The RAPTR furnace: a rapid heating and cooling sample furnace for in situ X-ray scattering studies of temperature-induced reactions. Corrigendum

Errors in the article by Hu et al. [J. Appl. Cryst. (2024), 57, 88–93] are corrected.

For the article by Hu et al. (2024), errors in the structural models fitted to the diffraction data for an example solid-state reaction are corrected. The corrections are confined to Section 5 of that article, which illustrates an application of the RAPTR furnace. A revised version of this section is provided below, together with the corrected Fig. 6.

For the reaction of PbO and WO₃, the WO₃ polymorph present at the reaction temperature was incorrectly modelled as the β polymorph rather than α-WO₃. In addition, the reaction intermediate was misidentified as Pb₃WO₆ instead of Pb₂WO₅. These errors affected the quantitative phase analysis, reported reaction evolution and mechanistic details.

5. Capturing the fast solid-state reaction of PbO and WO₃

The synthesis of the scheelite-type PbWO₄ was evaluated as a model stoichiometric solid-state reaction (PbO + WO₃ → PbWO₄). Traditionally, scheelite-type oxides are synthesized via conventional solid-state synthesis routes, heating in air at high temperatures (700–900°C) for long time periods (>2 h) (Mullens et al., 2023). Both Pb and W are heavy elements, and as such are expected to be relatively immobile, such that a slow reaction would be anticipated. An initial test reaction undertaken ex situ by heating a stoichiometric mixture of PbO (Puratronic, 99.999%) and WO₃ (NanoAmor, 99.9+%) at 750°C in air in a muffle furnace for 12 h yielded scheelite-type PbWO₄ as a phase-pure product.

Stoichiometric quantities of reagents were mixed and formed into pellets (∼70% packing density) to optimize contact between particles, before loading into the 1.1 mm OD SiO₂ glass capillary (Kamm et al., 2022). Capillary-loaded reaction mixtures were assembled within the RAPTR furnace, and fast time-resolved X-ray diffraction data were collected at beamline 11-ID-B of the Advanced Photon Source at Argonne National Laboratory (λ = 0.2116 Å, sample-to-detector distance ≃ 1.0 m) following translation of the mixture into the hot zone at 750°C. Data were collected with 1 s exposures during the first minute following translation, and then at increasing intervals at longer times: 2 s exposures (1–2 min), 5 s exposures (2–4 min), 10 s exposures (4–6 min), 30 s exposures (6–10 min), matching the data density to the anticipated rate of change for the reaction.

The time-resolved in situ diffraction data acquired following rapid heating to initiate the reaction using the RAPTR furnace showed that the solid-state synthesis of PbWO₄ proceeds via three steps (Fig. 1):

Figure 1 [Revised version of Fig. 6 of Hu et al. (2024).] Phase evolution during the reaction of PbO and WO3 at 750°C within the RAPTR furnace, from Rietveld refinement of the time-resolved powder diffraction data.

At room temperature, a three-phase mixture of α-PbO (P4/nmm, a ≃ 3.9 Å, c ≃ 5.0 Å), β-PbO (Pbcm, a ≃ 5.9 Å, b ≃ 5.5 Å, c ≃ 4.8 Å) and γ-WO₃ (P2₁/n, a ≃ 7.3 Å, b ≃ 7.5 Å, c ≃ 7.7 Å) is observed. During the first 10 s, as the reaction mixture heats to 750°C, WO₃ undergoes a series of structural phase transitions – P2₁/n (γ) to Pcnb (β, a ≃ 7.4 Å, b ≃ 7.6 Å, c ≃ 7.8 Å) to P4/ncc (α, a ≃ 5.3 Å, c ≃ 7.8 Å) – which are known to occur between 300 and 350°C, and between 720 and 850°C (Vogt et al., 1999), respectively. Crystalline PbO peaks disappear in tandem with the formation of crystalline products and an increase in diffuse scattering, either from disorder within the crystalline product or from a mixed PbO–WO₃ melt (pure PbO melts at 888°C and pure WO₃ melts at 1473°C). At 750°C, Rietveld refinement indicates that three phases are present: WO₃ (P4/ncc), the PbWO₄ product (I4₁/a) and an additional crystalline intermediate, which was determined to be Pb₂WO₅ (C2/m , a ≃ 14.3 Å, b ≃ 5.9 Å, c ≃ 7.6 Å, β ≃ 114°). The amount of WO₃ and Pb₂WO₅ decreased as the PbWO₄ product increased. The consumption of WO₃ and Pb₂WO₅ to form the PbWO₄ product occurs most rapidly in the early stages of reaction and gradually slows, with more than half the PbWO₄ product formed within the first 30 s. After 5 min, peaks from PbWO₄ are observed as a near-phase-pure reaction product.

The Pb-rich intermediate Pb₂WO₅ phase observed here is a metastable, kinetic product which would most likely have been overlooked in conventional experiments (Jantz et al., 2017). In this phase, lead oxide layers, consisting of edge- and face-sharing 7- and 9-coordinate polyhedra, are connected by WO₄ tetrahedra.

We note that the initial phase that forms, the Pb₂WO₅ phase, is richer in the lower valent, more mobile cation than the final product (i.e. Pb). That is, the Pb₂WO₅ intermediate has a higher Pb²⁺:W⁶⁺ ratio than the PbWO₄ product. This may suggest that cations from the more mobile species diffuse or insert into other phases faster than they redistribute. This observation leads to pertinent questions on how the reaction kinetics and stoichiometry of intermediates depend on the relative mobility of ions within the target product.

Related literature. The following references are cited only in the supporting information: Bush et al. (2023), Coelho (2018), Dinnebier et al. (2018), Jantz et al. (2020), Kaurova et al. (2016), Moreau et al. (1996), Moreau et al. (1999), Takai et al. (2004), Tatsumi (1997), Triantafyllou et al. (1997), Wang et al. (2012), Zagorac et al. (2019).

Footnotes

‡These authors contributed equally.