Ordering processes in Ln2TiO5 (Ln = Dy-Lu): The role of thermal history

We report the first synthesis of pyrochlore-like Ln2TiO5 with Ln = Er-Lu via cooling at a rate as slow as 5 deg. C/h in the range 1600-820 deg. C. The materials thus prepared differ in structure from the oxygen-ion-conducting Ho2TiO5 pyrochlore: the X-ray diffraction patterns of the pyrochlore-like Ln2TiO5 (Ln = Er-Lu) show weak 511 and 531 pyrochlore superstructure peaks and an unidentified peak at 2θ = 19.6 deg. (Cu Kα) (5% intensity). The Ln2TiO5 (Ln = Er, Tm) pyrochlores prepared via very slow cooling followed by quenching in liquid nitrogen from 840 deg. C contain, in addition to a pyrochlore-like phase, a small amount (up to 5%) of the low-conductivity high-temperature hexagonal phase β-Ln2TiO5. The pyrochlore Ho2TiO5 contains microdomains of the hexagonal phase β-Ho2TiO5, which does not affect its high oxygen ion conductivity. The conductivity of the pyrochlore-like Ln2TiO5 with Ln = Er and Yb is substantially lower than that of the fluorite-like Ln2TiO5 (Ln = Er, Yb) and oxygen-ion-conducting pyrochlore Ho2TiO5 obtained via furnace cooling (130 deg. C/h), which can be understood in terms of oxygen ordering in these compounds and the presence of the low-conductivity high-temperature phase β-Ln2TiO5 in the pyrochlore-like Ln2TiO5 (Ln = Er, Yb) at measurement temperatures (∼500-840 deg. C).