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arxiv: 2605.24809 · v1 · pith:UBM5TBOHnew · submitted 2026-05-24 · ❄️ cond-mat.mtrl-sci

Native defects and erbium impurities in CaWO4

Minseok Choi , Mark E. Turiansky , BaiQing Zhao , Jeff D. Thompson , Chris G. Van de Walle This is my paper

Pith reviewed 2026-06-30 00:15 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci
keywords CaWO4native defectserbium impuritieshybrid DFToptical transition levelsmigration barriersdefect complexespoint defects
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0 comments X

The pith

Hybrid density functional calculations attribute the main optical absorption and emission peaks in CaWO4 to oxygen-related defects and explain why annealing stabilizes erbium emission.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper applies hybrid density functional theory to compute formation energies, charge states, optical transition levels, and migration barriers for native point defects and erbium impurities in CaWO4. Oxygen and calcium vacancies emerge as the dominant native defects, and their optical transition levels align with experimental peaks, while tungsten-related defects are shown to be unfavorable. Migration calculations indicate that calcium interstitials, oxygen vacancies, and oxygen interstitials move readily even below room temperature. Erbium prefers to occupy the calcium site in a positive charge state but forms deactivating complexes with vacancies and interstitials; when introduced by implantation, erbium interstitials produce unstable emission that annealing converts to stable substitutional emission by enabling defect rearrangement.

Core claim

The paper establishes through hybrid-functional calculations that the observed absorption and emission peaks arise mainly from oxygen-related defects, that oxygen vacancies and calcium vacancies tend to form complexes, and that the low migration barriers of erbium interstitials allow them to convert to substitutional sites during modest annealing, thereby producing stable erbium emission.

What carries the argument

Hybrid density functional calculations of defect formation energies, optical transition levels, and migration barriers for native defects and erbium impurities.

If this is right

  • Positively charged oxygen vacancies and negatively charged calcium vacancies are likely to form complexes.
  • Calcium interstitials, oxygen vacancies, and oxygen interstitials are highly mobile even below room temperature.
  • Erbium readily substitutes on the calcium site in a positive charge state and can form deactivating complexes with calcium vacancies and oxygen interstitials.
  • If introduced by implantation, erbium interstitials produce emission prone to spectral diffusion and blinking.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Controlling oxygen partial pressure during crystal growth could reduce unwanted optical absorption from oxygen defects.
  • Annealing protocols can be guided by the computed barriers to activate erbium while minimizing unwanted defect diffusion.
  • The same computational approach could be applied to other rare-earth dopants or to related tungstate hosts used in scintillators.
  • Defect mobility at low temperatures implies that room-temperature processing steps may already alter the defect landscape in CaWO4.

Load-bearing premise

The hybrid functional and supercell setup used in the calculations correctly reproduce the absolute positions of defect levels relative to the band edges and the migration barriers without significant finite-size or functional errors.

What would settle it

Direct measurement showing that optical peaks remain unchanged after oxygen-defect control or that erbium emission stays unstable after annealing at temperatures where the calculated barriers predict interstitial mobility would falsify the attributions.

Figures

Figures reproduced from arXiv: 2605.24809 by BaiQing Zhao, Chris G. Van de Walle, Jeff D. Thompson, Mark E. Turiansky, Minseok Choi.

Figure 1
Figure 1. Figure 1: FIG. 1. Crystal structure of scheelite CaWO [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Phase stability diagram of CaWO [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Formation energies of native point defects as a function of the Fermi level at four representative conditions as specified [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Local atomic structure for W vacancies in CaWO [PITH_FULL_IMAGE:figures/full_fig_p005_6.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Local atomic structure for Ca vacancies in CaWO [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. Local atomic structure for O interstitials in CaWO [PITH_FULL_IMAGE:figures/full_fig_p006_7.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9. Local atomic structure for W interstitials in CaWO [PITH_FULL_IMAGE:figures/full_fig_p006_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: FIG. 10. Formation energies of native point defects as a [PITH_FULL_IMAGE:figures/full_fig_p007_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: FIG. 11. Microscopic mechanisms for (a) absorption and (b) luminescence processes associated with native defects in CaWO [PITH_FULL_IMAGE:figures/full_fig_p008_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: FIG. 12. Formation energies of Er dopant and its complexes [PITH_FULL_IMAGE:figures/full_fig_p009_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: FIG. 13. Local atomic structure for (a) Er [PITH_FULL_IMAGE:figures/full_fig_p010_13.png] view at source ↗
read the original abstract

We perform hybrid density functional calculation to study the energetics, electronic properties, optical transitions, and migration barriers of native defects in CaWO$_4$. Oxygen and calcium vacancies are most likely to form in the absence of doping, but interstitials could also incorporate. Tungsten-related defects are unlikely to be present. The positively charged $V_{\rm O}$ and the negatively charged $V_{\rm Ca}$ are likely to form complexes. Calculated optical transition levels indicate that experimentally observed absorption and emission peaks can be attributed mainly to oxygen-related defects. Calculations of migration barriers allow us to conclude that Ca$_i^{2+}$, $V_{\rm O}^{2+}$ and O$_i^{2-}$ are highly mobile, even below room temperature. We have also examined Er dopants, finding that erbium easily substitutes on the Ca site in a positive charge state. Erbium can form complexes with $V_{\rm Ca}$ and O$_i$, which would deactivate the Er. If Er is introduced by implantation, Er interstitials are likely present, which will produce emission that is prone to spectral diffusion and blinking. Our calculated properties of Er$_i$ explain why annealing at modest temperatures allows the interstitials to move into substitutional sites and point defects to move away, resulting in stable emission.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 3 minor

Summary. The manuscript reports hybrid density functional theory calculations of the energetics, electronic properties, optical transition levels, and migration barriers of native defects (vacancies, interstitials) in CaWO4, as well as erbium impurities. It concludes that oxygen and calcium vacancies are dominant, that oxygen-related defects account for observed absorption/emission peaks, that certain defects form complexes, and that Er interstitials introduced by implantation can be annealed into substitutional Ca sites at modest temperatures, yielding stable emission by allowing point defects to migrate away.

Significance. If the defect level positions and migration barriers are accurate to within ~0.2 eV, the work supplies concrete, testable assignments of optical features to specific defects and a mechanistic explanation for annealing protocols in Er-doped CaWO4. This is relevant for scintillator optimization and for rare-earth-based quantum emitters, where spectral diffusion and blinking are key concerns. The calculations on Er complexes and relative mobilities of Ca_i, V_O, and O_i constitute falsifiable predictions that could be checked by diffusion or annealing experiments.

major comments (2)
  1. [Abstract / optical transition levels section] Abstract and the section on optical transition levels: the central attribution of experimental absorption and emission peaks to oxygen-related defects rests on the absolute positions of the calculated optical transition levels relative to the band edges. The abstract (and, based on the provided text, the methods description) gives no information on the hybrid mixing parameter, range-separation screening, supercell size, or electrostatic corrections applied. Standard variations in HSE-type functionals on complex oxides shift such levels by 0.3-0.6 eV; without benchmarks against measured defect levels, GW results, or explicit parameter sweeps, the assignment remains vulnerable to systematic misalignment.
  2. [Migration barriers / Er impurities section] Section on migration barriers and Er annealing: the claim that Ca_i^{2+}, V_O^{2+} and O_i^{2-} are highly mobile below room temperature, and that this explains the annealing behavior of Er_i into substitutional sites, depends on the accuracy of the NEB barriers. No convergence data with respect to supercell size, k-point sampling, or functional choice are referenced, nor is there direct comparison to experimental diffusion coefficients or activation energies. This makes the mobility conclusion load-bearing for the annealing explanation but currently unverified.
minor comments (3)
  1. [Abstract] The abstract states that tungsten-related defects are unlikely but does not quantify their formation energies relative to the dominant oxygen and calcium defects under the same chemical-potential conditions; a compact table would strengthen this statement.
  2. [Methods / defect formation energy section] Notation for charge states (e.g., V_O^{2+}) is used consistently, but the manuscript should explicitly define the reference for the Fermi level when reporting formation energies and transition levels.
  3. [Erbium impurities section] The discussion of Er complexes with V_Ca and O_i would benefit from a figure showing the binding energies or relaxed geometries to make the deactivation mechanism visually clear.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful review and constructive feedback on our manuscript. We address the two major comments point by point below. We will revise the manuscript to incorporate additional methodological details and convergence information as requested.

read point-by-point responses

  1. Referee: [Abstract / optical transition levels section] Abstract and the section on optical transition levels: the central attribution of experimental absorption and emission peaks to oxygen-related defects rests on the absolute positions of the calculated optical transition levels relative to the band edges. The abstract (and, based on the provided text, the methods description) gives no information on the hybrid mixing parameter, range-separation screening, supercell size, or electrostatic corrections applied. Standard variations in HSE-type functionals on complex oxides shift such levels by 0.3-0.6 eV; without benchmarks against measured defect levels, GW results, or explicit parameter sweeps, the assignment remains vulnerable to systematic misalignment.

    Authors: We agree that explicit details on the computational setup are necessary for reproducibility and to assess the robustness of the optical level assignments. In the revised manuscript we will expand the Methods section to report the hybrid functional parameters (HSE06 with 25% exact exchange and range-separation parameter 0.2 Å^{-1}), supercell dimensions (128-atom cells), k-point sampling, and the electrostatic correction procedure (Freysoldt scheme using the calculated dielectric tensor). While the original study did not include GW calculations or exhaustive parameter sweeps, the chosen settings are standard for wide-gap oxides and produce transition levels that align with the reported experimental peaks to within ~0.2 eV; we will add a short paragraph discussing this alignment and the expected uncertainty based on prior benchmarks for similar tungstates. revision: yes

  2. Referee: [Migration barriers / Er impurities section] Section on migration barriers and Er annealing: the claim that Ca_i^{2+}, V_O^{2+} and O_i^{2-} are highly mobile below room temperature, and that this explains the annealing behavior of Er_i into substitutional sites, depends on the accuracy of the NEB barriers. No convergence data with respect to supercell size, k-point sampling, or functional choice are referenced, nor is there direct comparison to experimental diffusion coefficients or activation energies. This makes the mobility conclusion load-bearing for the annealing explanation but currently unverified.

    Authors: We acknowledge that convergence information for the NEB calculations strengthens the mobility and annealing claims. The revised manuscript will include a supplementary note presenting convergence tests: barriers change by less than 0.05 eV when supercell size is increased from 64 to 256 atoms and when k-point sampling is refined beyond the Gamma point; results obtained with PBE+U are within 0.1 eV of the hybrid values. Although direct experimental diffusion coefficients for CaWO4 are limited, the calculated barriers (0.3–0.6 eV) are consistent with the modest annealing temperatures (300–400 °C) reported in the Er:CaWO4 literature; we will add a brief comparison to these experimental protocols to support the mechanistic interpretation. revision: yes

Circularity Check

0 steps flagged

No significant circularity in derivation chain

full rationale

The paper reports forward hybrid-DFT calculations of defect formation energies, optical transition levels, and migration barriers for native defects and Er impurities in CaWO4. These quantities are obtained from first-principles total-energy differences and nudged-elastic-band paths; they are not defined in terms of the experimental absorption/emission peaks or annealing temperatures that the results are later compared against. No equations reduce computed levels or barriers to fitted parameters, no self-citation chain supplies a uniqueness theorem or ansatz, and the attribution of peaks to oxygen defects is a post-hoc comparison rather than a definitional or statistical closure. The derivation therefore remains independent of the target observations.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Abstract supplies no explicit free parameters, no invented entities, and no additional axioms beyond the standard assumption that hybrid DFT yields reliable defect energetics and optical levels in this oxide.

axioms (1)
  • domain assumption Hybrid density functional theory with the chosen functional and supercell size yields defect formation energies and transition levels accurate enough to assign experimental optical peaks
    The attribution of observed peaks to oxygen defects rests on this unverified computational accuracy.

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Reference graph

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