  The high-pressure structural behaviour of the fluoroperovskite KMgF_3 is
investigated by theory and experiment. Density functional calculations were
performed within the local density approximation and the generalized gradient
approximation for exchange and correlation effects, as implemented within the
full-potential linear muffin-tin orbital method. In situ high-pressure powder
x-ray diffraction experiments were performed up to a maximum pressure of 40 GPa
using synchrotron radiation. We find that the cubic Pm\bar{3}m crystal symmetry
persists throughout the pressure range studied. The calculated ground state
properties -- the equilibrium lattice constant, bulk modulus and elastic
constants -- are in good agreement with experimental results. By analyzing the
ratio between the bulk and shear modulii, we conclude that KMgF_3 is brittle in
nature. Under ambient conditions, KMgF_3 is found to be an indirect gap
insulator with the gap increasing under pressure.
