Phase diagram and valence state of iron in FeNb2O6 synthesized under controlled redox atmosphere

Mat. R e s . B u l l . , Vol. 21, p p . 231-236, 1986. P r i n t e d i n t h e USA. 0025-5408/86 $3.00 + .00 C o p y r i g h t (c) 1986 P e r g a m o n P r e s s L t d .

PHASE D I A G R A M SYNTHESIZED

Eiji

A N D V A L E N C E STATE OF IRON IN FeNb206 UNDER C O N T R O L L E D REDOX A T M O S P H E R E

Tokizaki, Y o s h i n o r i and K o z o N a g a s h i m a

Sugitani

I n s t i t u t e of Chemistry, U n i v e r s i t y of Tsukuba Sakura-mura, Ibaraki 305, Japan ( R e c e i v e d D e c e m b e r 4, 1985; C o m m u n i c a t e d b y M. Koizumi ABSTRACT FeNb206 c r y s t a l s were s y n t h e s i z e d by m e l t / s i n t e r i n g method under controlled g a s f l o w of CO2/H 2. P h a s e of t h e c r y s t a l s o b t a i n e d was d i f f e r e n t d e p e n d i n g on the p a r t i a l o x y g e n p r e s s u r e (PO 2 ). At h i g h p a r t i a l o x y g e n pressures, c o l u m b i t e type s t r u c t u r e was formed and the v a l e n c e state of iron was found to be both ferrous and ferric. At low p a r t i a l o x y g e n pressures, t r i r u t i l e type s t r u c t u r e was f o r m e d a n d t h e v a l e n c e s t a t e of i r o n w a s f o u n d to be ferrous. At i n t e r m e d i a t e p a r t i a l oxygen pressures, both the c o l u m b i t e type and the t r i r u t i l e type structures were found in the products. MATERIALS INDEX:

niobates, iron Introduction

FeNb206 h a s t w o p o l y m o r p h i c p h a s e s w h i c h a r e r e f e r r e d to as c o l u m b i t e and t r i r u t i l e phases (1,2). The former b e l o n g s to o r t h o r h o m b i c system w i t h the l a t t i c e s t r u c t u r e b a s i c a l l y c o n s t r u c t e d by the r e p e t i t i o n of three w o l f r a m i t e - t y p e s u b c e l l s a l o n g the a-axis (3). The l a t t e r b e l o n g s to t e t r a g o n a l system with the t r i r u t i l e type (or the t a p i o l i t e type) s t r u c t u r e and is s t a b l e at high t e m p e r a t u r e s ( 4 ) . T u r n o c k r e p o r t e d phase d i a g r a m s on F e - N b - O and F e - T a - O systems (5), as w e l l as on MnTa206 - F e T a 2 0 6 - O and others (3), studied under c o n t r o l l e d o x y g e n p a r t i a l p r e s s u r e in the s y n t h e t i c experiment. In his studies v a l e n c e state of Fe has been taken m o s t l y as d i v a l e n t , a l t h o u g h the p r e s e n c e of ferric ions has been c o n s i d e r e d in the d i s c u s s i o n about the s t r u c t u r e of the products. Since iron occurs both in d i v a l e n t and t r i v a l e n t states in FeNb206, the a t m o s p h e r i c c o n d i t i o n is e s s e n t i a l for the study of s i n g l e c r y s t a l growth and/or the study of r e l a t i v e phase s t a b i l i t y b e t w e e n the c o l u m b i t e and t r i r u t i l e phase. The p r e s e n t study r e p o r t s the effect of o x y g e n p a r t i a l p r e s s u r e on the f o r m a t i o n of FeNb206, as well as the v a l e n c e state of iron of the c r y s t a l s obtained. K.N.

deceased.

231

232

E. T O K I Z A K I , et a l .

Vo]. 21, No. 2

Experimental a)

Sample preparation

Fe203 and Nb205 (99.9% and 99.99% pure,respectively, Rare M e t a l l i c Co., Ltd.) were m e c h a n i c a l l y mixed to a desired ratio, and w e r e h e a t e d in a p l a t i n u m c r u c i b l e in air at 1 0 0 0 ~ for 24 hrs. The mixture was then taken into a platinum tube (5mm diameter, 20mm length) and suspended using thin platinum wire (0.1mm diameter) in a vertical quenching furnace. The temperature of the furnace was c o n t r o l l e d between (1200 - 1450) ±2°C using a Pt20%Rh-Pt40%Rh thermocouple as a detector. After 8-24 hours kept for equilibriation the specimen was quenched into water of room temperature placed at the bottom of the furnace. The specimens obtained were checked by chemical analysis and by X-ray method.

b)

Control of atmosphere

M i x e d gas of C O 2 and H 2 w i t h v a r y i n g c o m p o s i t i o n in the range log(CO2/H2) = -1.5-~ +1.5 was employed to control the oxygen partial pressure in the furnace. A gas mixer of JonstonWalker type (6) was used for this purpose. c)

Analysis

X-ray diffraction method was used to determine the phase of the product obtained. M~ssbauer spectra were measured at room temperature to check the v a l e n c e state of Fe using a 2048-channel analyzer and the cobalt 57 as F-ray source. The v e l o c i t y scale was calibrated by using Fe metal as a standard absorber.

Results and Discussion FeNb206 is k n o w n to h a v e an o r d e r e d l o w t e m p e t r a t u r e form with the columbite structure and an ordered high temperature form with the triruitle-type structure. The latter is e q u i v a l e n t to the ordered tapiolite structure. Schr~cke (7) reported that trirutile phase d e v e l o p e d from columbite phase at about 1250~C, suggesting that the trirutile phase is a high temperature polymorph of columbite phase. Our preliminary synthesis experiment (8) showed that the columbite type FeNb206 was always obtained in a reducing atmosphere, while FeNbO 4 was always obtained in an oxidizing atmosphere or in air. Further, FeNb206 synthesized in H 2 gas flow was found to change into the trirutile type w h e n k e p t in the gas f l o w for m o r e t h a n 10 h o u r s e v e n b e l o w 1000°C. W h e n kept at an e l e v a t e d t e m p e r a t u r e at I020°C o n l y a columbite-type phase was obtained, while when kept at temperatures between 1020 and I080°C, mixtures of the both phases

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Vol. 21, No. 2

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resulted. On the other hand, when FeNb206 was s y n t h e s i z e d under m i l d e r r e d u c i n g conditons, at 7 0 0 - 8 0 0 ~ C for two hours in H 2 gas f l o w f o l l o w e d b y k e e p i n g it in N 2 g a s f l o w at 1 0 0 0 - 1 3 0 0 ° C f o r 10 hours, o n l y c o l u m b i t e - t y p e phase was formed. These r e s u l t s w o u l d Suggest that FeNb206 c o u l d form either with the c o l u m b i t e or the t r i r u t i l e s t r u c t u r e d e p e n d i n g on the extent of r e d u c t i o n at the time of c r y s t a l l i z a t i o n . The extent of r e d u c t i o n w o u l d be r e s u l t e d in the r e l a t i v e amount of Fe(II) and Fe(IiI) in FeNb206, and c o n s e q u e n t l y w o u l d be r e f l e c t e d in the a v e r a g e ionic radius of A i o n in ANb206 w h e r e A = ( F e 2~, Fe 3+ ). On t h e b a s i s of t h e s e p r e l i m i n a r y results, s y n t h e s i s e x p e r i m e n t s under c o n t r o l l e d a t m o s p h e r i c c o n d i t o n s h a v e been c o n d u c t e d to i n v e s t i g a t e the r e l a t i v e s t a b i l i t i e s of c o l u m b i t e and t r i r u t i l e structures.

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FIG. I of F e N b 2 0 6 as a f U n c t i o n ratio CO2/H 2.

F i g u r e I s h o w s t h e r e s u l t s of t h e FeNb206 p h a s e o b t a i n e d in the s y n t h e s i s e x p e r i m e n t at v a r i o u s m i x i n g ratios of CO2/H 2 gas at v a r i o u s temperatures. At 1450°C l i q u i d phase was c o n f i r m e d to be p r e s e n t t o g e t h e r w i t h a s o l i d phase FeNb11029 b e f o r e quenching. The amount of the s o l i d phase i n c r e a s e d as CO2/H 2 decreased. At

234

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Vol.

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1400°C trirutile and columbite phases appeared, but FeNb11~9 was not observed. At solid region, there were three types of products: columbite phase, mixture of columbite and trirutile phases, and trirutile phase, as in the preliminary experiment. i

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In o r d e r to c h e c k the v a l e n c e state of iron in the synthesized FeNb206, M6ssbauer spectra were measured. Figures 2a) and 2b) shows the spectra for columbite and trirutile type FeNb206, respectively. Figure 2b) shows the typical pattern for Fe(II) in high spin state with the quadrupole splitting ~ 2 mm/s and the isomer shift 1.7 mm/s with respect to ~-Fe. On the

Vol. 21, No. 2

FeNb20 6

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other hand, Figure 2a) shows that both Fe(II) and Fe(III) exist in c o l u m b i t e type structure. The q u a d r u p o l e s p l i t t i n g is 2.8 mm/s, and the isomer shift 1.7 mm/s. Peaks o b s e r v e d in Fig. 2a) at v e l o c i t i e s at -5.5, -3.1, -0.8, +0.7, +3.0, a n d +5.3 m m / s are a t t r i b u t e d to the m a g n e t i c s p l i t t i n g of Fe(III). Quadrupole s p l i t t i n g of Fe(II) in c o l u m b i t e type FeNb206is s l i g h t l y larger than that in the t r i r u t i l e type FeNb206. This indicates that the a s y m m e t r y of e l e c t r i c f i e l d at F e - c a t i o n site in the c o l u m b i t e structure is larger than that in the t r i r u t i l e structure. M e t a l l i c Fe was not o b s e r v e d in the both spectra. These r e s u l t s are agreed with the e x p e r i m e n t a l r e s u l t s by Turnock (4), i.e. the c o l u m b i t e type FeNb206 o b t a i n e d at 1180°C i n c o r p o r a t e d a s m a l l amount of Fe3+Nb04 (ixiolite structure) so that it c o u l d be more p r e c i s e l y e x p r e s s e d as (Fe2+,Fe3+)Nb206 • C o n s e q u e n t l y the r e s u l t s in the present experiment may be s u m m a r i z e d as follows: a phase with the c o l u m b i t e type FeNb206 h a v i n g a s m a l l a m o u n t of Fe(III) w o u l d a p p e a r w h e n f o r m e d in a w e a k l y r e d u c i n g (low PO 2 ) atmosphere. This phase can be e x p r e s s e d as (Fe2+,Fe3*)Nb206 . Since the ionic radius of Fe(III) ion (0.64 A) is s m a l l e r t h a n t h a t of Fe(II) ion (0.74 A), the phase (Fe2+,Fe3÷)Nb206 is c o n s i d e r e d as h a v i n g an a v e r a g e ionic radius of F e - c a t i o n s m a l l e r than that in the phase Fe2+Nb206 . This is c o n s i d e r e d to f a v o r the formation of the c o l u m b i t e type phase, as in the case in the present experiment. On the other hand, a phase with the t r i r u t i l e structure FeNb206 w o u l d appear when formed in a strong r e d u c i n g a t m o s p h e r e (in H 2 gas f l o w at high temperatures). This phase contains o n l y Fe(II) ion and can be e x p r e s s e d as Fe2+Nb206 . The o c c u p a t i o n of o n l y Fe(II) ion at F e - c a t i o n s i t e of FeNb206 m a k e s the a v e r a g e i o n i c r a d i u s of Fecation larger than that in the phase (Fe2+,Fe3+)Nb206 . This favors the formation of the t r i r u t i l e phase (10). A possibility of f o r m i n g a N b ~ 4 p h a s e at a s t r o n g r e d u c i n g a t m o s p h e r e w o u l d a l s o c o n t r i b u t e to the f o r m a t i o n of t r i r u t i l e structure, because Nb204 has the r u t i l e s t r u c t u r e w h i c h is a b a s i c u n i t of the t r i r u t i l e structure. Acknowledgement Authors wish to express their thanks to Dr. Isamu Shindo, N a t i o n a l Institute for R e s e a r c h in Inorganic M a t e r i a l s , for his instructions and s u g g e s t i o n s for c o n t r o l l i n g redox a t m o s p h e r e in the experiment. References I.

C. P a l a c h e , Mineralogy"

H. B e r m a n , a n d C. F r o n d e l , Dana's " S y s t e m 7th ed. Vol.1, John Wiley, New York (1944).

2. H. S t r u n z " M i n e r a l o g i s c h e Verlag. L e i p z i g (1970). 3.

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Yon G. Bauer,

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5. A.C. Turnock, J. Amer. 49,177 (1966). 6. L . J o n s t o n (1925).

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and A. C. W a l k e r ,

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and K. N a g a s h i m a ,

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N i p p o n K a g a k u kaishi