The Electrical Conductivity of Molybdenite

Electrical conductivity of molybdenite (Mo${\mathrm{S}}_2$) as a function of voltage and temperature.—Measurements of the currents through narrow flat strips cut perpendicular to the crystallographic $c$-axis gave results for the range from 0° to about 200° C, in agreement with the equation: $i=K{V}^b{e}^{\frac{-k}{T}}$, where, for the mineral in the ordinary state, $k$ is about 3,000 and $b$ generally about 1.6. When the voltage is increased to a critical value, breakdown occurs, due to the electric heating. An expression is derived for the breakdown temperature, which is shown to increase as $k$ diminishes. Above 200° irregularities appear; and after heating to 400° and higher or after current heating beyond the breakdown point, the conductivity at ordinary temperatures is usually permanently improved, and $k$ is decreased. In fact, by the use of proper current cycles, a strip may be transformed by stages from a very poor conductor (resistance several megohms) into a reasonably good conductor (resistance only a few ohms), obeying Ohm's law ($b=1\mathrm{}$). At the same time, $k$ diminishes and the breakdown temperature rises until the break disappears at the limiting value of $k$ predicted by the equation; also the photoelectric sensitivity and dielectric polarization disappear. A high potential discharge likewise increases the conductivity. This remarkable permanent change of conducting state may indicate a corresponding change of structure from an $\alpha$ to a $\beta$ form, as suggested for selenium. If the constant $k$ is interpreted as being equal to $\frac{\varphi }{R}$, where $\varphi$ is the energy required to change a bound to a free electron and $R$ is the gas constant, then $\varphi$ is computed to vary from 0.26 volt for Mo${\mathrm{S}}_2$ in its normal state to 0.13 for the conducting state. This may be of interest in connection with the electron theory of conductivity.