ON STRUCTURAL SYSTEMATICS OF THE LANTHANIDES UNDER PRESSURE

HAL Id: jpa-00224325

https://hal.archives-ouvertes.fr/jpa-00224325

Submitted on 1 Jan 1984

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

ON STRUCTURAL SYSTEMATICS OF THE LANTHANIDES UNDER PRESSURE

W. Grosshans, W. Holzapfel

To cite this version:

W. Grosshans, W. Holzapfel. ON STRUCTURAL SYSTEMATICS OF THE LANTHANIDES UNDER PRESSURE. Journal de Physique Colloques, 1984, 45 (C8), pp.C8-141-C8-143.

�10.1051/jphyscol:1984825�. �jpa-00224325�

JOURNAL DE PHYSIQUE

Colloque C8, supplement au n ° l l , Tome 45, novembre 1984 page C8-141

ON STRUCTURAL SYSTEMATICS OF THE LANTHANIDES UNDER PRESSURE W.A. Grosshans and W.B. H o l z a p f e l

Faehbereich Physik, Univer>sittUt-GH-Paderborn, D-4790 Paderborn, F.R.G.

Resume - Pour tous les lanthanides t r i v a l e n t s r e g u l i e r s , on a observe sous pression j u s q u ' a 40 GPa l a sequence suivante de s t r u c t u r e s c r i s t a l l i n e s : hep - s t r u c t u r e type Sm - dhep - fee - fee d i s t o r d u e . De p l u s , on a prou- ve une d e l o c a l i s a t i o n des e l e c t r o n s f non seulement dans l e praseodyme mais egalement dans l e neodyme e t l e samarium. Les r e s u l t a t s pour 1'europium e t 1 ' y t t e r b i u m sont d i s c u t e s en envisageant 1'existence d'un e t a t de valence i n t e r m e d i a i r e .

INTRODUCTION

The l a n t h a n i d e metals scandium ( S c ) , y t t r i u m (Y) and lanthanum (La) t o l u t e c i u m (Lu) are a l l t r i v a l e n t i n the m e t a l l i c s t a t e a t ambient c o n d i t i o n s w i t h the exception of d i v a l e n t europium (Eu) and y t t e r b i u m ( Y b ) . The rare e a r t h c r y s t a l s t r u c t u r e sequence fee - dhep - Sm type - hep i s observed across the lanthanide s e r i e s w i t h i n c r e a s i n g atomic number. I t was r e a l i z e d i n e a r l i e r high pressure studies ( 1 - 4 ) t h a t the r a r e e a r t h c r y s t a l s t r u c t u r e sequence also appears i n i n d i v i d u a l elements by the a p p l i - c a t i o n of pressure. Duthie and P e t t i f o r (5) r e l a t e d the pressure induced s t r u c t u r a l sequence to the increase o f the d band occupancy N . from an sp - d t r a n s f e r under pressure. Furthermore, Johansson and Rosengren ( 6 ) p r e d i c t e d a t r a n s i t i o n towards the t r i v a l e n t s t a t e f o r Eu and Yb. The present studies were intended to c o l l e c t f u r t h e r experimental support f o r these general i d e a s .

EXPERIMENTAL

The lanthanide elements w i t h the exception o f cerium and promethium were i n v e s t i - gated to 40 GPa and above i n a gasketed diamond a n v i l c e l l by energy d i s p e r s i v e X-ray d i f f r a c t i o n w i t h synchrotron r a d i a t i o n . The d e t a i l s of the experimental techniques are given elsewhere ( 7 ) . The use of n i t r o g e n as pressure t r a n s m i t t i n g medium leads to nearly h y d r o s t a t i c c o n d i t i o n s over the whole pressure range. The Ruby R,-fluorescence method was used f o r pressure determination w i t h the c a l i b r a t i o n constant o f 365 pm/GPa ( 8 ) .

THE REGULAR LANTHANIDES

Experimental evidence f o r the r e g u l a r rare earth c r y s t a l s t r u c t u r e sequence has already been reported f o r various lanthanides i n the lower pressure region ( 9 , 1 0 ) . The present studies on the r e g u l a r t r i v a l e n t rare e a r t h metals Y, La, Pr, Nd, Sm, Gd, Td, Dy, Ho, E r , Tm and Lu c o n f i r m the s t r u c t u r a l sequence hep - Sm type - dhep - fee - d i s t . fee f o r a l l these elements i n the a v a i l a b l e pressure range ( 1 1 , 1 2 ) . Y, Gd and Tb undergo the whole s e r i e s o f t r a n s i t i o n s from hep to d i s t . f e e . A l l these elements show a smooth behaviour i n t h e i r P-V data w i t h o u t any n o t i c e d

volume change a t the phase t r a n s i t i o n s ( c f . f i g . l f o r e . g . Gd). The occurence o f the second order phase t r a n s i t i o n from fee to d i s t . fee ( 1 3 ) , the u l t i m a t e high

Abs t r a c t - The c r y s t a l s t r u c t u r e séquence hep - Sm type - dhep - fec - d i s t . fec has been observed f o r a i l the r e g u l a r t r i v a l e n t lanthanides under pressures up to 40 GPa. In a d d i t i o n , évidence f o r a d e l o c a l i z a t i o n o f the f é l e c t r o n s i s observed not only i n praseodymium, but a l s o i n neodymium and samarium. The data f o r europium and y t t e r b i u m are discussed w i t h respect to a p o s s i b l e mixed v a l e n t s t a t e .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984825

C8-142 JOURNAL DE PHYSIQUE

p r e s s u r e phase o f t h e r e g u l a r l a n t h a n i d e s , i s due t o a g r a d u a l d i s t o r t i o n o f t h e f c c l a t t i c e and can be r e l a t e d t o s o f t phonon modes n e a r t h e L p o i n t i n t h e B r i l l o u i n zone (13-15). Vohra e t a l . ( 1 6 ) a s s i g n e d t h e rhombohedra1 space group P3 2 1 t o t h i s d i s t o r t e d c u b i c l a t t i c e . F i n a l l y , i t s h o u l d be mentioned, t h a t t h e r e g u l t s on Y s u p p o r t t h e view t h a t no b o n d i n g c o n t r i b u t i o n f r o m f e l e c t r o n s i s r e q u i r e d f o r t h e s t a b i l i z a t i o n o f t h e r e g u l a r r a r e e a r t h s c r y s t a l s t r u c t u r e s . f BONDING I N RARE EARTH METALS

The s p e c i a l p r o p e r t i e s o f Ce ( 1 7 ) and t h e l i g h t a c t i n i d e s a r e g e n e r a l l y a t t r i b u t e d t o d e l o c a l i z e d f e l e c t r o n s . Furthermore, i t i s r e c o g n i z e d t h a t t h e a p p l i c a t i o n o f p r e s s u r e t o l i g h t l a n k h a n i d e s (Ce, Pr,. . . ) and heavy a c t i n i d e s (Am, Cm,. . ^{. ) can}

f o r c e t h e l o c a l i z e d f c o n f i g u r a t i o n t o p a r t i c i p a t e i n b o n d i n g which l e a d s t o t h e appearence o f l o w symmetry c r y s t a l s t r u c t u r e s . A t y p i c a l 1 i g h t a c t i n i d e c r y s t a l s t r u c t u r e ( a - u r a n i um) has been i d e n t i f i e d e x p e r i m e n t a l l y i n Ce between 5 GPa and 13 GPa ( 1 8 ) . 4 f d e l o c a l i z a t i o n was a l s o r e p o r t e d f o r P r ( 1 9 ) , where a new phase above 20 GPa was i n d e x e d as hexagonal. More r e c e n t l y (20,21), however, t h i s

s t r u c t u r e was i d e n t i f i e d as a-uranium. The P-V r e l a t i o n s h i p o f P r i s shown i n f i g . 1 t o g e t h e r w i t h d a t a f o r Gd f o r comparison. The d e l o c a l i z a t i o n i s accompanied by a volume c o l l a p s e o f a b o u t 9.3% and a s t e e p i n c r e a s e o f t h e b u l k modulus.

DIVALENT Eu AND Yb

i

'w ³⁰ . COLLAPSE:

L (3

\

?! 20 -

3

V ) V )

L

10

.

A d i f f e r e n t b e h a v i o u r i s observed i n t h e r a r e e a r t h elements Eu and Yb ( 9 ) , which a r e c l o s e t o t h e d i v a l e n t s t a t e under ambient c o n d i t i o n s . A t h i g h e r pressures, t h e s e elements were e c p e c t e d t o e x h i b i t normal t r i v a l e n t b e h a v i o u r w i t h t h e r e g u l a r s t r u c t u r a l sequence ( 6 ) . The observed sequences a r e however

f cc 4GPa, bcc -hcp 35GPa f o r Yb (22-24) and bcc - 12.5GPa ^{hcp -Eu} 32GPa 111 f o r Eu (23, 2 4 ) . The h c p - l i k e phase o f Eu I11 has been r e p o r t e d by Takemura e t a1 . ( 2 5 ) t o o c c u r a l r e a d y a t 18 GPa, whereas t h e p r e s e n t s t u d y found evidence f o r t h i s s t r u c t u r e o n l y above 32 GPa.

I n t h e p r e s e n t i n v e s t i - g a t i o n , a new h i g h p r e s s u r e s t r u c t u r e was found i n Nd and Sm. The t r a n s i t i o n p r e s s u r e s a r e 4 1 ( 1 ) GPa f o r Nd, b u t o n l y 3 3 ( 4 ) GPa f o r Sm.

The n a t u r e o f t h i s new phase i s n o t y e t c l e a r . b u t t h e symmetry seems t o be o r t h o r h o m b i c o r even l o w e r . The decrease i n t h e t r a n s i t i o n p r e s s u r e f r o m Nd t o Sm s t i m u l a t e d s i m i l a r expe- r i m e n t s on Gd and Tb, b u t no change i n c r y s t a l

2 0 2s 3 0 3 5 4 0 s t r u c t u r e s f r o m d i s t .

Atomic w m e / A ' f c c o c c u r e d i n Gd up t o 47 GPa and i n Tb up t o 40 GPa. I t s h o u l d be noted, t h a t t h e o c c u r - Fig.1: Volume vs. p r e s s u r e f o r praseodymi urn and gado- ence o f 1 ow symmetry

1 in i u m a t room temperature. The volume c o l l a p s e c r y s t a l s t r u c t u r e s i n a t t h e t r a n s i t i o n f r o m d f c c t o a-U i n P r i s P r and most p r o b a b l y

a b o u t 9.3% i n Nd and Sm f i t s i n t o

t h e s y s t e m a t i c s o f r a r e

e a r t h s , where such s t r u c t u r e s appears under p r e s s u r e a t a c r i t i c a l amount o f f

bonding, though t h e r e l a t i v e low t r a n s i t i o n p r e s s u r e i n Sm needs some f u r t h e r

t h e o r e t i c a l c o n s i d e r a t i o n s .

A comparison o f Eu and Yb w i t h i t s neighbours Gd and Lu p o i n t s t o a c o n t i n u o u s approach towards t h e t r i v a l e n t s t a t e as i n d i c a t e d a l s o by L edge a b s o r p t i o n s t u d i e s on Yb ( 2 6 ) . However, t h e occurence o f t h i s new phas&lin Eu may i n d i c a t e , t h a t europium shows e i t h e r an i n t e r m e d i a t e valence o v e r a wide range o f pressures o r even more complex b e h a v i o u r .

SCANDIUM

The 3d metal Sc does n o t undergo t h e r e g u l a r r a r e e a r t h s t r u c t u r a l sequence. Sc t r a n s f o r m s i n a s l u g g i s h t r a n s i t i o n between 20 GPa and 26 GPa f r o m hcp t o t e t r a g o n a l p-Np s t r u c t u r e ( 2 7 ) w i t h o u t any f u r t h e r t r a n s i t i o n up t o 40 GPa. T h i s d i f f e r e n c e be- tween Sc and Y i n d i c a t e s t h a t s i m p l e c a n o n i c a l band t h e o r y c a l c u l a t i o n s cannot e x p l a i n t h i s s t r u c t u r a l sequence i n d e t a i l .

ACKNOWLEDGMENT

The r a r e e a r t h m e t a l s have been k i n d l y s u p p l i e d by K.A. Gschneidner, Iowa S t a t e U n i v e r s i t y , Ames, Iowa USA.

T h i s work was performed a t HASYLAB, DESY and s u p p o r t e d i n p a r t by t h e Bundes- m i n i s t e r i u m f U r Forschung und Technologie.

REFERENCES

1 ) A. Jayaraman and R.C. Sherwood, Phys. Rev. L e t t . 12, 22 (1964) 2 ) A. Jayaraman and R.C. Sherwood, Phys. Rev. 134, A m 1 (1964) 3) D.B. McWhan and A.L. Stevens, Phys. Rev. 139,4682 (1965) 4 ) D.B. McWhan and A.L. Stevens, Phys. Rev. m, 438 (1967) 5 ) J.C. D u t h i e and D.G. P e t t i f o r , Phys. ~ e v . x t t . 38, 564 (1977) 6 ) B. Johansson and A. Rosengren, Phys. Rev. B 11, n 3 6 (1975)

7 ) W.A. GroRhans, E.F. Diising and W.B. Holzapfe7T H i g h Temp., H i g h Press. ( i n p r e s s ) 8 ) G.J. P i e m a r i n i , S. Block, J.D. B a r n e t t and R.A. Forman, J. Appl. Phys. 5 , 2774

(1975)

9 ) A. Jayaraman, i n Handbook on t h e Physics and Chemistry o f Rare Earths, eds. K.A.

Gschneidner and L. E y r i n g , Vol 1, pp. 707 (1978) and r e f e r e n c e s h e r e i n 10) A. Nakaue, J. Less Comm. Met. 60, 47 (1978)

1 1 ) Y .K. Vohra, H. 01 i j n y k , W.A. GroBhans and W.B. Hol z a p f e l , Phys. Rev. L e t t , 47,

1065 (1981)

12) W.A. GroRhans, Y.K. Vohra and W.B. H o l z a p f e l , J.M.M.M. 2, 282 (1982)

13) W.A. GroBhans, Y.K. Vohra and W.B. H o l z a p f e l , Phys. Rev. L e t t . 49, 1572 (1982) 14) C. S t a s s i s , C.-K. Loong and J. Zarestky, Phys. Rev. B 26, 5426 n 9 8 2 )

15) W.B. H o l z a p f e l , Phys. Rev. B 30, 2232 (1984)

16) Y.K. Vohra, V. v i j a y a k u m a r , m . Godwal and S.K. S i k k a , subm. t o Phys. Rev. B 17) €3. Johansson, P h i l . Mag. 30, 469 (1974)

1 8 ) W.H. Zachariasen and F . H . E l l i n g e r , Acta c r y s t . A 33, 155 (1977)

19) H.K. Mao, R.M. Hazen, P.M. B e l l and J. W i t t i g , J q p l . Phys. 52, 4572 (1981) 20) W.A. GroRhans, Y .K. Vohra and W.B. H o l z a p f e l , J.M.M.M. ( i n p r e s s )

2 1 ) G.S. Smith and J. A k e l l a , J. Appl. Phys. 53, 9212 (1982) 22) H.T. H a l l and L . Perrill, I n o r g . Chem. 2 , T 1 8 (1963)

23) W.B. H o l z a p f e l , T.G. Ramesh and K. Syasgen, J. Physique E 40, (1979) 24) W.A. GroRhans and W.B. H o l z a p f e l , J.M.M.M. ( i n p r e s s ) -

25) K. Takemura and K. Syassen, i n I n t . Symp. on S o l i d S t a t e Physics under Pressure, ed. S. Minomura ( T e r r a S c i e n t i f i c P u b l i s h i n g Corp., Tokyo), Izu-Nagaoka ( i n press )

2 6 ) K. Syassen, G. Wortmann, J . Feldhaus, K.H. Frank and G. K a i n d l , Phys. Rev.

4745 (1982)

27) Y .X. Vohra, W. GroRhans and W.B. H o l z a p f e l , Phys. Rev. - B 25, 6019 (1982)