Abstract: The optical properties of Yb3+-doped CrBr3 have been measured at various pressures, using Yb3+ as a spin-bearing luminescent probe of short-range magnetic order. The trends in Raman and optical absorption with pressure indicate compression of [CrBr6]3- octahedra with a uniform Cr3+-Br- bond-length contraction of --0.005 Å/GPa, despite the soft van der Waals gap. The f-f photoluminescence of Yb3+ redshifts with increasing pressure, attributed to enhanced Yb3+ f-orbital covalency leading to an increased sensitivity to Br- spin-orbit coupling at high pressure. An increase in the energy difference between Yb3+ up and down (pseudo)spin ground states is observed with increasing pressure, reflecting enhanced Yb3+-Cr3+ magnetic exchange coupling because of the shortened metal-halide bond lengths. A decrease in TC of CrBr3 with increasing pressure counteracts this effect. Exchange splitting of the Yb3+ ground state is still observed even above the magnetic ordering temperature of CrBr3 due to non-cancelling short-range Yb3+-Cr3+ exchange interactions. Analysis reveals a discontinuity in the pressure dependence of the Yb3+-Cr3+ magnetic exchange coupling strength (JYb-Cr) at -- 3.5 GPa, attributed to a loss of magnetic correlation and a reduction of TC under pressure. These results highlight the versatility of Yb3+ as an optical probe of bonding and magnetism in CrBr3, and more generally as a designer defect for spin-photonic functionalization in this class of 2D van der Waals magnets.

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