Control of multipolar and orbital order in perovskite-like [C(NH2)3]CuxCd1–x(HCOO)3 metal–organic frameworks
N L Evans, P M M Thygesen, H L B Boström, E M Reynolds, I E Collings, A E Phillips, and A L Goodwin
Journal of the American Chemical Society 138, 9393-9396 (2016)

We study the compositional dependence of molecular orientation (multipolar) and orbital (quadrupolar) order in the family of perovskite-like metal–organic frameworks [C(NH2)3]CuxCd1–x(HCOO)3. On increasing the fraction x of Jahn-Teller-active Cu2+, we observe first an orbital disorder/order transition and then a multipolar reorientation transition, each occurring at distinct critical compositions xo = 0.45(5) and xm = 0.55(5). We attribute these transitions to a combination of size, charge distribution, and percolation effects. The transitions we observe establish the accessibility in formate perovskites of novel structural degrees of freedom beyond the familiar dipolar terms responsible for (anti)ferroelectric order. We discuss the symmetry implica-tions of cooperative quadrupolar and multipolar states for the design of relaxor-like hybrid perovskites.