X-ray single-crystal study of the low-temperature structure of

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.