- Email: [email protected]
Magnetic properties of cobalt in oxide lattices
Magnetic properties of cobalt in oxide lattices P. COSSEE 1,ah(watorium voor Anori~mlsehe en Fyslsehe Chemie, RiJksunlversltelt, Leiden - Holhmd *
l ~ . The abhorrently low value of the magnetic susoeptJbfllty of Co804 a n d ZnCotO i a n d the mteertaJnty abettt the valence of the cobalt ions In these sptnels has led to th e I n v u t J n s t l o n of mixed orystals of CoO and ZnO. The mannetlc suseeptbility of the (Co, Zn)O mi xe d crystals, m e a s t m ~ up t o 1200.K, C could be represented by X == • + ~ w i t h a fairly larlie temporatm~.-tndepend~t l ~ r a m l n e t i s m (~ : 0.5 × 1 0 - - 3 e.m.u.) and a magnetic moment for t he Co2+ ions in t e t n t h e d r a l Intorstiees of 4 . 0 ~ B. The suseeptildlity measurements on several.samples of ZnC%0 i Indicated t h a t CoS + ions are present in the oetohedral holes. They showed only a small tempemtuee-lndependent ~ of 0.1 × 1 o - - 8 e.m.u. It appears t h a t iICthis s pht e l t e rvol e nt cobalt behaves In a similar way as in the nuJsterous s/x-coordinated e~mp6el eo~pounds. C The temperature dependence of the maim" smmeptlbUity of CouO4 : X = :: 0t "t- T--"--'O w i t h at ,=0.7 X 1 0 - - 8 e.sm.u, and I~, = 4.14~JI i s n o w eueUy e ~ on t k e mmml Mkm O m t t~Is C~ml~qmcl Is a ~ I 2.3 q d m l . T~ne teTwdent oobelt tons In the oeSmhadnd Intm..flees do not ezhlblt a permanent moment, wldeh explains t he low s u e m q ~ l U t y values. The Co $ + ions in the oetahedra of the perowskite LaCoOs seem t o h o l ~ r m a U y pantlma8netlc. This difference in behavioue of tervalent cobalt in t he o c t s h e d ~ of various ox]q~n Iottlees an d t h e behavloue of divalent eobalt in tetrahedra can be qtmlltaUvely understood w i t h the old of the • t r y s t s ! field • theory. INTRODUCTION.
The present investigation was initially undertaken to find a solution for some problems concerning CosO,. This substance crystallizes in the spinel structure, which is essentially a close-packed lattice of negative ions in which half of the octahedral and I/8 of the tetrahedral interstices are occupied by the positive ions. So far no satisfactory explanation has been given of the low paramagnetic susceptibility of this compound ( a mean Bohr magneton number of about 3.0 is reported in the literature [I]). Further, there has been no direct proof for one of the four possible distributions of cobalt ions over tetrahedral and octahedral interstices. Of these four possibilities: normal 2-3 spinel, inversed 2-3 spinel, normal 4-2 spinel and inversed 4-2 ( ' ) Present address: KomnldlJke/Shell-Lal~rntorimm, Amsta~lam.
4~4
I'. COSSEE
spinel, the first seems to be the most probable according to crystallographic investigations of Robin [2]. Starting from the assumption that the internal structure may be represented by the following formula Co'-'+[CosS+] 042- (octahedral ions are given in square brackets), a suitable approach to the problem is the determination of: a. magnetic properties of the spinel ZnCos04. In this compound the Z:~ "+ ions are very likely to occupy the tetrahedral interstices so that we m a y write Zn[Co2]O,. b. magnetic properties of Co s + ions in tetrahedral interstices of an oxygen lattice. This appears to be possible in mixed crystals of CoO and ZnO, containing less than 25 % CoO. These substances crystallize in the wurtzite structure, that is a hexagonal close-packed lattice of negative ions in which all positive ions occupy tetrahedral interstices. A combination of the properties of Co s÷ ions in tetrahedra and those of the CosO~= group m a y be compared with the actual magnetic properties of
Co20 ,.
EXPERIMENTAL.
Apparatus: All magnetic measurements were made with a torsion balance in a vacuum case by the Faraday method. The apparatus was calibrated with NiSO,.(NII~)~.SO~.6H,.O.For the measurements at higher temperatures up to 1200oK (;ds03was used as a secondary standard. !
X
.°I
o cOo~os Z.o.a9 5 o
zoo-
"~
Q c°~ °4
Q 1 I I
300
600
900
Fro. 1. - T e m p e r a t u r e - d e p e n d e n c e of t h e susceptlbilily of n Co-Zn oxide m i x e d crystnI a n d of CosO ~. Tire Z-vnlues a r e for Co-Zn oxide p e r Co-atom, for CosO4 p e r Mol CosO 4. T h e s t r u i g h t d o t t e d line gives the tlxeoretical • s p i n - o n l y , b e l m v l o t t r for a 3fol CoaO, c o n t a i n i n g tlu'ee • m n g n e t i c • Co Ions.
MAGNETIC PROPERTIES
O F C O B A L T IN O X I D E
LATTICES
485
Results: The susceptibility of the (Co, Zn)O mixed crystals as well C as of CosO, could be represented by ~( = ~ + T-------~(liquid air points 1
excluded). A plot of m aganst T is given in Fig. I. Z The properties of ZnCo20~ appeared to be highly dependent on the x.r
,o ~
3O0
Ii
[
T oh
F16. 2. - T e m p e r a t u r e - d e p e n d e n c e of Z . T for d i l l e r e n t ZitCatO, ~amples. - o - - 4 - T h i s u m p l e w a s p r e p a r e d a t a t o o low t e m p e r a t u r e . It w a s c o n v e r t e d d u r i n g t h e m e a s u r e m e n t t o a Smnlde c o n t a i n i n g less C o 2 + i o n s ; Aettmlly the best rumple which could be prelmred. This one was measured at a too hlsh t e m p e r a t u r e . D u r i n g t h e m e a s u r e m e n t c o n v e r s i o n t o o .~lmple w i t h 8 h i s h e r C~S ÷ c o n t e n t eemllT~t.
heat treatment but of all the samples the very low susceptibility could C be represented by 7, -----= + - ~ .
Actually, there is an optimal procedure
for the preparation of very weakly magnetic samples. A plot of ~.. T against T for different samples ZnCos0 4 demonstrates this peculiarity (Fig. 2). The small Curie-term C is attributed to Co s+ ions in the tetrahedral holes, the majority of cobalt being in the diamagnetic trivalent state. The temperature-independent term ~ seems to be due to the small impurities of Co s + ions as well as to the trivalent cobalt ions, which may be seen by extrapolating to a stoeehiometric sample containing no Co 2+ (Table I) (we were not able to prepare such a sample) * * .More det;til, , n d tllbk~ w i t h e x p e r i m e n t a l v , htes a r e 81yen in P. Cessee. Thesis L e i d e n 1936.
486
p. COSSEE TABLE I. --
and C in e.m.u. /or di~erent ZnCesO, samples.
=
Sample
'J
1
3
4
-
I
-~ Extrapo!atlon to
I
pure gnGotO4
I m.lO $ Cm,
o.37
o.33
11.28
0.26
]r
-~ 0 . 2
0.450
I;.372
o.237
0.183
I
0
TABLE "II. -- = in e.m.u, and ~ /or three di/lerent types o/ substances. ~.10s
Co ions in tetrahedeal holes (mean value for different dilutions of CoO in ZnO)
0.52
CotO, ffi (extrapolated from ZnC.~O4 samples)
0.2
.
.
.
.
.
.
.
.
.
.
.
.
C % 0 s (calculated from the above values)
.
.
.
.
.
.
.
.
.
.
.
.
. . . . . . . . . . .
. . . . . . . . . . . . .
Go,O, (experimental) . . . . . . . . . . . . . . . . . . . . . .
.
'
4.05 B.M.
o.72
4.05 B.M.
0.71
4 . 1 4 B.M.
The significant values obtained for the three different substances are combined in Table II, which amply demonstrates that CosO4 is a normal 2-3 spinel of which the trivalent ions in octahedral interstices exhibit no permanent magnetic moment. DISCUSSION. In oxide lattices we have to consider the following types of cobalt ions: a. Co 2+ ions in octahedral interstices. b. Co z + ions in tetrahedral interstices. c. Co s + ions in octahedral interstices (no examples are known in which Co s + ions occupy tetrahedral interstices).
We will briefly discuss the magnetic properties of the second and third types of ions, as Co 2 + ions in octahedral interstices have been satisfactorily treated by Abragam and Pryce [3]. Not many compounds containing Co z + ions in tetrahedral surroundings are known. A few halogenides are mentioned by Nyhblm [4]. Moments ranging from 4.4 to 4.7 B.M. are reported (our values are: ~ -~ 4.05 B.M., ---- 0.5 × 10 - s e. m.u.). Crystal field considerations [4, 5] show that the term scheme of energy levels of a Co 2 + ion in a tetrahedral hole has a singlet ground state (Fig. 3) (in octahedral interstices a triplet state is lowest), causing a c~spin-only magnetic moment. The departure from the exact ~,spin-only ~, value (3.87
MAGNETIC
PROPERTIES
OF COBALT IN OXIDE
LATTICES
487
B.M.) is due to the L.S. coupling, which is rather strong in the second half of the first row of transition elements. Above this singlet level are the two e m p t y triplet levels. As was shown by van Vleck [6] these empty levels may cause a temperature-independent contribution to .the susceptibility if the energy distance to the ground level is not too large. Our results on Co 8+ ions in octahedral interstices are in \ full agreement with those on the well-known hexacoordinated complexes. In these diamagnetic compounds a temCo # * OCr~IH_rOR4L Co ~ * r£T~AH£OR4~. perature-independent term FIG. 3 . - T e r r a s c h e m e s of Co='4- ions in, r e s p e c t i v e l y , varying between 0.03 and o e t a h e d r a l a n d t e t r s h e d r a l l n t e ~ t l e e s of a b o u t t h e s a ~ e SiZe. 0.17 × 10 - s e.m.u, has been found [7]. KsCoF 6 is the only complex in which trivalent cobalt exhibits a permanent moment of. 5.3 B.M. Among the oxides containing trivalent cobalt the perowskite LaCoOs t~t,~ is paramagnetic accoi'ding to Jonker and van $anten [8]. We shall have to explain this difference in behaviour of Co s+ ions in an oxidic spinel and a perowskite, respectively. According to Orgel [9] the octahedrally shaped electric field of the surrounding ions splits the 6/) level of the central ion into an upper doublet and a lower triplet. C~rSr4L . F/tLO If, however, the intensity of the crystal field gradually increases the F r o . 4. - , O q ~ l d i ~ m , f o r Co $ -~ Ions i n a c r y s originally much higher ~S level may t a l field of o e t a ~ d r a l s y m m e t r y . become the lower. As may be seen from Fig. 4 small variations in crystal field strength may alter the magnetic properties completely. If we compare the crystal structures of the spinels Cos04 and ZnCozO, with the arrangement in LaCoOs, we notice t h a t in both cases the central Co s+ ion is surrounded by an octahedron of 0 2 - ions, but in the perowskite the cube of 8 0 ~- ions at a distance of a ~-3 (Co-O distance - - a ) is replaced by a cube of 8 / ~ s + ions (Fig. 5). I
i
488
P. coss~-E
Considering one 0 2 - ion in the centre of one of the faces of the cubes, we notice that in the perowskite structure this ion is only surrounded by a square of La s+ ions and consequently strongly polarized in the plane of this face. A diminished polarizability perpendicular to this plane results so that
•
c~"~*
•
~ o z-
co Z*
~ o z"
FZ~. 5. - I m m e d i a t e e n v i r o u m e n l or n C~3 + ion ill ;! sl)inel Imd ill :! I~.'rowklle. T o the left: A part of the unil cell o( .111,2 -~-(;r)s:; -F O; '~-T o the right: Unit cell or LaCoO s.
these surrounding 0 2 - ions are now comparable to F - ions. The fact that the crystal field strength is highly dependent on the polarizability of the surrounding ions explains w h y KsCoF 6 and LaCoOs are the only compounds in which Co 3 + ions are paramagnetic * ACKNOWLEDGEMENT. The author wishes to thank the Management of the Koninklijke/Shell-Laboratorium, Amsterdam for the assistance in preparing this paper. - -
* R e c e n t l y It w a s shown tlmt in L i C . O s (on N a C l - l l k e structure) the C . 3-~ tons . r e diammgnetic, for w h i c h a similar e x p h m . l l o n wns given. See: P. F. Bongers, Thesis Leiden 1957. p. 4~2.
[1] [2] [3J [4] [5] [6] [7]
S. S. BHATNAGAR, B. IZRAKASH a n d M. A. QUA~','VM. ~ J. I n d h m Chem. S o c . . , 18, 3 ~ | (194(|).
J. ROn~N, • Compt. rend. ,, °35, 131|1, (1932); , These *. Pnris (1933). A. AnnAnAM a n d M. H . L. PRYc~:. , lh'o¢. P, oy. Soc. ,, ( L o n d o n ) A 20~. 173, (1931~. R. S. N v a o L u , • Q u a r t . l~evs. Chem. So¢. , (t.ondon) 7, :]77, (1953). C. J . GORTER, • Phys. l l e v . • 42, 437, (19:12). J . H . v a n VLEC.K, Theory o / E l e c l r i e and .'~lcqlnelie Susreplibilllies, London, O x [ o r d U n i v . Press, 1193")). R. W. ASMtTSSBN, • Thesis Copenhagen ,, p. 179-184, 11944); J . L. K[':RNAHAN a n d . . M . . I . SIENXO, • J . A m . Chem. Soc. ,, 77, 1978, 11953). [8] G. H..J().~n~.~:K a n d J . 11. v. SANT~:.~, • l)hystca ,, X I X , 120 119.5:]). [9] .L (IoGEL, , .I. Chem. F,~. ,, 47.56, (1932).
l ~ . The abhorrently low value of the magnetic susoeptJbfllty of Co804 a n d ZnCotO i a n d the mteertaJnty abettt the valence of the cobalt ions In these sptnels has led to th e I n v u t J n s t l o n of mixed orystals of CoO and ZnO. The mannetlc suseeptbility of the (Co, Zn)O mi xe d crystals, m e a s t m ~ up t o 1200.K, C could be represented by X == • + ~ w i t h a fairly larlie temporatm~.-tndepend~t l ~ r a m l n e t i s m (~ : 0.5 × 1 0 - - 3 e.m.u.) and a magnetic moment for t he Co2+ ions in t e t n t h e d r a l Intorstiees of 4 . 0 ~ B. The suseeptildlity measurements on several.samples of ZnC%0 i Indicated t h a t CoS + ions are present in the oetohedral holes. They showed only a small tempemtuee-lndependent ~ of 0.1 × 1 o - - 8 e.m.u. It appears t h a t iICthis s pht e l t e rvol e nt cobalt behaves In a similar way as in the nuJsterous s/x-coordinated e~mp6el eo~pounds. C The temperature dependence of the maim" smmeptlbUity of CouO4 : X = :: 0t "t- T--"--'O w i t h at ,=0.7 X 1 0 - - 8 e.sm.u, and I~, = 4.14~JI i s n o w eueUy e ~ on t k e mmml Mkm O m t t~Is C~ml~qmcl Is a ~ I 2.3 q d m l . T~ne teTwdent oobelt tons In the oeSmhadnd Intm..flees do not ezhlblt a permanent moment, wldeh explains t he low s u e m q ~ l U t y values. The Co $ + ions in the oetahedra of the perowskite LaCoOs seem t o h o l ~ r m a U y pantlma8netlc. This difference in behavioue of tervalent cobalt in t he o c t s h e d ~ of various ox]q~n Iottlees an d t h e behavloue of divalent eobalt in tetrahedra can be qtmlltaUvely understood w i t h the old of the • t r y s t s ! field • theory. INTRODUCTION.
The present investigation was initially undertaken to find a solution for some problems concerning CosO,. This substance crystallizes in the spinel structure, which is essentially a close-packed lattice of negative ions in which half of the octahedral and I/8 of the tetrahedral interstices are occupied by the positive ions. So far no satisfactory explanation has been given of the low paramagnetic susceptibility of this compound ( a mean Bohr magneton number of about 3.0 is reported in the literature [I]). Further, there has been no direct proof for one of the four possible distributions of cobalt ions over tetrahedral and octahedral interstices. Of these four possibilities: normal 2-3 spinel, inversed 2-3 spinel, normal 4-2 spinel and inversed 4-2 ( ' ) Present address: KomnldlJke/Shell-Lal~rntorimm, Amsta~lam.
4~4
I'. COSSEE
spinel, the first seems to be the most probable according to crystallographic investigations of Robin [2]. Starting from the assumption that the internal structure may be represented by the following formula Co'-'+[CosS+] 042- (octahedral ions are given in square brackets), a suitable approach to the problem is the determination of: a. magnetic properties of the spinel ZnCos04. In this compound the Z:~ "+ ions are very likely to occupy the tetrahedral interstices so that we m a y write Zn[Co2]O,. b. magnetic properties of Co s + ions in tetrahedral interstices of an oxygen lattice. This appears to be possible in mixed crystals of CoO and ZnO, containing less than 25 % CoO. These substances crystallize in the wurtzite structure, that is a hexagonal close-packed lattice of negative ions in which all positive ions occupy tetrahedral interstices. A combination of the properties of Co s÷ ions in tetrahedra and those of the CosO~= group m a y be compared with the actual magnetic properties of
Co20 ,.
EXPERIMENTAL.
Apparatus: All magnetic measurements were made with a torsion balance in a vacuum case by the Faraday method. The apparatus was calibrated with NiSO,.(NII~)~.SO~.6H,.O.For the measurements at higher temperatures up to 1200oK (;ds03was used as a secondary standard. !
X
.°I
o cOo~os Z.o.a9 5 o
zoo-
"~
Q c°~ °4
Q 1 I I
300
600
900
Fro. 1. - T e m p e r a t u r e - d e p e n d e n c e of t h e susceptlbilily of n Co-Zn oxide m i x e d crystnI a n d of CosO ~. Tire Z-vnlues a r e for Co-Zn oxide p e r Co-atom, for CosO4 p e r Mol CosO 4. T h e s t r u i g h t d o t t e d line gives the tlxeoretical • s p i n - o n l y , b e l m v l o t t r for a 3fol CoaO, c o n t a i n i n g tlu'ee • m n g n e t i c • Co Ions.
MAGNETIC PROPERTIES
O F C O B A L T IN O X I D E
LATTICES
485
Results: The susceptibility of the (Co, Zn)O mixed crystals as well C as of CosO, could be represented by ~( = ~ + T-------~(liquid air points 1
excluded). A plot of m aganst T is given in Fig. I. Z The properties of ZnCo20~ appeared to be highly dependent on the x.r
,o ~
3O0
Ii
[
T oh
F16. 2. - T e m p e r a t u r e - d e p e n d e n c e of Z . T for d i l l e r e n t ZitCatO, ~amples. - o - - 4 - T h i s u m p l e w a s p r e p a r e d a t a t o o low t e m p e r a t u r e . It w a s c o n v e r t e d d u r i n g t h e m e a s u r e m e n t t o a Smnlde c o n t a i n i n g less C o 2 + i o n s ; Aettmlly the best rumple which could be prelmred. This one was measured at a too hlsh t e m p e r a t u r e . D u r i n g t h e m e a s u r e m e n t c o n v e r s i o n t o o .~lmple w i t h 8 h i s h e r C~S ÷ c o n t e n t eemllT~t.
heat treatment but of all the samples the very low susceptibility could C be represented by 7, -----= + - ~ .
Actually, there is an optimal procedure
for the preparation of very weakly magnetic samples. A plot of ~.. T against T for different samples ZnCos0 4 demonstrates this peculiarity (Fig. 2). The small Curie-term C is attributed to Co s+ ions in the tetrahedral holes, the majority of cobalt being in the diamagnetic trivalent state. The temperature-independent term ~ seems to be due to the small impurities of Co s + ions as well as to the trivalent cobalt ions, which may be seen by extrapolating to a stoeehiometric sample containing no Co 2+ (Table I) (we were not able to prepare such a sample) * * .More det;til, , n d tllbk~ w i t h e x p e r i m e n t a l v , htes a r e 81yen in P. Cessee. Thesis L e i d e n 1936.
486
p. COSSEE TABLE I. --
and C in e.m.u. /or di~erent ZnCesO, samples.
=
Sample
'J
1
3
4
-
I
-~ Extrapo!atlon to
I
pure gnGotO4
I m.lO $ Cm,
o.37
o.33
11.28
0.26
]r
-~ 0 . 2
0.450
I;.372
o.237
0.183
I
0
TABLE "II. -- = in e.m.u, and ~ /or three di/lerent types o/ substances. ~.10s
Co ions in tetrahedeal holes (mean value for different dilutions of CoO in ZnO)
0.52
CotO, ffi (extrapolated from ZnC.~O4 samples)
0.2
.
.
.
.
.
.
.
.
.
.
.
.
C % 0 s (calculated from the above values)
.
.
.
.
.
.
.
.
.
.
.
.
. . . . . . . . . . .
. . . . . . . . . . . . .
Go,O, (experimental) . . . . . . . . . . . . . . . . . . . . . .
.
'
4.05 B.M.
o.72
4.05 B.M.
0.71
4 . 1 4 B.M.
The significant values obtained for the three different substances are combined in Table II, which amply demonstrates that CosO4 is a normal 2-3 spinel of which the trivalent ions in octahedral interstices exhibit no permanent magnetic moment. DISCUSSION. In oxide lattices we have to consider the following types of cobalt ions: a. Co 2+ ions in octahedral interstices. b. Co z + ions in tetrahedral interstices. c. Co s + ions in octahedral interstices (no examples are known in which Co s + ions occupy tetrahedral interstices).
We will briefly discuss the magnetic properties of the second and third types of ions, as Co 2 + ions in octahedral interstices have been satisfactorily treated by Abragam and Pryce [3]. Not many compounds containing Co z + ions in tetrahedral surroundings are known. A few halogenides are mentioned by Nyhblm [4]. Moments ranging from 4.4 to 4.7 B.M. are reported (our values are: ~ -~ 4.05 B.M., ---- 0.5 × 10 - s e. m.u.). Crystal field considerations [4, 5] show that the term scheme of energy levels of a Co 2 + ion in a tetrahedral hole has a singlet ground state (Fig. 3) (in octahedral interstices a triplet state is lowest), causing a c~spin-only magnetic moment. The departure from the exact ~,spin-only ~, value (3.87
MAGNETIC
PROPERTIES
OF COBALT IN OXIDE
LATTICES
487
B.M.) is due to the L.S. coupling, which is rather strong in the second half of the first row of transition elements. Above this singlet level are the two e m p t y triplet levels. As was shown by van Vleck [6] these empty levels may cause a temperature-independent contribution to .the susceptibility if the energy distance to the ground level is not too large. Our results on Co 8+ ions in octahedral interstices are in \ full agreement with those on the well-known hexacoordinated complexes. In these diamagnetic compounds a temCo # * OCr~IH_rOR4L Co ~ * r£T~AH£OR4~. perature-independent term FIG. 3 . - T e r r a s c h e m e s of Co='4- ions in, r e s p e c t i v e l y , varying between 0.03 and o e t a h e d r a l a n d t e t r s h e d r a l l n t e ~ t l e e s of a b o u t t h e s a ~ e SiZe. 0.17 × 10 - s e.m.u, has been found [7]. KsCoF 6 is the only complex in which trivalent cobalt exhibits a permanent moment of. 5.3 B.M. Among the oxides containing trivalent cobalt the perowskite LaCoOs t~t,~ is paramagnetic accoi'ding to Jonker and van $anten [8]. We shall have to explain this difference in behaviour of Co s+ ions in an oxidic spinel and a perowskite, respectively. According to Orgel [9] the octahedrally shaped electric field of the surrounding ions splits the 6/) level of the central ion into an upper doublet and a lower triplet. C~rSr4L . F/tLO If, however, the intensity of the crystal field gradually increases the F r o . 4. - , O q ~ l d i ~ m , f o r Co $ -~ Ions i n a c r y s originally much higher ~S level may t a l field of o e t a ~ d r a l s y m m e t r y . become the lower. As may be seen from Fig. 4 small variations in crystal field strength may alter the magnetic properties completely. If we compare the crystal structures of the spinels Cos04 and ZnCozO, with the arrangement in LaCoOs, we notice t h a t in both cases the central Co s+ ion is surrounded by an octahedron of 0 2 - ions, but in the perowskite the cube of 8 0 ~- ions at a distance of a ~-3 (Co-O distance - - a ) is replaced by a cube of 8 / ~ s + ions (Fig. 5). I
i
488
P. coss~-E
Considering one 0 2 - ion in the centre of one of the faces of the cubes, we notice that in the perowskite structure this ion is only surrounded by a square of La s+ ions and consequently strongly polarized in the plane of this face. A diminished polarizability perpendicular to this plane results so that
•
c~"~*
•
~ o z-
co Z*
~ o z"
FZ~. 5. - I m m e d i a t e e n v i r o u m e n l or n C~3 + ion ill ;! sl)inel Imd ill :! I~.'rowklle. T o the left: A part of the unil cell o( .111,2 -~-(;r)s:; -F O; '~-T o the right: Unit cell or LaCoO s.
these surrounding 0 2 - ions are now comparable to F - ions. The fact that the crystal field strength is highly dependent on the polarizability of the surrounding ions explains w h y KsCoF 6 and LaCoOs are the only compounds in which Co 3 + ions are paramagnetic * ACKNOWLEDGEMENT. The author wishes to thank the Management of the Koninklijke/Shell-Laboratorium, Amsterdam for the assistance in preparing this paper. - -
* R e c e n t l y It w a s shown tlmt in L i C . O s (on N a C l - l l k e structure) the C . 3-~ tons . r e diammgnetic, for w h i c h a similar e x p h m . l l o n wns given. See: P. F. Bongers, Thesis Leiden 1957. p. 4~2.
[1] [2] [3J [4] [5] [6] [7]
S. S. BHATNAGAR, B. IZRAKASH a n d M. A. QUA~','VM. ~ J. I n d h m Chem. S o c . . , 18, 3 ~ | (194(|).
J. ROn~N, • Compt. rend. ,, °35, 131|1, (1932); , These *. Pnris (1933). A. AnnAnAM a n d M. H . L. PRYc~:. , lh'o¢. P, oy. Soc. ,, ( L o n d o n ) A 20~. 173, (1931~. R. S. N v a o L u , • Q u a r t . l~evs. Chem. So¢. , (t.ondon) 7, :]77, (1953). C. J . GORTER, • Phys. l l e v . • 42, 437, (19:12). J . H . v a n VLEC.K, Theory o / E l e c l r i e and .'~lcqlnelie Susreplibilllies, London, O x [ o r d U n i v . Press, 1193")). R. W. ASMtTSSBN, • Thesis Copenhagen ,, p. 179-184, 11944); J . L. K[':RNAHAN a n d . . M . . I . SIENXO, • J . A m . Chem. Soc. ,, 77, 1978, 11953). [8] G. H..J().~n~.~:K a n d J . 11. v. SANT~:.~, • l)hystca ,, X I X , 120 119.5:]). [9] .L (IoGEL, , .I. Chem. F,~. ,, 47.56, (1932).