Copper rubidium diphosphate, Rb2Cu3(P2O7)2: synthesis, crystal structure, thermodynamic and resonant properties

Abstract

A new compound, Rb₂Cu₃(P₂O₇)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 P2₁/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 [CuO₆] polyhedra sharing cis-edges form together with diphosphate groups of two tetrahedra [P₂O₇] 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 Cs₂Cu₃(P₂O₇)₂. The regular changes in cell dimensions in the row Cs₂Cu₃(P₂O₇)₂ → Rb₂Cu₃(P₂O₇)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.