Pyrochlores. II. An investigation of La₂Ce₂O₇ by neutron diffraction

Abstract

The lattice parameter of La 2 Ce 2 O 7 fired at 1 400 °C was found to be 5.5700 ± 7 Å (at 20 ± 2 °C). Its crystal structure was of the disordered fluorite rather than the pyrochlore type. The degree of order in the structure could not be established from the neutron powder diffraction pattern, but it is likely that the metal and oxygen atoms were distributed at random, possibly with some tendency to short-range ordering. It would appear that for Ce 4+ the ionic size requirements for the formation of the pyrochlore structure are no longer satisfied, and that the cerates M 2 3+ Ce 2 4+ O 7 are outside the stability limits for 3–4 pyrochlores.