FLUORITE-RELATED PHASES IN RARE-EARTH OXIDE-TUNGSTEN TRIOXIDE SYSTEMS

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FLUORITE-RELATED PHASES IN RARE-EARTH

OXIDE-TUNGSTEN TRIOXIDE SYSTEMS

E. Summerville, J. Drennan, D. Bevan

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C7, supplkment au no 12, Tome 38, dkcembre 1977, page C7-73

FLUORITE-RELATED PHASES

IN

RAREEARTH OXIDETUNGSTEN

TRIOXIDE SYSTEMS

E. SUMMERVILLE, J. DRENNAN and D. J. M. BEVAN The Flinders University of South Australia, Bedford Park, South Australia

Resume. - Une etude nouvelle des Cquilibres de phases dans les systemes WO, - R 2 0 3

(R = Sm, Er : 1,7 < O/(W

+

R) < 2,O) montre qu'il n'y existe pas une phase Sm,WO12, qui avait Cte rapportee prtalablement avoir une maille pseudo-quadratique. Au lieu de Sm,WOl2 nous avons constate que la phase S m l o W 2 0 , , se produit. La diffraction electronique a mis en evidence le rapport entre la maille vraie et la sous-maille de type fluorine. La veritable symetrie des composCs R,,W202, n'est pas quadratique ; elle est probablement triclinique. Les rbultats de diffraction electronique pour les phases de type Rl4W4O,, )) sont complexes ; ils suggbrent qu'il y a une serie de composes avec les structures infinimenf aduptables, qui sont basees sur une sous-rnaille trigonale et presentent aussi unc similitude structurale avec la structure de type fluorine. Abstract. - New results on phase re1ationsh.ip.s and structures in the W 0 , - R 2 0 3 systems (R = Sm, Er) are presented for the region 1.7 < O/(W

+

R) < 2.0. The previously reported phase Sm,WO12, with a pseudotetragonal cell, was found not t o exist; instead, the previously unreported phase Sml0W2O2, was found. Electron diffraction studies have revealed the relationship of the supercell to the fluorite-type sub-cell. The true symmetry of the R l , W 2 0 2 , compounds is probably triclinic but close to tetragonal. Electron diffraction data on the (( R,,W403, )) phase type indicate the occurrence of a series of infinitely adaptive fluorite-related structures based on an ideal trigonal sub-cell.

Introduction. - There is much yet to be revealed

about the nature of order and disorder in solids whose parent structure is that fluorite. This is particularly true of fluorite-related mixed-cation systems, since, as is well known, cation self-diffusion processes are always many orders of magnitude slower than those of anions, and as a consequence, there is an inherent kinetic barrier in many cases to the achievement of equilibrium. This is particularly true at lower temperatures, where ordering might be favoured ener- getically over some disordered state produced initially in the sample preparation, but even at higher temperatures a stable ordered structure might not form if the initially disordered sample is not heated long enough at a sufficiently high temperature. Thus many of the reported phase relationships in such systems are significantly in disagreement (I).

Several mixed oxides of tungsten and the trivalent rare-earth elements have fluorite-related structures, and these constitute a case in point. Much detailed work on them has been reviewed by McCarthy and his colleagues [2]. In contrast to the M 0 2 - R 2 0 3 (M = Th, Ce, Zr, Hf : R = rareearth) systems [3-71, no grossly non-stoichiometric, disordered fluorite- type phase is found in the W0,-R203 systems for the comparable composition range

(') A detailed discussion of this problem is given in reference [I].

instead, three fluorite-related superstructure phases have been reported at 1 400 OC [2], which have the following ideal formulae and distributions :

R,WO,, for La-Lu. Y : R,,W,O,, for Gd-Ho. Y : R14W403, for Nd-Lu, Y. However, in some systems at least

(e.g. W0,-Sm,03 [8], and W0,-Gd,O, [2])

,

a range of composition for one or more of these phases has been reported.

The structures of these phases, apart from the rhombohedral R 6 W 0 , , compounds for Tb-Lu (which are apparently isostructural with UY601, [9]), are unknown. The X-ray powder difhaction patterns of R6WOI2 phases for La-Pr are reported as being cubic or pseudocubic, but pseudotetragonal for Nd-Gd. All the Rl,W,O,, phases studied by Mc- Carthy et al. [2] were indexed as pseudotetragonal also, having powder patterns very similar to those of the pseudotetragonal R6WO12 phases, while the powder diffraction data for R14W403, phases were partially indexed on the basis of a pseudorhombohe- dral cell identical with that of the rhombohedral R6W0,, phases. However, the symmetry of many of these R,,W403, compounds is clearly lower. The results of McCarthy et al. [2] in general confirm pre- vious work (reviewed in their paper) or supersede it, but this paper reports and discusses further develop- ments, and discrepancies between new and extant data.

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C7-74 E. SUMMERVILLE, J. DRENNAN AND D. J. M. BEVAN

Experimental. - The starting materials used were WO,, Sm20,, and Er203, all 99.9

%

pure and obtained from Koch-Light Laboratories. Samples were prepared by direct weighing of WO, and R 2 0 3 after prior ignition at 800 OC, and were thoroughly mixed in a Glen Creston vibratory mixer. Firings were carried out either in air (pelletized samples) or in sealed platinum tubes of 3 mm diameter : at high temperatures (> 1 600 OC) there was evidence of loss of W 0 3 from air-heated samples. A Johnson-Matthey Pt (40

%

Rh)-wound furnace was used for temperatures up to 1 350 OC and a molybdenum-wound furnace for temperatures between 13500C and 1 700 OC. The products of the various heat treatments were studied by powder X-ray diffraction, for which a Guinier-Hiigg focussing camera and strictly mono- chromatic CuKcr, radiation were used, and by electron diffraction in an AEI 802 electron microscope fitted with a high-tiltlrotate goniometer stage. For measu- rement purposes, X-ray diffraction patterns of some samples were obtained with either

T h o 2 (a = 5.596 9 A) or CeO, (a = 5.41 1 0 A) added as reference substance.

Results and discussion. - In this preliminary report

attention is focussed on two systems W03-Sm203 and W0,-Er203.

THE SYSTEM W03-Sm?03. - Samples of varying

compositions, as summarized in table I, were prepared

and reacted in air, first at 1 300 OC for 15 hours, and subsequently at 1 550 OC for 3 days. The close similarity between the two sets of product diffraction patterns suggests that the same products are formed at the? temperatures, that there was little loss of W 0 3 at the higher temperature, and that reaction was complete even after the first heating. Samples of total composition O/(W

+

Sm) = 1.714 and

O/(W

+

Sm) = 1.750

were also heated in sealed Pt capsules at 1 600 OC :

the same products were obtained here also. However, with the exception of the composition

O/(Sm

+

W) = 1.75 ( S m , , W 2 0 2 , ) . these product diffraction patterns were diphasic in character (table I), the predominant phase being the pseudotetragonal SmloW202,, and the minor phase B-type Sm203. Thus the phase Sm,WO,, does not exist under these conditions. (A similar experiment with samples of total composition

+

Sm)

-

1.833

Summary of phase relationships

Sm,O,-WO,

T OC Phases O/(W

+

+

12/21 1 350 7/12 1 680 7/12

(0) 18/33 refers to the cc Rl,W,O,, N phase type.

( b ) 12/21 refers to the R,,W,O,, compound.

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FLUORITE-RELATED PHASES IN RARE-EARTH TRIOXIDE SYSTEMS C7-75

were identical. Figures la, lb, lc, and Id show electron diffraction patterns common to both samples, and are

FIG. 1 . -Typical electron diffraction patterns from Sm,,W,O,,. For both the super-cell and the _fluorite sub-cell these zones are la [loo], Ib [OlO], lc [001], Id [IIO]. Fluorite sub-cell indices are

indicated.

of four fluorite-type sub-cell zones containing superstructure reflections : from these and others-the unit cell was obtained. This is an apparently tetragonal superstructure of the fluorite-type sub-cell with two axes doubled and the third trebled : the sub-cell distortion is very small. The diffraction, patterns indicate a primitive cell and the presence of an n-glide perpendicular to one of the doubled sub-cell axes. The constancy (within experimental error) of the d-

spacings obtained from powder diffraction patterns of this phase occurring at different total compositions suggests that it is a fine-phase.

However, more comprehensive data (see below) obtained from ErloW,O2, (there are many more superstructure reflections from this than from SmloW202,) suggest that the true symmetry of SmloW20,, is lower than tetragonal.

The existence of the pseudo-rhombohedra1 phase reported by McCarthy et al. [2] at the ideal composi-

tion Sm14W403, was confirmed in this work, but closer examination reveals a much more complex situation than that previously proposed. Further discussion of the (( R,,W,O,, )) phases (the quotes

now indicating a phase type) IS given below.

THE SYSTEM W0,-Er203. - The compositions studied and the phases found are shown in table I. The existence of the rhombohedra1 compound Er6WO12 and the (< ErI4W,O3, )) phase was confirm-

ed, and samples of these compositions heated in sealed Pt capsules to 1 680 O C produced these same phases, which suggests that both are stable to high temperatures. Samples of total composition interme- diate between these limits, when reacted at 1 350 OC, (yielded a diphasic mixture of the two : a similar

result was obtained by McCarthy et al. [2] at 1 400 OC. However, when these same samples were heated in sealed Pt capsules to 1 680 OC, the pseudotetragonal Er, ,W, O,, phase was formed, either occurring uniquely at this composition, or coexisting with Er6WOl2 or (( Er14W,03, D, depending on the over-

all composition. All attempts to reverse this change by long annealing (7-10 days) at 1 3500C have failed : once formed, this phase is apparently very stable.

Electron diffraction patterns from this phase are identical with those obtained from SmloW2021, except for the slightly larger reciprocal lattice spacings. The X-ray powder diffraction data (0 < sin2 8 < 0.18) from a sample of Erl0W2O2, formed in a sealed Pt tube at 1 600 OC over 12 hours are shown in table I1 :

many superstructure lines were observed. However, an attempted tetragonal indexing based on the unit cell determined by electron diffraction was unsuccess- ful, 8 of the 31 measured lines being unindexed. In this attempt the 3-fold extension of the fluorite-type sub- cell was necessarily designated the c-axis, so that the two reflections, with relative intensities in the ratio 2 : 1,

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C7-76 E. SUMMERVTLLE, J. DRENNAN A N D D. J. M. BEVAN

TABLE I1

Partially indexed X-ray powder diflraction pattern of Er,,W202, based on an orthorhombic cell with refined parameters

a = 10.543 f 0.003 A , b = 10.477 f 0.003 A ,

c = 15.818

+

0.004 A , V = I 747.1

+

2.0

A3

Intensity h k I SinZ O(obs.)

2 2 6 0.128 30 W 4 2 3 0.128 30 M

r

3 4 0 0.13460 3 3 4 0.134 60 FUZZY 3 4 1 0.137 07 FUZZY 4 3 2 0.143 59 W 0.148 39

{i i

i

0.155 92 W _{0.155 92} M 2 2 7 0.159 14 M 0.167 90 M 4 0 6 0.170 68 4 3 4 0.171 89 S 4 4 0 0.171 89 0 4 6 0.171 89

(400, 040), and 006,. A second attempt (see table 11) made use of an orthorhombic cell with the indices of the split 200, reflection assigned as (400, 006), and 040, : only 2 of the measured lines were unindexed on this cell, which is nevertheless still unsatisfactory. Subsequently, a very sharp X-ray powder pattern was obtained from a sample of Er,,W,O,, heated for 15 hours in a sealed Pt tube at 1 680 O C . This

showed additional weak superstructure lines, and close splitting into doublets of others previously measured as singlets, although there was no obser- vable change in the sub-cell reflections. This pattern has'not yet been indexed, but it suggests that the symmetry is even lower than orthorhombic. Viewed in the context of the higher temperature of preparation of this sample, the diffraction data suggest further that a greater degree of order is achieved with increas- ing temperature.

Electron diffraction studies on the cr ErI4W4O3, ))

phase have also been carried out and are described below.

THE (( R,,W,O,, )) PHASES. - The simplest ideal unit cell of these phases, as determined from electron diffraction studies, is primitive trigonal. The following relationships apply :

a = [ I ~ o ] , and a* = 1/6[4E];

Figures 242 and 2b show electron diffraction pat-

terns of a ( 11 1 ), and a ( 110 ), fluorite-type sub- cell zone respectively, obtained from a sample of overall composition O/(W

+

Er) = 1.79 : the super-

cell reciprocal axes are indicated. Table I11 gives the indexing of the powder X-ray diffraction pattern. The unit cell contains 12 cations if it is assumed, as found with other fluorite-related superstructures, that the cation sub-lattice remains intact. However, a sample of overall composition O/(W

+

Er) = 1.833 heated in a sealed Pt capsule at 1 680 OC gave electron diffraction patterns with offsets in the rows of superstructure reflections, indicating a very large supercell of the simple trigonal cell. .Figure 3 shows such a pattern of a ( 110 ), zone (cf. figure 2b). These differences correlate qualitatively with differences in the powder diffraction patterns from the two samples, which are shown in figure 4 : the trigonal or pseudo- trigonal splitting of the strong sub-cell reflections, and the superstructure reflections, are clearly different for the two samples. Moreover, the 220, reflection is split into 3 lines (not visible in figure 4) for the sample with O/(W

+

Er) = 1.833, indicating a triclinic distortion of the fluorite-type sub-cell.

More extensive data have been obtained for

(( Sm,,W,O,, D. However, in this system the simple

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FLUORITE-RELATED PHASES IN RARE-EARTH TRIOXIDE SYSTEMS C7-77

TABLE I11

fndexed X-ray powder dzflraction pattern of the ideal trigonalphase (( ErliW403j )) from a sample of

composition O/(W

+

Er) = 1.79. ReJned cell para- meters are

a = 7.375

+

0.001 A , c = 9.308

+

0.002 A ,

V = 438.4 f 0.2

A3

Intensity h k I Sin2 B(obs.) SinZ %(talc.)

- - - - - - W 1 0 2 0.041 94 0.941 94 W 1 1 1 0.050 48 0.050 48 S 0 0 3 0.061 66 0.061 64 S 2 0 1 0.065 07 0.065 02 W 1 0 3 0.076 19 0.076 18 S 2 0 2 0.085 54 0.085 57 W 2 1 1 0.108 62 0.108 65 W 1 0 4 0.124 1 1 0.124 12 W 3 0 1 0.137 70 0.137 74 W 3 0 2 0.158 31 0.158 28 W 2 1 3 0.163 25 0.163 44 S 2 0 4 0.167 85 0.167 75 S 2 2 0 0.174 62 0.174 52

zone in which rows of superstructure reflections in this zone (or very nearly so) pass through each sub-cell reflection, but their direction is no longer [422], as in

FIG. 3. - A ( 110 )F electron diffraction pattern from a sample of

composition O/(W

+

Er) = 1.833 showing an equivalent supercell zbne to that in figure 2b. However the-c_ontinuous rows of spots between the origin and both 11 1, and 4 2 5 are now split into two

off-set rows.

FIG. 2. -Axial electron diffraction patterns of the ideal trigonal phase Erl4W4O,,. 2a ( 1 1 1 ), = [OO1],,ig,, 2b ( 110 )F = [O1O],,i,,. Reciprocal lattice axes are indicated tpgether with fluorite sub-

cell indices.

FIG. 4. - Guinier photographs of Er,,W403, type phases from

samples of composition O/(Er

+

W) = 1.79 (left) and 1.833 (right). The left pattern contains lines from tetragonal Er,,W20,1, the right pattern lines from CeO, standard - these are indicated in the figure along with fluorite indices. Note the differing super-

structure lines and sub-cell line-splittings.

the simple trigonal case

_{- -}

: the approximate direction is [33, 17, 151, and the spacing between reflections in

_-

these rows is

-

1/49 [33, 17, 151,. Figure 5b shows the diffraction pattern of the same [231], zone of a different crystal, but here the rotation of the superstructure rows from the ideal [4221,, is clearly less than that shown in figure 5a. This situation is very reminiscent

of that observed by Bursill, Hyde, and Philp [lo] for the system Ti0,-Cr,03. Figure 6 shows more electron diffraction patterns, this time of equivalent ( 110 ),

zones of <( Sml,W403, N, which indicate different

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C7-78 E. SUMMERVILLE, J. DREN [NAN AND D. J. M. BEVAN

FIG. 5. -Two equivalent [231], electron diffraction patterns from different crystals of (( Sm14W403, )) showing variability in the degree of rotation of the off-set superstructure rows. The length

of these rows probably lndlcates that they lie in the zone.

unique superstructure for each composition. However, the situation is even more complex in that different supercells have been observed in electron diffraction for different crystals from the same sample. This implies compositional heterogeneity within a given sample or different ordered states from one crystal to another.

There are only two possible structures for the_ ideal trigonal << Er14W403, )) if the space group is P3. The

more likely of these has special six-coordinated (tungsten) cations and anion vacancies in strings parallel to the c-axis ([I 111,). These strings are arrang- ed in a highly symmetrical, two-dimensional array (see figure 7) analogous to that of the strings in UY601, [9]. Attempts are being made to determine this structure directly.

Order and disorder. - The large difference between

the formal ionic radii of W6+ (0.58

A

in six-coordination [ll]) and

R3+

(0.96

A

and 0.88

A

for Sm3+ and Er3+ resp. in six-coordination [l 11) should favour cation ordering, and this almost certainly occurs in Er6WOI2. However, the isostructural compound Sm6W0,, does not form. The tetragonal SmloW202,, on the other hand, forms readily at 1 300 O C , but a

d )

FIG. 6. -Typical [OITL diffraction patterns from different crystals of a Sm,,W4033 )), analogous to those of (( ErI4W4O3, M in figure 3. The super-lattice rows now no longer lie in the zone and are evident only because of spiking.~ A variety of different supercells based on the trigonal Er14W403, cell are indicated by these

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FLUORITE-RELATED PHASES IN RARE-EARTH TRIOXIDE SYSTEMS C7-79

FIG. 7. -Projection along [OOI],,,, = [Ill], of the ideal atom sites of Er,,,3W,,,0,,(Erl,W,0,, or M,,O,,). In the fluor~te structure in the same projection cations would occur at levels along the c-axis of 0 (intersections), 113 (solid circles) and 213 (open circles). The ideal trigonal structure probably has a tungsten ion at the origin and strings of vacancies along the trigonal axis.

A fluorite cell is shown on the left.

much higher temperature is required to form Er1,W2O2,, although once formed this does not seems to disproportionate at 1 350 OC where Er, WO,, and (( Erl,W403, )) are formed ab initio. This again

suggests an ordered structure. Detailed structural studies are clearly required in order to resolve these issues, and these are being undertaken. In this context, the systems MOO,-R,O, will also be explored since in them the formation of single crystals might well be easier, and the greater difference in scattering factors between Mo and R in any .case makes them more favourable for study. The problems uncovered by the new results reported here on the (( R14W,03, ))

phase are formidable, but may be amenable to solu- tion by high resolution lattice imaging. In summary, however, it is clear that much still remains to be done. Acknowledgments. - Financial support for this work from the Australian Research Grants Committee is gratefully acknowledged.

References [I] BEVAN, D . J. M. and SUMMERVILLE, E., Handbook o j Physics

and Chemistry of the Rare-Earths (North Holland) (in press).

[2] MCCARTHY, G. J., FISHER, R. D., JOHNSON, G. G. and Goo-

DEN, E., N.B.S. Special Publication 364 (1972) 397. [3] SIBIEUDE, R. and FOEX, M., J . NucI. M ~ t e r . 56 (1975) 229. [4] KELLER, C., BERNDT, U., ENGERER, H. and LBITNER, L.,

J . Solid State Chem. 4 (1972) 453.

(51 BEVAN, D. J. M., BARKER, W. W., MARTIN, R. L. and PARKS, T. C., Proc. o j the Fourth Conference on Rare Earth Research (Gordon and Breach., N.Y.) 1965, p. 441.

[6] THOKNBER, M. R., REVAN, D. J. M. and SUMMI~RVIL.I.E, E., J . Solid State Chem. 1 (1970) 545.

[7] ROUANET, A.. Rev. Int. Htes Temp Rifr. 8 (8971) 161. [8] CHANG, L. L. Y., SCROGER, M. G. and PHILLIPS, B., J . Inorg.

Nucl. Chem. 28 (1966) 1 179.

191 AITKEN, E. A., BARTRAM, S. F. and JUENKE, E. F., Inorg. Chem. 3 (1 964) 949.

[lo] BURSII.~., L. A,, HYDE, B. G. and PHILY, D. K., Phil. Mug. 23 (1971) 1501.