Phase Boundary of Silicon Dioxide SiO 2 under High-pressure and -temperature Determined by In Situ DAC Laser Heating Technique The physical properties and structural evolution of silicon dioxide ( SiO 2 ) at high pressure and high temperature have attracted attention in geophysical s c i e n c e b e c a u s e s i l i c o n d i o x i d e i s a p r i m a r y component of minerals in the interior of the Earth. T h e m a n t l e o f t h e E a r t h c o n s i s t s o f m o s t l y S i O 2 , M g O , F e O , A l 2 O 3 , a n d C a O c o m p o n e n t s . Therefore, there is a possibility that SiO 2 plays key r o l e i n t h e s t r u c t u r e a n d d y n a m i c s o f t h e E a r t h ’ s m a n t l e . T h e p o s s i b i l i t y o f a p r e s s u r e - i n d u c e d t e t r a g o n a l - o r t h o r h o m b i c ( P 4 2 / m n m - P n n m ) p h a s e t r a n s i t i o n i n S i O 2 w a s s u g g e s t e d b y c r y s t a l c h e m i c a l a r g u m e n t s [ 1 ] . T h e t r a n s i t i o n o c c u r s i n the vicinity of 50 GPa at room temperature and has now been investigated from both experimental [2,3] an d th eo re ti ca l pe rs pe ct iv es [4 ,5 ]. Al th ou gh de ta il ed T : Tetragonal phase; O : Orthorhombic phase; P : Platinum 2 θ θ Intensity 10 15 20 O ( 1 1 0 ) P ( 1 1 1 ) O ( 1 0 1 ) , ( 0 1 1 ) O ( 2 1 0 ) , ( 1 2 0 ) O ( 2 1 1 ) O ( 1 2 1 ) O ( 2 2 0 ) P ( 2 2 0 ) O ( 0 0 2 ) O ( 3 0 1 ) O ( 0 3 1 ) , ( 1 1 2 ) P ( 3 1 1 ) O ( 1 1 1 ) O ( 3 1 1 ) , ( 1 3 1 ) P ( 2 0 0 ) T ( 1 1 0 ) P ( 1 1 1 ) T ( 1 0 1 ) P ( 2 0 0 ) T ( 2 1 0 ) T ( 2 1 1 ) T ( 2 2 0 ) P ( 2 2 0 ) T ( 0 0 2 ) T ( 3 0 1 ) T ( 3 1 1 ) T ( 1 1 1 ) T ( 1 1 2 ) P ( 3 1 1 ) k n o w l e d g e h a s b e e n a c c u m u l a t e d o n t h e h i g h - pressure behavior of SiO 2 , most studies have been l i m i t e d t o r o o m t e m p e r a t u r e . T h e s e c o n d - o r d e r t e t r a g o n a l - t o - o r t h o r h o m b i c t r a n s i t i o n a t h i g h pressure and high temperature has been studied in G e O 2 [ 6 ] , w h i c h i s r e g a r d e d a s a n a n a l o g u e o f SiO 2 . In situ observation at high pressure and high t e m p e r a t u r e i s r e q u i r e d t o d e t e r m i n e t h e p h a s e boundary of the high-pressure phases, because the o r t h o r h o m b i c p h a s e i n S i O 2 i s u n q u e n c h a b l e , converting to the tetragonal phase on decrease of temperature and release of pressure. An in-situ X-ray diffraction measurement system under high pressure and temperature with use of a dia mon d anv il cel l ( DAC ) app ara tus has bee n set u p a t b e a m l i n e B L 1 0 X U [ 7 ] . T h e s a m p l e s w e r e heated with a multimode continuous wave Nd :YAG laser using double-sided laser heating techniques, whi ch min imi zed the tem per atu re gra die nts of the h e a t e d a r e a . T h e s a m p l e t e m p e r a t u r e w a s me as ur ed fr om on e si de of th e sa mp le us in g th e sp e ct ro ra d i o me tr i c me th o d . Th e h e a te d sa mp l e s we re pr ob ed by an gl e- di sp er si ve X- ra y di ff ra ct io n t e c h n i q u e . T h e i n c i d e n t X - r a y b e a m w a s Fig. 1. X-ray diffraction patterns for silicon dioxide phases using angle-dispersive technique with DAC experiments. 59 40 50 60 70 80 90 100 500 1000 1500 2000 2500 P ( GPa ) = (51 2) + (0.012 0.004) × T (K) Pressure ( GPa ) Temperature (K) Tetragonal phase Orthorhombic phase monochromatized to wavelength of 0.4127 Å. The X-ray beam size was collimated to 1 6 - 2 0 μ m d i a m e t e r . P r e s s u r e w a s d e t e r m i n e d f r o m t h e m e a s u r e d u n i t c e l l vo lu me of pl at in um us in g th e eq ua ti on of state for platinum mixed with the sample. We conducted two runs at pressures b e t w e e n 4 5 t o 9 2 G P a a n d a t t e m p e r a t u r e s b e t w e e n 3 0 0 K t o 2 2 0 0 K . T h e t y p i c a l diffraction patterns of the tetragonal ( P4 2 / m n m ) a n d t h e o r t h o r h o m b i c ( P n n m ) p h a s e s a r e r e p r o d u c e d i n F i g . 1 . T h e c h a n g e s f r o m d o u b l e p e a k s i n t h e o r t h o r h o m b i c p h a s e ( 2 1 1 - 1 2 1 a n d 3 0 1 - 0 3 1 ) t o s i n g l e p e a k s i n t h e t e t r a g o n a l p h a s e i n d i c a t e d t h a t t h e o r t h o r h o m b i c p h a s e t r a n s f o r m e d t o t h e t e t r a g o n a l p h a s e . O u r d e t e r m i n a t i o n s o f t h e tetragonal and orthorhombic stability fields are summarized in Fig. 2 . Zhang et al. [8] r e p o r t e d t h a t a n e q u i l i b r i u m p h a s e b o u n d a r y b e t w e e n c o e s i t e ( m o n o c l i n i c ) and stishovite ( tetragonal ) of SiO 2 at about 1 0 G P a c o u l d n o t b e d e t e r m i n e d b e l o w 1 2 7 3 K b e c a u s e o f t h e k i n e t i c s o f t h e p h a s e t r a n s i t i o n . H o w e v e r , t h e s e c o n d - o r d e r p h a s e t r a n s i t i o n b e t w e e n t h e t e t r a g o n a l a n d o r t h o r h o m b i c p h a s e s i n G e O 2 o c c u r s w i t h o u t p r e s s u r e a n d t e m p e r a t u r e h y s t e r e s i s [ 6 ] . T h e r e f o r e , i t w a s p o s s i b l e t o d e t e r m i n e t h e p h a s e b o u n d a r y b e t w e e n t h e t e t r a g o n a l a n d o r t h o r h o m b i c p h a s e o f S i O 2 a t l o w temperature in this study. References [1] L. Nagel and M. O’Keeffe, Mateer. Res. Bull. 6 (1971) 1317. [2] K. J. Kingma et al. , Nature 374 (1995) 243. [3] D. Andrault et al. , Science 282 (1998) 720. [4] C. Lee and X. Gonze, J. Phys. Condens. Matter 7 (1995) 3693. [5] D. M. Teter and R. J. Hemley, Phys. Rev. Lett. 80 (1998) 2145. [6] S. Ono, K. Hirose, N. Nishiyama and M. Isshiki, Am. Mineral. 87 (2002) 99. [7] T. Watanuki et al. , Rev. Sci. Instrum. 72 (2001) 1289. [8] J. Zhang et al. , Phys. Chem. Mineral. 23 (1996) 1. [9] S. Ono, K. Hirose, M. Murakami and M. Isshiki, Earth Planet. Sci. Lett. 197 (2002) 187. Shigeaki Ono Japan Marine Science & Technology Center E-mail: sono @ jamstec.go.jp Fig. 2. The experimental results and a phase boundary determined by in situ observation. The solid circles and s q u a r e s r e p r e s e n t c o n d i t i o n s w h e r e t h e t e t r a g o n a l a n d orthorhombic phases were stable [9]. The open symbols represent data from Andrault et al. [3]. The solid line is the inferred phase boundary between the tetragonal and orthorhombic phases in silicon dioxide. 60
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