ON PHASE TRANSITIONS AND STRUCTURAL SYSTEMATICS I N ALKALINE EARTH METALS UNDER PRESSURE

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ON PHASE TRANSITIONS AND STRUCTURAL SYSTEMATICS I N ALKALINE EARTH METALS

UNDER PRESSURE

H. Olijnyk, W. Holzapfel

To cite this version:

H. Olijnyk, W. Holzapfel. ON PHASE TRANSITIONS AND STRUCTURAL SYSTEMATICS I N ALKALINE EARTH METALS UNDER PRESSURE. Journal de Physique Colloques, 1984, 45 (C8), pp.C8-157-C8-159. �10.1051/jphyscol:1984829�. �jpa-00224329�

JOURNAL DE PHYSIQUE

Colloque C 8 , supplement au n ° l l , Tome 45, novembre 1984 page C8-157

ON PHASE TRANSITIONS AND STRUCTURAL SYSTEMATICS IN ALKALINE EARTH METALS UNDER PRESSURE

H. O l i j n y k and W.B. H o l z a p f e l

Faahbereich Physik, liniversitaet-GH-Paderborn, D-4790 Padevbovn, F.R.G.

A b s t r a c t - The c r y s t a l s t r u c t u r e s o f Mg.Ca.Sr and Ba were s t u d i e d 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 a t room temperature and pressures up t o 50 GPa.

Mg transforms around 50 GPa from hep t o b c c . S t r u c t u r a l t r a n s i t i o n s to bec and to a simple cubic s t r u c t u r e are observed i n Ca a t 19.5 and 32 GPa r e - s p e c t i v e l y . S r shows the well known t r a n s i t i o n from fee to bcc a t 3.5 GPa and a d d i t i o n a l phase t r a n s i t i o n s around 26, 35 and 46 GPa, which lead u l t i m a t e l y t o the B a - I V - s t r u c t u r e . T h e experiments on Ba gave some evidence f o r the existence o f a new phase,Ba-V,above 46 GPa.Structural systematics and t h e i r r e l a t i o n t o s-=>d t r a n s f e r are discussed.

The present studies used 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 e i t h e r conven- t i o n a l X-ray sources / l / or w i t h synchrotron r a d i a t i o n (HASYLAB) / 2 / , a diamond a n v i l high pressure c e l l / 3 / , a n d ruby luminescence f o r the pressure determination w i t h the P i e r m a r i n i scale / 4 / .

D i f f r a c t i o n patterns o f Mg a t va- r i o u s high pressures are reproduced i n f i g . 1 . T h e s e patterns c l e a r l y show the occurance of a new phase f o r Mg w i t h bcc structure.Smal1 ad- mixtures o f the lower pressure hep phase are i n d i c a t e d by arrows.The l a t t i c e parameter f o r bgc-Mg a t 58 GPa is a = 2.953(3) A.The volume change a t the t r a n s i t i o n i s around U.

D i f f r a c t i o n patterns of Ba a t high compression are shown i n fig.2.Some o f the d i f f r a c t i o n l i n e s o f Ba-IV disappeared on i n c r e a s i n g pressure around 46 GPa,and also d r a s t i c changes i n the i n t e n s i t y r a t i o s were observed a t t h i s p r e s s u r e . T y p i c a l patterns o f Ba-IV were recorded on decreasing pressure only below 26 GPa.These changes i n the d i f f r a c t i o n patterns present f i r s t evidences of a new high pressure phase,Ba-V.

F i g . 1 - D i f f r a c t i o n patterns of Mg a t various pressures.

Résumé - Les structures cristallines de Mg,Ca,Sr et Ba ont été étudiées par diffraction de rayons X en dispersion d'énergie à température ambiante et à des pressions jusqu'à 50 GPa. Mg se transforme vers 50 GPa de hc en ce.

Des transitions structurales en ce et en cubique simple ont été observées dans Ca respectivement à 19,5 et 32 GPa. Sr subit la transition bien connue c f c — c e à 3,5 GPa, d'autres transitions vers 26, 35 et 46 GPa qui conduisent finalement à la structure Ba-IV. Les expériences sur Ba ont donné des indications sur l'existence d'une nouvelle phase, Ba-V au-dessus de 46 GPa. Une étude des structures en relation avec les transferts s—-d est discutée.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984829

C8-158 JOURNAL DE PHYSIQUE

2 0 40 6 0

ENERGY ( k e V )

A summary o f t h e p r e s e n t l y known phase t r a n s i t i o n s i n a l k a l i n e e a r t h m e t a l s a t room t e m p e r a t u r e and h i g h p r e s s u r e i s g i v e n i n t a b l e 1 toge- t h e r w i t h r e c e n t d a t a f o r t h e d i - v a l e n t r a r e e a r t h m e t a l s Eu and Yb /5,16,17/.

The s y s t e m a t i c s o f these phase t r a n s i t i o n s a r e more c l e a r l y seen i f one s c a l e s t h e p r e s s u r e depen- d i n g a t o m i c r a d i i RA b y t h e f i x e d i o n i c r a d i i RI a n d uses t h e s e r a d i u s r a t i o s f o r t h e p r e s e n t a - t i o n o f t h e phase t r a n s i t i o n s as i l l u s t r a t e d i n f i g . 3 . 0 n e can no- t i c e t h a t Ca,Yb and S r a l l t r a n s - f o r m f r o m f c c t o bcc s t r u c t u r e a t t h e same RA/RI~l.9.Yb,Eu and Ba show t h e t r a n s i t i o n f r o m bcc t o hcp s t r u c t u r e a t r a t i o 1.6 and a l s o S r t r a n s f o r m s a t a b o u t t h e same v a l ue f r o m bcc t o Sr-111 .

F i r s t p r i n c i p l e c a l c u l a t i o n s c o r r e l a t e d t h e s t r u c t u r a l sequence f c c + bcc + hcp f o r these d i v a l e n t m e t a l s t o an e l e c - t r o n i c s+d t r a n s f e r /11,12/ ,and one can assume i n a n a l o g y t o t h e b e h a v i o u r o f t h e r a r e e a r t h m e t a l s t h a t t h e r a d i u s r a t i o g i v e s a general measure f o r t h e s j d t r a n s f e r /13/ .Furthermore, these c a l c u l a t i o n s /12,15/ show t h a t t h e s+d t r a n s f e r s h o u l d be completed f o r Ba a t 50% compre- ssion,where i n f a c t % a - I V becomes most s t a b l e . S r - V occurs i n t h i s B a - I V - s t r u c t u r e a t t h e same r a d i u s r a t i o and t h e r e f o r e t h e B a - I V - s t r u c t u r e may be r e - F i g . 2 - D i f f r a c t i o n ;patterns o f Ba a t h i g h garded as t y p i c a l f o r t h e heavy compression.The spectrum a t 14 GPa was r e - a l k a l i n e e a r t h m e t a l s a f t e r com- c o r d e d on u n l o a d i n g . leti ion o f s - > d t r a n s f e r . I f s + d

SC : I I C P ---- 11 ORTHORH.

f l ~ . H C P - - - - 50 BCC

!9.5 32

CA : FCC - - - - BCC ---- SC

3.5 26 36 16

Sa . FCC - - - - BCC ---- S R - I I I ---- SR-IV - - - - . ~ A - I V "

12 46

3a: ?L?- " ( p -...-..-...---- 3A.ly .... BA-V

13 32

Eu : s c c - - - - HCP ---- ^Eu-Ill

I 35

Y B : FCC - - - - hCC ---- HCP

t r a n s f e r i n Ca i s a l s o completed a t a r a d i u s r a t i o R~/R~:;s1.4 as i n S r and Ba,one may e x p e c t a n o t h e r phase t r a n s i t i o n around 70 GPa.However,Ca has no d-core and t h i s may 1 ead t o d e v i a t i o n s f r o m t h e b e h a v i o u r o f t h e h e a v i e r a l k a l i n e e a r t h m e t a l s a t h i g h compression.

-

Be: Ref .6 Eu,Yb:Ref.5,16,17 Ca,Sr:Ref . 8 Ba:Ref.9,8

- - - - -

hcp 1 ^bce I ^fee (

bcc 1 ^rcc I

6avIBal~l hcp( bcc 1

. . . . . . ^{2,.0 - . - . 2:5} . . . ^I

Ratio of atomic to ionic radii:

-

I f the analogy between heavy a l k a l i n e e a r t h metals and Eu and Yb would continue t o h o l d a t h i g h e r pressures ,the l a t t e r two elements should transform t o the Ba-IV-structure around 35 and 55 GPa r e s p e c t i v e l y . A c t u a l 1 y Eu-111, which i s s t a b l e above 32GPa and

up t o a t l e a s t 45 GPa /5/ i s d i f f e r e n t from the Ba-IV-struc- t u r e .

Obviously, the 1 ack o f d-bands near t h e Fermi-level i n Mg e x p l a i n s e a s i l y w i t h i n t h i s framework a l s o the d i f f e r e n t s t r u c t u r a l behaviour o f t h i s l i g h t d i v a l e n t metal 1141.

F i g . 3 - S t r u c t u r a l systematics.

The values f o r RI were taken from r e f . 10 Z denotes the atomic number.

This work was supported i n p a r t by HASYLAB and the Sundesministerium f u r Forschung und Technologie.

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