  Neutron star mergers (NM) are a plausible source of heavy r-process elements
such as Europium, but previous chemical evolution models have either failed to
reproduce the observed Europium trends for Milky Way thick disc stars (with
[Fe/H] ~ -1) or have done so only by adopting unrealistically short merger
timescales. Using analytic arguments and numerical simulations, we demonstrate
that models with a single-phase interstellar medium (ISM) and
metallicity-independent yields cannot reproduce observations showing [Eu/alpha]
> 0 or [Eu/Fe] > [alpha/Fe] for alpha-elements such as Mg and Si. However, this
problem is easily resolved if we allow for a 2-phase ISM, with hot-phase
cooling times \tau_{cool} of order 1 Gyr and a larger fraction of NM yields
injected directly into the cold star-forming phase relative to alpha-element
yields from core collapse supernovae (ccSNe). We find good agreement with
observations in models with a cold phase injection ratio f_{c,NM}/f_{c,ccSN} of
order 2, and a characteristic merger timescale \tau_NM=150 Myr. We show that
the observed super-solar [Eu/alpha] at intermediate metallicities implies that
a significant fraction of Eu originates from NM or another source besides
ccSNe, and that these non-ccSN yields are preferentially deposited in the
star-forming phase of the ISM at early times.
