Steady-state and time-resolved photoluminescence measurements were performed on an Edinburgh Instruments FLS1000 luminescence spectrometer equipped with a 450 W Xe lamp, double Czerny-Turner monochromators in the excitation and emission compartment (2 x 325 mm focal length) with ruled gratings blazed at 400 nm (excitation) and 500 nm (emission) and a thermoelectrically cooled (-20 °C) PMT-980 single-photon counting detector. The emission spectra were corrected with respect to the grating efficiency and PMT sensitivity For time-resolved measurements, VPL-450 (λex = 450.9 nm, avg. peak power = 53.0 mW) and VPL-510 lasers (λex = 515.0 nm, avg. peak power = 12.0 mW, both lasers from Edinburgh Instruments) with adjustable temporal pulse width (0.1 μs to 1 ms) and variable repetition rate The as-synthesized Gd3In2Ga3O12 was found to crystallize in the cubic garnet-type structure with space group 𝐼𝑎3 ̅ 𝑑 (# 230), which could be confirmed via Rietveld analysis of the XRD patterns and is in agreement with previously reported in the literature . A schematic representation of the garnet-type structure of Gd3In2Ga3O12 together with a representation of the [GdO8], [InO6] and [GaO4] units is shown in Figure -b, while the corresponding refined diffraction pattern is depicted in Figure -a. All RE-activated materials were found to be isostructural to the parent compound. Figure S 1 in the Supplementary Material illustrates the Rietveld refinements of powdered Gd3In2Ga3O12: RE 3+ (RE: Nd and Ho) samples with different varying activator concentrations and with lattice parameters a ranging from 12.67847(4) Å to 12.64176( ) Å. The lattice parameter a and the unit cell volume V of undoped and doped samples were determined accordingly and are summarized in Table . The lattice parameters decrease or increase approximately linearly according to the activator content x (see Figure and). The slopes of the curves, da/dx, closely correspond to the difference in ionic radii between Gd 3+ (r8-c= 1.053 Å) and the respective Nd 3+ (r8-c= 1.109 Å) and Ho 3+ (r8-c= 1.015 Å) cations (r(Gd CN=8 3+ )r(RE CN=8 3+ )), as shown in Table 1, which indicates substitutive replacement of Gd 3+ by the different species. Except for decreasing or increasing of lattice parameters, no significant alterations in the crystal structure were indicated according to the Rietveld analysis. Moreover, no crystalline impurities were identified in any of the doped samples. Hence, we conclude that the formation of a pure Gd3In2Ga3O12 phase via solid-state reaction is achievable, and the RE 3+ cations (Nd 3+ and Ho 3+ ) are successfully incorporated into the slightly distorted dodecahedral coordinated A site . ) Crystal structure of Gd3In2Ga3O12 derived from Rietveld refinement of the powder XRD using VESTA software (c) Powder XRD patterns of Gd3In2Ga3O12:RE 3+ (RE = Nd and Ho) of crushed single crystals. Variation of lattice parameters (a, and V) with different activator concentrations of (d) Gd3In2Ga3O12:Nd 3+ and (e) Gd3In2Ga3O12:Ho 3+ obtained from Rietveld refinements