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Hydrated calcium oxalates: crystal structures, thermal stability and phase evolution Alina R. Izatulina, Vladislav V. Gurzhiy, Maria Krzhizhanovskaya, Mariya A. Kuz’mina, Matteo Leoni, and Olga V. Frank-Kamenetskaya Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00826 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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Crystal Growth & Design

Hydrated calcium oxalates: crystal structures, thermal stability and phase evolution Alina R. Izatulina,*,† Vladislav V. Gurzhiy,† Maria Krzhizhanovskaya,† Mariya A. Kuz’mina,† Matteo Leoni, ‡ Olga V. Frank-Kamenetskaya† †

Institute of Earth Sciences, St. Petersburg State University, 199034, University emb. 7/9, St.

Petersburg, Russian Federation ‡

University of Trento, 38123, via Mesiano 77, Trento, Italy

Calcium oxalate; Whewellite; Weddellite; Caoxite; Crystal structure; Thermal expansion; Structural complexity; X-ray diffraction

Thermal stability, structural evolution pathways and phase transition mechanisms of the calcium oxalates whewellite (CaC2O4·H2O), weddellite (CaC2O4·(2+x)H2O) and caoxite (CaC2O4·3H2O) have been analyzed using single crystal and powder X-ray diffraction (XRD). During single crystal XRD heating experiments, α-CaC2O4 and the novel calcium oxalate monohydrate have been obtained and structurally characterized for the first time. The highest thermal expansion of these compounds is observed along the direction of the hydrogen bonds, whereas the lowest expansion and even contraction of the structures occur due to the displacement of neighbor layered complexes towards each other and to an orthogonalization of the monoclinic angles.

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Within the calcium oxalate family, whewellite should be regarded as the most stable crystalline phase at ambient conditions. Weddellite and caoxite transform to whewellite during dehydrationdriven phase transition promoted by time and/or heating.

Introduction Oxalic acid is the smallest and simplest dicarboxylate and is ubiquitous in the environment (in soil, water, and aerosols1-2), which explains the widespread distribution of oxalates. Calcium oxalates are notably common biominerals and can be found e.g. in coal basins, bituminous shale, bottom sediments, on the contact of rocks with guano, lichens, fungi or some higher plants and even on a surface of monuments3-10. This is mostly due to the affinity of oxalates to bivalent cations, which is reflected in the ability to form insoluble precipitates, and because the sources of calcium (carbonate rocks and subfossils) interact more actively with weak organic acids. Calcium oxalates are also found among the pathogenic mineral precipitates in human bone marrow, myocardium, joints, lungs, liver, thyroid gland, intestinal mucosa, eyes, and urinary system11-13. Oxalates span therefore several fields (medicine, biology, mineralogy, materials science, etc.), which is reflected in a large number of publications. Nevertheless, many questions remain unresolved. For instance, the role of water in the formation of calcium oxalate crystal structures as well as the mechanisms of phase transition is still unclear. Calcium oxalates are represented in nature by three hydrated forms: whewellite (CaC2O4·H2O; COM)14-15, weddellite (CaC2O4·(2+x)H2O; COD)16-17 and caoxite (CaC2O4·3H2O; COT)18-19. The anhydrous forms are known only as synthetic compounds20, so the H2O molecules obviously play a significant role in crystal growth. COM is the most prevalent mineral, while COD and


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COT are regarded as metastable phases, which is confirmed by a number of studies and observations21-22. On the other hand, COD and COT are regarded as precursors for COM crystals formation23-24. Thus, the composition of oxalate renal stones varies during storage at room temperature. For instance, pure COD samples of our collection after 4 years of storage contain, on average, about 30% of COM. A similar example was recently described in Conti et al. 2015; as a result, we can say that any aged caoxite crystal in museums or private collections is most likely not COT, but other calcium oxalates. A number of works have also been devoted to the dehydration of calcium oxalates21, 25-32, but they mainly rely on TGA, DSC or SEM. These techniques are very sensitive to changes in the amount of water, but at the same time, they give a very rough idea on the state of crystalline matter, which is often the priority. Crystal structures of hydrated calcium oxalates were intensively studied during last half a century, which results are summarized in Table 1. However, there is still no clear understanding about the correlation between the structural architectures of the calcium oxalates. Thus, it is quite essential to determine the ways of dehydration-driven structural evolution and phase transition mechanisms among the aforementioned compounds.

Table 1. Crystallographic data for calcium oxalates. Mineral name

Chemical formula

S.G.

Z

a, Å / α, °

b, Å / β, ° 14.471(2) / 109.978(5) 14.5884(4) / 107.05(2) 7.295(1) / 107.07(3)

Whewellite

CaC2O4·H2O

P21/c

8

6.250(1)

Whewellite

CaC2O4·H2O

P21/n

8

9.9763(3)

I2/m

4

9.978(1)

I4/m

8

12.378(1)

12.378(1)

P-1

2

6.110(1) / 76.43(1)

7.164(1) 70.19(1)

Weddellite Caoxite

CaC2O4·H2O (Т= 55 °C) CaC2O4·(2+x)H2O (x = 0.13-0.43) CaC2O4·3H2O


/

c, Å / γ, °

V, Å3

10.114(2)

859.7(3)

6.2913(3)

875.38(11)

[35]

6.292(1)

437.82(12)

[36]

7.366(1)

1128.6(1)

8.442(2) / 70.91(1)

325.3(1)

Ref. [33-34]

[33, 3739] [24, 34, 40]

3


β-CaC2O4

P2/m

4

6.1644(3)

7.3623(2) / 90.24(2)

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9.5371(5)

432.83(4)

[20]

Herein we report on the thermal stability of crystalline compounds and phase transition mechanisms within the family of calcium oxalates, which shows a highly complex nature for such a chemically simple system. During single crystal XRD heating experiments α-CaC2O4 and the novel calcium oxalate monohydrate has been obtained and structurally characterized.

Experimental Calcium oxalate crystalline samples Single crystals of COM and COD have been extracted from the oxalic renal stones of our collection, which consist of about 2000 samples removed from residents of St. Petersburg, Russian Federation. About 74.5% of samples within the collection consist of calcium oxalates and about 2% are pure weddellite stones. A fraction of the pure COM and COD stones (Figure 1) was also crushed for the powder X-ray diffraction (PXRD) experiments at non-ambient temperatures. No natural COT was found. COM, COD, and COT were also synthesized from sodium oxalate (Na2C2O4, 98 %, Vekton), anhydrous calcium chloride (CaCl2, 99%, Vekton), citric acid monohydrate (C6H8O7·H2O, 99%, Vekton), magnesium sulfate heptahydrate (MgSO4·7H2O, 99%, Vekton), sodium hydroxide (NaOH, 99%, Vekton), and hydrochloric acid (HCl, 35-38 wt. % in H2O, 99.9%, Vekton). The synthesis was performed at room temperature (22–25 °C) by precipitation from aqueous solutions of the correspondent reagents in 500 ml of deionized water at different pH in the range 4.5–7.5). The solutions were kept for 5 days to complete the precipitation. The pH was adjusted using sodium hydroxide and hydrochloric acid solutions. The resulting precipitates were filtered, washed with deionized water and dried in air at room temperature.


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In particular, the COM powder was synthesized by precipitation from aqueous solution of 0.268 g (2 mmol) of sodium oxalate and 0.222 g (2 mmol) of calcium chloride at a pH range of 5.50–5.65. Conversely, a crystalline powder of predominantly COD (with minor amounts of COM) was prepared by precipitation from aqueous solution of 0.201 g (1.5 mmol) of sodium oxalate and 0.833 g (7.5 mmol) of calcium chloride with addition of 0.672 g (3.2 mmol) of citric acid at a pH range of 5.95–6.15. Finally, single crystals of COT were prepared by precipitation from aqueous solution of 0.201 g (1.5 mmol) of sodium oxalate, 0.666 g (6.0 mmol) of calcium chloride with addition of 1.365 g (6.5 mmol) of citric acid and 0.246 g (1.0 mmol) of magnesium sulfate at a pH range of 5.95–6.15. The major portion of the obtained precipitate consisted of the mixture of COM spherulites (up to 20-30 µm) and bipyramidal crystals of COD (up to 50 µm), while the minor portion of precipitate consisted of larger in size (up to 200 µm, Figure 1) prismatic crystals of COT, which were manually selected under the optical microscope.


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Figure 1. Calcium oxalate crystals and renal stones: fragments of the whewellite monophase renal stone (a); single crystals of weddellite (b, c); twin of caoxite crystals (d); caoxite crystal after measurement at 40 °C (e; the loss of transparency is due to the phase transition to compound 14, Table 2); weddellite renal stone (f; the surface of the stone is formed by sharp edges and smooth, reflecting the light, faces of COD crystals).

Single crystal X-ray studies Single crystals of COM, COD, and COT were selected under an optical microscope, encased in oil-based cryoprotectant and mounted on glass fibers. Data were collected using a Rigaku Oxford Diffraction Xcalibur diffractometer equipped with an Eos CCD area detector operated


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with monochromated MoKα radiation (λ[MoKα] = 0.71073 Å) at 50 kV and 40 mA. Diffraction data were collected at different temperatures in the range ‒173 to +100 °C (Table 2) with frame widths of 0.5° in ω and φ, and exposures of 30, 5 and 3 s spent per each frame within the series of measurements of COM, COD, and COT initial crystals, respectively. Data were integrated and corrected for background, Lorentz, and polarization effects. An empirical absorption correction based on spherical harmonics implemented in the SCALE3 ABSPACK algorithm was applied in CrysAlisPro program41. The unit-cell parameters (Tables 2 and S1) were refined by the leastsquares techniques. The structures were solved by direct methods and refined using the SHELX programs42 incorporated in the OLEX2 program package43. The final models included coordinates and anisotropic displacement parameters for all atoms. The H atoms of H2O molecules were localized from difference Fourier maps and kept fixed during the refinement. The positions of the H atoms of COD ‘zeolite’ H2O molecules were not localized. Supplementary crystallographic data have been deposited in the Inorganic Crystal Structure Database (Table 2) and can be obtained from Fachinformationszentrum Karlsruhe via www.fizkarlsruhe.de/en/leistungen/kristallographie/kristallstrukturdepot/order-form-request-fordeposited-data.html.

No. of SC XRD experiment

Temperature, °C

Table 2. Crystallographic data and refinement parameters for 1 – 14.

Crystal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Chemical formula

1

-80

CaC2O4·H2O

P21/c 6.2906(4)

2

20

CaC2O4·H2O

P21/c 6.2949(3)

S.G.

a, Å / α, °

COM

b, Å / β, °

c, Å / γ, °

V, Å3

14.5876(9) / 10.1208(9) 875.27(11) 109.536(6) 14.6029(7) / 10.1225(7) 877.06(9) 109.513(5)


R1 (|Fo| ≥ 4σF)

ICSD

0.0305

434200

0.0360

434201

7


I2/m 6.3031(3)

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7.3026(3) / 10.0038(4) 440.11(3) 107.102(5) 7.3994(12) / 6.236(4) 442.6(3) 100.86(4)

3

80

CaC2O4·H2O

0.0314

434202

4

100

CaC2O4

0.0719

434203

5

-173 CaC2O4·(H2O)2.25 I4/m 12.3094(6) 12.3094(6)

7.3131(5)

1108.08(13) 0.0242

434204

6

-120 CaC2O4·(H2O)2.25 I4/m 12.3086(6) 12.3086(6)

7.3290(5)

1110.34(13) 0.0256

434205

7

-80 CaC2O4·(H2O)2.25 I4/m 12.3187(5) 12.3187(5)

7.3363(5)

1113.29(12) 0.0256

434206

8

-40 CaC2O4·(H2O)2.26 I4/m 12.3333(3) 12.3333(3)

7.3392(3)

1116.36(8)

0.0267

434207

C2/m 9.767(2)

COD

9

0

CaC2O4·(H2O)2.24 I4/m 12.3432(3) 12.3432(3)

7.3469(3)

1119.33(7)

0.0263

434208

10

20

CaC2O4·(H2O)2.19 I4/m 12.3398(3) 12.3398(3)

7.3475(3)

1118.81(7)

0.0264

434209

11

-80

CaC2O4·(H2O)3

0.0261

434210

12

-20

CaC2O4·(H2O)3

0.0286

434211

13

10

CaC2O4·(H2O)3

0.0284

434212

14

40

CaC2O4·H2O

0.1358

434213

COT

6.1052(7) / 7.1262(7) / 8.4328(9) / 321.55(7) 76.319(9) 70.002(10) 70.319(10) 6.1054(7) / 7.1406(6) / 8.4431(8) / P-1 323.31(6) 76.418(8) 70.135(9) 70.535(9) 6.1027(7) / 7.1489(7) / 8.4440(8) / P-1 323.93(6) 76.457(8) 70.209(10) 70.639(9) 7.234(5) / C2/m 9.315(9) 7.606(9) 475.8(9) 111.84(12) P-1

Powder X-ray studies at non-ambient temperatures Pieces of monomineral COM and COD renal stones, as well as synthetic samples of hydrated calcium oxalates, were ground in an agate mortar for the in situ examination using a Rigaku Ultima IV powder X-ray diffractometer (CoKα radiation; 40 kV / 30 mA; Bragg-Brentano geometry; PSD D-Tex Ultra detector). A Rigaku SHT-1500 chamber was employed for experiments in air in the range of +25 – +150 °C; a Pt strip (20×12×2 mm3) was used as heating element and sample holder. Low-temperature experiments (‒173 to +170 °C) were performed in vacuum in a Rigaku R 300 chamber using a Cu sample holder (20×12×2 mm3). The temperature steps varied from 2 to 10 °C depending on compound and temperature range. The heating rate was 2 °C/min. The collection time at each temperature step was about 30 min. The reversibility of the observed phase transformations was checked by collecting PXRD data both on heating and cooling.


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Phase identification was carried out using the ICDD PDF-2 Database (release 2016). The unit cell parameters were refined by the Pawley method using TOPAS 4.2 software44. The background was modelled using a Chebychev polynomial of 12th order. The peak profile was described using the fundamental parameters approach. The zero shift parameter was refined at every step, and it was usually increased by 0.01-0.02 °2θ because of the sample holder expansion on heating. The main coefficients of the thermal-expansion tensor were determined using a second-order approximation of temperature dependencies for the unit cell parameters by means of the TEV program45. The same software was also employed to determine the orientation of the principal axes of the thermal expansion tensor and for visualization purposes.

Results Structure descriptions Whewellite structure evolution The first investigation of whewellite crystal structure was performed by Cocco14. Afterward, the structural model of COM has been refined several times either in P21/c or P21/n space groups33-35. The works of S. Deganello on thermal treatment of whewellite crystals should be noted36, 46. The crystal structure of COM (1‒3) is based on Ca-bearing sheets, arranged parallel to (100) in case of whewellite structure (1 and 2; Figure 2a), and parallel to (101) in case of its hightemperature modification (3; Figure 2b). The Ca2+ cations are arranged at the center of a distorted square antiprism that shares edges with three adjacent antiprisms to form layers with lens-like voids, occupied by the oxalate groups. These layers are linked to each other via oxalate


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groups and hydrogen bonds, which involve terminal H2O molecules of Ca polyhedra that are directed to the interlayer space. There are two crystallographically nonequivalent Ca positions in the structure of whewellite (1 and 2), which are coordinated by eight O atoms that belong to one H2O molecule and five oxalate groups. H2O molecules in the structures of 1 and 2 occupy two sites, each of which is disordered over two crystallographically nonequivalent positions (Figure 3a) with the total site occupancy factor (s.o.f.) equal to 1.0. The Ca‒O distances vary in the range 2.425(2)‒2.527(3) Å with an average value equal, respectively, to 2.426 and 2.447 Å for the Ca1- and Ca2-centered polyhedra in the structure of 2.


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Figure 2. Layered complexes build of Ca-centered polyhedra and oxalate groups in the structures of whewellite (a), HT whewellite (b), novel monohydrate phase 14 (c), α-CaC2O4 (d), βCaC2O4

(e); and chains of edge-sharing Ca-polyhedra in the structure of COD (f) . Legend: Ca-

polyhedra = lilac; O atoms = red; C atoms = white; H atoms = grey.


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HT modification of whewellite (3) possess the same structural motif as in 1 and 2, and even symmetry remains monoclinic, however, the lattice type changes from primitive to I-centered and the b unit cell parameter halves (Table 2). These changes are the result of slight temperatureinduced atomic shifts that make the Ca polyhedra and oxalate groups arranged right one after another on the projection along the [010] in comparison to those for whewellite (Figure 4a,c). Such structural transformation can hardly be followed using PXRD due to the high similarity of the experimental patterns (Figure 5c, S1), but is well seen on the unwrapped precession images of the reciprocal space (Figure 5a,b): extinction of reflections along the [010], resulting from the halving of the b unit cell parameter. The Ca atoms have the same coordination environment in the structure of 3 as in 1 and 2 i.e. a distorted square antiprism built by 8 O atoms with Ca‒O ranging between 2.439(2)‒2.476(4) Å and an average distance equal to 2.450 Å. Besides the positional disorder of H2O molecules in the structure of 3, a splitting of the O3 site (O atom of the oxalate group) is observed (Figure 3b), which is the result of symmetry operation and could be explained by cumulative effect of imposition of O7 and O4 atoms in the structure of low temperature COM modification (1 and 2). Recently47,48, it was proposed that the variations of the whewellite unit cell parameters can be explained by the presence of additional H2O molecules arranged in between the Ca-bearing sheets and creating linked chains of oxalate groups. As those sites are substantially vacant and disorderly distributed, they can’t be seen in the difference Fourier synthesis maps. It is likely that phase transition from LT to HT whewellite occurs after release of these H2O molecules, whose presence in the structure causes the slight distortions leading to a doubling of the b unit cell parameter.


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Figure 3. Coordination of Ca atoms in the structures of whewellite (a), HT whewellite (b), novel COM phase 14 (c), α-CaC2O4 (d), β-CaC2O4 (e), weddellite (f) and caoxite (g). Legend: Ca atoms = lilac; O atoms = red; C atoms = grey; H atoms = white.


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Further heating of the COM single crystal sample up to 100 °C results in the release of H2O and in the transition to the novel anhydrous calcium oxalate structural type (4). Its structure is quite remarkable due to high similarity with the previous monohydrates (Figure 4e). There is one crystallographically nonequivalent Ca2+ atom in the structure of 4, coordinated by seven O atoms from two nonequivalent oxalate groups (Figure 3d). In comparison to the monohydrated structures of 1‒3, the Ca‒O bonds in 4 are shorter, as a result of the loss of one atom in the coordination sphere (Ca‒O = 2.326(5)‒2.461(3) Å and = 2.405 Å). The Ca-centered polyhedron share the edges with three adjacent polyhedra to form a layered complex arranged parallel to (001). This resembles the projection of the monohydrated phases (Figure 2d), taking into account the strong shift of one of the O atoms in the Ca-dimers caused by the release of apical H2O molecules.


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Figure 4. Projections along the layered complexes and the arrangement of figures of thermal expansion/contraction coefficients in the structures of whewellite (a, b), HT whewellite (c, d), αCaC2O4

(e, f), β-CaC2O4 (g) and novel COM phase 14 (h). Legend: see Fig. 2; figures of TEC are

arranged relative to the structure, expansion = green, contraction = red. Weddellite structure evolution


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The first structural data on weddellite were obtained by Sterling 17, who localized the position of “zeolite” H2O molecule. Tazzoli and Domeneghetti33 refined the structural data and observed the disordered distribution of “zeolite” H2O molecules along the z axis within the channel. The most recent works37-39 show a significant variation in the amount of “zeolite” H2O molecules and a clear positive correlation between the latter and several crystal chemical parameters, including the a unit cell parameter. There is one crystallographically nonequivalent Ca2+ atom in the structure of weddellite (5‒10) coordinated by six O atoms of oxalate groups and two O atoms of cis-H2O molecules, to form slightly distorted square antiprisms (for instance, Ca‒O = 2.326(2)‒2.498(1) Å and = 2.455 Å in the structure of 9). Each Ca-centered antiprism shares common O1‒O1 edges with two adjacent polyhedra to form infinite chains arranged along the [001] direction. These chains are linked together via oxalate groups to form two types of channels arranged along the c axis (Figure 6) and different in the internal diameter. The walls of the large channel are formed by OW1 H2O molecules, while OW2 molecules are arranged within the smaller one. The diameter of the small channel could be determined as the distance between two OW2 molecules arranged at the same level and is equal ~3.0 Å. The diameter of the large channel is determined as the diagonal distance between OW1 atoms and is equal ~4.6 Å (four OW1 molecules are arranged in the plane parallel to (001)). At the center of the large channel, on the fourfold axis, there is an additional position of “zeolite” H2O molecule that is split into two nonequivalent and essentially vacant sites (OW3 and OW31) separated by ~0.6 Å. The sites of the OW1 and OW2 H2O molecules are fully occupied. These molecules are involved in the hydrogen bonding system with the O atoms of the oxalate groups: OW1‒H1···O1 and OW2‒H2···O2. The “zeolite” H2O


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molecules in the structures of weddellite vary from 0.13 to 0.43 p.f.u.; the resulting structural features and correlations have been discussed in detail in recent works37-39.

Figure 5. Unwrapped precession images of the reciprocal space of the structure of HT whewellite (a), whewellite (b); experimental and calculated powder diffraction patterns of COM single crystal. Caoxite structure evolution The crystal structure of COT has been reported for the first time by Deganello et al.18. The structure has been subsequently described with an alternative triclinic cell setting by Basso et al.19 and further refined to better R-values by assigning the H atoms positions24,34,40. There is one crystallographically nonequivalent Ca2+ atom in the structure of caoxite (11‒13) coordinated by eight O atoms to form slightly distorted square antiprisms. Three of these oxygens are of nonequivalent H2O molecules, whereas the other five belong to oxalate groups. The Ca‒O bond lengths correlate well with the other structures of calcium oxalates that possess


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the same antiprism coordination geometry (thus, Ca‒O = 2.406(2)‒2.538(1) Å and = 2.456 Å in the structure of 13). However, the presence of three cis-H2O molecules in the coordination sphere of Ca2+ cation (Figure 3g) modifies the angular parameters of the polyhedron. The crystal structure of caoxite is based on dimers of edge-sharing Ca-polyhedra, connected via oxalate groups into layered zigzag units arranged parallel to the (010) plane (Figure 7). The linkage between these layers is provided by the H-bonding system only.

Figure 6. Crystal structure of COD along the [001] direction (a) and the arrangement of figures of thermal expansion/contraction coefficients (b, c). Legend: see Figure 4. Heating of the COT single crystal up to 40 °C resulted in the release of two H2O molecules and the corresponding transition to the novel calcium oxalate monohydrate (14). In this structure, the Ca2+ cations possess a quite usual distorted square antiprism coordination geometry (Ca‒O = 2.420(8)‒2.548(12) Å and = 2.469 Å). However, the detailed analysis shows a


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remarkably different arrangement of its ligands in comparison to all other hydrated calcium oxalates: the oxygens of the H2O molecules, in fact, contribute to bridge two neighboring Ca atoms (Figure 3c). Moreover, the linkage of the neighboring Ca-centered polyhedra occurs through the sharing of common OW1‒OW1 edge. It also worth noting that the geometry of the Ca-centered polyhedra is close to those in the structure of COD with a planar square face O1‒O1‒O2‒O2 arranged in front of the edge of two H2O molecules (OW1‒OW2 and OW1‒OW1 in the structure of COD and 14, respectively). The crystal structure of 14 is based on the layered complexes of edge-shared Ca antiprisms with lens-like voids occupied by the oxalate groups that are quite similar to the sheets described in the structures of other monohydrates and anhydrous calcium oxalates. The layers are arranged parallel to (001) and linked to each other by the oxalate groups. Most likely the particular type of Ca-polyhedra linkage through the common OW1‒OW1 edge results in another orientation of the oxalate groups in between the layers that is well seen on the projections of the structures along the layers (Figure 4h).


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Figure 7. Crystal structure of COT along the [100] (a) and [010] directions (b) and the arrangement of figures of thermal expansion/contraction coefficients relative to the respective projections of the structure (c, d). Legend: see Figure 4. Anhydrous oxalate structure The only known to date crystal structure of anhydrous calcium oxalate (β-CaC2O4) has been solved using a combination of atomistic simulation and Rietveld method refinements from PXRD data

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. The calcium atoms are surrounded by seven O atoms from the oxalate groups

with a geometry similar to the one found in the structure of 4. Due to the release of H2O molecules from the structure, a rearrangement of the oxalate groups occurs, and rather short O4‒O4 edge-shared bonds of 2.6 Å appear between the adjacent polyhedra (O1‒O1 = 3.08 Å in the structure of 4). Each Ca-centered polyhedron shares three edges with the neighbor polyhedra


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to form a layered complex arranged parallel to (100), whose projection is similar to the sheets in the structure of 4. The difference between the structures of anhydrous calcium oxalates is in the arrangement of “shifted vertices” within the layers caused by the release of apical H2O molecules. According to Figures 2d,e, such vertices are alternating (up- or downward) within the chain of Ca polyhedra arranged along the [010] direction in the structure of 4, whereas in the structure of β-CaC2O4 the vertices are oriented only in one direction. This in turn results in a different shape of the layers (Figure 4e,g): in the structure of 4 all the Ca polyhedra dimers within the layers are co-directed along the [10-1] direction, whereas the layers in β-CaC2O4 are corrugated and the dimers have alternate [101] and [-101] orientation.

Thermal behavior and phase transitions The analysis of the powder diffraction patterns obtained at room temperature revealed single COM and COD phases in the respective powder samples, while a very small amount of COM (less than 5%) was found within the COT sample (as indicated by the presence of a small diffraction maximum at around 17° 2θ; Figure 10). PXRD patterns of the COM sample as a function of temperature are shown in Figure 8. As already mentioned, the most noticeable difference between the classic and the HT whewellite powder data (compounds 2 and 3 and Figure 5c), is the presence (or absence) of two weak peaks at around 2θ = 30°. However, only diffuse low-intensity humps are seen in the experimental patterns at the lower temperatures, with subsequent smoothing in this angular range.


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Figure 8. Powder X-ray diffraction patterns of COM as a function of temperature (23 – 400 °C) under heating in air. At 130 °C a phase transition to a new phase that was assigned to the anhydrous calcium oxalate (compound 4) is clearly observable. The similarity of PXRD pattern calculated from the structural data of 4 with the card № 00-018-0295 (PDF-2, 2016) allows us to assign the phase and our structural model of 4 to the α-CaC2O4 compound. At 150 °C the COM phase almost disappears. A further transformation to β-CaC2O4 (card № 00-018-0296, PDF-2, 2016) starts at around 310 °C, and above 340 °C the sample doesn’t contain the α-CaC2O4 anymore.


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It should be mentioned that recently four possible models of anhydrous calcium oxalates were proposed using quantum mechanical methods,25 among which one model (COA-IV) is very close to the structural model of compound 4. It is of interest that Zhao W. et al.25 have suggested another (COA-III) model to be the α-CaC2O4. This inconsistency could be easily explained by the difficult comparison of nearly similar calculated diffraction patterns with not so clear experimental one. The assignment of compound 4 to the first among the anhydrous phases is obvious, since such transition was detected in situ while heating the single crystal of COM, but on the other hand, we could expect the rest of predicted models appear as additional transitional architectures between the α- and β- or β- and γ-CaC2O4. As clearly seen from the graphs of the COM unit-cell parameters as a function of temperature (Figure S2), the essential change in the temperature dependence character starts at 70 °C. Similar inflection points around 70 °C are observed if the powder patterns are indexed in the unit cell of HT whewellite modification (3). Combining the SC and PXRD data, it is most likely that the transition from LT to HT whewellite occurs in the 55 – 75 °C range. Thus, indexing of COM powder diffraction patterns have been performed in the following temperature ranges: whewellite 23 – 70 °C, HT whewellite 70 – 120 °C, α-CaC2O4 150 – 300 °C (Table S2, Figure S2-S4). According to the theory of thermal behavior49-51, the maximal thermal expansion should be along the direction of the weakest bonding. Taking into account the layered structural motif of whewellite and its thermal derivatives, one could expect the highest expansion in the direction perpendicular to the sheets of Ca-bearing polyhedra. However, the linkage of sheets via oxalate groups and the oblique symmetry of the lattice modify the thermal behavior. The maximal expansion is observed towards the bisector of the β angle as the result of the shear stress52-53 of


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the layers (Figure 4a-f). The strongest contraction occurs perpendicular to the (10-1) plane and is due to the orthogonalization of the monoclinic angle. The thermal behavior of the HT whewellite phase is governed by layers shifting followed by the release of structural water, which decreases the interlayer spacing. Further diagonal expansion/contraction in the structure of α-CaC2O4 results in the undulation of the Ca-bearing layers and almost final orthogonalization of the structure (β = 90.24(2) Å), as it is shown in Figure 4g.

Table 3. The main coefficients of the thermal expansion/contraction αii (i =1-3) and orientation of the main axes in the structures of calcium oxalates. Temp., °C α11 COM at LT range 23 12.5 30 11.5 50 6.5 70 ‒0.6

α22

α33

< α11a

Hydrated calcium oxalates: crystal structures ... - ACS Publications

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