X-ray diffraction measurements of Bragg and diffuse scattering associated with charge ordering in the inorganic compound alpha(')- NaV2O5 show a continuous phase transition at a temperature of about 33.1 K. Many of this material's properties suggest a spin-Peierls transition, as established in CuGeO3 and MEM(TCNQ)(2). We compare the order parameter as well as fluctuations in the order parameter in these materials, and conclude that alpha(')- NaV2O5 is dissimilar, and that the transition is two dimensional in nature, due to charge ordering associated with the fractional average valence at the vanadium site within the orthorhombic a- b plane.

We have performed measurements of the critical neutron scattering on CsCo0.83Mg0.17Br3, a dilute stacked triangular lattice (STL) Ising antiferromagnet (AF). A two component line shape associated with the critical fluctuations appears at a temperature coincident with T(N1) observed in pure CsCoBr3. Such scattering is indicative of fluctuations in prototypical random field Ising model (RFIM) systems. The random field domain state arises in this case due to geometrical frustration within the STL Ising AF, which gives rise to a three sublattice Néel state, in which one sublattice is disordered. Magnetic vacancies nucleate AF domains in which the vacancies reside on the disordered sublattice thereby generating a RFIM state in the absence of an applied magnetic field.

A single-crystal x-ray study on the low-temperature structure of proves the existence of two different crystal structure distortions, as originally found by neutron scattering. One phase is hexagonal and does not differ much from the room temperature structure. It appears through a second-order phase transition. The other phase is orthorhombic and appears through a first-order phase transition. A model is proposed for the orthorhombic phase, which fits our x-ray measurements on as well as on . The model gives rise to a magnetic configuration that maps on to the row model of Zhang et al.