Structural and double magnetic transitions in the frustrated spin-$\frac{1}{2}$ capped-kagome antiferromagnet (RbCl)Cu${}_5$P${}_2$O${}_{10}$

Abstract

The structural and magnetic properties of the geometrically frustrated spin-$1/2$ capped-kagome antiferromagnet (RbCl)Cu${}_5$P${}_2$O${}_{10}$ are investigated via temperature dependent x-ray diffraction, magnetization, heat capacity, and ${}^{31}$P NMR experiments on a polycrystalline sample. It undergoes a structural transition at around $T_{\mathrm{t}}\simeq 310$ K from a high temperature trigonal ($P\bar{3}m1$) to a low temperature monoclinic ($C2/c$) unit cell, where the low temperature structure features the capped-kagome geometry of Cu${}^{2+}$ ions. Interestingly, it shows the onset of two successive magnetic transitions at $T_{N1}\simeq 20$ K and $T_{N2}\simeq 7$ K. The shape of the ${}^{31}$P NMR spectra unfold 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 $f\simeq 8$, ensuring strong magnetic frustration in the compound. From the ${}^{31}$P NMR spin-lattice relaxation rate, the leading antiferromagnetic exchange coupling is estimated to be $J/k_{\mathrm{B}}\simeq 117$ K. These unusual double magnetic transitions make this compound beguiling for further investigations.

Accepted in Phys. Rev. B (2023), 12 pages, 10 figures, 2 tables