Fig. 1: The crystal structure and DC electronic properties of CeMnAsO1-xFx.

a The tetragonal unit cell of CeMnAsO1-xFx, which crystallises in the P4/nmm space group. Insulating CeO1-xFx layers are situated between tetrahedral layers of MnAs. b The temperature-dependent resistivity (ρ) of CeMnAsO1-xFx, showing an insulator-insulator transition for x > 0.03. The data are normalised so that the transitions are apparent. The inset shows the variation of log(resistivity) versus inverse temperature where the insulator-insulator transition is observed for x = 0.035, 0.050 and 0.075.