  A model of diluted random field sustained by quenched volume deformations is
shown to reproduce puzzling physical features found in X(CN)_{x}Y_{1-x} mixed
cyanide crystals. X is an alkali metal (K, Na or Rb) and Y is a spherical
halogen ion (Br, Cl or I). Critical thresholds x_c at which associated first
order ferroelastic transitions disappear are calculated exactly. The diluted
random field is shown to compete with compressibility in making the transition
first order. Transitions are then found to remain first order down to x_c
except in the case of bromine dilution where they become continuous. All the
results are in excellent agreement with available experimental data.
