Optical spectra of Ni(II) in Ni2GeO4 and germanate spinels

Volume 185,number I ,2

CHEMICALPHYSICSLETTERS

11October 1991

Optical spectra of Ni( 11) in Ni,Ge04 and germanate spinels M. Lenglet Laboratoire de Physicochimie des MatPriaux, INSA de Rouen, B. P. 08, 76131 lZlontSaini Aignan Cedex, France

and C.K. Jorgensen Dipartement de Chrmie hfininPra!e,Analytique et AppliquCe, Universite de Gen?ve, CH-121 I Geneva 4. Switzerland

Received 14January 199I : in final form 6 June 199I

A structural reinvestigation of Ni,GeO, has been performed by means of EXAFSand ligand-field spectroscopyand reveals Iwo opticallydifferent nickel sites. The presence of germanium on either tetrahedral or octahedral sites in mixed germanatesincreases the ionicity ofthe Ni( II),,,,,-0 or Cr(III)-0 bonds.

1. Introduction We have published the characterization of the Ni(II)-0 chemical bond for tetrahedral and octahedral coordination in spinels by X-ray photoelectron spectroscopy, X-ray spectrometry and ligandfield spectroscopy [ 1 1. The characteristic parameters of the Ni(II),,,,- 0 bond are not very sensitive to the Jahn-Teller effect, unlike those of Cu (II)-0 bonds [2,3]. .4mong the inverse spinels, Ni,Ge04 shows a specific optical absorption spectrum with remarkable bond-splitting effects. The purpose of this paper is a structural reinvestigation of Ni,GeO, and mixed spinels containing high-valency cations by ligand-field spectrometry and X-ray spectrometry.

2. Optical absorption spectroscopy Tarte et al. [4] suggest that the Pauling electrostatic rule should be added to the classical factors which determine the cation distribution in spinels containing several high-valency cations (at least one trivalent and one tetravalent). These authors have synthesized a series of complex germanates including the following compositions: Li,5ZnCro.,Ge0,, Li,,NiCro,Ge04 and Li, ,MU,5Cr,,,Ge0,504 (M” = Elsevier Science Publishers B.V.

Table I Distribution in germanates deduced from IR pattern Spine1composition

Cation distribution

Li0,5NiCr0.,GeOd Li(l.SM0.~CrI.JGe0.504

Ge [ Li,,NiCrOs 10, L&M~.s[Crl.sGec.~10,

Mg’+, Co’+, Ni*+, Zn’+); and they have proposed the distribution shown in table 1 deduced from the IR pattern. Tarte comes to the conclusion that whenever the expected cation distribution leads to the simultaneous occurrence of significant quantities of highvalency cations on both tetrahedral and octahedral sites, the distribution is in contradiction with the known preferences of the cations for the tetrahedral and octahedral environments. We have accordingly studied a series of spine1 complex germanates among which we shall consider here the following compositions: ( 1 -x)Ni,Ge04-,uNiGa204,( 1 -x)Ni,Ge04 -xNiCr204, Li,,,NiCro.,GeOl and Lio.sNio.sCr,.5Ge,,O,. 2.1. Octahedral Ni’+ environment: Ni,GeO, and Niz_xGa2xGe,_xOl spinels An examination of the Tanabe-Sugano diagram for Ni2+ in an octahedral environment shows that 111

I I October I991

CHEMICALPHYSICSLETTERS

Volume185,number 1,2

1 (ID

t 'CI.5

.i

terms of two optically different nickel atoms in two inequivalent crystallographic environments. The six intense bands have, therefore, been assigned in groups of two, each pair arising from the same spin-allowed transition in the two cation sites; see table 2. The replacement of the Ge4+ ions by Ga3+ leads to a very pronounced decrease in the splitting of the 3A2g+3Tlg (0,) transition (fig. 1). The large values of the Racab parameters (table 3) are consistent with literature data relative to oxidized compounds with similar Ni( II)-0 distances; see table 4. 2.2. Tetrahedral and mixed Nizf environment: Ni2_,Cr2,Gei_,0~ and other mixed germanates

c

L

c

t L

+-----T--&7-zT 25000

Fig. 1. Optical spectraabsorptionof Ni,_,Ga,,Ge, -,O, compounds: (1) x=0; (2) x=0.25: (3) x=0.5; (4) x=0.75; (5) .X=1. up to eight transitions are possible in the energy range of interest, three spin-allowed and five spin-forbidden. Spin-allowedbands are usually much more intense than spin-forbidden bands, so one would predict three intense and five weak bands if Ni2+ occupied only one type of cation site. In this case, however, six of the seven observed bands are moderately intense (curve 5, fig. 1). Thus, the optical absorption spectrum of Ni,GeO, may be interpreted in

In this system, an inversion of the cation distribution is observed. The normal and inverse mixed spinels are separated by a miscibility gap from each other as in the Ni,_,Zn,Ge,O, system [S] _ In the range O
transitions of Ni(H,O)z+, (Niz,:,) and (NiC14)2-, (Ni&) ranges between 1 and 10 cm-’ mol-’ and 10m2 and 10m3 cm-’ mol-‘, respectively. Near-infrared bands of Ni& and Ni$& (respectively located at a 8000 and = 9000 cm-‘) are of equal intensity in NiCr,,,Ga,.20h and Ni,.sCro.zGeo,sOJ spinels characterized by the following distribution:

Table2 Sites

1

2

112

Parameters (cm- ’)

Energyofthe transition from the 3A21level (cm-‘)

‘T*,

‘Tlr and ‘E,

IT,,

3T,,

4

B

8000 9100

13100 15300

20600

23200 26100

800 910

830 920

a) (I),

Ni&ko.a [Nil 00.4104 L&N& 5 [C~I sGeo.5 1%

(2) mean (site l), (site 2).

x=0.2 Li~.5Nio.5Cr,.5Geo.504

tetrahedral or mixed environment ( 1 -x)Ni2Ge04-xNiCr,04 x=1 Ni[CrllOa

Ga[NiGa]O, Ga~.SGe~.,[Ni,.5Gao.5104 Gao.2sGeo.71 [Nil 75Gao 25 104 Gc[Ni2]04 Ge[ Li,,.5NiCr0.5]04 Zn [Ni&n~.2GellO,

octahedral environment ( 1 -x)Ni2Ge0,-xNiGa,O, X=1 x=0.5 ~~0.25 x=0 LiO ,NiCr,.,GeO_+ Ni0.Jn,.zGe04

distribution

parameters

Cation

energy and ligand-field

Formula

Table 3 Optical transition

a)

13 13

13.15

‘E*

‘T*

Energy of the transition

9.2 9.5

8.15

(2)

9.25 9.1

9.3

a)

8 8

(I)

3T**

Energy of the transition

of mixed germanates

16.3

15.8 15.35 15.25

(2)

22.5 20.6 20.6

‘TI,

8.5

3A2

I)

23

24 23.4

(1)

3T~,

25.9

26.5

15.76

3TI

from the ‘-I’, level ( 10’ cm-‘)

13.1 13

(1)

3T,,

from the ‘A,, level ( 1O3 cm-

26.2

26.1 26.1

(2)

950

945

415 390 2450

4

Optical

800 800 820

(1)

D,

Optical

840 830 805

(1)

815 800 850 845

B

parameters

925 9-l 0 910 920

(2)

B

parameters

900 920 920 920

(cm- ’)

860

905

(2)

(cm-’ )

[ill

t21

Ref.

<

2 2 E

2 :

3

n 3 s 5 P

‘id

5 B

1

c”

00

2

5

Volume 185, number 1,2

CHEMICAL PHYSICS LETTERS

Table 4

Ni(OH)* nepouite N&M&, 9A1204 NiGa,O,

Ni-0 (nm) a’

Q,

B

Ref.

0.205 0.203

850 910 980 945

930 905 900 905

[S] [5,6] [7] this study

0.204

a’ From EXAFS studies.

Fig. 2. Optical

spectra

of mixed nickel germanates:

Lio.sNi&, &0.504; (2) Li,,NiCr,,GQ,; (4) LiO.,ZnCrO.,GeO,.

tetra Ni~fGa~,$

octa Ni~$Cr$Ga~,~O:

Nii.TGe$

Ni:iCri.$O:-

(I )

(3) Ni0.Jn,.,Ce04;

From the experimental data, one can conclude that the molar extinction coefficient of Ni(I1) tetrahedral is at least ten times stronger than that of Ni (II) octahedral in spinels. The detection limit of fourfold coordinated Ni( II) is estimated to be of the order of l-2 at%. The coordination of nickel in other mixed germanates, as illustrated in fig. 2, reveals the validity of Tarte’s conclusions about factors determining the cation distribution in spinels containing several highvalency cations. For Li0.5ZnCr0.,Ge0,, the expected cation distribution (in fact, realized) is determined by the very strong preference of zinc for tetrahedral sites. The reverse situation is found in Lio.sNiCro.sGe04, owing to the strong preference of nickel for octahedral sites. A completely different situation is observed for the spinels Li~.SMo.sCr,.SGeo.s04with very surpris114

I I October 1991

ing behavior of the nickel compound, owing to the strong preferences of Ni2+ and Cr3+ for octahedral environment, the expected distribution is clearly Li0.5Ge,,5[Ni0.iCr1.5]04 with, in addition, a high probability of a I : 1 ordering of lithium and germanium, which would give an increased stability to this structure. A satisfactory explanation may be found in the Pauling electrostatic rule [ 41. In the spine1 structure, every oxygen is bound to one tetrahedral and three octahedral cations. Tarte has accordingly determined the contributions of one tetrahedral plus one octahedral cation of moderate to high valency to the bonding capacity of the oxygen atom and deduced the possible contribution for the two remaining cations. It is clear that the bonding capability of the oxygen atom to the remaining cations decreases significantly if the two first atoms, one tetrahedral and one octahedral have both a high valency. For a given combination of high-valency cation, the situation is worst when the highest valency cation is located on a tetrahedral site. This interpretation explains the behavior of the Znz_,MGeO, (06~~2): M=Co, Ni: threesolid solutions are observed for x values near 0 (phenakite structure), E 1 and 2 (normal and inverse spinels). These three domains of solid solutions are separated by two wide miscibility gaps. The second one (between .W 1 and x= 2), apparently surprising, is easily understood by means of the Pauling electrostatic rule. Assignments of the optical transitions and crystal field parameters are reported in table 3. This study allows the following conclusions: ( 1) The Pauling electrostatic rule may explain the large miscibility gap observed in systems such as Zn,GeO,-Ni#eO,; Ni,GeO,-NiCr,O,. (2) The presence of germanium on either tetrahedral or octahedral sites in mixed germanates increases the ionicity of the Ni(II),,,,-0 and the Cr(III)-0 bonds (ZnCr,O,, B=640 cm-‘; Li,,ZnCr,,,GeO,, B= 680 cm-‘). (3) In germanate spinels containing only tetrahedral germanium, the optical absorption spectrum reveals two nickel atoms, optically different, in two different crystallographic environments.

Volume 185,number I,2

11October 1991

CHEMICALPHYSICSLETTERS

3. EXAFS study of Ni,GeO* According to Reinen [ 121, the specific optical absorption spectrum of Ni2Ge04 can be explained by the crystallographically plausible assumption that the coordinating atoms of Ni2+ are compressed along their trigonal axes. He has developed a crystal-field formalism which allows a quantitative treatment of spectra. Nevertheless, this hypothesis does not induce two different Ni-0 lengths. Bertaut et al. [ 131 have studied the magnetic structure and properties of nickel germanate. In Ni2Ge0,, an antiferromagnetic order sets in below T= 15 IL The propagation vector of the magnetic structure is k= [ 1f f ] and the spin value S( Ni) = 1.11. The true symmetry is rhombohedral, splitting the B, sites U= I, 2, 3,4) in two antiferromagnetic lattices b=B, and e= B2tB3+B4. In an attempt to get a better insight in the local environment of nickel an investigation by means of X-ray absorption spectroscopy (XANES, EXAFS) has been performed. The EXAFS spectra of polycrystalline samples were recorded at Orsay, using the X-ray beam delivered by the DC1 storage ring. DC1 was operated at 1.85 GeV and about 250 mA. The Bragg monochromator was a two-crystal (Si 3 11) device. Absorption spectra were measured in the transmission mode at room temperature. Data collection consisted in stepping the monochromator in progressively increasing energy increments: 0.25 eV for XANES and 2 eV over

the EXAFS range (8200-9200 eV). The EXAFS data have been analyzed using the now well-known procedure [ 141. The phase-shift function and scaling factor induced by the central-atom absorber Ni and oxygen backscatterers were computed from experimental data using the classical EXAFS formula within the plane wave approximation. They were computed with NiFe,O, as reference compound. At first, the NiFe,O, spectrum is treated using the Teo and Lee parameters [ 151. The solution: R= 0.204 ? 0.002 nm, CN = 6 is in good agreement with the experimental data determined from X-ray and neutron diffraction studies: R = 0.203 nm. The adjusted parameters resulting from the various calculations are listed in table 5. The EXAFS data relative to Ni2Ge04 have been analyzed on the basis of one or two Ni (II)-0 distances. The best-fit phase parameters were obtained with an equal number of Ni-0 distances at 0.201 and 0.207 nm (table 5). Ni K X-ray absorption and XPS Ni 2p,,, of NiGa,O, and Ni,GeO, are compared in fig. 3. The characteristic parameters of the Ni(II),,,,-0 bond in Ni,GeO, are not compatible with a single site for Ni*+ ions (table 5). It is well established that the peak and satellite structures of N12~,,~ spectra are strongly influenced by different parameters characterizing the chemical bond: coordination nature of the neighbouring cations and magnetic structure. The XPS Ni 2p,,, spectra in Ni,GeO, recorded at 293 and 593 K are sim-

Table 5 XPS, XANESand EXAFSparameters of nickel a) XPS Ni 2p,,, main peak &

0 1s Eb satellite

X-ray absorption spectroscopy EXAFS

XANES

1,/I,

fwhm AE,, fwhm

Ai% &,d.,,,j

A&,,,d*

R (A)

______ (1)

NiFe>O, NiGa204 855.7 2.5 Ni#eO, 856.3 3.5

6.3 6.1

3.7 z5.5

0.40 0.49

530.8 531.2

10.7 18.1 10.2 18.9

15.7 78.6

(2)

Q

CN

(1) (2)

2.04 2.04j 2.04 2.07 2.01 3

6 6 6 3

0.43x lo-2 2.7~10-~ 0.95x W

a) hE, = energyseparation in eV between the main line and satellite; Is/I,,,= relative intensity ratio of the satellite to the main line is the energy separation in eV between the pre-edge ratio of the peak height; AEK=EK_cdse (compound) -EK_cdge(metal) in eV;A&3d_mlx)= and the main peak.

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CHEMICAL PHYSICS LETTERS

Volume 185, number 1,2

II October 1991

shifted towards higher energy and shows a splitting more pronounced than that of NiGa204. This structure may be attributed either to a site distortion or to the existence of two different sites. In conclusion, from this structural reinvestigation of N&GeO,, by EXAFS and ligand-field spectroscopy, we suggest the presence of two nickel atoms optically different in two different crystallographic environments (the low-energy component of each of the three bands may be attributed to the site 1, Ni-0: 2.07 A). 4

eV

880

References [ 11M. Lenglet, A. d’Huysser, J.P. Bonnslle, J. Diirr and C.K. Jerrgensen, Chem. Phys. Letters 136 ( 1987) 478. [ 21 M. &jet, A. d’Huysser, I. .4rsene, J.P. Bonnelle and C.K. Jorgensen, J. Phys. C 19 ( 1986) L363.

[ 31 J. Arsene, M. Lenglet and C.K. Jergensen, Mat. Res. Bull.

‘,

1,’ ‘,’

,F,’

l--H,

8325

/,

,

8350

,>

eV

Fig. 3. Ni 2p3,? spectra (a) and Ni K-edge XANES (b) in NiGaz04 ( I ) and Nl,GeO., ( 2).

ilar. Thus, magnetic interactions do not influence XPS spectra unlike those of NiO and ferrimagnetic compounds [ 161. The Ni 2p,,, spectrum is characterized by a large overlap on the satellite with the main peak. The two lines are quasi-symmetric but too large for only one octahedral nickel environment. The Ni K-edge of NizGeO, is broadened,

116

I9 (1984) 1281. [4] P. Tarte and A. Rulmont, J. Mater. Sci. Letters 7 (1988) 551. [ 51A. Manceau and G. Calas, Clay Minerals 21 (1986) 341. [ 61 A. Manceau and G. Calas, Clay Minerals 22 (1987) 357. [ 710. Schmitz-Dumont, A. L&and D. Reinen, Ber. Bunsenges. Physik. Chem. 69 (1965) 76. [ 81D. Reinen, Z. Anorg. Allg. Chem. 356 (1968) 172, 182. [ 91 M. Lenglet, A. d’Huysser and C.K. Jorgensen, Inorg. Chim. Acta 133 (1987) 61.

[IO] M. Lenglet and A. d’Huysser, Compt. Rend. Acad. Sci. (Paris) 310 (1990)483.

[I I] D. Reinen, Struct. Bonding 7 (1970) 114. [ 121 D. Reinen, Theoret. Chim. Acta 8 (1967) 260. [ I31 E.F. Bertaut, Vu Van Qui, R. Pauthenet and A. Murasik, J. Phys. (Paris) 25 (1964) 5 16.

[ 141 D. Raoux, I. Petiau, P. Bondot, A. Fontaine, P. Lagarde, P. Levitz,G. Louplas and A. Sadoc, Rev. Phys. Appl. IS ( 1980) 1079. 1151 B.K. Te0andP.A. Lee, J. Am. Chem. Sot. 101 (1979) 2815. [ 161 M. Lenglet, A. d’Huysser and C.K. Jorgensen, Chem. Phys. Letters, submitted for publication.