Speaker
Description
Atomic size mismatch in oxide crystals has long been treated as a barrier, linked to phase instability, dopant segregation, and uncontrolled defect formation during melt growth. This project overturns that limitation by establishing ionic size mismatch and melt nonstoichiometry as controllable variables that define phase structure and functional properties in rare-earth aluminate high-entropy oxides. The central problem addressed is the absence of predictive composition-structure-property relationships in compositionally complex oxides, where conventional doping models fail and equilibrium phase diagrams provide no guidance for the multiphase and non-stoichiometric regimes that deliver the highest performance.
The work introduces a unified strategy based on three independently controlled mismatch parameters: (i) ionic-radius incompatibility on shared lattice sites, (ii) growth-rate-controlled phase partitioning, and (iii) melt-to-crystal stoichiometric deviation. These variables are accessed through micro-pulling-down ($\mu$-PD) crystal growth, directional solidification, and controlled melt chemistry, combined with synchrotron spectroscopy, electron paramagnetic resonance, thermoluminescence, and temperature-dependent luminescence. The $\mu$-PD method enables rapid exploration of off-equilibrium compositions and direct mapping of phase evolution beyond the limits of conventional Czochralski growth.
This approach yields direct control over phase architecture and functionality. In Ce$^{3+}$-doped YAG-YAP eutectics, growth rate defines lamellar spacing and dopant partitioning, enabling control of light propagation through the material by adjusting the microstructure. Large domains transmit blue light, while finer structures enhance scattering, enabling tuning of emission characteristics for high-power white laser diode operation. The same dual-phase structure enables ratiometric thermometry, with relative sensitivity up to $1.1\%~\mathrm{K}^{-1}$ under X-ray excitation, demonstrating self-activated optical temperature sensing under ionizing radiation.
A key advance is the use of strong cation size mismatch to engineer defect distributions and energy transfer. In Pr$^{3+}$-doped Lu$_3$(Al,Sc)$_5$O$_{12}$, Sc$^{3+}$ substitution introduces local lattice distortion that suppresses segregation, increases compositional uniformity, and localizes excitons. This directly enhances energy transfer to Pr$^{3+}$ centers, producing a six-fold increase in scintillation light yield ($11{,}200~\mathrm{ph/MeV}$). At higher Sc content, a critical threshold ($x \approx 1.5$) is reached where the garnet lattice destabilizes, generating a hypoeutectic garnet-perovskite structure. This transition arises from site instability under extreme size mismatch and produces a distinct defect configuration that further improves charge-carrier dynamics. At even higher mismatch, ferroelastic inclusions form that undergo symmetry changes under neutron irradiation, indicating potential for selective neutron radiation memory applications.
Melt nonstoichiometry provides an additional means of controlling phase composition and performance. In Ce$^{3+}$-doped (Tb,Y)$_3$Al$_5$O$_{12}$, moderate Y$_2$O$_3$ deficiency induces a garnet-Al$_2$O$_3$ dual-phase structure with improved thermal stability and luminous efficacy up to $158~\mathrm{lm/W}$, suitable for high-power white lighting based on laser diode excitation. At higher deficiency, a garnet--perovskite structure forms, increasing scintillation output by up to $80\%$ relative to the standard single-phase Y$_3$Al$_5$O$_{12}$:Ce scintillator through modified rare-earth partitioning and energy transfer.
The outcome is a predictive Crystal Phase Engineering framework in which mismatch is used to control phase formation, defect structure, and carrier dynamics from the atomic to the microstructural scale. This enables targeted design of materials with combined optical and radiation-detection properties. The resulting materials provide a pathway toward next-generation scintillators, high-power phosphors, self-activated thermometers operating in ionizing radiation fields, and ferroelastic materials for neutron-responsive memory in nuclear and space technologies.