K2NiF4 TYPE OXIDES : THE COMPOUNDS La2-xSrxNiO4-y : CRYSTAL CHEMISTRY AND ELECTRICAL PROPERTIES

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K2NiF4 TYPE OXIDES : THE COMPOUNDS La2-xSrxNiO4-y : CRYSTAL CHEMISTRY AND

ELECTRICAL PROPERTIES

M. Khairy, P. Odier, J. Choisnet

To cite this version:

M. Khairy, P. Odier, J. Choisnet. K2NiF4 TYPE OXIDES : THE COMPOUNDS La2-xSrxNiO4-y :

CRYSTAL CHEMISTRY AND ELECTRICAL PROPERTIES. Journal de Physique Colloques, 1986,

47 (C1), pp.C1-831-C1-835. �10.1051/jphyscol:19861127�. �jpa-00225524�

JOURNAL DE PHYSIQUE

C o l l o q u e C I , s u p p l e m e n t a u n ° 2 , Tome 4 7 , f e v r i e r 1 9 8 6 p a g e c i - 8 3 1

K

N i F

TYPE OXIDES : THE COMPOUNDS L a

_

S r

N i 0

_ : CRYSTAL CHEMISTRY ftND ELECTRICAL PROPERTIES

M. KHAIRY, P . ODIER a n d J . CHOISNET

Centre dé Recherches sur la Physique des Hautes Températures, C.N.R.S., F-45045 Orléans Cedex, France

Résumé - La s u b s t i t u t i o n du lanthane par le strontium dans La NiO a é t é étu- d i é e . Une famille d'oxydes mixtes de formulation La2_

SrxNi04_„ a é t é obtenue à l ' a i r avec un large domaine d'homogénéité ( 0 ^ x 4 1.2). Les p r o p r i é t é s s t r u c t u r a l e s e t é l e c t r i q u e s sont discutées à p a r t i r des r é s u l t a t s de d i f f r a c - t i o n X e t de conductivité é l e c t r i q u e . En p a r t i c u l i e r , n o u s mettons en évidence une forte c o r r é l a t i o n entre l a d i s t o r t i o n géométrique des octaèdres de nickel e t l e s p r o p r i é t é s é l e c t r i q u e s (o i» 100 (Q.cm) pour 0.8 < x •$ 1 ) .

A b s t r a c t - The s u b s t i t u t i o n o f lanthanum f o r s t r o n t i u m h a s been s t u d i e d i n L a

N i 0

. A f a m i l y of mixed o x i d e s w i t h t h e f o r m u l a t i o n L a

_

S r N i 0

_

h a s been a i r o b t a i n e d w i t h a w i d e homogeneity r a n g e (0 ^ x < 1.2) . S t r u c t u r a l and e ' l e c t r i c a l p r o p e r t i e s a r e d i s c u s s e d from X r a y d i f f r a c t i o n r e s u l t s and e l e c t r i c a l c o n d u c - t i v i t y m e a s u r e m e n t s . We show up e s p e c i a l l y a s t r o n g r e l a t i o n s h i p between t h e g e o m e t r i c a l d i s t o r t i o n of t h e Ni o c t a h e d r a and t h e e l e c t r i c a l p r o p e r t i e s

{o T. 1 0 0 t e . c n ) ) -

f o r 0 . 8 < x < 1) . I - INTRODUCTION

Several oxides with the A

B0

composition are isotype of the K2NiF

family which essentially is an intergrowth of AB0

perovskite and sodium chloride A0 layers.Such mixed oxides as La

Cu0

(1 ) and La

Ni0

(2) show simultaneously a rather high elec- trical conductivity (about 10(ftcm)

) and a good thermal stability in air: La

Ni0

congruently melts at approximately 1700°C. The partial substitution of lanthanum by strontium in these two oxides has been studied. It results either in a non-stoi- chiometric compounds: La2_

Sr

CuQ

_

(3) or in stoichiometric compounds:La2_

Sr

Ni0 (4), depending on the preparation conditions. The copper compound exhibits and en- hancement of the electrical conductivity to a value as high as lOSfccm) for x = 1/3 (5) related to a large amount of C u

. Unlike this result, the values reported for the electrical conductivity of nickel based compounds are lower than that of pure La

Ni0

(4,6,7). This result is inconsistent with the increase in the N i

content of the material expected after the Sr substitution. Accordingly the crystal chemis- try and electrical properties of strontium substituted lanthanum nickelates has been reinvestigated on air prepared materials. More specifically, we emphasize the strong relationship between the geometrical distortion of the Ni octahedron and the elec- trical behaviour (semi-conductor or metallic), earlier evidenced in La

Ni0

(8)(ll).

II - EXPERIMENTAL METHODS

The synthesis of mixed oxides La

_

Sr

Ni0

_

has been carried out using mixtures of powders having the composition(2-x)/2 La203 + x SrC03 + NiO. The mixtures were cal- cined in platinum crucibles for lh at 920°C followed by 15h at 1120°C all being con- ducted in air. This last heating was repeated with intermittent grinding twice or three times for 0.5 < x 4 1.2. The products were air quenched after each firing.

The compositlonswith x = 0.01, 0.2, 0.5, 0.6, 0.8, 1.0, 1.2 were air sintered at 1180°C after compacting the powder at 480 MPa in an isostatic press. Rods were thus machined for electrical measurements." The electrical measurements were performed in air between 20 and 900°C by the four probe technic under dc current. Platinum elec- trodes sintered at 800°C were used, which permit a good reproducibility of the data.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19861127

JOURNAL DE PHYSIQUE

The crystallographicanalysis was made using a Guinier Nonius camera. The diffracto- grams were recorded with a Philips powder goniometer with the CuKa radiation.Infra- red spectra were obtained in transmission in the range 800-200 cm-1 with a Beckman 4250 spectrometer with the KBr disk technic.

111 - RESULTS AND DISCUSSION

A family of strontium lanthanum mlxed nickelates was synthesised following the above described method. These compounds crystallize within the K2NiF4 isotypic family;

a large homogeneity range is observed for the compounds La2-xSrxNi04-y, x ranging between 0 and 1.2. The preparation in air prevents the full oxidation of N ~ I I in N~I'I and probably brings about a non-stoichiometric defect on the oxygen sublattice as previously shown up for the copper compounds (3). Two composition ranges can be distinguished from the X ray patterns.

A: 0 6 x 60.8: The indexation has beenomade in a tetrggonal cell with parameters

---

very similar to those of La2Ni04: a s3.8A and c s 12.7 A. As indicated in an intro- ductary paper (9) the a parameter progressively decreases with x while c increases, then c remains constant (0.54 x 6 0.7) and finally smoothly decreases. Consequently the c/a ratio displays a clear maximum reaching 3.335 for x

0.8.

B: 0.8 x 4 1.2. The occurence of e~tra~diffraction peaks which cannot be indexed

---

in the previous tetragonal cell (a% 3.8 A)and show an increasing intensity with x, is the signature of a superstructure. The samephenomenonhas been yet observed for LaOe8Srl 2Cu0 (3), its actual tetragonal cell being a, = 5a s 18.8 a ^and

cs= c

15.9 the same 5a x 5a terragon;bl cell is also a good model for La0.8Srl.2 Ni04-y: as = 5a s 19.1 a ^{and cs}

^c -12.4 A,with tne same indexation as for the copper com~ound.Nevertheless.this result should be confirmed bv an electron diffraction stu- dy. ~ a b i e I

Parameters of the tetragonal cell and c/a ratio of Lag-xSrxNiOu-y

(*)Values for the sub-cell

I I I

I x a I ^c(%) I ^c/a

I 0 '3.860(1)' I 12.672(2)'3.283 I I

/ ^0.1 / ^3.853111, 12.679!2~) 3.290 _I I

3.842(1)1 12.690( 5) 3.300 I

I '3.827(1) 12.710(2)1 3.321 1

1 0.5 '3.819(1) 12.723(3) 3.331 I 1

I I

I 0.6 13.816(1) 12.715(4) 3.332 _{I I} I

13.810(1) 12.709(4) 3.335 I I I

/ 3.813(1) 12.708(3) 3.331 I I 1

11.2(*) 3.824(1) 12.365(5) 3.234 I I I 1 I

Table I brings together a,c and the c/a ratio for all the compositions studied. Table I1 shows up the good agreement between calculated and measured dhkl values for La0.8 Srl.2 NiO

Unlike the A range, the parameter a increases 4-Y while an abrupt decrease of c occurs between x

1 and 1.2. The c/a ratio sharply falls to 3.234 for x = 1.2. This trend must be related to a progressive ordering of oxygen vacancies in the (001) planes with simultaneously a de- crease of the anisotropy of the NiO6 octahedra

Table 11 Indexation of dhkl for Lao. 8Sr1, 2Ni04-y up to 1.58

( * ) hkl of the sub-cell

I hkl' I 1 I

I I dabs I dcalc I

I I ( * ) 002 6.183 / ^{6.1826 I}

400 I 4.787

1 4.7801

I

I 410 4.641 I

I , ^{( * ) 501 1 3.652 1 ~:~~~~ I}

531 I 3.1706 1 ^{3.1696 I} / ^{( * ) 004 3.0908} _{1 3.0913} ^I

602 2.8295 I

I ( * ) 503 2.8041 ::: E:: ¹

I(*) 550 2.7016 2.7040 I

1 ^{( * ) 552} 1 ² - ⁴⁷⁶³ 1 ^2.4775 I

111-2 Structural study of the compositions 1 a=b=19.120(7) c=12.365(5) 1 ...

x

0.5 and x

0.8.

---

The crystal chemistry of these mixed oxides has been specified for the particular

compositionsx = 0.5 and x

0.8 using 29 X ray reflexions for the former (38 hkl)

and 17 (21 hkl) for the latter. The more symmetric space group, i.e. I4/mmm was cho- osen which involves the only limitihg condition on (hkl): h+k+l

2n.

The effect of stoichiometry was taken into account by searching the best refinement in two extreme cases: - i) fully oxidized N ~ I I I (y=O) and -ii) fully non-stoichiorne- tric compound with only N ~ I I (y=x/2). In that particular case, the possibility of or- dering oxygen vacancies on each oxygen family O(1) and O(2) (see table 11) has been considered. After refinement of the atomic parameters, the z values of (La,Sr), Ni, O1 and O2 are reported in table I11 together with their isotropic thermal factor.

Table I11

Atomic parameters and RI discrepancy factors for L ~ ~ - ~ s (x=0.5; ~ ~ N x=0.8) ~ O ~ - ~ Stoichiometric and non-stoichiometric models.

Atoms sites x y z I Lal. 5Sr0. 5Ni04-y I I

Lal. zSrO. 8Ni04-y I

La,Sr 4(e) 0 0 z I z;0.3605(5) B=1.4(1) 1 ; ) ' 1 z=0.3599(6) 8=0.9(2) (A) O 2 1

Ni 2(a) 0 0 0 1 ~=0.1(3) (1;)' ~=0.7(6) ( A ) * I

O(1) 4(e) 0 0 z I z=O.178 (4) I z=O.175 (5) I

O(2) 4(c) % ^{0 0} I , ^I I I

I

I

Stoichiometric 1

y=o y=o I

model 1 I I

B]O(I) 1 (h2 I I 4'8(9)1~~

^{0.053 1}

B~O(Z) 1 :::j:iiR1 ^{= 0.077} 1 ^{1.2(7) I}

I I I I

INon stoichiometric 1

model y=O. 25 I _{y=O. 40} 1

I I I I

I

I

Lanthanum and strontium are statistically distributed on the 4(e) sites.The confi- dence factors RI are rather similar: 0.072-0.088 for x

0.5 and 0.05-0.066 for x

0.8 due to the relatively weak contribution of oxygen atoms to the X ray diffrac- tion.However the thermal factor is of particular interest for discussing the struc- ture. As an example in the non-stoichiometric model if RI is slightly smaller when oxygen vacancies are located on the 0(2) family i.e. in the (001) plane,the B(O(2)) becomes negative which is totally unrealistic. On contrary the RI increases when the oxygen vacancies are located on the O(1) family. The stoichiometric model is thus considered as the more significant one; we however do not exclude the possibility of a weak non-stoichiometry with oxygen vacancies in the (001) plane.

The high value of the thermal factor B (O(1))

3.5 to 4.8(;12 for the stoichiome- tric model can be related to the strong anisotropy zf the Ni octahedron having two long axial Ni-O(;) distances: 2.26 1 (x=0.5), 2.23 A (x=0.8) and four short Ni-O(2) distagces: 1.91 .A (x=0.5 and 0.8). The axial (La,Sr)-O(1) distance is very short:

2.32 A and this suggests a competition between Ni-O(1) bonds and (La,Sr)-O(1) bonds, which can take into account the large axial therm2l motion of O(1) gtoms. A similar effect is observed in La2Cu04 (10): La-0(1)=2.37 A, Cu-O(l)

2.40 A and a B(O(1))

4 ( f i ) ' . We should emphasize chat such a strong anisotropy ofothe (Ni06) does no& exist in LaSrNi04 obtained by Demazeau et a1 (6) (Ni-O(1) = 1.97 A and Ni-0(2)=1.91 A).The shortening of the axial Ni-O(1) distance could be due to the high pressure (100 MPa of 02) used during the synthesis. The enhancement of the anisotropy in our compounds is an interesting fact that further studies will probably clarify; it can be attribu- ted either to a Jahn-Teller effect of the N ~ I I I d ion or a pure geometrical distor- tion as observed in LaSrV04 (lo), an other strontium K2NiF4 type oxide.

The study of IR spectra (800-300 cm-l) of La2-xSrxNi04-y (0 6 x

0.8) gives further

informations on the (La,Sr)-O(1) and Ni-O(1) bonds. As shown on Fig.1, two main

features must be noticed. -i) The highest frequency band shifts to larger energies

with the introduction of strontium (650-680 cm-I). This is in agreement with the

structural observations and the assignment of that band to the very short

JOURNAL DE PHYSIQUE

(La,Sr-O(1)) bond,as proposed bjr Goodenough and a1 (11). The enhancement of the (La,Sr)-O(1) bond produces a weaker Ni-O(1) bond and a strenghened aniso- tropy of Ni octahedra-ii).There is a progressive weakening of the vibrational bands when x increases, a feature correlated with an increase of the carrier concentration with the substitu- tion rate as it will be discus-

m - sed in the following.

C , .

$ ? Fig.1-IR spectra of

E m

z

La2-xSrxNi04-y

m

h

cy-'

111-3 Electrical Properties. ...

Results of electrical measurements are plotted in Fig. 2 . The resistivity variation with the temperature is given for various substitution rates: x

0.01, 0.2,

0.8, 1 , 1.2. The insert represents the conductivity (log a) versus 1/T for 20°< T <

200°C. The following can be put forward: i) The resistivity decreases with the para- meter x; the minimum value of

= 1.10-~(Q cm) ( a

100(Q~m)-~) is obtained for x=0.8;

ii) In the temperature range 20-900°C the compounds display a progressive transition from a semi-conducting behaviour to a metallic like state in the high temperature range. The activation energy in the semi-conductive regime, although not precisely determinable, gradually decreases from 0.086 e V for x

0.01 to zero for x

0.8 and the coefficient in the metallic state is very small for x>0.8.

The compounds having low substitution rates, i.e. x

0.01 behave similarly to La2Ni04 (2,12.) but with a larger resistivity (twice at the pmin of La2Ni04); for large x ( > 0.8) the compounds are metal-

lic like LaNiO with also a larger plncm)

NI resistivity. One of the major effects 3

of substituting La by Sr is to oxidize

Nil1 in ~ i " ' ; simultaneously the re- ^'qoirrmf. _E02- sistivity decreases (the conductivity _{m.102 -} za ---

increases) in proportion of the hole concentration carriers brought by the NilI1. However, as pointed out pre- viously, some non-stoichiometry exists on the oxygen sites which reduces the hole concentration by trapping elec- tron holes or equivalently reducing some N ~ I I I in N ~ I I . The minimum resis- tivity occurs for x=0.8 while it is expected for x=l corresponding to LaSrNi04 (all Ni in the Nil11 state).

For that particular composition the carrier concentration is in a first approximation equal to that of LaNiO ; on an other hand the charge transpor?

essentially takes place in the (001) plane (a perovskite layer of LaNi03) (13) and so the hole mobility is ex- pected to be of the same order of ma-

gnitude than for LaNi03. The resis- ₀

_{T I'CI} tivity of LaSrNiOa should thus be

comparable to that of LaNi03: it is

in fact twice larger. One reason for Fig.2: Resistivity of La2-,Sr NiO

x 4-Y versus

a higher resistivity is the non- T. Logo (conductivity)- 1/T 1s plot-

stoichiometry on the oxygen sites ted in the insert.

IV- CONCLUSION

The tetragonal unit cell of La2-xSrxNi04-y is progressively elongated by the intro- duction of Sr. This is related to an increasing anisotropy of the Ni06 octahedra in the (001) plane and to a particularly short (La,Sr)-O(1) bond in the c direction.

The resistivity is decreasedto a minimum value by the introduction of Sr, a metal- lic state being reached for Lal.zSr0.8Ni04-y which resistivity is only twice that of LaNi03, but in a much stable environment.

already mentioned above. The ten-

REFERENCES

11) Evande B.,Mtiller-Buschbaum H.K and Schweizer M.Z.Anorg.Allg.Chem.428,120 (1977).

dency of the oxygen vacancies to cla' order within the (001) plane con- tributes to create scattering cen- 335- ters for the electronic wave func- t o n and thus to increase the re- sistivity. It is clear than an ex- tension of this work requires the direct measurement of N ~ I I I con-

centration. 3 30

In addition to the charge carrier influence, the structure has an important effect on the resisti- vity, especially the geometrical

Ganguly P. and RaoC.N.RMater.Res.Bul1 8 , 405 (1973).

Nguyen N. ,Choisnet J. ,Hervleu M.and ~ a v e a u B. J.Solid State Chem.2, 120, (1981).

Gopalakrishan J.,Colsmann G. and Reuter B.J.Solid State Chem.z,145,(1977).

Nguyen N.Studer F. and Raveau B.,J.Phys.Ghem.Solids 44 (5), 389, (1983).

Demazeau G.,Pouchard M. and HagenmUller P.,J.Solid State Chem.E,159, (1976).

King H.W., Murphy G.J. and Castelliz K.M. J.Canad.Ceram.Soc.z,8, (1983).

Odier P., Nigara Y.,Coutures J.and Sayer M. J.Solid State Chem.56, 32 (1985).

Odier P.,Bouraly J.P.,Plessier J.M.and Choisnet J.Sllicates Indus.L1.2,17(1985) Longo J.M. and Raccah P.M. J.Solid State Chem.6, 526, (1973).

Singh K.K., Ganguly P. and Goodenough J.B. ~ . ~ o l i d State Chem.z, 254, (1984).

Nigara Y., Odier P. and Anthony A.M. Science of Ceramics V o l . g , 551,the Swe- dish Society of Ceramics, Stockholm (1981).

Rao C.N., Buttrey D.J., Otsuk N.Ganduly P.,Harrison H.R.,Sandberg C.J. and Honig J.M., 3 . Solid State Chem. 51, 266, (1984).

Vasanthacharaya N .Y. , Ganguly P. , ~ G d e n o u ~ h J.B. and Honig J .M. J .Physics C. 17, 2745 (1984).

I I

-08 1

el mdall,c(

- I

0 01

distortion of the Ni06 octahedra. 3.25 10 10-2

20.10-2

30 10-2

_{p t} nmr As a matter of fact, we can use

the value of the c/a ratio as an Fig.3 - c/a-resistivity at room temperature for imperfect measure of this distor- various compositions of La2-xSrxNi04-y.

tion. So, the elongation of the

unit cell (c/a ratio) is related to the hole transport as recently pointed out (8).

In Fig.3 is plotted the resistivity of La Sr NiO at room temperature versus 2-x .

the c/a ratio of these compounds. The resistlvlFy f x ' %

0.01 very well correlates with others if one takes the c/a value of La2Ni04 which is very reasonable.It ap- pears that for semi-conducting compounds (~(0.6) the resistivity decreases with increasing c/a; in other words the more elongated the NiO6 octahedra are, the less resistive is the material. This correlation is in good agreement with the IR data and explains the low resistivity found by others for LaSrNi04 (6,11).It is interes- ting to notice that the more anisotropic material (x=0.8) has the smaller resistivi- ty. The composition x=0.6 is at the border of a metallic state and very close to the minimum metallic conductivity as it can be calculated from the Mottfs theory (14).

For larger x the resistivity is metallic with a nearly nu1 temperature coefficient

(6,8) while for smaller x the temperature has a marked influence probably connected

with the thermal deformation of the unit cell. All these general features a e : in

agreement with the results obtained on similar compounds with copper; furthermore

the stability with respect to the oxygen, is better than for the copper compounds.