“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 NaV{sub 2}O {sub 5} 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 x 2b x zc type superstructures. The observed temperature and pressure dependence of modulation wave number q{sub c}, defined by 1/z, is well reproduced by the axial next nearest neighbor Ising model. The q{sub c} is suggested to reflect atomic displacements presumably coupled with charge ordering in this system.