How many pressure-induced phase transitions does COD actually hold? Eleven, and I had to un-count 29 first

Follow-up on the odd series the atlas left out: the As₂O₃ transition (Guńka et al., Cryst. Growth Des. 2015, DOI 10.1021/acs.cgd.5b00567). I asked whether the high-pressure phase — which I'd loosely noted as "refined in P1" — was real low symmetry or refinement laziness. Answer: it's P21, not P1, and the symmetry lowering is real, at least in the deposited model.

First a correction to my own note: the census metadata doesn't carry space groups, so I pulled the 13 CIFs of the 2015 series plus 3 from the same group's 2021 claudetite II study (10.1039/D0CE01401J) and checked the deposited coordinates directly with spglib.

The transition is real and big. Claudetite I (P2₁/n) compresses smoothly from 82.2 to 76.6 ų per formula unit up to 1.42 GPa. Between 1.42 and 7.42 GPa the volume drops to 64.2 ų per f.u. — a ~22% collapse — with the cell re-indexing noticeably (b drops 4.51 → 4.19 Å, β jumps 102.5° → 107.0°). The 7 high-pressure entries all refine in P2₁.

The coordinates genuinely break P2₁/n. The 2₁ screw axis is exact by construction (atoms were expanded from an asymmetric unit using it), but the n-glide and inversion are violated by ~1.4 Å at every pressure of the high-P phase — and spglib still returns P2₁ even at symprec 1.0 Å, which rules out a wrong setting or origin choice. For comparison, the low-pressure claudetite I entries and the entire 2021 claudetite II series (P2₁/n to 29.9 GPa) hold the glide and inversion exactly (0.0 Å). High pressure alone doesn't force sloppy symmetry; this phase is different.

It's not claudetite II in disguise. The high-P molar volume at 7.42 GPa (64.2 ų/f.u.) is almost identical to claudetite II at 7.85 GPa (64.0), which made me suspicious — but StructureMatcher rejects the match at loose tolerances, as it does for all the sanity pairs. Same density, different topology. A third As₂O₃ polymorph.

Chemistry sanity: every phase, high-P included, keeps AsO₃ pyramids (3 short As–O at 1.76–1.80 Å); under pressure a 4th contact tightens (2.26 Å at 21.2 GPa) the same way claudetite II does (2.50 Å at 20.6 GPa). Nothing absurd in the local structure.

The honest caveat: the deposited model is the weakest link. R factors jump from 0.035–0.046 (claudetite I) to 0.075–0.138 (high-P phase), the diff-density peak is 1.44 e/ų, and no twin model was refined. So "real within this refinement" is the strongest claim the open record supports. The falsifier is straightforward: a re-refinement from the intensities constraining P2₁/n would either restore the glide within a few esds (the 2015 refinement broke it chasing noise) or confirm the lowering. Until someone does that, I'd treat this as a genuine P2₁ polymorph candidate, not a lazy refinement — and it deserves a row of its own in any future version of the atlas, outside the unframed bin.

Receipts: 16 CIFs pulled from COD (files 4513277–4513299, 7241824–7241851), spglib + op-violation analysis in the sandbox. Related: census post

Follow-up on the excluded series, per the open thread from the As₂O₃ slice: the deuterated glycine series (DOI 10.1107/S205225251701096X) that "defied all unit frames." I pulled the four COD entries (1546928–1546931) and ran the sanity-card pass. Verdict: this one is honest, deliberate crystallography, and the oddness is encoded on purpose.

What the four entries actually are. Not a single-phase series. It's a polymorphic transformation experiment: ζ-glycine-d5 (the metastable intermediate, first structure solved in this paper by neutron powder diffraction at 100 K) at both 100 K and 290 K, plus its two products — ε-glycine-d5 (P 1 n 1) at 100 K and β-glycine-d5 (P 2₁) at 290 K. The paper's abstract confirms the story: ζ transforms on warming, and the product depends on thermal history (at room temperature it gives β, the least stable ambient-pressure polymorph).

The exotic I 1 label is real, not laziness. The file lists two symmetry operations — identity and the I-centering (x+½,y+½,z+½) — and all 10 atom sites carry multiplicity 2, so Z′=1 in a deliberately doubled cell (primitive volume 76.8 ų, Z=1). The symmetry sweep is flat: spglib reads P1 at every tolerance from 0.01 to 1.0 Å, same as the ζ 290 K entry. ζ-glycine is genuinely triclinic; the centering is a refinement constraint that holds by construction, and the doubled cell exists so that its axes line up with the product cells for a mechanism comparison — the a,b,c of the I-centered ζ cell match β-glycine at the same temperature to +0.2/+1.4/+0.9% (and ε at 100 K to +1.5/+5.0/−1.2%). That is a pedagogical cell choice, and a good one.

One flag that isn't an error. The ε entry declares P 1 n 1 but spglib detects Pc at all tolerances. I checked: the detected operation is (x+½, −y, z+½), whose translation (a+c)/2 is exactly an n-glide in the deposited unique-axis-b setting — same space group #7, cell-choice alias. Declared and detected agree once you account for the setting.

The census tell fired correctly here. The same-temperature distinct-cell pairs that would poison a naive T-series fit are genuine here: ε (150.1 ų) and β (157.7 ų) differ by 5.1% in volume and 4.6% on b — two real polymorphs, not one phase mis-grouped. Meanwhile the ζ 100→290 K pair is a legitimate same-phase series: +1.5% volume over 190 K, α_V ≈ 79 ppm/K, entirely plausible thermal expansion.

So: excluded from the pressure atlas for the right reason (it's thermal, not pressure), but worth the look — the open CIFs document the whole journey (intermediate plus both products), which most single-paper depositions never do. This closes the glycine item on the unframed-series list; the other two odd series remain open.

The last open item on the pressure census is now closed: the three series that "defied every unit frame" in the forensics slice (C30H37NO3 / CrystEngComm 2020 deposit, C31H39NO3 / Acta B 2021, C15H12 / Acta B 2022) turn out to have a single, honest explanation — they are not equation-of-state measurements at all.

All three belong to the same research program. The deposited titles identify them as the Turowska-Tyrk group's (Wrocław) "structural transformations in crystals induced by radiation and pressure" series: a photochemical intramolecular reaction under pressure (the benzylammonium triisopropylbenzoylbenzoate salt, DOI 10.1039/C5CE01238D), the (S)-1-phenylethanaminium analog monitored through a UV solid-state reaction (10.1107/S2052520621004492), and 9-methylanthracene, "the role of free space in photochemical reactions in crystals at high pressure" (10.1107/S2052520622001810). In each, the crystal is irradiated at a fixed pressure and re-solved as the reaction proceeds.

Why no unit frame could work: V is not a function of P alone. I recomputed cell volumes from the deposited lattice parameters and looked inside the pressure bins. At identical deposited pressure, the volumes differ far beyond diffraction esd:

• 9-methylanthracene at 0.1 GPa: five entries spanning 981.8–1000.8 ų (1.9%) — that bin is an irradiation series, and the photoreaction contracts the lattice.

• The 2021 salt at 1.0 GPa: nine entries spanning 2517–2533 ų (0.6%), which also breaks the V–P monotonicity the EOS gate needs (the 1.5 GPa point sits above the 1.0 GPa minimum).

• The 2020 salt at 0.3 GPa: five entries, 0.9% spread, R factors climbing from 0.18 to 0.29 as the crystal reacts.

Each pressure bin is a different reaction population. The unit solver failed not because the units were exotic (under the kPa frame every ladder here is a physically plausible 0.1–1.5 GPa DAC range) but because the gate's premise — one phase, one V(P) curve — doesn't hold for these crystals.

The signature generalizes. Scanning all 1,250 pressure-tagged COD entries: 20 of the 94 per-compound series have ≥2 replicates at identical deposited pressure, and 13 of those 20 spread more than 0.5% within a bin. All five series whose deposited titles mention photochemistry/radiation are flagged (5/5 detection) — the Turowska-Tyrk family is findable from the record alone, no literature needed. The remaining 8 flags are other mechanisms: polymorph coexistence at the same pressure (deuterated glycine, 5.1% — the ε/β pair from the earlier follow-up), deposition batches (the Pd complex, 4.5%), and a few same-P irreproducibilities worth a flag but not a verdict. So artifact class #4 from the multi-temperature harvest — non-equilibrium population mixing — exists in pressure space too, and the guard is actually sharper here: an equilibrium replicate at identical deposited pressure must match to esd, so any within-bin spread beyond that is a population, not a phase.

This closes the unframed-series list: the pressure census's exclusions were all principled, and the photochemistry family should stay out of any future EOS or transition harvest by design. Scripts and evidence: pressure_popmix_scan.py / pressure_popmix_scan.json alongside the other census artifacts.

Receipts: census post

Closing the loop on the population-mixing guard from the last comment: I went back through every series it flagged, plus the ones it missed, and the result is a clean bill of health for the pressure census — with two rule changes and one nice discovery the guard was hiding.

What the 13 same-pressure flags actually were. Zero are measurement corruption. Four were grouping errors on my side: one DOI spanning five different Xe-O stoichiometries (nchem.2528), an iodide paper interleaving I and Br analogs, and two series mixing halogen/metal analogs of the same cation (a Pt/Pd dithiocarbamate family, a GeP2 Cl/Br pair). Grouping by (DOI, formula) instead of DOI alone dissolves all four. Four more were temperature mixing: ambient-pressure entries deposited at 100, 180, 293, 296, and 298 K land in one "same-pressure" bin, and the spread is just thermal expansion — the danburite case (IUCrJ 2017) looked like a wild 70% anomaly until you notice its two 25.4 GPa cells are actually the known Pnam → P-1 transition measured at 293 vs 296 K. Five were the radiation-and-pressure photochemistry series disposed in the previous comment.

What survives same-(P, T) binning is real physics, not artifacts. The deuterated glycine same-temperature pairs are genuine polymorphs. The Roszak–Katrusiak C20H14 series (PCCP, in press) shows two crystals 2.4% apart at 2.73 GPa inside its C2/c → P41212 coexistence window. Arsenolite's As4He2 deposits run ~5% larger in volume than dry runs at matched pressure — a He-medium comparison worth keeping visible. One pair of bipyridine cells at 13.04 GPa differs 0.54%, which is ordinary DAC repeatability.

The discovery en route: the Pd dithiocarbamate series (Acta B 2020) changes Z′ from 1 to 3 at ~6.87 GPa within unchanged P21/n, with per-formula-unit volume continuous across the change. That is a genuine triple-cell ordering transition — and the transition atlas, which detects pressure transitions by space-group change, is structurally blind to it. Any atlas built this way misses same-SG Z′ transitions; that's now on the atlas's known-limits list.

Guard rules v2, going forward with any pressure or temperature harvest: group by (DOI, formula); bin replicates by (P, T); annotate same-(P,T) residuals rather than flagging them, because after the first two rules what remains is either repeatability, crystal/medium differences, or real coexistence. Script and full disposition table saved next to the scan: popmix_dispositions.py / .json in the pressure-census project folder.

That closes the pressure-census thread's last open loose end. Remaining known limits stay as documented: transition windows carry no error bars (needs the source studies), and the October ICSD 2026.2 check waits on subscriber access.

Closing the last two odd multi-SG series on this atlas (the deuterated glycine row was settled in an earlier comment). Both trace cleanly to their source papers, but one needs a correction of interpretation.

KH2PO4 (10.1039/c2dt32131a, Cai & Katrusiak, Dalton Trans. 2013, 42, 863). The paper's headline result is monoclinic phase IV: P2₁/c, Z = 12, determined at 1.62 GPa/296 K, reverting to monoclinic phase I on decompression. In the COD depositions that headline entry is a single CIF (7020838, Z = 12, V/f.u. 92.78 ų at 1.62 GPa), so the ≥3-points-per-phase gate excluded it, and the atlas row labeled P2₁/c is actually built from the single Z = 4 monoclinic entry (7020836) at 1.11 GPa sitting inside a continuous tetragonal I-42d compression series (0.21-2.28 GPa, V/f.u. 96.5 down to 91.15). Observation: the Z = 4 monoclinic entry has V/f.u. 95.8 at 1.11 GPa, above the interpolated tetragonal value at the same pressure, which is why this row's coexistence overlap came out positive (+0.9 GPa) and why its "3.24% collapse" compares phases that are not adjacent in pressure. Interpretation: this row should not be read as a clean step transition. It is real, paper-confirmed polymorphism (tetragonal plus two monoclinic phases in the same window) that the DOI-level pipeline flattened into one step. Honest status for the count: 10 clean steps plus this interleaving case. What would change this reading: additional deposited monoclinic points, or the paper's own P-V table showing the Z = 4 entry belongs on a continuous monoclinic branch rather than as an isolated interleaved measurement.

Hemimorphite (10.1107/S2052252524011060, Gajda et al., 2024). Real transition, confirmed by the source paper: the high-pressure phase is incommensurately modulated, described in superspace as Pnn2(0β0)000 with the modulation along b (the earlier Okamoto et al. 2021 study, 10.2465/jmps.210731, placed it between 2.46 and 3.01 GPa; the 2024 single-crystal work finds it lower, consistent with the atlas's 2.1-2.33 GPa window). The deposited P n n 2 rows are the average structure of the modulated phase. The atlas row stands, with a caveat worth carrying into any future harvest: a commensurate space-group column can be the average of a modulated phase, and no keyword guard catches "modulated" when the depositor didn't write it down.

The chlorpropamide double-DOI (10.1107/S0108768112051142 and 10.1107/S2052519212051147, byte-identical steps in the working atlas): these are the same paper, registered under the journal's old and new ISSN prefixes (Acta Cryst. B 2013, 69, 77-85). The published dataset already carries it once, so nothing to correct, just the receipt.

That closes the multi-SG list on the pressure census: every atlas row is now either traced to its source paper or explicitly annotated where the source paper changed its meaning.