PHYSICS

S. S. KABALKINA, S. V. POPOVA, N. R. SEREBRYANAYA,
Corresponding Member of the Academy of Sciences of the USSR L. F. VERESHCHAGIN

ON A NEW MODIFICATION OF Ag₂O WITH A LAYERED STRUCTURE

Recently we established that at pressures \(p > 80\) kbar and \(t > 600^\circ\text{C}\) cuprous oxide decomposes according to the equation \(\mathrm{Cu}_2\mathrm{O} \to \mathrm{CuO} + \mathrm{Cu}\) (in press). It was natural to assume that the structural analogue of \(\mathrm{Cu}_2\mathrm{O}\)—\(\mathrm{Ag}_2\mathrm{O}\)—would behave identically under the same conditions. However, the investigation unexpectedly led to a different result.

Table 1

Results of experiments at high pressures and temperatures *

Table 2

Interplanar spacings and structural amplitudes of \(\mathrm{Ag}_2\mathrm{O}\) II

Silver oxide was prepared by the action of caustic soda on a solution of silver nitrate. The product obtained, washed by decantation, after drying gave a Debye pattern of pure \(\mathrm{Ag}_2\mathrm{O}\) (cuprite structure \((^1)\)). The substance under study was placed in a platinum ampoule in a high-pressure vessel; heating was carried out with the aid of a graphite heater introduced into the chamber. At \(p = 115\)—\(125\) kbar** and \(t = 1400 \pm 200^\circ\text{C}\) (Table 1) a new phase, \(\mathrm{Ag}_2\mathrm{O}\) II, was obtained. The Debye pattern of \(\mathrm{Ag}_2\mathrm{O}\) II contains 25 lines (Fig. 1), which were indexed in the \(\mathrm{CdJ}_2\) structure (see Table 2). The unit-cell parameters of the new phase were determined from the photograph

* In a number of experiments at high temperature, in addition to \(\mathrm{Ag}_2\mathrm{O}\) II, a small amount of greenish-yellow \(\mathrm{Ag}_2\mathrm{CO}_3\) and a white powder were formed, giving on the X-ray pattern a system of unknown lines.

** The pressure values were taken according to the Kennedy scale \((^2)\).

To the article by S. S. Kabalkina, S. V. Popova, N. R. Serebryanaya, L. F. Vereshchagin, p. 853

Fig. 1. a — X-ray diffraction pattern of Ag₂OI (cuprite structure); b — X-ray diffraction pattern of Ag₂OII (CdI₂ structure). Filtered copper radiation, RKU-114 camera

To the article by N. A. Balashova, St. Rashkova, p. 896

Fig. 1. Microphotographs of cobalt precipitates obtained at different pH values (120×).
a — pH 1.7, b — pH 4.1

$\mathrm{Ag_2O}$ II in a mixture with NaCl. Their values in hexagonal axes are: $a = 3.072 \pm 0.003$ Å, $c = 4.941 \pm 0.004$ Å, $c/a = 1.608$. The space group is $D^3_{3d}=C\bar{3}m$; $\rho_{\text{rentg}} = 9.53$ g/cm$^3$, $\rho_{\text{exp}} = 9.5 \pm 0.3$ g/cm$^3$, $\rho_{\text{starting phase}} = 7.3$ g/cm$^3$ (density measurements of $\mathrm{Ag_2O}$ II were carried out at the Institute of Mineral Raw Materials by means of a pycnometric method developed for application to small quantities of substance). The change in density in the transition $\mathrm{Ag_2O I} \to \mathrm{Ag_2O II}$ is $\sim 30\%$. The atoms in the crystal are arranged as follows: O occupies the position 000, and the two Ag atoms occupy $\frac{1}{3}\ \frac{2}{3}\ z$, $\frac{2}{3}\ \frac{1}{3}\ \bar{z}$. Thus, determination of the structure reduces to finding the parameter $z$ for Ag. For this purpose, a linear section of the three-dimensional Patterson synthesis was constructed in the direction $\left[\frac{1}{3}\ \frac{2}{3}\ z\right]$. Summation of the series was carried out with the aid of $3^\circ$ strips. The linear synthesis (Fig. 2) revealed two peaks with coordinates $z = 0.25$ and $z = 0.50$, of which the first corresponds to the interatomic vector Ag—O, and the second to Ag—Ag. The Ag parameter, calculated from both maxima, is $z = 0.25$.

Fig. 2. Linear section of the three-dimensional Patterson synthesis along the direction $\left[\frac{1}{3}\ \frac{2}{3}\ z\right]$.

In comparing the experimental and calculated structural amplitudes, the discrepancy factor $R$ proved to be equal to $-0.25$ (for $F_{\text{calc}}$ the temperature correction was not taken into account). Thus, it may be considered proven that the new phase has the $\mathrm{CdI_2}$ structure and, consequently, is a close-packed hexagonal packing of silver atoms with filling by oxygen atoms of half the octahedral voids according to the motif: filled layer, empty layer. It may be added that $\mathrm{Ag_2F}$ ($^3$) also has a structure of the $\mathrm{CdI_2}$ type. The distances between atoms in $\mathrm{Ag_2O}$ II (Ag—O 2.60 Å, Ag—Ag 2.86 Å) indicate the ionic character of the bond between the silver and oxygen atoms within a layer and the similarity of the interaction of neighboring silver atoms to the interaction in metallic silver (metallic radius Ag = 1.44 Å). The distances between atoms in the structure of $\mathrm{Ag_2O}$ I (cuprite) (Ag—Ag 3.34 Å and Ag—O 2.04 Å) characterize an obvious change in the bonds during the phase transformation. Thus, the transition $\mathrm{Ag_2O I} \to \mathrm{Ag_2O II}$ is accompanied by a change in the type of packing, the coordination number (2 and 4 to 3 and 6), and the character of the bond between individual atoms.

The appearance of a metallic luster and the Ag—Ag distance in the new phase permit the supposition that $\mathrm{Ag_2O}$ II is a layered semiconductor with a comparatively small band gap.

In conclusion, the authors consider it their duty to thank T. B. Zdorik for assistance rendered in determining the density of the substance studied.

Institute of High Pressure Physics
Academy of Sciences of the USSR

Received
14 VI 1963

CITED LITERATURE

1. G. B. Bokii, Introduction to Crystal Chemistry, Moscow, 1954.
2. G. C. Kennedy, P. N. La Mori, Progress in Very High Pressure Research, N. Y., 1961.
3. H. Terrey, H. Diamond, J. Chem. Soc., 131, 2820 (1928).