Flux growth and characterization of single crystals of the perovskites Pb2FeTaO6 and Pb2CoWO6

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Flux growth and characterization of single crystals of the perovskites Pb2FeTaO6 and Pb2CoWO6

BRIXEL, Wolf Dietrich, BOUTELLIER, Roland, SCHMID, Hans

Abstract

Single crystab of the perovskites Pb2 FeTa06 and Pb2CoW06 (abbreviated by PFT and PC:W in this text) have been grown from a PbO flux in sealed platinum crucibles of 30-180 mL volume, with and without using the accelerated crucible rotation technique (ACRT). The typical crystal size was 0.5-5 mm edge length for PFT (aggregates of intergrown cubes) and 3-10 mm for PCW (cubes and cubo-octahedra). Both compounds are black with a metallic luster in reflected light and are red-brown in transmission. The crystals were subsequently characterized by means of X-ray powder diffraction, chemical analysis, optical domain studies, birefringence, absorption spectra, and elec. resistivity. [on SciFinder(R)]

BRIXEL, Wolf Dietrich, BOUTELLIER, Roland, SCHMID, Hans. Flux growth and characterization of single crystals of the perovskites Pb2FeTaO6 and Pb2CoWO6. Journal of Crystal Growth, 1987, vol. 82, no. 3, p. 396-404

DOI : 10.1016/0022-0248(87)90330-7

Available at:

http://archive-ouverte.unige.ch/unige:30941

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1 / 1

396 Journal of Crystal Growth 82 (1987) 396-404 North- Holland, Amsterdam

FLUX GROWTH AND CHARACTERIZATION OF SINGLE CRYSTALS OF THE PEROVSKITES Pb₂FeTa0₆ AND Pb₂CoW0₆

W. BRIXEL, R. BOUTELLIER and H. SCHMID

..-..Department of Mineral, Analytical and Applied Chemistry, UniversilJ' of Geneva, CH-1211 Geneva 4, Switzerland Received 6 December 1985; manu:,:L:ript received in final form 5 r-..iay 1986

Single crystab of the perovskites Pb₂FeTa0₆ and Pb₂CoW0₆ (abbreviated by PFT and PC:W in this text) have been grown from a PbO flux in sealed platinum cmcibles of 30-lSO ml volume, with and without using the accelerated crucible rotation technique (ACRT). Growth temperature ranges were typically 1200 ~ 950°C: for PFT and 1220 ~ 800°C for PCW with cooling rates of O.S-0.8°Cjh. The typical size of crystals was 0.5-5 mm edge length for PFT (aggregates of intergrown cubes) and 3-10 mm for PCW (cubes and cubo-octahedra). Both compounds arc hla\.:k with a metallic lustre in reflected light and are red-brown in transmission. The crystals were subsequently characterized by means of X-ray powder diffraction, chemical analysis, optical domain studies, birefringence, absorption spectra, electrical resistivity.

1. Introduction

Over the past 25 years there have been syn- thetized more than 200 complex perovskites with the general formula A₂BB'0_{6 ,} where the large A cation is 12-fold coordinated with oxygen, and the smaller B and B' cations are 6-fold coordinated

--

. ,vith oxygen. If the difference in charge and size between the B and B' cations is large enough, they will order three-dimensionally, thus leading to a doubled unit cell parameter and with space group FmJm in the ideal cubic case [1]. In practice, however, slight distortions of the cubic structure often lead to lower symmetry phases. The majority of these compounds shows one or more structural phase transitions associated with anti- or ferro- electric ordering. In addition anti- or ferromag- netic ordering may also set in if the concentration of paramagnetic ions is sufficiently high. Ferro- electric fcrromagnets are of particular interest.

Their symmetry allows both the linear and higher order magnetoeletric effects (see e.g. Type FE IjFM I in table 2 of ref. [2]) and a variety of other crystallo-physical properties (table 1 of ref.

[2]). So far only boracites [3], some perovskites [4,5] and magnetite [6] have been found to belong to this category of materials. Until recently most

work on magnetically ordered perovskites has been done on ceramics. However, in order to achieve further progress, the synthesis of single crystals is mandatory. Pb₂FeTa0₆ is of interest because the homologous phase Pb₂FeNb0₆ was reported to be both ferroelectric and ferromagnetic below 9 K [7]. The same is true for Pb₂CoW0₆ [8,9]. Further- more, the latter compound is of particular interest because it seems to be the first example of an oxygen-perovskite in which an incommensurate phase is found and moreover, to set on at a first order transition [10-12].

The objective of this work was to grow single crystals of perovskites of sufficient size and homo- geneity for both optical and electrical measure- ments [10-12]. As both compounds melt incon- gruently and cannot be synthetized by means of chemical vapour transport, flux growth seems to be the most appropriate and simple growth tech- nique. In a further step we investigated their phase transitions by different physical methods [10,11, 13]. These studies are still in progress. The ex- istence of the (anti)-ferroelectric and (anti)-ferro- magnetic properties of these two perovskites as well as some of their phase transitions have been known for a long time and are discussed in many papers [9,14-18], but our knowledge is still incom- 0022-0248/87 j$03.50 '0 Elsevier Science Publishers B. V.

(North-Holland Physics Publishing Division)

w·_ Brixel et al. I Flux growth and characterization of perovskites 397 Table 1

Phase transitions of PFT and PCW

IV Ill ¹¹ I

Pb2CoW0

6 9K sec.order 235K first 298K first

order order

Co2+ and w6• I

ferromagnetic I

paramagnetic

-

^ordered undetermined monoclinic cubic Fm3m

·--

Pb/eTao6

143K sec.order 205K first 248K second

order order

Fe 3•and Ta5+ anti ferro-

I I

disordered magnetic paramagnetic

' rhombohedral monoclinic ^cubic Pm3m

plete. PCW has an ordered elpasolite-like struc- ture in its cubic phase (Fm3m) [10] whereas PFT shows no ordering, the Fe³+ and Ta⁵+ ions being - randomly distributed (Pm3m) [19], as is the case for the related Pb₂FeNb0₆ [20] with probably a short-range ordering effect [21]. In table 1 we have collected the so far known data in the phases and phase transitions of PFT and PCW.

2. Experimental

2.1. Growth experiments 2.1.1. Experimental procedure

In order to optimize the growth parameters we investigated part of the systems PFT- PbO and PCW- PbO by differential thermal analysis, con- sidering them in first approximation as pseudo-bi- nary in analogy with the system PFN-PbO (in this text, Pb₂FeNb0₆ is abbreviated by PFN) [22]. In practice, however, these systems revealed them- selves to be much more complex then expected. In both cases phases which do not belong to the

__

.,

pseudo-binary systems crystallize out. Further- more both perovskites melt incongruently [23] so that the approximation of a binary eutectic system becomes irrelevant. For these reasons and with a view to finding the optimum growth conditions, the influence of both the flux composition and soak temperature were systematically investigated.

After optimization of these two parameters the influence of the cooling rate (varied between 3 and 0.45 K h-¹) and use of the ACRT (see section 2.1.1) were studied.

The choice of PbO as flux [14,24] was justified as being a very good high-temperature solvent for these perovskites and itself a component of them which avoids introducing foreign ions into the lattice of the grown crystals. The chief disad- vantages are its toxicity and its high volatility above 1000°C which required sealing of the cruci- ble to avoid weight loss. Although some authors [25,26] observed an attack of the platinum crucible by PbO fluxes, we noticed no kind of corrosion except for a very small weight loss of the crucible, probably due to the formation of volatile Pt oxides at high temperatures [27].

398 J.t'. Bri.v.el et al. / Flux gro»,tlz and characterization ofperouskites

The starting oxides used were PbO specpure from Johnson-Matthey, Ta₂0₅ and W0₃ both ?

99.9% from Fluka, Fe₂0₃ > 99.99% from Koch- Light; stoichiometric CoO was made by air oxida- tion (8d, 1420°C) of 99.9% cobalt cathodes purchased from Falconbridge (instead of CoO, Co₃0₄ Merck p.a., was used in the first two PCW crystal batches). In a typical run stoichiometric ratios of the powdered oxides were thoroughly mixed by shaking in a glass container until the mixture appeared homogeneous under the micro- scope. Platinum crucibles of 30 and 180 ml volume, specially shaped to be sealed and opened several times, and 15 ml standard-shaped crucibles for preliminary experiments were used. These cruci- bles were filled to 3/4 of their height (84 and 54 mm overall height for 180 and 30 ml crucibles, respectively) by premclting the oxides (e.g. in wt%

37.8 PFT +59 PbO

+

2.2 Fe₂0₃ or 65 PCW

+

35 PbO) at 1000 o C and scaling the lid by means of argon-arc welding. In order to equilibrate the in- ternal pressure with the atmospheric one, a small hole (about 50 !Lm diameter) was drilled through the lid. The crucibles were placed into a 9 kW furnace C''v1oSi ₂ heating elements), regulated by a proportional integral differential (PID) unit and programmed by an electronic ramp generator. The furnace has been equipped with an accelerated crucible rotation technique (ACRT) driving sys- tem constructed by one of the authors (R.B.). This technique permits vigorous stirring inspite of the closed crucible and thus enhancing the homogene- ity of the solution and improving the growth con- ditions [2829). Usually each crystal batch was realized in an independent run and the crucible was placed in the middle of the furnace chamber.

Four PtjPt-10% Rh thermocouples were located on either side, on top and bottom of the crucible.

The temperature was raised to the set point (typi- cally 1210-1230 ° C), held at this value for 24 h, and lowered at a constant rate of 0.5-0.7 o Cjh.

The initial temperature cycling technique [30], an effective method to reduce the number of nuclei in the early stage of crystallization, was not at- tempted in this work as in neither case the liqui- dus curve of the perovskite was sufficiently well known. When the temperature fell to 940 ° C (PFT) and 780 o C (PCW), the crucible was removed

from the furnace, two holes were pierced in the lid and the remaining liquid was quickly decanted before it could solidify. The crucible cooled natu- rally down to room temperature. The crystals were removed by hand or by careful hammering on the crucible walls. The PFT crystals were subse- quently washed in hot 30% nitric acid to remove flux residues, whereas the PCW crystals could only be cleaned by scratching away the soft flux, as all common acids strongly attack PCW.

2.1.2 Growth results

Both perovskites were obtained as black crystals with semi.metallic lustre as a sign of high absorp- tion and refractive index. PFT crystallizes in the form of cubes of 0.5-5 mm edge length, mostly intergrown and forming oriented aggregates (fig.

1) which disintegrated into many small crystal fragments after the HN0₃ treatment. The presence of raised edges, hoppers and a hollow (skeletal) morphology is evidence for growth out of equi- librium [31). Beside the perovski.te there was formed in all PFT batches an iron deficient pyro- chlore phase of composition Pb₂Ta₂_xFe₂x0_{7 •} x;z (x ~ 0.16) crystallizing in the form of brown

Fig. 1. PFT crystal aggregate; hatch PPT-5b.

-

~V. Brixel er al. / Flux growth and dwraoerizutwn of paouskiws 399

Fig. 2. PCW crystals with (100) and (111) facets; batch PCW-6.

Marker represents 1 cm.

transparent octahedra which could easily be sorted out by hand from the black perovskite cubes.

Their relative amount was a function of both the flux composition and the soak temperature, a higher soak temperature favouring the crystalliza- tion of the pyrochlore phase (in agreement with ref. (14]).

PCW crystallizes in the form of cubo-octahedra on the crucible walls (fig. 2) and mostly as inter- grown distorted cubes on the bottom. The size was typically 3-10 mm edge length, one sample reaching 17 mm.

The (100) facets were always smooth and bright, whereas the (111) facets were rough (fig. 3). The different morphology of the two types of facet seems to indicate that their growth mechanism is different; this supposition is supported by the presence of pyramidal (100) and (111) growth

Fig. 3. Rough surface structure of an as-grown (111) facet of PCW.

sectors differing in properties (see section 2.2.3).

In some batches also some CoO crystallized a' dark grey rounded octahedra, forming a layer below the PCW cubes. This phase was separated easily by hand from the perovskite one.

The efficiency of the stepwise increased cooling rate, providing a constant supersaturation of the solution and hence a stable growth [32] could not be demonstrated for PFT or PCW. The ACRT however affected drastically the growth conditions for both compounds: for PFT it only reduced the number of nuclei. leading to bigger crystals but without suppressing their skeletal shape, whereas in the case of PCW the ACRT both reduced the number of nuclei and flux inclusions. The size of the crystals became greatly increased. ACRT con- ditions used in this work were: cycle period of 160 s, linear change of rotation speed with maximal amplitude of

±

100 rpm.

2.2. Characterization 2.2.1. X-ray diffraction

The different phases having crystallized in the batches, perovskites PFT and PCW, pyrochlorc Pb₂Ta₂_xFe₂x07+x;_{2 ,} Co₃0₄ and CoO, have been identified by X-ray powder diffraction (Guinier-

Huber camera, Cu Kii) The pyrochlore phase was found to be isomorphous with Pb₂Ta₂0 ₇ [33]. In agreement with earlier literature, only PCW showed extra lines of a doubled perovskite cell, due to ordering of

cd+

and W²+ ions [14J5].

2.2.2. Chemical analysis

The identification of the perovskites and the pyrochlore has been complemented by chemical analysis of the cations in order to know their exact stoichiometry. As the compounds were only par- tially soluble in acids, a method to dissolve them was developed. It consists in an attack of the powdered samples by fused KHS0_{4 ,} and com- plexation of Pb, Fe and Co by EDTA at pH 10. In this medium hydrated Ta₂0₅ precipitates, whereas tungsten remains in solution as WOJ . Pb, Fe and Co were determined by atomic absorption spec- troscopy and Ta gravimetrically as Ta₂0_{5 .} A method to precipitate selectively W0₃ is still in development, since W does not give a reproduci-

400 W Brixel et al. / Flux growth and characterization of perouskiTes

ble atomic absorption signaL PCW has also been analysed by electron probe microanalysis, with good accuracy for Co and W but too low for Pb.

It was found that the stoichiometry of PFT is in close agreement with the ideal one, whereas the ' composition of the pyrochlore phase differs appre-

~ ciably from Pb₂Ta₂0_{7 ;} it contained a significant amount of iron corresponding to the formula

Pb2^Ta~.83^Fe0^_34^0x (0 was not analysed). The small polarizing FeJ+ ion replaces TaS+ on the B sites of the lattice, the size of these two ions being very similar. Slight deviations from ideal stoichiometry were also found in PCW. Atomic absorption mea- surements gave a CojPb ratio of 0.46

±

0.02 and electron probe microanalysis - CojW ratio of 0.95

±

0.02. By normalizing the Pb coefficient to

2, one obtains Pb₂Co_{0_92}W_{0_97}0,. The microprobe

a

method was unable to detect a difference in com- position between both types of growth sector in PCW.

The specific gravity of the perovskites was mea- sured at 20 ± 0.1 o C using a glass pyknometer (V= 10.725 ml) and toluene Merck p.a. as immer- sion medium. The samples used were single crystals of PFT (4.553 g) and PCW (3.404 g). The values found were 9.44

±

0.01 and 9.84

±

0.01 g cm-³ for PFT and PCW, respectively. The calculated values are 9.64 (a₀

=

4.007

A)

[14] for PFT and 9.73 ( a₀ = 8.0103

A)

[10] for PCW. The slightly higher measured specific gravity of PCW is significant and consistent \Vith the chemically found Co de- ficiency.

2. 2. 3. Optical studies

PFT and PCW single crystals were cut into thin (100) and (110) platelets and polished with 3 and 0.25 p.m diamond paste. Typical dimensions of platelets were 2-4 mm edge length and 50-60 p.m thickness for PFT and 25-30 p.m for PCW, the absorption of which is stronger. Special care was taken during sample preparation because of the brittleness of the materials (especially PCW). The Vickers microhardness was measured and lies in the range of 250-300 and 650-700 for PCW and PFT, respectively. Furthermore, a bad polishing introduces mechanical stress which in turn changes the domain pattern of the non-cubic phases and often leads to cracking of the whole sample. The

0

b +

Fig. 4. (a) (100) PFT platelet showing truncated (100) growth pyramids, cut close below the natural crystal surface; thickness of sample 55 I'm; room temperature. (b) Schematic cross-sec- tion of (100) growth sectors corresponding to (a), with extinc- tion directions indicated. Contrast in (a) by means of crossed

polars and compensation of hi refringence in vertical bands.

platelets were studied in transmission using a Leitz Orthoplan polarizing microscope [10,13] equipped with a He-flow cryostat, allowing cooling down to 4K.

In unpolarized white light PFT appears homo- geneous and of dark brown colour, typical of

W. Brixel et al. / Flux JVOWlh and characJerizalion of perm.>s/,jtcs 401

Fe3+ ions. Between crossed polarizers pyramidal (100) growth sectors become observable (fig. 4).

The very weak nearly temperature independent birefringence (.:ln

=

^10-4) of these sectors is at- tributed to growth defects [13].

The study of ferroelectric domain patterns, spontaneous birefringence, optical absorption spectrum in the visible and DC-resistivity of PFT were some of the subjects of a recent paper [13].

The PCW platelets are composed of two types of growth sector differing strongly in absorption;

they are visible both in polarized and unpolarized light (fig. 5). In unpolarized white light those parts belonging to (100) growth pyramids appear dark red-brown and those ones of (111) pyramids arc weakly absorbing and appear red-orange. The sec-

b

-(100) natural face

(100) sectors

Fig. 5. (a) (lOO) PCW platelet showing clear (lll) and dark (100) pyramidal growth sectors; cut closely beneath natural (100) facet; thickness of sample 27 I'm; phase 11; crossed polars parallel to [1001. (b) Schematic (100) and (111) growth sectors with a monoclinic domain pattern, corresponding to

(a).

....

'

⁰ ....

"'

.!:!

-

,., _c

_..

^~

.,

c ~

c. 0

6 . - - - ,

5 4 3 2

0

Pb₂CoW0₆ (100)0 cut, 27~m thick

500

dark (lOO) growth sector

/

clear (Ill) growth sector ¹

600

wavelength [nm

J

⁷⁰⁰

Fig. 6. Visible absorption spectrum of PCW (thickness of sample 27 J.tffi, T ~ 298 K).

tor boundary does not influence the domain pat- tern in the non-cubic phases, i.e domain walls are found to run freely across the sector boundary. In the same sample the temperature of the mono- clinic to cubic phase transition was observed at 21.5

±

0.5°C in the dark (100) sector and at 25

±

0.5 o C in the clear (111) sector with a thermal hysteresis of ,;:; 0.5

°

C on both sectors.

Absorption spectra have been recorded on both types of sector between A= 490 nm and A= 750 nm, on a circular area (diameter= 300 j!m) of a (100) platelet, using a microscope combined with a microphotometer (Leitz MP1), a prism mono- chromator (Zeiss M4QIII) and a Xe lamp (fig. 6).

The spectra of both sectors are similar, with ab- sorption shoulders at A= 510 nm and A"" 560 nm, but a stronger wavelength-independent back- ground on the (100) sectors. The two shoulders at 510 and 560 nm are attributed respectively to the

4T₁g--. ⁴ T₁g(P) and ⁴T_1g--. ⁴ A_2g crystal-field tran- sitions of oxy-octahedrally coordinated high-spin Co²+ [34]. The strong rise in absorption at A< 500 nm is probably due to the Co²

++

W⁶

+--+

Co3+

+

W S+ charge transfer. Further optical studies have been carried out on PCW (birefringcnce of phase II and Ill, domain studies of phase II) and have been published elsewhere [10].

2.2.4. DC resistivity of PCW (resistivity of PFT, see ref [13})

In analogy with PFT, PCW shows a significant

402 W. Brixel et al. / Flux growth and chaructaizution of perovskites

13

12 cleor(lll)

E 11 growth s e c t a /

u

"'

^{10 '} /-dark ( 100)

-

Phase , Phase

'"'

^{n m}

-

⁹ growth sector

·;;

- ^.,

B

·;;;

.,

~

u ₀ 7

~

0 6

"'

0 5

3 4 5 6 7 B 9 10

reciprocal temperature 1000/T (K'') Fig. 7. DC electric resisti\ity of (100) and (111) growth sectors of PCW versus reciprocal temperature (applied field: 1.5 V, thickness of samples 34 I'm, cut from the same (lOO) platelet).

photoconductive effect, which could however only be evidenced qualitatively, as light absorption raises the sample temperature by several degrees above the set one. The observed decrease in re- sistivity under illumination was therefore a com- bination of thermal and photoconductive effects.

Dynamic measurements of the increasing resisitiv- ity by quick switching off of the light source are currently underway, permitting to separate the

Tahle 2

Some properties of PFT and PCW Properties (room temperature if not otherwise specified)

PFT

13,---,

_12

E i

Pb2CoW0

6 (lOO) cut,lhickn_: 34,um , T"'1751<

0

" 11

;..

\

~10

^\

~

clearC111) growth sector

~: ~- --~~

o

--==:===========d•=•k:I=IO=O=l~o•=~=t=h~•o~c~ro~r~==::J

£ 7 +---~~---··

0 5 10 15 20 25 30 35 40 45 50

applied potential (volts)

Fig. R. DC electrric resistivity of PCW versus applied field ('f -175 K, same samples as in fig. 7).

"photoinduced" effect from the thermal one, due to their different time constants. Two samples formed of a (111) and a (100) growth sector, respectively, were cut from a (100) platelet (34 p.m thick), electroded with transparent gold layers and contacted with gold wire. For resistivity measure- ments, a constant potential of 1.5 V was applied to the sample in darkness and the resulting cur- rent was measured with a picoamperemeter (ftg.

7). The higher resistivity of the (111) sector is consistent with its higher transition temperature to the cubic phase and the lower absorption, indicat- ing a higher degree of perfection of (111) sectors.

In contrast with PFT, which shows a linear cur-

PCW

Crystal structure of high temperature phase

Cubic, Pm3m, a_{0 -} 4.007 A

at RT

Cubic, Fm3m, a _{0 -} 8.010(3) A at 306 K

Colour

Fracture Cleavage

Hardness (Mohs' scale) Hardness (Vickers' scale)

Specific gravity at 293 K (g cm-³)

DC resistivity at 300 K ( Q cm)

Black with semi-metallic lustre.

brnwn in transmission (thickness, ..; 100 I'm), powder yellow-brown Conchoidal

None 5.5-6 650-700

9.44 (calculated 9.64) 1.57Xl0⁶

Black with semi-metallic lustre, red- brown in transmission (thickness

..; 40 Jtm), powder dark yellow-brown Conchoidal

11(100) and (110), only on thin platelets 3.5-4 (very brittle)

250-300

9.84 (cakulated 9.73) 2.82 X 10⁶, clear sectors 1.12 X 10⁵, dark sectors

W. Brixel et al. / Flux f(rowth and characterizatwn of pervu:skites 403 rent-voltage characteristic down to at least 5

V jcm, a strong increase of resistivity was ob- served in both (lOO) and (111) growth sectors in fields below - 3000 V /cm (corresponding to about 10 V on our samples; fig. 8). This behaviour is not yet understood.

..._ 3. Conclusions

The application of the ACRT method, for the first time, to the growth of PCW from a PbO flux was found to be highly advantageous, yielding individual well facetted (1 00 j111) single crystals if very slow cooling rates (0.4-0.7° C/h) were used. Analogous growth experiments of PFT from a PbO flux were less satisfactory, yielding much smaller and intergrown crystals of hopperlike shape, although flux composition, stirring, cooling rate and crucible size were varied systematically.

Further improvement of the growth of both PCW and in particular PFT single crystals from a PbO flux will require a detailed determination of the respective ternary phase diagrams Pb0-Co0- W03 and Pb0-Fe₂0 3- Ta20_{5 ,} as our DTA mea- surements have shown that the PCW-PbO and PFT-PbO sections of these diagrams are not of - pseudo-binary character.

Polarized light microscopy was found to be useful in the study of growth sectors, domain structures, structural transitions and the de- termination of the point groups of some phases [10,13]. Some properties of the PFT and PCW single crystals are shown in table 2.

Acknowledgments

The authors wish to thank Mrs. E. Despland and Dr. J. Bertrand for electron probe microanal- ysis measurements, Professor H.-J. Scheel and Dr.

J.-P. Rivera for helpful discussions, Mr. R .. Cros and Mr. E. Burkhardt for technical assistance, and Mrs. 0. Hirth for typing. This research was sup- ported by the Fonds National Suisse de la Re- cherche Scientifique (Projects No. 2.833-0.83 and No. 2.231-0.84).

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