Electrical conduction in rare earth orthophosphates, LnPO4 (Ln = La, Pr, Nd or Sm), with the monazite structure (P21/n) was investigated by using conductivity measurements at 500 to 925C. From the conductivities of undoped and 1mol%Sr-doped LnPO4 under wet (H2O and D2O) and dry atmospheres, it was found that LnPO4 began to conduct protons under wet atmosphere by substituting Sr for Ln. The conductivity behavior of 1mol%Sr-doped LnPO4 versus p(H2O) and p(O2) was discussed in terms of the defect equilibria. It was concluded that protonic conduction was dominant in the materials though electron holes contributed slightly to the total conductivity as temperature increased. All of the 1mol%Sr-doped LnPO4 investigated here exhibited similar electrical conduction regardless of the rare earth element used.

Protonic Conduction in Rare Earth Orthophosphates with the Monazite Structure. N.Kitamura, K.Amezawa, Y.Tomii, N.Yamamoto: Solid State Ionics, 2003, 162-163, 161-5