Compositional nanodomain formation in hybrid formate perovskites

Abstract

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...

Ferroic multipolar order and disorder in cyanoelpasolite molecular perovskites

“…This is precisely what happens in guanidinium formate perovskites, where ferroic quadrupole order even has a particular functional consequence. The interplay of collective Cu 2+ Jahn Teller distortions with ferroquadrupolar order in [C(NH) 2 ) 3 ]Cu(HCOO) 3 drives inversion symmetry breaking via a so-called hybrid improper ferroelectric mechanism [31][32][33]. Indeed the various possible types of multipolar order that are more generally accessible to molecular perovskites seem to be particularly useful ingredients for generating polar (and, in turn, ferroelectric) phases [34].…”

Compositional inhomogeneity and tuneable thermal expansion in mixed-metal ZIF-8 analogues

“…22 and 38, except that we have used atomic absorption spectroscopy (AAS) as a means of identifying the experimental Zn:Cd ratios in our final samples. 42,45 Despite employing a set of transition-metal precursors with evenly-spaced Cd compositions (x = 0,0.1,0.2,. . .,1), we found some significant variability between nominal and actual compositions, with ∆x = |x nom − x exp | 0.1.…”

“…In order to do so, we employed a recently-developed IR/NMF approach which exploits the sensitivity of linker vibrational frequencies to the chemical nature of the metals to which linkers are bound. 42 The in-plane mIm bending region of the IR absorption spectrum of Zn 1−x Cd x (mIm) 2 is an excellent candidate for this type of analysis, since one expects distinct signatures for bending modes of Zn-mIm-Zn, Zn-mIm-Cd, and Cd-mIm-Cd linkages. 48 Our experimental IR measurements show a continuous evolu- tion in absorption profile with composition x [Fig.…”

Tilts and shifts in molecular perovskites

“…71 This compound has been predicted to be ferroelectric and B-site substitution leads to the formation of compositional nanoregions reminiscent of relaxor ferroelectrics. 71,72 Analogous structures without a Jahn-Teller distortion adopt the centrosymmetric space group Pnna. 70 ; Ph = phenyl, C 6 H 5 ) all contain the correct ingredients to couple to a M 3 + Jahn-Teller distortion to break the inversion symmetry.…”