Crystal structures, frustrated magnetism, and chemical pressure in Sr-doped Ba3NiSb2O9 perovskites

Abstract

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 ${\mathrm{Ba}}_{3\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Ni}{\mathrm{Sb}}_{2}{\mathrm{O}}_{9}$ 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 $(x=0)\ensuremath{\rightarrow}6H$ with $\mathrm{NiSb}{\mathrm{O}}_{9}$ pairs $(x=0.5)\ensuremath{\rightarrow}3C$ (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 $6H {\mathrm{Ba}}_{2.5}{\mathrm{Sr}}_{0.5}\mathrm{Ni}{\mathrm{Sb}}_{2}{\mathrm{O}}_{9} (x=0.5)$ 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 ${\mathrm{Ba}}_{1.75}{\mathrm{Sr}}_{1.25}\mathrm{Ni}{\mathrm{Sb}}_{2}{\mathrm{O}}_{9} (x=1.25)$ crystallizes in a $3C$ double perovskite ${A}_{2}B{B}^{\ensuremath{'}}{\mathrm{O}}_{6}$ cubic structure where $A, B$, and ${B}^{\ensuremath{'}}$ sites are occupied by (Ba + Sr), Sb, and $(\frac{2}{3}\mathrm{Ni}+\frac{1}{3}\mathrm{Sb})$ atoms, respectively. The ${B}^{\ensuremath{'}}$ 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 ($\ensuremath{\approx}\ensuremath{-}65$ and $\ensuremath{\approx}\ensuremath{-}213$ 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 $(\ensuremath{\mu}\mathrm{SR})$ measurements identify a dynamic magnetic state down to the base temperature (95 mK), consistent with a previously published inelastic neutron scattering study. For $3C {\mathrm{Ba}}_{1.75}{\mathrm{Sr}}_{1.25}\mathrm{Ni}{\mathrm{Sb}}_{2}{\mathrm{O}}_{9}, \ensuremath{\mu}\mathrm{SR}$ and $^{121}\mathrm{Sb}$ nuclear magnetic resonance measurements both indicate the presence of a transition to a static magnetic state below $11(1)$ 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 ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ 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 ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ 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.