  Phase diagrams are the most reliable way of predicting phase changes in a
system. In this work, we create the phase diagram of the Ga(20-x)Al(x)+
bimetallic cluster system, for which the monometallic clusters display
greater-than- bulk melting behaviour. Employing first-principles
Born-Oppenheimer molecular dynamics in the microcanonical ensemble, we describe
the solid-liquid-like phase transition in two distinct compositions Ga11Al9+
and Ga3Al17+. The clusters show peaks in specific heat at 824 K and 922 K
respectively which is above the corresponding bulk alloy melting temperatures.
Mean squared displacements, diffusion coefficients and atoms-in-molecules
analysis emphasise the differences between the environments of the internal and
surface atoms. An Arrhenius-like description of the 'ease' with which internal
and surface sites (or atoms) can exchange positions with temperature is used to
quantify the greater-than-bulk melting character which corroborates the view of
a Ga(20-x)Al(x)+ cluster being a remnant of the corresponding bulk alloy
material with additional characteristics specific to its size and composition.
