The present resonant x-ray scattering has been performed on a monoclinically split single domain of NaV(2)O(5). The observation of a critically enhanced contrast between V(4+) and V(5+) ions has led us to the unequivocal conclusion of the charge-order pattern of the low-temperature phase of NaV(2)O(5) below T(c) = 35 K. In spite of the possible four types of configuration of the zigzag-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.

The effect of pressure is studied on highly Mg-doped CuGeO 3 , which at ambient pressure shows an antiferromagnetic transition but no spin-Peierls transition. The magnetic susceptibility under pressure was measured; the broad maximum around 60 K shifts to lower temperatures and the spin-Peierls ͑SP͒ transition reappears. The temperature-concentration phase diagram at various pressures was constructed and it was found that the SP phase region enlarges with pressure. These results mean that the enhancement of frustration due to nearest-neighbor and next-nearest-neighbor exchange interactions overcomes the effect of interchain interaction, and that the spin-Peierls phase is more stabilized than the paramagnetic or antiferromagnetic phases. In the temperature-pressure-concentration phase diagram, the compositional phase transition between the dimerized-antiferromagnetic phase and the uniform-antiferromagnetic phase is second order at PϾ0.2 GPa, but first order at PϽ0.2 GPa. We propose that the change in order of this transition is also due to enhancement of the frustration. We show an example of frustration effects in a one-dimensional quantum spin system.