Philippe Mendels

The effects of chemical pressure on the structural and magnetic properties of the triple perovskite ${\mathrm{Ba}}_3\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$ are investigated by substituting ${\mathrm{Sr}}^{2+}$ ions for ${\mathrm{Ba}}^{2+}$ ions. Two ${\mathrm{Ba}}_{3-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$ phases could be stabilized via a solid-state reaction at ambient pressure (AP) in air. The $6H$ with ${\mathrm{Sb}}_2{\mathrm{O}}_9$ pairs $\left(x=0\right)\to 6H$ with $\mathrm{NiSb}{\mathrm{O}}_9$ pairs $\left(x=0.5\right)\to 3C$ (cubic with corner-sharing octahedral, $x=1.25$) sequence of structural phases occurs with increasing Sr content, i.e., chemical pressure, which is like that previously reported for pure samples of ${\mathrm{Ba}}_3\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$ obtained under increasing high physical pressure (HP). For the $6H {\mathrm{Ba}}_{2.5}{\mathrm{Sr}}_{0.5}\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9 \left(x=0.5\right)$ phase, using combined Rietveld refinements of powder x-ray and neutron diffraction patterns, precession electron diffraction tomography data collected on thin crystals, aberration-corrected high-angle annular dark field scanning transmission electron microscopy coupled to energy dispersive x-ray spectroscopy mapping, we reach the conclusion that the structure features corner-sharing $\mathrm{Sb}{\mathrm{O}}_6$ octahedra and $\mathrm{NiSb}{\mathrm{O}}_9$ pairs of face-shared octahedra (or Ni-Sb dumbbells) with either a random orientation of the Ni-Sb dumbbells or nanosized chemical correlations for the dumbbell arrangement. As observed in HP ${\mathrm{Ba}}_3\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$ produced through synthesis at 9 GPa, AP ${\mathrm{Ba}}_{1.75}{\mathrm{Sr}}_{1.25}\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9 \left(x=1.25\right)$ crystallizes in a $3C$ double perovskite $A_{2}B{B}^{\prime }{\mathrm{O}}_6$ cubic structure where $A, B$, and $B^{\prime }$ sites are occupied by (Ba + Sr), Sb, and $\left(\frac{2}{3}\mathrm{Ni}+\frac{1}{3}\mathrm{Sb}\right)$ atoms, respectively. The $B^{\prime }$ sites, which are randomly occupied by spin-1 ${\mathrm{Ni}}^{2+}$ and diamagnetic ${\mathrm{Sb}}^{5+}$, form a face-centered-cubic (FCC) sublattice where the ${\mathrm{Ni}}^{2+}$ amount stays above the site percolation threshold. Weiss temperatures ($\approx -65$ and $\approx -213$ K for ${\mathrm{Ba}}_{2.5}{\mathrm{Sr}}_{0.5}\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$ and ${\mathrm{Ba}}_{1.75}{\mathrm{Sr}}_{1.25}\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$, respectively) indicate that dominant magnetic interactions between ${\mathrm{Ni}}^{2+}$ spins are antiferromagnetic with magnitudes like those observed in the corresponding HP phases of pure ${\mathrm{Ba}}_3\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$. As for the $6H$ HP ${\mathrm{Ba}}_3\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$ compound, in $6H {\mathrm{Ba}}_{2.5}{\mathrm{Sr}}_{0.5}\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$, muon spin relaxation $\left(\mu \mathrm{SR}\right)$ measurements identify a dynamic magnetic state down to the base temperature (95 mK), consistent with a previously published inelastic neutron scattering study. For $3C {\mathrm{Ba}}_{1.75}{\mathrm{Sr}}_{1.25}\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9, \mu \mathrm{SR}$ and ${}^{121}\mathrm{Sb}$ nuclear magnetic resonance measurements both indicate the presence of a transition to a static magnetic state below $11\left(1\right)$ K with a significant amount of disorder in this frozen state, in contrast to the spin-liquid state previously suggested for the $3C$ HP phase of ${\mathrm{Ba}}_3\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$. Consistently, a broad maximum is observed in the specific heat at the same temperature. Building on the structural data, the magnetic properties of HP $6H {\mathrm{Ba}}_3\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$ and AP $6H {\mathrm{Ba}}_{2.5}{\mathrm{Sr}}_{0.5}\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$ are discussed in light of recent works on triangular and $J_1\text{−}{J}_2$ honeycomb systems with or without quenched disorder. We are led to the conclusion that the driving force toward a spin-liquid-like state is quenched disorder which needs to be incorporated in $J_1\text{−}{J}_2$ honeycomb models. Our evidence of a magnetic transition to a frozen magnetic ground state for the AP Sr-doped $3C$ phase is in line with models for geometrically frustrated FCC antiferromagnets. This calls for a better experimental and possibly theoretical understanding of the HP $3C$ phase.

Measuring the specific heat of herbertsmithite single crystals in high magnetic fields (up to 34 T) allows us to isolate the low-temperature kagome contribution while shifting away extrinsic Schottky-like contributions. The kagome contribution follows an original power law $C_p(T\to 0)\propto T^{\alpha }$ with $\alpha \sim 1.5$ and is found to be field independent between 28 and 34 T for temperatures $1\le T\le 4\mathrm{K}$. These are serious constraints when it comes to replication using low-temperature extrapolations of high-temperature series expansions. We manage to reproduce the experimental observations if about 10% of the kagome sites do not contribute. Between 0 and 34 T, the computed specific heat then has a minute field dependence supporting an algebraic temperature dependence in zero field, typical of a critical spin-liquid ground state. The need for an effective dilution of the kagome planes is discussed and is likely linked to the presence of copper ions on the interplane zinc sites. At very low temperatures and moderate fields, we also report some small field-induced anomalies in the total specific heat and start to elaborate on a phase diagram.

We report x-ray diffraction, magnetic susceptibility, heat capacity, ${}^1\mathrm{H}$ nuclear magnetic resonance (NMR), and muon spin relaxation $\left(\mu \mathrm{SR}\right)$ measurements, as well as density-functional band-structure calculations for the frustrated $S=3/2$ triangular lattice Heisenberg antiferromagnet (TLHAF) $\alpha \text{−}{\mathrm{HCrO}}_2$ (trigonal, space group: $R\overline{3}m$). This compound undergoes a clear magnetic transition at $T_{\mathrm{N}}\simeq 22.5$ K, as seen from the drop in the muon paramagnetic fraction and concurrent anomalies in the magnetic susceptibility and specific heat capacity. Local probes (NMR and $\mu \mathrm{SR}$) reveal a broad regime with slow fluctuations down to $0.7T_{\mathrm{N}}$, this temperature corresponding to the maximum in the $\mu \mathrm{SR}$ relaxation rate and in the NMR wipe-out. From the comparison with ${\mathrm{NaCrO}}_2$ and $\alpha \text{−}{\mathrm{KCrO}}_2$, the fluctuating regime and slow dynamics below $T_{\mathrm{N}}$ appear to be hallmarks of the TLHAF with $ABC$ stacking. We discuss the role of interlayer frustration, which may have impacted recent spin-liquid candidates with triangular geometry.

The quantum kagome antiferromagnets ${\mathrm{YCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{O}}_x{\mathrm{Cl}}_{3-x}$ ($x=0,1/3$) are produced using a unified solid state synthesis route for polycrystalline samples. From structural refinements based on neutron diffraction data, we clarify the structure of the ${\mathrm{Y}}_3{\mathrm{Cu}}_9{\left(\mathrm{OH}\right)}_{18}{\mathrm{OCl}}_8$ ($x=1/3$) compound and provide a revised chemical formula. We use muon spin relaxation, as a local probe of magnetism, to investigate the exotic low-temperature properties in the two compounds. In agreement with the low-temperature neutron diffraction data, we find no evidence for long range ordering in both materials but they exhibit distinct ground states: while disordered static magnetism develops in the $x=0$ compound, we conclude on the stabilization of a quantum spin liquid in the $x=1/3$ one, since the local fields remain fully dynamical. Our findings are in contrast to previous reports based on thermodynamical measurements only. We then discuss their origin on the basis of structural details and specific heat measurements. In particular, the $x=1/3$ compound appears to realize an original spatially anisotropic kagome model.