CHEMISTRY

Yu. M. UKRAINSKII and Corresponding Member of the Academy of Sciences of the USSR A. V. NOVOSELOVA

MOLYBDENUM AND RHENIUM DISELENIDES

MoSe₂ was first obtained by heating molybdic acid with selenium in a stream of hydrogen (¹), and ReSe₂ by decomposing Re₂Se₇ in vacuum at 325–330° (²). Judging from the literature, the crystallographic and electrical properties of these compounds have not been studied up to the present time. Since the disulfides of molybdenum and rhenium (³) have the same type of crystal structure, and MoS₂ has long been known as a semiconductor, it was of interest to study from this point of view the properties of the diselenides of molybdenum and rhenium.

In our work, the synthesis of MoSe₂ and ReSe₂ was carried out by sintering powdered molybdenum and rhenium (both of approximately 99% purity) with a stoichiometric amount of selenium (99.9% principal substance), in quartz ampoules sealed under vacuum, at 700° for 100 hours. The resulting preparations were gray powders with a metallic luster, quite stable in air. X-ray photographs of the samples were taken by the powder method in RKD-57 type cameras using copper radiation. Table 1 gives the results of indexing the powder pattern of MoSe₂. Molybdenum diselenide has a hexagonal unit cell with parameters: \(a = 3.28_4\) kX; \(c = 12.8_8\) kX.

Table 1

Indexing of the powder pattern of MoSe₂, Cu \(K\alpha = 1.539\) kX radiation

The pycnometric density for MoSe₂ was found to be \(6.90 \pm 0.05\) g/cm³.

If it is assumed that in the unit cell of MoSe₂, as in MoS₂, there are two formula units, then the calculated density of MoSe₂ is 7.0 g/cm³, which agrees rather well with the experimentally found density. The majority of the lines of the MoSe₂ powder pattern

are indexed with the same indices \(hkl\) as in the powder pattern of \(\mathrm{MoS_2}\). Moreover, in both cases the same extinction conditions hold for reflections of the types \(hhl\) and \(00l\) for even \(l\). All this makes it possible to regard molybdenum diselenide and disulfide as isostructural compounds.

The powder pattern of \(\mathrm{ReSe_2}\) has its own set of interplanar spacings, which indicates that the reaction between rhenium and selenium has gone to completion. It contains a considerably larger number of lines than \(\mathrm{MoSe_2}\), and it is not possible to fit them into a unit cell of the \(\mathrm{MoS_2}\) type. The structure of rhenium diselenide is probably of lower symmetry.

The electrical conductivity and thermoelectric power of \(\mathrm{MoSe_2}\) and \(\mathrm{ReSe_2}\) were measured potentiometrically, by a compensation circuit.* The samples were cylinders 4 mm in diameter and 12–15 mm high, pressed from powders under a pressure of \(8\ \mathrm{T/cm^2}\). Measurements of electrical conductivity were carried out in the temperature range \(20\text{--}70^\circ\). At \(22^\circ\) the specific electrical conductivity of \(\mathrm{MoSe_2}\) proved to be \(1.23 \cdot 10^{-4}\ \Omega^{-1}\cdot\mathrm{cm}^{-1}\); with an increase in the temperature of the sample it rose noticeably and at \(62^\circ\) was \(2.30 \cdot 10^{-4}\ \Omega^{-1}\cdot\mathrm{cm}^{-1}\). For \(\mathrm{ReSe_2}\) at \(24^\circ\) it was \(6.75 \cdot 10^{-5}\ \Omega^{-1}\cdot\mathrm{cm}^{-1}\), and at \(64^\circ\) increased to \(10.7 \cdot 10^{-5}\ \Omega^{-1}\cdot\mathrm{cm}^{-1}\). The positive temperature coefficient of electrical conductivity and its magnitude point to the semiconducting properties of \(\mathrm{MoSe_2}\) and \(\mathrm{ReSe_2}\), which is also confirmed by the high thermoelectric power that we found in these compounds. The thermoelectromotive force of \(\mathrm{MoSe_2}\) and \(\mathrm{ReSe_2}\) samples was measured by the integral method \((^5)\), relative to alumel. The temperature difference between the ends of the sample was varied from 25 to \(150^\circ\). For \(\mathrm{MoSe_2}\), under these conditions the thermoelectric power correspondingly varied in the range 610–430 \(\mu\mathrm{V/deg}\), and for \(\mathrm{ReSe_2}\) from 1200 to 880 \(\mu\mathrm{V/deg}\). In both cases the hot probe (alumel) became negatively charged; on this basis it may be concluded that the molybdenum and rhenium diselenides synthesized by us are \(p\)-type semiconductors \((^5)\).

In conclusion, we consider it our pleasant duty to express our gratitude to Yu. P. Simanov for discussion of the results of the X-ray investigation.

Moscow State University
named after M. V. Lomonosov

Received
4 V 1961

REFERENCES

\(^1\) E. Wendehorst, Zs. anorg. Chem., 173, 268 (1928).
\(^2\) H. V. A. Briscoe, R. L. Robinson, E. M. Stoddart, J. Chem. Soc., 134, 1439 (1931).
\(^3\) J. Lagrenaudie, J. phys. et radium, 15, 299 (1954).
\(^4\) Yu. M. Ukrainskii, A. V. Novoselova, Yu. P. Simanov, ZhNKh, 1, 148 (1959).
\(^5\) U. Daniel, Introduction to the Physics of Semiconductors, IL, 1959.

* The measurement method was described by us in \((^4)\).