Electron diffraction study of the low temperature phase in lead orthovanadate

Mat. R e s . B u l l . , Vol. 22, p p . 193-199, 1987. P r i n t e d in the USA. 0025-5408/87 $3.00 + .00 C o p y r i g h t (c) 1987 Pergamon J o u r n a l s L t d .

ELECTRON DIFFRACTION STUDY OF THE LOW TEMPERATURE PHASE IN LEAD ORTHOVANADATE C.Manollkas I, G.Van Tendeloo and S.Amelinckx 2 University of Antwerp (RUCA) Groenenborgerlaan 171 , 2020 Antwerpen

(Belgium)

( R e c e i v e d J u n e 9, 1986; R e f e r e e d )

ABSTRACT Using electron diffraction and transmission electron microscopy it is shown that lead orthovanadate can follow two different transformation paths on cooling. The high temperature Y-phase transforms according to the scheme ~ ~ B ÷ a. The metastable Y' phase, which results from frozen-in Y, transforms into a different low temperature phase, the a'-phase. In the literature, two different structures, both called a, were proposed for the low temperature phase. It turns out that both structures (here called a and a') occur, but under different conditions. MATERIALS INDEX : ferroelastic material- lead orthovanadate.

I. Introduction The different phases of the compound Pb3(VO~)2, of the related material Pb3(PO~)~, and of the mixed crystals have been the subject of numerous papers (I-15). In this short note we shall first describe some new observations on lead orthovanadate using mainly electron diffraction and subsequently we shall compare the phase transitions in the vanadate with those in the phosphate. 2. Lead Orthovanadate It seems to be well established that lead orthovanadate occurs in at least three phases of which the crystallographic characteristics are summarized in table I. Different sets of lattice parameters have been proposed for the same phase; in particular it is noteworthy that two different structures for the low temperature a-phase have been described in the literature; their unit cells are both given in table I. Moreover it was recently suggested that an intermediate phase called Y' occurs between Y and B (6). The characteristics of this phase are summarized in table I as well. The ~'-phase is observed at room temperature in parts of the specimen which have not been transformed into the B-phase, although this is the stable phase at this temperature. On approaching the Y ~ Y' transition temperature diffuse spots already appear in the Y-phase at positions which will afterwards sharpen and form the Y'-phase diffraction pattern. I) Permanent address : University of Thessaloniki 2) Also at : SCK/CEN, Mol (Belgium) 193

(Greece)

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Similar diffuse spots have also been observed in the rhombohedral B-phase of lead phosphate and were attributed to a three-fold degenerate zone-boundary soft vibration mode. One of the three components of this mode would give rise to the static monocllnlc distortion observed in the monoclinic a-phase C2/c

FIG.I Cooling sequence of a_ s~keclmen of lead orthovanadate. The diffraction pattern is taken along the [ 1 1 2 ] * R zone. (a) At T >377°K the specimen is in the Y-phase; diffuse spots are presumably due to a zone boundary soft mode; (b) 371 K < T < 377 K : the diffuse spots have sharpened, the specimen is in the Y'-phase (6) ; (c) at room temperature the specimen contains both Y' and B-phase areas; (d) at room temperature : one variant of the B-phase. TABLE I : Crystallographic data of Pb,(VO~), according to the literature

Phase

T(K)

space group

a(nm)

Y

388 368

R3m * **

0.7560(I) 0.5760(i) 0.7555(I)

375

***

1.50

*

1.15

Y'

S

*

298

b(nm)

c(nm)

0.5757(I)

2.0368(3) 0.9969(3)

a or B

Ref.

44.78(I)

[I] [I] [2]

116.09(I) 44.7

4.o7

[5] [5]

P2,/n P2/c,Pc or P2~/c 295 P2~/c 261.5 P2~/e

0.7515(I) 1.397 0.7514(I) 0[7508(I)

0.61061(5) 0.589 0.6107(I) 0.6115(I)

0.9283(I) 0.939 0.9526(I) 0[9153(I)

111.86(I) 101 i15.20(I) 115.18(I)

[I] [3] [2] [2]

261.5 203 77 203

0.7508(I) 0.7478(I) 0.7460(I) 1.5042

0.6115(I) 0.6181(I) 0.6191(I) 0.6106

0.9153(I) 0.9387(I) 0[9348(I) 1.9073

115.18(I) 116.60(I) 116.63(I) 115.20

[2] [2] [2] [4]

A2 A2 A2 P2~

: cell constants in hexagonal setting

** : cell constants in terms of the monoclinic B-Pb,(VO,)2 type cell *** : only point group 3m has been reported.

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(11,12), whereas the two other components would be associated with remaining zone boundary modes in this monoclinic phase. The diffuse spots in the vanadate have presumably a similar interpretation as in the phosphate. The lead orthovanadate behaves somewhat differently. On cooling the diffuse spots become sharp Y' spots; dark field images in such spots show no fragmentation in orientation variants, but translation variants have been observed (6). The Y' spots are sometimes observed, superposed on the B-phase pattern showing that both phases may occur simultaneously.

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FIG.2 Comparison of the reciprocal lattices of B, ~ and ~' as viewed along three different zones for each phase. (a) along the normal to the basal plane F ~ L111J*R.; (b) along the normal to the (a*, b*) plane; (c) along the normal to the (a*, c*) plane. The different zones are chosen so as to correspond as closely as possible in the different phases. Full dots refer to nodes in the plane of the drawing, open dots refer to extinctions.

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Fig. 1 represents a cooling sequence from above 377°K to room temperature, as observed along the [112]* R zone (the index R refers to the rhombohedral parent phase). In (a) which represents a pattern due to the Y-phase , diffuse spots are present, which have become sharp in (b), which refers to the Y'phase. In (c) both Y' and B spots are visible at room temperature, whereas in (d) o~ly the B-phase is present. The regions of Y' in Pb3(VO~)2 were shown to result from regions of residual Y, which were "frozen-in" during the Y ÷ B transformation. It was shown in (5) by means of electron microscopy and electron diffraction that residual Y-phase OCCurS either in wedge shaped areas centered on the domain walls separating two different orientation states of the B-phase, or in triangular areas in the centre of "star patterns" of the B-phase. Repeatedly recycling the specimen through the B ~ Y transition by beam heating in the microscope increases the frozen-in fraction of Y' but on reheating some Y'-reglons may also transform in B before retransforming into Y.

phas e. ~ng the 3ng the ~rlants ng the ~esence basic

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The simultaneous occurrence of microdomains belonging to the high and low temperature phases was also inferred from x-ray diffraction in Pb30 ~ (]6). Some new observations of the low temperature s-phase in lead orthovanadate, which allow to reconcile the two conflicting descriptions of the s-phase referred to above in table I, are directly related to the occurrence of the Y'phase. When cooling a specimen which consists of a mixture of Y'-phase and of B-phase, it is found that below about I0°C, the B-regions transform into the s-phase described in (2). On the other hand a fraction of the ~' regions transforms into a phase producing a diffraction pattern which cannot be indexed with respect to the a-unit cell given in (2); we shall call the corresponding phase ~'. Along the [112]R direction a fourfold long period is present (fig.3) • Different sections of the diffraction patterns of the B, ~ and ~' phases are compared in fig.2. Whereas the diffraction pattern of the s-phase is rather closely related to that of the B-phase, the ~'-phase has a rather different diffraction pattern. It turns out that it can be indexed on the unit cell proposed for the ~-ph~se in (4). There is no simple relationship between the two descriptions given in (2) for the s-phase and in (4) for the ~'-phase (using our terminology). However, when choosing the c*-axis, through the (110) R reciprocal lattice point the relationship becomes closer (fig.2). The basal section of the ~'-phase (i.e. the [111]R zone) often exhibits three orientation variants (fig.3c) related by 120 ° rotations. This proves that the parent phase (i.e. the Y'-phase) has a threefold symmetry axis, which is consistent with our assumption that Y' is rhombohedral and has the same pointgroup as Y (6).

FIG.4 Diffraction pattern of lead orthophosphate. (a) one single variant of the monoclinic s-phase viewed along the [111]* R zone; (b) three different orientation variants. Dark field images produced in spots such as I, 2 and 3 reveal three different orientation variants; (c) Rhombohedral B-phase : note the diffuse spots at the same sites as in (b). Dark field images made in these spots do not reveal orientation vari ants.

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FIG.5 Dark field images of different orientation variants of the ~phase of lead orthophosphate imaged successively in spots such as I, 2 and 3 in fig.4

3.Lead Orthophosphate The evolution of the diffraction pattern and of the image is somewhat different in the lead orthophosphate. When making dark field images in the different diffuse spots in the phosphate always the complete area of the specimen is bright, which means that there is no fragmentation in orientation domains; this is consistent with the assumed dynamic origin of the spots as proposed in (11,12). When cooling until the diffuse spots appear sharp the rhombohedral B-phase has transformed into three variants of the monoclinlc ~-phase (fig.4). This is illustrated in fig.5 which reproduces dark field images made in such spots as I, 2 and 3 of fig.4b. Occasionally a single variant can be selected (fig.4a). Such a fragmefltation is not observed in the Y'-phase of lead orthovanadatei it only appears in the a'-phase.

377K

371K

283K

~- phase .-~ ~phase .-~-phase --~ ~-phase I

~ F=phase--~=phase R~

~283K

FIG.6 Phase transition paths of Pb3(VO~) 2

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4. Conclusions We conclude from these observations that the phase transitions in the orthophosphate and in the orthovanadate exhibit similarities, but also characteristic differences. The phase transitions in the vanadate are of particular interest since it appears that on cooling two different transition paths are possible; they are represented schematically in fig.6. One of the paths refers to the thermodynamically stable form; it is schematically represented as ~ ~ B ~ and the second one refers to the frozen-in metastable form; it can be schematised as Y ~ ~' ~ ~'. A somewhat similar situation is known to exist in certain transition metal dichalcogenides (such as TaS2, TaSe2) (17-19) where the IT form, which can be obtained at room temperature as a metastable phase, undergoes a number of phase transitions on cooling, which are different from those of the stable 2H form at room temperature. The two structures proposed for the low temperature phase (