Spin-orbit coupling controlled ground states in the double perovskite iridates <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>A</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi>B</mml:mi></mml:mrow><mml:msub><mml:mi mathvariant="normal">IrO</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:math> (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>A</mml:mi><mml:mo>=</mml:mo></mml:mrow></mml:math> Ba, Sr; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>B</mml:mi><mml:mo>=</mml:mo></mml:mrow></mml:math> Lu, Sc)

Iridates with the $5{d}^{4}$ electronic configuration have attracted recent interest due to reports of magnetically ordered ground states despite longstanding expectations that their strong spin-orbit coupling would generate a $J=0$ electronic ground state for each ${\mathrm{Ir}}^{5+}$ ion. The major focus of prior research has been on the double perovskite iridates ${\mathrm{Ba}}_{2}{\mathrm{YIrO}}_{6}$ and ${\mathrm{Sr}}_{2}{\mathrm{YIrO}}_{6}$, where the nature of the ground states (i.e., ordered vs nonmagnetic) is still controversial. Here, we present neutron powder diffraction, high-energy-resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS), resonant inelastic x-ray scattering (RIXS), magnetic susceptibility, and muon spin relaxation data on the related double perovskite iridates ${\mathrm{Ba}}_{2}{\mathrm{LuIrO}}_{6}, {\mathrm{Sr}}_{2}{\mathrm{LuIrO}}_{6}, {\mathrm{Ba}}_{2}{\mathrm{ScIrO}}_{6}$, and ${\mathrm{Sr}}_{2}{\mathrm{ScIrO}}_{6}$ that enable us to gain a general understanding of the electronic and magnetic properties for this family of materials. Our HERFD-XAS and RIXS measurements establish $J=0$ electronic ground states for the ${\mathrm{Ir}}^{5+}$ ions in all cases, with similar values for Hund's coupling ${J}_{\mathrm{H}}$ and the spin-orbit coupling constant ${\ensuremath{\lambda}}_{\mathrm{SOC}}$. Our bulk susceptibility and muon spin relaxation data find no evidence for long-range magnetic order or spin freezing, but they do exhibit weak magnetic signals that are consistent with extrinsic local moments. Our results indicate that the large ${\ensuremath{\lambda}}_{\mathrm{SOC}}$ is the key driving force behind the electronic and magnetic ground states realized in the $5{d}^{4}$ double perovskite iridates, which agrees well with conventional wisdom.

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