Copper rubidium diphosphate, Rb2Cu3(P2O7)2: synthesis, crystal structure, thermodynamic and resonant propertiesElectronic supplementary information (ESI) available: Photo of single crystals synthesized and crystallographic data in CIF. See DOI: 10.1039/c3nj00422h

A new compound, Rb 2 Cu 3 (P 2 O 7 ) 2 , has been obtained from the melt in the Rb-Cu-P-O system. Its monoclinic crystal structure was determined by single-crystal X-ray diffraction: space group P 2 1 / c , Z = 2, a = 7.7119(8) Å, b = 10.5245(9) Å, c = 7.8034(9) Å, β = 103.862(5)° at 293 K, R = 0.030. The copper ions show coordination number (CN) 6 (4+2, distorted tetragonal bipyramidal). Trimers of [CuO 6 ] polyhedra sharing cis -edges form together with diphosphate groups of two tetrahedra [P 2 O 7 ] a microporous 3D framework with channels open along the c direction. The rubidium ions positioned in the channels show CN 10. The new phase is isotypic to Cs 2 Cu 3 (P 2 O 7 ) 2 . The regular changes in cell dimensions in the row Cs 2 Cu 3 (P 2 O 7 ) 2 → Rb 2 Cu 3 (P 2 O 7 ) 2 are caused by the compression of channel volumes due to decrease of the Cu-O-P angles in the framework windows. An electron spin resonance study indicates appearance of short range magnetic correlations below ∼120 K, long range magnetic order takes place at T N = 9.2 K as follows from magnetization and specific heat measurements. First principles calculations of the magnetic exchanges indicate that the effective Cu-Cu hopping interactions corresponding to super-super-exchange paths involving P atoms are much stronger than those within the edge-sharing Cu2-Cu1-Cu2 trimer units. The results of the X-ray study of a novel Rb 2 Cu 3 (P 2 O 7 ) 2 compound, its physical properties and theoretical calculations to elucidate the magnetic exchange interactions in a complicated d 9 -Jahn-Teller system are presented.