  The cubic-to-orthorhombic structural transition occurring in CsH at a
pressure of about 17 GPa is studied by ab initio calculations. The relative
stability of the competing structures and the transition pressure are correctly
predicted. We show that this pressure-induced first-order transition is
intimately related to a displacive second-order transition which would occur
upon application of a shear strain to the (110) planes. The resulting
instability is rationalized in terms of the pressure-induced modifications of
the electronic structure.
