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1 xo ISSN 070-6, Russian Journal of General Chemistry, 007, Vol. 77, No. 0, pp. 677!68. + Pleiades Publishing, Ltd., 007. Original Russian Text + A.A. Selyutin, N.P. Bobrysheva, N.V. Chezhina, A.V. Shchukarev, A.O. Kozin, 007, published in Zhurnal Obshchei Khimii, 007, Vol. 77, No. 0, pp. 608!6. ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ State of Atoms of the Transition Group Elements in Dilute Solid Solutions Based on LiMO (M = Sc, Ga, Al): II. Magnetic Characteristics of Nickel Atoms in LiNi x Sc! Solid Solutions A. A. Selyutin, N. P. Bobrysheva, N. V. Chezhina, A. V. Shchukarev, and A. O. Kozin St. Petersburg State University, Universitetskii pr. 6, St. Petersburg, 9850 Russia Received March, 007 Abstract - The state of nickel atoms in LiNi x Sc! x O solid solutions was studied. The anomalous magnetic characteristics can be accounted for only by the presence of dilution-resistant clusters containing Ni(III) in two states: high- and low-spin. DOI: 0./S We reported in the previous work [] that LiMO oxides (M is a d element) attract increased interest due to their diverse magnetic and electrochemical characteristics. The functional properties are largely determined by the valence and spin states of d elements, which still are open to discussion in the majority of cases. According to the general formula of the oxide, the oxidation state of transition elements (M = Mn, Fe, Co, Ni) must be equal to three, whereas their spin state can vary. However, published magnetic parameters provide insufficient evidence to conclude which particular variant is realized for each d element. The spin and valence states are difficult to determine because most works deal with magnetically concentrated systems in which cooperative interactions mask the states of separate atoms. In the case of cobalt, for example, not only high- and low-spin configurations, but even another oxidation state, Co(IV), have been reported []. Special researcher s interest has been attached to LiNiO. First, its electrochemical and magnetic properties depend on the method and procedure of synthesis [, ]. In our opinion, this fact is in itself indirect evidence for the possibility of various spin states. Second, there is no magnetic ordering in this oxide below the Noel temperature. Several explanations of for this phenomenon have been offered, but none of them is accepted as exhaustive. In particular, the nickel ion located in the LiO plane is supposed to tend to make parallel the spin of neighboring nickel For communication I, see []. atoms located in the NiO plane [5]. This interferes with antiferromagnetic interactions between nickel atoms in the nickel layer, which results in distortions in the antiferromagnetic order in the layer. With the aim of revealing special features of the magnetic behavior of d elements in LiMO oxides (M = Mn, Fe, Co, Ni) we performed magnetic dilution of the latter. For the diamagnetic solvent we chose LiScO and synthesized LiFe x Sc! x O, LiFe x Ga! x O, LiNi x Sc! x O, LiNi x Ga! x O, LiCo x Sc! x O, LiCo x Ga! x O, and LiMn x Ga! x O solid solutions containing 0.58 mol % of the transition metal. The manganese, iron, cobalt, and nickel atoms in the solutions are in the octahedral surrounding of oxygen atoms, like in magnetically concentrated oxides. X-ray phase analysis showed that the solutions all have the designated structure LiScO. Chemical analysis for the paramagnetic element and magnetic susceptibility measurements in the temperature range 7700 K were also performed. The oxidation states of elements were determined by X-ray photoelectron spectroscopy both for the above series and for a different variant of magnetic dilution, namely for the solid solutions based on the diamagnetic solvent LaGaO. The Mp / (M = Mn, Fe, Co, Ni) binding energies for all the solid solutions under study are given in the table. The binding energies of reference substances are given for comparison. The resulting data allow us to conclude that the oxidation state of the paramagnetic atoms is three. The change of the oxygen surrounding from octahedral to tetrahedral is reflected in the spectra by a 677

2 678 SELYUTIN et al. c Ni 0 0 6, emu mol! x Fig.. Plot of c Ni vs. x for the LiNi x Sc! x O system. T, K: () 90, () 60, () 0, and () eV line shift. This effect is absent in the case of nickel, which seems to be associated with enhanced stability and with the size of nickel clusters. Evidence for the presence of clusters is given below. This communication is devoted to the magnetic behavior of nickel atoms in LiNi x Sc! x O solid solutions. The magnetic properties of solid solutions containing iron, cobalt, and manganese will be considered in the following communications. The dependences of the paramagnetic component of magnetic susceptibility per mol of nickel atoms (c Ni ) and of the effective magnetic moment (m eff )on the concentration of the solid solution and on the temperature have several anomalies. Mp / (M = Mn, Fe, Co, Ni) binding energies in solid solutions ÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄ Solid solution ³ E b, ev ³ Reference ³ E b, ev ³ ³ substance ³ ÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄ LiFe 0.05 Sc 0.95 O ³ 70.7 ³ Fe O ³ 7.0 LiFe 0.05 Ga 0.95 O ³ 7. ³ ³ LiNi 0.05 Sc 0.95 O ³ 855. ³ NiO ³ 85.0 LiNi 0.05 Ga 0.95 O ³ 855. ³ ³ LiCo 0.05 Sc 0.95 O ³ 780. ³ Co O ³ LiCo 0.05 Ga 0.95 O ³ ³ ³ LiMn 0.05 Ga 0.95 O ³ 6. ³ Mn O ³ 6.9 ÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄ m eff,bm T, K Fig.. Plot of m eff vs. T for LiNi x Sc! x O solid solutions. x: () 0.085, () 0.090, () 0.00, () 0.005, and (5) The plot of magnetic susceptibility vs. composition (Fig. ) is typical for dilution of antiferromagnetics only at x > 0.0. At x ~0.0, a maximum is observed and then the susceptibility decreases with decreasing concentration. There can be several reasons for the decrease in the susceptibility. The first is the presence of two competing contributions to the susceptibility: ferro- and antiferromagnetic. Judging from the Weiss constant, ferromagnetic interactions do take place at low solution concentrations. This also follows from the temperature dependence of the effective magnetic moment at various solution concentrations (Fig. ). The effective magnetic moment increases with temperature up to x > However, the m eff of the most dilute solid solutions decreases with temperature. Let us consider the possible spin states of nickel atoms, which could be responsible for such magnetic characteristics. The nickel(iii) atom (d 7 ) in the octahedral surrounding can be present in two spin states: high (t 5 g e g, T g, S /) and low (t 6 g e g, E g, S /). The pure magnetic spin moments for the high-spin and low-spin states are.87 and.7 BM, respectively. For the T g state, the moment depends on temperature. In the crystal field of the octahedral symmetry it increases 5 RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No

3 m eff,bm STATE OF ATOMS OF THE TRANSITION GROUP ELEMENTS... : II kt/x Fig.. Plots of m eff vs. kt/x for the spin equilibrium of Ni (S /) and Ni (S /) in the medium-strength crystal field. Calculation for E/x: () 0, ().5, and ().5. () Experiement. from.0 to 5. BM as the temperature increases. The m eff of the E g ground term is independent of temperature and is usually.7.77 BM. The x $ 0 m values for each measured temperature were found by extrapolation of c Ni and m eff to zero concentration. The m x $ 0 values decrease from.87 to.99 MB in the temperature range under study (77 00 K) and are between the theoretical values for the high-spin and low-spin states. If the obtained experimental magnetic moments are defined by a superposition of the low-spin (Ni S /) and high-spin (Ni S /) states, m x $ 0 at the temperature T will be determined by formula (). m x$0 (T) = am Ni(S /) + ( a)m Ni(S /) (T). () Here a is the mole fraction of nickel atoms in the lowspin state. Since m Ni(S /) increases with temperature and m Ni(S /) is constant, the total magnetic moment should increase with temperature, but not decrease, as is observed in the experiment. One more variant is coexistence of atoms with the / and / spins, i.e. spin equilibrium. Such situation can take place when the high-spin and low-spin states are close in energy. Therefore, the relative occupancies of these states are comparable and vary with temperature. In this case, the magnetic moment should be affected by spinorbit coupling which is described by the mkt/x dependence, where k is Boltzmann s constant, T, absolute temperature, and x, one-electron spin-orbit coupling constant. The magnetic moment was calculated by formula () [6]. x x (00x + 500) + (70x 58)exp ( 7) + (0x 97)exp ( 7) + 50xexp [( 5 + e )x] m 6 x = , eff x x 5x{ + exp( 7) + exp( 7) + 5 e exp[(7 + 7)x]} 6 x x = x/kt. () The calculations were performed with several E/x values (from /5 to /5) covering the whole range of variation of the spin-orbit coupling constant in complex oxides. As seen from Fig., the resulting theoretical curves fail to fit the experimental data. Thus, the obtained results cannot be accounted in terms of complete disaggregation of nickel atoms on magnetic dilution. Therefore, we should consider variants in which some aggregates of nickel atoms are preserved in the course of dilution. In the region of the lowest concentrations, the most probable variant is dimers of nickel atoms in various spin states. Since decreases as the temperature increases, for LiNi x Sc! x O we should consider a dimer with the ferromagnetic type of exchange. Therewith, three dimer compositions are possible: Ni (S/) ONi (S/), Ni (S/) O Ni (S/), and Ni (S/) ONi (S/). The magnetic susceptibility of a dimer can be determined by Eq. (). m x $ 0 Ng b 5S(S + )(S +)e!e(j,s)/kt S c dim = , kt5(s +)e!e(j,s)/kt S E(J, S) =J[S(S +)S a (S a +)S b (S b + )]; () RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No

680 SELYUTIN et al. S = S a + S b, S a + S b,..., S a S b, g a + g b g a g b S g(s) =777 + 777, a (S a +)S b (S b +) 777777777.

The calculation by this equation can be represented as a dependence of the magnetic moment on reduced temperature kt/j, where J is the exchange parameter.

4 680 SELYUTIN et al. S = S a + S b, S a + S b,..., S a S b, g a + g b g a g b S g(s) = , a (S a +)S b (S b +) S(S +) Here S is the atom spin; g, g factor; and T, absolute temperature. The calculation by this equation can be represented as a dependence of the magnetic moment on reduced temperature kt/j, where J is the exchange parameter. The calculated dependences of effective magnetic moment on reduced temperature for the three types of dimers [7] are given in Fig.. The experimental dependence of magnetic moment at infinite dilution is also shown in this figure. The reduced temperatures, as an example, are calculated for J 0 cm!. As seen, the calculated dependence for none of the dimers in the pure state fits the experimental x $ 0 m T dependence. A situation is also possible, when the magnetic susceptibility at infinite dilution is a sum of the susceptibilities of variuos dimers and single nickel atom in one of the two spin states. Such situation can be described in the general way by formula (). m = a m [Ni(S /)] + a m [Ni(S /)] + a m [Ni(S /)ONi(S /)] + a m [Ni(S /)ONi(S /)] + a 5 m [Ni(S /)ONi(S /)]. () Here a, a, a, a, and a 5 are the mole fractions of nickel atoms in various spin states and the mole fractions of the corresponding dimers. m eff,bm The calculated dependences of magnetic susceptibility of all the possible clusters of two nickel atoms with various exchange parameters, as well as their combinations with each other and with single nickel atoms fail to fit the experimental dependences. Hence it follows that we should consider the presence of clusters containing a greater number of nickel atoms in the infinitely dilute solid solutions. We can do nothing but estimate magnetic moments for such clusters according to the theoretical values calculated as functions of reduced temperature kt/j [7]. The best fit of calculation to experiment is observed for a cyclic hexamer of nickel atoms (S /) (Fig. 5). The empirical formula of such cluster is [Ni 6 Li O 0 ] 9!. The enhanced resistance of nickel clusters to dilution is associated with additional electrostatic interaction between the lithium atom having no unpaired electrons and the nickel atom in the low-spin state. EXPERIMENTAL The solid solutions of LiNiO in the diamagnetic solvent LiScO were synthesized by the ceramic procedure from stoichiometric mixtures of corresponding components: special purity grade Li CO, analytical grade NiO, and special purity grade Sc O, which were tested for ferromagnetic admixtures that distort measurement results. Lithium carbonate was used in a 5% excess. The optimal conditions of synthesis were chosen on the basis of X-ray phase analysis and magnetic susceptibility measurements kt/j Fig.. Plot of effective magnetic moment vs. reduced temperature. Calculation for () Ni(S /)ONi(S /), () Ni(S /)ONi(S /), and () Ni(S /)ONi(S /). () Experiment. Fig. 5. Structure of the [Ni 6 Li O 0 ] 9! cluster. RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No

5 STATE OF ATOMS OF THE TRANSITION GROUP ELEMENTS... : II. 68 The powder X-ray patterns were recorded on a DRON- X-ray diffractometer (CuK = radiation). The powder patterns were identified using the Powder Diffraction File [8]. The magnetic susceptibility was measured by the Faraday method in the temperature range 7700 K. The optimal duration of synthesis of homogeneous LiNi x Sc! x O solid solutions is 0 h at T 7 K. According to the X-ray phase analysis, homogeneous solid solutions with the designated structure and with unit cell parameters corresponding to the structure of the solvent were obtained. The equilibrium in the distribution of paramagnetic atoms in the obtained solid solutions was proved by magnetic susceptibility measurements. The stability of the nickel oxidation state was proved by the fact that samples obtained in different gas media (air, nitrogen, and oxygen) had the same magnetic characteristics. The oxidation state of iron is three, which was proved by Moessbauer, ESR, and X-ray photoelectron spectroscopy, and by magnetic susceptibility measurements. The X-ray photoelectron spectra were recorded at the University of Umeo (Sweden) on a Kratos Analytical Axis Ultra electron spectrometer (England) with excitation with monochromatic AlK = emission (hn 86 ev). The surface potential was controlled by means of the surface charge neautralization system. The scale of binding energies (E b ) was calibrated by the Cs line of hydrocarbon admixtures at 85.0 ev. Analysis of the solid solutions for paramagnetic components was performed by atomic absorption spectroscopy. The magnetic susceptibilities were measured by he Faraday method in the temperature range K. The error in relative measurements was %. The paramagnetic susceptibilities calculated per mole of the paramagnetic atom (c M ) were corrected for diamagnetism with regard to the experimental susceptibilities of the diamagnetic matrix. The selected procedures for the extrapolation of magnetic characteristics to infinite dilution provided an error of no more than %. REFERENCES. Bobrysheva, N.P. and Selyutin, A.A, Zh. Obshch. Khim., 007, vol. 77, no. 6, p Montoro, L.A. and Rosolen, J.M., Electrochim. Acta, 00, vol. 9, p... Alcantara, R., Lavela, P., Tirado, J.L., Zhecheva, E., and Stoyanova, R., J. Solid State Electrochem., 999, vol., p... Hirano, A., Kanie, K., and Ichikawa, T., Solid State Ionics, 00, vols. 55, p Yu, A, Subba Rao, G.V., and Chwdari, B.V.R., Solid State Ionics, 000, vol. 5, p.. 6. Martin, R.L. and White, A.N., Transition Metal Chemistry, Carlin, R.L., Ed., 968, vol., p.. 7. Rakitin, Yu.V. and Kalinnikov, V.T., Sovremennaya magnetokhimiya (Contemporary Magnetochemistry), St. Petersburg: Nauka, Powder Diffraction File, Swarthnore (USA): Int. Center Diffr. Data. RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No

Transition Elements. pranjoto utomo

Transition Elements. pranjoto utomo Transition Elements pranjoto utomo Definition What is transition metal? One of which forms one or more stable ions which have incompletely filled d orbitals. 30Zn? Definition Zink is not transition elements