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HIGH PRESSURE POSITRON ANNIHILATION STUDIES OF Fe IN A DIAMOND ANVIL CELL
H. Radousky, R. Reichlin, R. Howell
To cite this version:
H. Radousky, R. Reichlin, R. Howell. HIGH PRESSURE POSITRON ANNIHILATION STUDIES OF Fe IN A DIAMOND ANVIL CELL. Journal de Physique Colloques, 1984, 45 (C8), pp.C8-369-C8-372.
�10.1051/jphyscol:1984865�. �jpa-00224367�
HIGH PRESSURE POSITRON ANNIHILATION STUDIES OF Fe IN A DIAMOND ANVIL CELL
H.B. Radousky, R.L. R e i c h l i n and R.H. Howell
University of California, Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A.
Résumé - Nous avons mesuré l'élargissement Doppler du spectre de Fe, jusqu à la pression de 40 GPa, en utilisant une cellule haute pression à enclumes de diamant (DAC). Cette expérience représente la première réalisation d'une mesure d'annihilation de positrons utilisant la technique des DAC.
Abstract - The Doppler broadening spectrum of Fe has been measured to a pressure of 40 GPa using a high pressure diamond anvil cell (DAC). This represents the first positron annihilation experiment which utilizes DAC technology.
Une goal of high pressure physics is to understand the electronic structure of materials, and how it affects observable physical quantities. An experimental technique both capable of determining the electronic structure of a material and applicable to high pressure studies is the Doppler broadening of positron annihilation radiation. Doppler broadening and angular correlation techniques can provide detailed information concerning electron momentum distribution in a solid, and have become a standard tool for such studies at ambient
pressure.1 Although some applications of positron annihilation have lower electron momentum resolution than comparable methods such as de Haas-Van Alphen,2 the positron techniques can be applied to situations which are otherwise intractable.
Some early positron measurements have been made on a few simple metals below 10.0 GPa using Bridgeman anvil devices.3.5 The more recent advent of the diamond anvil cell (DAC) has made experimental studies up to pressures of many tens of gigapascals possible.6 In this experiment, the Doppler broadening spectrum of Fe has been measured to 40.0 GPa.
The Doppler broadening technique was chosen for our first experiment because it allows us to obtain a high count rate, while still having the small sample required to get to high pressures with the diamond cell. Due to the
annihilation with electrons of various momenta, the sharp annihilation line at 511 KeV is broadened, with the electron momentum information being contained in the 1ine shape.
The experimental design for the positron annihilation studies is illustrated in Fig. 1. In our experiment, we used a Mao and Bell7 type cell with diamonds which had cutlets of 0.8mm. The sample consisted of a sandwich of twp pieces of Fe foil with positron emitting Na^2 placed in between. The foils were 99.998% purity and were annealed under an argon atmosphere to
remove any defects from the rolling process. The starting sample size was 350 microns in diameter and 100 microns thick. The sample was placed in the hole of a hardened type 301 stainless-steel gasket without any pressure medium.
Small ruby chips (5-20 microns) were placed in the sample chamber for pressure
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984865
JOURNAL DE PHYSIQUE
/
Sample/
1 u a
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Fe foil ( 5 0 ~ )3 5 0 ~ diameter
I
511 KeV yDetector
LEI
F i g . 1
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E x p e r i m e n t a l c o n f i g u r a t i o n .c a l i b r a t i o n u s i n g t h e r u b y f l u o r e s c e n c e technirque.8 Pressure g r a d i e n t s were measured a t v a r i o u s p r e s s u r e s and were f o u n d t o be a t most 3.0 GPa a t 40.0 GPa. E r r o r s i n t h e p r e c i s i o n o f t h e p r e s s u r e measurements a r e e s t i m a t e d t o b e l e s s t h a n 0.5 GPa a t a l l pressures, based m a i n l y on b r o a d e n i n g o f t h e r u b y R1 l i n e caused b y n o n - h y d r o s t a t i c pressures.
T h i s e x p e r i m e n t d i f f e r s f r o m p r e v i o u s p o s i t r o n work3-5 a t h i g h p r e s s u r e i n t h a t t h e samples a r e n e c e s s a r i l y s m a l l i n o r d e r t o r e a c h 40.0 GPa (.35mm as opposed t o 3-8mm i n p r e v i o u s e x p e r i m e n t s ) and t h e s o u r c e was d e p o s i t e d d i r e c t l y on t h e sample f o i l s , r a t h e r t h a n o n a s u p p o r t i n g m a t e r i a l . The D o p p l e r broadening d a t a were reduced t o t h e r a t i o o f l o w momentum e v e n t s t o t o t a l events, S. The p l o t o f t h e S parameter
2.
p r e s s u r e shown i n F i g . 2 can be v i s u a l l y d i v i d e d i n t o t h r e e r e g i o n s . Below t h e t r a n s i t i o n pressure, S f a l l s i n a c o n t i n u o u s manner f r o m t h e i n i t i a l p r e s s u r e o f 2.4 GPa t o t h e o n s e t o f t h e bcc ( a ) t o hcp ( E ) t r a n s i t i o n a t 14.0 GPa. The t r a n s i t i o n i ss l u g g i s h and t a k e s a p p r o x i m a t e l y 10 GPa t o complete. T h i s has been seen f r o m p r e v i o u s x-ray9 and r e s i s t i v i t y data,lO and i s apparent i n o u r p o s i t r o n d a t a as w e l l . From 14 t o 24 GPa t h e Fe appears t o be i n a two phase r e g i o n . T h i s r e g i o n i s c h a r a c t e r i z e d b y a c u s p - l i k e f e a t u r e i n t h e d a t a w h i c h i s most l i k e l y a s s o c i a t e d w i t h d e f e c t s , r a t h e r t h a n any i n t r i n s i c p r o p e r t y o f t h e m a t e r i a l . I t i s i n t e r e s t i n g t o n o t e t h a t a s i m i l a r c u s p - l i k e f e a t u r e i s a l s o observed i n t h e p o s i t r o n spectrum o f Fe as i t i s heated t h r o u g h i t s bcc t o f c c t r a n s i t i o n n e a r 1 3 0 0 ~ . l l Above 24.0 GPa, Fe appears t o b e c o m p l e t e l y i n t h e hcp phase and t h e d a t a once more show a smooth decrease o f t h e S parameter w i t h i n c r e a s i n g pressure.
h i g h e r energy e l e c t r o n s a t h i g h pressures. T h i s can be understood from t h e f o l l o w i n g arguments. The Doppler broadening s p e c t r a of metals have been shown t o be made up o f two components, a valence c o n t r i b u t i o n and a c o r e
component.1 Where t h e valence c o n t r i b u t i o n goes t o zero i s t h e Fermi energy. As t h e volume decreases, t h e Fermi energy moves t o h i g h e r values, which causes t h e p o s i t r o n a n n i h i l a t i o n r a t e w i t h valence e l e c t r o n s o f h i g h e r energy t o increase. I n a d d i t i o n , t h e f r a c t i o n o f a n n h i l a t i o n s w i t h c o r e e l e c t r o n s increases as t h e volume i s compressed.
An i n t e r e s t i n g f e a t u r e o f t h e d a t a i s t h a t t h e r a t e o f change o f S w i t h pressure i s v e r y d i f f e r e n t before and a f t e r t h e t r a n s i t i o n . Conversion o f t h e d a t a t o S
G .
volume u s i n g t h e P-V data o f Mao&
&.I2 showed a s i m i l a r d i f f e r e n c e , i n d i c a t i n g t h a t t h e d a t a a r e s e n s i t i v e t o changes i n t h e e l e c t r o n momenta d i s t r i b u t i o n due t o o t h e r e f f e c t s , such as t h e l o s s of magnetism o r changes i n t h e Fermi topology.ACKNOWLEDGEMENT
Work performed under t h e auspices o f t h e U.S. Department o f Energy by Lawrence Livermore N a t i o n a l Laboratory under c o n t r a c t #W-7405-Eng-48.
JOURNAL DE PHYSIQUE
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