Structural, magnetic, and optical properties tuning in [HPPh3]2[Zn1-xMnxBr4] solid solutions

A series of [HPPh₃]₂[Zn_1-xMn_xBr₄] solid solutions (where HPPh₃ is a triphenylphosphonium cation, 0 ≤ x ≤ 1) was obtained. These solid solutions crystallize in the orthorhombic Pbca space group. The unit cell volume and metal-halide distances obey Vegard's law, confirming the formation of a substitutional solid solution. Electron paramagnetic resonance (EPR) spectroscopy shows a concentration dependence of the spin-Hamiltonian parameters. Decreasing the Mn(II) content leads to a reduction in the zero-field splitting value from $D$ = 2950 MHz to practically zero at low concentrations. This results in a resolved manganese hyperfine structure with $A$ (⁵⁵Mn) = 265 MHz. The isotropic $g$-tensor value exhibits a similar decrease, from 2.01 to 1.99. Minor Mn(II) doping significantly alters the luminescence properties, shifting them toward those of the pure Mn(II) compound. The photoluminescence quantum yields exhibit an S-type dependence on Mn(II) concentration, increasing from 4 % to 86 % under 275 nm excitation and from 0 to 58 % under 455 nm excitations. The bandgap energy demonstrates a non-monotonic dependence on manganese(II) content, reaching its maximum value at 20 % Mn doping. These findings highlight the tunability of the structural, magnetic, and optical properties in these mixed-metal zinc(II)-manganese(II) solid solutions, which are extremely sensitive to even slight distortions in the metal ion environment. The observed effects are consistent with crystal field theory.