STRUCTURAL CHEMISTRY OF Mn, Co AND Ni IN SOME NATURAL MANGANESE OXIDES

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STRUCTURAL CHEMISTRY OF Mn, Co AND Ni IN SOME NATURAL MANGANESE OXIDES

A. Manceau, S. Llorca, G. Calas

To cite this version:

A. Manceau, S. Llorca, G. Calas. STRUCTURAL CHEMISTRY OF Mn, Co AND Ni IN SOME

NATURAL MANGANESE OXIDES. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-703-C8-

707. �10.1051/jphyscol:19868132�. �jpa-00226032�

STRUCTURAL CHEMISTRY OF Mn, Co AND Ni IN SOME NATURAL MANGANESE OXIDES,

A . MANCEAU, S . LLORCA* and G . CALAS

Laboratoire d e Mineralogie-Cristallographie, Universites Paris 6 et 7 , U A CNRS 0 9 , 4, Place Jussieu, F-75252 Paris Cedex 0 5 ' ~ a b o r a t o i r e de Mineralogie-Cristallographie, 39, Allee J. Guesde, F-31400 Toulouse, France

Deux oxydes de manganese cobaltiferes et nickeliferes -une lithiophorite et une asbolane- representatifs des minerais continentaux, ont ete etudies par spectroscopie d'absorption X. Les differences portent A la fois sur le degre d'oxydation et I'ordre $ I courte distance. Les atomes de cobalt sont toujours trivalents et hexacoordonnes quel que soit I'oxyde de manganese. Bien que leur localisation ne soit pas connu avec exactitute, les atomes de cobalt sont tres certainement segregC dans une structure localement tres ordonnee. Les atomes de nickel sont au contraire divalents. Ils construisent des il6ts d'hydroxyde Ni(OH)2 dans I'asbolane et sont inseres dans le feuillet hydroxy-alumineux AI(OH)3 de la lithiophorite.

Two Ni- and Co-containing manganese oxides, a lithiophorite and an asbolane, which are representative

continental ore deposits have been studied by XAS. The distinct behaviours of Ni and Co are emphasized and concern both oxidation states and local structures. The electronic structure and short range order around Co atoms do not depend on the nature of the bearing phase. Co atoms are found to be trivalent and 6-fold coordinated. Results lead to rejection of a substitution of cobalt in manganese sites and strongly support a segregation of Co atoms in a locally ordered structure. Unlike cobalt, nickel exhibits distinct surroundings in both phases: it builds partial Ni(OH)2 layers in asbolane and is located within the hydrargillite layer AI(OH)3 in lithiophorite.

INTRODUCTION

Manganese oxides are ubiquitous phases found in the marine environment as well as in continental deposits.

They host a variety of minor elements, some of which (Co,Ni,Cu,Zn) may reach concentrations up to several percents. In spite of their ubiquitous character and economic importance, the crystal chemistry of minor elements in manganese oxides remains largely unknown for two reasons. Firstly, manganese oxides contain numerous structural defects making crystal structure determination by X-ray diffraction methods extremely difficult; secondly, associated transition elements cannot be individually recognized in the Mn matrix. Both these reasons make XAS particularly well suited to study separately the oxidation state and atomic environment of associated minor elements.

Two minerals have been studied: a lithiophorite and an asbolane. Their fundamental structure unit consists of Mn02 layers. These alternate with aluminous sheets (All-x,Lix)(OH)g in lithiophorite (1) and islands of Ni(OH)2 sheets in asbolane (2,3) (Fig. 1). Spectra were recorded at the Mn, Co and Ni K-edges using synchrotron radiation of DCIILURE (France).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868132

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m C a n H 2 O

STRUCTURE O F LlTHlOPHORlTE STRUCTURE O F ASBOLAN

Figure 1.

STRUCTURAL CHEMISTRY OF COBALT

Co K XANES. The normalized Co k-edge spectra of asbolane and lithiophorite are shown in Fig. 2a with those of references: CoOOH (co3+oct.), ( c o ( H ~ o ) ~ ) ~ + (co2+oct.) and CoCr204 (Co2+tet.). The threshold energy measured at the first inflexion point is the same in Mn oxides and CoOOH and shifted posiiively by 3eV compared to that of the octahedral co2+ complex. Taking the similar metal-oxygen bonding character in CoOOH and Mn02 into consideration, the edge energy indicates that cobalt occurs as C O ~ + ions in these Mn oxides. In fact the oxidation state is more certainly determined from the pre-edge energy as it concerns transitions towards empty bound states of 3d character. An energy shift of the pre-edge of I ,OeV is observed at the Co K-edge between Co2+ and Co3+ cations (Fig. 2b) and confirms the trivalent state of cobalt in Mn oxides. The site geometry of Co atoms can be determined examining the main edge structure and the amplitude of the pre-edge. Transition elements in a tetrahedral site are known to yield two resolved structures on the upslope of the edge moving the crest to highest energy. Besides, the pre-edge is about 5-7 times enhanced in 4-fold vs. 6-fold coordination because of the greater 4p-3d mixing when the absorbing atom is at the center of a cluster without inversion symmetry (4). It comes out from these considerations and examination of figures 2a and 2b that C O ~ + ions are 6-fold coordinated.

Fig. 2: Co K-threshold and pre-edge structures of Mn oxides and reference compounds.

EXAFS. Of significance in the normalized EXAFS spectra is their overall shape which is nearly identical for

lithiophorite and asbolane at both Co and Mn K-edges (Fig. 3). Before any analysis it may be concluded that the

local structure around Go atoms does not depend on the nature of the Co-bearing Mn oxide and is very close to

that of Mn atoms. The RDF (Fig. 4) show two prominent peaks located at the same apparent R value. Their

magnitude is lowered at the Mn K-edge which indicates different atomic orders around Mn and Co atoms as it will

be further discussed. Interatomic distances have been determined on the Fourier filtered kX(k) by a least square

procedure using theoretical phase shift functions tabulated by Teo and Lee (5).

coordination and greater for a divalent cobalt atom. This result is in line with XANES results. The similarity of the Mn-(0,OH) and Co-(0,OH) distances militates for a close agreement between the ionic radii of ~ n and ~ +

co3+ and, hence, a low-spin configuration of

ions (radius ~n~+=0.54A; C O ~ + L S = O . ~ ~ ~ ; CO~+HS=O.GIA).

The filtered kX(k) spectra of the second shell of asbolane, lithiophorite and CoOOH have the same zero crossings (Fig. 5). For asbolane this unambigously indicates that the cation-cation distance is close to that of COOOH (d(Co-Co)=2.81A) whatever the assumption on the chemical constitution of the second shell (Co or Mn).

In lithiophorite this result strongly supports Co or Mn second neighbours at the same distance as in asbolane since waves backscattered by At and 3d elements are out of phase at low k range. In fact this phase shift difference might be compensated by different Co-(Co,Mn) and Co-AI distances but in this hypothesis the Co-AI bond length is set at an unrealistic value. Furthermore the experimental envelopes of the filtered X(k) functions of lithiophorite and asbolane are similar and characteristic of a 3d backscattering atom.

4 6 8 : O K ( : ' )

Fig. 3: Co and Ni K-EXAFS spectra.

Fig. 4: Radial structure functions at the Co and Mn K-edges.

Fig . 5: Fourier filtered X(k) functions of

the first shell at the Co K-edge.

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The enhancement of the amplitude of the backscattered waves which affects simultaneously the first and the second atomic shells at the Co K-edge in Mn oxides is to be outlined. This difference comes from a different local structure around Mn and Co atoms in Mn oxides since both atoms are always 6-fold coordinated. In the Mn matrix the density of defects is known to be high and it lowers the amplitude of the signal at the Mn K-edge. This result leads to rejection of a substitution of cobalt in manganese sites and strongly supports a segregation of Co atoms in a locally ordered structure. Finally, in both asbolane and lithiophorite the Co-Co distance is found to be 2.79A. The quasi identity of the Mn-Mn (2.81A) and Co-Co (or Co-Mn) distances rules out the possibility of a localization of Co atoms above or below Mn4+ vacancies of the Mn02 layers as suggested by many authors.

UI K-'THRESHOLD

N,(OH)2

._iflflj

r

Fig. 6: RDF at the Ni K-edge.

STRUCTURAL CHEMISTRY OF NICKEL

Few problems arise from the oxidation state of nickel in this geological environment. Ni K-edge spectra are identical to that of Ni(OH)2 which confirms its divalent state. Ni K-EXAFS spectra of asbolane and lithiophorite are clearly distinct (Fig. 3). The first one is like that of Ni(OH)2 whereas the second does not resemble any other seen up to now. Looking at the structure of raw EXAFS spectra it may be yet deduced that the site location of nickel in lithiophorite and asbolane is clearly distinct to that of Co atoms.

In asbolane the RDF presents two peaks which are shifted to shorter R value as compared with Ni(OH)2 (Fig. 6). Ni-OH and Ni- Ni distances in asbolane are found to beequal to 2.00 and 3.03A as compared with 2.05 and 3.09A for Ni(OH)2. The diminution of the Ni-OH and Ni-Ni distances in asbolane is explained by the necessary reduction of the unit cell dimension of the Ni(OH)2 layer to fit with that of Mn02. Hydrogen bonds between successive layers contribute to this reduction (2,3). The presence of Ni-(Mn,Co) atomic pairs is excluded as these would have given interatomic distances shorter than the Ni-Ni one and larger bond length at the Mn and Co K-threshold than those observed for Ni-free (Mn,Co)(O,OH)2 layers like CoOOH and

lithiophorite.

. . .

.02

:

I

I

4 6 0 10 I t l k ' l

Fig. 7: Fourier filtered X(k) functions at the Ni K-edge.

CONCLUSION

The second peak of the RDF of lithiophorite is at shorter value than that of asbolane (Fig. 6). The corresponding partial X(k) functions are out of phase for low k values (Fig. 7).

Both phase and amplitude of X(k) of lithiophorite leads support to a second shell filled with a light element, that is AI. Ni-(0,OH) and Ni-AI distances derived from the EXAFS analysis are equal to 2.01A and 2.94A.

The distinct structural chemistry of Ni and Co is outlined by XAS and concerns both oxidation state and

local structure. Co atoms are found to be trivalent and 6-fold coordinated. Co atoms are suspected to build

highly ordered oxyhydroxide layers whatever the nature of the Co-bearing phase and the existence of a

random Co for Mn subsitution is unlikely. By contrast with cobalt, nickel exhibits distinct surroundings in

asbolane and lithiophorite. In asbolane, Ni atoms build a Ni(0H)~like local structure whereas in lithiophorite,

they are located in the AI(OH)3 layer.

(2) Chukhrov F.V., Gorshkov A.I., Vitovskaya I.V., Drits V.A., Sivtsov A.I., and Dikov-Yu.P. (1980a) Izvest. Akad. Nauk. SSSR, Ser. Geol. 9,108-1 20.

(3) Chukhrov F.V., Gorshkov A.I., Vitovskaya I.V., Drits V.A., Sivtsov A.I., and RudnitskayaYe.S. (1980b) An SSSR Izvesfiya, Ser. Geol. 6, 73-81.

(4) Calas G. and Petiau J. (1983) Solid. State Com. 48, 625-629.

(5)Teo B. K. and Lee P.A. (1 980) Amer. Chem. Soc. 101,2815-2830.