Non-stoichiometry and optical spectra of Nd(III) substituted PbTiO3

Mat. Res. Bull. Vol. Ii, pp. 663-668, 1976. Printed in the United States.

P e r g a m o n Press, Inc.

NON-STOICHIOMETRY AND OPTICAL SPECTRA OF Nd(III) SUBSTITUTED PbTiO 3 J. Bouwma and M.A. Heilbron Twente University of Technology, Department of Chemical Engineering, Laboratory of Inorganic Chemistry and Materials Science, P.O. Box 217, Enschede, the Netherlands

(Received April 19, 1976; C o m m u n i c a t e d by G. H. Jonker)

ABSTRACT

The non-stoichiometry of the perovskite (ABO3)-type phase in the system PbO-TiO2-Nd203 has been studied. Monophasic compounds of composition Pbl_~xNdxTiO3+x( 1 5_e) with x ~ 0.21 and 0.09 ~ e ~ 1.5 were prepared. The'zerroelectric Curie temperature (T c) shows a decrease of 18.5 K/at% Nd with increasing value of x. T c shows an increase of 3.5 K/mol % PbO with decreasing value of (increasing content of PbO). The observed effect of on optical spectra can be interpreted by assuming that Nd(III) ions partly occupy B sites in compounds with < 1.5.

Introduction The perovskite (ABO3)-type phases in the systems PbO-TiO2-Ln203 (Ln = La,Sm) are highly non-stoichiometric (1,2). Unit-cell dimensions and dielectric properties of these phases depend Strongly on composition. We have interpreted this non-stoichiometry on the basis of a defectstructure with variable numbers of both Ln(III)-ions and vacancies distributed over A and B sites in the perovskite structure (I). A defectstructure of the same type has been proposed for the perovskite-type phase in the system SrO-TiO2-La203 which shows comparable non-stoichiometry (3). The defectstructures proposed are based on the results of X-ray diffraction studies. Additional evidence for these defectstructures, especially concerning the occurrence of Ln(III) at two different crystallographic sites, might be obtained from optical spectra of these titanates substituted with suitable rare earth ions. Lindop and Goodwin (4) studied EPR and optical spectra of CaO.2AI203 doped with Nd(III) ions (4f 3 electronic configuration). They concluded that Nd(III) ions occupy four different sites in this compound. 663

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In this paper we report results of our investigations on the homogeneity range, Curie temperature and optical spectra of Nd(III) substituted PbTiO 3. Experlmental

Par t

ceramic samples of composition Pbl_~xNdxTi03 +x(l .5-u) with x = 0.10, 0.18 and 0.21, respectively, and various values of (0.8 ~ u ~ 1.5) were prepared using standard techniques (i). Sintering was performed at 1520 K. The measured densities of the products were higher than 95% of the theoretical densities. The compositions of the samples were determined by X-ray fluorescence analysis. Homogeneity and unit-cell dimensions were determined from X-ray powder diffraction patterns. Curie temperatures were deduced from DTA curves. Diffuse reflectance, normal transmission and scattered transmission spectra in the frequency range 6,000-25,000 cm -I at room temperature were recorded using a Varian Cary 171 spectrophotometer. Low-temperature (down to 32 K) transmission spectra were recorded with a Perkin Elmer El4 spectrophotometer. Thin (~ I00 ~m) polished ceramic slices were used for transmission measurements. Homogeneity range and Curie temperature Table I gives a survey of the unit-cell dimensions and Curie temperatures of monophasic samples. For compounds with x = 0.21 no Curie temperatures could be detected from DTA measurements. The heat effects accompanying the ferroelectrlc-paraelectric transition are obviously below detection limit for this composition. From the results of the investigations on homogeneity range and Curie temperature of the perovskite-type phase, the following conclusions can be drawn. i) Pure compounds with 0.9 I a ~ 1.5 could be prepared. This means that the width of the homogeneity range (expressed in units of ~) at fixed Nd/Ti ratio (x) far from the solubility TABLE I Unit-cell Compounds x

Parameters and Curie Temperatures for various of Composition Pb, -~xNdxTiO3+x~ ,5-a) "

a(~)

c(~)

c/a

v(~3)

w (K) C

O.lO

1.42 1.25 i.09 0.99

3.9042 3.9048 3.9065 3.9095

4.0555 4.0559 4.0579 4.0637

1.039 1.039 1.039 1.039

61 . 8 2 61 . 8 4 61 . 9 2 62.11

589 591 598 603

0.18

1.54 1.42 1.20 1.10 0.99

3.9064 3.9066 3.9139 3.9175 3.9177

3.9733 3.9788 3.9866 3.9923 3.9958

I

017 !.018 1.019 1.019 1.020

60.63 60.72 61 . 0 7 61 . 2 7 61 . 3 3

422 433 444 454 462

0.21

1.48 0.95

3.9074 3.9211

3.9573 3.9786

1.013 1.015

60.42 61 . 1 7

Vol. II, No. 6

Nd SUBSTITUTED

LEAD

TITANATE

665

limit of Nd(III) in PbTiO 3 (Xma x ~ 0.4) equals almost the c o r r e s p o n d i n g widths of the h o m o g e n e i t y ranges of La(III) and Sm(III) substituted PbTiO 3 (I). ii)

An increase of the Nd/Ti ratio (x) of the phase at fixed value of e leads to a slight increase of the a-axis, and to d e c r e a s i n g values of c-axis, axial ratio c/a, unit-cell volume V and Curie temperature T c. The decrease of T c is about 18.5 K/at.% Nd(III). The corresponding values of La(III) and Sm(III) substitutions are 19.5 and 18.0, r e s p e c t i v e l y (i).

iii) An increase of the PbO content (decreasing ~) at fixed Nd/Ti ratio leads to greater values of a,c,c/a,V and T c. The increase of T c is about 3.5 K/mol% PbO. The corresponding values for La(III) and Sm(III) substitutions are 2 and 5, respectively (i). Obviously, the effect of Nd(III) substitution on various properties is intermediate between those of La(III) and Sm(III) substitutions. This is in accordance with the intermediate value of the ionic radius of Nd(III). Optical

spectra

The spectra show absorption bands at frequencies which are in good agreement with literature data on spectra of Nd(III) ions (4,5). Figures 1 and 2 show transmitted intensity (It), in arbitrary units, vs. wave number (~) for the two compounds with x = 0.21 (Table I). In the figures and in the d i s c u s s i o n of the spectra values of 1.5 and 1.0, respectively, are used for the sake of simplicity. Fig. 1 shows the influence of temperature on the shape of the absorption band around 17,000 cm -I, which is due to transitions from 419/2 qround state levels to 4G5/2 and 2G7/2 levels. Two effects can be distinguished. i) Resolution becomes better with d e c r e a s i n g temperature. ii) The absorption peak at about 16,600 cm -I, which is present in the spectra recorded at room temperature and at 100 K, is absent in the spectrum recorded at 32 K. This indicates that this peak is due to a transition from a ~tark level (belonging to the ~I9/2 manifold) which must be close (of the order of 100 cm -I) above the ground level.

I

i

I It

x lOCi:m4) ~s

\ d5

#o FIGURE

|

Transmitted intensity vs. wave number for the absorption band around 17,000 cm -I of a compound with x = 0.21 and ~ = 1.0 at 32, I00 and 295 K, respectively.

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J. B O U W M A

and M . A. H E I L B R O N

I

I

Vol. ii, No. 6

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T r a n s m i t t e d i n t e n s i t y vs. w a v e n u m b e r for two c o m p o u n d s w i t h x = 0.21, and ~ = 1.5 and 1.0, r e s p e c t i v e l y , at 32 K in the w a v e n u m b e r r e g i o n s l l t 0 0 0 - 1 4 , 0 0 0 cm -I (a), 1 6 , 5 0 0 - 1 8 , 0 0 0 cm -I (b) and 1 8 , 5 0 0 - 1 9 , 9 0 0 cm T M (c). T h e a r r o w s i n d i c a t e the a d d i t i o n a l a b s o r p tion p h e n o m e n a in the c o m p o u n d w i t h ~ = 1.0.

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Nd SUBSTITUTED

LEAD

TITANATE

667

The e f f e c t of u o n the s p e c t r u m of c o m p o u n d s w i t h x = 0.21 at 32 K c a n be s e e n f r o m Fig. 2 (a,b and c), w h i c h s h o w s d i f f e r e n t s p e c t r a l r e g i o n s . T h e s p e c t r a of the two c o m p o u n d s s t u d i e d , viz. w i t h ~ = 1.5 and 1.0, are v e r y similar, e x c e p t for some a d d i t i o nal small a b s o r p t i o n p e a k s and s h o u l d e r s in the s p e c t r u m of the c o m p o u n d w i t h u = 1.0. T h r o u g h o u t the s p e c t r u m t h e s e a d d i t i o n a l a b s o r p t i o n s are f o u n d at s l i g h t l y l o w e r w a v e n u m b e r s than t h o s e of the m a i n a b s o r p t i o n peaks. W i t h d e c r e a s i n g e l e c t r o n - e l e c t r o n r e p u l s i o n e n e r g y or i n t e r n a l e l e c t r i c f i e l d s t r e n g t h for a p a r t i c u l a r site, all a b s o r p t i o n p e a k s of a s p e c t r u m are s h i f t e d to l o w e r w a v e n u m b e r s . T h i s i n d i c a t e s t h a t a small p a r t of the Nd(III) ions o c c u p i e s a s e c o n d site in this c o m p o u n d . F r o m spectra, r e c o r d e d at r o o m t e m p e r a t u r e (not s h o w n here), it can be seen t h a t the a b s o r p t i o n b a n d s c o r r e s p o n d i n g to the s e c o n d site, d i s t i n c t l y i n c r e a s e w i t h d e c r e a s i n g ~ values. However, the r e l a t i v e l y l a r g e s p e c t r a l b a n d w i d t h s do not a l l o w to c a l c u l a t e the o s c i l l a t o r s t r e n g t h s of the i n d i v i d u a l bands. T h e r e f o r e , a q u a n t i t a t i v e i n t e r p r e t a t i o n c a n n o t be given. The f e a t u r e s , w h i c h the s p e c t r a of c o m p o u n d s w i t h d i f f e r e n t v a l u e s of e h a v e in common, m u s t be due to Nd(III) ions at A sites. M e r e l y on the b a s i s of the s p e c t r o s c o p i c r e s u l t s , it cannot be c o n c l u d e d y e t t h a t the s e c o n d site, w h i c h Nd(III) o b v i o u s l y o c c u p i e s in c o m p o u n d s w i t h u < 1.5, is i n d e e d the B site of the p e r o v s k i t e s t r u c t u r e . H o w e v e r , this c o n c l u s i o n a p p e a r s to be j u s t i f i e d by the r e s u l t s of our X - r a y d i f f r a c t i o n s t u d i e s on La(III) s u b s t i t u t e d P b T i O ~ (I) and SrTiO. (3). T h e s e r e s u l t s h a v e s h o w n that a d e f e c t s ~ r u c t u r e w i t h b ~ t h La(III) ions and vac a n c i e s d i s t r i b u t e d o v e r A and B sites is m o s t l i k e l y for t h e s e compounds. In the d e f e c t s t r u c t u r e p r o p o s e d no B - s i t e d e f e c t s e x i s t in c o m p o u n d s w i t h ~ = 1.5 (Table 2). Hence, in t h e s e c o m p o u n d s the rare e a r t h ions o c c u p y A s i t e s only, g i v i n g r i s e to r a t h e r s i m p l e o p t i c a l spectra. The d e f e c t c o n c e n t r a t i o n s in c o m p o u n d s w i t h < 1.5 are not f u l l y d e t e r m i n e d by c o m p o s i t i o n . Two e x t r e m e c a s e s w h i c h are c h a r a c t e r i z e d by a n e g l i g i b l e n u m b e r of rare e a r t h ions at B sites (case I), and a n e g l i g i b l e n u m b e r of B - s i t e v a c a n c i e s (case II), r e s p e c t i v e l y , m u s t be d i s t i n g u i s h e d . The TABLE

2

N u m b e r of D e f e c t s per Unit Cell for Ln(lll) s u b s t i t u t e d PbTiO^ or SrTiO_ with x = 0.20 and ~ = 1.5 and 1.0, resp e c t i v e l y . For ~he C o m p o u n d with e = 1.0 s e p a r a t e N u m b e r s are g i v e n for the two e x t r e m e Cases I and II. defect type

number ~ = 1.50

number ~ = 1.00 case

Ln A VA Ln B VB

0.20 O.iO

0.194 0.032 0.032

I

case 0.162 0.064 0.032

II

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Vol. ii, No. 6

defect concentrations in a compound with x = 0.20 and ~ = 1.0, are given in Table 2 for these cases. The actual defect concentrations are intermediate (1,3). It is likely that this defectstructure also holds for Nd(III) substituted PbTiO.. Therefore, it is to be expected that ~ome 10% of the Nd(III~ ions occupy B sites in a compound with x = 0.21 and m = 1.0, giving rise to additional optical absorption phenomena. This means that the observed effect of e on optical spectra supports, at least qualitatively, the defectstructure proposed. Acknowledgements We are highly indebted to H.J.L. van der Valk of the University of Groningen for making available the Perkin Elmer spectrometer and for his assistance in the low-temperature measurements. We wish to thank G.C. Tiekstra, H. Kruidhof and F.J. Carelsen for their technical assistance and A.J. Burggraaf and P.J. Gellings for useful discussions. References 1

K.J. de Vries, K. Keizer, J. Bouwma Science of Ceramics 8, 187 (1975).

and A.J.

2

D. Hennings,

(1971).

3

J. Bouwma, K.J. de Vries and A.J. Phys. Stat. Sol. (a) 35, (1976).

4

A.J. Lindop and D.W. 6, 1818 (1973).

5

G.H. Dieke, Spectra and energy crystals, Interscience (1968).

Mat.

Res.

Bull.

6,

Goodwin,

329

Burggraaf,

Burggraaf,

accepted

by

J. Phys.

C: Solid State Phys.

levels

of rare earth ions in