Verneuil-grown SrTiO3_3 single crystals produced from a starting powder with Sr/Ti=1.04Sr/Ti = 1.04 deform plastically to more than 10% strain at room temperature even at a strain rate of 10−1 s−110^{-1}\,\mathrm{s^{-1}}, a behavior attributed to fewer point defects and hence higher dislocation mobility than in crystals grown from stoichiometric powder. This attribution, however, rests entirely on inference: the actual atomic Sr/TiSr/Ti ratios and the concentrations and species of point defects (Sr vacancies, Ti antisites, oxygen vacancies) in the as-grown crystals have never been measured; the fraction of mobile dislocations entering the Orowan strain-rate balance is unknown, leaving the dislocation stress exponent only bracketed; the defect species that actually pin moving dislocations and their interaction energies with dislocations are unidentified; and whether Sr/Ti=1.04Sr/Ti = 1.04 is the optimal starting composition is untested. The open problem is to close this processing →\to point-defect →\to dislocation-mobility →\to plasticity chain quantitatively. Later work (2020–2026) via defect-chemistry engineering (reduction treatments and Nb/Fe doping) has established species-level trends — Sr vacancies strongly hinder dislocation motion while oxygen vacancies favor dislocation nucleation — but no study has measured the defect populations of composition-controlled crystals or quantified defect–dislocation interaction energies, and no composition optimization has been performed, so the source question remains open.

Background

Strontium titanate (SrTiO3_3) is a perovskite oxide (a cubic crystal structure with Sr at cube corners, Ti at the body center, and oxygen at face centers) that has become the model system for an unexpected phenomenon: dislocation-mediated plasticity of a brittle ceramic at room temperature. Plastic flow of crystals is carried by dislocations (line defects whose motion constitutes slip); in most ceramics the lattice resistance to dislocation glide is so high at ambient temperature that cracks form before any appreciable plastic strain. SrTiO3_3 was the first perovskite oxide reported (2001) to show macroscopic plastic deformation under uniaxial compression at room temperature, on the {110}⟨110⟩\{110\}\langle 110\rangle slip systems, and the related perovskites KNbO3_3 and KTaO3_3 were later found to behave similarly (Frisch et al., 2025). At room temperature, dislocation motion in SrTiO3_3 proceeds by the kink-pair mechanism over the Peierls barrier (the periodic lattice resistance to glide), so the flow stress is governed by how easily dislocations nucleate, multiply, and glide. Point defects — here Sr vacancies (VSrV_{Sr}), Ti antisites on Sr sites, and oxygen vacancies (VOV_O) — interact elastically and electrostatically with dislocation cores and can form Cottrell atmospheres (clouds of solutes dragged by or pinning a dislocation), acting as obstacles that harden the crystal and promote early fracture. In SrTiO3_3 this is not a hypothetical concern: Sr and O vacancies have been imaged at and near dislocation cores, Sr vacancies have particularly low formation energies, and Sr is lost by evaporation at the solidification front during crystal growth, so Verneuil-grown crystals (grown by melting a powder feed through an oxyhydrogen flame onto a rotating seed) are intrinsically Sr-deficient. Nakamura et al. (2017) exploited this: comparing Verneuil crystals grown from powders with Sr/Ti=1.00Sr/Ti = 1.00 and Sr/Ti=1.04Sr/Ti = 1.04 (compressed along [001][001] at 297 K at strain rates from 10−510^{-5} to 10−1 s−110^{-1}\,\mathrm{s^{-1}}), they found markedly lower flow stress, larger fracture strains, and almost strain-rate-independent flow in the Sr/Ti=1.04Sr/Ti = 1.04 crystals, with nearly identical dislocation substructures (∼1013 m−2\sim 10^{13}\,\mathrm{m^{-2}} by TEM). Interpreting this through the Orowan relation γ˙=ρmbvˉ\dot\gamma = \rho_m b \bar{v} (shear strain rate equals mobile dislocation density times Burgers vector times mean dislocation velocity) and the power law vˉ∝τm\bar{v} \propto \tau^m (with mm the stress exponent), they attributed the high mobility of dislocations in the Sr/Ti=1.04Sr/Ti = 1.04 crystals to fewer point defects, speculating that excess Sr in the feed compensates the growth-induced Sr deficiency. Subsequent defect-chemistry studies established that the point-defect population does control room-temperature dislocation behavior: increasing the oxygen-vacancy concentration by reduction favors dislocation nucleation but impedes dislocation motion (Stich et al., 2021), vacancy concentration can even reverse the relative yield strength of SrTiO3_3 between the nano- and microscale (Fang et al., 2020, Scripta Materialia), theory predicts oxygen vacancies lower the Peierls stress via local misfit-energy remodeling (Li et al., 2022), and a length-scale-bridging comparison of Nb-doped (Sr-vacancy compensated) versus Fe-doped (oxygen-vacancy compensated) crystals showed Sr vacancies strongly suppress dislocation motion while oxygen vacancies promote incipient plasticity (Okafor et al., 2026). What no later study has done is return to the original composition-controlled crystals and measure the defects directly.

Problem Statement

For SrTiO3_3 single crystals whose room-temperature plasticity is controlled by the growth composition, establish quantitatively the causal chain from starting-powder Sr/TiSr/Ti ratio to point-defect population to dislocation mobility to macroscopic plasticity, as posed by the open issues of Nakamura et al. (2017). The single overarching objective — a closed, quantitative processing-to-plasticity account — requires four jointly linked results: (1) direct measurement of the actual atomic Sr/TiSr/Ti ratios and of the concentrations and species of point defects (Sr vacancies, Ti antisites on Sr sites, oxygen vacancies) in as-grown crystals produced from controlled starting compositions, so that the assumed link between feed composition and in-crystal defect populations becomes a measured one; (2) determination of the density (or fraction) of mobile dislocations during room-temperature deformation entering the Orowan balance γ˙=ρmbvˉ\dot\gamma = \rho_m b \bar{v}, so that the dislocation velocity-stress relation and stress exponent mm are fixed rather than bracketed by hypotheses; (3) identification of which point-defect species actually pin moving dislocations and how their interaction with dislocations affects dislocation mobility, addressing the gap noted in the source that a direct, quantitative identification of the pinning species and of their interaction energies with dislocations is lacking for the composition-controlled crystals, the 2020–2026 defect-chemistry studies having established the species-level trends only for doped or reduced crystals; and (4) determination of whether the starting composition Sr/Ti=1.04Sr/Ti = 1.04 is optimal for maximum room-temperature plasticity — taken as the largest plastic (fracture) strain attainable at a fixed strain rate — over starting Sr/TiSr/Ti compositions and growth conditions, or identification of the composition that is. Each of the four results is an independently refutable sub-claim, and establishing any subset of them advances but does not by itself close the account: the objective above is met only when all four are established jointly for composition-controlled crystals. As an explicitly secondary extension, the source also asks whether the same composition/point-defect control enhances room-temperature plasticity in other oxide materials; answering the SrTiO3_3 core above is complete without it, but a demonstrated transfer of the defect-population-to-mobility relationship to a second oxide chemistry would substantially strengthen the account.

References

  1. Atsutomo Nakamura, Kensuke Yasufuku, Yuho Furushima, Kazuaki Toyoura, K. Peter D. Lagerlöf, Katsuyuki Matsunaga, 'Room-Temperature Plastic Deformation of Strontium Titanate Crystals Grown from Different Chemical Compositions', Crystals 7(11), 351 (2017), DOI: 10.3390/cryst7110351, https://doi.org/10.3390/cryst7110351
  2. Xufei Fang, Kuan Ding, Stephan Janocha, Christian Minnert, Wolfgang Rheinheimer, Till Frömling, Karsten Durst, Atsutomo Nakamura, Jürgen Rödel, 'Nanoscale to microscale reversal in room-temperature plasticity in SrTiO3 by tuning defect concentration', Scripta Materialia 188, 228-232 (2020), DOI: 10.1016/j.scriptamat.2020.07.033, https://doi.org/10.1016/j.scriptamat.2020.07.033
  3. Xufei Fang, Lukas Porz, Kuan Ding, Atsutomo Nakamura, 'Bridging the Gap between Bulk Compression and Indentation Test on Room-Temperature Plasticity in Oxides: Case Study on SrTiO3', Crystals 10(10), 933 (2020), DOI: 10.3390/cryst10100933, https://doi.org/10.3390/cryst10100933
  4. Stephan Stich, Kuan Ding, Qaisar Khushi Muhammad, Lukas Porz, Christian Minnert, Wolfgang Rheinheimer, Karsten Durst, Jürgen Rödel, Till Frömling, Xufei Fang, 'Room-temperature dislocation plasticity in SrTiO3 tuned by defect chemistry', Journal of the American Ceramic Society 105(2), 1318-1329 (2021), DOI: 10.1111/jace.18118, https://doi.org/10.1111/jace.18118
  5. Yi Li, Xiangyang Liu, Peng Zhang, Yi Han, Muzhang Huang, Chunlei Wan, 'Theoretical insights into the Peierls plasticity in SrTiO3 ceramics via dislocation remodelling', Nature Communications 13, 6925 (2022), DOI: 10.1038/s41467-022-34741-4, https://doi.org/10.1038/s41467-022-34741-4
  6. Xufei Fang, Wenjun Lu, Jiawen Zhang, Christian Minnert, Junhua Hou, Sebastian Bruns, Ulrike Kunz, Atsutomo Nakamura, Karsten Durst, Jürgen Rödel, 'Harvesting room-temperature plasticity in ceramics by mechanically seeded dislocations', Materials Today 82, 81-91 (2025; online 2024), DOI: 10.1016/j.mattod.2024.11.014, https://doi.org/10.1016/j.mattod.2024.11.014
  7. Alexander Frisch, Chukwudalu Okafor, Oliver Preuß, Jiawen Zhang, Katsuyuki Matsunaga, Atsutomo Nakamura, Wenjun Lu, Xufei Fang, 'Room-temperature dislocation plasticity in ceramics: Methods, materials, and mechanisms', Journal of the American Ceramic Society 108(9), e20575 (2025), DOI: 10.1111/jace.20575, arXiv:2504.04581, https://doi.org/10.1111/jace.20575
  8. Chukwudalu Okafor, Kohei Takahara, Svetlana Korneychuk, Isabel Huck, Sebastian Bruns, Ruoqi Li, Yan Li, Karsten Durst, Atsutomo Nakamura, Xufei Fang, 'Dislocation-point defect interaction on plasticity across the length scale in SrTiO3', Journal of Materiomics 12(4), 101232 (2026; online 2025), DOI: 10.1016/j.jmat.2026.101232, arXiv:2510.18620, https://doi.org/10.1016/j.jmat.2026.101232