Structure and magnetic properties of the AB(HCO₂)₃ (A = Rb⁺ or Cs⁺, B = Mn²⁺, Co²⁺ or Ni²⁺) frameworks: probing the effect of size on the phase evolution of the ternary formates

Abstract

The AB(HCO2)3 (A = Rb+ or Cs+ & B = Mn2+, Co2+ and Ni2+) frameworks adopt either a perovskite-like or chiral hexagonal structure with dominantly antiferromagnetic interactions. This clarifies the phase evolution of the ternary formates with cation size.

Transition‐Metal Dependent Cation Disorder in the Chiral Cubic AB(HCO2)3 Metal‐Organic Frameworks (A = Li or Na, B = Mn or Co)

“…Calculating these values for the four frameworks examined in this study give tolerance factors of 0.56, 0.57, 0.59, and 0.60 for LiMn(HCO 2 ) 3 , LiCo(HCO 2 ) 3 , NaMn(HCO 2 ) 3 , and NaCo(HCO 2 ) 3 , respectively, 0.56 is the smallest tolerance factor for a AB (HCO 2 ) 3 framework reported to date [Note that for consistency with previously reported tolerance factor calculations for this family of compounds we have used the ionic radii of the high spin state six‐coordinate transition metals and eight coordinate ionic radii for the alkali metals]. Comparison with the behavior of the remainder of the AB (HCO 2 ) 3 frameworks, suggests that this cubic phase is likely adopted by any other AB (HCO 2 ) 3 frameworks that form with a tolerance factor in this range. Frameworks with tolerance factors significantly higher than 0.60 most likely give rise to the polar Pna 2 1 structure adopted by NH 4 Cd(HCO 2 ) 3 , which has a tolerance factor of 0.63; this has been referred to as the so‐called Perovskite III structure, because of its resemblance to a perovskite phase with every second channel in the structure doubly occupied by the A ‐site cations.…”

“…The confirmation of occupational disorder of the A ‐ and B ‐site cations in the Co frameworks examined in this study is the first example of such disorder in the AB (HCO 2 ) 3 frameworks . This is likely made possible by the much smaller size of the alkali metals on the A sites in these materials.…”

“…While several of the AB (HCO 2 ) 3 structure types are modifications of the well‐known pervoskite structures two unique chiral structures are formed only by this family . Of these the chiral cubic structure forms when A = Na, the smallest A ‐site cation incorporated into the formate frameworks thus far .…”

“…[2a] Amongst the most promising frameworks with such functionalities are the AB (HCO 2 ) 3 frameworks, in which A is a monovalent alkali metal, ammonium or a protonated alkylamine and B is typically a first row transition metal. [1c], [1d] This family of frameworks is known to exhibit five different structure types with three dimensional covalent connectivity, all of which feature octahedral B site cations linked in a corner‐sharing fashion through the formate ligands, with the A site typically sitting in channels found in these structures …”

“…Its highly unusual windmill magnetic topology, however, is very rare and cases with magnetic cations on this sublattice are almost exclusively limited to β‐Mn and its various doped alloys . The NaMn(HCO 2 ) 3 [5a] framework is therefore interesting in that it achieves the same magnetic topology in a coordination framework, where naively, the structure formed might be expected to be more likely to be robust to significant changes in the B ‐site cation; this is typically found for the other architectures adopted by AB (HCO 2 ) 3 frameworks . This could allow β‐Mn analogues to be formed with a wider range of paramagnetic cations with strongly frustrated magnetic coupling.…”