The search for candidate Kitaev materials has largely focused on $4d$ and $5d$ transition-metal compounds with various lattice geometries. In contrast, investigations of rare-earth $4f$ systems have thus far been restricted mainly to honeycomb and triangular lattices. In this work, we investigate the rare-earth-based double perovskites ${\mathrm{Ba}}_2{\mathrm{YbNbO}}_6$ and ${\mathrm{Ba}}_2{\mathrm{NdNbO}}_6$, which crystallize in a face-centered cubic structure. Magnetization and heat-capacity measurements establish isolated $j_{\mathrm{eff}}=1/2$ Kramers doublet ground states arising from strong spin-orbit coupling (SOC) and crystal electric-field effects, which are further supported by density functional theory calculations. Millikelvin-temperature thermodynamic measurements reveal long-range magnetic order with moderate frustration in both compounds. The emergence of magnetic order may be understood within an order-by-disorder scenario, as theoretically proposed for rare-earth fcc lattices with finite Kitaev interactions. Our results thus identify ${\mathrm{Ba}}_2{\mathrm{YbNbO}}_6$ and ${\mathrm{Ba}}_2{\mathrm{NdNbO}}_6$ as promising rare-earth spin-orbit-entangled magnets and motivate further experimental and theoretical investigations aimed at determining the complete exchange tensor to elucidate the microscopic origin of the underlying magnetic interactions.

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• Frustrated magnetism
• Magnetic phase transitions
• Magnetic susceptibility
• Specific heat
• Rare-earth magnetic materials
• Density functional theory