ABSTRACT

The β-HIO₃ polymorph, previously difficult to detect and whose existence was questioned, has been structurally characterized. The crystal structure of β-HIO₃ was solved in the same space group as α-HIO₃ (P212121); however, it was found that the unit cell axes were all different by about 1 Å. Similar to that of α and γ phases, the unit cell contains only a single HIO₃ molecule in the asymmetric unit with I-O bond lengths ranging from 1.786(5) to 1.903(7) Å. The I(V) atom is further coordinated by three oxygen atoms of neighboring acid molecules forming a distorted octahedral with a range of I-O distances (2.498(6) - 2.795(7) Å). The one structural difference that separates the β phase from the α and γ phases is that the hydroxyl group is bridging between two I(V) atoms, resulting in a smaller hydrogen bonding distance (O-O distance: 2.559 Å (β), 2.665 Å (γ) and 2.696 Å (α)) and presumably a different crystalline energy. Similar to γ-HIO₃, β-HIO₃ is metastable and slowly converts to α-HIO₃. It is hypothesized that β-HIO₃ is a transition step in the formation of α-HIO₃ and β-HIO₃ is a result of trapped water inside particles during crystallization.

Keywords:

X-Ray Diffraction, Iodates, Acids, Crystal Structure, Oxides

1. Introduction

Iodine(v) oxides (i.e. I₂O₅, HIO₃, HI₃O₈, I₄O₉) are compounds with unique optical properties and potential for high energy release. The unique optical properties of iodine oxides, specifically iodic acid (HIO₃), are a result of the non-centrosymmetric space group P2₁2₁2₁ [1] , and have led to the synthesis of different aluminum iodate species [2] [3] . Iodine oxides are also appealing for use as an oxidizer when combined with aluminum fuel particles. In fact, Smith et al. [4] showed detonation velocities could be achieved from aluminum iodate mixtures. The aluminum and iodine oxide reactions also have the potential to disperse high temperatures and aerosolized iodine species that can kill bacterial agents [5] . The optical properties and potential for energy release of iodine oxides are controlled by the crystalline structure of precipitated HIO₃ product [2] [3] [4] .

There is only one commonly accepted polymorph for both I₂O₅ and HI₃O₈; however, there are four reported polymorphs of HIO₃ in literature: α, β, γ, and δ [1] [6] [7] [8] [9] . The most commonly reported structure of iodic acid is α-HIO₃. The γ-HIO₃ polymorph was first reported by Fischer et al. [1] and structurally determined by single crystal diffraction on crystals produced by mixing iodic acid with chromium. The γ-HIO₃ polymorph is thought to be a result of chromium in solution favoring the formation of dimers and trimers of HIO₃ that form γ-HIO₃ as solutions of HIO₃ and chromium precipitate. On the other hand, the β-HIO₃ polymorph was first reported in 1960 by Halasz et al. [8] with peak intensities determined from a powder diffraction experiment (Table S1); however, the diffraction pattern was not indexed. The β-HIO₃ polymorph is also mentioned briefly in Selte et al. [7] , but is later questioned by Fischer et al. [1] and to our knowledge, these are the only known reports of a β polymorph of HIO₃. The δ polymorph was recently reported in Wu et al. [9] , but detailed crystal information was not given. It is suggested that δ-HIO₃ is metastable at elevated temperatures and slow heating converts δ-HIO₃ directly into I₂O₅. The dehydration steps for the formation of HIO₃ are not well understood, but the formation of dimers, trimers and higher order polymers have been related to concentration of HIO₃ [10] [11] [12] . Many studies show polymerization of ${\text{IO}}_3^{-}$ from solution [10] [11] [12] , but a link between polymerization and formation of α-HIO₃ has not been established.

In this study, we will report on the single crystal structure of β-HIO₃ that has not previously been reported and understand the polymorphic physical behavior of β-HIO₃. To this end, the objectives were to synthesize β-HIO₃ and characterize its structure using single crystal X-ray diffraction (XRD).

2. Experimental

2.1. Sample Preparation

The synthesis method to form β-HIO₃ begins with dissolving commercially available I₂O₅ powder, supplied by Sigma Aldrich (St. Louis, MO), in distilled water. Commercial I₂O₅ was mixed to a 1:1 wt. % ratio of I₂O₅ to deionized water and placed in a beaker on a magnetic stirrer and stirred for 10 minutes at 200 RPM to allow complete I₂O₅ dissolution into solution. The iodate solution was then dried in a vent hood with an average relative humidity (RH) of 35% at 23˚C, until solid crystals precipitated from solution. Multiple samples (i.e., >5) were made to establish repeatability.

2.2. Single Crystal XRD

Single crystal X-ray diffraction data were collected on a Bruker PLATFORM three circle diffractometer equipped with an APEX II CCD detector and operated at 1500 W (50 kV, 30 mA) to generate (graphite monochromated) Mo Kα radiation (λ = 0.71073 Å). Crystals were transferred from the reaction vial and placed on a glass slide in polyisobutylene. A Zeiss Stemi 305 microscope was used to identify a suitable specimen for X-ray diffraction from a representative sample of the material. The crystal and a small amount of the oil were collected on a MῑTiGen cryoloop and transferred to the instrument where it was placed under a cold nitrogen stream (Oxford) maintained at 100 K throughout the duration of the experiment. The sample was optically centered with the aid of a video camera to ensure no translations were observed as the crystal was rotated through all positions.

A unit cell collection was then carried out. After it was determined that the unit cell was not present in either the CCDC or ICSD databases a sphere of data were collected. Omega scans were carried out with a 10 sec/frame exposure time and a rotation of 0.50˚ per frame. After data collection, the crystal was measured for size, morphology, and color. These values are reported in Table S2.

2.3. Single Crystal XRD Refinement Details

After data collection, the unit cell was re-determined using a subset of the full data collection. Intensity data were corrected for Lorentz, polarization, and background effects using the Bruker program APEX 3. A semi-empirical correction for adsorption was applied using the program SADABS [13] . The SHELXL-2014 [14] , series of programs was used for the solution and refinement of the crystal structure. The hydrogen atom bound to O₁ was constrained with a DFIX command and a thermal parameter of −1.2. An extinction coefficient of 0.0298 was also applied during the final refinement.

2.4. Powder Diffraction XRD

All powder diffraction data were collected on a Rigaku Ultima III powder diffractometer. X-ray diffraction patterns were obtained by continuously scanning a 2θ range of 15˚ - 60˚, step size = 0.02˚, and scan time ranging from of 1.5 - 3 degrees/minute depending on the scan. The X-ray source was Cu Kα radiation (λ = 1.5418 Å) with an anode voltage of 40 kV and a current of 44 mA. The beam was then discriminated by Rigaku’s Cross Beam parallel beam optics to create a monochromatic parallel beam. Diffraction intensities were recorded on a scintillation detector after being filtered through a Ge monochromator. Samples were prepared as standard powder mounts and diffractograms were processed through the software JADE v9.1.

3. Results

During the initial PXRD studies of what was presumed to be α-HIO₃, it was discovered that the resulting patterns did not match any indexed crystal structure. Single crystal studies were then carried out on the large blocky crystals. The β-HIO₃ phase reported in Halasz et al. [8] is shown in Supplemental Information. Table S1 and is similar to the lattice parameters shown here. Because of the similarities between our results and Halasz et al. [8] , and to avoid adding any more Greek letters to describe the phases of HIO₃, it is assumed crystals shown here are the β-HIO₃ polymorph. Since unit cell data is not reported for δ-HIO₃ in Wu et al. [9] , only a visual comparison between β-HIO₃ and δ-HIO₃ could be made and is discussed further below. Unit Cell data, bond lengths and angles, atomic coordinates, anisotropic displacement parameters, hydrogen coordinates, and hydrogen bonds for β-HIO₃ determined by single crystal data is shown in Supplemental Information Tables S2-S7. While the crystal structure of β-HIO₃ was also solved in the space group α-HIO₃ (P2₁2₁2₁), it was found that the unit cell axes were all different by about 1 Å. Similar to that of α and γ phases, the unit cell contains only a single HIO₃ molecule in the asymmetric unit with I-O bond lengths ranging from 1.786(5) to 1.903(7) Å (Table S3). The I(V) atom is further coordinated by three oxygen atoms of neighboring acid molecules forming a distorted octahedral with a range of I-O distances (2.498(6) - 2.795(7) Å). The one structural difference, shown in Figure 1 (e.g., α-HIO₃ shown in Supplemental Information, Figure S1), that separates the β phase from the α and γ phases is that the hydroxyl group is bridging between two I(V) atoms, resulting in a smaller hydrogen bonding distance (O-O distance: 2.559 Å (β), 2.665 Å (γ) and 2.696 Å (α)) and presumably a different crystalline energy.

4. Discussion

During repeatability testing, β-HIO₃ readily converted to α-HIO₃, indicating β-HIO₃ is metastable, similar to the γ phase. The metastable nature of β-HIO₃ is shown in Figure 2. The XRD measurements in Figure 2 are from crystals ground into powder using mortar and pestle before data collection. In Figure 2, the top curve was collected immediately after drying. This sample is referred to as Sample 1 initial. The middle curve in Figure 2 was initially ground after drying and allowed to sit in a vent hood at 20% RH for 8 days before analysis. The middle curve in Figure 2 is pure α-HIO₃ and matches PDF 97-006-6643 [15] and the top curve is β-HIO₃. The bottom curve in Figure 2 shows XRD measurements that were taken from Sample 1 that was put in a vent hood for 8 days prior to grinding and then ground for XRD measurements. The bottom curve in Figure 2 shows that when Sample 1 is placed in a vent hood for 8 days as a single crystal prior to grinding and then ground for XRD measurements, it remained as β-HIO₃.

Figure 2 shows that β-HIO₃ is metastable and changes to α-HIO₃ over time only if the samples are ground prior to aging. Since all samples are ground before placement in the XRD, the change from β-HIO₃ to α-HIO₃ is not directly related to the physical force of the grinding and is a time dependent process related to increased surface area from grinding. It is assumed that trapped water results in the formation of β-HIO₃. Upon grinding, trapped water is released and drying the solid material readily converts β-HIO₃ into α-HIO₃. The validity of the assumption that trapped water results in the formation of β-HIO₃ is discussed below.

Figure 3 shows Sample 1 with curves from PDF #00-045-0872 labeled as I₄O₉XH₂O from Wikjord et al. [6] . Figure 3 shows that the sample labeled I₄O₉XH₂O is β-HIO₃.

In Wikjord et al. [16] , the sample labeled I₄O₉XH₂O was I₄O₉ hydrated by atmospheric water. In Smith et al. [17] , it was shown that when I₄O₉ is exposed to 20% RH for 4 hours, I₄O₉ hydrates to HIO₃. Because β-HIO₃ is seen during hydration of I₄O₉ into α-HIO₃ and β-HIO₃ is seen here when I₂O₅ is mixed with water, β-HIO₃ is assumed to be a transition step in the formation of α-HIO₃. We have shown that β-HIO₃ is metastable and converts to α-HIO₃ when ground prior to aging. Because β-HIO₃ is seen during the formation of α-HIO₃ and β-HIO₃ is metastable when ground prior to aging, we propose that the formation of β-HIO₃ is an intermediate step in the formation of α-HIO₃ caused by water trapped inside the precipitated crystals.

Visual comparison of diffraction patterns between δ-HIO₃ shown in Wu et al. [9] and diffraction patterns shown here for β-HIO₃ are similar. Also, similar comparisons between these phases and I₄O₉XH₂O have been made here and in Wu et al. [9] , suggesting δ-HIO₃ and β-HIO₃ are the same phase. The 6 reported I-O bond distances for δ-HIO₃ are similar (less than 0.03 Å difference) to bond distances reported here for β-HIO₃. Similar bond distances and visual comparisons of diffraction patterns suggest δ-HIO₃ and β-HIO₃ are the same crystal structure. In Wu et al. [9] , it is reported that δ-HIO₃ dehydrates directly into I₂O₅, not HI₃O₈ during slow heating conditions of DSC analysis. Because of this direct dehydration into I₂O₅ during slow heating, δ-HIO₃ is reported as metastable. This can be explained by the metastable nature of β-HIO₃ reported here and results from Smith et al. [18] showing that HIO₃ dehydrates directly into I₂O₅ and HI₃O₈ is formed during heating. Since comparison between measurements in Halasz et al. [8] and diffraction patterns obtained, using modern XRD are difficult to quantify, it is probable that δ-HIO₃ identified by Wu et al. [9] is actually β-HIO₃.

5. Conclusion

The β-HIO₃ polymorph, previously difficult to detect and whose existence was questioned, has been synthesized and structurally characterized. When crystals are ground before aging, β-HIO₃ is metastable and slowly converts to α-HIO₃. Experiments were designed to study the metastable nature and based on XRD analysis. β-HIO₃ may be a transition step in the formation of α-HIO₃ and result from trapped water inside HIO₃ crystals.

Acknowledgements

The authors are grateful for support from the Army Research Office under award W911NF-14-1-0250 and encouragement from our program manager, Dr. Ralph Anthenien.

Cite this paper

Smith, D.K., Unruh, D.K. and Pantoya, M.L. (2018) Discovery of β-HIO₃: A Metastable Polymorph of HIO₃. Advances in Materials Physics and Chemistry, 8, 246-256. https://doi.org/10.4236/ampc.2018.85016

References

Supplemental Materials