Single crystal growth and magnetism of ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$: Tracking the $J=0$ ground state of ${\mathrm{Ir}}^{5+}$

Single crystals of ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$ have been successfully grown using hydroxides flux. ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$ crystallize in the ${\mathrm{K}}_4\mathrm{Cd}{\mathrm{Cl}}_6\text{-type}$ structure with the space group $R\text{−}3c$ (no. 167). ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$ are electrically insulating with estimated activation gaps of 0.68 and 0.80 eV, respectively. ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$ show paramagnetic behavior down to 2 K. In this work, the observed effective moments, ${\mu }_{\mathrm{eff}}$, for ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ single crystals are $0.31{\mu }_{\mathrm{B}}$ for $H\perp c$ and $0.28{\mu }_{\mathrm{B}}$ for $H\left|\right|c$, which are much smaller than that of $0.49{\mu }_{\mathrm{B}}$ previously reported for the polycrystalline ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ samples. For ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$ single crystals, a much larger value of ${\mu }_{\mathrm{eff}}=0.57{\mu }_{\mathrm{B}}$ is observed as compared with ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ single crystals. The x-ray absorption spectroscopy and low-temperature specific heat data indicate that the Ir in ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ has an almost pure ${\mathrm{Ir}}^{5+}$ valence state, while the Ir in ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$ is slightly lower than +5. The estimated low limits of magnetic impurity ${\mathrm{Ir}}^{4+}$ are about ∼1.7% and ∼9.2% for ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$, respectively. These magnetic impurities are likely to fully explain the observed ${\mu }_{\mathrm{eff}}$ values for ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$ single crystals, supporting the $J=0$ ground state of ${\mathrm{Ir}}^{5+}$ in ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$.

Figure 1

Crystal structures of ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$(${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$) view along (a) [110] and (b) [001] directions. Panels (c) and (d) show optical photos of ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$ single crystals, respectively.

Figure 2

$\text{Ir-}{L}_3$ XAS spectra of ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$ in comparison with an ${\mathrm{Ir}}^{4+}$ reference ${\mathrm{La}}_2\mathrm{CoIr}{\mathrm{O}}_6$ and an ${\mathrm{Ir}}^{5+}$ reference ${\mathrm{Sr}}_2\mathrm{CoIr}{\mathrm{O}}_6$.

Figure 3

The temperature dependence of resistivity for (a) ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and (b) ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$. The insets show corresponding data fitted with the Arrhenius equation.

Figure 4

Temperature dependence of magnetic susceptibility and the inverse magnetic susceptibility for (a) ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ ($H\perp c$), (b) ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ ($H\left|\right|c$), and (c) ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$.

Figure 5

Temperature dependence of specific heat $C_p$ for (a) ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and (b) ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$. The insets show the corresponding $C_p/T$ vs $T^2$ curves.

Figure 6

Temperature-dependent $C_p/T$ data under varied magnetic fields for (a) ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and (b) ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$. Temperature-dependent $C_{\mathrm{mag}}/T$ and $S_{\mathrm{mag}}$ data for (c) ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and (d) ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$.