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.Compositional heterogeneity is an essential ingredient in a number of important classes of functional materials. 1 Arguably the clearest example is that of the relaxor ferroelectrics, such as (1 − x)Pb(Mg 1/3 Nb 2/3 )O 3 -xPbTiO 3 (PMN-PT): 2 here an inhomogeneous distribution of B-site cations allows the formation of polar nanoregions (PNRs), 3-5 the collective motion of which is responsible for the giant electromechanical response observed and exploited experimentally. 6,7 Recently, the same ideas have been extended to relaxor ferromagnets, 8 which are in turn conceptually related to cluster spin glasses, 9 known for their exotic magnetic memory effects. 10 Likewise, in some porous metal-organic frameworks (MOFs), inhomogeneous vacancy distributions 11 affect gas uptake 12 via a mechanism that is analogous to ion conduction pathways in compositionally-heterogeneous solid-oxide fuel-cell materials. 13 From a materials design perspective, the existence of nanoscale compositional inhomogeneities relies on a delicate balance of interaction strengths at the atomic scale. If the interaction between [18][19][20] It was in this context that we sought to explore cation distributions in the mixed-metal formate perovskites [Gua]Cu x M 1−x (HCOO) 3 (Gua + = C(NH 2 ) + 3 ; M = Mn, Zn, Mg). It has been known for some time that many formate perovskites show relaxor-like dielectric behaviour, [21][22][23] which is usually associated with glassy dynamics of the organic (A-site) cation. 24,25 Indeed a recent 13 C NMR study of [(CH 3 ) 2 NH 2 ]Zn(HCOO) 3 even demonstrated the spontaneous formation of fluxional PNRs during the onset of ferroelectric order. 26 By contrast, the exploration of mixed-metal formate perovskites is rather less well developed. 27-31 Our focus here on the [Gua]M(HCOO) 3 family is motivated by the link in this particular system between polarization and cooperative Jahn-Teller (JT) order for M = Cu [ Fig. 1]. 28,32,33 In particular, we anticipated that substitution on the Cu site by JT-inactive cations (i.e., Mn, Zn, Mg; note Cd gives a different structure type 28,34 ) might be expected to reduce the length-scale of polar order, and-if clustering were found to occur-then favour the formation of PNRs in Cu dilute compositions.Moreover, a computational study of [Gua]Mn 0.5 Cu 0.5 (HCOO) 3 has recently suggested strong enhancement in both polarization and magnetization relative to the Mn-and Cu-containing end-members. 35 Our focus here is on the synthesis and structural char...