Crystal Structure, Polymorphism, and Anisotropic Thermal ...

Crystal Structure Polymorphism and Anisotropic ThermalExpansion of α‑Ca(CH3COO)2Published as part of a Crystal Growth and Design virtual special issue on The Rietveld Refinement Method Halfof a Century Anniversary

Sebastian Bette Gerhard Eggert Sebastian Emmerling Martin Etter Thomas Schleidand Robert E Dinnebier

Cite This Cryst Growth Des 2020 20 5346minus5355 Read Online

ACCESS Metrics amp More Article Recommendations sı Supporting Information

ABSTRACT Dehydration of calcium acetate monohydrate (Ca-(CH3COO)2middotH2O) by heating to 300 degC leads to the formation ofanhydrous α-Ca(CH3COO)2 During heating and cooling cycleshigh- and low-temperature forms of α-Ca(CH3COO)2 werediscovered The reversible first-order phase transformationbetween the two forms occurs in a temperature range between150 and 170 degC The crystal structures were solved from laboratorypowder X-ray diffraction (PXRD) data The low temperature formof α-calcium acetate (LT-α-Ca(CH3COO)2) crystallizes at roomtemperature in a primitive triclinic unit cell with space group P1with lattice parameters of a = 87168(3) Aring b = 126408(3) Aring c =123084(3) Aring α = 1174363(17)deg β = 77827(2)deg γ =115053(2)deg and a unit cell volume of 109023(6) Aring3 High-temperature α-calcium acetate (HT-α-Ca(CH3COO)2) crystallizes at 300 degC in a rhombohedral unit cell with space group R3lattice parameters of a = 211030(5) Aring c = 87965(2) Aring and a unit cell volume of 339258(17) Aring3 In both crystal structures edgesharing polyhedra of calcium cations and acetate anions that coordinate in both a mono- and bidentate way build up channel-likemotifs During the phase transition the coordination mode of a bridging acetate anion changes from monodentate to bidentate andthe elliptical channels of the low temperature form become circular This leads to both negative and positive thermal expansion alongdifferent principal axes in the crystal structure of LT-α-Ca(CH3COO)2 and to an overall considerably big volumetric thermalexpansion

INTRODUCTION

Calcium acetate salts are seemingly simple compounds that areused for various purposes such as a binder for phosphorus for thetreatment of hyperphosphatemia1 or as a food additive2 Innature pure calcium acetate hydrates do not occur as mineralsand only a mixed calcium copper acetate hydrate named paceite(CaCu(CH3COO)4middot6H2O) has been found yet3 Recently itwas demonstrated that calcium acetate monohydrate (Ca-(CH3COO)2middot1H2O) occurs as an intermediate during themechanochemical synthesis of paceite or of a mixed cadmiumcalcium acetate hydrate (CaCd(CH3COO)4middot6H2O)

4 Calciumacetate hydrate salts Ca(CH3COO)2middotnH2O with n = 12 and 1occur as efflorescence phases on calcareous heritage objects suchas eggs mollusca shells5 or ancient pottery6 The acetate saltsform from the reaction of acetic acid which is emitted by woodfrom wooden storage furniture and showcases7 with calciumcarbonate This phenomenon is known as Bynersquos disease8 andhas been observed since the end of the 19th century Calciumcarboxylate zigzag chains form the structural backbone of the

efflorescence phases9 which enables the incorporation of otherions such as formates10 or nitrates11 into the coordinationsphere of calcium or the intercalation of chloride anions12minus14 orwater molecules1516 in between the chainsThe simple binary system Ca(CH3COO)2-middotH2O exhibits a

surprisingly large variety of solid phases In aqueous solutionCa(CH3COO)2middotH2O is the thermodynamically stable phase617

There are three different polymorphs of this monohydrate18minus20

but only one crystallizes from a pure aqueous calcium acetatesolution18 Upon heating Ca(CH3COO)2middotH2O releases waterin three discrete steps leading to calcium acetate subhydratesCa(CH3COO)2middotnH2O with water contents of n = 12 and n =

Received April 25 2020Revised June 11 2020Published June 12 2020

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This is an open access article published under a Creative Commons Attribution (CC-BY)License which permits unrestricted use distribution and reproduction in any mediumprovided the author and source are cited

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1421 and finally to anhydrous calcium acetate which alsoexhibits three different polymorphs γ- β- and α-Ca-(CH3COO)2

22 Up to 2020 only the crystal structures of themonohydrates have been determined18minus20 Recently we wereable to solve the crystal structure of Ca(CH3COO)2middot12H2O

23

At ambient conditions this hemihydrate crystallizes in a largeunit cell of almost 12 000 Aring3 and exhibits a collagen-like triplehelix motif During the thermal decomposition of thiscompound we observed the formation of β- and α-Ca-(CH3COO)2 as short living intermediates While the powderX-ray diffraction (PXRD) data for the β-form showed goodagreement with the reference data we obtained a completelydifferent powder pattern of α-Ca(CH3COO)2 as compared tothe reported ones by Walter-Levy22 or Panzer21 This and thelack of structural knowledge of ldquosimplerdquo compounds such asanhydrous calcium acetate motivated us to perform a detailedinvestigation on α-Ca(CH3COO)2 In this paper we present thecrystal structures of the low (LT-α-Ca(CH3COO)2) whichmatches the previously published reference data for α-Ca(CH3COO)2 and the high-temperature form (HT-α-Ca-(CH3COO)2) of α-calcium acetate and a detailed study on itsanisotropic thermal expansion

EXPERIMENTAL SECTIONSynthesis Ca(CH3COO)2middotH2O was used as a precursor material

for α-Ca(CH3COO)2 For the production of well crystalline Ca-(CH3COO)2middotH2O a saturated calcium acetate solution produced bydissolving Ca(CH3COO)2middotH2O (VWR gt99) in deionized water wasfilled in a terracotta pot that was placed in desiccators with a volume of 2L at room temperature (22minus24 degC) In the desiccator a relativehumidity (RH) of 75 was obtained by adding a vessel filled withsaturated sodium chloride solution The desiccators were stored for 6weeks During the storage white efflorescence crystals formed on theouter surface of the pots The purity of the recrystallized Ca-(CH3COO)2middotH2O was proven by PXRD (Supporting InformationFigure S1)For the production of α-Ca(CH3COO)2 Ca(CH3COO)2middotH2O was

heated in an argon stream of 50 mLmin up to 350 degC with a heatingrate of 2 Kmin The temperature was kept constant for 1 h Afterwardthe solid was cooled down in the argon stream to room temperaturewith a cooling rate of 2 KminPhase Characterization Scanning electron microscopy (SEM) of

the calcium acetate hydrate phases was performed with a TESCANVega TS 5130 MM (20 kV accelerating voltage) scanning electronmicroscope after the samples were coated with gold Infrared spectrawere recorded in attenuated total reflection (ATR) geometry on aldquoPerkinElmer Spectrum Twordquo device equipped with a diamond crystalThe background spectrum was measured separately and subtractedElemental analyses of carbon hydrogen sulfur and nitrogen wereperformed with a Vario Micro Cube analyzer (Elementar) Thermalanalysis was carried out using a STA 449 F5-Jupiter (Netzsch) devicefor TG measurements Approximately 32 mg of the sample was placedin an Al2O3 crucible and heated from 30 to 350 degC subsequently cooledto 30 degC heated again to 350 degC and finally cooled to 30 degC with aheating and cooling rate of 5 Kmin in a 50 mLmin Ar-stream Anempty Al2O3 crucible was used as a reference For backgroundmeasurements 32 mg of corundum (α-Al2O3) was subjected to thesame temperature program Prior to the measurement the corundumwas calcinated at 1000 degC for 2 h Sorption measurements wereperformed on a Quantachrome Instruments Autosorb iQ 3 withnitrogen (N2) at 77 K or carbon dioxide (CO2) at 273 K as adsorbateThe samples were activated under high vacuum at either roomtemperature or 120 degC for 12 h before measurement The BrunauerminusEmmettminusTeller surface area (SBET) was determined from the nitrogenisotherm from the linear regime in the pressure range 01 lt pp0 lt 03Prior and after the sorption measurement PXRD patterns wererecorded (Figure S2) in order to ensure that the substance can

withstand the high vacuum atmosphere during the activation processPXRD patterns for phase identification were collected at roomtemperature on a laboratory powder diffractometer in DebyeminusScherrergeometry (Stadi P-diffractometer (Stoe) CuminusKα1 radiation fromprimary Ge(111)-Johann-type monochromator triple array of Mythen1 K detectors (Dectris)) The samples were sealed in 05 mm diameterborosilicate glass capillaries (Hilgenberg glass no 0140) which werespun during the measurements Each pattern was measured in a 2θrange from 10deg to 110deg applying a total scan time of 1 h The PXRDpattern for the crystal structure solution of LT-α-Ca(CH3COO)2 wascollected using the same device by only using one Mythen 1 K detectorof the triple array and applying a scan range from 20deg to 110deg 2θ and atotal scan time of 20 h Temperature-dependent in situ X-ray diffractionexperiments were performed on a D8-Advance diffractometer (BrukerCuminusKα1 radiation from primary Ge(111)-Johann-type monochroma-tor Lynx Eye position sensitive detector (Bruker)) in DebyeminusScherrergeometry using a water-cooled furnace (mri capillary heater (25minus1000) degC) for heating the capillary The sample was loaded into a 05mm diameter quartz glass capillary (Hilgenberg) which was also spunduring the measurements The patterns were measured with a scanrange of 50deg 2θ to 400deg 2θ employing a step size of 0005 and a totalscan time of 4 h A delay time of 30 min was applied prior to eachmeasurement to ensure thermal equilibration The PXRD pattern forthe crystal structure solution of HT-α-Ca(CH3COO)2 was collectedusing the same device Thereto the sample was heated to 300 degC andmeasured applying a scan range from 50deg to 90deg 2θ and a total scantime of 20 h Temperature-dependent in situ synchrotron PXRDmeasurements were performed in DebyeminusScherrer geometry at awavelength of λ = 020713 Aring (sim60 keV) on beamline P021 at theDeutsches Elektronen-Synchtrotron (DESY)PETRA III synchrotronin Hamburg Germany The sample was loaded in an open 05 mmquartz capillary that was heated by an in-house built ceramic heater upto 350 degC with a heating rate of 2 Kmin The temperature was keptconstant for 30 min and then the sample was cooled with a cooling rateof 2 Kmin Diffraction patterns were recorded in 5 K intervals For thispurpose a delay of 5 min was applied prior to each measurement inorder to allow thermal equilibration For the measurements the samplewas illuminated for 1 min The scattered X-rays were detected by a 2Ddetector (PerkinElmer XRD1621 CN3-EHS with a pixel size of 200 times200 μm2 and a pixel area of 2048 times 2048 pixels) that was mountedorthogonal to the beam path with a sample-to-detector distance of 1378mm The collected DebyeminusScherrer rings were subsequentlyazimuthally integrated with the pyFAI-software24 to one-dimensionalpowder diffraction patterns in Q [nmminus1] and 2θ [deg] versus intensityParameters for intensity integration were determined from a NISTsilicon reference sample (NIST 640d) Further mask generationpolarization correction and azimuthal integration of the 2D patternswere performed using the xpdtools software25

Crystal Structure Solution The program TOPAS 6026 was usedto determine and refine the crystal structures of LT- and HT-α-Ca(CH3COO)2 Indexing of the powder patterns was carried out byiterative use of singular value decomposition (LSI)27 and led to aprimitive triclinic cell for the low temperature phase with P1 and P1 asmost probable space groups and to a rhombohedral cell for the hightemperature phase with R3 R3 R3m and R3m as most probable spacegroups The lattice parameters are given in Figure 2 Le Bail28 fitsapplying the fundamental parameter approach of TOPAS29 wereemployed to determine the peak profile and the precise latticeparameters For modeling of the background Chebychev polynomialsof sixth order were used The refinements converged quickly

The crystal structures were solved by applying the globaloptimization method of simulated annealing (SA) in real space as itis implemented in TOPAS30 Considering the determined unit cellvolume the space group symmetry and the packing density of therelated calcium acetate monohydrate18minus20 the molar content of the unitcells was expected to be around Z = 6 for LT-α-Ca(CH3COO)2 andaround Z = 18 for HT-α-Ca(CH3COO)2 Accordingly and with respectto the space symmetry for the low temperature phase three calcium andsix acetate ions and for the high temperature phase one calcium and twoacetate ions were put into the unit cell and freely translated and rotated

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The acetate related atom sites were constrained by using rigid bodies inz-matrix notation Because of the limits of the PXRD method theacetate related hydrogen sites were omitted Amerging radius of 07 Aring31

was used to check for atoms situated on special positions (inversioncenters) or occupying identical sites The global optimizations werecarried out several times with different starting sets of rigid bodies andions and led to identical results within the error limits each time For thefinal Rietveld refinements32 all profile and lattice parameters werereleased iteratively and positions of the calcium atoms were subjected tofree unconstrained refinement The bond lengths and angles of the rigidbodies were refined restraining them to reasonable values The finalagreement factors are listed in the Supporting Information (Table S1)the atomic coordinates and selected bond lengths are given in Table S2and Table S3 and the fit of the whole powder pattern is shown inFigures S3 and S4 The crystallographic data have been deposited atCCDC deposit numbers 1999002minus1999003Refinement of the Temperature-Dependent in situ PXRD

Patterns The temperature -dependent in situ PXRD patterns weresubjected to fully weighted Rietveld refinements The crystal structureof HT-α-Ca(CH3COO)2 that was determined at 300 degC was used asthe starting model for the laboratory data of the first cooling cyclestarting at 310 degC All lattice parameters and the positions of thecalcium cations were freely refined The positions of all acetate relatedatoms were constrained by rigid bodies that were allowed to freelyrotate and translate The refined crystal structure was then used as thestartingmodel for the next temperature step in an iterative process Thisprocedure was conducted for all heating and cooling cycles Thesynchrotron data were refined by an analogous procedure The volumethermal expansion coefficients and all axes expansion coefficients werecalculated from the refined lattice parameters by using the PASCalsoftware33

RESULTS

Phase Characterization The crystallization of Ca-(CH3COO)2middotH2O by diffusion of a saturated aqueous solutionof calcium acetate through the pores of a terracotta vessel leadsto the formation of a well crystalline material (SupportingInformation Figure S1 calculated C = 273 wt H = 46 wt found C = 272(1) wt H = 45(1) wt ) The crystallitesexhibit a pronounced needle-like morphology with lengths of upto more than 100 μm (Figure 1a) and diameters ofapproximately 4 μm (Figure 1 c) After thermal dehydrationthe crystallites are considerably smaller (Figure 1d) Duringheating they seem to split along the long axis and the crystallite

surface appears to be rougher after the thermal treatment(Figure 1ef)The diffraction pattern of α-Ca(CH3COO)2 that was

recorded at room temperature (Figure 2a) is in good agreementwith the reference data given by Panzer21 (red lines) Some peakintensities differ which is attributed to the fact that the referencedata correspond to flat plate (BraggminusBrentano Geometry)

Figure 1 SEM images of the starting material Ca(CH3COO)2middotH2O (aminusc) and of the calcinated material at room temperature α-Ca(CH3COO)2 (dminusf)

Figure 2 PXRD patterns of α-Ca(CH3COO)2 at room temperature(a) and at 310 degC (b) including reference data of α-Ca(CH3COO)2

21

space group symmetries and lattice parameters obtained by LSIindexing27

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measurements and therefore may be influenced by preferredorientation effects There are also additional Bragg-peakspresent in the recorded diffraction pattern As they cannot beassigned to any known byproduct such as Ca(CH3COO)2middotH2O

18 β-Ca(CH3COO)221 or Ca(CH3COO)2middot12H2O

23 andas the elemental analysis indicated the presence of pureanhydrous calcium acetate (calculated C = 304 wt H =38 wt found C = 304(1) wt H = 38(1) wt ) weincluded all measured reflections into the indexing processwhich led to a triclinic unit cell of 109023(6) Aring3 and the latticeparameters given in Figure 2a Heating of the solid to 310 degC ledto a drastic change in the diffraction pattern (Figure 2b)Indexing of the diffraction pattern revealed a rhombohedral unitcell with a tripled volume and lattice parameters as given inFigure 2b The c-axis (87966(2) Aring) of the rhombohedral cellcorresponds to the a-axis (87168(3) Aring) of the triclinic one As aconsequence there is a low- and a high-temperaturemodification of α-Ca(CH3COO)2 A comparison with theliterature data revealed that HT-α-Ca(CH3COO)2 (ldquohightemperaturerdquo α-Ca(CH3COO)2) has already been observed asan intermediate during the thermal decomposition of bothCa(CH3COO)2middot12H2O

23 and Ca3(CH3COO)4(HCOO)2middot4H2O

10

IR spectroscopy at ambient conditions additionally proves thecomplete release of water molecules after the calcinationprocess as all water related OminusH stretching and bendingmodes disappear in the spectrum of the calcinated material(Figure 3 (1minus3) (7) (8)) Except for the water related bandsthe IR-spectra of Ca(CH3COO)2middotH2O and LT-α-Ca-(CH3COO)2 are very similar The comparatively small splittingof the most intense symmetrical and antisymmetrical carbox-ylate related CminusO stretching modes (Figure 3 Table 1 (11)(14)) ofΔν = 134 cmminus1 indicates that the majority of the acetateanions in LT-α-Ca(CH3COO)2 exhibit a bidentate and bridgingcoordination behavior34 However additional asymmetrical CminusO stretching modes of lower intensities at higher wavenumbers(Figure 3 Table 1 (9) (10)) point to monodentatecoordination behavior of some of the acetate groups in thecrystal structures of the anhydrous calcium acetates The overallsplitting of the CminusO and CminusC related stretching modesindicates the presence of at least three to four symmetricallyindependent acetate anions in the crystal structure of LT-α-Ca(CH3COO)2Temperature-dependent in situ PXRD andDTA analysis were

employed to investigate the transformations between HT- and

LT-α-Ca(CH3COO)2 (Figure 4) The transformation startsbetween 150 and 170 degC and is completely reversible LT-α-Ca(CH3COO)2 transforms endothermically into the high-temperature form (Figure 4b) while heating and cooling ofthe low-temperature form a pronounced shift of the Bragg peakscan be observed (Figure 4a) which indicates a strong thermalexpansion

Crystal Structure Description LT-α-Ca(CH3COO)2crystallizes in a centrosymmetric triclinic lattice with six formulaunits per unit cell and all atoms located on general positionsHence there are three symmetrically independent calcium sites

Figure 3 Excerpts form the IR-spectra of the starting material Ca(CH3COO)2middotH2O (blue line) and the calcinated material LT-α-Ca(CH3COO)2(black line) the complete IR-spectrum is presented in the Supporting Information (Figure S5) and a tentative band assignment is given in Table 1

Table 1 Band Positions Shapes and Tentative Assignmentsin the IR Spectra of the Starting Material Ca(CH3COO)2middotH2O and the Calcinated Material LT-α-Ca(CH3COO)2According to Literature Data35minus38

bandno

Ca(CH3COO)2middotH2Opositioncmminus1

LT-α-Ca(CH3COO)2positioncmminus1 assignment

(1) 3470 br ν(OminusH) [H2O](2) 3270 br(3) 3167 br(4) 3003 s ν(CminusH) [CH3](5) 2984 sh(6) 2933 m 2929 m(7) 1690 m δ(OminusH) [H2O](8) 1652 m(9) 1601 m ν(CminusO)as(10) 1575 sh 1581 sh(11) 1542 m 1550 m(12) 1454 sh ν(CminusO)s(13) 1444 m 1447 m(14) 1409 m 1416 m(15) 1341 s 1347 s δ(CminusH) [CH3](16) 1054 m 1048 m ρ(CminusH) [CH3](17) 1024 m 1022 m(18) 961 s 958 m ν(CminusC)(19) 945 s 948 m(20) 931 s 938 m(21) 674 s 670 s δ(OminusCminusO)(22) 659 s 654 s(23) 618 m 615 s(24) 488 m ν ρ ω(CaminusO)

lattice modes(25) 479 sh 474 sh(26) 471 m 465 m

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that are coordinated by acetate related oxygen atoms Two ofthese sites Ca(1) and Ca(3) exhibit a 7-fold coordinationsphere with acetate anions coordinating in both a mono- andbidentate fashion (Figure 5a) The other calcium site Ca(2)shows a distorted octahedral coordination sphere with acetateanions coordinating exclusively in a monodentate way HT-α-Ca(CH3COO)2 crystallizes in a centrosymmetric rhombohedrallattice with 18 formula units per unit cell and also all atomslocated on general positions Because of the higher space groupsymmetry all calcium sites are symmetrically equivalent in thehigh-temperature phase Seven acetate related oxygen atomscoordinate the calcium cation with one acetate coordinating in abidentate way and all other acetates showing monodentatecoordination (Figure 5b) Acetate anions bridge neighboringcalcium cations forming channel-like motifs of 12 edge sharingcalcium carboxylate polyhedra in both structures In the crystalstructure of LT-α-Ca(CH3COO)2 these channels run in a-direction and the 12-member rings are not completely closed asneighboring [CaO6]-octahedra are bridged by acetate anionsbut not in an edge sharing way (Figure 5c blue polyhedra) Thisis well reflected by the Ca(2)minusCa(123) distances Whereasthe separation between Ca(2) and Ca(1) or Ca(3) iscomparatively short with 389(1) Aring and 372(1) Aring respectivelydue to edge sharing of polyhedra the distance betweenneighboring Ca(2) sites is with 468(1) Aring considerably largerIn the structure of the high-temperature phase the channels arecompletely closed as all calcium-carboxylate polyhedra are nowsharing edges (Figure 5d) The distance between the calciumsites situated in polyhedra that become edge sharing after thephase transition shortens from 468(1) Aring to 445(1) Aring evendespite heating

Acetate related methyl groups are situated within the channelsin the crystal structures of the α-Ca(CH3COO)2 phases (Figure6) The effective size of the channels cannot be determineddirectly from the crystal structure since due to the limits of thePXRD method we were not able to determine the positions ofthe hydrogen atoms In addition it is very likely that the methylgroup is disordered especially in the crystal structure of HT-α-Ca(CH3COO)2 which was determined at 300 degC Thereforewe decided to use the distance between the methyl relatedcarbon atom and the center of the channels (Figure 6 magenta

Figure 4 Temperature-dependent in situ PXRD patterns of α-Ca(CH3COO)2 during heating and cooling cycles (a) and DTA curvesduring heating (b) and cooling (c) in the gray highlighted temperatureintervals the phase transition occurs

Figure 5 Calcium coordination in (a) LT-α-Ca(CH3COO)2 and (b)HT-α-Ca(CH3COO)2 and packing diagrams of (c) LT-α-Ca-(CH3COO)2 and (d) HT-α-Ca(CH3COO)2 with Ca(1) polyhedrapresented in yellow Ca(2) polyhedra presented in green and Ca(3)polyhedra presented in blue

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and green dashed lines) as ameasure of the channel radius and itshould be noted that the real channel radii are up to 1 Aring smallerIn the crystal structure of LT-α-Ca(CH3COO)2 the channelsexhibit an elliptical shape This results in distances between thechannel center andmethyl related carbon atoms of 31 and 45 AringBecause of the trigonal lattice symmetry of the high-temperaturephase the channels show a circular shape with all methyl relatedcarbon atoms being located at a distance of 38 Aring to the channelcenter The channels are not accessible for any gas loading as it isshown by the comparatively small BET surface of 047(8) m2gand the isotherms (Supporting Information Figures S 6 and S 7)can be classified as type II which is typically found fornonporous materials39

Thermal Expansion The thermal expansion of α-Ca-(CH3COO)2 was investigated by temperature-dependent in situlaboratory and synchrotron PXRD measurements (Figure 7aopen and filled symbols) The unit cell volume of LT-α-Ca(CH3COO)2 shows a comparatively big expansion as itexpands by heating from 25 to 150 degC (ΔT = 125 K) by morethan 16 (Figure 7a blue symbols) After the phase transition

both the unit cell volume and the crystallographic density show adiscontinuous jump which is indicative for a first-order phasetransition This is additionally confirmed by the peaks observedin the DTA curve (Figure 4bc) The thermal expansion of HT-α-Ca(CH3COO)2 progresses considerably more slowly asheating from 175 to 300 degC (ΔT = 125 K) only leads to avolume expansion of 09 (Figure 7a red symbols)Accordingly the linear thermal expansion coefficient α of thelow-temperature phase was calculated as being doubled incomparison to that one of the high-temperature phase (Table2) Detailed analyses of the principal directions of the thermal

expansion reveal that LT-α-Ca(CH3COO)2 expands anisotropi-cally The material shows a large positive thermal expansion inone direction (α(X3) = 217(9) times 10minus6 K Table 2) whereas theexpansion in an orthogonal direction is negative (α(X1) =minus95(6) times 10minus6 K Figure 7b red and blue grids) In contrastHT-α-Ca(CH3COO)2 exhibits positive thermal expansionexclusively (Figure 7c) with similar thermal expansioncoefficients for all principal axes (Table 2) and thereforeexpands almost isotropically

Figure 6 Comparison of the channel motifs and radii (indicated bydistances of the methyl related carbon atoms and the channel centers)in the crystal structures of LT-α-Ca(CH3COO)2 (left) and HT-α-Ca(CH3COO)2 (right) with Ca(1) polyhedra presented in yellowCa(2) polyhedra presented in green and Ca(3) polyhedra presented inblue The methyl related carbon atoms situated within the figure planeare indicated by green dashed lines the methyl related carbon atomsexhibiting an offset with respect to the figure plane are indicated bymagenta dashed lines

Figure 7 Evolution of the lattice parameters and the crystallographic density of LT- and HT-HT-α-Ca(CH3COO)2 upon heating (a) plots showingthe variation of the thermal expansion coefficientαwith the principal directions X1 X2 and X3 (Table 2) of LT- (b) andHT-α-Ca(CH3COO)2 (c) redlines indicate positive and blue lines negative thermal expansion

Table 2 Volume Expansion Coefficients and All AxesExpansion Coefficients of LT- and HT-α-Ca(CH3COO)2

a

axes α 10minus6 Kminus1 σ(α) 10minus6 Kminus1 a b c

LT-α-Ca(CH3COO)2X1 minus948827 5612 01141 06001 07917X2 208118 10435 09893 01399 minus00424X3 2177281 84507 minus02997 minus08808 03665V 1471779 52854

HT-α-Ca(CH3COO)2X1 202163 02807 08179 05753 0X2 202163 02807 minus02994 09541 0X3 317141 07337 0 0 1V 724323 02288

aα is the linear coefficient of the thermal expansion with σ(α) beingthe corresponding estimated standard deviation a b and c are theprojections of the principal directions Xn on the unit cell axes Plots ofthe thermal expansion along the principal axes and of the overallvolume expansion are given in Figure S8 of the SupportingInformation

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A closer look into the crystal structures of α-Ca(CH3COO)2provides an explanation of its anisotropic thermal expansion andthe change in the expansion behavior after the phase transitionThe X1 and X3 axes (Figure 8ab red and magentas arrows) areoriented orthogonally to the channels in the crystal structureswhereas the X2 axis shows parallel orientation to the channelsAccordingly for the low-temperature phase the absolute valuefor the expansion coefficient along the channel direction is thelowest and these coefficients are within the error range identicalfor both LT- and HT-α-Ca(CH3COO)2 (Table 2) Thenegative thermal expansion or in other words the thermalcontraction along the X1 direction in the crystal structure of thelow-temperature phase leads to a shortening of Ca(2)minusCa(2)distances (Figure 8a blue polyhedra) and finally makes the[CaO7] polyhedral edge sharing after the phase transition(Figure 8b) This is also reflected by the calciumminusoxygendistances The acetate anions bridging these calcium cations areclearly coordinating in a monodentate fashion as one of theacetate related oxygen atoms exhibits a distance of 330 Aring to oneof the Ca2 sites (Figure 8c dashed gray bond) During heatingall other CaminusO distances increase while this distance decreasesto 290 Aring Therefore this oxygen atoms enters the coordinationsphere of calcium (Figure 8d) and the incorporation of anadditional ligand atom into the Ca2+-coordination sphere mustbe the driving force for the negative thermal expansion along theX1-axis In the orthogonal X3-direction (Figure 8a red arrow)LT-α-Ca(CH3COO)2 shows a considerably high thermalexpansion coefficient of 218(8) 10minus6 Kminus1 which severelydecreases to 72(1) 10minus6 Kminus1 after the phase transition (Table 2)This expansion counteracts the elliptical shape of the channels inLT-α-Ca(CH3COO)2 (Figure 6) and thus enables the increaseof the lattice symmetry from P1 to R3 After the transition toHT-α-Ca(CH3COO)2 the principal X1 and X3 axes becomesymmetrically equivalent

DISCUSSION

The crystal structures of the high- and low-temperature form ofα-Ca(CH3COO)2 with their channel-like structural main motifsappear to be special despite the fact that the overall structuralknowledge of anhydrous divalent metal acetates is very limitedThe crystal structures of β- and γ-Ca(CH3COO)2 are stillunknown Because of their hygroscopic behavior and theirsensitivity toward hydrolysis often only the crystal structures ofthe acetate hydrates1240 or of oxy-41minus43 or hydroxyacetates44minus46 as so-called ldquobasic acetatesrdquo are known Bariumacetate which is the only known anhydrous alkaline earth metalacetate exhibits different structural motifs since interconnectedBa4(CH3COO)8 units build up a three-dimensional networkwith much smaller channels formed by six-membered rings ofbarium carboxylate polyhedra47 Anhydrous zinc(II)48 andiron(II) acetate49 crystallize as two-dimensional coordinationpolymers and in anhydrous chromium(II)50 copper(II)51

molybdenum(II) 52 and rhodium(II)53 dinuc learM2(CH3COO)4 paddle wheel complexes form one-dimensionalchains whereas the crystal structures of anhydrous manganese-(II) cobalt(II) and nickel(II) acetate are still unknownThe thermal expansion behavior of LT-α-Ca(CH3COO)2

that completely changes after the transformation into the high-temperature phase is remarkable A large positive thermalexpansion is well-known for coordination compounds withmorecomplex carboxylate ligands such as [Cu2(benzoate)4(3prime-fluoro-4-styrylpyridine)2]

54 which shows a volumetric thermalexpansion of 2857 times 10minus6middotKminus1 It is uncommon for solids toexhibit both negative and positive thermal expansion coefficientssimultaneously There are some examples of metal organicframeworks such as [Zn(trans-2-(4-pyridyl)-4-vinylbenzoate)2]middotDMF55 or [Cd(3-(pyridin-4-yl)benzoate)(4-(pyridin-4-yl)-benzoate)]middotDMFDMA56 showing this effect but they are allbuilt from much more complex ligand systems than simpleacetate anions A prominent and comparatively simplerepresentative for colossal positive and negative thermalexpansion is Ag3[Co(CN)6]

57 In this case weak argentophilic

Figure 8 Illustration of the orientation of the principal axes X1 and X3 (Table 2) of the thermal expansion of LT- (a) andHT-α-Ca(CH3COO)2 (b) intheir crystal structures changes in the Ca-coordination during the phase transition from LT- (c) to HT-α-Ca(CH3COO)2 (d)

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5352

(Ag+middotmiddotmiddotAg+) interactions and the flexible Co-CN-Ag-NC-Cointeractions govern the thermal behavior whereas in α-calciumacetate the incorporation of an additional ligand atom into theCa2+-coordination sphere is the driving force for the anisotropicthermal expansion

CONCLUSIONS

Thermal dehydration of Ca(CH3COO)2middotH2O and additionalheating to 300 degC lead to the formation of the α-polymorph ofanhydrous calcium acetate This polymorph exhibits a high- andlow-temperature form with the latter matching the PXRDreference data for α-Ca(CH3COO)2 The phase transitionbetween the high- and low-temperature form is completelyreversible LT-α-Ca(CH3COO)2 crystallizes in a primitivetriclinic lattice whereas HT-α-Ca(CH3COO)2 exhibits arhombohedral unit cell with tripled volume In both crystalstructures edge sharing polyhedra of calcium cations and acetateanions that coordinate in both a mono- and bidentate way buildup channel-like motifs The only difference between the crystalstructures is the coordination number In HT-α-Ca-(CH3COO)2 all calcium cations are coordinated by sevenacetate related oxygen atoms whereas in the low-temperatureform one out of three calcium sites exhibits only a 6-fold oxygencoordination During the phase transition an additional acetaterelated oxygen atom enters this coordination sphere and theelliptical channels of the low-temperature form become circularThis leads to the presence of both negative and positive thermalexpansion along different principal axes in the crystal structure ofLT-α-Ca(CH3COO)2 Therefore the cheap and easily acces-sible α-Ca(CH3COO)2 shows interesting thermal propertiesand a flexibility in the coordination sphere of the cation whichopens up potential usage for different applications by furthermodifications and engineering on its crystal structure

ASSOCIATED CONTENT

sı Supporting InformationThe Supporting Information is available free of charge athttpspubsacsorgdoi101021acscgd0c00563

Crystallographic and Rietveld refinement data of LT- andHT-α-Ca(CH3COO)2 Complete IR spectrum of LT-α-Ca(CH3COO)2 CO2-adsorption and desorption curvesof LT-α-Ca(CH3COO)2 Plots of the thermal expansionalong the principal axes and of the overall volumeexpansion of LT- and HT-α-Ca(CH3COO)2 (PDF)

Accession CodesCCDC1999002minus1999003 contain the supplementary crystallo-graphic data for this paper These data can be obtained free ofcharge via wwwccdccamacukdata_requestcif or by email-ing data_requestccdccamacuk or by contacting The Cam-bridge Crystallographic Data Centre 12 Union Road Cam-bridge CB2 1EZ UK fax +44 1223 336033

AUTHOR INFORMATION

Corresponding AuthorSebastian Bette minus Max Planck Institute for Solid State Research70569 Stuttgart Germany State Academy of Art and Design70191 Stuttgart Germany Institute for Inorganic ChemistryUniversity of Stuttgart 70569 Stuttgart Germany orcidorg0000-0003-3575-0517 Email SBettefkfmpgde

AuthorsGerhard Eggert minus State Academy of Art and Design 70191Stuttgart Germany

Sebastian Emmerling minus Max Planck Institute for Solid StateResearch 70569 Stuttgart Germany Department of ChemistryLudwig Maximilian University of Munich 81377 MunichGermany

Martin Etter minus Deutsches Elektronen-Synchtrotron (DESY)22607 Hamburg Germany

Thomas Schleidminus Institute for Inorganic Chemistry University ofStuttgart 70569 Stuttgart Germany

Robert E Dinnebier minus Max Planck Institute for Solid StateResearch 70569 Stuttgart Germany

Complete contact information is available athttpspubsacsorg101021acscgd0c00563

FundingDFG project ldquoIn search of structurerdquo (Grant EG 1379-1)NotesThe authors declare no competing financial interest

ACKNOWLEDGMENTSMarie-Louise Schreiber is gratefully acknowledged for perform-ing the elemental analyses and the IR-spectroscopy measure-ments Viola Duppel for taking the SEM-images and MaxwellW Terban for integrating the 2D diffraction data (all MaxPlanck Institute for Solid State Research) The Max PlanckSociety provided open access funding

ABBREVIATIONSHT-α-Ca(CH3COO)2 high temperature α-Ca(CH3COO)2LT-α-Ca(CH3COO)2 low temperature α-Ca(CH3COO)2PXRD powder X-ray diffraction

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(55) Chen Z Gallo G Sawant V A Zhang T Zhu M Liang LChanthapally A Bolla G Quah H S Liu X Loh K P DinnebierR E Xu Q H Vittal J J Giant Enhancement of Second HarmonicGeneration Accompanied by the Structural Transformation of 7-Foldto 8-Fold Interpenetrated MetalminusOrganic Frameworks (MOFs)Angew Chem Int Ed 2020 59 833minus838(56) Zhou H-L Zhang Y-B Zhang J-P Chen X-MSupramolecular-jack-like guest in ultramicroporous crystal for excep-tional thermal expansion behaviour Nat Commun 2015 6DOI 101038ncomms7917(57) Goodwin A L Calleja M Conterio M J Dove M T EvansJ S O Keen D A Peters L Tucker M G Colossal Positive andNegative Thermal Expansion in the Framework Material Ag3[Co-(CN)6] Science 2008 319 794minus797

Crystal Growth amp Design pubsacsorgcrystal Article

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