“…The best known examples are compounds of general formula [cat][M(HCOO)3], where M = Mn, Ni, 3 Co, Fe, and cat denotes ammonium, methylammonium, dimethylammonium (DMA), guanidinium (Gua), formamidinium (FMD) or azetidinium cation [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Many of [cat][M(HCOO)3] compounds undergo structural phase transitions, which are often associated with the appearance of electric order and pronounced dielectric anomalies [8][9][10][11][12][13][14][15][16][17]. It is worth to mention that up to now the highest phase transition temperature (435 K) in the family of dense MOFs has been found for imidazolium analogue, [C3N2H5][Mn(HCOO)3] [14].…”