Synthesis and symmetry transformation in the perovskite compounds PbHfO3 and CdHfO3

Mat. R e s . Bull. Vol. 10, pp. 187-192, in the U n i t e d S t a t e s .

1975.

Pergamon Press,

Inc.

Printed

SYNTHESIS AND SYMMETRY TRANSFORMATION IN THE PEROVSKITE COMPOUNDS PbHfO 3 and CdHfO 3 P. D. Dernier and J. P. Remeika Bell Laboratories Murray Hill, New Jersey 07974

( R e c e i v e d J a n u a r y Z, 1975; R e f e r e e d )

ABSTRACT From high temperature X-ray diffraction experiments the perovskite compounds PbHf03 and CdHf0~ have been shown to undergo a series of structural phase transformations, oAt 298°K PbHfO~oiS orthorhombic with cell dimensions a = 5.8572(5)A, b = II.689(I)A, c = 4.0971(4)A and most probable space group Pnam. At 450°K it is rhombohed~al with hexagonal cell dimensions a = 5.854(I)A and c = 7.145(2)A]and at 520 K PbHfO 3 is cubic perovskite with a = 4.1354(4)A. CdHfO 3 ~s also orthorhombic at 298°K w~th cell dimensions a = 5.5014(8)A, b = 5.6607(8)A, c = 7.969(1)A, and space group Pbnm. At 1075°K CdHfO~ is rhombohedral with hexagonal cell dimensions a = 5.747(4)A and ~ = 13.49(1)A

Introduction For many years considerable experimental work has been carried out on the perovskite compounds PbHfO 3 and to a lesser extent CdHf03 but very little X-ray diffraction data has been reported for the various phases at elevated temperatures. In previous X-ray powder diffraction studies of PbHfO 3 by Shirane and Pepinsky (1) and of CdHf03 by Averyanova et al. (2) tetr~gonally distorted intermediate temperature phases have been suggested. However from our studies of other ABOq compounds, such as the LnGa03 series (3), LnFe03 series (4), and particularly the symmetry transformation across the LnAlO 3 series (5) the tetragonal symmetry of these high temperature hafnates appeared unlikely. In this paper we report the synthesis and X-ray powder diffraction data for PbHf03 and CdHfO~ at ambient and elevated temperatures. In addition some preliminary assignmefits of most probable space groups are presented based on single crystal precession photographs and comparative analysis. Experimental Single crystals of PbHfO~ were grown using PbO and B203 as the high temperature solvent. With thls technique the common ion effect was utilized in that part of the PbO became a major component of the compound. Powders of HfO 2, (1.50 gms), B203, (4.00 gms), and PbO, (60.00 gms), were weighed into a 187

188

PEROVSKITE

CORIPOUNDS

Vol. I0, No. 3

lO0 c.c. platinum crucible. The crucible was covered with a platinum lid and loaded into a horizontal, resistively powered furnace. The ambient atmosphere was air. The temperature was raised to 1325°C, held for 16 hours, and then controlled cooling at a rate of 5.0°C/hr. was initiated. Typically the furnace was control cooled to ~500°C when the crucible was removed and allowed to cool to room temperature. Extraction of the crystals was achieved using a volume ratio of 1:3 of HNOq and H20. Resulting crystals were light tan in color and up to 4 mm on an ~dge of a prismatic rectangle. Calibrated X-ray fluorescence analysis gave in atomic weight percent, Pb = 48.2 and Hf = 40.7. Calculated quantities for PbHfO 3 are Fo = 47.76 and Hf = 41.17. Attempts to prepare CdHfO B from this solvent did not succeed. PbHfO 3 appears to be the more stable phase and little or no Cd was found in the resultant crystals. Small crystals, (~ 1 mm2 X 0.2 mm), were obtained using CdF 2 as the solvent. CdO (5.14 gm~), HfO 2 (8.42 gins), and CdF 2 (28.00 gms), were weighed into a lO0 c.c. platinum crucible and heated to 1300°C, held for 4 hours and cooled at 3.O°C/hr. to ~600°C. Extraction was done using about l:l, CHBCOOH and H20. Analysis gave Cd = 32.4, Hf = 53.5 with calculated quantities for CdHfO 3 being Cd = 31.16 and Hf = 52.68. All powder diffraction experiments at elevated temperatures were performed on crushed crystals which were mixed with silicon powder as an internal standard. At 298°K Guinier and Debye-Scherrer films were taken of PbHfO 3 and CdHfO 3 respectively. High temperature diffraction patterns were obtained by use of a platinum resistance strip device mounted on a Philips/Norelco X-ray diffractometer. The scanning rate was 1/4 ° per minute with 1 ° divergence slit. The radiation used was CuK with a bent LiF crystal monochromator on the detection arm of the diffractometer. Complete 2e scans were taken of PbHfO 3 at 450 ° and 520°K. These temperatures were well within the stability ranges of the respective phases (6). The high temperature transition in CdHf03, > 900°K, was near the l~m~ts of temperature control for the resistance device and this necessarily restricted the data collection to a set of unique lines in the front reflection region. These included the orthorhombic cell reflections (004), (220), (020), (ll2), and (200). Precession photographs of small rhombic crystals of PbHfO 3 and CdHfO B were taken at 298°K with MoKG radiation. For both crystals the layers taken were hkO, hkl, and hh&. The first layer of PbHfO 3 however was anomalously weak even after 72 hours of exposure. This layer was repeatedwith unfiltered radiation over 48 hours in order to increase the intensities. In addition small crystals of PbHfO 3 and CdHfOq were checked for piezoelectricity at room temperature using a modified Giebe~Scheibe circuit (7). However, no piezoelectric signals were detected by this method indicating that the crystals may be centrosymmetric or the coupling coefficient may be too small. Results In Table 1 observed and calculated d-spacings are reported for PbHfO 3 at 298°K, 450°K, and 520°K. The room temperature parameters were taken from a Guinler film with CuK~ radiation and correctedwith a KC1 internal standard. Pertinent unit cell data are as follows: 298°K ~50°K 520°K

a =

5.8572(5)

b = 11.689(i) c = 4.0971(4) = 90.0 ° 0 = 90.o 7 = 90.0 Vol./(cubic cell~ 70.13 (A) 3

5.85 (i) 5.854(1) 7.145(2) 90.0 ° 90.0 120.0 70.68 (~)3

.1354(4) 4.1354(4) 4.1354(4) 90.0 ° 90.0 90.0 o 70.72 (A) 3

Vol. I0, No. 3

PEROVSKITE

COMPOUNDS

189

TABLE 1 Interplanar d-spacings for PbHfO 3 2~8 K

ii0 iii 200 040 121 210 201 211 240 0O2 31o 151 311 321 161 2O2 042 170 1 00 080 242 360 ~40 280 123

~3

402 082

vw vw

5.241 3.227

ms vs w vw vvw m wm

2.927 2.912 2.839 2.382 2.335 2.069 2.049 1.922

VWbr. vw

1.456 1.381 1.309 1.307 1.297

5.237 3.227 2.929 2.922 2.911 2.840 2.382 2.335 2.069 2.049 1.925 1.918 1.742 1.687 1.685 1.679 1.677 1.606 1.606 1.46~ 1.461 1.456 1.379 1.309 1.307 1.297

1.191

1.190

1.741

ms

1.686

m

1.678

vw

1.605 1.46~

Wbr. wm vvw w wm m w

4~0

1.191

K

~20

K

ak~

I --

~bs.

~al~.

-a -k ~

--

z

~bs.

a --calc.

ii0 012

vs

2.920

2.924

2.916

2.927 2.920

Ii0

vs vw

2.382

111

vvw

2.385

2.387

s

2.067

2.388 2.382 2.067

200

vs

2.064

2.068

030 122 104

ms

1.686

1.688

1.686 1.683

1.689 1.688 1.684

211

s s

220 024

wm wm

1.461 i.~58

1.463 1.460

220

wm

1.461

1.462

312 214

wm wm

1.307 1.302

1.308 1.306

310

m

1.307

1,307

vw

1.191

222

w

1o193

1.193

~n%r.

i.i03

vvw

i.o]3

wm w wm wm wm vw

0"97~m 0.9722 0.97o7 0.9247 0.9239

1.194 1.191 1.106 1.1o5 l.lOh 1.1o3 1.o34 0.9756 0.9754 0.9746 0.9733 0.9253 0,9236 0.9028 0.9020

003 021 202

006 042 140 116 134 232 404 350 5o2 324 ~06 ~22 226 235 333 152 o54 018 600 244 208 520 342 514 146 128

w wm wm w ~ w

0.9017 0.8812 o.88Ol 0.8783 o.8~41 0.8436 o.8415

wm

0.8111

wm

0.8099

0.8823 o.8818 0.8795 o.8h49 0.8442 0.8423 o.8118 0.8116 0.8112 0.8104

wm

0.8090

0.809#

321

wm

1.i05

1.105

400 410 322 330 411

vw

1.033

vvw

1.0o3

m

0.9743

1.033 i.oo3 i.o0~ o.97~7 o.9747

420

wm

0.92#5

0.9247

421

w

0.9022

0.9024

332

w

0.8815

0.8816

422

w

0.8~i

o.8#41

51o #31

m

o.81o9

o.811o

In Table 2 observed and calculated d-spacings are reported for CdHfO 3 at 298 K taken from a Debye-Scherrer film with Cr, Ks radiation. Lattice parameters and volumes for a few select temperatures are as follows: 298°K

a =

~.5o14(8) ~

b = 5.6607(8) c = 7.969(1) = 90.0 ° = 90.0 7 = 90.0

875°K 5.5~8(2) 5.714(2) 7.994(3) 90.0 ° 90.0 90.0

VOl./(cubic cell~ 62.04 (A) 3

63.47 (~)3

i075°K O

5,747(4) A 5.747(4) 13.~9(~) 90.0 °

90.0 120.0 64.31 (~)3

190

PEROVSKITE

I

r

I

] (024)

Interplanar d-spacings for CdHfO 3 °K

975°K k

751K

(22.010 ~(oo4)o

(zZO~o loo%

~. ;

675*K \

I 47 °

4G• ~

28

,

TABLE 2

H

~~

Vol. I0, No. 3

I

/ ~5

_

COMPOUNDS

L

45 °

FIG. 1 X-ray powder d i f f r a c t i o n profile~ of the CdHfO 3 orthorhombic cell reflections (220) and (004) versus temperature.

hk~

~

~bs.

!calc.

ii0 020 112 200 i13 004 220 023 221 130 114 222 131 024 132 204 312 133 040 224 041 400 042 134

w m s wm vvw ms ms vw vw w w w w

3.922 2.812 2.786 2.733 2.195 1.985 1.965 1.930 1.908 1.779 1.773 1.762 1.737

ms m ms w w ms vw w w w

1.625 1.609 1.594 1.479 1.413 1.399 1.392 1.373 1.332 1.328

3.945 2.830 2.803 2.751 2.203 1.992 1.973 1.937 1.915 1.784 1.778 1.768 1.742 1.629 1.629 1.614 1.598 1.481 1.415 1.402 1.393 1.375 1.333 1.329

s

1.258

m vw m vw w wm w w ms

1.248 1.242 1.236 1.222 1.202 1.199 1.181 1.178 1.154

24o 116 043 332 241 420 421 026 242 422 333 044

1.259 1.258 1.249 1.249 1.243 1.237 1.222 1.202 1.200 1.181 1.178 1.154

Figure i shows the orthorhombic reflections (oo4) o and (22O) o coalescing into the hexagonal cell reflection (024)H as a function of increasing temperature. In a similar fashion the reflections (ll2)o and (200) o coalesce into the (104)Hwhile the (020)o becomes the (llO)H. In Fig. 2 the relative volume of the CdHfO 3 unit cell is plotted as a function of T. The rapid increase in volume over the 200 ° span prior to the orthorhombic-rhombohedral transition should be noted since it is strongly correlated with the dielectric permeability previously reported by Averyanova et al. (2). • i "C From precesslon photographs of a small rnombl crystal of PbHfO 3 at 298~K the following types of reflections were observed: (hkg) no conditions, (hkO) no conditions, (hO~) h = 2n, (hO0) h = 2n, (OkO) k = 4n, (OOg) ~ = 2n, (Okl) absent. Apparently due to stron~ Dseudocubic symmetry the intensities of the reflections (O,2n,O) and (O,k,2n+l) were not strong enough to be

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3

PEROVSKITE

COMPOUNDS

191

observed even after I [ 1 long exposures (72 hrs.). In addition none of these TRtGONAL reflections were observed in t.0,1 the Guinier, t .03 Debye-Scherrer, or diffractometer 1.02 tracings. To test ~ ,.o, for a possible doubling of the tOO' c-axSs, which would 0.99 suggest that PbZrO 3 and FbHfO 3 are isostructural (8), long exoosures of [ I I [ the (Ok~2) level 500 700 900 t 100 were made. However, no reflections were TEMPERATURE (*K) observed and the only space groups FIG. 2 remaining, which are commensurate with Relative volume of the CdHfO 3 pseudo-cubic unit cell is plotted against temperature. these observations, are Priam or the corresponding polar Pna2 I.

I I

I I

The intermediate temperature phase of PbHfO 3 has rhombohedral symmetry, expressed as a hexagonal cell, as clearly seen by the high angle triplets reported in Table I. These triplets are derived from the unique cubic reflections (332)c and (422)c. A tetragonal distortion of the perovskite cell as was reported (i) cannot account for these observed d-spacings. The distortion from cubic dimensions is very slight indeed. An ideal perovskite cell expressed as a hexagonal cell has a (c/a) 2 ratio of 1.5000 whereas PbHfO 3 at 450 K has a corresponding value of 1.4895 or an 0.7% distortion. By comparison the perovskite NdAIO 3 (5) has a 1.9% index of distortion. The deviation of PbHfO 3 from the ideal structure is also very slight since no weak superlattice reflections were observed which would require a doubling of the c-axis (as was true in the case of NdAl03). In fact, from powder diffraction data the choice of space group is still ambiguous even with the smaller cell since the extinctions are commensurate with groups R-3m, R3m, R3c, and R3c. Unfortunately the only distinguishing structural characteristic is the particular distortion that the oxygen array assumes. Precession photographs of a small CdHfO 3 crystal at 298°K were somewhat simpler to index and interpret. The following types of reflections were observed: (hk~) no conditions, (hkO) h + k = 2n, (hOt) h + ~ = 2n, (Ok~) k = 2n, (hO0) h = 2n, (OkO) k = 2n, (006) g = 2n. These extinctions are consistent with the same space groups chosen for PbHfO 3 but with different cell orientations, Pbum and Pbn21, and cell dimensions. Presumably the c-centering restriction on the (hkO) reflections is due to some structural pseudorelationship since the total set of extinctions is not commensurate with any other orthorhombic space group. The high temperature phase of CdHfO 3 also has rhombohedral symmetry which again disagrees with previous work where a tetragonal distortion has been proposed (2). The reflections (104)H and (l!O)H can be thought of as being

19Z

PEROVSI~/TE

COMPOUNDS

Vol. I0, No. 3

derived from the perovskite reflection (llO)c while the (O24)H is generated from the (2OO)c. On can ~mmediately see that if a tetragonal symmetry were to hold the (200)c would become a clearly resolved doublet while the (llO)c would become less well resolved. The powder diffraction data show just the reverse. The distortion from cubic cell dimensions is relatively great in the case of CdHfOq, even at 1075°K, since the (c/a)2 ratio is 5.51 as compared to the ideal 6.00 value. This leads to an index of distortion of ~ 8.2% as campared to 1.9% for NdAlO 3 and 0.7% for PbHfO 3. It is quite likely that the structural distortion is also large and hence the most probable space groups are RBc and the corresponding polar group RBc. In these space groups the larger cell with doubled c-axis permits greater freedom of distortion in the oxygen array. Conclusion In contrast to previous results the intermediate temperature phase of PbHfO B and the high temperature phase of CdHfO 3 are rhombohedral. For PbHfO 3 the pgoper symmetry sequence with increasing T-is orthorhombic-rhombohedralcubic. The same sequence holds for CdHfO B but the cubic phase has not yet been observed. The most probable space groups for the corresponding phases of PbHfO~ with increasing temperature are Pnam - (R-Bin, R~c) - PmBm. From the negatlve results of the piezoelectric measurements the sequence Pna21 - (RSm, RBc) - PmBm appears less likely. For CdHfOq the most probable sequence is Pbnm - R3c. The metric and structural distortions from cubic symmetry are much greater for the respective phases of CdHf03 than for PbHfO 3 as would be expected from radius ratio considerations. HoweVer, the final assignment of space group and quantitative structural determinations must await future single crystal X-ray diffraction studies. Acknowledgment s We wish to thank E. M. Kelly for assistance in crystal preparation and A. S. Cooper for X-ray powder diffraction photographs. References 1.

G. Shirane and R. Pepinsky, Phys. Rev. 91, 812 (1953).

2.

L. N. Aver'yanova, I. N. Belyaev, Yu. I. Gol'tsov, L. A. Solov'ev, R. I. Spinko and O. I. Prokopalo, Sov. Phys.-Solid State lO, 2698 (1969).

3.

M. Marezio, J. P. Remeika and P. D. Dernier, Inorg. Chem. 7, 1337 (1968).

4.

M. Marezio, J. P. Remeika and P. D. Dernier, Acta Cryst. B26, 2008 (1970).

5.

M. Marezio, P. D. Dernier and J. P. Remeika, Solid State Chem. _~, ll (1972).

6.

G. A. Samara, Phys. Rev. B1, 3777 (1970).

7.

"Piezoelectric Crystals and Their Application to Ultrasonics". W. P. Mason, D. Van Nostrand Co., Inc. pg. h8 (1950).

8.

F. Jona, G. Shirane, F. Mazzi and R. Pepinsky, Phys. Rev. 105, 849 (1957).