DISCOVERY OF THE "DEVIL'S FLOWER" IN A CHARGE-ORDERING SYSTEM — SYNCHROTRON X-RAY DIFFRACTION STUDY OF NaV2O5

Abstract

We review recent synchrotron diffraction studies of Na V 2 O 5. The resonant X-ray scattering performed on a monoclinically-split single domain of Na V 2 O 5 shows a critically enhanced contrast between V 4+ and V 5+ ions. The results has led us to the unequivocal conclusion of the charge-order pattern of low-temperature phase of Na V 2 O 5 below T c =35 K. In spite of the possible four types of configuration of the zig-zag-type charge-order patterns in the ab-plane (A, A′, B and B′), the stacking sequence along the c-axis is determined as the AAA′A′ type by comparison with model calculations. By assigning the A and A′ configurations to Ising spins ↑ and ↓, one can reasonably explain the previously discovered "devil's staircase"-type behavior with respect to the modulation of layer-stacking sequences at high pressures and low temperatures, which clearly resembles the global phase diagram theoretically predicted by the ANNNI model, called "devil's flower". This is the first case that the devil's flower appears in such a charge-ordering system, where charge-order patterns are regarded as Ising spins.