A diffuser for decelerating a compressed gas, is provided, which includes a ring divided at circumferential positions around the ring into several arcuate sections, the complete ring is assembled from these sections. The ring defines one or more passages, the cross-section of which increases in the direction of flow of a fluid through the diffuser. The interface between adjacent sections is configured so that relative movement between the sections is prevented. The interface is configured to have a series of interlocking serrations along the axial length of the diffuser. In an embodiment, there are two arcuate sections involving two interfaces, and the peaks and troughs of the serrations of each of the two sections are arranged at an angle to a longitudinal axis of the diffuser. A small relative movement between the sections is achieved when the two angles at the two interfaces have opposite signs.