am. Chem. Soc., 77, 6132 (1955).

6. A.C. Sakowski-Cowley, K. Lukaszewicz and H.D. Megaw, Acta Cryst. B25, 851 (1969).

7. Y. Miyamoto and S. Kume, J.P. 'Doumerc and P. Hagenmuller, Mat. Res. Bull., 18, 1463 (1983).

8. R.D. Shannon and C.T. Prewitt, Acta Cryst. B25, 925 (1969).

9. C.N.W. Darlington and H.D. Megaw, Acta Cryst. B29, 2171 (1973).