  We report the synthesis and structural characterisation of three mixed-metal
formate perovskite families [C(NH$_2$)$_3$]M$_{1-x}$Cu$_x$(HCOO)$_3$ (M = Mn,
Zn, Mg). Using a combination of infrared spectroscopy, non-negative matrix
factorization, and reverse Monte Carlo refinement, we show that the Mn- and
Zn-containing compounds support compositional nanodomains resembling the polar
nanoregions of conventional relaxor ferroelectrics. The M = Mg family exhibits
a miscibility gap that we suggest reflects the limiting behaviour of nanodomain
formation.
