X-Ray Diffraction Study on Phase Relation of BaZnGeO 4

A single-crystal diffraction study has been carried out at 145 K, 195 K and 300 K to investigate the phase relations of BaZnGeO 4 . The space group of phase VI has been found to be P6 1 22 or P6 5 22 with the lattice parameters a VI =5.32 Å and c_VI=26.1 Å a t 145 K . T h e s y m m e t r y i s h i g h e r t h a n t h a t o f t h e r o o m - t e m p e r a t u r e p h a s e I I I . A n e w p h a s e , V I I , h a s b e e n f o u n d i n t h e s a m p l e l a r g e r t h a n 0.2 m m a n n e a l e d i n t h e t e m p e r a t u r e r a n g e o f 100-160 K f o r a b o u t 1 m o n t h . T h i s p h a s e i s t h o u g h t t o h a v e a h e x a g o n a l s y m m e t r y ( P 6_3 o r P 6_3/ m ) w i t h t h e l a t t i c e p a r a m e t e r s a_VII=10.68 Å a n d c_VII=8.69 Å. S i n c e p h a s e V I d i d n o t a p p e a r i n t h e c a s e o f c r y s t a l s s m a l l e r t h a n 0.2 m m , t h e p a r t i c l e s i z e e f f e c t o n t h e V - V I p h a s e c h a n g e h a s b e e n c o n f i r m e d . T h e r e l a t i o n s h i p s a m o n g p h a s e s I , I I , I I I , I I I ', I V , V , V I a n d V I I o f B a Z n G e O _4 a r e d i s c u s s e d .