MAGNETIC PROPERTIES OF OXIDIC SPINELS CONTAINING TETRAVALENT MANGANESE

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1 R490 Philips Res. Repts 18, , 196: MAGNETIC PROPERTIES OF OXIDIC SPINELS CONTAINING TETRAVALENT MANGANESE Abstract by G. BLASSE Measurements of the magnetic susceptibility as a function of the temperature are carried out for a number of oxidic spinels containing tetravalent manganese. The magnetic BB interaction between Mn4+ ions in these oxides is found to be weak and negative. This 90 interaction is compared with analogous interactions between other d3 ions. 1. Introduetion, ' Some new oxidic spinels containing tetravalent manganese were recently reported by the present author 1), viz. L4/3MnÎ304, LiMe3+Mn4+04 (Me = = Al, Cr, Fe, Co, Ga, Rh) and LiMe2+o.sMn4+ 1 S04 (Me = Mg, Co, Ni, Cu, Zn). In these compounds the Mn H ions (electron configuration 3d3) occupy octahedral sites (B sites) without exception, as is to be expected. I some of them the Mn 4 + ion is the only kind of paramagnetic ion present. We decided therefore to study the paramagnetic behaviour of these compounds to determine sign and strength of the magnetic interaction between the MnH ions. Since the Mn H ions are on B sites, the reciprocal susceptibility-vs-temperature curves of these compounds give direct information about the Mn4+-MnH BB interaction in the spinel lattice. 2. Experimental The preparation and the chemical and X-ray analysis of the materials under consideration have been described elsewhere 1). Susceptibility measurements were performed in the region K using the Faraday method. The apparatus was constructed by Dr. U. Enz of this laboratory from a Mettler balance and an electromagnet producing fields up to Oe. The experimental susceptibilities were corrected for diamagnetism using the values for the ions given by Selwood 2), and for the temperature-independent paramagnetism ofthe Rhê+ion using a value of eu. This value was obtained from our own measurementse) on MgRh2Û4. The corrected susceptibilities follow, the Curie-Weiss law, I/x = (T- BA)/C, in the investigated temperature region;. BA is the asymptotic Curie temperature and C the Curie constant. Table I presents the cation distributions, the results of the chemical analyses, the Curie constants per gramion Mn 4 +, and the asymptotic Curie temperatures of the materials investigated. 3. Interpretation of the resnlts The value of the asymptotic Curie temperature BA is a measure of the strength of the magnetic interaction inside the octahedral sublattice, since only spinels

2 V2+-CI- 'TABLE I Cation distribution, chemical analysis, Curie constant per gramion Mn 4 +, and asymptotic Curie temperature of, some oxidic spiriels containing Mn4+,.,, eper', % active oxygen compound cation distribution *) gramion 8AeK) experimental theoretical Mn **) Li4/aMns/a04 Li[Lh/aMns/a]: ,-55 LiZno.sMnl.S04 Lio.sZno.s [Lio.sMnl's] ,9, LiRhMn04 Li [Rh Mn] ) octahedral site cations between 'brackets;,...) to be compared with the spin-only value 1'87. ; >,0, o d I i Io o TABLE 11 On sign and strength of the 90 exchange interaction in the right-angled configuration Me(da)-anion-Me(d a ), ' electronegativity compounds ratio of ' charge of ofthe anion magnetic interaction studied reference cation-anion the ionic the cation according to (found experimentally) experimentally radii Pauting strongly negative VCb 9. Cra+-CI weakly positive CrCta 10 Cra+-Br g. weakly positive CrBra 11 Cra+-Oz '3,5 strongly negative ZnCrZ04, MgCrZ04 5 Cr3+-S very weakly positive _' ZnCrZS4 uc,o,.. 4 Mn weakly negative this paper - - Mn4+-SZ (prediction:) positive -,, 12 0' Io _, I...

3 with diamagnetic ions on tetrahedral sites are involved. The configuration 01 importance for this interactionis the right-angled system B-site cation/anion/bsite cation. From the values of (JA in table I it follows that the magnetic interaction in the right-angled configuration Mn4+-02-Mn4+ is weak and negative. this agrees with the measurements of Bongers 4) on LbMnOa. The structure of this compound can be described by a layer-wise type of superstructure in the rocksalt lattice. Consequently the only con:figuration of importance for the magnetic interaction in this compound is a right-angled con:figuration Mn Mn 4 +. From the x-l-vs-t curve of Li2MnOa, Bongers obtained C = 1.78 and (JA = -28 K. It is concluded, therefore, that the Mn4+-02-Mn4+_900 interaction is weak and. negative in this structure too.- This result is noteworthy, since the magnetic interaction in the isoelectronic configuration Cr Cr3+ (90 ) is very strongly negative: the spinels Mg[Cr2]04 and Zn[Cr2]04 have (JA = -370 K and -350 K, respectively 5). The ionic radii ofthe Cr3+ and the Mn 4 + ion are 0 63 and 0 60 A, respectively (values according to Ahrens 13». Although the latter value is probably a little too high, it seems likely that not only the difference of the ionic radii but also other factors determine the strength of the interaction in these isoelectronic configurations. These will be discussed now. 4. DIscussion. Two mechanisms have been proposed for the 90 exchange interaction between octahedrally surrounded metal ions with electron configuration d3: (1) cation-cation interaction (Goodenough 6», which will be negative in this. case;. (2) cation-anion-cation interaction (Kanamori 7», which will be positive in this case. The sign of these interactions is also consistent with the theory of exchange given by Anderson 8». The strength ofthe cation-cationinteractionis determined by the overlap of the t2g orbitals of the cations and consequently by the size of the cation and the anion involved (fig. I). The strength of the cation-anion- Fig. 1. Overlap of the t2g orbitals of two octahedrally surrounded cations with angle cationanion-cation 90. The anions are presented by the black points A.

4 MAGNETIC PROPERTIES OF OXIDIC SPINELS CONTAINING TETRAVALENT MANGANESE 403 Fig. 2. The overlap in the 90 superexchange interaction between octahedrally surrounded, cations; A = anion. cation interaction is determined by the degree ofcovalencyof the cation-anion bond, i.e., by the overlap of the cation and the anion orbitals (fig. 2). Consequently the interaction is expected to be strongly negative in the case of metal ions, which are not too small relative to the anions and are bonded I ionically (i.e., charge of the cation is low and electronegativity of the anion is large), since cation-cation interaction predominates. However, a weaklynegative, or even positive interaction is expected in the case of relatively small metal ions, of which the bond with the anions has a certain covalent character (i.e., charge of the cation is high and electronegativity of the anion is small), since cationanion-cation interaction predominates. These rules are nicely confirmed by the experimental results. In table 11some examples are presented. The ratio between the cation and the anion ionic radii has been taken as a rough measure of the cation-cation interaction. The degree of covalency can be estimated from the charge of the cation and the electronegativity of the anion. Of course, only those compounds are tabulated in I which the 90 exchange between octahedrally surrounded metal ions is the dominating exchange mechanism. Finally we wish to remark that the 90 Mn4+-S2--Mn4+ interaction is expected to be positive. Unfortunately we did not succeed in preparing sulphides in which this configuration occurs. Eindhoven, November 1963

5 404 - G.BLASSE.REFERENCES 1) G. Blasse, J. inorg, nucl. Chem. 25, ,1963 and Philips Res. Repts, to be published. 2) P. W,. Selwood, Magnetochemlstry, New York, Interscience, ) G. Blasse, Philips Res. Repts 18, , ' ) P. F. Bongers, Thesis, University of Leiden, 1957, chap. 5. 5) T. R. McGuire, L. N. Roward and J. S. Smart, Ceramic Age 60, 22-24, ) J. B. Goodenough, Phys. Rev. 117, , ) J. Kanamori, Phys, Chem. Solids 10, 87-98, ) P. W. Anderson, Phys. Rev. 115, 2-13, ) C. Starr, F. Bitter and A. R. Kaufmann, Phys. Rev. 58, , ) J. W. Cable, M. K. Wilkinson and E. O. Wollan, Phys. Chem. Solids 19,29-34, ) I. Tsubokawa, J. phys. Soc. Japan 15, , ) F. K. Lotgering, Philips Res. Repts 11, , ) L. R. Ahrens, Geochim. cosmochim. Acta 2, , I.