How does crystalline solids form from solution? Crystallization of lanthanide(III) hydroxides

doi:10.21203/rs.3.rs-3145015/v1

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How does crystalline solids form from solution? Crystallization of lanthanide(III) hydroxides

https://doi.org/10.21203/rs.3.rs-3145015/v1

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Technology and innovation requires new high performance materials, however our knowledge of how crystalline materials are formed is limited.^1-5

We can control the growth of single crystals from melts and make semiconductors, optical fibers, and jet turbine blades.⁶ And we can with atomic precision make molecular architectures in the lab,⁷ or by exploiting biotechnological advances.⁸

However, where we for the creation of molecular species have a detailed mechanistic understanding, we do not have a succinct model that describes the formation mechanism of crystalline materials from solutions containing the molecular building blocks.^9-11

Here we show that the formation of lanthanide(III) hydroxide crystallites does not take place in solutions,^12-17 but happens upon dehydration on the filter. The data reveals that no solids are present in solution and that no crystallization occur. In solution a strong, washable gel phase is formed. Using total scattering,^18-21 it is documented that no long range order is present in solutions, and that the gel is constructed from clusters of lanthanide(III) ions.

Despite the emergence of studies of the structure of crystallites in solutions and in melts,^22-31 the early stages of formation of solids from ions and molecules in solution remains poorly understood.^{21, 32-34}

The process from solvated ion to cluster to agglomerate can be described like any other chemical transformation, and must follow a reaction profile with energy barriers, intermediates, and transition states—and will be governed by reaction kinetics and thermodynamics. The first step is to map all species involved between solute and nanocrystal, and here studying materials made from lanthanide(III) ions are useful as luminescence, electron miscopy and scattering is strong with these ions.

Physical sciences/Chemistry/Inorganic chemistry/Solid-state chemistry

Physical sciences/Materials science/Nanoscale materials/Synthesis and processing

Physical sciences/Chemistry/Physical chemistry/Reaction kinetics and dynamics

Undergraduate chemistry textbooks teach reaction kinetics and thermodynamics, which often is depicted as a reaction coordinate showing the species involved in the chemical transformation on an energy axis. The visualization neatly summarises energetics, kinetics, and the atomic movement in a reaction. Although the bookkeeping involved in a transformation from species in solution to species in a solid structure is more complex, these general concepts from reaction kinetics must be applicable to transformations from molecules in solution to molecules in the solid. Nevertheless, we are yet to have a strong model for how these transformations occur.^{4,5, 21-34}In particular the formation of crystalline solids remains elusive, despite the importance in e.g. drug delivery.³⁵

In the field of lanthanide coordination chemistry, a transformation from ions in solution to a—assumed crystalline—solid precipitate is one of the main tools in making and purifying complexes. The excess of lanthanide ions is removed by precipitation by addition of aqueous hydroxide. The reaction product is a solid hydroxide that can be removed by filtration after 30-60 minutes. We assumed that the long timescale of this well-known and frequently used reaction, combined with the high electron density and luminescence of the lanthanides, would make lanthanide(III) hydroxides an ideal model system for investigating the transformation from solution to crystalline solid.

Lanthanide(III) hydroxides are a well known crystal system, where the final form of the crystals can be engineered extensively. We chose a system that contained only Ln(III), nitrate, and sodium. In this system we investigated the precipitation of lanthanum(III) and neodymium(III) hydroxide, by adding sodium hydroxide to a solution made by dissolving lanthanide(III) oxides in nitric acid.

Following the established theories,^{9-11, 21-34} we would expect that pre-nucleation clusters would form followed by seed crystal formation and then crystal growth (Figure 1, top). Optical spectroscopy, pH, and X-ray total scattering can follow changes in coordination geometry; optical spectroscopy, light scattering, electron microscopy, and X-ray diffraction can follow crystal formation. By combining these techniques, we are able to monitor the entire process in detail, and when we do, we find that the process is very different to what the prevalent theory describes.

Figure 1 show the crystal formation process as a reaction coordinate. We start by forming a hydrated Ln(OH)₂NO₃ complex (Figure 3A) by addition of base to the solution containing aqueous lanthanide(III) nitrate, this species immediately form an opaque solution containing gel-like particles. If this gel is isolated, washed with water and dried, a crystalline Ln(OH)₂NO₃(H₂O)_x material is formed, vide infra. This indicates that the nitrate ions are an integral part of the mechanically stable gel particles. If more base is added, or if the gel is heated to 140 °C in a sealed vessel, the aqueous gel reacts and takes a more solid form. Here, the nitrate ions are removed from the inner coordination sphere of Ln³⁺ forming the solid gels [Ln(OH)_2/3(H₂O)₇]NO_3(s-gel)and [Ln(OH)_2/3(H₂O)₇]OH_(s-gel). If this solid gel is collected and dried two phases are formed: Ln(OH)₃ and NaNO₃. If the gel is collected, washed, and dried pure Ln(OH)₃ is isolated. The dehydration can take place on the filter or in solution with high hydroxide concentrations (pH > 12) at elevated temperature. The crystalline material can form in the autoclave at pH ≥ 12. In summary, the crystallization occurs from a solid gel, which is formed from an aqueous gel, and the solid crystals are formed upon dehydration at high temperatures or by evaporation on the filter. No crystallization occurs in solution at ambient conditions.

Before establishing the individual steps of the mechanism proposed in Figure 1, we turn to the crystal growth that occur in and after the last step. The crystallites obtained after filtering and drying the solid gels formed at 140 °C were all Ln(OH)₃ (Extended Data Figure 1). The morphology of the final hexagonal crystals is strongly pH-dependent (Figure 2C, Extended Data Figure 4). This imply that the gel formed within the autoclave has a composition (Ln(OH)_2/3(H₂O)_x) that reflects the hydroxide concentration in the reaction mixture. We can explain the morphology using a simple additive growth model (Figure 2B), where the hydroxide concentration dictates the rate of adsorption of Ln(III) at the open site on the two surfaces (Figure 2A). At high pH, the higher hydroxide concentration favours one growth direction resulting in long nanowires, where short hexagonal nanorods are formed at low pH. The aspect of the nanocrystals can thus be tuned using pH (see extended data), but is limited by the total lanthanide(III) amount in each gel particle.

The formation of the aqueous and solid gel phases was studied in detail using neodymium(III).

The two gel phases [Ln(OH)_2/3(H₂O)₇]NO_3(s-gel)and [Ln(OH)_2/3(H₂O)₇]OH_(s-gel) was identified from the two different solid products formed, but was also identified in situ. Figure 3A shows the pH titration that is the starting point for the gel formation and Figure 4 shows the gel formation kinetics. Figure 3B displays the spectral fingerprints of all observed structures in the reaction coordinate in Figure 1. In Figure 3C, powder X-ray diffraction data from the different products, Nd(NO₃)(OH)₂(H₂O)_z and Nd(OH)₃, formed from the gels are shown. The nitrate containing and hydrated phase has not been previously reported but was identified using X-ray total scattering and pdf analysis of the final powder. PDF-modelling was done with five different structures (Extended Data Figure 3), and although the long range order above 20 Å was poorly described, the layered structure motif found in Nd(OH)₃, Nd(NO₃)(OH)₂, and Nd(NO₃)(OH)₂(H₂O) describes the data well. These three structures and the best-fit refinement is highlighted in Figure 3D and we conclude the chemical composition to be: Nd(NO₃)(OH)₂(H₂O)_z.

In solution, gel formation was followed using light scattering, see Figure 4, and changes in the coordination geometry of the lanthanide(III) ions was probed using absorption spectroscopy.³⁶ As indicated by the morphologies of the final products (Figure 2), the data show a continuum of species with varying hydroxide concentration from the aqueous gel Nd(OH)₂NO₃(H₂O)_z to the solid gels [Nn(OH)_2/3(H₂O)₇]NO_3(s-gel)/[Nn(OH)_2/3(H₂O)₇]OH_(s-gel). The absorption spectra allows easy identification of the initial Nd(H₂O)³⁺ and final Nd(OH)₃ species. The intermediate Nd(OH)₂NO₃(H₂O)_z and [Nd(OH)_2/3(H₂O)₇]OH_(s-gel) gels also have distinct features in the absorption spectra (Figure 4B), most readily recognized in a comparing to the spectra from the more readily isolated species (Figure 3B).

The unique features in the absorption spectra allow us to follow the change from one gel to another, while light scattering shows the formation of a scattering material, here associated to the formation gels. The latter is easily recognized, as the solution turns opaque as the gel forms. While the complicated speciation does not allow for creation of a kinetic model at this stage, the gel formation was explored in detail. Figure 4A and 4D show kinetic traces recording using an immersion probe, and how light scattering and absorption spectra was isolated from the same data. The kinetic traces and absorption in Figure 4A and 4B show the effect of pH on the gel formation, and that the scattering of the gels are different. Note that the solutions are stirred, thus gel particles are formed instead of a gel phase.

At neutral pH the Nd(OH)₂NO₃(H₂O)_zgel is formed as soon as base is added and the solution reaches equilibrium after 30-40 seconds. At high pH, the more scattering Nd(OH)₂NO₃(H₂O)_z gel is formed first, and then converted to the less scattering [Nd(OH)_2/3(H₂O)₇]OH_(s-gel) gel. This assignment is corroborated by the traces recovered from the absorption spectra. In Figure 4A, the relative absorption at 792-795 nm shows the change from the aqua complex to the different gels. Here, the molar absorption coefficients are ordered εNd(H₂O)³⁺ > ε(Nd(OH)₂NO₃(H₂O)_z) > ε([Nd(OH)_2/3(H₂O)₇]OH_(s-gel)) and the data show the same difference as the light scattering data. The gel particles are so stable that they can be imaged in SEM (Figure 4C). Figure 4E shows that dehydration of the gel leads to crystal formation upon exposure to the e-beam.

To investigate the gel formation in detail, the kinetic traces were recorded at pH 13 for five different neodymium(III) concentrations. The data in Figure 4D show that the amount of gel formed depends on the neodymium(III) concentration and that two gel phases are made, but that the coordination environment of the neodymium(III) centre does not change between the two gel phases. We are thus able to conclude that we at high pH 13 see the reaction from Nd(H₂O)³⁺ in solution via the Nd(OH)₂NO₃(H₂O)_z gel to the [Nd(OH)_2/3(H₂O)₇]OH_(s-gel) gel, which is the final product in solution at ambient conditions. If the solution with the [Nd(OH)_2/3(H₂O)₇]OH_(s-gel) gel is heated to 140 °C—or if the gel is filtered, washed, and dried—crystallization to Nd(OH)₃ occur. Figure 5A shows the X-ray diffraction data of the [Nd(OH)_2/3(H₂O)₇]OH_(s-gel) gel, the dispersion of Nd(OH)₃ formed after heating, and the Nd(OH)₃ powder.

Returning to the model in Figure 1, the reaction was followed from aqueous solution to crystals at high pH, and it was shown that crystallization occur in solution from the [Nd(OH)_2/3(H₂O)₇]OH_(s-gel) gel at elevated temperatures.³⁷

At lower pH the product in solution at ambient conditions contains nitrate, while it after heating becomes pure lanthanide hydroxide. To investigate this change, we performed in situ X-ray total scattering. The raw data are shown in Figure 5B (and Extended Data Figure 9). The data show that the observed atomic structure has no long-range order and the short-range order is different from that in the crystal phase (Figure 5C). We interpret the data in (Figure 5B) as we at t = 0 have the aqueous gel formed from Nd(OH)₂NO₃(H₂O)_z at room temperature, which upon heating becomes a solid gel formed by [Nn(OH)_2/3(H₂O)₇]NO_3(s-gel). The data (Figure 5D) show that each neodymium(III) center becomes less hydrated as, we see a decrease of the characteristic distance of the second coordination sphere with water at 4.5 Å,³⁸ and we see an increase in the distance that corresponds to a Nd-Nd distance of 6 Å if the ions are bridged by nitrate. This indicates the formation of closer packed clusters we associated with the solid gel in Figure 1. Interestingly, the X-ray total scattering data shows a fully reversible reaction (Extended Data Figure 9). From the different products formed before and after heating, and from the absorption spectra of the two gels, we know that a change from Nd(OH)₂NO₃(H₂O)_z to [Nn(OH)_2/3(H₂O)₇]NO_3(s-gel) occurs upon heating. Thus, the reversibility pertains only the structure of the gels in solution, [Nn(OH)_2/3(H₂O)₇]NO_3(s-gel) is the thermodynamic product, and as the main effect of heating appears to be dehydration of the neodymium(III) centers we propose that this is the crucial step of the crystal formation. Further, at neutral pH the hydroxide concentration must be too low to allow crystals to form. As extended heating does not change the outcome of the reaction, high local hydroxide concentration— either through addition of base or evaporation of solvent in the gel particles—must be a thermodynamic requirement for crystallization to occur.

In conclusion, it was shown that lanthanide(III) hydroxides do not form in solution. The purification of numerous lanthanide(III) complexes in the literature has been performed by filtering of a solid gel formed after 40-60 seconds, and the different structures of Ln(III) hydroxides reported in the literature was most likely formed at very high local pH. More importantly, the results reported here show that crystallization can be regarded as any other chemical process although the molecular species needed to describe the reactions kinetics are a lot harder to define.

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Chemicals

Deionized water from the in-house reverse osmosis system was used without further purification. All chemicals were of analytical grade and used as received without further purification. These are La₂O₃, Nd₂O₃, KOH, NaOH, and 70 mass-% HNO₃.

Crystal systems

The identified crystals systems are: Lanthanide hydroxide: Ln(OH)₃. CCDC reference: 2106882; Lanthanide dihydroxy nitrate: Nd(OH)₂NO₃. CCDC reference: 63550; and Lanthanide dihydroxide nitrate(V) hydrate: La(OH)₂(NO₃)(H₂O). CCDC reference: 68554.

Lanthanide hydroxide is of the hexagonal crystal structure with space group P63/m (no. 176). Combining three unit cells each rotated 120 ° a Miller-Bravais cell can be constructed. The Miller-Bravais cell contains two distinct surfaces, the a- and c-surfaces, which are perpendicular to the a₁, a₂, and a₃, and c axes respectively. The growth model considered in this work is based on the distinction between the surface structures and the corresponding sites for the adsorbing of neodymium(III) ions: the a-site and the c-site. The a-site contain three oxygen atoms exposed to the solution, while the c-site contain six oxygen atoms. All oxygen atoms are assumed protonated and in equilibrium with the solution. Monomer adsorption to the a-site will result in the capture of a single hydroxide into the crystal structure, while adsorption to the c-site will result in the capture of three hydroxides. In combination with the monomer equilibria, this difference in the number of hydroxide required for crystal growth forms the basis of the kinetic model shown in Figure 2.

Lanthanide dihydroxy nitrate and lanthanide dihydroxide nitrate(V) hydrate are not of the hexagonal crystal structure, but contains the same structural motif as the lanthanide hydroxide structure. The hexagonal structure is split in two units containing this motif separated by a layer of interstitial nitrates and water. The three structures can therefore compared as they have the same layered motif separated by: nothing (Lanthanide hydroxide), nitrates (Lanthanide dihydroxy nitrate), and nitrates and water (Lanthanide dihydroxide nitrate(V) hydrate).

Crystal morphology. Lanthanide(III) hydroxides nanocrystals have been investigated in detail. The studies have focused on morphology motivated by applications such as photocatalytic activity,^1-5 and antibacterial properties.⁶ Different morphologies have been achieved using hydrothermal synthesis: nanorods,^6-14 nanosheets,^7,11,15 nanobelts,¹⁶ nanowires,^10-12 nanospindles,¹⁷ and nanotubes have been shown to form.^12,13,18 The Ln(OH)₃ nanocrystals can be converted to Ln₂O₂CO_3,¹⁹ LnO₂S,¹² and Ln₂O_3,^{7,9,15-17,19-22}and Ln(OH)₃are used as a precursor for hydrothermal treatment synthesis of LnF₃.^12,23 In the conversion of Ln(OH)₃ to other materials, LnOOH has been shown to form as a stable and isolatable intermediate.²⁰ The morphology of the Ln(OH)₃ is important, as it has been shown to be conserved in most cases. Thus, the Ln(OH)₃ formation mechanism is relevant for the synthesis of these materials.

Crystal Growth. The formation of anisotropic nanostructures has been proposed to arise from a reaction mechanism where kinetic control from capping agent determines growth direction.²⁴ This mechanism can explain why these morphologies can be made using hydrazine and amines as bases.^3,10,21 However, this mechanism fails to explain why the same morphologies are obtained using hydroxide as base.^11-13 And while most studies follow a hydrothermal synthesis, it has been shown that Ln(OH)₃ nanocrystals can be formed directly upon addition of base.^25-27

Synthesis of lanthanide(III) nanostructures

Synthesis via titration.

Two different experiment types where performed for the synthesis via titration.

Experiment type 1: 500 mg La₂O₃ or 505 mg Nd₂O₃ was dissolved in 20 mL 2.4 M HNO3 solution after 10 minutes on magnetic stirrer. The resulting lanthanide(III) concentration was 100 mM. When fully dissolved, 2 M KOH was added at a rate of 1 mL pr. minute with magnetic stirring.

Experiment type 2: 500 mg La₂O₃ or 505 mg Nd₂O₃ was dissolved in 20 mL 2.8 M solution after 10 minutes on magnetic stirrer. The resulting lanthanide(III) concentration was 100 mM. When fully dissolved, 1.6M NaOH was added at a rate of 1 mL pr. minute with magnetic stirring.

The titration was done with an EasyPlusTM syringe pump from Mettler Toledo using a 20 mL syringe. The system was primed to ensure that the no volume loss in the tube occurred. The pH was probed using a pH Sensor InLab® Expert Pro-ISM from Mettler Toledo. A suspension appears at pH=3 and the titration is stopped at pH values of 7, 8.5, 10, 12, and 13 for the lanthanum(III) samples and at pH values of 6, 7.5, 12, and 13 for the neodymium(III) samples.

Synthesis via fast injection. 500 mg La₂O₃ or 505 mg Nd₂O₃ was dissolved in 20 mL 1.6 M HNO3 solution after 10 minutes on magnetic stirrer. The resulting lanthanide(III) concentration was 100 mM. When fully dissolved, between 4.9 and 5.5 mL 7M KOH was injected over the course of 6 s using a 10 mL syringe and KD Scientific KDS Legato 100 (78-8100) Infusion Syringe Pump. The pH was determined by measuring the pH of the suspension after 2 min with a pH Sensor InLab® Expert Pro-ISM from Mettler Toledo.

Isolation of powders without hydrothermal synthesis. The reaction mixtures (25 – 50 mL depending on experiment type) was filtered through a grade 3hw filter from Sartorius using vacuum filtration. The filter cake was washed twice with water. The filter cake samples were dried overnight at 60°C on the filter, and crushed to fine powder with a mortar and pestil.

Isolation of powders with hydrothermal synthesis. 20 mL of the suspensions at the specific pH-values are transferred to an autoclave and heated at 140 °C for approximately 19 h. The autoclaves were allowed to cool to room temperature, and the samples were filtered through a grade 3hw filter from Sartorius using vacuum filtration. The filter cake was washed twice with water. The filter cake samples were dried overnight at 60°C on the filter, and crushed to fine powder with a mortar and pestil.

Scanning electron Microscopy (SEM)

The SEM was run with a voltage of 2-20 kV. The SEM samples were prepared by first cleaning small pieces of a silicon wafers with soapy water, followed by a rubbing with a dust free paper towel. The silicon substrate was then rinsed with first ethanol then water twice. It was dried with a stream of nitrogen, and then placed ina UV/ozone champer for 10 min. Finally, each wafer was rinsed with first ethanol then water and dried with a stream of nitrogen. All water used was milli-Q water and the ethanol was HPLC grade.

The crystalline powders where dispersed in water and a single drop were transferred to a wafer. After 5 minutes the water was carefully removed with piece of dust free paper towel, and the sample was dried with a stream of nitrogen. The samples were stored in an airtight sample holder before imaging in the SEM.

The gel samples were transferred as a single drop to the wafer from the stirred suspension 2 min after the 7M KOH injection. After 5 minutes the water was carefully removed with piece of dust free paper towel. The samples were stored in an airtight sample holder before imaging in the SEM. The SEM imaging was done within three hours of creation of the gel.

Powder X-ray Diffraction (PXRD)

Powder X-ray diffraction was done on a Bruker D8 X-ray diffractometer with a Cu anode, providing an x-ray wavelength of λ = 1.5418Å). Several measurements were performed with acquisition parameters of 2θ = 10-80, Δ2θ = 0.06, and T = 30 minutes, or 2θ = 5-70, Δ2θ = 0.01, and T = 60 minutes. All diffractograms are normalized by division with Imax.

Rietveld Refinements

Rietveld refinements were done in Full Prof.²⁸ The refinements used the Thompson-Cox-Hastings pseudo-Voigt * Axial divergence asymmetry peak shape with a background generated by a linear interpolation between background points. The background points were manually chosen, and their intensities was included as fitting parameters. For simplicity a simple model for the refinements were used, where only the following parameters were refined: a, c, overall B-factor, the FWHM parameters U and Y. The refinement model does therefore not take defects or anisotropy into account and the crystallite shapes calculated are therefore not accurate. The instrumental contributions to the peak width were obtained using a Si standard.

Ex situ x-ray Total Scattering for generating Pair Distribution Functions (PDF)

The powder samples were transferred to a 0.3 mm borosilicate capillary type with Ø: 0.3 mm, length: 80 mm, and thickness: 0.01 mm. The tube was sealed by melting the end with a butane gas burner. The gel samples were allowed settle in the bottom of the container in which they were synthesized for 1 hour. Then the concentrated gel was transferred to a quartz tube with Ø: 0.8 mm, length: 80 mm, and thickness: 0.01 mm with a 0.3 mm borosilicate capillary tube using the capillary effect. The tube was sealed by melting the end with a butane gas burner.

X-ray total scattering data for ex-situ PDF was obtained using a Panalytical Empyrean Series 3 instrument with an Ag source and a and a GaliPIX detector. The exposure time was 7.5 h in the 2θ range of 3–56° and 22.5 h in the 2θ range of 54–110°. A wavelength of λ = 0.56 Å and the following parameters was used for generating the PDF: Qmin = 0.62 Å^-1, Qmax = 16 Å^-1, Qmax,inst = 18.4 Å^-1, rpoly = 0.9 Å.

In situ Total Scattering

In situ X-ray total scattering data were obtained at beamline ID11, ESRF, France using a wavelength of λ = 0.2115Å. The synthesis was done in a custom-made reaction cell for in situ X-ray studies of solvothermal synthesis, which is similar in design reported by Becker et al.²⁹ The precursor suspension was injected into a fused silica tube with 0.7 mm inner diameter and 0.09 mm wall thickness. The pressure during reaction was kept stable at 100 bar throughout the experiment using an HPLC pump. A heat gun was used to heat the capillary containing the precursor. Data from three samples were obtained; one at pH = 6 and one at pH = 7.5where each measurement were started within the first hour of mixing. Additionally, another sample at pH = 7.5 where measured after 24 h of mixing. All samples show similar results. Experiments at higher pH than 10 could not be done, as the reaction cell could not sustain the high pH. The samples were measured at 20 °C for 120 s before heating, then heated by the heat gun up to 140 °C with a linear increase up to 500 s. the sample was left at this temperature for 1000 s. Furthermore, background data were obtained from measuring data from a capillary with water from 25 °C to 140 °C for 500 s using a linear increase in temperature.

The 2D data were obtained by a RA-PDF³⁰ setup with a short distance to the detector. The collected data were reduced to 1D by radial integration and converted to PDFs by normalizing and Fourier transformation in xPDFsuite.³¹ The PDF converter used the following parameters: Qmin = 0.89 Å^-1, Qmax = 16 Å^-1, Q_max,inst = 24 Å^-1, rpoly = 0.9 Å. The experimental time resolution was 1 second. In the data shown this has been averaged to a resolution of 10 seconds. Background subtraction with scattering data from the fused silica capillary with pressurized water was used. Details are described in supporting information, where the experimental setup and the treatment of the background for PDF generation is described.

Pair Distribution Functions (PDF)

The obtained PDF data were modelled using PDFgui³², where real-space Rietveld refinements were performed. The models refined are based on data base crystal structures or clusters with no long range order build in VESTA³³.

Absorption Spectroscopy

Absorption measurements were performed on a Lambda 1050 double-beam spectrophotometer from PerkinElmer using a halogen lamp. All measurements were performed using air as a reference. The absorption spectra were measured on an absolute scale from a solvent baseline (100% transmission; Abs = 0) to a blocked beam (0% transmission; Abs = ∞). Slits where fixed at 1 nm. For solutions, the measurements were performed in 10 mm quartz cuvette using the Lambda 1050 three detector module. For suspensions of gels and powders the measurements were done using the Lambda 1050 integrated sphere module. Suspensions were injected into 10 mm quartz cuvettes and powders where trapped between two borosilicate glass coverslips from Epredia that was glued together with Depend O2 transparent nail polish. All samples were placed in the center of the integrating sphere and no lenses has been used.

In-situ Absorbance Spectroscopy

In situ absorbance measurements, where made with a Halogen light source (HL-2000-FHSA) from Avantes. The spectrometer used was a STS-VIS miniature spectrometer from Ocean Optics. Transmittance measurements were done with a 1s time resolution using a transmission dip probe with 10 mm sample opening from Thorlabs. The total transmitting distance of 20 mm was used.

In-situ absorbance measurements was done using three methods: 1) by mixing a nitric acid neodymium solution quickly with a 7 M NaOH solution under vigorous magnetic stirring. 2) By titrating in 7M NaOH using a syringe pump. And 3) by peforming fast (XX s) injections using the syringe pump. Details on the data treatment is available as supporting information.

Modelling of pH titrations

pH titrations has been modelled based on the equilibrium equations of lanthanides(III) in water.³⁴

This set of equilibrium equations is solved numerically by the implementation of a Newton-Raphson algorithm based on the reaction extends as described by Paz-Garcia et al.³⁴

Method References

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How does crystalline solids form from solution? Crystallization of lanthanide(III) hydroxides

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