PHYSICAL CHEMISTRY

A. A. Slinkin, E. A. Fedorovskaya

APPEARANCE OF FINE STRUCTURE IN THE E.P.R. SPECTRUM OF CHROMIUM OXIDE DOPED WITH $\mathrm{Li}^+$ IONS

(Presented by Academician A. A. Balandin, 13 V 1964)

In work ($^1$) it was found that, in the reaction of various potassium compounds with $\mathrm{CrO_3}$ at high temperatures, the products formed give a very complex e.p.r. spectrum. It was assumed that such a spectrum is due to the fine structure of the $\mathrm{Cr^{3+}}$ ion; however, the question remained unclear as to why the fine structure is preserved in a matrix of $\mathrm{Cr_2O_3}$.

To clarify this question, and also the possibility of obtaining such spectra for products of the interaction of $\mathrm{CrO_3}$ with salts of other monovalent ions, in the present work we investigated the products of the reaction of $\mathrm{CrO_3}$ with $\mathrm{LiNO_3}$.

Table 1

The reaction was carried out in the solid phase in air at a rate of temperature rise of $\sim 7$ deg/min, and the mixture was held for 1 hour at $800^\circ$. Some samples were obtained under the same conditions, but in a hydrogen atmosphere. The concentration of $\mathrm{Li_2O}$ in the final mixture was 0.01; 0.1; 0.5; 1.0; 3.0; 5.0; 10.0; 15.0 wt. %. The e.p.r. spectra were recorded on an RE-1301 apparatus with $\nu = 9326$ MHz at $t = 20,\ 150,\ -196^\circ$. The magnetic susceptibility was measured by the Faraday method in the interval $H = 1500$–$4500$ oersted and $t = 20$–$160^\circ$. The electrical conductivity of some samples was studied at direct current in vacuum of $10^{-5}$ mm Hg and in the interval $t = 20$–$300^\circ$.

Figure 1 shows the e.p.r. spectra of samples containing 0.1–15 wt. % $\mathrm{Li_2O}$. All the e.p.r. spectra (except the spectrum of the sample with 15 wt. % $\mathrm{Li_2O}$) have an appearance clearly reminiscent of the fine structure (f.s.) of the $\mathrm{Cr^{3+}}$ ion in chromium–potassium alum. Indeed, from the data given below (Table 1) it is seen that the maxima of the absorption lines observed by us coincide with the maxima calculated for $\mathrm{KCr(SO_4)_2\cdot 12H_2O}$. Consequently, the complex character of the e.p.r. spectrum of $\mathrm{Cr_2O_3}$ samples doped with $\mathrm{Li_2O}$ is indeed associated with the fine structure of the $\mathrm{Cr^{3+}}$ ion. In this connection it is necessary to answer two questions: first, in which sites of the crystal lattice are the $\mathrm{Cr^{3+}}$ ions located that give rise to the f.s. of the e.p.r. spectrum, and what role is played by the $\mathrm{Li^+}$ ions; second, why the f.s. is observed in a magnetically concentrated system, in which exchange and dipole–dipole interactions are strong.

In chromium–potassium alum the $\mathrm{Cr^{3+}}$ ions are in the cubic crystal field of an octahedron of $\mathrm{H_2O}$ molecules. The presence of an axial component leads to splitting of the levels in zero field and to the appearance of f.s. Consequently, in $\mathrm{Cr_2O_3}$—$\mathrm{Li_2O}$ samples part of the $\mathrm{Cr^{3+}}$ ions must be located in fields of the same symmetry. This is easily realized in the given system, since it is well known ($^3$) that the interaction of lithium salts with $\mathrm{Cr_2O_3}$ at high temperatures leads to the formation of the $\mathrm{LiCrO_2}$ structure. Of the samples studied, the sample with 15 wt. % $\mathrm{Li_2O}$ corresponds exactly to such a compound. The arrangement of $\mathrm{Cr^{3+}}$ and $\mathrm{Li^+}$ ions in such a structure is shown in Fig. 2, $a$*, from which it is seen that the $\mathrm{Cr^{3+}}$ ions and the $\mathrm{Li^+}$ ions occupy oc-

* The figure shows one half $(c/2)$ of the hexagonal unit cell of $\mathrm{LiCrO_2}$.

Table 2

Dependence of the physical properties of the specimens on the content of Li\(_2\)O

| | \multicolumn{8}{c}{Concentration of Li\(_2\)O, wt.%} |
|---|---:|---:|---:|---:|---:|---:|---:|---:|
| | 0 | 0.01 | 0.1 | 1.0 | 3.0 | 5.0 | 10.0 | 15.0 |
| \(\chi^* \cdot 10^6\) | 25.7 | 25.6 | 48.2 | 72.1 | 89.2 | 87.5 | 51.0 | 26.0 |
| \(\lg \rho_{5150^\circ\mathrm{C}}\) | 12.5 | — | — | — | 6 | — | — | — |
| \(E_\sigma\), eV | 1.40 | — | — | — | 0.33 | — | — | — |
| Color | Light green | Light green | Green | Dark green | Black | Black | Dark green | Green |

* The value of \(\chi\) is given for \(H = 3500\) oersted; specimens with 0.1–10% Li\(_2\)O are ferromagnetic.

tetrahedral voids. However, this unit cell is much more complex than the cell of chromium–potassium alum (Fig. 2, c), and the Cr\(^{3+}\) ions in it cannot give an e.p.r. spectrum similar to the spectrum of alum.

The conditions under which the investigated specimens were obtained apparently led to part of the Li\(^+\) ions being used not for the formation of LiCrO\(_2\) compounds, but entering the interstices of the Cr\(_2\)O\(_3\) lattice, causing strong oxygen absorption by the specimens. The formation of a large amount of nonstoichiometric oxygen led to a sharp increase in magnetic susceptibility and electrical conductivity, as is seen from the data of Table 2; the largest amount of nonstoichiometric oxygen is present in specimens with a content of 3 and 5 wt.% Li\(_2\)O.

Under these conditions, apparently, a structure is readily formed with an excess of oxygen ions and with an arrangement of Cr\(^{3+}\) ions analogous to their arrangement in alums (Fig. 2, b), while the remaining Cr\(^{3+}\) ions (Fig. 2, a) are oxidized to Cr\(^{6+}\) ions, indicated in Fig. 2, b by dots.

Fig. 1. E.p.r. spectra of Cr\(_2\)O\(_3\) specimens containing additions of Li\(_2\)O: 1 — 0.1%; 2 — 1%; 3 — 3%; 4 — 5%; 5—10%; 6 — 15% Li\(_2\)O

Fig. 2. Structure of the unit cell of LiCrO\(_2\) and KCr(SO\(_4\))\(_2\)·12H\(_2\)O. a — unit cell of LiCrO\(_2\) (half); b — possible arrangement of the ions in the “defective” LiCrO\(_2\) structure; c — unit cell of KCr(SO\(_4\))\(_2\)·12H\(_2\)O

Consideration of Fig. 3 shows that the f.s. of the e.p.r. spectra is indeed associated with nonstoichiometric oxygen. Specimens with a content of 3 and 5 wt.% Li\(_2\)O, prepared in an H\(_2\) atmosphere, give only a narrow e.p.r. line without f.s. (Fig. 3, 1 and 4). Heating the obtained specimens in an air stream leads to the appearance of f.s. A decrease in the amount of nonstoichiometric oxygen and, correspondingly, a decrease in f.s. also occurs upon introducing increasing amounts of Al\(^{3+}\) ions into the specimens (Fig. 4), which hinder the penetration of Li\(^+\) ions into the interstices of the Cr\(_2\)O\(_3\) lattice.

The introduction of Li\(^+\) ions into the Cr\(_2\)O\(_3\) lattice leads not only to the appearance of nonstoichiometric oxygen, but also substantially affects the indirect ex—

exchange interaction in $\mathrm{Cr_2O_3}$ and the formation of an antiferromagnetic structure. The presence of $\mathrm{Li^+}$ ions in the $\mathrm{Cr_2O_3}$ structure leads to the disappearance of long-range antiferromagnetic order and to a strong shift of the transition point toward low temperatures. Such a weakening of the exchange interaction makes it possible to observe h.f.s. in the EPR spectra. The antiferromagnetic interaction appears at low temperatures, and this leads to a smearing of the h.f.s. in the EPR spectra.

The $\mathrm{Li^+}$ ions prevent the formation of long-range antiferromagnetic order, but the indirect exchange interaction in the chain $\mathrm{Cr^{3+}—O^{2-}—Cr^{3+}}$ remains to some extent. The presence of such exchange is confirmed by the small line width (130 oersted) of the sample with 15 wt.% $\mathrm{Li_2O}$ (Fig. 1), which in the case of dipole–dipole interaction should have been about 1200 oersted.

Fig. 3. EPR spectra of $\mathrm{Cr_2O_3—Li_2O}$ samples obtained in $\mathrm{H_2}$ and treated with air.
1 — 3% $\mathrm{Li_2O}$ in an $\mathrm{H_2}$ atmosphere at $800^\circ\mathrm{C}$; 2 — the same, heated at $800^\circ$ for 1 hour in air; 3 — the same at $900^\circ$ for 2 hours in air; 4 — 5% $\mathrm{Li_2O}$ in an $\mathrm{H_2}$ atmosphere; 5 — the same, heated at $800^\circ$ for 2 hours in air.

Fig. 4. Influence of additions of $\mathrm{Al^{3+}}$ ions on the form of the EPR spectra of a $\mathrm{Cr_2O_3}$ sample containing 3 wt.% $\mathrm{Li_2O}$.
1 — 3%; 2 — 6%; 3 — 12% $\mathrm{Al_2O_3}$.

The detailed mechanism of the influence of $\mathrm{Li^+}$ ions on the exchange interaction in $\mathrm{Cr_2O_3}$ is still not clear; however, it may be assumed that the interaction of $\mathrm{Li^+}$ ions with oxygen ions to some extent prevents the participation of the oxygen $p$-electrons in the indirect exchange interaction.

Institute of Organic Chemistry named after N. D. Zelinsky
Academy of Sciences of the USSR

Received
29 IV 1964

REFERENCES CITED

1. A. A. Slinkin, E. A. Fedorovskaya, DAN, 150, 328 (1963).
2. C. A. Whitmer, R. T. Weidner et al., Phys. Rev., 74, 1478 (1948).
3. N. Rudorff, Zs. Naturforsch., 96, 614 (1954); E. F. Bertaut, I. Dulac, J. Phys. and Chem. Solids, 21, 118 (1961).