Gap states in insulating LaMnPO$_{1-x}$F$_{x}$ (x = 0 - 0.3)

Infrared transmission and electrical resistivity measurements reveal that single crystals of LaMnPO$_{1-x}$F$_{x}$ (x $leq$ 0.28) are insulating. The optical gap obtained from transmission measurements is nearly unaffected by doping, decreasing only slightly from 1.3 eV in undoped LaMnPO to 1.1 eV for x = 0.04. The activation gaps obtained from electrical resistivity measurements are smaller by at least an order of magnitude, signalling the presence of states within the optical gap. At low temperatures, the resistivity is described well by variable range hopping conduction between these localized gap states. Analysis of the hopping conduction suggests that the gap states become slightly more delocalized with fluorine content, although metallic conduction is not observed even for fluorine concentrations as large as x = 0.28.