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Local Structure of Zr(OH)4 and the Effect of Calcination Temperature from X‑ray Pair Distribution Function Analysis Graham King,† Jennifer R. Soliz,*,‡ and Wesley O. Gordon‡ †

Independent Research Consultant, 4596 Fairway Drive, Los Alamos, New Mexico 87544, United States Edgewood Chemical Biological Center, 5183 Blackhawk Road, Aberdeen Proving Ground, Maryland 21010-5424, United States

‡

ABSTRACT: Analysis of X-ray pair distribution function data has provided a detailed picture of the local structure of amorphous Zr(OH)4 and its thermal decomposition into ZrO2. In the untreated phase, the Zr atoms tend to be coordinated by six or seven oxygen atoms. The Zr centered polyhedra connect to each other primarily by sharing edges, but also with a significant amount of corner sharing, to form two-dimensional sheets in which the Zr are connected to an average of about five other Zr. This local structure is related to the structure of monoclinic ZrO2 and can be derived from it by removing certain Zr neighbors to form sheets and reduce the corner to edge sharing ratio. The maximum correlation length in Zr(OH)4 is about 12 Å. Heating up to 125 °C results in significant water loss but does not alter the network of Zr and bridging O atoms. Additional water loss caused by heating to 250 °C triggers a reorganization into a new type of amorphous phase with a three-dimensional network and a greater number of Zr−Zr neighbors. Further heating to 330 °C causes crystallization into a mixture of tetragonal and monoclinic ZrO2, with the minor tetragonal phase having a smaller average domain size. The tetragonal component vanishes by 900 °C.

1. INTRODUCTION

exposure to environmentally relevant concentrations of CO2 and H2O, it has emerged as a top choice decontaminant.20 Unlike ZrO2, which has well established crystal structures, little is known about the structure of Zr(OH)4, as it lacks any long-range order. There have been a few studies which look at the surface structure, porosity, and particle morphology.12,21 One study has used the extended X-ray absorption fine structure (EXAFS) method to look at the short-range order and proposed a square grid model where each Zr is connected to four others through double hydroxyl bridges.22 However, a detailed picture of the local structure is still missing, especially concerning the interatomic distances beyond ∼5 Å. In this study, we use the pair distribution function (PDF) to provide a more complete picture of the short-range order and also to monitor the process of how Zr(OH)4 transforms into ZrO2 upon heating.

Zirconium oxides and hydroxides are inorganic compounds of major scientific and industrial importance. One of the prime examples is yttria-stabilized zirconia, which has been extremely well studied due to its excellent ionic conduction and mechanical properties.1−4 ZrO2 has also more recently been attracting a great deal of attention as a possible replacement for SiO2 in complementary metal-oxide-semiconductor devices, as its higher dielectric constant permits the creation of thinner layers.5−8 Many studies have also looked at ZrO2 as a potential catalyst for the breakdown of species such as CO.9,10 ZrO2 exists in three crystalline forms.11 At room temperature, it has a monoclinic crystal structure with P21/c space group symmetry. At high temperature (>1214 °C), it transforms to a tetragonal structure with P42/nmc symmetry, and then at very high temperature (>2377 °C), it transforms to the cubic fluorite structure with Fm-3m symmetry. The tetragonal and cubic phases can also be stabilized at room temperature by doping with a lower valent cation. In addition to being a precursor material for the synthesis of zirconia, zirconium hydroxide has many uses of its own. Its high surface area and porous nature make Zr(OH)4 useful for adsorption, filtration, and chemical removal applications.12−14 Zirconium hydroxide, as well as a number of Zr based metal− organic frameworks, have been found to be exceptionally good at breaking down certain toxic substances, such as chemical war agents.15−19 As Zr(OH)4 is cheap, commercially available, environmentally safe and retains some activity even after © XXXX American Chemical Society

2. EXPERIMENTAL SECTION Two commercially available types of Zr(OH)4 particles were purchased from MEL Chemicals (Flemington, NJ) which are designated in this study as type 1 and type 2. These have manufacturer designations of 1247 (type 1) and 1501/06 (type 2). Samples of each type were studied as-received (AR), upon being treated in 80% relative humidity (80RH) and after being calcined at temperatures of 40, 125, 250, 330, 500, or 900 °C in air with a ramp rate of 10 °C/min and a dwell time of 6 h (designated by annealing temperature). The highest Received: December 21, 2017

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DOI: 10.1021/acs.inorgchem.7b03137 Inorg. Chem. XXXX, XXX, XXX−XXX

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negligible. While the fits with the cubic and monoclinic phases did not match the data, fits with the tetragonal phase provided a moderately good fit which was able to account for the positions of all of the observed peaks. From this, it is concluded that sample 2-AR contains nanoparticles or has nanosized regions with the tetragonal ZrO2 structure. The PDFs of samples 1-AR and 1-80RH as well as 2-AR and 2-80RH were compared to assess the effects of humidity on the structures. The PDFs of the samples treated in 80% relative humidity look extremely similar to those of the untreated samples. For sample 1-80RH, there is a very small increase in intensity between 2.6 and 3.1 and also between 4.5 and 5.1 compared to sample 1-AR (Figure 2). These regions of the PDF are known to have large contributions from O−O distances. This indicates that these samples may contain slightly more oxygen (in the form of water) than the untreated samples. However, this extra water most likely resides in pores, as it does not seem to have any appreciable effect on the connectivity of the Zr atoms. The PDF of sample 2-80RH looks very similar to that of sample 2-AR at low r, but then at medium r, it tends to have slightly stronger peaks from the tetragonal ZrO2 phase (Figure 2). Overall, the type-2 material seems to show more changes as a result of humidity. 3.2. Local Structure of Amorphous Zr(OH)4. The PDF of sample 1-AR was analyzed to gain a detailed understanding of the structure of the untreated amorphous phase. The PDF is most sensitive to Zr atom positions, since these have the largest X-ray scattering power, but it also has some sensitivity to the O atom positions. The H atoms scatter too weakly to make an appreciable contribution to the PDF, and thus, they must be left out of the analysis. At low r, the PDF of sample 1-AR bears a strong resemblance to the PDF of monoclinic ZrO2 (Figure 3). The low-r features of the PDF of 1-AR differ considerably from what would be expected from tetragonal or cubic ZrO2. It is therefore helpful to first review some of the structural features of monoclinic ZrO2, as some of these are clearly shared with amorphous Zr(OH)4. In monoclinic ZrO2, there is a single crystallographically unique Zr position which is coordinated to seven O atoms. There are two unique O positions, one of which is coordinated to three Zr atoms and the other which is coordinated to four Zr atoms. These ZrO7 polyhedra share either edges or corners with each other. Each Zr is connected to 11 other Zr through oxygen bridges. There are seven edge sharing connections where the Zr are connected to each other through two O bridges and four corner sharing connections where the Zr are connected through a single O bridge. The edge sharing connections result in Zr−Zr distances of ∼3.5 Å,

calcination temperature was chosen due to the upper heating limit of the box furnace. X-ray total scattering data was collected on the 11-ID-B instrument at the Advanced Photon Source at Argonne National Lab. The ex situ treated samples were loaded into Kapton capillaries and measured at room temperature using λ = 0.2113 Å radiation. The 2D diffraction data was integrated into 1D patterns using the FIT2D software.23 The pair distribution functions were generated from the total scattering data using the PDFgetX2 software with a Qmax value of 24 Å−1.24 The PDF data were fit using the PDFgui and RMCProfile software.25,26 For this study, we use the following definitions of the pair distribution function: G(r) = 4πr(ρ(r) − ρ0), R(r) = rG(r) + 4πr2ρ0, where ρ(r) is the microscopic pair density, ρ0 is the average pair density, and r is the radial distribution.

3. RESULTS AND DISCUSSION 3.1. Comparison of Types and the Effect of Humidity. The PDFs of the as-received samples of type 1 and type 2 look remarkably similar at low r, but above ∼4.5 Å, major differences begin to appear (Figure 1). The PDF of sample 1-AR falls off

Figure 1. Comparison of the PDFs of the as-received samples of type 1 and 2.

quickly as r increases, becoming essentially flat by ∼12 Å. In contrast, the PDF of sample 2-AR shows structural correlations up to ∼37 Å. While sample 1-AR is clearly a purely amorphous material, the longer range order in sample 2-AR could indicate either (1) that it is also a pure material just with longer range correlations or (2) that it contains the same amorphous phase as sample 1-AR but with an additional component. The PDF of sample 2-AR was fit between 12 and 26 Å with the known phases of ZrO2 to see if any of these phases could account for the peaks at higher r. This r range was chosen, as it contains peaks of appreciable intensity but the contribution from an amorphous component with the structure of 1-AR would be

Figure 2. Comparison of the PDFs of the as-received samples and those treated at 80% relative humidity. B

DOI: 10.1021/acs.inorgchem.7b03137 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. RMC fit (red line) to the experimental G(r) of sample 1-AR (black dots).

Figure 3. Comparison of the PDFs of amorphous Zr(OH)4 (1-AR) with monoclinic ZrO2 (1-900C). The peaks from nearest neighbor distances are labeled.

A number of statistics were extracted from the final RMC configuration. The Zr atoms are connected to an average of 4.82 other Zr through O bridges. About 8.95% of the O atoms were found not to be coordinated to any Zr atom, 41.45% of the O were bonded to just a single Zr atom, 39.42% of the O were found to be bridging two Zr atoms, 9.51% were bonded to three Zr, and 0.64% were bonded to four Zr. It seems most likely that the uncoordinated O are present as H2O, those bonded to one or two Zr are terminal and bridging OH− groups, respectively, and those bonded to three or four Zr are O2− ions. Indeed, the number of uncoordinated H2O and the number of three or four coordinate O2− is roughly the same in the RMC configuration, meaning that the overall Zr(OH)4 composition would be maintained. This shows that what we are calling zirconium hydroxide is actually somewhere between a hydroxide (Zr(OH)4) and a hydrated oxide (ZrO2·2H2O), although closer to a hydroxide. The distinction between these two types of phases has been previously considered by others.27,28 The number of terminal hydroxyls matches fairly closely with a recent spectroscopic study which concluded that 38% of the OH− groups are terminal in Zr(OH)4.21 Inspection of the bond length and bond angle distributions in the RMC configuration can give insight into some of the features in the PDF beyond the nearest neighbor length scale. The small broad peak at ∼4.95 Å appears to be primarily due to O−O distances, while the stronger broad features around ∼5.9 and ∼6.7 Å are due to Zr−Zr distances. The Zr−O−Zr bond angle distribution has peaks at ∼102 and ∼144° which are what would be expected to produce Zr−Zr distances of ∼3.5 and ∼4.0 Å with Zr−O bonds around 2.15 Å. The O−Zr−O bond angle distribution has a sharp peak at ∼72° and then a flat, broad distribution of angles that starts to fall off above ∼140°. The second nearest neighbor (snn) Zr−Zr peaks can give us additional insight into how the Zr centered polyhedra are connected together. There are just two ssn Zr−Zr peaks present at ∼5.9 and ∼6.7 Å, each with a low-r shoulder. Monoclinic ZrO2 also has these same Zr−Zr distances but also has several additional snn Zr−Zr distances starting as low as 4.5 Å and with larger concentrations of distances around 5.2 and 6.2 Å. The Zr(OH)4 PDF is near a minimum at 5.2 and 6.2 Å, showing that such distances are uncommon. This would seem to indicate that certain configurations of polyhedra present in monoclinic ZrO2 predominate in Zr(OH)4 while others are nearly absent. Figure 5 shows two arrangements of three ZrO7 polyhedra in monoclinic ZrO2 which lead to the snn distances around 5.9 and 6.7 Å. To understand how it is possible to have

while the corner sharing connections result in Zr−Zr distances of ∼4.0 Å. The PDFs of amorphous Zr(OH)4 and monoclinic ZrO2 are shown in Figure 3. The first peak in all of the PDFs is from Zr− O distances. The peak has its greatest intensity at 2.15 Å in sample 1-AR and the position and intensity of this peak remains nearly the same for all samples, indicating the Zr coordination number is similar in the oxide and hydroxide. Next, there are weak features around ∼2.5 and ∼2.7 that correspond to O−O distances. This O−O region has greater intensity in the PDF of the hydroxide. This is primarily due to the overall greater concentration of oxygen in this phase, although differences in sample density also have some effect. The next two peaks at 3.47 and 4.0 Å are due to Zr−Zr distances with some minor contributions from second nearest neighbor Zr−O and O−O. In the hydroxide, these Zr−Zr peaks have less intensity, with the 4.0 Å peak decreasing by a greater amount. It appears that the number of Zr−Zr neighbors is lower in the hydroxide and the ratio of corner to edge sharing connections has decreased. Beyond the peak at 4.0 Å, all features in the PDF are from distances greater than nearest neighbors and contain contributions from all types of atom−atom pairs. To determine the coordination number of Zr by O in Zr(OH)4, the G(r) function was converted to the R(r) function and the area of the first peak was integrated. This gave a coordination number of 6.3, showing that the coordination number is similar to that seen in ZrO2 but possibly slightly lower. To get more quantitative information about the structure, reverse Monte Carlo (RMC) modeling was done on the G(r) of sample 1-AR. A large atomic configuration of 3375 Zr and 13 500 O atoms was produced and then randomized. The density of the configuration was based on the established value 3.25 g/cm3. The refinement was done in a multistep process. The configuration was initially refined using an average coordination number constraint for Zr based on the value obtained from R(r) and a large maximum move limit but without using the data. The atoms were then given an intermediate maximum move and refined against the data with the constraint still in place. In the final stage, a smaller maximum move was used and the coordination constraint was replaced with Zr−O and O−O interatomic potentials. These potentials were necessary to drive realistic nearest neighbor distance distributions, especially for the O−O partial, as it makes only a minor contribution to the total PDF. The final fit is shown in Figure 4. C

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is composed of mostly ZrO6 and ZrO7 polyhedra, with possibly a few Zr having slightly higher or lower coordination numbers. These polyhedra primarily share edges with each other and occasionally corners. They tend to form sheets that are nearly two-dimensional, and some of the second nearest neighbor distances seen in ZrO2 are observed much more than others. The second nearest neighbor distance distributions are broad due to variation in the coordination number of Zr which varies the Zr−Zr−Zr angles, and due to there being different patterns of connectivity present even though some predominate. These sheets of polyhedra are connected to each other through hydrogen bonding, and there is an occasional water molecule in gaps between these structural units. The variety of possible patterns quickly eliminates any longer range order, as features from third nearest neighbors are very weak and the PDF goes to 0 by ∼12 Å. The structure derived from the PDF data can be contrasted with an earlier model based on EXAFS data.22 The two models are similar in that they both have two-dimensional arrays of Zr atoms bridged by two hydroxyl groups, but there are a number of important differences. The earlier model has each Zr connected to four other Zr in a square grid, while our PDF analysis has shown that Zr has an average of about five Zr neighbors. The EXAFS study also does not show the presence of any single bridged Zr−Zr neighbors, which are clearly present in our PDF data. The presence of some single bridged Zr pairs is directly related to the fact that there are five neighbors, which cannot pack in a regular way while still all sharing edges. Having five neighbors within the plane also more naturally leads to a rapid loss of order as r increases compared to a square grid structure, which would be expected to show more medium range order than is observed. We did not observe a significant concentration of Zr−Zr distances at ∼5.0 Å, as reported in ref 20. Instead, we observe only a very weak peak at this distance which we attribute mainly to O−O distances, while the much stronger second nearest neighbor Zr−Zr peaks occur at longer distances. The differences between these models could be due to the samples having different local structures as different synthesis methods were used,29 or it could be that the lower noise of our data and the access it gives us to longer correlation lengths has allowed us to develop a more accurate model. Our five-neighbor model seems energetically plausible, as it can be considered a fragment of the ground state structure of ZrO2. 3.3. Decomposition of Zr(OH)4. Upon heating, Zr(OH)4 loses water until it eventually decomposes into ZrO2 through the following process: Zr(OH)4(s) → ZrO2(s) + 2H2O(g). The mass loss with heating has been measured by thermogravimetric analysis (TGA) and is reported in a previous publication.21 The PDFs of the samples heated to 40 and 125 °C appear to be extremely similar to those of the PDFs of the AR samples of the corresponding type (Figure 7). This is perhaps not surprising for the samples heated to 40 °C, as little water has been lost at this temperature. However, the previous TGA analysis shows that about 65% of the mass loss needed to convert to ZrO2 has occurred by 125 °C. What has likely occurred is that the uncoordinated H2O and some of the terminal OH− groups have been lost while the bridging oxygens all remain. The network of Zr and bridging O atoms remains nearly unaltered despite this loss, which results in minimal changes to the appearance of the X-ray PDFs. The samples heated to 250 °C show significant changes in their PDFs relative to the unheated samples (Figure 7). These

Figure 5. Polyhedral connectivity that leads to the two most commonly observed second nearest neighbor Zr−Zr distances in amorphous Zr(OH)4. The left figure shows the most common arrangement with Zr−Zr distances of ∼6.75 Å and a Zr−Zr−Zr angle of ∼157°. The right figure shows the second most common arrangement and has Zr−Zr distances of ∼5.83 Å and a Zr−Zr−Zr angle of ∼119°. Green atoms are Zr, and red atoms are O.

large numbers of these configurations present while still having the correct total number of Zr neighbors, an exercise was performed where a ZrO7 polyhedron in the monoclinic ZrO2 structure was chosen and then all neighboring Zr atoms which make Zr−Zr distances not observed in the Zr(OH)4 PDF were deleted. Removing these neighbors left five remaining neighbors, in good agreement with the RMC results. An important feature of this configuration, shown in Figure 6, is that it is nearly planar. By combining the above results, it is possible to provide a detailed description of the structure of amorphous Zr(OH)4. It

Figure 6. Typical environment around a Zr centered polyhedron in amorphous Zr(OH)4. The figure was generated from the monoclinic ZrO2 structure by removal of neighboring polyhedra which had second nearest neighbor Zr−Zr distances not observed in the sample 1-AR PDF. The central Zr polyhedron has five neighbors which are mostly edge sharing but with some corner sharing connections. The relative ratio of corner to edge sharing for this configuration is only half of that seen in monoclinic ZrO2. Only second nearest neighbor distances of 5.83, 6.38, and 6.75 Å are present with other distances being avoided by only keeping this nearly two-dimensional section of the structure. Green atoms are Zr, and red atoms are O. D

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Figure 7. Evolution of the PDFs upon heating each type prior to crystallization.

coordinated by two, three, or four Zr atoms. This means that on a local level this phase looks fairly similar to the phases of ZrO2, although the coordination numbers are still somewhat lower as a consequence of the O:Zr ratio being slightly above 2. The coordination numbers obtained match fairly closely with those obtained in a theoretical study of amorphous ZrO2, and the PDF of this phase looks similar to the previously measured amorphous ZrO2 PDF.8,30 The largest change to snn Zr−Zr distances is that the peak around 6.7 Å is nearly gone while a new peak has appeared around 6.35 Å (Figure 7). The peak around 5.9 Å has also shifted very slightly to higher r and increased slightly in intensity. The observed peaks for this phase do not match up with those from any single ZrO2 polymorph. Tetragonal ZrO2 would be expected to produce a strong peak around 6.3 Å; however, none of the features in the PDF at lower r values match up well with the tetragonal structure but instead most closely resemble the monoclinic structure. We have noticed that the 101 planes of the tetragonal structure contain only one type of snn Zr−Zr distance at ∼6.3 Å. It could be that there are single layers of Zr atoms arranged in a similar fashion as in these 101 planes but with the Zr atoms still having the coordination environments and connectivity in other directions similar to that seen in the monoclinic phase. Such a pattern also produces third nearest neighbor distances at ∼7.3 Å, and indeed, a weak peak does appear at this distance in the PDF. The presence of such planes could facilitate the formation of a metastable tetragonal phase upon further heating, as discussed below. 3.4. Crystalline Phases. Samples heated to 330 °C or above are all crystalline ZrO2, as indicated by the appearance of Bragg peaks in the diffraction patterns and the observation of PDF peaks out to high r values. However, there are still large differences in the PDFs of the samples heated to 330, 500, and 900 °C (Figure 9). There are clear correlation length differences, with the samples heated to higher temperatures showing stronger peaks that persist to higher r values. This increase in crystallinity is expected with greater heat treatment. There are also some minor differences in the positions and relative intensities of the peaks among the samples treated at different temperatures. To understand the origin of these differences, the PDFs were fit using the crystalline forms of ZrO2 as models. For the samples heated to 900 °C, the best fits were obtained when using just the monoclinic phase, showing these samples have reached their thermodynamic equilibrium. The samples heated to either 330 or 500 °C could be best modeled as a mixture of monoclinic and tetragonal phases. The relative amounts of tetragonal phase are greatest for the 350 °C samples, then decrease for the 500 °C samples, and then by 900

show a decrease of intensity in the nearest neighbor O−O region, a decrease in intensity around ∼5 Å, an increase in intensity for the Zr−Zr peak at 4.0 Å, and a distinctly different shape for the region corresponding to second and third nearest neighbor Zr−Zr distances. The correlation length does not seem to have increased significantly, so while the samples heated to 250 °C have a different local structure heating to this temperature has not led to longer range order. It appears that by this temperature enough water has been lost that the network of Zr and bridging O has collapsed upon itself and rearranged to create a new type of local structure. To understand the structure of this phase, RMC modeling was done on the G(r) of sample 1-250C. The TGA analysis shows that the mass loss is about 87% complete at 250 °C, which corresponds to a composition of ZrO2.23H0.46 or alternately ZrO1.77(OH)0.46. To create the supercell, this was approximated by having a composition of Zr4O9. A large configuration was constructed with 5324 Zr atoms and 11 979 O atoms. As the density is not well-known, a guess was made that had a number density between that of Zr(OH)4 and ZrO2. Then, several RMC runs were carried out using slightly different densities and the one that gave the best fit and most reasonable looking results was chosen for analysis (Figure 8).

Figure 8. RMC fit (red line) to the experimental G(r) of sample 1250C (black dots).

The RMC configuration had a coordination number for Zr by O slightly above 6, which is basically the same as that seen for the unheated samples. However, the coordination number of Zr by itself has increased substantially and is now 8.07. Most of this increase comes from additional corner sharing connections between Zr centered polyhedra as the network becomes three-dimensional. The O atoms are almost all E

DOI: 10.1021/acs.inorgchem.7b03137 Inorg. Chem. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC0206CH11357. We thank Drs. Kevin Beyer and Karena Chapman for assistance in the data collection and Dr. Alex Balboa for assistance with sample preparations. The authors would also like to thank Dr. Michael Ellzy, Dr. Bruce King, Gregory Peterson, and Darren Emge for their support. The authors gratefully acknowledge the Joint Program Executive Office under the Chemical Biological and Detection Program as well as the Defense Threat Reduction Agency under program number CB10123 for support of this effort.

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Figure 9. Change in the PDFs of the type-1 crystalline samples upon heating.

REFERENCES

(1) Shri Prakash, B.; Senthil Kumar, S.; Aruna, S. T. Properties and development of Ni/YSZ as an anode material in solid oxide fuel cell: a review. Renewable Sustainable Energy Rev. 2014, 36, 149−179. (2) Shim, J. H.; Chao, C. C.; Huang, H.; Prinz, F. B. Atomic Layer Deposition of Yttria-Stabilized Zirconia for Solid Oxide Fuel Cells. Chem. Mater. 2007, 19, 3850−3854. (3) Ahamer, C.; Opitz, A. K.; Rupp, G. M. Fleig, Revisiting the Temperature Dependent Ionic Conductivity of Yttria Stabilized Zirconia (YSZ). J. Electrochem. Soc. 2017, 164, F790−F803. (4) Jaipal, M.; Chatterjee, A. J. Relative Occurrence of OxygenVacancy Pairs in Yttrium-Containing Environments of Y2O3-Doped ZrO2 Can Be Crucial to Ionic Conductivity. J. Phys. Chem. C 2017, 121, 14534−14543. (5) Besling, W. F. A.; Young, E.; Conard, T.; Zhao, C.; Carter, R.; Vandervorst, W.; Caymax, M.; Gendt, S. D.; Heyns, M.; Maes, J.; Tuominen, M.; Haukka, S. Characterisation of ALCVD Al2O3−ZrO2 nanolaminates, link between electrical and structural properties. J. NonCryst. Solids 2002, 303, 123−133. (6) Chim, W. K.; Ng, T. H.; Koh, B. H.; Choi, W. K.; Zheng, J. X.; Tung, C. H.; Du, A. Y. Interfacial and bulk properties of zirconium dioxide as a gate dielectric in metal−insulator−semiconductor structures and current transport mechanisms. J. Appl. Phys. 2003, 93, 4788−4793. (7) Mahajan, A. M.; Khairnar, A. G.; Thibeault, B. J. High dielectric constant ZrO2 films by atomic layer deposition technique on germanium substrates. Silicon 2016, 8, 345−350. (8) Vanderbilt, D.; Zhao, X.; Ceresoli, D. Structural and dielectric properties of crystalline and amorphous ZrO2. Thin Solid Films 2005, 486, 125−128. (9) Karwacki, C. J.; Ganesh, P.; Kent, P. R. C.; Gordon, W. O.; Peterson, G. W.; Niu, J. J.; Gogotsi, Y. Structure−activity relationship of Au/ZrO2 catalyst on formation of hydroxyl groups and its influence on CO oxidation. J. Mater. Chem. A 2013, 1, 6051−6062. (10) Kouva, S.; Honkala, K.; Lefferts, L.; Kanervo, J. Monoclinic zirconia, its surface sites and their interaction with carbon monoxide. Catal. Sci. Technol. 2015, 5, 3473. (11) Howard, C. J.; Hill, R. J.; Reichert, B. E. Structures of ZrO2 polymorphs at room temperature by high-resolution neutron powder diffraction. Acta Crystallogr., Sect. B: Struct. Sci. 1988, 44, 116−120. (12) Peterson, G. W.; Rossin, J. A.; Karwacki, C. J.; Glover, T. G. Surface chemistry and morphology of zirconia polymorphs and the influence on sulfur dioxide removal. J. Phys. Chem. C 2011, 115, 9644− 9650. (13) Peterson, G. W.; Karwacki, C. J.; Feaver, W. B.; Rossin, J. A. Zirconium hydroxide as a reactive substrate for the removal of sulfur dioxide. Ind. Eng. Chem. Res. 2009, 48, 1694−1698. (14) Singh, J.; Mukherjee, A.; Sengupta, S. K.; Im, J.; Peterson, G. W.; Whitten, J. E. Sulfur dioxide and nitrogen dioxide adsorption on zinc oxide and zirconium hydroxide nanoparticles and the effect on photoluminescence. Appl. Surf. Sci. 2012, 258, 5778−5785.

°C the tetragonal phase is gone. It was also observed that the refined tetragonal phase fraction varied slightly depending on the r range used in the refinement. The phase fraction was greatest when refining at low r, indicating that the correlation length of the tetragonal component is smaller than that of the monoclinic component. The samples of type 2 were found to have a greater amount of tetragonal phase, presumably since they contained some prior to heat treatment. Other studies using different experimental methods have previously noted the appearance of metastable tetragonal ZrO2 when zirconia is produced by the decomposition of a precursor.21,31−33

4. CONCLUSION The low noise level of our data and the access it gives us to longer range correlations has allowed us to propose a new and more detailed description of Zr(OH)4 than has previously been achieved. In contrast to some earlier studies which have suggested that amorphous zirconium hydroxide is structurally similar to cubic or tetragonal ZrO2, our PDF data show unambiguously that Zr(OH)4 is most closely related to monoclinic ZrO2. We propose a model where the Zr are coordinated to six or seven O atoms and connected to about five other Zr atoms. The Zr centered polyhedra are mostly connected by sharing edges, but there is also a small but important number of corner sharing connections. Heating to temperatures up to 125 °C causes some water loss, but the Zr connectivity is unaffected. Heating to 250 °C causes a transition to a new amorphous phase with a similar correlation length. In this phase, the Zr have about eight Zr neighbors, with the additional connections being almost entirely corner sharing. This new phase also appears to have some structural similarities to the tetragonal ZrO2 phase. Further heating results in a mixture of tetragonal and monoclinic ZrO2, with the tetragonal component disappearing as the annealing temperature is further increased. This structural knowledge will assist in understanding the mechanism of useful behaviors such as the breakdown of chemical warfare agents.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (410) 436-5290. ORCID

Graham King: 0000-0003-1886-7254 Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.inorgchem.7b03137 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b03137 Inorg. Chem. XXXX, XXX, XXX−XXX