“Devil’s staircase”-type phase transition in NaV₂O₅ under high pressure

The “Devil’s staircase”-type phase transition in the quarter-filled spin-ladder compound NaV₂O₅ has been discovered at low temperature and high pressure by synchrotron radiation x-ray diffraction. A large number of transitions are found to successively take place among higher-order commensurate phases with 2a × 2b × zc type superstructures. The observed temperature and pressure dependence of modulation wave number q_c, defined by 1/z, is well reproduced by the axial next nearest neighbor Ising model. The q_c is suggested to reflect atomic displacements presumably coupled with charge ordering in this system.