2500 2000 1500 1000 500 120 0 20 40 60 80 100 Temperature (K) Pressure ( GPa ) R B8 B1 B8 + B1 B1 H i g h P r e s s u r e a n d H i g h T e m p e r a t u r e P h a s e T r a n s i t i o n s o f H i g h P r e s s u r e a n d H i g h T e m p e r a t u r e P h a s e T r a n s i t i o n s o f FeO Fig. 1. Phase diagram of FeO. The B1-R and R-B8 transition boundaries are from Fei and Mao [2]. B1, NaCl-type ; B8, NiAs-type ; R, rhombohedral structure. Fi g. 1 ). Th e Fig . 2 . The rel ati ve F e O i s o n e o f t h e m o s t a b u n d a n t o x i d e c o m p o n e n t s i n E a r t h ’ s m a n t l e a n d i s a l s o l i k e l y incorporated in the core. FeO crystallizes with the NaCl-type structure ( B1 ) under ambient condition s and transforms to a rhombohedral phase above 17 GPa at 300 K [1]. The resistance-heated diamond anvil cell ( DAC ) experiments showed the transition f r o m r h o m b o h e d r a l t o N i A s - t y p e ( B 8 ) s t r u c t u r e above 90 GPa at 600 K [2]. Fei and Mao [2] further s u g g e s t e d t h e t r a n s i t i o n f r o m t h e B 1 t o t h e B 8 s t r u c t u r e a r o u n d 7 0 G P a a t h i g h t e m p e r a t u r e ( > 1 0 0 0 K ) t o b e c o n s i s t e n t w i t h e a r l i e r s h o c k c o m p r e s s i o n s t u d i e s [ 3 ] a n d e l e c t r i c a l r e s i s t a n c e m e a s u r e m e n t s [ 4 ] . T h i s B 1 t o B 8 t r a n s i t i o n h a s never been confirmed at high temperatures. Hi gh -p re ss ur e an d hi gh -t em pe ra tu re ex pe ri me nt s w e r e p e r f o r m e d a t b e a m l i n e B L 1 0 X U u s i n g D A C co up le d wi th he at in g by a Nd :Y AG la se r [5 ]. We collected X-ray diffraction patterns of the sample at in situ high pressure and temperature conditions up to 87 GPa and 1730 K. A fine powder of Fe 0.954 O mi xe d wi th Fe (2 :1 by mo le ra ti o) wa s us ed as a starting material. It was loaded into a 100 - μ m hole d r i l l e d i n a r h e n i u m g a s k e t p r e i n d e n t e d t o a t h i c k n e s s o f 5 0 μ m , t o g e t h e r w i t h a p r e s s u r e medium of Al 2 O 3 polycrystalline pellets. The size of the heating spot was more than 50 μ m. The X-ray diffraction data were collected from an area 20 μ m i n d i a m e t e r . E x p o s u r e t i m e s w e r e o n e t o f i v e minutes. We observed a transition from the B1 to the B8 st ru ct ur e ab ov e 70 GP a at 16 00 K ( typical diffraction patterns of FeO with either the B1 or B8 str uct ure are sho wn in in te ns it ie s of di ff ra ct io n pe ak s of th e B8 st ru ct ur e ma y in di ca te th e me ta ll ic po ly ty pe Ni As st ru ct ur e, w h i c h i s a m i x t u r e o f i n v e r s e ( O a t N i s i t e ) a n d no rm al st ru ct ur e s (F e at Ni si te ), as pr op os ed by M a z i n e t a l . [ 6 ] . T h e m e t a l l i c f e a t u r e s o f t h e polytype NiAs can explain the metallization of FeO 72 F i g . 2 . E x a m p l e s o f X - r a y d i f f r a c t i o n patterns showing the B1 and B8 structures of FeO together with ε -Fe and corundum. References [1] T. Yagi et al. , J. Geophys. Res. 90 (1985) 8784. [2] Y. Fei and H. Mao, Science 266 (1994) 1678. [3] R. Jeanloz and T. Ahrens, Geophys. J. R. Astr. Soc. 62 (1980) 505. [4] E. Knittle and R. Jeanloz, Geophys. Res. Lett. 13 (1986) 1541. [5] T. Watanuki et al. , Rev. Sci. Instrum. 72 (2001) 1289. [6] I. Mazin et al. , Am. Miner. 83 (1998) 451. inferred from the resistance measurements [ 4 ] . T h e t r a n s i t i o n t o a m e t a l l i c p h a s e o f F e O c o u l d e n h a n c e o x y g e n s o l u b i l i t y i n m o l t e n i r o n . A s t r u c t u r a l d i f f e r e n c e between the B1 structure of MgO and the B 8 s t r u c t u r e o f F e O c o u l d l e a d t o a n i m m i s c i b i l i t y g a p i n t h e M g O - F e O s o l i d s o l u t i o n a b o v e 7 0 G P a . T h e l o w e r m o s t m a n t l e c o u l d b e e n r i c h e d w i t h F e O a s a r e s u l t o f c h e m i c a l r e a c t i o n s b e t w e e n s i l i c a t e p e r o v s k i t e a n d l i q u i d i r o n . T h e F e O - r i c h ( M g , F e ) O p h a s e w i t h t h e B 8 s t r u c t u r e i s l i k e l y t o b e p r e s e n t t o g e t h e r w i t h M g O - r i c h f e r r o p e r i c l a s e i n t h e d e e p l o w e r m a n t l e . T h e p r e s e n c e o f t h e B 8 p h a s e s h o u l d i n c r e a s e t h e e l e c t r i c a l a n d thermal conductivity , which may explain the n a t u r e o f a h i g h l y e l e c t r i c a l l y c o n d u c t i v e l a y e r o b s e r v e d a t t h e b o t t o m 2 0 0 m o f t h e Earth’s mantle. Kei Hirose Tokyo Institute of Technology E-mail: kei @ geo.titech.ac.jp 73 8 9 1 0 1 1 1 2 1 3 1 4 c o r ( 1 0 4 ) B 1 ( 1 1 1 ) c o r ( 1 1 0 ) ε i r o n ( 1 0 0 ) B 1 ( 2 0 0 ) c o r ( 1 1 3 ) c o r ( 2 0 2 ) c o r ( 0 2 4 ) ε i r o n ( 0 0 2 ) ε i r o n ( 1 0 1 ) I n t e n s i t y 2 θ c o r ( 1 1 0 ) c o r ( 1 0 4 ) ε i r o n ( 1 0 0 ) B 8 ( 0 0 2 ) B 1 ( 1 1 1 ) B 8 ( 1 0 1 ) B 1 ( 2 0 0 ) c o r ( 1 1 3 ) c o r ( 2 0 2 ) ε i r o n ( 0 0 2 ) ε i r o n ( 1 0 1 ) B 8 ( 1 0 2 ) c o r ( 0 2 4 ) 6 5 G P a , 1 6 0 0 K 8 6 G P a , 1 7 3 0 K
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