Structural and double magnetic transitions in the frustrated spin- 12 capped-kagome antiferromagnet (RbCl)Cu5P2O10

The structural and magnetic properties of the geometrically frustrated spin-$\frac{1}{2}$ capped-kagome antiferromagnet $(\mathrm{RbCl}){\mathrm{Cu}}₅{\mathrm{P}}₂{\mathrm{O}}₁₀$ are investigated via temperature-dependent x-ray diffraction, magnetization, heat capacity, and $³¹\mathrm{P}$ NMR experiments on a polycrystalline sample. It undergoes a structural transition at around ${T}_{\mathrm{t}}\ensuremath{\simeq}310$ K from a high-temperature trigonal $(P\overline{3}m1)$ to a low-temperature monoclinic $(C2/c)$ unit cell, where the low-temperature structure features the capped-kagome geometry of ${\mathrm{Cu}}²⁺$ ions. Interestingly, it shows the onset of two successive magnetic transitions at ${T}_{\mathrm{N}1}\ensuremath{\simeq}20$ K and ${T}_{\mathrm{N}2}\ensuremath{\simeq}7$ K. The shape of the $³¹\mathrm{P}$ NMR spectra unfolds the possible nature of the transitions below ${T}_{\mathrm{N}1}$ and ${T}_{\mathrm{N}2}$ to be of incommensurate and commensurate antiferromagnetic type, respectively. A large value of the Curie-Weiss temperature as compared to ${T}_{\mathrm{N}1}$ sets the frustration parameter $f\ensuremath{\simeq}8$, ensuring strong magnetic frustration in the compound. From the $³¹\mathrm{P}$ NMR spin-lattice relaxation rate, the leading antiferromagnetic exchange coupling is estimated to be $J/{k}_{\mathrm{B}}\ensuremath{\simeq}117$ K. These unusual double magnetic transitions make this compound beguiling for further investigations.