Structural and double magnetic transitions in the frustrated spin-1/2 capped-kagome antiferromagnet (RbCl)Cu₅P₂O10

The structural and magnetic properties of the geometrically frustrated spin-1/2 capped-kagome antiferromagnet (RbCl)Cu5P2O10 are investigated via temperature-dependent x-ray diffraction, magnetization, heat capacity, and 31P NMR experiments on a polycrystalline sample. It undergoes a structural transition at around T-t similar or equal to 310 K from a high-temperature trigonal (P (3) over bar m1) to a low-temperature monoclinic (C2/c) unit cell, where the low-temperature structure features the capped-kagome geometry of Cu2+ ions. Interestingly, it shows the onset of two successive magnetic transitions at T-N1 similar or equal to 20 K and T-N2 similar or equal to 7 K. The shape of the P-31 NMR spectra unfolds the possible nature of the transitions below T-N1 and T-N2 to be of incommensurate and commensurate antiferromagnetic type, respectively. A large value of the Curie-Weiss temperature as compared to T-N1 sets the frustration parameter f similar or equal to 8, ensuring strong magnetic frustration in the compound. From the P-31 NMR spin-lattice relaxation rate, the leading antiferromagnetic exchange coupling is estimated to be J/k(B) similar or equal to 117 K. These unusual double magnetic transitions make this compound beguiling for further investigations.