Work function of the mixed-valent manganese perovskites

We have performed resistivity and work function measurements on a series of samples with Ln(0.7)D(0.3)MnO(3) composition, where Ln is a lanthanide (La, Pr, or Nd) and D is a nominally divalent ion (Sr, Ca, or Pb). The resistivity measurements are, within some small variation, in agreement with those previously reported for polycrystalline samples of similar composition. The measurement of a work function in air is often difficult to reproduce, mainly owing to the fact that it is a surface probe and can be modified significantly by attachment of contaminants from the atmosphere. Here we have obtained reproducible results that are slightly more stable than the reference metals used in the measurements. This required the introduction of a higher order correction to a common Kelvin probe experimental procedure. Another factor contributing to the high quality of the results is the low carrier density of the conducting oxides. This allows greater penetration of quasistatic electric fields. The work function results here are in overall agreement with previous photoemission work that determined an approximate electron binding energy. We find the best correlation of increased work function is to strain introduced by larger divalent ions and smaller Lanthanide ions. We conclude that the work function is dominated by electrons near the divalent site, and this is interpreted in terms of strain modifying the coulomb energy of the electronic states. (C) 2004 American Institute of Physics.