Complex octahedral tilt phases in the ferroelectric perovskite system Li_xNa_1–xNbO₃

High-temperature phase behavior in the system LixNa1−xNbO3 has been studied by using high-resolution neutron powder diffraction. Each of the three compositions studied in the Na-rich part of the phase diagram (viz., x=0.03,0.08, and 0.12) shows evidence for distinct and complex structural modulations based on different tilting schemes of NbO6 octahedral units. Whilst octahedral tilting is prevalent in the structural chemistry of perovskites the details of the long-range tilt phases are distinct from those previously observed in NaNbO3 itself. A phase with a well-defined fourfold superlattice is observed for the composition Li0.12Na0.88NbO3, and yet more complex phases with modulations based on 20-fold and 30-fold repeats are observed for Li0.03Na0.97NbO3 and Li0.08Na0.92NbO3, respectively. This peculiar structural frustration makes the system LixNa1−xNbO3 a most structurally complex “simple” perovskite.