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Superconductivity in hexagonal tungsten bronzes
Volume24A, number 11
PHYSICS LETTERS
SUPERCONDUCTIVITY
IN H E X A G O N A L
22 May 1967
TUNGSTEN
BRONZES
J.P.REMEIKA,
T.H.GEBALLE, B.T.MATTHIAS*, A.S.COOPER, G. W. H U L L and E. M. K E L L Y Bell Telephone Laboratories, Murray Hill, New Jersey, USA Received 18 April 1967
It is possible, by etching in various acids, to remove alkali-metalions from hexagonal phase alkalitungsten bronzes, to change their latticeconstants and to increase their superconductingtransitiontemperatures substantially.
W e have found superconductivity in the hexagonai phases of some alkali metal tungsten bronzes to occur at temperatures above 6°K, considerably higher then previously reported values [1]. The transition is an inverse function of alkali metal content over at least some range of concentration. The c parameter and, consequently, the c/a ratio as well, is smaller for the higher transition temperature material. The measurements were m a d e on single crystals of R b x W O and K x W O 3 prepared by electrolysis of either molten R b 2 W O 4 or K 2 W O 4 and W O 3 as described by Sienko and Morehouse [2]. The superconducting measurements were made by following the a.c. susceptibility of the sample as a function of temperature using the SchawlowDevlin resonant-frequency method at 16 kHz [3]. Measurements of the m o m e n t in a magnetic field were also m a d e using a ballistic method. Intreated single crystals of hexagonal R b x W O 3 were found to become superconducting at about 2°K in agreement with Sweedler et al. [1]. The hexagonal K x W O 3 had broad transitions starting around 2.5°K which is higher than previously found [I]. R was found that a standard volume of material prepared by powdering the freshly-grown untreated crystals to a particle size of < 0.1 m m did not give a full superconducting signal thus indicating the presence of some nonsuperconducting material. Consequently, the procedure for r e m o v i n g u n r e a c t e d s t a r t i n g m a t e r i a l suggested by Sienko and Morehouse [2] was (partly) followed. The powdered c r y s t a l s were boiled in dilute NH4OH solution, m e a s u r e d and then boiled in HF * And at the University of California, La Jolla, California.
solution. We then d i s c o v e r e d that this t r e a t m e n t not only r e s u l t e d in a full s u p e r c o n d u c t i n g signal (i.e., the e n t i r e volume then excluded flux) but that it r a i s e d the s u p e r c o n d u c t i n g t r a n s i t i o n t e m p e r a t u r e a s m u c h a s a factor of 3. Subsequently it was found that any m i n e r a l acid would produce s i m i l a r effects. The r e s u l t s for a n u m b e r of s a m p l e s a r e given in table 1. The l a t t i c e p a r a m e t e r s d e t e r m i n e d by s t a n d a r d powder X-ray diff r a c t i o n t e c h n i q u e s a r e a l s o given. They were computed f r o m the positions of about 10 l i n e s in the b a c k - r e f l e c t i o n r e g i o n . The p a r a m e t e r s for unetched s a m p l e s a r e in good a g r e e m e n t with those r e p o r t e d by Magneli [4] for Rb0.29WO 3 and K0.31WO3 . Both c h e m i c a l and X-ray f l u o r e s c e n c e a n a l y s i s indicate that the etching p r o c e d u r e r e duces the Rb ion c o n c e n t r a t i o n by } to ½ of its value. T h i s r e s u l t is r a t h e r s u r p r i s i n g s i n c e b r o n z e s a r e c o n s i d e r e d to be i n e r t to attack by acid [5]. We have not been able to r a i s e the t r a n s i t i o n t e m p e r a t u r e of any b r o n z e phases other than the hexagonal WO 3 phase by etching. In p a r t i c u l a r , the t e m p e r a t u r e of t e t r a g o n a l NaxWO3 r e m a i n e d at the 0 . 5 5 ° r e p o r t e d by Raub et al. [6]. The CsxWO3 hexagonal phase was much m o r e r e s i s tant to acid attack than the Rb or K compound. By p r o l o n g e d etching in HC1 it was p o s s i b l e to r a i s e a p o r t i o n of the s a m p l e to above 4.7 °, although the bulk of the t r a n s i t i o n r e m a i n e d below 2°K. The ease with which the hexagonal phase can be a l t e r e d is u n d e r s t a n d a b l e in t e r m s of M a g n e l i ' s s t r u c t u r e . He finds that the alkali m e t a l a t o m s lie in the open hexagonal channels which exist p a r a l l e l to the c axis. T h e s e channels can a c c o m modate up to two alkali m e t a l a t o m s per unit cell. They a r e a p p r o x i m a t e l y the d i a m e t e r of an Rb + ion at t h e i r m o s t c o n s t r i c t e d region. Evidently 565
Volume 24A n u m b e r 11
PHYSICS
LETTERS
22 May 1967
Table 1 Composition
Sample #
Treatment
T c Range (OK)
RbxWO 3
7090
powder
1.97-1.88
RbxWO 3
7090
powder etched NH4OH
2.84-2.36
RbxWO 3
7090
powder etched HF
5.70-4.41
RbxWO 3
7090
powder etched HC1
5.15-3.90
RbxWO 3
7146
powder etched H2SO4
6.55-5.45
RbxWO 3
7146
powder etched aqua regia
6.15-5.51
RbxWO 3
7146
powder etched HNO 3
6.25-5.35 4.80-3.31
Lattice constants (/~) a
b
7.384
7.569
7.390
7.511
RbxWO 3
7201
powder etched H3PO 4
RbxWO 3
7225
c r y s t a l etched H2SO4
6.40-6.14
KxWO 3
7123
powder
2.52-1.0 *
7.379
7.538
KxWO 3
7123
powder etched HF
5.70-3.31
7.372
7.484
CsxWO 3
7268
powder etched H2SO4 + HC1
~4.76-2.26
1.77-1.39
* M e a s u r e m e n t made after etching in NH4OH; only ½ volume was superconducting. t h i s s p a t i a l a r r a n g e m e n t p e r m i t s r e m o v a l of K + a n d R b +, b u t n o t t h e l a r g e r C s + ion. P r e l i m i n a r y m e a s u r e m e n t s of t h e m a g n e t i c m o m e n t a t 1 . 4 ° K r e f l e c t t h e a n i s o t r o p y of t h e l a t t i c e . A p p r o x i mately three times as much flux is trapped at 12 kG f o r H p a r a l l e l to t h e c a x i s t h a n f o r H p e r p e n d i c u l a r . A t 4 . 2 ° K H c 2 ~ 24 kG f o r H p a r a l l e l t o t h e c a x i s a n d H c 2 ~ 16 kG f o r H p e r p e n d i c u lar. These preliminary measurements which have been made on single crystal cubes ~ 2ram on a n e d g e s u f f e r s o f a r , b e c a u s e t h e e t c h i n g i s not complete throughout the crystal. The powder d i f f r a c t i o n r e s u l t s q u o t e d i n t a b l e 1, on t h e o t h e r hand, lead to the conclusion that the powdered material is etched throughout. A n u m b e r of m o d e l s c a n b e o f f e r e d t o e x p l a i n t h e r o l e of t h e a l k a l i m e t a l ion c o n t e n t on t h e s u p e r c o n d u c t i v i t y of t h e h e x a g o n a l b r o n z e s . F u r t h e r e x p e r i m e n t s a r e b e i n g p e r f o r m e d in o r d e r to d e c i d e w h i c h , if a n y , of t h e m i s a p p l i c a b l e .
566
W e a r e i n d e b t e d to M i s s S. V i n c e n t f o r t h e Xr a y f l u o r e s c e n c e q u a l i t a t i v e a n a l y s i s , to T . Y . Kometani for chemical analysis, to M.Marezio and E. A.Wood for their valuable advice conc e r n i n g c r y s t a l s t r u c t u r e , t o A. R . S w e e d l e r f o r h e l p f u l d i s c u s s i o n s , a n d t o K. A n d r e s a n d H . J . Williams for experimental cooperation.
References 1. A . R . S w e e d l e r , Ch. J. Raub and B. T. Matthias, Phys. L e t t e r s 15 (1965) 108. 2. M . J . S i e n k o and S . M . M o r e h o u s e , Inorg. Chem. 2 {1963) 485. 3. A . L . S c h a w l o w a n d G . E . D e v l i n , Phys. Rev. 113 (1959} 120. 4. A.Magneli, Acta Chem. Scand. 7 {1953) 315. 5. M . J . S i e n k o , Nonstoichiomctric compounds, ed. R . F . G o u l d , {American Chemical Society. 1963} p. 224. 6. Ch. J. Raub, A . R . Sweedler, M.A. Jensen, S. B r o a d ston and B . T . M a t t h i a s , Phys. Rev. L e t t e r s 13 (1964) 746.
PHYSICS LETTERS
SUPERCONDUCTIVITY
IN H E X A G O N A L
22 May 1967
TUNGSTEN
BRONZES
J.P.REMEIKA,
T.H.GEBALLE, B.T.MATTHIAS*, A.S.COOPER, G. W. H U L L and E. M. K E L L Y Bell Telephone Laboratories, Murray Hill, New Jersey, USA Received 18 April 1967
It is possible, by etching in various acids, to remove alkali-metalions from hexagonal phase alkalitungsten bronzes, to change their latticeconstants and to increase their superconductingtransitiontemperatures substantially.
W e have found superconductivity in the hexagonai phases of some alkali metal tungsten bronzes to occur at temperatures above 6°K, considerably higher then previously reported values [1]. The transition is an inverse function of alkali metal content over at least some range of concentration. The c parameter and, consequently, the c/a ratio as well, is smaller for the higher transition temperature material. The measurements were m a d e on single crystals of R b x W O and K x W O 3 prepared by electrolysis of either molten R b 2 W O 4 or K 2 W O 4 and W O 3 as described by Sienko and Morehouse [2]. The superconducting measurements were made by following the a.c. susceptibility of the sample as a function of temperature using the SchawlowDevlin resonant-frequency method at 16 kHz [3]. Measurements of the m o m e n t in a magnetic field were also m a d e using a ballistic method. Intreated single crystals of hexagonal R b x W O 3 were found to become superconducting at about 2°K in agreement with Sweedler et al. [1]. The hexagonal K x W O 3 had broad transitions starting around 2.5°K which is higher than previously found [I]. R was found that a standard volume of material prepared by powdering the freshly-grown untreated crystals to a particle size of < 0.1 m m did not give a full superconducting signal thus indicating the presence of some nonsuperconducting material. Consequently, the procedure for r e m o v i n g u n r e a c t e d s t a r t i n g m a t e r i a l suggested by Sienko and Morehouse [2] was (partly) followed. The powdered c r y s t a l s were boiled in dilute NH4OH solution, m e a s u r e d and then boiled in HF * And at the University of California, La Jolla, California.
solution. We then d i s c o v e r e d that this t r e a t m e n t not only r e s u l t e d in a full s u p e r c o n d u c t i n g signal (i.e., the e n t i r e volume then excluded flux) but that it r a i s e d the s u p e r c o n d u c t i n g t r a n s i t i o n t e m p e r a t u r e a s m u c h a s a factor of 3. Subsequently it was found that any m i n e r a l acid would produce s i m i l a r effects. The r e s u l t s for a n u m b e r of s a m p l e s a r e given in table 1. The l a t t i c e p a r a m e t e r s d e t e r m i n e d by s t a n d a r d powder X-ray diff r a c t i o n t e c h n i q u e s a r e a l s o given. They were computed f r o m the positions of about 10 l i n e s in the b a c k - r e f l e c t i o n r e g i o n . The p a r a m e t e r s for unetched s a m p l e s a r e in good a g r e e m e n t with those r e p o r t e d by Magneli [4] for Rb0.29WO 3 and K0.31WO3 . Both c h e m i c a l and X-ray f l u o r e s c e n c e a n a l y s i s indicate that the etching p r o c e d u r e r e duces the Rb ion c o n c e n t r a t i o n by } to ½ of its value. T h i s r e s u l t is r a t h e r s u r p r i s i n g s i n c e b r o n z e s a r e c o n s i d e r e d to be i n e r t to attack by acid [5]. We have not been able to r a i s e the t r a n s i t i o n t e m p e r a t u r e of any b r o n z e phases other than the hexagonal WO 3 phase by etching. In p a r t i c u l a r , the t e m p e r a t u r e of t e t r a g o n a l NaxWO3 r e m a i n e d at the 0 . 5 5 ° r e p o r t e d by Raub et al. [6]. The CsxWO3 hexagonal phase was much m o r e r e s i s tant to acid attack than the Rb or K compound. By p r o l o n g e d etching in HC1 it was p o s s i b l e to r a i s e a p o r t i o n of the s a m p l e to above 4.7 °, although the bulk of the t r a n s i t i o n r e m a i n e d below 2°K. The ease with which the hexagonal phase can be a l t e r e d is u n d e r s t a n d a b l e in t e r m s of M a g n e l i ' s s t r u c t u r e . He finds that the alkali m e t a l a t o m s lie in the open hexagonal channels which exist p a r a l l e l to the c axis. T h e s e channels can a c c o m modate up to two alkali m e t a l a t o m s per unit cell. They a r e a p p r o x i m a t e l y the d i a m e t e r of an Rb + ion at t h e i r m o s t c o n s t r i c t e d region. Evidently 565
Volume 24A n u m b e r 11
PHYSICS
LETTERS
22 May 1967
Table 1 Composition
Sample #
Treatment
T c Range (OK)
RbxWO 3
7090
powder
1.97-1.88
RbxWO 3
7090
powder etched NH4OH
2.84-2.36
RbxWO 3
7090
powder etched HF
5.70-4.41
RbxWO 3
7090
powder etched HC1
5.15-3.90
RbxWO 3
7146
powder etched H2SO4
6.55-5.45
RbxWO 3
7146
powder etched aqua regia
6.15-5.51
RbxWO 3
7146
powder etched HNO 3
6.25-5.35 4.80-3.31
Lattice constants (/~) a
b
7.384
7.569
7.390
7.511
RbxWO 3
7201
powder etched H3PO 4
RbxWO 3
7225
c r y s t a l etched H2SO4
6.40-6.14
KxWO 3
7123
powder
2.52-1.0 *
7.379
7.538
KxWO 3
7123
powder etched HF
5.70-3.31
7.372
7.484
CsxWO 3
7268
powder etched H2SO4 + HC1
~4.76-2.26
1.77-1.39
* M e a s u r e m e n t made after etching in NH4OH; only ½ volume was superconducting. t h i s s p a t i a l a r r a n g e m e n t p e r m i t s r e m o v a l of K + a n d R b +, b u t n o t t h e l a r g e r C s + ion. P r e l i m i n a r y m e a s u r e m e n t s of t h e m a g n e t i c m o m e n t a t 1 . 4 ° K r e f l e c t t h e a n i s o t r o p y of t h e l a t t i c e . A p p r o x i mately three times as much flux is trapped at 12 kG f o r H p a r a l l e l to t h e c a x i s t h a n f o r H p e r p e n d i c u l a r . A t 4 . 2 ° K H c 2 ~ 24 kG f o r H p a r a l l e l t o t h e c a x i s a n d H c 2 ~ 16 kG f o r H p e r p e n d i c u lar. These preliminary measurements which have been made on single crystal cubes ~ 2ram on a n e d g e s u f f e r s o f a r , b e c a u s e t h e e t c h i n g i s not complete throughout the crystal. The powder d i f f r a c t i o n r e s u l t s q u o t e d i n t a b l e 1, on t h e o t h e r hand, lead to the conclusion that the powdered material is etched throughout. A n u m b e r of m o d e l s c a n b e o f f e r e d t o e x p l a i n t h e r o l e of t h e a l k a l i m e t a l ion c o n t e n t on t h e s u p e r c o n d u c t i v i t y of t h e h e x a g o n a l b r o n z e s . F u r t h e r e x p e r i m e n t s a r e b e i n g p e r f o r m e d in o r d e r to d e c i d e w h i c h , if a n y , of t h e m i s a p p l i c a b l e .
566
W e a r e i n d e b t e d to M i s s S. V i n c e n t f o r t h e Xr a y f l u o r e s c e n c e q u a l i t a t i v e a n a l y s i s , to T . Y . Kometani for chemical analysis, to M.Marezio and E. A.Wood for their valuable advice conc e r n i n g c r y s t a l s t r u c t u r e , t o A. R . S w e e d l e r f o r h e l p f u l d i s c u s s i o n s , a n d t o K. A n d r e s a n d H . J . Williams for experimental cooperation.
References 1. A . R . S w e e d l e r , Ch. J. Raub and B. T. Matthias, Phys. L e t t e r s 15 (1965) 108. 2. M . J . S i e n k o and S . M . M o r e h o u s e , Inorg. Chem. 2 {1963) 485. 3. A . L . S c h a w l o w a n d G . E . D e v l i n , Phys. Rev. 113 (1959} 120. 4. A.Magneli, Acta Chem. Scand. 7 {1953) 315. 5. M . J . S i e n k o , Nonstoichiomctric compounds, ed. R . F . G o u l d , {American Chemical Society. 1963} p. 224. 6. Ch. J. Raub, A . R . Sweedler, M.A. Jensen, S. B r o a d ston and B . T . M a t t h i a s , Phys. Rev. L e t t e r s 13 (1964) 746.