Governing principles of hydration of mixed proton conducting Co-based double perovskites

Domain: biology
Published: 2026-03-23
Editorial outcome: major_revision

Abstract
Proton ceramic electrochemical cells PCECs hold promise for efficient, sustainable production and use of
hydrogen. The positive electrodes are mixed proton conducting perovskites that facilitate water splitting and
oxygen reduction, but the factors that determine the protonation are poorly understood. Here, we establish the
governing principles of protonation through a study of hydration of 45 double perovskites with the general
formula 𝐴I𝐴IICo O , having Ba or Ba+Sr on AI and a mix of rare earths (Y and lanthanides Ln = La, Pr, Nd, Sm,
2 6−𝛿
Gd, Dy, Tb, Lu) on AII. We show how hydration is coupled to the A-site basicity and disorder as well as population
of electron holes in the Co-O bond (Co oxidation state), promoted by a closed or semi-closed Ln 4f shell, i.e., Ln =
La, Gd, Lu.

Introduction
There is an increasing interest in the hydration of mixed proton electron conductors (MPECs) as positive
electrodes (positrodes) of proton ceramic electrochemical cells (PCECs)1–10, but quantification of the minor proton
concentrations, as well as description and experimental verification of how the local chemical and electronic
structure affects hydration, are ambiguous. Atomistic modelling shows that proton and hydroxide affinities are
related to the ionicity of the cation-oxygen bonds and that proton affinity dominates over hydroxide affinity in
hydration thermodynamics11. Ionic bonds with complete cation-to-oxygen electron transfer are thus pivotal for
hydration and significant proton concentrations.

In this study, we experimentally show how the A-site cation influences the charge transfer in double perovskites
with AI and AII ordering and how the A-site composition is directly linked to proton stability in the structures. We
show how lanthanides (Ln) with closed (La or Lu) or semi-closed (Gd) 4f shells situated on the AII site influence
electron states in the Co-O bond and – through an extensive experimental matrix – how these particular
compositions promote hydration. Furthermore, our results reveal how hydration may be over- or underestimated
through additional oxidation or reduction as the crystal structure equilibrates during the hydration process.

Hydration of Ba-based mixed conducting transition metal (TM) perovskites has been investigated in a series of
studies, focusing on how the electron structure of the TM-O bond influences hydration12–17. There has been focus
on cation acidity and basicity, repulsion between protons and electron holes, and how materials with electron
holes as dominating positive defects are reduced through what becomes hydrogenation12. Moreover, if an
electron hole is partly or fully transferred from the TM to oxygen – so-called negative charge transfer – proton
stability decreases. Trends of negative charge transfer versus TM-O bond lengths and TM-O-TM angles have been
established using Density Functional Theory (DFT) calculations and X-ray Absorption Spectroscopy13–15, concluding
that longer TM-O bonds and increasing deviation from TM-O-TM = 180° increase the ionic character of perovskites
and thus oxide ion basicity and, in turn, hydration.

Many Fe- and Co-based perovskites exhibit considerable hydration2,4,6,18. In this work, hydration has been
systematically studied in 45 different AIAIICo O double perovskites with mainly Ba on AI, a range of rare earths
2 6-δ
and lanthanides on AII, and additional substitutions of Sr or lanthanides on the A| site and Fe or Ti for Co on the B
site. Figure 1 shows an orthorhombic a2b2c Pmmm unit cell of BaGd La Co O (BGLC82) with Ba in AI, Gd and
0.8 0.2 2 6-δ
La in AII, Co in B, and with seven oxygen sites refined from synchrotron radiation powder X-ray diffraction (SR-
PXD) and Neutron Powder Diffraction (NPD)19. Common for all compositions is that oxygen vacancies locate and
tend to order to and within the Ln layer, resulting in a doubled b-axis and orthorhombic structure19,20. Oxygen
vacancies at O7 renders the Co positioned above it in pyramidal coordination with oxidation states +2 and +3
(labelled Co ). Co above the O3 position is in octahedral coordination with oxidation states +3 and +4 (labelled
red
Co )21.
ox

We report experimental evidence for the effect of A-site cation basicity – reflected by their individual Allred-
Rochow electronegativity through increasing Ln ionic radius –, TM-O-TM bond-character, and B-site oxidation
state on proton concentration. Most importantly, we show that these factors alone are not sufficient: hydration
in Co-based double perovskites requires that the Ln 4f-shell must be closed (empty La or full Lu) or semi-closed
(half-full Gd).

The results show that the trends reported for Fe-based materials also apply for Co-based compositions, but also
that proton concentrations are easily over- or underestimated from thermogravimetric experiments: The
hydration process is coupled to slow A-site cation disorder that alters oxidation equilibria, resulting in additional
weight gain from oxygen uptake in humid, oxidising atmosphere.

Results
Hydration of A|A||Co O as a function of compositional A-site matrix
2 6-δ
Figure 2a shows a selection of compositions screened for hydration using TG at 300°C in dry and humidified air
(data partly adapted from a previous work20). Only compositions with closed- or semi-closed Ln 4f-shell on the AII-
site (La, Gd, Lu) show significant hydration. Figure 2b displays hydration for all compositions (with abbreviations
listed in Supplementary Table 1), including multiple substitutions on AI, AII, and B versus the averaged Ionic
Radius22 Ratio (IRR) between the cations on AI and AII sites. It is evident that proton concentrations diminish with
the introduction of Ln with open 4f shells. Mixtures of more than two closed-shell Ln on AII and substitution of Sr
for Ba on AI also suppress hydration. Some of the compositions – with Sr on AI or high concentrations of La on AII
– crystallize in a cubic ABO structure, also suppressing hydration. Acceptor doping of Sr for Ln, giving an overall
3
A-site valence below +2.5, increases the concentration of oxygen vacancies and, hence, the degree of hydration,
as seen for cubic BaLa Sr Co O , which shows a six-fold higher hydration than cubic BLC. Substituting 10% Ti
0.75 0.25 2 6-δ
or Fe for Co lowers the mass gain during hydration. All in all, even though proton concentrations for the hydrating
compositions are relatively low – up to around 1 mol% under oxidising conditions – the results show a clear trend:
Significant hydration is seen only for compositions where the lanthanides have either empty (La), half-full (Gd),
or full (Lu) 4f-shells.

X-ray absorption spectroscopy (XAS); Effect of Co electron structure and oxidation state
The indirect effect of the Ln 4f shell on the Co-O electron structure is illustrated in Figure 2c-h, where we present
X-ray Absorption Spectra with spectral lines from 526 to 532 eV, representing the pre-edge structure of the
oxygen K-edge in hydrating Ba La CoO (BLC) and BaGdCo O (BGC), non-hydrating BPC and BNC (c), and
0.5 0.5 3 2 6-δ
BaGd La Co O (x = 0, 0.3, 0.5, 0.8, and 1) (d-h) after treatments in dry and humid atmospheres. The first
x 1-x 2 6-δ
maximum at ~528 eV represents the overlap of Co3d t and O2p orbitals, while the second maximum at ~530 eV
2g
represents Co3d e and O2p overlap23. The first peak position shifts along the Ln series (Figure 2c), reflecting how
g
the Co3d t and O2p energy, scales with Ln radius22. The average Co oxidation state depends on the Ln radius20,24–
2g
26, which, in turn, affects the bond energy levels.

The spectral weight at ~528 eV indicates electronic depopulation of the Co3d t -O2p bond27. Non-hydrating BNC
2g
and BPC show higher intensity, reflecting more electron holes, while the closed-shell BLC and BGC exhibit lower
intensity and thus more electrons and/or less covalent character for Co3d t -O2p.
2g

Figure 2d-h show how the electron structure in the closed-shell compositions shifts before and after exposure to
wet atmosphere in air at 300°C, revealing a significant increase in the intensity of the Co3d t -O2p signal for BLC,
2g
BaGd La Co O (BGLC37), BGLC82 and BGC, after exposure to H O. The absence of signal for BGLC37 and
0.3 0.7 2 6-δ 2
BGLC82 in the dry state represents either a fully ionic bond, with no Co3d t -O2p overlap, or an overlapping bond,
2g
with only occupied states in a low-spin configuration. The e orbital is directed in the ab plane between Co and
g
oxygen, while t is directed diagonally towards the A-site cation28. Repulsion from protons situated at the nearby
2g
oxide ion causes hole transfer to t and a decrease in the Co3d e -O2p hole population upon hydration. This
2g g
decrease is higher for BGLC37 and BGLC82 than for BGC and BLC, indicating lower proton concentrations in the
latter two.

The exception from the closed-shell hydration trend is BGLC55, which shows no hydration from TG (Figure 2b),
high spectral weight for Co3d t -O2p, and no increase in t intensity after exposure to H O (Figure 2f). The
2g 2g 2
symmetrical BGLC55 may be seen as a special case with respect to electronic structure, displaying elongated a-
and b-unit cell parameters19 and more unoccupied states in the Co2d t -O2p orbitals, reflecting a high-spin
2g
configuration.

Effects of structure: Diffraction studies
Most compositions adopt A-site order, where Ba and Ln prefer AI and AII positions, respectively, and the structures
can be refined to either tetragonal P4/mmm or orthorhombic Pmmm structures (Figure 1). One exception is
Ba La CoO , which can be cubic BLC(C) with an average Co oxidation state +3.43, or A-site-ordered
0.5 0.5 3
orthorhombic BLC(O) with an average oxidation state +3.3 depending on thermal and atmospheric history20.
Figure 3a shows how hydration depends on the A-site ordering, where the ordered – and more reduced –
orthorhombic BLC(O) exhibits four times higher mass gain in wet atmosphere than the disordered – and more
oxidised – cubic BLC(C)20.

In order to further characterize how proton stability is connected to bond angles and cation-anion distance13–15,
we employed SR-PXD and NPD to evaluate differences between open- and closed-shell compositions and locate
preferable proton sites. There is a slight tendency to longer TM-O bonds and a higher O-TM-O angle deviation
from 180° for the closed-shell elements, especially for O4 and O6 (Figure 1 and Supplementary Figure 1). Taking
the previous studies into account, we suggest that there is less negative charge transfer and higher proton stability
at O4 and O6 in the orthorhombic Pmmm structure.

Figure 3b displays Co oxidation state (y1-axis) and oxygen non-stoichiometry (y2-axis) vs. average AII-site Ln radius.
It shows hydrating (blue) and non-hydrating (red) compositions distributed over the range of average Ln ionic
radii, underlining the pivotal role of closed Ln 4f shell over the roles of non-stoichiometry, Ln basicity, and average
Co oxidation state.

Figure 3d and e compare hydration at 300°C in wet air for BGLC37 after reductive annealing in Ar at 1000°C for
10 h and subsequent equilibration in dry air at 350°C (BGLC37 ) and for as-prepared BGLC37. X-ray diffraction
red
(XRD) (Supplementary Figure 2) shows that A-site ordering is more complete in BGLC37 . As for BLC and
red
Ba La FeO 16, reduced BGLC37 shows significantly more hydration than oxidised.
0.85 0.15 3-δ

2H isotope exchange; Quantification of protonation
The proton concentration was estimated by fast and reversible isothermal H O-D O exchanges in order to
2 2
separate protonation and hydration from additional oxidation or reduction (Figure 3d and e). The interpretation
of the increased weight from such isotope shifts as proton concentration is associated with several potential
sources of error (buoyancy, lower vapour pressures of D O vs H O, different solubilities of humidity-reducing
2 2
wetting stage salts (here KBr), and different hydration thermodynamics). We still qualitatively interpret the results
to show that approximately half of the weight gain after long-term hydration in wet air is associated with
oxidation.

Comparing proton concentrations in BGLC82 and BGLC37 at 300°C in air by H O-D O exchange (Figure 3f), it can
2 2
be inferred that even though BGLC82 has the lower Co oxidation state, the higher La/Gd ratio in BGLC37 leaves
the latter with more protons, displaying dominance of Ln basicity over Co oxidation state.

To further quantify hydration in BGLC82 and BGLC37, 2H (D) concentration profiles were studied by exposing
polished sintered pellets to D O (0.023 atm) and performing Secondary Ion Mass Spectrometry (SIMS).
2
Quantification is done for BGLC82 by calibrating 2H profiles using an ion-implanted reference sample, ensuring
less than ±10% relative error30. The results are shown in Figure 3c for samples exposed at 400°C in Ar and at 600°C
in Ar and air. Raw counts/s for BGLC82 are compared to BGLC37 in Supplementary Figure 3a and b. As expected,
red
BGLC37 has orders of magnitude more 2H in hydrated than in dry state, confirmed also by isotope back-
red
exchange in H O atmosphere (Supplementary Figure 3b). Raw counts/s for BGLC37 are 50 times higher than in
2 red
BGLC82 at 600°C in air, reflecting the higher proton concentration in the former. The results show significant
protonation of BGLC bulk up to 600°C. For BGLC37 , the concentration of 2H is uniform throughout the sub-
red
surface and bulk, while BGLC82 shows a concentration gradient through the sub-surface. The fast back-exchange
from 2H to 1H for BGLC37 (blue line in Supplementary Figure 3b) indicates fast proton transport and that the
red
concentration gradients seen in Figure 3c represent equilibrium at each condition.

Parallel hydration and oxidation; two processes with different time constants
Hydration of MPEC materials in wet atmospheres is often measured in long-term isothermal5 or isobaric1,18 TG
experiments. Depending on the method and on the relative stability of oxygen vacancies, electron holes, and
protons, mass exchange of water, hydrogen, and oxygen may be indiscriminately interpreted as water, as is
customary for redox-stable materials, where the oxygen vacancy concentration is fixed by a dopant and electronic
defects are in the minority. We have assessed this by comparing isobaric and isothermal mass gain of BGLC82 in
wet air and compared proton concentrations calculated under the assumption that mass gain in wet atmosphere
stem from hydration alone in both cases (Figure 4a and b). This approach results in a significantly over-estimated
proton concentration after isobaric measurements. This is due to additional – and un-accounted – oxidation upon
long-term exposure to wet air during isobaric measurements. H O-D O exchange reveal fast initial hydration
2 2
coupled with slow additional oxidation, and short isothermal exposure to H O is therefore necessary for proper
2
determination of proton concentration from hydration under oxidising conditions.

The protonation and redox equilibria were thus further investigated for BGLC82 and BGLC37 by switches between
dry and wet air at 300, 400 and 500°C and with H O-D O switches during equilibration. The results showed that
2 2
both compositions exhibit a combination of hydration and subsequent oxidation upon exposure to water
(Supplementary Figure 4). By returning to dry conditions, new equilibria were established, indicating a structural
change after exposure to water. Proton incorporation in BGLC37 was also examined using dry air and N with
red 2
H O-D O switches.31 In oxidising conditions, hydration is coupled with slow oxidation, giving rise to mass gain over
2 2
150 h under wet conditions. H O-D O switches showed a proton concentration of 3 mol% for BGLC37 in wet air
2 2 red
at 300°C. Proton concentration was then measured after dwelling for 400 h in dry N . Isothermal switches
2
between dry N , H O, and D O showed that 75% of the weight gain originates from hydrogenation (Figure 4c and
2 2 2
d). Measuring proton concentration with increasing pH O and pD O (Figure 4e) revealed an increase in the
2 2
fractional hydration with higher pH O, resulting in a proton concentration of 1.5 mol% at pH O = 0.016 atm in N
2 2 2
at 300°C (Figure 4f). Hydrogenation has previously also been shown for BGLC82 substituted with 10% Ti for Co on
the B-site.32

A-site order-disorder; Atomic resolution STEM and EDS
On basis of the acquired understanding, we compared as-prepared and hydrated BGLC37 by STEM-EDS with
atomic mapping and compared the results to SR-PXD and NPD to unify local and global characterisation of A-site
order-disorder upon hydration and oxidation. Figure 5 a) shows atomic maps of BGLC37 in dry and hydrated
states, verifying the A-site ordering of Ba-Ln in dry state and A-site disorder after hydration. Ba is ordered on the
AI-site with a minor occupancy also on AII in the dry state (Supplementary Figure 5). Hydration induces the A-site
disordering, i.e., BGLC37 shows a transition from A-site order to disorder after exposure to humid atmosphere at
300°C. Around 30 grains were examined without detecting A-site order in the hydrated state. SR-PXD Bragg
reflections are, however, undistinguishable between dry and wet states both for BGLC37 and BGLC8219 but may
also be refined with a Pmmm 2a3b3c supercell (Supplementary Figure 6a and b), which is oxidised and fully A-site
disordered. The combination of SR-PXD, NPD (Supplementary Figure 7) and STEM EDS mapping reveals a
transition from reduced, A-site ordered Pmmm a2b2c for BGLC37 and P4/mmm ab2c for BGLC82 in the dry state,
into an oxidised, A-site disordered Pmmm 2a3b3c supercell for both compositions upon hydration (Figure 5b).

Oxygen vacancies in the Ln layer are suggested to compensate for the difference in ionic radii between the cations
occupying the AI and AII sites24. Hence, large Ln’s such as La will oxidise and disorder more easily due to the smaller
difference in ionic radius between Ba and La. For BGLuC82, the two lanthanides are sufficiently small to maintain
the same oxidation equilibrium before and after hydration. This is reflected by the full reversibility when reverting
from wet to dry conditions (Figure 2a).

The hydrated supercell is oxidised, but also contains oxygen sites such as O10 that – due to higher bond angle
deviation from 180° and elongated Co-O bond13–15 – will have a more complete Co-O electron transfer and can be
protonated more easily than O4 and O6 in the a2b2c phase. As a result, both hydration and oxidation become
more favourable when the structure changes from a2b2c to 2a3b3c, and hydration – a process consuming oxygen
vacancies (reaction A in Figure 5c) – induces A-site disorder and oxidation (Reaction B in Figure 5c) in a combined
A+B hydroxidation reaction.

Under inert conditions, exposure to wet atmosphere is not enough to trigger the A-site disorder. This is most
likely due to the larger concentration of oxygen vacancies in the Ln layer stabilizing the a2b2c structure at low
pO . Instead of A-site disorder and oxidation, hydration is combined with reduction in an A-B hydrogenation
2
reaction (Figure 4e, f).

Methods
Synthesis: Ba Sr Ln Ln’Co O ceramics (Ln, Ln’ – lanthanides or Yttrium, 0 ≥ x ≥ 1) were synthesized using the
1-x x 1-x x 2 6-δ
solid state reaction method according to the protocol established in our previous work:20 La O (99.99% Alfa
2 3
Aesar, Pr O (99.99% Aldrich) and Nd O (99.9%, Chempur) were preheated at 900°C for 5 h. Additionally,
6 11 2 3
Sm O (99.9%, Chempur), Gd O (99.98% Alfa Aesar), Tb O (99.99% Aldrich), Dy O (99.9%, Chempur) and Y O
2 3 2 3 4 7 2 3 2 3
(99.9%, Chempur) were used as the Ln/Ln’ sources. The Ln/Ln’ oxides, BaCO (99.9% Sigma Aldrich) and
3
Co O (99.98% Alfa Aesar) were mixed in an agate mortar in stoichiometric proportions and then pelletized. The
3 4
synthesis was performed in a muffle furnace at 1150°C for 48 h. To form the tetragonal BaLaCo O ,
2 6-δ
the as-synthesised sample was heat treated in Ar at 1050°C for 24 h and then re-oxidized in air at 350°C for 3 h.
BGLC37 and BGLC82 was commercial (Marion technology, France).

SR-PXD patterns were obtained at the Swiss-Norwegian Beamline (SNBL) BM01 and BM31, ESRF, Grenoble with
a Pilatus 2M 2-dimensional (2D) detector (BM01) and Dexela Perkin Elmer 2923 CMOS 2D detector (BM31) with
wavelengths of 0.78956 Å and 0.31232 Å, respectively. 1D data were obtained by integration of the 2D diffraction
patterns using the Bubble program. Additional measurements were performed at the Diamond Light Source in
Didcot, UK, at beam line I11 in high-resolution mode by 45 photomultiplier detectors, each with a Si(111) analyser
crystal (λ = 0.82 Å), cfr. an earlier publication for details.20

X-ray diffraction (XRD) was made with an X’Pert Phillips diffractometer with Cu Kα radiation (1.5418 Å).

NPD data were collected at RT using the constant-wavelength diffractometer PUS (λ =1.555 Å) at the JEEP II
reactor (Kjeller, Norway) and the time-of-flight (TOF-NPD) instrument POWGEN at SNS (Oak Ridge, US). The
sample was loaded in vanadium cans and results analysed using the Rietveld method refined simultaneously to
the SR-PXD and NPD data, when available, see an earlier publication for further methodological details20 and
refinement parameters in Supplementary Methods.

Thermogravimetric hydration measurements were performed at University of Oslo (UiO) and at Gdánsk
University of Technology (GUT). At UiO a Netzsch STA 449 F3 Jupiter was coupled to a custom-built gas mixer34
equipped with H O and D O wetting stages, where dry N was humidified with 2.6% H O or 2.3% D O. In order to
2 2 2 2 2
keep pO constant in dry and wet atmospheres, the humidified N was mixed with dry O and N to reach a final
2 2 2 2
composition of 1.6% H O (or 1.4% D O) in air. At GUT, a Netzsch Jupiter® 449 F1 simultaneous thermal analyser
2 2
was used for water uptake studies isothermally at 300°C. As-prepared powders, crushed from pellets, were
heated at a rate of 5°C/min to 800°C and kept for 5 h to remove the residual water. Then the sample was cooled
to 300°C at the same rate and stabilized for 2 h before the purge gas atmosphere was switched from dry to
humidified (1.9% H O). This was maintained for 2 h, followed by the reverse switch to dry gas. Synthetic air was
2
used as a purge gas and N as a protective gas. H O-D O exchanges were performed for selected compositions to
2 2 2
support estimation of proton concentrations based on hydration as the initial dominating process during
equilibration in wet conditions. The isobaric hydration experiment presented in Figure 4b was performed by
heating the powder in dry air from room temperature to 900°C, dwelling for 1 hour, and stepwise cooling at
3°/min and equilibrating at each step temperature. The process was then repeated in wet atmosphere and the
weight difference between wet and dry atmosphere at each temperature was interpreted as pure hydration. See
also the Supplementary Discussion of Hydration of a secondary phase.

SIMS: Four sintered BGLC82 pellets sintered at 1190 °C for 10 h for sufficient relative density were used for the
SIMS experiments. Two samples were equilibrated for 48 h at 600 in 2.6% H O in air and Ar, respectively before
2
switching to 2.3% D O for 2 h. A third sample was equilibrated at 400°C and 2.6% H O in Ar for 48 h before
2 2
switching to 2.3% D O for 4 h. A fourth sample was annealed in dry air at 600°C for 48 h and used for calibration
2
by ion implantation. For BGLC37 , the same powder was used as had been used during the long-term hydration
red
shown in Figure 3d). The powder was pressed to pellets and sintered in ambient air for 10 h at 1190°C. The pellets
were polished and annealed in dry air at 900°C for six h and then for 53 h in dry air at 600°C before being cooled
to room temperature. Following this treatment, one pellet was heated to 600°C in a TG in dry air and equilibrated
for 15 h before introducing 2.3% D O for 280 minutes. A second pellet was exposed to the same treatment, now
2
keeping the pellet for 50 h in 2.3% D O. A third pellet was kept in its dry state. A Cameca IMS7f SIMS equipped
2
with a 15 keV O + primary ion beam source was used to record the concentration vs. depth profiles of 2H. The
2
raster size was set to 150 m and data was collected from the central 33 m. Absolute concentration of 2H were
obtained by measuring a separate 2H implanted reference sample.

The SIMS reference standard was prepared by ion implantation using a 1MeV Tandem ion accelerator from
National Electrostatics Corporation (NEC). Deuterium ions were extracted from a gas source and accelerated to
an energy of 200 keV and implanted into a separate sample to a fluence of 5×1014 cm-2. The sample was used
as a reference sample to convert the measured deuterium signal into absolute concentration. The SIMS reference
sample was measured with the same instrument parameters as the sample of interest, utilizing the implantation
fluence to calculate the conversion factor, i.e. the relative sensitivity factor of deuterium.

For depth calibration, the sputtered crater depths were determined by a Dektak 8 stylus profilometer and a
constant erosion rate was assumed. After having obtained 2H concentration profiles by SIMS, one deuterated
pellet was heated in a TG to 600°C in 2.3% D O in air, equilibrated at 600°C for 15 minutes before switching to
2
2.6% H O for 20 minutes. The 2H profile was subsequently collected after this reversed isotope exchange. All
2
pellets were rapidly cooled to room temperature after exposure to D O.
2

XAS: X-ray Absorption Spectroscopy was performed at the Solaris National Synchrotron Radiation Centre in
Kraków, Poland. A dedicated PEEM/XAS bending magnet beamline was utilized to measure O-K edges. Powder
samples were mounted on carbon tape and placed Omicron plates. The measurements were performed under
ultra-high vacuum at room temperature. The presented data were collected in the XAS fluorescence mode. The
data were normalized according to the conventional procedure to the edge intensity after subtracting the
background signal fitted with the first-order polynomial fitted to the pre-edge regime. Before the measurement
samples were dried for 24 hrs at 900°C in air to obtain the ‘dry’ state. Samples labelled as ‘wet’ were exposed to
humidified air for 24 hrs at 300°C.

STEM/EDS: The microstructure and composition of the samples were characterized by annular bright field (ABF)
and high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) coupled to X-ray
energy dispersive spectroscopy (EDS). This work was performed with a DCOR Cs probe-corrected FEI Titan G2 60-
300 instrument, with 0.08 nm nominal spatial resolution when operated at 300 kV, equipped with a Bruker SuperX
EDS system comprising four silicon drift detectors. Sample preparation involved depositing powder suspended in
isopropanol on lacey carbon copper grids followed by shielded plasma cleaning.

Iodometric titration was used to determine the oxygen stoichiometry and average cobalt oxidation state at room
temperature, performed as described in previous works:20,29 15–20 mg of the sample and a surplus of potassium
iodide (ca. 0.2 g) were put in a three-neck flask, flushed with nitrogen. When the air was removed, 15 ml 2 M HCl
was added to dissolve the sample, and iodine ions titrated in an inert atmosphere with 0.01 mol/dm3 Na S O
2 2 3
from a 10 ml microburette (±0.05 ml) using starch as indicator.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Overall I have a positive tendency to recommend this paper for publication in Nat. Comm. Nevertheless I also suggest significant revision on various levels.\nThe topic is highly relevant and suitable for Nat Comm. Mixed conductors which include also proton conductivity are key functional materials for protonic ceramic fuel and electrolyzer cells, which will acquire an increasingly important role in a H2-based energy system. Several groups find that one one hand, Co-rich materials have a tendency of less pronounced proton uptake, on the other hand Co is typically beneficial for the catalytic activity of such electrode materials. Thus, an improved understanding of a Co-based material as given here is highly welcome.\nThe double perovskites BGLC and related RE-substituted materials have been investigated by the present authors' groups over an extended time. The present paper finalizes these investigations and is able to draw a quite clear picture of a materials system that is really complex. For example, it is found that - even for non-redoxactive RE3+ cations - the amount of hydration depends on RE3+ being f0, f7 or f14 configuration. And while the amount of proton uptake is quite low, it can still modify the ordering of the cations and propensity for oxidation, which leads to relaxation processes after pH2O step on multiple time scales.\nTherefore the paper is recommended for publication, but in order to be well comprehensible for the readers the presentation of this complicated story needs some improvement.\nPresentation:\n(1) the paper covers a large range of samples, several data measured also in preceding publications. It would be extremely helpful to collect all lattice parameters, other structural information (space group, presence/absence of A site order etc, quality of the Rietveld fitting) in a table in the SI, indicating also exact state of sample (as-prepared, dry annealing, O2 or N2 annealing, hydrated state...)\n(2) the paper shows that TG weight change may originate from hydration, hydrogenation, oxidation, and how the actual proton content can be determined from H2O/D2O exchange. Table S1 states \"hydration mol% from weight gain in H2O\" - so it seems the numbers there might over/underestimate the real proton concentration? Please add a column giving the real [H+] as determined from H2O/D2O. Labelling in fig 2b is misleading: axis label says \"mol% H\" which sounds like real proton conc. (from H2O/D2O) while cation says \"weight gain in H2O\" which is sum of all processes. BGLC82 value in fig 2b differs from that in fig 4b\n(3) fig 2b is quite puzzling with the colored arrows. better use just red symbols for \"hydrating\" and black for \"non-hydrating\" samples. Fig 3d,e: giving absolute weight changes in mg is not very suitable (magnitude depends on absolute sample weight used). I understand that not always the precise [H+] is known owing to different reactions, but at least use \"weight change/%\" instead of \"weight change/mg\". Generally, the colored backgrounds in the TG plots (also fig 4) are too strong. Please use lighter colors, and try to simplify the figures as much as possible (i.e. not using color gradients when within such a field the conditions are not changing)\n(4) fig 4: it would be helpful to insert the sample name abbreviation in each of the plots. Fig 4ab please discuss in more detail in the text that/why the isothermal process (does it proceed by hydration or by hydrogenation, which reaction is assumed for converting weight change into proton conc.) yields lower proton conc. than isobaric (does this imply the isobaric value is overestimated). Fig 4cd please indicate T. It would be nice to directly indicate in this fig. which fraction of H is from hydration and which from hydrogenation. Fig. 4e please indicate T, that it is in N2, and what are the pH2O and pD2O.\n(5) fig 5 middle part: please write \"dry\" and \"hydrated\" close to the respective structures. Bottom part: the small coordinate system with axes labelled pO2 and pH2O is easily overlooked. Please indicate more prominently that horizontal axis refers to proton concentration, and vertical axis to oxygen uptake (oxidation). Please indicate more clearly that the \"hydroxidation\" happens only because the hydration leads to A site disorder which then favors further oxidation (without this it is not conceivable why hydration and oxidation should occur in parallel since they both consume oxygen vacancies)\nAdditional discussion/explanation:\n(6) within the presented data set there is a strong correlation that only f0, f7 or f14 RE3+ materials show large hydration. However, based on the usual \"textbook knowledge\" that f orbitals have a very small radius and are thus \"quite inside the atom\" it is not so obvious how the details of f occupancy (6 or 7 or 8 electrons) would affect the t2g orbitals of Co which at at approx. 3.5 Angstrom distance. Might it help to think about Jahn-Teller-like effects at RE which could modify RE-O bonding, or generally how f occupancy could affect RE-O bonds (and this then the Co-O bonds)? Along this line, maybe look also at RE-O bond lengths and angles (similar to Co-O in fig A1ab)?\n(7) fig 2d-h: it is impressive but to some degree also astonishing how a small proton uptake of about 1% (0.01 H per AA'B2O6 !) strongly modifies the intensity of the O-prepeak (which is averaging over all O's, most being unprotonated). Any further explanation is welcome\n(8) Co is known to have several spin states. What is known about spin of Co in present samples, it may also affect nature of Co-O bond and thus tendency for protonation? maybe add some information in the SI\n(9) hydration vs hydrogenation are quantified from the H2O/D2O experiment. However, even when pH2O=pD2O are used, the hydration mass action constants for H and D might slightly differ because zeropoint vibration effects. At least mention that this might add a little uncertainty\nFurther points:\n- fig 1b please write \"ion radius ratio\" instead of \"IRR\"\n- caption fig 2 there is no \"g-k\", should be d-h?\n- fig 3b: authors seem to use a different scale for ionic radii than in [20] - why? the radii in [20] are the wellknown Shannon radii, with 1.16 A for La in CN=8\n- ion implantation is not a standard lab technique. Thus it must be specified how/here the D implantation as SIMS standard was done\n- pg 9 line 225 should probably read \"hydration causes A site disordering\"?\n- lines 263 and 267 please mention the figure numbers which show the respective correlations\n- fig S6 maybe indicate more clearly which are the specific reflections that evidence the A site ordering. maybe use log scale for intensity to give more weight to small reflections\n- standard sample preparation is 1150°C for 48h - what is resulting relative density. Does this directly yield sufficiently dense samples for the SIMS experiment, or were they made by modified process?\n- I guess \"powder\" for TG mentioned in line 340 is obtained by crushing the 1150°C for 48h pellets ? then please write that. Any idea of resulting grain/particle size?", "reviewer_2": "This paper studies Co-based double perovskites with mixed A2+ and A3+ cations in the A-site. It employs a variety of structural, thermogravimetric and electrochemical measurements to draw general conclusions about the hydration behavior.\nAlthough the manuscript contains some interesting techniques, I found it to be unorganized in the presentation of data and results, and this regrettably influenced clarity. In general, the discussion and conclusions are insufficiently supported by the results. In addition, the discussion frequently resorts to data that was already published, or even not shown.\nI recommend that this paper is submitted to a more specialized journal after its several problematic aspects are amended.\nSpecific points follow.\n1) Most figures are assembled out of different panels, which can help condensing information, but then the meaning of the large Figure 1 is unclear.\n2) The authors mention that both neutron (NPD) and sychrotron (SR-XRD) powder diffraction have been used to \"unambiguously show that hydration triggers a transition into an oxidised A-site disordered\" supercell (page 11). I could not find NPD in either the manuscript or the Supporting information. On the other hand, page 1 only mentions SR-XRD as the technique to refine the position of seven different oxygen sites.\n3) The abstract anticipates that hydration is linked to A-site basicity, but this is actually never discussed directly.\n4) One of the main hypotheses put forward is that closed or semi-closed 4f states (La3, Lu3+, Gd3+) influence hydration, and the authors use open/closed symbols in Fig 2b and Table S1. There seems to be inconsistency here: why is BGLYC721 marked as closed shell but BGLuC721 is not? Why are BGLSrC(C) or BGLuC721 not closed shell? The trend seems less clear than it is presented by the authors, as very similar compositions show markedly different hydrations.\n5) Fig 2a is mostly taken from ref. 20. Fig2b is plotted against IRR as the x-axis: what is the meaning of the radius ratio and why should hydration be plotted against it? This is never explained. In Figure 3b the x-axis seems to be way too low, and I suspect the ionic radii for an octahedrally coordination were incorrectly considered.\n6) As the authors underline how the extensive composition matrix (page 2) supports the correlations, one would expect that the XAS spectra on Fig 2c-h would cover the same samples as in Fig 2b, while it only Gd/La compositions are shown. Also since the absolute intensity of XAS peaks is discussed, some more information about measurements (is it electron yield? Are surface states expected to be the same as the bulk?) and data reduction (how are the spectra normalized?) would be good. Also panels c-e and f-h are shown with a common x-axis, but the tickmarks don't align.\n7) At least laboratory XRD with Rietveld refinements should be shown for each sample in the Supporting information. The only diffraction data actually shown is Fig S6, which is too small to appreciate any difference between the two panels. There is also no goodness of fit or statistical significance reported in order to tell the two models apart. Previous data by the same authors (refs. 20 and 28) is much better presented in comparison.", "reviewer_3": "This study presents the governing principles of protonation by systematically analyzing the hydration trends of cobalt-based double perovskite, A^I A^II Co_2 O_(6-δ). The authors report that lanthanide A-site dopants with a closed or semi-closed Ln 4f shell (i.e., Ln = La, Gd, Lu) induce electron hole transfer from eg to t2g in the Co-O bond, as supported by XAS measurements. Additionally, STEM-EDS, SR-PXD, and PND clearly demonstrate that hydration triggers a transition to an oxidized A-site disordered 2a3b3c Pmmm supercell, which affects both oxidation and hydration equilibria. This work is well-designed, and the analysis is meaningful, particularly in designing mixed proton-electron positrodes that maximize hydration and surface reaction kinetics. Such advances could offer substantial contribution to the PCEC community. Therefore, I would recommend that this paper is suitable to be published in this journal after revision with following comments.\n- In Figure S3, the concentration profile of BGLC37red with D2O exchanged at 600°C shows a distinct pattern of initially increasing, then decreasing, and finally reaching saturation. This behavior is not mirrored in BGLC82. Could the authors elaborate on the underlying reasons for this unique trend in BGLC37red?\n- The authors state that the hydrated supercell is oxidised and contains oxygen sites like O10, which are more easily protonated than O4 and O6 in the a2b2c phase, leading to enhanced hydration and oxidation in the 2a3b3c structure. Could the authors provide further explanation or references to support this assertion? Understanding the specific reasons why O10 sites are more susceptible to hydration compared to O4 and O6 would clarify this aspect of the study.\n- Regarding a STEM-EDS results as provided in Figure 5a, the authors mentioned that the ordering structure in BGLC37 become disorder after hydration. I’m simply wondering if this phenomenon may be reversed. Does hydrated BGLC37 back into an ordered structure after dehydration? Furthermore, do you have a change to check the chemical stability under hydration conditions?\n- In figure 2a, I think there are some missing TGA results. Please show all TGA data with all compositions even in supplementary figure.\n- Why the mass change of BLC with cubic structure and BLC with ordered structure is more than 4 times different. Is it because the unit cell volume has increased? Please clearly explain this point.\n- Based on the TGA data in Figure 2a, fully Gd and La doped in BaLnCo2O6-d samples seem to exhibit the highest mass change under both dry and wet conditions. Nonetheless, Gd and La co-doped sample, BGLC, is used as representative material in this work. There is a lack of clear explanation regarding this.\nMinor points:\n- There are errors in Figure 2. In caption b), “See also Table S2 “ need to be changed “See also Table S1”. In figure 2b, there is duplicate data point with an IRR close to 1.15 (BSrPC). Please check it.\n- There are typos on page 6. In figure 3b, y1-axis is Co oxidation state and y2-axis is oxygen non-stoichiometry. And blue arrows indicate hydrating and red arrows indicate non-hydrating in figure 3b. It seems to be incorrectly labeled, unlike the manuscript. Please revise these errors.", "reviewer_4": "This work presents a new governing principle for the hydration and proton transport in Co-based double perovskites. This principle is extracted from a comprehensive experimental campaign that includes the preparation of a large material library (experimental matrix based on Ln combinations) and fundamental characterization looking at the atomic-level structural and electronic aspects together with proton-transport methods.\nThe quality and completeness of the presented results are of the highest quality. The work is generally well described, and the discussion is well articulated and succinct. I am convinced that this article will impact the research community working on proton ceramics and electrochemistry. I recommend the publication of the paper subjected to minor revision, taking into account the following list of comments and suggestions:\n1- Fig. 2b. This chart is difficult to read, and I needed to go to Table S2 (back and forward) a couple of times. Is there a simple way to include the composition information directly in Fig. 2.\n2- Page 6. When comparing proton concentrations in BGLC82 and BGLC37, it is properly concluded that there is a dominance of Ln basicity over Co oxidation state. For the general reader, I recommend the authors explain more in detail the term ‘Ln basicity’ in this context and its implications in the proton and electronic transport.\n3- Last paragraph, page 6. Please, use a unique symbol for deuterium. Now, D and 2H are used simultaneously in the text, Fig. 3 and Fig. S3\n4- Page 9. There is a very interesting conclusion: “the two lanthanides are sufficiently small to maintain the same oxidation equilibrium”. Please, could you elaborate more on this and extrapolate to other combinations and systems, i.e., as a general principle?\n5- Fig. 5. I suggest the authors label each individual image and re-arrange it to improve the overall readability of the figure. In addition, as a general conclusion of the figure, it is stated, “We note that pure hydration in mixed valence oxides like the ones we consider here occurs only under specific combinations of pO2 and pH2O.” Can the authors be a bit more specific on the type of combinations pO2-pH2O enabled hydration and the associated materials properties that determine this ‘hydration space’.\n6- To illustrate the potential and versatility of the proposed principles, the authors could comment on the potential application of these principles to the design and understanding of other mixed oxides exhibiting hydration and/or proton transport, if possible, providing specific examples of composition and crystalline structures. Can this be connected with disorder or entropic effects in different classes of oxides?"}, "author_response": "To the Reviewers,\nWe herewith submit our revision, with apologies for the long time it has taken, not only due to\nconsiderable work in response to the reviewers’ suggestions, but also due to a long sick leave by the\nfirst author. Only with his return, we have been able to finalise the revision.\nResponses to the Reviewers’ comments and suggestions\nWe are grateful for the many insightful reviews and constructive comments, which we have all used to\nimprove the manuscript. Reviewers’ comments are reproduced as is and our responses and actions are\ninserted in boldface.\nReviewer #1:\nOverall I have a positive tendency to recommend this paper for publication in Nat. Comm. Nevertheless\nI also suggest significant revision on various levels.\nThe topic is highly relevant and suitable for Nat Comm. Mixed conductors which include also proton\nconductivity are key functional materials for protonic ceramic fuel and electrolyzer cells, which will\nacquire an increasingly important role in a H2-based energy system. Several groups find that one one\nhand, Co-rich materials have a tendency of less pronounced proton uptake, on the other hand Co is\ntypically beneficial for the catalytic activity of such electrode materials. Thus, an improved\nunderstanding of a Co-based material as given here is highly welcome.\nThe double perovskites BGLC and related RE-substituted materials have been investigated by the\npresent authors' groups over an extended time. The present paper finalizes these investigations and is\nable to draw a quite clear picture of a materials system that is really complex. For example, it is found\nthat - even for non-redoxactive RE3+ cations - the amount of hydration depends on RE3+ being f0, f7\nor f14 configuration. And while the amount of proton uptake is quite low, it can still modify the ordering\nof the cations and propensity for oxidation, which leads to relaxation processes after pH2O step on\nmultiple time scales.\nTherefore the paper is recommended for publication, but in order to be well comprehensible for the\nreaders the presentation of this complicated story needs some improvement.\nPresentation:\n(1) the paper covers a large range of samples, several data measured also in preceding publications. It\nwould be extremely helpful to collect all lattice parameters, other structural information (space group,\npresence/absence of A site order etc, quality of the Rietveld fitting) in a table in the SI, indicating also\nexact state of sample (as-prepared, dry annealing, O2 or N2 annealing, hydrated state...)\nResponse: We agree and have followed the reviewer’s advice and expanded Table S1.\n(2) the paper shows that TG weight change may originate from hydration, hydrogenation, oxidation,\nand how the actual proton content can be determined from H2O/D2O exchange. Table S1 states\n\"hydration mol% from weight gain in H2O\" - so it seems the numbers there might over/underestimate\nthe real proton concentration? Please add a column giving the real [H+] as determined from H2O/D2O.\nLabelling in fig 2b is misleading: axis label says \"mol% H\" which sounds like real proton conc. (from\nH2O/D2O) while cation says \"weight gain in H2O\" which is sum of all processes. BGLC82 value in fig 2b\ndiffers from that in fig 4b\nResponse: The numbers (mol% H) in fig 2b are taken from the initial (2 h) weight gain of hydration\nfrom dry to wet air at 300°C. As can be seen from Fig S4, the initial weight gain is mostly hydration.\nSince we do not have H O/D O shifts on all samples, we use the fast initial weight gain to estimate\n2 2\nthe proton concentration, relying on the experimental verification from D O experiments on some\n2\nof the compositions. The caption of Fig. 2b is edited to explain this better, and we have expanded\nthe elaboration on it in the methods section. The difference for BGLC82 in Fig 2b and 4b reflects the\nchallenges of accurate determination in two different measurements. Figure 4b highlights the\ndifference between measuring isothermal weight changes, reflecting mostly only hydration of an\notherwise static structure, and isobaric hydration with decreasing temperature, where the hydration\nis enhanced by secondary structural changes. This is expanded on in the text. We have added a\ncolumn in S1 on the D O/H O results, as suggested.\n2 2\n(3) fig 2b is quite puzzling with the colored arrows. better use just red symbols for \"hydrating\" and black\nfor \"non-hydrating\" samples.\nResponse: We assume the reviewer means Fig. 3b. In that case we agree and have amended the\nfigure according to the reviewer’s suggestion.\nFig 3d,e: giving absolute weight changes in mg is not very suitable (magnitude depends on absolute\nsample weight used). I understand that not always the precise [H+] is known owing to different\nreactions, but at least use \"weight change/%\" instead of \"weight change/mg\". Generally, the colored\nbackgrounds in the TG plots (also fig 4) are too strong. Please use lighter colors, and try to simplify the\nfigures as much as possible (i.e. not using color gradients when within such a field the conditions are\nnot changing)\nResponse: We agree and have amended Figs. 3 d) and e) correspondingly.\n(4) fig 4: it would be helpful to insert the sample name abbreviation in each of the plots.\nResponse: We agree and have marked Figs. 4a and b with BGLC82 and Figs. 4c and d with BGLC37.\nFig 4ab please discuss in more detail in the text that/why the isothermal process (does it proceed by\nhydration or by hydrogenation, which reaction is assumed for converting weight change into proton\nconc.) yields lower proton conc. than isobaric (does this imply the isobaric value is overestimated).\nResponse: Only hydration is assumed for converting weight gain to proton concentration. This will\ngive approximately correct proton concentration for isothermal measurements with short periods in\nwet atmosphere at each temperature. Hydration is also used as basis for calculation of proton\nconcentrations in the isobaric measurements. In this mode, the material is subjected to wet\natmosphere for a longer time, and proton concentrations will be overestimated due to additional\noxidation. We see no way to do this differently, and the presentation of the results is meant to\nhighlight the importance of isothermal measurements to get the right result.\nFig 4cd please indicate T. It would be nice to directly indicate in this fig.\nResponse: We agree and have updated the figure with information of temperature (300°C).\nwhich fraction of H is from hydration and which from hydrogenation.\nResponse: In Figs 4c and d we don't give fractions of H. In oxidizing conditions, the weight gain (mg)\nis a combination of hydration and oxidation. The additional weight gains in D O give the total mass\n2\nof H, which are converted to mol% from the total sample mass. Under inert conditions we do the\nsame, and the total mass of D again allows us to calculate mol% H. The ratio between mass gain in\nD O vs H O is much larger in N atmosphere, revealing that most of the protons come from\n2 2 2\nhydrogenation.\nFig. 4e please indicate T, that it is in N2, and what are the pH2O and pD2O.\nResponse: We have improved the caption for 4e. pH O for each step is given in Fig.4f. T is 300°C.\n2\n(5) fig 5 middle part: please write \"dry\" and \"hydrated\" close to the respective structures. Bottom part:\nthe small coordinate system with axes labelled pO2 and pH2O is easily overlooked. Please indicate\nmore prominently that horizontal axis refers to proton concentration, and vertical axis to oxygen uptake\n(oxidation). Please indicate more clearly that the \"hydroxidation\" happens only because the hydration\nleads to A site disorder which then favors further oxidation (without this it is not conceivable why\nhydration and oxidation should occur in parallel since they both consume oxygen vacancies)\nResponse: We have improved the figures as suggested and clarified in the associated text.\nAdditional discussion/explanation:\n(6) within the presented data set there is a strong correlation that only f0, f7 or f14 RE3+ materials\nshow large hydration. However, based on the usual \"textbook knowledge\" that f orbitals have a very\nsmall radius and are thus \"quite inside the atom\" it is not so obvious how the details of f occupancy (6\nor 7 or 8 electrons) would affect the t2g orbitals of Co which at at approx. 3.5 Angstrom distance. Might\nit help to think about Jahn-Teller-like effects at RE which could modify RE-O bonding, or generally how\nf occupancy could affect RE-O bonds (and this then the Co-O bonds)? Along this line, maybe look also\nat RE-O bond lengths and angles (similar to Co-O in fig A1ab)?\nResponse: The reviewer’s comment is pointing to an important alternative causing the f-shell\ninfluence on the Co-O bond. We have revisited our structural data to look at the RE-O bond and see\nif there is any link to f0, f7 and f14. We did not find any clear trend, and figures of bond-lengths for\nRE-O and bond-angles for Co-O as a function of Ln radius are now added as Figure S1 c and d,\nrespectively. This does not, of course, rule out a Jahn-Teller-like effect from the RE-O bond, but our\nstudy does not support any conclusion on this. We have, however, included reference to a recent\npublication where a Ln4f-TM3d hybridization is reported for a perovskite structure. We have made a\ncomment on this in the text.\n(7) fig 2d-h: it is impressive but to some degree also astonishing how a small proton uptake of about\n1% (0.01 H per AA'B2O6 !) strongly modifies the intensity of the O-prepeak (which is averaging over all\nO's, most being unprotonated). Any further explanation is welcome\nResponse: It is indeed intriguing. In our opinion this may be related to the degenerate band structure\nand how small changes affects the configuration of electrons. It seems that the O-K pre-edge\nstructure is highly sensitive to even small changes in protonic defect concentration. The ability to\nincorporate protonic defects, being govern by the electronic structure of O 2p-Co 3d hybridized\norbital, manifests itself with changes in the averaged signal. Since we do not have a reliable answer\nto the question, we have decided to avoid speculations in the manuscript, and left this issue un-\ncommented.\n(8) Co is known to have several spin states. What is known about spin of Co in present samples, it may\nalso affect nature of Co-O bond and thus tendency for protonation? maybe add some information in\nthe SI\nResponse: Our previous studies shown that the mixture of Co3+ HS, and Co4+ LS is present in the\ninvestigated materials, however it is difficult to calculate the concentration of cobalt ions at each\noxidation and spin state on the basis of only XAS results. The correlation between cobalt spin state\nand tendency for protonation was not observed, since no significant changes in Co spin state were\nrevealed for hydrating and non-hydrating materials. However, fully LS cobalt is not present for any\nof the studied materials contrary to e.g. in the LaCoO system [ref. Dalton Transactions 52 (17), 5771-\n3\n5779; Materials 13 (18), 4044 ]\n(9) hydration vs hydrogenation are quantified from the H2O/D2O experiment. However, even when\npH2O=pD2O are used, the hydration mass action constants for H and D might slightly differ because\nzeropoint vibration effects. At least mention that this might add a little uncertainty\nResponse: The effects suggested are expectedly negligible here in view of the qualitative approaches\nin this article, but we have improved the description of actual levels of H O and D O in the gases in\n2 2\nthe Methods section and throughout the text and – to meet the reviewer’s suggestion – added a\nsentence on the uncertainties stemming from multiple minor effects in isotope exchange\nexperiments.\nFurther points:\n- fig 1b please write \"ion radius ratio\" instead of \"IRR\"\nResponse: Implemented.\n- caption fig 2 there is no \"g-k\", should be d-h?\nResponse: Thanks for spotting the mistake. Corrected.\n- fig 3b: authors seem to use a different scale for ionic radii than in [20] - why? the radii in [20] are the\nwellknown Shannon radii, with 1.16 A for La in CN=8\nResponse: We use https://www.sciencedirect.com/science/article/abs/pii/002245969190388X as\nref on ionic radii. This gives us a full set of 12-coordinated Ln radii. We use this in a follow-up\nmanuscript (in preparation) on stability in steam. Shannon radii don’t give values for the whole\nmatrix in 12-coordination.\n- ion implantation is not a standard lab technique. Thus it must be specified how/here the D\nimplantation as SIMS standard was done\nResponse: We have described the ion implantation for fabrication of the SIMS reference standard by\nincluding the following paragraph:\n“A SIMS reference standard was prepared by ion implantation using a 1MeV Tandem ion accelerator\nfrom National Electrostatics Corporation (NEC). Deuterium ions were extracted from a gas source and\naccelerated to an energy of 200 keV and implanted into a separate sample to a fluence of of\ncm-2. The sample was used as a reference sample to convert the measured deuterium signal in𝟏to𝟒\n𝟓×𝟏𝟎\nabsolute concentration. The SIMS reference sample was measured with the same instrument\nparameters as the sample of interest, utilizing the implantation fluence to calculate the conversion\nfactor, i.e. the relative sensitivity factor of deuterium. “\n- pg 9 line 225 should probably read \"hydration causes A site disordering\"?\nResponse: Thanks for pointing out this serious mistake and, hence, the general inconsistency of that\nparagraph. The text in that paragraph has been corrected accordingly.\n- lines 263 and 267 please mention the figure numbers which show the respective correlations\nResponse: Done.\n- fig S6 maybe indicate more clearly which are the specific reflections that evidence the A site ordering.\nmaybe use log scale for intensity to give more weight to small reflections\nResponse: There are no reflections that evidence A-site ordering. The two Bragg-patterns are the\nsame and can be fitted to both dry and hydrated state. Our conclusions on A-site disorder comes\nfrom having studied more than 30 spots and grains with STEM-EDS without finding A-site order in\nthe hydrated material. Refinements to a 2a3b3c supercell is the confirmation that unifies XRD and\nSTEM-EDS findings.\n- standard sample preparation is 1150°C for 48h - what is resulting relative density. Does this directly\nyield sufficiently dense samples for the SIMS experiment, or were they made by modified process?\nResponse: The pellets for SIMS were sintered at 1190°C for 10 hours to obtain sufficient relative\ndensities for SIMS measurements. We have added this information.\n- I guess \"powder\" for TG mentioned in line 340 is obtained by crushing the 1150°C for 48h pellets ?\nthen please write that. Any idea of resulting grain/particle size?\nResponse:The powders were indeed from crushed and hand milled pellets, the average grain sizes\nhaven’t been measured, but the BET measurements showed that the surface area was in the range\nof 1-5 m2/g reflecting that the powders were coarse enough to represent bulk properties.\nReviewer #2:\nThis paper studies Co-based double perovskites with mixed A2+ and A3+ cations in the A-site. It\nemploys a variety of structural, thermogravimetric and electrochemical measurements to draw general\nconclusions about the hydration behavior.\nAlthough the manuscript contains some interesting techniques, I found it to be unorganized in the\npresentation of data and results, and this regrettably influenced clarity. In general, the discussion and\nconclusions are insufficiently supported by the results. In addition, the discussion frequently resorts to\ndata that was already published, or even not shown.\nI recommend that this paper is submitted to a more specialized journal after its several problematic\naspects are amended. Specific points follow.\n1) Most figures are assembled out of different panels, which can help condensing information, but then\nthe meaning of the large Figure 1 is unclear.\nResponse: We are not sure how to respond to this comment. We have left Figure 1 as it is, given its\nimportance in giving a detailed overview of occupancies for cations and oxygen, and also for labelling\nof Co sites. We believe the reader will benefit from having a large structure to consult when reading\nthe manuscript.\n2) The authors mention that both neutron (NPD) and sychrotron (SR-XRD) powder diffraction have been\nused to \"unambiguously show that hydration triggers a transition into an oxidised A-site disordered\"\nsupercell (page 11). I could not find NPD in either the manuscript or the Supporting information. On\nthe other hand, page 1 only mentions SR-XRD as the technique to refine the position of seven different\noxygen sites.\nResponse: We have used NPD refinements as starting point for refining SR-PXD results on some\nselected materials. Since our hydrating materials generally contain various amounts of Gd, which is\na neutron absorber, we had to make separate 160Gd enriched samples especially for NPD. Hence, the\nquanta are small and different samples were used for NPD and SR-PXD, where we focused on the\nhydrated and non-hydrated samples without 160Gd enrichment. However, in the particular case of\nBGLC37, we performed NPD and SR-PXD on both dry and D O-exposed 160Gd enriched sample. We\n2\nhave inserted a figure in SI with examples of NPD results for BGLC28 and for BGLC37 in dry and wet\n(D O) atmospheres.\n2\n3) The abstract anticipates that hydration is linked to A-site basicity, but this is actually never discussed\ndirectly.\nResponse: We discuss it in the introduction of the manuscript. Based on the reviewer’s comment, we\nnow bring it in more explicitly also in the Discussion.\n4) One of the main hypotheses put forward is that closed or semi-closed 4f states (La3, Lu3+, Gd3+)\ninfluence hydration, and the authors use open/closed symbols in Fig 2b and Table S1. There seems to\nbe inconsistency here: why is BGLYC721 marked as closed shell but BGLuC721 is not? Why are\nBGLSrC(C) or BGLuC721 not closed shell? The trend seems less clear than it is presented by the authors,\nas very similar compositions show markedly different hydrations.\nResponse: BGLSrC has Sr substituted for Ln, and it is thus not relevant to label it closed-shell. It is\nalso cubic. BGLLuC622 and BGLLuC721 are closed-shell. We have corrected this accordingly in the\nfigure and table.\n5) Fig 2a is mostly taken from ref. 20. Fig2b is plotted against IRR as the x-axis: what is the meaning of\nthe radius ratio and why should hydration be plotted against it? This is never explained. In Figure 3b\nthe x-axis seems to be way too low, and I suspect the ionic radii for an octahedrally coordination were\nincorrectly considered.\nResponse: Yes, 70 % of the data in Fig 2a is from [20]. We have inserted a comment and reference on\nthis in the text. Regarding Fig 2b: There is no other relevant x-axis, at least we don't have another\nsuggestion for x-axis. The ratio between the averaged ionic radii for the cations on AI and AII sites is\nof interest for structural and stability reasons. Since there is no other relevant x-axis, we hope for\naccept to keep it as it is. In fig.3b we use tabulated values for 12-coordinated Ln from\nhttps://www.sciencedirect.com/science/article/abs/pii/002245969190388X as ref on ionic radii (see\ncomment also for Reviewer #1.)\n6) As the authors underline how the extensive composition matrix (page 2) supports the correlations,\none would expect that the XAS spectra on Fig 2c-h would cover the same samples as in Fig 2b, while it\nonly Gd/La compositions are shown.\nResponse: We only chose examples of hydrating (BLC, BGC, BGLC37 and BGLC82) and non-hydrating\n(BGLC55, BPC and BNC) due to limited beam-time.\nAlso since the absolute intensity of XAS peaks is discussed, some more information about\nmeasurements (is it electron yield? Are surface states expected to be the same as the bulk?) and data\nreduction (how are the spectra normalized?) would be good. Also panels c-e and f-h are shown with a\ncommon x-axis, but the tickmarks don't align.\nResponse: The presented data were collected in the XAS fluorescence mode, thus it refers not only\nto the most outer layers of the materials, but also to the volume of samples. The data were\nnormalized according to the conventional procedure to the edge intensity after subtracting the\nbackground signal fitted with the first-order polynomial fitted to the pre-edge regime. This\ninformation was added to the experimental section. The tick-marks shift in the figure 2 resulted from\nthe slight changes in the graphs scale. The error was corrected. What is more, the line types used in\nfigure 2c were changed to be more distinguishable in a monocolour printed version.\n7) At least laboratory XRD with Rietveld refinements should be shown for each sample in the\nSupporting information. The only diffraction data actually shown is Fig S6, which is too small to\nappreciate any difference between the two panels.\nResponse:\nWe have expanded table S1 with structural information for all samples.\nThere is also no goodness of fit or statistical significance reported in order to tell the two models apart.\nPrevious data by the same authors (refs. 20 and 28) is much better presented in comparison.\nResponse:We have inserted statistical data for all refinements in table S1.\nReviewer #3:\nThis study presents the governing principles of protonation by systematically analyzing the hydration\ntrends of cobalt-based double perovskite, A^I A^II Co_2 O_(6-δ). The authors report that lanthanide A-\nsite dopants with a closed or semi-closed Ln 4f shell (i.e., Ln = La, Gd, Lu) induce electron hole transfer\nfrom eg to t2g in the Co-O bond, as supported by XAS measurements. Additionally, STEM-EDS, SR-PXD,\nand PND clearly demonstrate that hydration triggers a transition to an oxidized A-site disordered\n2a3b3c Pmmm supercell, which affects both oxidation and hydration equilibria. This work is well-\ndesigned, and the analysis is meaningful, particularly in designing mixed proton-electron positrodes\nthat maximize hydration and surface reaction kinetics. Such advances could offer substantial\ncontribution to the PCEC community. Therefore, I would recommend that this paper is suitable to be\npublished in this journal after revision with following comments.\n- In Figure S3, the concentration profile of BGLC37red with D2O exchanged at 600°C shows a distinct\npattern of initially increasing, then decreasing, and finally reaching saturation. This behavior is not\nmirrored in BGLC82. Could the authors elaborate on the underlying reasons for this unique trend in\nBGLC37red?\nResponse: Previous TEM studies have revealed a certain segregation of Ba cobaltite or Ln oxides or\nhydroxides to the surfaces or sub-surfaces. These segregations depend on composition (Gd/Ln),\ntemperature and atmosphere. Hence, any differences in the proton concentration profile from\nsurface to bulk between compositions may arise from cation concentration gradients. We have\ndecided to leave this issue for further studies. We value the comment and agree on its importance.\nNevertheless, we prefer to leave this topic out from the text.\n- The authors state that the hydrated supercell is oxidised and contains oxygen sites like O10, which are\nmore easily protonated than O4 and O6 in the a2b2c phase, leading to enhanced hydration and\noxidation in the 2a3b3c structure. Could the authors provide further explanation or references to\nsupport this assertion? Understanding the specific reasons why O10 sites are more susceptible to\nhydration compared to O4 and O6 would clarify this aspect of the study.\nResponse: This is a good point. We base this statement on previous studies that show a more\ncomplete TM-O electron transfer when the bond angle deviates from 180°C and the bond is\nelongated. We have made an entry on this and inserted references.\nOxidation is a result of – or causing – the phase transition itself. In the a2b2c phase, oxygen vacancies\ncompensate for the AI vs AII size mismatch. In the 2a3b3c phase there is no difference between the\nA-sites, and hence no need for oxygen vacancies.\n- Regarding a STEM-EDS results as provided in Figure 5a, the authors mentioned that the ordering\nstructure in BGLC37 become disorder after hydration. I’m simply wondering if this phenomenon may\nbe reversed. Does hydrated BGLC37 back into an ordered structure after dehydration? Furthermore,\ndo you have a change to check the chemical stability under hydration conditions?\nResponse: We have tried to reverse the ordering, but it has not been successful. Dehydration /\nreduction is much slower than hydration / oxidation, so it may be a matter of sufficient time. We\nhave a manuscript under preparation on chemical stability for the whole composition matrix in\nsteam.\n- In figure 2a, I think there are some missing TGA results. Please show all TGA data with all compositions\neven in supplementary figure.\nResponse: We don't show all TG curves as it would be too chaotic. In 2a we only show examples. All\nresults are in Table S1 and Fig. 2b.\n- Why the mass change of BLC with cubic structure and BLC with ordered structure is more than 4 times\ndifferent. Is it because the unit cell volume has increased? Please clearly explain this point.\nResponse: The four-fold increase in mass gain for ordered BLC reflects the different electronic\nstructure and different oxidation state for the two polymorphs. The more reduced and ordered\nstructure hydrates more because the more reduced Co ions transfer fewer holes to O. This\nphenomenon is indirectly supported by the findings in Fig d-h, where we show how depopulation of\nelectron holes in the Co orbitals correlates with susception to protons in the Co-O surroundings.\nt2g\nThe example we give in Fig. 2a is supported by the same effect for reduced and oxidised BGLC37 as\nshown in Fig. 3d and e. We have clarified with an entry in the manuscript.\n- Based on the TGA data in Figure 2a, fully Gd and La doped in BaLnCo2O6-d samples seem to exhibit\nthe highest mass change under both dry and wet conditions. Nonetheless, Gd and La co-doped sample,\nBGLC, is used as representative material in this work. There is a lack of clear explanation regarding this.\nResponse: Even though BLC and BGC gain more mass in wet atmospheres, we believe this is mostly\ndue to oxidation and that proton concentrations are small. H2O/D2O shifts may confirm this, but we\ndon't have them. One can also see that BLC and BGC maintain all the mass gained in wet conditions\nafter reverting to dry. Finally, XAS measurements of BLC and BGC in dry and wet sates indicate less\nrepulsion on electron holes from protons in these compositions, indicating lower proton\nconcentrations than in the BGLC compositions. Hence, we interpret the weight gain in wet\natmosphere as mostly oxidation. Furthermore, both fully La- and Gd-doped compositions are\nunstable in wet atmospheres: Orthorhombic BLC reverts to cubic (oxidised) structure in wet oxidising\nconditions, while BGC decomposes under water pressures relevant for its potential use in proton\nceramic cells. The introduction of mixed La/Gd composition stabilises the ordered structure while\nmaintaining the compositional integrity in high steam pressures. This is, however, the scope of our\nnext publication which is currently under preparation. We have made an entry in the manuscript\nregarding BLC(O) and BGC.\nMinor points:\n- There are errors in Figure 2. In caption b), “See also Table S2 “ need to be changed “See also Table S1”.\nIn figure 2b, there is duplicate data point with an IRR close to 1.15 (BSrPC). Please check it.\nResponse: Thanks for spotting these – they are corrected.\n- There are typos on page 6. In figure 3b, y1-axis is Co oxidation state and y2-axis is oxygen non-\nstoichiometry. And blue arrows indicate hydrating and red arrows indicate non-hydrating in figure 3b.\nIt seems to be incorrectly labeled, unlike the manuscript. Please revise these errors.\nResponse: Thanks for spotting this collection of our errors. We have corrected them in the text.\nReviewer #4:\nThis work presents a new governing principle for the hydration and proton transport in Co-based\ndouble perovskites. This principle is extracted from a comprehensive experimental campaign that\nincludes the preparation of a large material library (experimental matrix based on Ln combinations)\nand fundamental characterization looking at the atomic-level structural and electronic aspects together\nwith proton-transport methods.\nThe quality and completeness of the presented results are of the highest quality. The work is generally\nwell described, and the discussion is well articulated and succinct. I am convinced that this article will\nimpact the research community working on proton ceramics and electrochemistry. I recommend the\npublication of the paper subjected to minor revision, taking into account the following list of comments\nand suggestions:\n1- Fig. 2b. This chart is difficult to read, and I needed to go to Table S2 (back and forward) a couple of\ntimes. Is there a simple way to include the composition information directly in Fig. 2.\nResponse: We don't know how to make I clearer. We have tried to limit the numbering and labelling\nin the figure and left it as an overview rather than a source of data. However, we have expanded\ntable S1 with more information on all samples investigated.\n2- Page 6. When comparing proton concentrations in BGLC82 and BGLC37, it is properly concluded that\nthere is a dominance of Ln basicity over Co oxidation state. For the general reader, I recommend the\nauthors explain more in detail the term ‘Ln basicity’ in this context and its implications in the proton\nand electronic transport.\nResponse: We have emphasised this aspect – with Ln basicity based on electronegativity – in the\nmanuscript. The reviewer mentions transport, but the relevant discussion here deals with\nthermodynamics – not transport – of defects.\n3- Last paragraph, page 6. Please, use a unique symbol for deuterium. Now, D and 2H are used\nsimultaneously in the text, Fig. 3 and Fig. S3\nResponse: Thanks. We have made sure it is consistent. 2H is used for the hydrogen atomic isotope in\nthe SIMS results, while D O is used for deuterium oxide as is the standard abbreviation. These two\n2\nabbreviations are the ones most commonly used for the two purposes, and keeping them makes the\nmanuscript easier to read.\n4- Page 9. There is a very interesting conclusion: “the two lanthanides are sufficiently small to maintain\nthe same oxidation equilibrium”. Please, could you elaborate more on this and extrapolate to other\ncombinations and systems, i.e., as a general principle?\nResponse: It means that the TG profile is fully reversible for BGLuC. Hence, no oxidation or structural\nchanges and no slow mass gain in wet conditions. We have elaborated on it a bit more but have not\nbeen able to extrapolate to other systems or a general principle.\n5- Fig. 5. I suggest the authors label each individual image and re-arrange it to improve the overall\nreadability of the figure. In addition, as a general conclusion of the figure, it is stated, “We note that\npure hydration in mixed valence oxides like the ones we consider here occurs only under specific\ncombinations of pO2 and pH2O.” Can the authors be a bit more specific on the type of combinations\npO2-pH2O enabled hydration and the associated materials properties that determine this ‘hydration\nspace’.\nResponse: We have labelled each part. The conclusion is based on the measured hydroxidation and\nhydrogenation in oxygen and N , respectively, for BGLC37 red(uced). The combination of pO and\n2 2\npH O is dependent on the average Ln radius and how resistant the structure is to A-site disorder. We\n2\nhave added a sentence on this in the manuscript, and the issue is also treated in the elaborated\ncomment on BGLuC (reviewer comment 4 above) in the manuscript.\n6- To illustrate the potential and versatility of the proposed principles, the authors could comment on\nthe potential application of these principles to the design and understanding of other mixed oxides\nexhibiting hydration and/or proton transport, if possible, providing specific examples of composition\nand crystalline structures. Can this be connected with disorder or entropic effects in different classes\nof oxides?\nResponse: The most important consequence of these findings is how the TM-O bond can be\nengineered to adjust the population of electrons and holes. This means adjusting the TM-O charge\ntransfer, and thus the ionicity of the bond. The secondary – or less detrimental – governing principles\nstate that Ln basicity is more important than Co oxidation state, as illustrated by the higher proton\nconcentration in the more oxidised, but also more basic BGLC37 than in the more reduced, but less\nbasic than BGLC82 and the even more reduced and even less basic BGLuC82. We also confirm the\nimportance of bond-angles, as has previously been only theoretically determined. It is difficult for us\nto suggest possible structures or compositions here, but we have added a sentence in the text on the\npractical importance of the findings.\nResponses to reviewers #1, #3, and #4:\nThese reviewers recommended the manuscript revision 1 to be published without comments.\nResponse to reviewer #2\nReviewer #2 general comment: In the response letter and revised paper, the authors have partially\naddressed my earlier comments. However, my general assessment of this paper has not changed. I\nfind the conclusions to be not adequately supported by the structural data shown here, and these data\nthemselves are quite uneven, and presented and discussed in an unsatisfactory way. Since most of the\ndata have already been published, the necessity of a rapid communication is not evident.\nReviewer #2 comment 1): In my earlier review, I observed a disconnection between very general\nclaims (\"[...] NPD unambiguously show that hydration triggers a transition\") and the lack of actual data\nshown. Such a disconnection still seems to be there. Now two NPD traces (one sample out of all)\nare shown in fig S7, but there is still no actual NPD refinement, or evidence of a different phase being\npresent in the hydrated vs. dry state beyond all the oxygen positions being full and the protons being\nin the lattice. The XRD patterns of fig S6, which are fitted to different supercells, look identical down\nto the pixel level, so the choice of different supercells remains unexplained. Summing up, both the XRD\nand NPD data shown here do not seem to support the very general conclusions.\nReply: We agree that structural data alone does not give an unambiguous conclusion on the structural\ntransition. It is the combination between STEM-EDS elemental mapping and XRD / NPD that allow us\nto make such a bold statement. It has been challenging to arrive to this conclusion given a clear Ba-Ln\norder-disorder transition on the A-site as shown by STEM-EDS and at the same time no change in the\nXRD diffractogram upon hydration. Seeing that a disordered supercell gives the same Bragg reflections\noffer a logical and sound explanation to both of these observations.\nRegarding the number samples characterised by NPD, we here only present two – BGLC37 and BGLC28.\nThe reason for this is the sample cost, as these samples must be made with a non-neutron absorbing\n160Gd isotope. None of these results are previously published, and since we believe the hydroxidation\nreaction to be a general reaction for mixed conducting electrode materials, we find the results highly\nrelevant and worthy of rapid communication.\nHaving said that, we fully understand the reviewer’s comment regarding NPD – not only SR PXD –\nrefinements to support our claim. We have therefore also included refinements of NPD on the dry and\nhydrated BGLC37 isotope sample. The refinements show a slightly – maybe insignificantly – better fit\nto the disordered 2a3b3c supercell with Rwp value slightly lowered for both dry and hydrated samples\nas compared to refinement to a2b2c orthorhombic structure. Figure S7 is now modified to include NPD\nrefinements results. We have also included the NPD refinement results in S.I.\nReviewer #2 comment 2): The authors have now included more details in the Methods section\nconcerning the XAS measurements. However, the Lanthanide M4,5 and Co L2,3 edges are also\nmentioned, but they are neither shown nor discussed.\nReply: We apologize for the inconsistency in the experimental description. The information on the\nother edges has been removed from the manuscript.\nReviewer #2 comment 3): The data in fig 2c are adapted from ref 29. However, spectra of BGLC37, 55\nand 82 in fig 2e-g also seem to have been published already (doi:10.1039/D2DT03989C) by the same\nauthors: this latter paper however is not cited here, nor is the interpretation concerning spin states,\nwhich might be relevant also for the present manuscript. The spectra also look different (BLGC37 peaks\nat lower energies than both BGLC55 and BGCL82 in this manuscript, but at higher energies in the\npublished paper; BGLC55 is much sharper than the others in this manuscript, but the shapes are all\nvery similar in the published paper). Can the authors comment on the apparently inconsistent\npresentation and discussion of the spectra?\nReply: The data presented in doi:10.1039/D2DT03989C and this article were measured in different\nprojects under different grants and under non-identical conditions. Data presented in the above-\nmentioned paper were measured in TEY mode and under a narrower energy range (510-570 eV) (Fig.\n1 below), which covers mostly the pre-edge features of the spectra. Consequently, these data could\nnot be normalized to the edge regime, but we decided to conduct the normalization to the pre-edge\nfeature around 528 eV. We are aware that with this approach we lost the information on the relative\nintensities of the spectra, but we decided to conduct a quantitative analysis of pre-peak position, which\nis related to the Co oxidation and spin state. What is more, since the data published in\ndoi:10.1039/D2DT03989C were acquired in the TEY mode they differ from the data presented in this\nwork.\nFigure 1 XAS spectra in the whole measured energy range, presented in doi:10.1039/D2DT03989C.\nFor this study, we used the FY mode, which is more representative for the bulk properties than the TEY\nmode. We also expanded the energy range to 475-750 eV, which enables us to normalize data to the\nedge regime. The acquisition mode was also changed to FY. See Fig. 2 below. This approach gives\ninformation not only on the present spin and oxidation state but also the relative density of unoccupied\nstates. Furthermore, this study aimed to show the difference between the material before and after\nexposure to a humidified atmosphere. For this purpose, the samples were initially dried at 900 °C for\n24 h and then exposed to water vapour. The spectra of dried and humidified materials are presented\nin this manuscript. In the previous studies, as-synthesized materials were characterized. We\nelaborated on materials preparation in the experimental section. The difference in both sample\npreparation and measurement conditions is the reason for the observed differences between the two\ndata sets.\nFigure 2 XAS spectra in a wider energy range\nReviewer #2 comment 4): Using a ratio between ionic radii as an x-axis for fig 2b is left unexplained,\neven in the rebuttal letter. Since there is no apparent relation between hydration and IRR, the choice\nof IRR is even more confusing.\nReply: We believe we answered this comment in the previous rebuttal letter. We there replied “There\nis no other relevant x-axis, at least we don't have another suggestion for x-axis. The ratio between the\naveraged ionic radii for the cations on AI and AII sites is of interest for structural and stability reasons.\nSince there is no other relevant x-axis, we hope for accept to keep it as it is.” We have at present no\nfurther explanation to offer and again think it should be left as it is.\nReviewer #2 comment 5): As I already pointed out in my earlier comments, the ionic radii used in fig\n3b (and fig S1d) seem to be wrong, and they are not consistent with the source provided by the authors\n(Jia, J Solid State Chem 1991, 95, 184). Even if Shannon radii are not available for the whole lanthanide\nseries, the dataset by Jia agrees with the Shannon radii to a large extent.\nReply: We are embarrassed, the reviewer is right. The x-axis values for average Ln radius were indeed\nwrong and not according to our reference. The error is now corrected and figure 3b shows the correct\nvalues for average Ln radii.\nReviewer #2 comment 6): As the O K-edge spectra are collected in fluorescence mode, the penetration\ndepth at 530 eV, assuming an atomically flat surface, is on the order of 150 nanometres. A quantitative\nassessment of how this can be considered as a representative average of the whole sample volume\nwould be good.\nReply: The two basic modes of acquisition in X-ray absorption are TEY and FY. The latter, used in this\nwork, is the most suitable for assessing the volume properties of the material. A 150 nm penetration\ndepth is far deeper than surface space charge layers (and accompanying segregation) in such a material\nwith high defect and conductivity levels. It is also far deeper than expected diffusion would reach at\nambient temperatures during storage and transfer of samples prior to XAS. We equilibrated the\nsamples in either dry or wet conditions before the measurement and may expected on basis of the\nabove that deviations near the surface play a minor role in XAS/FY compared to the bulk.\nReviewer #2 comment 7): The authors state that the normalization procedure for XAS follows \"the\nconventional procedure to the edge intensity\", but the actual edge intensity is expected to be visible\nonly above ca. 550 eV. Cfr. also doi:10.1039/D2DT03989C, where the same spectra were apparently\nnormalized in a very different way. In any case, showing just a 6 eV range is not an acceptable XAS data\npresentation. This is a very important point as the authors discuss the absolute intensity of these peaks.\nThe oxygen near-edge spectra must be shown in full in the SI so the normalization procedure can be\nappreciated, and the peaks due to the interactions with Ln 5d states are also shown.\nReply: As described in detail in the answer to comment no 3, we agree with the Reviewer that the\nactual edge intensity is visible at energies higher than 550 eV. The data presented in this manuscript\nwere normalized to the post-edge intensity level (550-570 eV), after background subtraction. We\ninserted the spectra in the wider energy range in the SI."}, {"round": 2, "reviewer_comments": {"reviewer_1": "the revision has sufficiently resolved my questions, now it is recommended for publication", "reviewer_2": "In the response letter and revised paper, the authors have partially addressed my earlier comments. However, my general assessment of this paper has not changed. I find the conclusions to be not adequately supported by the structural data shown here, and these data themselves are quite uneven, and presented and discussed in an unsatisfactory way. Since most of the data have already been published, the necessity of a rapid communication is not evident.\n1) In my earlier review, I observed a disconnection between very general claims (\"[...] NPD unambiguously show that hydration triggers a transition\") and the lack of actual data shown. Such a disconnection still seems to be there. Now two NPD traces (one sample out of all) are shown in fig S7, but there is still no actual NPD refinement, or evidence of a different phase being present in the hydrated vs. dry state beyond all the oxygen positions being full and the protons being in the lattice. The XRD patterns of fig S6, which are fitted to different supercells, look identical down to the pixel level, so the choice of different supercells remains unexplained. Summing up, both the XRD and NPD data shown here do not seem to support the very general conclusions.\n2) The authors have now included more details in the Methods section concerning the XAS measurements. However, the Lanthanide M4,5 and Co L2,3 edges are also mentioned, but they are neither shown nor discussed.\n3) The data in fig 2c are adapted from ref 29. However, spectra of BGLC37, 55 and 82 in fig 2e-g also seem to have been published already (doi:10.1039/D2DT03989C) by the same authors: this latter paper however is not cited here, nor is the interpretation concerning spin states, which might be relevant also for the present manuscript. The spectra also look different (BLGC37 peaks at lower energies than both BGLC55 and BGCL82 in this manuscript, but at higher energies in the published paper; BGLC55 is much sharper than the others in this manuscript, but the shapes are all very similar in the published paper). Can the authors comment on the apparently inconsistent presentation and discussion of the spectra?\n4) Using a ratio between ionic radii as an x-axis for fig 2b is left unexplained, even in the rebuttal letter. Since there is no apparent relation between hydration and IRR, the choice of IRR is even more confusing.\n5) As I already pointed out in my earlier comments, the ionic radii used in fig 3b (and fig S1d) seem to be wrong, and they are not consistent with the source provided by the authors (Jia, J Solid State Chem 1991, 95, 184). Even if Shannon radii are not available for the whole lanthanide series, the dataset by Jia agrees with the Shannon radii to a large extent.\n6) As the O K-edge spectra are collected in fluorescence mode, the penetration depth at 530 eV, assuming an atomically flat surface, is on the order of 150 nanometers. A quantitative assessment of how this can be considered as a representative average of the whole sample volume would be good.\n7) The authors state that the normalization procedure for XAS follows \"the conventional procedure to the edge intensity\", but the actual edge intensity is expected to be visible only above ca. 550 eV. Cfr. also doi:10.1039/D2DT03989C, where the same spectra were apparently normalized in a very different way. In any case, showing just a 6 eV range is not an acceptable XAS data presentation. This is a very important point as the authors discuss the absolute intensity of these peaks. The oxygen near-edge spectra must be shown in full in the SI so the normalization procedure can be appreciated, and the peaks due to the interactions with Ln 5d states are also shown.", "reviewer_3": "It seems that the significance of this work has been improved through the revision stage. The authors clearly responded to all the reviewer's comments, and the revision has significantly enhanced this manuscript. I have no further remarks regarding this manuscript.", "reviewer_4": "The current manuscript is suitable for publication"}, "author_response": ""}, {"round": 3, "reviewer_comments": {"reviewer_2": "The authors have amended the technical shortcomings in the manuscript concerning diffraction and XAS data presentation."}, "author_response": ""}]

Decision letter