Precursor stoichiometry controls dopant structure, defect chemistry, and valence in Mn-doped ZnGa2O4 nanocrystals
Abstract
The luminescence of doped nanocrystals is usually optimised through the activator; here we show that the host cation ratio is an equally decisive, and largely overlooked, design variable. Mn-doped ZnGa2O4 nanocrystals (Mn:Zn = 4×10−4) were grown hydrothermally at nominal Zn/Ga ratios between 0.30 and 0.75 (stoichiometric: 0.50) and post-annealed at 400--1100∘C, and the resulting composition--temperature matrix was mapped by X-ray- and UV-excited luminescence, EPR, Rietveld-refined X-ray diffraction, Raman spectroscopy and SEM. The as-grown crystals are phase-pure spinel at every composition and accommodate their off-stoichiometry internally as a point-defect inventory, reflected in a 0.4\% variation of the lattice parameter; on annealing, the excess gallium is exsolved as β-Ga2O3 in amounts set by the solution composition. The Mn2+ emission separates into two optima, near-stoichiometric crystals annealed at 400--600∘C and Zn-excess crystals annealed at 1100∘C, the latter being twice as bright under X-ray excitation. Under X-ray and 365~nm excitation the two optima are separated by a minimum at 800∘C that coincides with a collapse of the Mn2+ EPR population and has no counterpart in the Raman linewidths, identifying transient, defect-compensated oxidation of the dopant rather than lattice degradation as its origin. The most Zn-rich composition shows neither Mn2+ nor Mn4+ emission at any annealing stage, whereas its trace Cr3+ emission is strong, indicating that Mn is rejected already during growth. Solution stoichiometry therefore acts on two levels, dopant capture and native-defect inventory, while annealing controls dopant valence and lattice order. The brightest high-temperature phosphor is thus not the nominally stoichiometric one, but the composition whose excess is consumed on its way to stoichiometry.
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BibTeX
@misc{rudolf2026precursor,
title = {{Precursor stoichiometry controls dopant structure, defect chemistry, and valence in Mn-doped ZnGa$_2$O$_4$ nanocrystals}},
author = {Mátyás M. Rudolf and Gábor Bortel and Nikoletta Jegenyés and Árpád Jakab and László Bencs and Ferenc Simon and Adam Gali and Dávid Beke},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.14195},
eprint = {2610.14195},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.14195},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Precursor stoichiometry controls dopant structure, defect chemistry, and valence in Mn-doped ZnGa_2O_4 nanocrystals
AU - Mátyás M. Rudolf
AU - Gábor Bortel
AU - Nikoletta Jegenyés
AU - Árpád Jakab
AU - László Bencs
AU - Ferenc Simon
AU - Adam Gali
AU - Dávid Beke
PY - 2026
DA - 2026/10/09
PB - arXiv
DO - 10.48550/arxiv.2610.14195
UR - https://arxiv.org/abs/2610.14195
AB - The luminescence of doped nanocrystals is usually optimised through the activator; here we show that the host cation ratio is an equally decisive, and largely overlooked, design variable. Mn-doped ZnGa_2O_4 nanocrystals (Mn:Zn = 4×10^−4) were grown hydrothermally at nominal Zn/Ga ratios between 0.30 and 0.75 (stoichiometric: 0.50) and post-annealed at 400--1100^∘C, and the resulting composition--temperature matrix was mapped by X-ray- and UV-excited luminescence, EPR, Rietveld-refined X-ray diffraction, Raman spectroscopy and SEM. The as-grown crystals are phase-pure spinel at every composition and accommodate their off-stoichiometry internally as a point-defect inventory, reflected in a 0.4\% variation of the lattice parameter; on annealing, the excess gallium is exsolved as β-Ga_2O_3 in amounts set by the solution composition. The Mn^2+ emission separates into two optima, near-stoichiometric crystals annealed at 400--600^∘C and Zn-excess crystals annealed at 1100^∘C, the latter being twice as bright under X-ray excitation. Under X-ray and 365~nm excitation the two optima are separated by a minimum at 800^∘C that coincides with a collapse of the Mn^2+ EPR population and has no counterpart in the Raman linewidths, identifying transient, defect-compensated oxidation of the dopant rather than lattice degradation as its origin. The most Zn-rich composition shows neither Mn^2+ nor Mn^4+ emission at any annealing stage, whereas its trace Cr^3+ emission is strong, indicating that Mn is rejected already during growth. Solution stoichiometry therefore acts on two levels, dopant capture and native-defect inventory, while annealing controls dopant valence and lattice order. The brightest high-temperature phosphor is thus not the nominally stoichiometric one, but the composition whose excess is consumed on its way to stoichiometry.
ER -