For measuring electrical conductivity, the oxide powders were pressed into pellets and the conductivity was determined at 25° C. by a 2 point probe method. First, 1 g of the powder samples were inserted into the Teflon tube with stainless steel bottom (electrode) of a measuring cell. After the filling is completed, a second stainless steel electrode was inserted on the top, and the filled test cell is inserted in between the pressure gauge. The pressure is increased to 40 MPa and the resistance is measured at said pressure via the 2 point method with an Agilent 3458A multimeter. From the measured resistance R (in Ohm), the electrical conductivity is calculated according to:
Conductivity=d/(R A)

• • d: distance of the 2 electrodes
  • R: measured resistance
  • A: electrode area (0.5 cm²)

The resistance is the sum of the following contributions: electrode contact resistance, intragrain (bulk) resistance and intergrain resistance.

In the present invention, electrical conductivity is determined at a pressure of 40 MPa.

Amount of Iridium, Ruthenium, Tin and the Optional Metal Dopant

The amounts of iridium, ruthenium, metal dopant and tin are determined by elemental analysis according to the following method: 0.04 to 0.5 g of the sample is mixed with 10 g of a mixture of 84% Li₂B₄O₇, 1% LiBr and 15% NaNO₃. Using a Claisse Fluxer M4, a mixed pellet is formed. After cooling to room temperature, the elemental composition is determined using wavelength dispersive X-ray fluorescence.