Crystal chemistry and physical properties of the A2M3(H2O)2[B4P6O24(OH)2] (A = Cs, Rb; M = Ni, Cu, (Ni, Fe)) borophosphate family
Three new transition metal borophosphates, Cs2Cu3(H2O)2[B4P6O24(OH)2] (denoted (I)), Cs2Ni3(H2O)2[B4P6O24(OH)2] (II) and mixed Rb2Ni2.5Fe0.5(H2O)2[B4P6O24(OH)2] (III) were synthesized from boric acid flux at 473-493 K. X-ray single-crystal diffraction study confirms their isotipy with (Cs/Rb)2Co3(H2O)2[B4P6O24(OH)2] borophosphates crystallizing in the orthorombic space group Pbca. The crystal structures of all compounds are based on 3D frameworks made from corrugating borophosphate layers [B4P6O24(ОН)2] and trimers of MO4(H2O)2 and two MO6 edge-sharing octahedra. The negative charge of the framework is compensated by alkali metal cations, Rb+ or Cs+. Structural analysis reveals the flexibility of mixed anionic frameworks of described structural type. The Cu-member demonstrates a strong distortion of the units cell due to the Jahn-Teller effect of the d9 configuration of Cu2+ cation. The possibility of solid solution between different transition metals (Fe, Ni) with formation of crystals with mixed structural positions is confirmed by an example of (III) phase. It is shown, that Fe2+ cations prefer to occupy the M2 position with more distorted oxygen enviroment, as compared to the M1O6 octahedra. Magnetic studies of (I), (II) and (III) prove that all compounds are paramagnets down to lowest temperatures of measurements, 2 K. (I) and (II) compounds are thermally stable up to 500°C. Different mechanisms of their thermal decomposition are discussed. The possibility of a wide isomorphous substitution at the transition metal sites of the title structure type opens the way to properties’ modification in the discussed series of borophosphates.