Disorder-driven magnetic duality in the spin-$\frac{1}{2}$ system ktenasite, Cu$_\text{2.7}$Zn$_\text{2.3}$(SO$_\text{4}$)$_\text{2}$(OH)$_\text{6}\cdot$6H$_\text{2}$O

Abstract

Disorder in frustrated quantum systems can critically influence their magnetic ground states and drive exotic correlated behavior. In the $S = \frac{1}{2}$ system ktenasite, Cu$_\text{2.7}$Zn$_\text{2.3}$(SO$_\text{4}$)$_\text{2}$(OH)$_\text{6}\cdot$6H$_\text{2}$O, we show that structural disorder drives an unexpected dimensional crossover and stabilizes a rare coexistence of distinct magnetic states. Neutron diffraction reveals significant Cu/Zn mixing at the Cu2 site, which tunes the Cu$^{2+}$ sublattice from a two-dimensional scalene-distorted triangular lattice into a one-dimensional spin-chain network. Magnetic susceptibility, neutron diffraction, ac susceptibility, and specific heat measurements collectively indicate magnetic duality: a coexistence of incommensurate long-range magnetic order below $T_\text{N} = 4\,$K and a cluster spin-glass state with $T_\text{f} = 3.28\,$K at $ν= 10\,$Hz. Our findings highlight ktenasite as a rare platform where structural disorder tunes the effective dimensionality and stabilizes coexisting ordered and glassy magnetic phases, offering a unique opportunity to explore the interplay of frustration, disorder, and dimensional crossover in quantum magnets.

Figures (13)

• Figure 1: (a) Refined crystal structure of Cu$_\text{2.7}$Zn$_\text{2.3}$(SO$_\text{4}$)$_\text{2}$(OD)$_\text{6}\cdot$6D$_\text{2}$O in monoclinic $P2_1/c$ from D2B data at 10 K. (b) Single $[\ce{Cu(Cu,Zn)(OD)3O}]^{2-}$ layer of edge-sharing, Jahn–Teller–distorted octahedra. (c,d) Magnetic sublattice showing (c) an anisotropic triangular arrangement of Cu^2+ ions, which evolves into (d) corrugated 1D chains along the $b$ axis at Cu1 sites due to random site disorder, with Cu–-Cu distances indicated.
• Figure 2: Rietveld-refined powder (a) neutron and (b) x-ray patterns for ktenasite.
• Figure 3: Structural model fit of single-crystal x-ray diffraction data collected at 180 K. $F_{\text{calc}}^{2}$ and $F_{\text{obs}}^{2}$ represent the calculated and observed structure factors, respectively. The inset shows an optical microscope image of several submillimeter-sized ktenasite crystals.
• Figure 4: (a) Temperature dependence of Cu$_\text{2.7}$Zn$_\text{2.3}$(SO$_\text{4}$)$_\text{2}$(OH)$_\text{6}\cdot$6H$_\text{2}$O magnetization under ZFC and FC conditions at selected fields. The inset is a zoomed-in view showing the onset of ZFC–FC divergence. (b) ZFC $M/H$ as a function of temperature at 1 T. The inset presents the temperature derivative, revealing a peak near 4 K. (c) Inverse susceptibility ($H/M$) at $\mu_0H = 1$ T, with the Curie–Weiss fit shown as a solid red line. (d) Isothermal magnetization at 2 and 50 K, with Brillouin function fit shown as solid line. The inset shows the field derivative, exhibiting slope changes at 1.25 and 6 T.
• Figure 5: Temperature and frequency dependence of the (a) real ($\chi^{\prime}_{\mathrm{ac}}$) and (b) imaginary ($\chi^{\prime\prime}_{\mathrm{ac}}$) components of the ac susceptibility. The inset in (a) shows the frequency dependence of the freezing temperatures fitted using a power law, while the inset in (b) shows fits using both Néel-Arrhenius and Vogel-Fulcher laws.