Electrical conductivities of strontium-doped rare earth ultraphosphates and oxyphosphates

Electrical conductivity of lanthanum ultraphosphate and neodymium oxyphosphate both doped with strontium, La1-xSrxP5O14-δ (x = 0, 0.01, 0.03 and 0.05 in nominal formulae) and (Nd1-xSrx)3PO7-δ (x = 0 and 0.03), respectively, was investigated with a two-probe ac technique. The electrical conductivity was measured as functions of oxygen partial pressure and water vapor pressure in the temperature range of 523 to 673 K for the former and 973 to 1273 K for the latter. La1-xSrxP5O14-δ have considerable protonic conductivity, which was evidenced by isotope effect of hydrogen and deuteron on the electrical conductivity, over the wide range of oxygen partial pressure such as oxygen and hydrogen containing atmospheres. La1-xSrxP5O14-δ exhibit protonic conductivities of 4.7 × 10−5−2.2 × 10−3 S cm−1 in the temperature range of 523–673 K, which is comparable to the other phosphate based protonic conductors. Although protonic defects are considered to be induced and a major positive defect in (Nd0.97Sr0.03)3PO7-δ, attributed to small protonic mobility, (Nd0.97Sr0.03)3PO7-δ is a mixed oxide ion and electron hole conductor at 1173–1273 K, then, it turns to be electron hole conductor below temperatures of 1073 K. (Nd0.97Sr0.03)3PO7-δ in 1.0 × 10−2 atm oxygen–1.9 × 10−2 atm water vapor had smaller conductivity than that in unhumidified 1.0 × 10−2 atm oxygen. The conductivities were 9.7 × 10−3−3.1 × 10−1 S cm−1 and 2.8 × 10−3−2.5 × 10−1 S cm−1 in the above gases, respectively, in the temperature range of 973–1273 K. (Nd0.97Sr0.03)3PO7-δ had larger activation energy such as 165 kJ mol−1 than proton conducting La1-xSrxP5O14-δ (76 kJ mol−1) including other proton conducting phosphates (68–86 kJ mol−1).