on phase transitions and structural systematics in ...

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. .

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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. Holzapfel Faahbereich

Physik,

liniversitaet-GH-Paderborn,

D-4790 Padevbovn,

F.R.G.

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. 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 studied 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 to 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 to the Ba-IV-structure.The 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 to 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 convent 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 Piermarini scale / 4 / . D i f f r a c t i o n patterns o f Mg a t various high pressures are reproduced i n fig.1.These 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 admixtures 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 at the t r a n s i t i o n i s around U.

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

D i f f r a c t i o n patterns of Ba a t high compression are shown in 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 increasing 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 pressure.Typical 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.

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

JOURNAL DE PHYSIQUE

C8-158

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 toget h e r w i t h recent data f o r the 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 depend 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 radius r a t i o s f o r the presentat 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 not 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 transforms a t about the same v a l ue f r o m bcc t o Sr-111 .

20

40

60

ENERGY ( k e V ) 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 compression.The spectrum a t 14 GPa was r e c o r d e d on u n l o a d i n g .

SC :

11

IICP

----

ORTHORH.

50

f l ~ . H C P ----

CA Sa

: FCC

.

FCC

BCC

!9.5

---3.5

----

BCC

BCC

3a: Eu

32

---26

SC

scc

?L?-

"(p

-.....-..-.....---3A.ly .... BA-V

HCP

----

Y B : FCC

----

----

hCC

----

HCP

SR-IV - - - - . ~ A - I V "

12

32

35

I

16

SR-III

13

:

36 ----

----

Eu-Ill

46

F i r s t principle calculations correlated the 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 behaviour o f the r a r e earth metals t h a t the radius 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% compression,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 Ba-IV-structure a t the 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 garded as t y p i c a l f o r t h e heavy a l k a l i n e e a r t h m e t a l s a f t e r comleti ion o f s - > d t r a n s f e r . I f s + d 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 1ead t o d e v i a t i o n s from the behaviour o f the heavier a l k a l i n e e a r t h metals a t high compression.

-

Table 1: Phase t r a n s i t i o n s and t r a n s i t i o n pressures. Be: Ref .6 Eu,Yb:Ref.5,16,17 Ca,Sr:Ref . 8 Ba:Ref.9,8

------------------hcp

6avIBal~lhcp(

. .

.

1 bce Ifee ( bcc 1 rcc I bcc

-

1

...

-

-

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-structure.

2,.0

- . - .

2:5

. . .

Ratio of atomic to ionic radii: RA /RI

I

Obviously, the 1ack o f d-bands near t h e Fermi-level i n Mg explains 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.

REFERENCES:

/1/ HOLZAPFEL W.B. and MAY W. in:High Pressure Research i n Geophysics,eds S.Akimot o and M.H.Manghnani (Centre o f Academic Publication,Tokyo,l982) p. 73 /2/ STAUN OLSEN J.,BURAS B.,GERWARD L. and STEENSTRUP S.,J. Phys. E: S c i . I n s t r 14 (1981),1154 /3/ SYASSEN K. and HOLZAPFEL W.B. ,Europhys. Conf. Abstr. 1A (1975) ,75 /4/ PIERMARINI G.J.,BLOCK S.,BARNETT J.D. and FORMAN R.A.,J. Appl. Phys. 46 (1975) 2774 /5/ GROSSHANS W.A. and HOLZAPFEL W.B.,J. Mag. Mag. Mat. (1984),in press /6/ MING L.C. and MANGHNANI M.H.,J. Phys. F: Met. Phys. 14,(1984),L1 /7/ VIJAYAKUMAR B.K..GODWAL B.K.,VOHRA Y.K.,SIKKA S.K. and CHIDAMBARAM R..J. Phvs. F:Met. Phys. 14 (1984),~65 /8/ OLIJNYK H. and HOLZAPFEL W.B.,Phys. L e t t . lOOA (1984),191 /9/ AKIMOTO S.,YAGI T.,IDA Y.,INOUE K. and SAT0 Y.,High Temp. High Press. 7 (1975) / l o / "Modern C r y s t a l l o g r a p h y IU,B.K. Vainshtein,Springer Series i n S o l i d S t a t e Sciences 15 (1980) /11/ SKRIVER H.L.,Phys. Rev. L e t t . 49 (1982),1768 /12/ DUTHIE J.C. and PETTIFOR D.G.,Sol. St. Comm. 27 (1978),613 /13/ DUTHIE J.C. and PETTIFOR D.G.,Phvs. Rev. L e t t . 38 (1977),564 /14/ MCMAHAN A.K. and MORIARTY J.A. ,Phys.~ev. B 27 (1983),3235 /15/ HERBST J.F. ,Phvs. Rev. B 22 (1980) .4600 /16/ SYASSEN K. a n d HOLZAPFEL W.B: , i n ~ i Pressure ~ h Science and Technology,Vol 1, eds. K.D.Timmerhaus and M.S.Barber (Plenum Press,N.Y.,1979),p.223 /17/ TAKEMURA K. and SYASSEN K. , I n t e r n a t i o n a l Symbosi um on "Sol i d S t a t e Physics under Pressure", Izu-Nagaoka,Japan,Abstr. 20m05 (1984)

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