ESR and endor study of vanadium pentoxide gels

C o l l o i d . a n d Surfaces. 11 (1984) 119--128 Elsevier Science Publishers B.V.. Amsterdam - . , - , . . 119 Printed in The Netherlands . " . . ESR AND ENDOR STUDY OF VANADIUM PENTOXIDE _ . . . k L GELS P. B A R B O U X , D. GOURIER and J. LIVAGE Spectrochimte d u Solide, L A 302, Universit~ Pierre et ltfarie Curie, 4. place Ju~ieu, 7 5 2 3 0 Paris ~ e d e x 0 5 (France) (Received 20 October 1983; accepted in final form 4 January 1984) ABSTRACT An E~R and i~NDOR study o f the vanadium pentoxtde gels was performed during the first stages o f the process o f swelll,g ~sith water. It shows that water molecules belong to the coordination polyhedron of the V(IV) ions giving rise to hydrated vanadyl species. Swelling can be described as the Intercalation o f water layers between vanadium oxide ribbons. Some Brownish m o t i o n o f the vanadyl ions at the surface o f the gel o ~ u r s a s soon as the mean distance between the ribbons becomes larger than fi X. INTRODUCTION V a n a d i u m p e n t o x i d e gels e x h i b i t a l a y e r e d s t r u c t u r e [ 1 ] . E l e c t r o n a n d X-ray diffraction studies [2,3] have shown that the colloidal particles are a c t u a l l y f l a t r i b b o n s a b o u t 103 A l o n g a n d 102 A wide.. T h e t w o - d i m e n s l o n a l cell p a r a m e t e r s a ffi 2 7 A a n d b ~ 8 . 6 A s u g g e s t t h a t t h e o r g a n i z a t i o n w i t h i n t h e r i b b o n is c l o s e l y r e l a t e d t o t h e l a m e l l a r s t r u c t u r e o f o r t h o r h o m b i c V = O s [ 2 ] . S o m e s t a c k i n g o f t h 0 r i b b o n s is o b s e r v e d w h e n t h e g e l is d e p o s i t e d o n t o a substrate leading to a turbostratfc on,dimensional order along the e direction, perpendicular to the surface of the ribbons. The basal spacing d along t h e o d i r e c t i o n d e p e n d s o n t h e a m o u n t o f w a t e r : d -- 1 1 . 5 A fGr a x e r o g e l dried at room temperature and containing about 1.6 H=O per V=Os. Under v a c u u m o r u p o n h e a t i n g , a n o t h e r x e r o g e i c o r r e s p o n d i n g t o V , O s , 0 . 5 H = O is obtained, exhibiting a d spacing of 8.7 A [1], By comparison with similar layered clay systems [3], the 2.8 A shortening of the d spacing was atrrio b u t e d t o t h e d e p a r t u r e o f o n e i n t e r f o l i a r w a t e r l a y e r . A s w e l l i n g p r o c e s s in w h i c h t h e d s p a c i n g i n c r e a s e s p r o g r e s s i v e l y is o b s e r v e d w h e n w a t e r is a d d e d to the xerogel, leading to a colloidal solution [4] . . . . V a n a d i u m p e n t o x i d e gels, V2Os* n H 2 0 , a p p e a r t o b e m i x e d c o n d u c t o r s . Electronic conductivity arises from the hopping of small polarons between v a n a d i u m i o n s in d i f f e r e n t v a l e n c e s t a t e s , n a m e l y V ( V ) a n d V ( I V ) [ 5 ] , w h i l e i o n i c c o n d u c t i v i t y arises f r o m t h e d i f f u s i o n o f p r o t o n s t h r o u g h t h e g e l [ 6 ] , T h e s e geis c a n b e r U S e d as a s t a r t i n g m a t e r i a l f o r m a k i n g t h i n layer3 b y t h e s o l - - g e l p r o c e s s [ 7 ] . S u c h l a y e r s c o u l d l e a d t o n e w a p p l i c a t i o n s in t h e 0166-6622/841508.00 O 1984 Elsevier Science Publishers B.V. 120 . . ~L ,~ • . " . ~ . ...... - field o f material ~cienee as antistatic coatings [8] o r s w i t c h i n g devices [9,101. T h e e l e c t r o n i c c o n d u c t i v i t y o f t h e gel d e p e n d s o n t h e m n o t m t o f param a g n e t i c V ( I V ) ions, w h i l e t h e p r o t o n i c c o n d u c t i v i t y ' d e p e n d s o n t h e a m o u n t o f water. This p a p e r will t h e r e f o r e p r e s e n t an ESR mxd E N D O R s t u d y of t h e i n t e r a c t i o n s b e t w e e n w a t e r m o l e c u l e s a n d V ( I V ) c e n t e r s a t t h e interface b e t w e e n t h e colloidal particles a n d the interfoliar water. " PREPARATION OF THE GELS V a n a d i u m o x i d e gels can b e o b t a i n e d b y acidification o f a s o d i u m m e t a v a n a d a t e s o l u t i o n at a p H o f a b o u t 2. In o r d e r t o avoid foreig31 ions, acidification is p e r f o r m e d b y p r o t o n e x c h a n g e t h r o u g h an i o n - e x c h a n g e resin ( D o w e x 50W-X2, 5 0 - - 1 0 0 mesh). Decavanadic acid• s o l u t i o n s arc thus obt a i n e d [ 1 1 | . T h e y p o l y m e r i z e s p o n t a n e o u s l y at r o o m t e m p e r a t u r e , giving rise t o a red viscous gel a f t e r a few hours. S o m e r e d u c t i o n occurs d u r i n g t h e process, a n d a b o u t 1% o f t h e v a n a d i u m ions are in t h e V ( I V ) o x i d a t i o n state [ 1 2 ] . Most o f t h e w a t e r can b e r e m o v e d a t r o o m t e m p e r a t m ' e , giving a p o w d e r y xerogel t h a t can be d e s c r i b e d by the r o u g h f o r m u l a V 2 O s . n H ~ O , w h e r e n ~ 1.6. T h e r m a l analysis o f this xerogel s h o w s t h a t t h e r e m a i n i n g w a t e r can b e r e m o v e d in t w o steps [ 1 ] . T h e first begins at r o o m t e m p e r a t u r e m~d e x t e n d s to a b o u t 150°C, w i t h a m a x i m u m a r o u n d 130°C. I t c o r r e s p o n d s to weaklyb o n d e d water. More s t r o n g l y - b o n d e d w a t e r c o r r e s p o n d i n g t o V~O~* 0.5 H 2 0 is r e m o v e d b e t w e e n 2 4 0 a n d 2 7 0 ° C . Crystallization into o r t h o r h o m b i c V2Os t h e n occurs above 300°C. ELECTRON sPIN RESONANCE L o w temperature experiments O n c e the gel is m a d e , it d o e s n o t seem t o b e possible t o r e m o v e all t h e w a t e r b y h e a t i n g w i t h o u t d e s t r o y i n g its lamellar s t r u c t u r e a n d getting crystalline V2Os. However, in a previous s t u d y we s h o w e d t i m t a m o r p h o u s V2Os c o u l d b e o b t a i n e d b y splat c o o l i n g o f t h e m o l t e n o x i d e [13]. This a m o r p h o u s o x i d e also e x h i b i t s a fibrous t e x t u r e a n d could lead t o a gel o r a colloidal s o l u t i o n w i t h t h e a d d i t i o n o f w a t e r [ 14]. Figure Xa s h o w s t h e ESR s p e c t r u m o f an a n h y d r o u s a m o r p h o u s V2Os o13rained b y splat cooling. It" is r a t h e r p o o r l y resolved b u t e x h i b i t s t h e hypero fine features typical o f a V ( I V ) i o n ( S - - 1 / 2 , I = 7 / 2 ) in an axially d i s t o r t e d crystal field a n d can be d e s c r i b e d b y t h e usual spin H a m i l t o n i a n Fig. 1. ESR (b) VzO:*0.5 spectra r~-arded at low temperature (T=,77 HzO xeregel; (¢) VzOs- 1.5 HzO xerogel. K). (a) Amorphous Y2Os; • 121 la H c Ga..s~ 2SO0 3000 3500 g.._]l lb | Fig. 1. ! 3OOO ! ~00. H(8i 122 = g , H . S z + g , (iIxSx ÷ H y S y ) + AtSzlz + Ai(SXlx-+ Syly) :~ The ESIt parameters deduced from the spectrum are gt = 1.913 At - 176G g. = 1.981 A~ --- 6 6 G T h e g p a r a m e t e r s a p p e a r t o b e v e r y c l o s e t o t h o s e o f c r y s t a l l i n e V2Os [ 1 5 1 , s u g g e s t i n g t h a t t h e s h o r t r a n g e o r d e r is a l m o s t t h e s a m e i n b o t h c o m p o u n d s , i.e., a VOs s q u a r e p y r a m i d w i t h a v e r y s h o r t V ~ O d o u b l e b o n d ( 1 . 5 8 A ) a l o n g t h e z axis [ 1 6 ] . Figure lb shows the ESR spectrum of a V2Os. 0.5 H~O xerogel obtained b y d r y i n g t h e gel u n d e r v a c u u m . I t is still r a t h e r p o o r l y n.,solvc,d. A s b e f o r e , it c o r r e s p o n d s t o a V ( I V ) in a n a x i a l l y d i s t o r t e d crysta~ field a n d c a n b e described b y the same spin H a m i l t o n i a n with g# --1.925 gl -- 1 . 9 8 2 Af = 196G Ai = 76G T h e s e p a r a m e t e r s are s l i g h t l y d i f f e l ' e n t f r o m t h e p t e v i o l m o n e s , s h o w i n g t h a t t h e c o o r d i n a t i o n p o l y h e d r o n o f t h e V ( I V ) ions has b e e n m o d i f i e d b y hydration. F i g u r e l c s h o w s t h e E S R s p e c t r u m o f a V a O s " 1.6 H a O x e r o g e l d r i e d a t r o o m t e m p e r a t u r e . I t is m u c h b e t t e r r e s o l v e d t h a n t h e p r e v i o u s s p e c t r u m . T h e E S l t l i n e w i d t h a p p e a r s t o b e m u c h s m a l l e r . T h e p e a k - t o - p e a k ilnew i d t h o f t h e parallel h y p e r f i n e lines d e c r e a s e s f r o m 8 0 t o 16 G. T h i s E S R s p e c t r u m is still t y p i c a l o f a V ( I V ) i o n in a n a x i a l ligand field, b u t t h e E S R parameters are quite different gn -- 1 . 9 3 5 AS --- 2 0 4 G g1 = 1.986 AI = 78G T h e y a r e n o w q u i t e s i m i l a r t o t h o s e p u b l i s h e d b y Ballhmx~en a n d G r a y f o r the hydrated vanadyl ion VO(H,O)]* [17|. T h i s suggests t h a t w a t e r m o l e c u l e s n o w b e l o n g t o t h e c o o r d i n a t i o n p o l y h e d r o n o f tile V ( I V ) i o n t h a t s h o u l d b e s u r r o u n d e d b y six o x y g e n n e a r e s t n e i g h b o u r s w i t h a l o n g e r V = O d o u b l e b o n d (VffiO = 1 . 6 7 A). Room temperature experiments T h e r o o m t e m p e r a t u r e E S R s p e c t r u m o f a V 2 O s • 0.5 H ~ O x e r o g e l is s h o w n it! Fig. 2a. N o m o d i f i c a t i o n is o b s e r v e d w h e n t h e r e c o r d i n g t e m p e r a t u r e increases, a n d this s p e c t r u m is q u i t e s i m i l a r t o t h e o n e ~ h o w n in Fig. l b . A d r a s t i c m o d i f i c a t i o n is o b s e r v e d , h o w e v e r , w h e n t h e i x e r o g e l c o n t a i n s 1.6 H a O p e r V a O s . F i g u r e 2 b s h o w s t h e E S R s p e c t r u m o f a V ~ O s . 1.6 H z O x e r o g e l . I t e x h i b i t s o n l y e i g h t lines "arising f r o m t h e h y p e r f i n e c o u p l i n g o f one "unpaired electron (8 =1/2) with' the nuclear, spin (I=7/2) -of, the v a n a d i u m a t o m a n d c a n b e d e s c r i b e d b y t h e i s o t r o p i e s p i n :Ham-.'lt0nian ~:g#HS+AS~ ~ ~ " ~ ~:~ '" " .... " ~th g = I,967 A = 117 G _20 _ -' ~oo ~o H(6) Ft~'. 2. IF~IR spectra recorded at room temperature ( T = 3 0 0 K). (a) V~Os.O.5 HjO; (b) V,O, "1.6 H,O. A n intensity d i s t r i b u t i o n is observed among the h y p e r f i n e lin~.s. Thei¢ w i d t h appears to depend o n the corcesponding nuclear s p i n q u a n t u m number M. Such a behavior is typical o f a f a s t t u m b l i n g o f the V ( I V ) ions under study. This could be due to a Brownian m o t i o n o f the molecular species containing the V ( I V ) ion. A n analysis o f this m o t i o n can be performed according to the model developed b y Wilson and Kivelson [ 1 8 ] . I n the fast t u m b l i n g referee, ~ the l i n e w i d t h Z~H(Jtf) o f - e a c h h y p e r f i n e component, can be e x p r e s s e d as w h e r e ~ , Q, ~ a n d 6 p a r a m e t e r s d e p e n d o n t h e A a n d g t e n s o r s , t h e m i c r o , 124 wave f r e q u e n c y ~ , a n d t h e c o r r e l a t i o n t i m e ~ j o f t h e m o t i o n a n d t h e . ~ residual l i n e w i d t h a t . A d e t a i l e d analysis o f s u c h a m o t i o n in t h e case o f h y d r a t e d a m o r p h o u s V2Os has a l r e a d y b~en p u b l i s h e d [ 1 9 ] . T h e s a m e analysis p e r f o r m e d fo r a V 2 0 s - 1.6 Hz O gel leads t o (i) an a c t i v a t i o n e n e r g y E ffi6 kcal m o l e -t a n d a c o r r e l a t i o n t i m e re ffi 0 . 8 - 1 0 t3 s in t h e case o f a t h e r m a l l y a c t i v a t e d m o t i o n d e s c r i b e d h y re = T0 exp -E/I~T. (it) a n e q u i v a l e n t r a d i u s a = 5.2 A in t h e case o f a B r o w n i a n m o t i o n d e s c r i b e d b y r e - - 4 / 3 - = a 3 . ~ / k T , wllere q c o r r e s p o n d s t o t h e viscosity o f water T h e t h e r m a l a c t i v a t i o n e n e r g y de c reas es t o a b o u t 2 k c a l m o l e -z , w h i l e t h e e q u i v a l e n t radius r e m a i n s b e t w e e n 4 mid ~ A w h e n t h e w a t e r c o n t e n t increases in t h e gel [ 1 2 ] . This suggests t h a t t h e m o t i o n d o e s n o t involve t h e w h o l e r i b b o n b u t r e m a i n s l o c a l i z e d a r o u n d t h e V (IV) a t o m s . ENDOR D u r i n g a n E N D O R e x p e r i m e n t t h e static m a g n e t i c field H0 o f t h e E S R s p e c t r u m is k e p t c o n s t a n t w h i l e a r a d i o - f ~ q u e n c y field is swept. T h e ENI~OR s p e c t r u m t h e n c o r r e s p o n d s t o t h e i n t e n s i t y v a r i a t i o n o f t h e E S R line o n w h i c h H0 is s e t versus t h e f r e q u e n c y o t t h e r t field. A t y p i c a l p r o t o n E N D O R s p e c t r u m e x h i b i t s several d o u b l e t s c e n t e r e d o n t h e free p r o t o n n u c l e a r f r e q u e n c y Vp. T h e i r s e p a r a t i o n c o r r e s p o n d s , i n a first o r d e r approxim a t i o n , t o t h e e l e c t r o n - - p r o t o n h y p e r f i n e co u p lin g . T h e E S I t spec.tmm o f t h e V 2O s , n H 2 0 gels exhibits an axial s y m m e t r y . O n e o f t w o sets of E S R lines c a n t h e n be c h o s e n fo r E N D O ) t , t h e parallel or p e r p e n d i c u l a r h y p e r f i n e lines. W h e n t h e m a g n e t i c field H0 is s et o n a parallel line ( f o r i n s t a n c e M l ffi - 7 / 2 ) , t h e E N D O t t s p e c t r u m s h o w s o n l y t h e V4*--H couplings c o r r e s p o n d i n g t o t h e v a n a d i u m sites having t h e i r V--O s h o r t e s t b o n d parallel ( o r n e a r l y parallel) t o t h e H0 d i r e c t i o n . T h e E N D O R s p e c t r u m t h e n l o o k s like a "single crystnl s p e c t r u m " [ 2 0 ] . W h e n H0 is set o n o n e o f t h e seven p e r p e n d i c u l a r h y p e r f i n e lines, all t h e V 4. ions having t h e i r v = o b o n d p e r p e n d i c u l a r t o t l o will c o n t r i b u t e t o t h e E N D O R s p e c t r u m . We t h e n have a " t w o - d i m e n s i o n a l s p e c t r u m " [ 2 0 ] . E N D O R e x p e r i m e n t s w e r e perl~ormed o n a B n l k e r 2 2 0 D s p e c t r o m e t e r e q u i p p e d with a B r u k e r E N D O R a c c e s s o r y a n d an O x f o r d I n s t r u m e n t E S R 9 c o n t i n u o u s h e l i u m flow c r y o s t a t . T h e o p t i m u m t e m p e r a t u r e for r e c o r d i n g o f t h e E N D O I t s p e c t r u m was f o u n d t o b e a r o u n d 10 K. O t h e r i n s t r u m e n t a l p a r a m e t e r s w e r e as follows: m i c r o w a v e p o w e r N 2.5 roW, r a d i o f r e q u e n c y p o w e r 100 W ( t h e m a x i m u m p o w e r available With o u r r f amp lifier), m o d u l a t i o n 100 kHz. Figure 3 s h o w s t h e p r o t o n E N D O R s p e c t r a o f • V2 O s , 1.6 H 2 0 xerogel. T h e m a g n e t i c field H o was set o n t h e M i = - ' / [ 2 l~mrallel a n d M i -~ + 7 / 2 p e r p e n d i c u l a r c o m p o n e n t s o f t h e E S R s p e c t r u m . - O n l y t w o pairs o f lines, • . 125 a r o u n d i.0.8 , a n d s 6 , 4 MHz,~ican b e seen in t h e almost~crystal-like E N D O R s p e c t r u m : (Fig,-3a)...Thel pair ~w i t h i t h e larger ~separationl arises f r o m ' t h e c o u p l i n g w i t h p r o t o n s clo~e t o t h e V = O axis, w h i l e t h e o t h e r o n e s h o u l d c o r r e s p o n d t o p r o t o n s s i t u a t e d in o r n e a r a plane p e r p e n d i c u l a r tO this V - O axis. T h e u n p a i r e d e l e c t r o n b e i n g in a p u r e l y dxy orbital1 i t s h y p e r f i n e c o u p l i n g w i t h an axial~ p r o t o n (situated near t h e z axis) s h o u l d he m a i n l y dipolar, t h e c o n t a c t t e r m b e i n g nearly zero in this case [ 2 1 , 2 2 ] . T h e value o f t h e h y p e r f i n e c o u p l i n g s h o u l d t h e n be close t o 2 Ap, c o r r e s p o n d i n g t o t h e d i p o l e - d i p o l e e l e c t r o n - - p r o t o n c o u p l i n g along t h e z d i r e c t i o n . T h e parallel c o u p l i n g for t h e axial p r o t o n s s h o u l d t h e n be (At)axial "" 2 A p -- 6.4 MHz. (MHz) 3b.,, ~,~ .L Fig. 3. Proton ENDOR spectra of V~Ot* 1.6 HzO xero~el. (a) Parallel spectrum; (b) perpendieular spectrum. T h e p e r p e n d i c u l a r E N D O R s p e c t r u m (Fig. 3b) exhibits several pairs o f lines separated b y 0.8, 3.1, 4.9, 8.3 a n d 15.5 MHz. T h e 3.1 MHz pair closely c o r r e s p o n d s t o t h e p e r p e n d i c u l a r c o u p l i n g o f t h e axial p r o t o n s (At)axlal ~ - A p ffi - 3 . 1 MHz. T h e s e p r o t o n s p r e s u m a b l y c o r r e s p o n d t o an axial water m o l e c u l e b o n d e d t o t h e V 4÷ ion a l o n g t h e z axis, o p p o s i t e t o t h e V = O s h o r t b o n d [ 2 3 ] . All o t h e r E N D O I t lines can be a t t r i b u t e d t o equatorial p r o t o n s [ 2 4 ] . T h e s e p r o t o n s m a y b e l o n g t o w a t e r m o l e c u l e s situated i n t h e equatorial plane, a s f o r V O ( H 2 0 ) ] ~ . It is m o r e probable, h o w e v e r , t h a t s o m e p r o t o n a t i o n o f t h e bridging o x y g e n occurs. These E N D O K spectra, t o g e t h e r with t h e m e a s u r e d ESR parameters, are q u i t e similar t o t h o s e a l r e a d y pub- 126 l i s h e d f o r V O ( H 2 0 ) ] -~ in f r o z e n s o l u t i o n s [ 2 1 ] o r i n T u t t o n salts [ 2 2 ] : a n d f o r V O 2÷ a d s o r b e d o n a Y Z e o l i t e [ 2 3 ] . We m a y a s s u m e t h e n t h a t t h e l i g a n d f i e l d a r o u n d V ( I V ) i n t h e V 2 O s " 1 . 6 H 2 0 x e r o g e l is q u i t e s i m i l a r t o t h a t o f VO(H20)~*. . . . . Figure 4 shows the proton ENDOR spectra of a V~Os. 0.0 H20 xeregel obtained by dehydration under vacuum at room temperature. On each side of the central line we see pairs of peaks corresponding to _ A s ffi 6 . 9 M H z i n t h e p a r a l l e l s p e c t r u m A t -- 2 . 9 M H z in t h e p e r p e n d i c u l a r s p e c t r u m This proton ENDOR spectrum appears to be 8omewhat different from the p r e v i o u s o n e a n d f r o m t h e V O ( H 2 0 ) ] + E N D O R s p e c t r u m . T h e A t -- 6 . 9 MI~:r, a n d A t ffi 2 . 9 M H z h y p e r f i n e c o u p l i n g s s u g g e s t t h a t s o m e p r o t o n s r e m a i n in a n a p p r o x i m a t i v e l y a x i a l p o s i t i o n . T h e e q u a t o r i a l p r o t o n s , h o w e v e r , are n o l o n g e r d e t e c t a b l e , 4o/ -7/2// Fig. 4. Proton E N D O R spectra o f V=Oj* 0.5 H~O xeroget$. (a) Parallel spectrum; (b) perpendicular spectrum. T h e E N D O R c e n t r a l l i n e t h a t c a n b e s e e n a r o u n d .the p r o t o n f r e q a e n c y , p in b ~ t h p a r a l l e l a n d p e r p e n d i c u l a r s p e c t r a s h o u l d c o r r e s p o n d t o t h e soc a l l e d " m a t r i x l i n e . " i t arises f r o m t h e c o u p l i n g w i t h t h e p r o t o n s s i t u a t e d ffLr f r o m t h e p a r a m a g n e t i c V 4. i o n . CONCLUSION • : . X-ray a n d n e u t r o n d i f f r a c t i o n e x P e r i m e n t s have s h o w n t h a t t h e first stages o f h y d r a t i o n c o r r e s p o n d ~t o t h e i n t e r c a l a t i o n o f w a t e r : l a y e r s b e t w e e n ~t h e r i b b o n s o f t h e gel [ 4 ] . T h e basal s pacin g d alo n g t h e ¢ d i r e c t i o n t h e n i n c r e a s e s b y steps of~ 2.8 A; This c o u l d e x p l a i n w h y t h e l i n e w i d t h o f t h e h y p e r f i n e lines decreases u p o n h y d r a t i o n . T h e m e a n d i s t a n c e b e t w e e n V ( I V ) ions s i t u a t e d a t t h e Surface o f t h e r i b b o n s inCreases, d e c r e a s i n g t h e d i p o l a r i n t e r a c t i o n s b e t w e e n p a r a m a g n e t i c centers. As s h o w n b y E N D O R , t h e V2 Os- 1.6 H 2 0 E S R l i n e w i d t h is m a i n l y g o v e r n e d b y n o n r e s o l v e d h y p e r f i n e interactions with protons. R o o m temperature ESR spectra show that some m o l e c u l a r m o t i o n , c h a r a c t e r i z e d b y an e q u i v a l e n t radius o f a b o u t 5 A, o c c u r s w h e n t h e w a t e r c o n t e n t b e c o m e s g reater t h a n 1.6 I120 p e r V2Os. S u c h a col" ,,osition c o r r e s p o n d s t o t h e i n t e r c a l a t i o n o f t w o w a t e r layers, w i t h an i n t e r l a y e r s e p a r a t i o n o f 2.8 × 2 ~ 5.6 A [ 3 ] t allowing t h e r o t a t i o n o f t h e v a n a d y l ions a t t h e s u r f a c e o f t he gel. Magnetic r e s o n a n c e e x p e r i m e n t s give s o m e m o r e i n f o r m a t i o n a b o u t t h e i n t e r a c t i o n b e t w e e n V ( I V ) ions a n d w a t e r m o l e c u l e s at t h e su rface o f t h e gel. T h e ESIt a n d E N D O R s p e c t r a o f t h e V 2 O s - l . 6 H 2 0 x ero g el are q u i t e similar to t h o s e o f t h e h y d r a t e d v a n a d y l io n VO(H20)~+. T h e y suggest t h a t s o m e w a t e r belongs t o t h e c o o r d i n a t i o n p o l y h e d r o n o f t h e V ( I V ) ions s i t u a t e d a t t h e s u r f a c e o f t h e ribbons. T h e s e V ( I V ) ions are s u r r o u n d e d b y six o x y g e n n e a r e s t n e i g h b o r s , t h e sixth o x y g e n arising f r o m o n e w a t e r m o l e c u l e s i t u a t e d a l o n g t h e c axis, o p p o s i t e to t h e V--O d o u b l e b o n d . W h e n r e m o v e d , this w a t e r m o l e c u l e w o u l d leave a five-fold• c o o r d i n a t i o n . This agrees w i t h t h e t h e r m a l analysis o f t h e x ero g el s h o w i n g t h a t , a f t e r d e h y d r a t i o n a bove 300°C, o r t h o r h o m b i c V2Os is o b t a i n e d in w h i c h t h e v a n a d i u m ls s u r r o u n d e d b y five o x y g e n o n l y •[16]. T h e E S R a n d E N D O R s p e c t r a o f t h e V2Os, 0.5 H 2 0 are s o m e w h a t d i f f e r e n t . T h e y still e x h i b i t p r o t o n hyperfine c o u p l i n g s p r e s u m a b l y arising f r o m azial p r o t o n s , b u t t h e c o o r d i n a t i o n p o l y h e d r o n is c l e a r l y d i f f e r e n t f r o m w h a t is o b s e l v e d in c r y s t a l l i n e V20~ a n d VO(H20)~+. In particular, w e m a y p o i n t o u t t h a t t h e E N D O R signal d u e to e q u a t o r i a l p r o t o n s are n o l o n g e r d e t e c t a b l e . All t h e s e results agree q u i t e well w i t h a previous infrared s t u d y p e r f o r m e d o n V~O5 gels [ 2 5 ] . As a c o n c l u s i o n , w e p o i n t o u t t h e fo llo w in g features: T h e stronglyb o n d e d w a t e r m o l e c u l e s (V2Os • 0.5 H 2 0 ) s e e m t o be l i n k e d n e a r t h e V ( I V ) ions, leading t o s o m e m o d i f i c a t i o n o f t h e g a n d A tensors. T h e basal spacing o f t h e r i b b o n s is d ffi 8.{$ A. T h e i n t e r l a y e r spacing r e m a i n s q u i t e small so t h a t d i p o l a r i n t e r a c t i o n s are still i m p o r t a n t an d n o m o t i o n is possible. S o m e swelling is o b s e r v e d w h e n m o r e w a t e r is a d d e d . I t leads t o an increase o f t h e i n t e r l a y e r spacing (d " 1 1 . 5 A). T h e d i p o l a r i n t e r a c t i o n s d e c r e a s e a n d s o m e local m o t i o n s a r o u n d V ( I V ) ions b e c o m e possible. E N D O R e x p e r i m e n t s clearly s h o w t h a t t h e c o o r d i n a t i o n o f V ( I V ) is t h e n q u i t e close t o t h a t o f VO(H 20)~+ ion. N o significant m o d i f i c a t i o n o f t h e E S R s p e c t r u m is o b s e r v e d w h e n m o r e w a t e r is a d d e d . T h e V ( I V ) c o o r d i n a t i o n p o l y h e d r o n d o e s n o t seem t o vary. 128 REFERENCr~S 1 P. Aldebert, N. Baffler, N. Gharbl and J. Livage, Mater. Res. Bull., 16 ( 1 9 8 1 ) 669. 2 J.J. Legendre and J. Livage, d. Colloid Interface SeI., 94 (1083) 75. 3 J.J. Legendre, P. Aldebert, N. Baffler and J. L[vage, J. Colloid i nt erface Sei., 94 (19831 84. 4 P..]Lldehert, HAV. Hnesslin, N. Barrier and J. Livoge, J. Colloid Int erface Hei., 98 (1~84) 478. 5 C. Sanchez, F. Babonneau, R. Morfneau, J. L[vage and J. Bullot, Philos. Mag. B, 47 (19831 279. 6 P. B a r b o u x , N. Baffler, R. Morineau, J. Livage, Solid S t a t e foni©s, 9, 10 (19831 1073. 7 J. Livage and J. Lemerle, Annu. Re','. Mater, Sci., 12 ( 1 9 8 2 ) 103. 8 Kodak PathS, French p a t e n t B F. 2429252, 1979. 9 J. Bullot and J. Liva~e, French patent 81. 13665, 1981. 10 J. Bullot, O. Gallais, M. G a ut hl e r and J. Livage, Phys. S t a t u s S o l l d i A, 71 (19821 KL. 11 J. L e m e d e , L. Ne|em and J. Lefebvre, J. Chem. R e s . , ( i 9 7 8 ) 525301. | 2 N. Gharbi, C. Sanehez, J. I,[vage, J. Lemerle, L . N e j e m and J. Lefebvre, Inorg. Chem., 21 ( I 9 8 2 1 2768. 13 L. Rivoalen, A. Revc.~levs©hi, J. L|vage and R. Collongues, J. N o n - t r y s t . Solids, 21 (19761 171. 14 J. Livage, N. Gharhi. 11LC. L e r o y and M. Mlehattd, Mater. Res. Bull., 1 3 ( 1 9 7 8 1 1117. 15 E. Glllis and ~-. Boesman, Phys. St at us Solldi, 14 (19661 337. 16 H.G. Bachmann, F.R. A h m e d and W.H. Barnes, Z. Krist., 115 (19611 110. 110. 17 C.J. Rmllhausen and H.R. Gray, Inorg. Chem., I (10621 111. 18 R. W i l s o n and D. Kivel~on, J. Chem. Phys., 44 ( 1 9 6 6 ) i b4. 19 P. Tougne, A.P. Legrand, C. Sanchez and J. Livage, J. Phys. Chem. Solids, 42 ( 1 9 8 1 ) 101. ~10 | l . g . Oersmann and J.D. Swallen, J. Chem. Phys., 36 (1~]621 3221. Ol H. van Will;gen, J. ~|aRn. Reson., 39 (1980) 37. 22 23 24 2~ N.M. A t h e r t o n and J . F . S h a e k [ e t o n , Mol. Phys., 39 (1980) 1471. !1. van Willigen and T.K. Chandrashekar, J. Am. Chem. Soc., 1 U 5 ( 1 9 8 3 1 4 2 3 2 . H. van Wllllgen, C.F. Mulks and N.M. A t h e r t o n , Inorg. Chem., 21 (19821 1709. M.T. Vandenhorre, R. Pros,, E. Huard and J. Livage, Mater. Res. Bull., 18 ( 1 9 8 3 ) !133.