Eu(O2C‐C≡C‐CO2): An EuII Containing Anhydrous Coordination Polymer with High Stability and Negative Thermal Expansion

Abstract Anhydrous EuII–acetylenedicarboxylate (EuADC; ADC2− = −O2C‐C≡C‐CO2 −) was synthesized by reaction of EuBr2 with K2ADC or H2ADC in degassed water under oxygen‐free conditions. EuADC crystallizes in the SrADC type structure (I41/amd, Z=4) forming a 3D coordination polymer with a diamond‐like arrangement of Eu2+ nodes (msw topology including the connecting ADC2− linkers). Deep orange coloured EuADC is stable in air and starts decomposing upon heating in an argon atmosphere only at 440 °C. Measurements of the magnetic susceptibilities (μ eff=7.76 μB) and 151Eu Mössbauer spectra (δ=−13.25 mm s−1 at 78 K) confirm the existence of Eu2+ cations. Diffuse reflectance spectra indicate a direct optical band gap of E g=2.64 eV (470 nm), which is in accordance with the orange colour of the material. Surprisingly, EuADC does not show any photoluminescence under irradiation with UV light of different wavelengths. Similar to SrADC, EuADC exhibits a negative thermal volume expansion below room temperature with a volume expansion coefficient α V=−9.4(12)×10−6 K−1.


Introduction
Amongt he many ligands that have been used for the construction of coordinationp olymers (CPs) and metal-organic frameworks (MOFs)a cetylenedicarboxylate ( À O 2 C-CC-CO 2 À , ADC 2À )b elongs to the simplesto nes. It consistso fashort, rigid and linear C 4 carbonb ackbonew ith al ength of approx. 4.1 and two donating carboxylate groups (see Figure 1). Due to the free rotation around theC ÀCs ingle bond,t hese carboxylate groups can adopt all torsion angles between ac oplanar (08, D 2h symmetry) and perpendiculara rrangement (908, D 2d symmetry). This is in contrast to the terephthalate linker (BDC 2À ), which is widely used for the synthesis of MOFs like MOF-5 [1] or MIL-53. [2] For BDC 2À torsion angles between the carboxylate groups and the phenylr ing close to zero are found in most compounds. [3] The length of the carbon backbone in the BDC 2À linker is approx.5 .75 .S o, it is not surprising that many MOFs with permanentp orosity were found with the latter,b ut only very few examples with the ADC 2À linker. [4] Next to spatial restrictions also the low thermal stabilityofa cetylenedicarboxylic acid (H 2 ADC) unders olvothermal conditions, which are most widely used for the synthesis of porous MOFs, [5] hinders the preparation of these materials. Very recently, Janiak and co-workersu sed this low stability for an in situ hydrohalogenation of H 2 ADC and as traight-forward synthesiso fc hlorofumarate based MOFs with MOF-801 topology. [4a, b] However,a sp ointed out by Cheetham and Rao, "apart from porous hybrid frameworks, the field has other opportunities to offer." [6,7] Accordingly,n umerousn on-porous CPs with ADC 2À ligandsw ere synthesized with interesting magnetic, [8] synthetic, [9] luminescence [10] or conductivity [11] properties. In all exam- ples additional ligands, mainly N-donor ligandso rw ater molecules, were used to crystallize these interesting materials. On the other hand, the field of anhydrous homoleptic CPs with the ADC 2À ligand is extremely small. At the moment only reports on the synthesis and crystal structure of SrADC, [12] PbADC, [13] CaADC, [14] and Li 2 ADC [15] are available. All compounds with composition M II ADC crystallize in the SrADC type structure (I4 1 /amd,n o. 141, Z = 4) with ad iamond-likea rrangement, that is, the metal cations are arranged in an almost cubic diamond-like fashion. The ADC 2À ligandsc onnect these cations resulting in aM II O 8 coordination sphere. It is remarkable that the carbon backbones of these linkers are all aligned along the tetragonal [001] axis. Thus, at etragonal symmetry results, but the metric of the unit cell is still close to cubic ( ffiffi ffi 2 p a tet % c tet ). [12] For SrADC, an egative thermalv olume expansion (NTE) below room temperature was reported. [12] The resulting thermale xpansion coefficient is rather small (a V = À4.7 (13) x 10 À6 K À1 )a nd by af actor of % 6s maller than the corresponding coefficients in otherw ell-known NTE materials like ZrW 2 O 8 (a V = À27.2 10 À6 K À1 ). [16] The occurrence of NTE in SrADC was explained by a" guitar string vibration" of Sr···O···Sr units thusr educing the Sr···Sr distances with increasing temperaturea nd increasingt ransverse vibrational motion of the bridging oxygen atom. [12] Kepert and co-workers developed a model for NTE in an umber of Prussian Blue analogues, M II Pt IV (CN) 6 with M II = Mn, Fe, Co, Ni, Cu, Zn, Cd. [17] Here, the absolute values of a V increase with increasing size of the M II cation. This wasc orrelated to the strengths of the metal-cyanide bonding interaction leading to different energies of the transverse vibration of the cyanidebridge (i.e.,"guitar string vibrations"). [17] Accordingly,e nhanced NTE behaviour is expected for more flexible structures.
To prove these concepts we aimed at the synthesis of EuADC, which is expected to crystallize isotypically to SrADC due to the very similar ionic radii of Eu 2 + and Sr 2 + . [18] However, because of the oxidations ensitivity of Eu II ,t he synthetic procedure forS rADC being performed in water and air could not simply be transferred to the respective Eu II compound and a new approach had to be developed.

Synthesis of EuADC
Eu II as am ild reducing agent (e 0 (Eu II /Eu III ) = À0.35 V) is oxidized in air to form Eu III .T herefore, in first experiments we reacted the electride of Eu in liquid ammonia with H 2 ADC under inert conditions to avoid contact with water and oxygen. The yellow-orange precipitate that formed after addition of an equimolar amount of H 2 ADC to the blue electride was investigated by XRPD ( Figure S1, SupportingI nformation). The comparisonw ith ac alculated pattern for SrADC showed that isotypic EuADC had formed. However, the reflectionsa re very broad pointingt oalow crystallinity of the material. Annealing at mild temperatures did not improve the crystallinity significantly and elemental analysisrevealed up to 2% nitrogen obviously due to NH 3 that could not be removed completely.
Samples with ah igh crystallinitya nd sharpr eflectionsw ere obtained by the reactiono fK 2 ADC with EuBr 2 in degassed water (room temperature, Argon atmosphere)a ccording to Equation (1): The orange precipitate wasw ashed with degassedw ater, but even after several washing cycles small amountso fK Br remained in the final material( cp. weak reflection at 2q % 8.78 in Figure 2). From aR ietveld refinement an amount of 3.5 weight %K Br was determined. As ingle phase sample was obtained by the reactiono fH 2 ADC with EuBr 2 in degassed water (room temperature, Ar atmosphere)a ccordingt oE quation (2): This also orange-coloured product ( Figure S2, Supporting Information)w as used for all measurements reported in the following( with the exception of the synchrotron powder diffraction data recorded at the ESRF,w here samples obtaineda ccording to Equation (1) wereu sed). EuADC is stable in air.E ven after several months no signs of degradation were detectable.

Crystal Structure of EuADC
In contrast to SrADC [12] no single crystalss uitable for an X-ray structure analysis were obtainedf or EuADC in the syntheses described above. Therefore, high-resolution synchrotron powderd iffraction data was used to refine the crystal structure of EuADC. It crystallizes isotypically with SrADC. [12] The latter was used as as tartingm odel for the refinement. In Figure 2 the plot of the Rietveld fit is shown confirming av ery good agreement between the observed and refined data. Details of this refinement as well as some structural detailsa re summarized in Table 1, fractional atomic coordinatesa nd selected in- teratomic distances are given in Ta bleS1( Supporting Information).
The resultingc rystal structure of EuADC is depicted in Figure 3. It shall only be briefly discussed, as an extended descriptionw as already presented fori sotypic SrADC. [12] The unit cell volume of EuADC is slightly smaller than that of SrADC [12] (cp. Table 3). The Eu II cations form ad iamond-like arrangement with four identical Eu-Eu distances (4.43531(4) )a nd Eu-Eu-Eu angles very close to the ideal tetrahedral angles:1 09.768 (4 ) and 108.898 (2 ). These Eu II cations are connected to each other by the ADC 2À linkers. It is remarkable that the carbon backbones of these linkers are all alignedp arallel to [001] ( Figure 3). This leads to at etragonal symmetry,b ut the cubic symmetry is "preserved" by a c/a ratio very closet o ffiffi ffi 2 p " (cp. Ta ble 3). Note that I4 1 /amd,t he space group of the crystal structure of EuADC, is ad irect subgroup (translationengleich, index 3) of Fd3 m the space group of diamond. Each Eu II cation in EuADC is surrounded by eight oxygen atomso ft he carboxylate groups of six different ADC 2À ligands. Twoc arboxylate groups coordinate in ab identate chelatingm ode and four monodentately.T he Eu II O 8 polyhedron is best described as a snub diphenoid (J 84 = JSD-8, CShM = 6.725). [20] However,t he large CShM value (continuous shape measures) [20] already indicates that this polyhedron is highly distorted.E ach Eu II O 8 polyhedroni sc onnected by common edges to four further Eu II O 8 polyhedra resulting in a3 Df ramework structure with open channels. These channels are filled with the carbon atoms of the ADC 2À linkers. On the other hand, each linker coordinates to six Eu II cations,t hree by each carboxylate group in ac helating, bridging (m 3 -h 1 :h 2 :h 1 )m ode. The overall coordination can be described by the Nigglif ormula 3 1 [Eu(ADC) 6/6 ]. AT oposPro analysis [21] including the Eu 2 + nodes and the connecting ADC 2À linkersl eads to the rare unimodal msw topologya ccounting for 6-connectedv ertices and two different kinds of edges. [22] Magnetic Properties To confirm the divalent oxidation state of europium in EuADC temperature-dependent magnetic susceptibilities were measured in an externalf ield of 0.1 T. The resulting c = f(T)a nd c À1 = f(T)c urves are shown in Figure 4.
The linear curve in the c À1 = f(T)p lot indicates aC urie-Weiss behaviour for an (almost) pure paramagnet. From aC urie-Weiss fit according to Equation (3): with c dia = À67 10 À6 emumol À1 representing the diamagnetic contribution of EuADC, q P = À0.31 Ka nd an effective magnetic moment m eff = 7.76 m B were obtained.T he very small (negative) q P value points to an almost pure paramagnetism with negligible antiferromagnetic interactions. These interac-  [19] 1897411    6 ], ground multiplet 7 F 0 ), ac ompletely different magnetic behaviour with av an Vleck susceptibility is expected [24] so that the occurrence of larger amounts of Eu 3 + in EuADC can be ruled out.

Eu Mçssbauer Spectroscopy
The 151 Eu Mçssbauer spectra of EuADC at 6, 78, and 293 Ka re presented in Figure5;t he corresponding fitting parameters are listed in Ta ble 2. EuADC shows am ain signal at an isomer shift of À13.25 mm s À1 (78 Kd ata), indicating divalente uropium and ah ighly ionic bondings ituation. Similar negative isomer shifts are typically observed for divalent europium halides, halide glasses or borate glasses. [25][26][27] The tiny second signals around d = 0mms À1 indicate an Eu 3 + contribution,m ost likely arising from trace amounts of surfaceo xidation products.
These signals (around1 -2 %o ft he total area) were included as as imple Lorentzian within the fitting procedure.
The signals of divalent Eu II were all symmetrical and we obtained stable fits without any constraints resulting in the line width and electric quadrupole splitting parameters listed in Ta ble 2. The numerical values as well as as uperpositiono ft he spectra (not shown here) clearly reveal aw eak broadening of the signal from ambient temperature down to 6K.I ti sw ell knownt hat for such weak broadening effects one observes correlation between the line width and the quadrupole splitting parameter,h ampering ad oubtless separate refinement. As at est we tried af it of the 6Kspectrum with the line width fixed at the room temperature value. However, the simulated signal did not reproduce the experimental spectrum well. Thus, the broadening of the signal most likely relies on ad istributiono fs lightly different quadrupole splittings.
To test the local coordination of each Eu II cation in dependence of the temperature we refined the synchrotron powder diffraction data obtaineda tt he ESRF and DELTA down to 100 Ku sing the Rietveld method. The resulting structural parameters werea nalysed using the SHAPE algorithm. [20] However,n oc lear trend of the CShM values for aJ SD-8 polyhedron was obtained in dependence of the temperature. Ap ossible reason for the increasing line widths with decreasing temperatures in the Mçssbauer spectra might be relatedt ot he negative thermale xpansion observed in EuADC (see below). This effect is connected to as trongly increasing vibrational motion of the connecting oxygen atom ("guitar string vibration") [28] with increasing temperature.S uch am otion will "smear" out the coordinationa roundac entral metal cation thus leadingt o av irtually highers ymmetric coordinations phere. However,t o confirm this assumption am ore detailed structural analysisi s necessary,w hich should be based on X-ray single crystal data. Up to now,w eh ave been unable to obtain single crystalss uitable for such an analysis.

Thermal Analysis
As already mentioned, EuADC shows asurprisingly high chemical stability for an Eu II containing compound,a se ven after exposure to (humid) air for severalm onths no signs of degradation were observed in the XRPD patterns. To examinet he thermal stability of EuADC, DSC/TGA measurements were performed in an argon atmosphere up to 1000 8C. In Figure 6t he   Figure S13, Supporting Information. EuADC shows a sharp exothermic signalatapprox. 440 8Cpointing to adecompositiono ft he material. This decomposition temperature is only slightly lower than that reported for SrADC. [12] In Figure S14 (Supporting Information) the DSC/TGA curveso f EuADC and SrADC are compared, measured under the same conditions. Accordingly,t he decomposition temperature in SrADC is about 15 8Ch igher than in EuADC. Ad ecomposition temperature of ca. 440 8Cf or an Eu II containing coordination polymeri ss till remarkable, but not completely unprecedented. [29,30] However,f or recently discovered ADC-based MOFs [4] much lower decomposition temperatures were reported. For SrADC the decomposition of the materialu pon heating was thoroughly investigated:S rADC (= SrC 4 O 4 )r eleases CO upon heating and forms amorphous carbon as well as SrCO 3 . The latter decomposes to SrO at higher temperatures releasing CO 2 . [12] For EuADC the release of CO should lead to am assl oss of 10.6 %, which was reached at approx.6 30 8C, but with no clear plateau.E ven up to 1000 8Cn oc lear plateau was observed ( Figure S13, Supporting Information). To clarify this point, severals amples of EuADC were heated to 500, 700, and 1000 8C, and the resulting residues were investigated by XRPD. For the two lower temperatures (500 and 700 8C) only an amorphous solid was obtained, whereas for the residue obtained after heating to 1000 8Ct he XRPD pattern clearly shows the reflectionso fb oth modifications of Eu 2 O 3 ( Figure S15, Supporting Information). Obviously,S rADC and EuADCs how ad ifferent decomposition reaction.

Thermal Expansion
As mentioned in the introduction, SrADC shows an egative thermal( volume) expansion below room temperature. However,t he resulting thermale xpansion coefficient is small (a V = À4.7(13) 10 À6 K À1 ) [12] compared to other NTE materials. [31] In this respect, it was interesting to see how the substitution of Sr 2 + by Eu 2 + in isotypic compounds with very similar unit cell dimensions affects the NTE properties. In Figure 7t he unit cell volumes of EuADC as obtainedf rom temperature-dependent synchrotron powder diffraction data (beamline BL9, DELTA, Dortmund/Germany) are plotted. All lattice parameters are listed in Ta ble S2 (Supporting Information) and the respective Le Bail fits are given in Figures S3-S10. It should be noted that at the SNBL (ESRF,G renoble/France) with as ignificantly better resolution ad ecomposition of EuADCa fter several hours within the high-flux beam of this beamline was observed, as indicated by ab lack colouro ft he materiale xposed to the beam. Furthermore, the lattice parameters obtained at room temperature before and after cooling show large deviations, which make these measurements unsuitable for ap recise determination of the temperature-dependence of these lattice parameters. Within the lower flux of the DELTA synchrotron also as light blackening of EuADC after several hours within the synchrotron beam was observed, but the lattice parameters are still reproducible, as can be seen in Figure 7.
The unit cell volumes plottedi nF igure 7c learly indicate a negative thermal volume expansion in EuADC. From al inear fit between 293 and 100 K, an egative thermal expansionc oefficient a V = À9.4(12) 10 À6 K À1 was obtained, which is significantly larger than in SrADC (a V = À4.7(13) 10 À6 K À1 ). [12] As in both compounds the occurrenceo fN TE is attributedt ot ransverse M II ···O···M II vibrations ("guitar string vibrations"), [28,32] its thermale xpansion coefficient a V will mainly depend on the flexibility of the structure, that is, the strength of the M···O bond.I fa ni onic character of this bond is assumed, its strength will dependo nt he M···O distances, as was pointedo ut by Kepert andc o-workers in am odel they developed for NTE in a number of Prussian Blue analogues,M II Pt IV (CN) 6 with M II = Mn, Fe, Co, Ni, Cu, Zn, Cd. [17] Here, the absolutev alues of a V increase with increasing size of the M II cation.A ccordingly,t he a V values in SrADC and EuADC should be very similar, as the M···O distances are almost the same in both compounds (cp. Ta ble 3). The different masses of Sr II and Eu II should only have am inor influence. In agreement with this simple model Kepert and co-workersf ound very similar a V valuesi naseries of LnCo(CN) 6 materials for Ln = Y III and Ho III ,w hichh ave very similar ionic radii, but differentm asses. [33] In the view of these re-  sults the different a V values for SrADC and EuADC are surprising, buti ts hould be noted that the determinationo fp recise lattice parameters in this structure type is difficult, since the pseudo-cubic symmetry (see c/a ratio in Ta ble 3) leads to an overlap of many reflections of the tetragonal unit cell (cp. Figure 2). However,m easurements on ah igh-resolution synchrotrond iffractometer are not applicable here, as these compounds tend to decompose in high-flux synchrotron beams (see above). At the moment we are workingtoset up alaboratory X-ray diffractometerw ith high resolution to examine this interesting class of compounds in more detail.

OpticalSpectroscopy
Microcrystalline EuADC has an orange colour( Figure S2, Supporting Information). This appearance is in agreement with its diffuse reflectance spectrum that is shown in Figure 8a and indicatesa no nset of increasing reflectance R above around 450 nm. The opticalb and gap was more accurately characterized with aid of as o-called Tauc plot [34,35] (see Figure 8b,u pper panel).I nt here, f(R)d enotes the Kubelka-Munkf unction [36] that is related to the diffuse reflectance by Equation (4): and is am easure fort he absorption coefficient of the powder assuming ac onstant scattering contribution. Consequently,t he absorption cross section s abs is directly proportionalt of ( R)·E. It may then be easily concluded that above the opticalb and gap E g for ad irect band gap, al inear relationb etween (f(R) . E) 2 and E should follow Equation (5): which is simply ac onsequence of the energy dependence of the electronic density of states (DOS)i nabulk solid and its proportionality to s abs by Fermi's goldenr ule. Al inear leastsquares fit to the linear part of the Ta uc plot and extrapolation to the abscissa allows the determination of ad irect optical band gap with the value E g = 2.64(1)eV( or l g = 470 nm). The statistical deviation of the data from the linear fit as indicated in the residual plot (see Figure 8b)i ndicates the validity of the assumption of ad irect band gap in contrastt oa ni nf irst in-stance similarly accurately fitted indirecto ptical interband transition (see Figure S16, Supporting Information). The value of the band gap agreesp erfectly with the observed complementary orange colouro fs olid EuADC. It shouldb en oted that, for example, EuCl 2 and EuBr 2 ,a re colourless compounds indicating much larger band gaps. It is well-known that Eu II containing compoundsa re important phosphors with al argev ariety of different applications. [37] The broad 4f 6 5d 1 !4f 7 transition is dependentu pon the ligand field around Eu II andt hus in the focuso fm any research groups world-wide. [38][39][40][41][42] In contrast to those typical findings, no Eu II -related luminescencew as observed in EuADC, when excited with different energiese ven at low temperatures (10 K). The absence of any emission due to the 4f 6 5d 1 !4f 7 transition of Eu II is probably caused by concentration quenching. Am ore detaileda nalysis( e.g. investigation of doped compounds) of this unexpected finding is underway.

Conclusions
In summary,weh ave synthesized orange coloured Eu II -acetylenedicarboxylate (Eu[O 2 C-CC-CO 2 ] = EuADC) as as ingle-phase 2.758(5), 4 PbADC [13] 7.284(1) 10.325 (2)  microcrystallinep owder.I tcrystallizesi nt he SrADC type structure (I4 1 /amd, Z = 4) [12] and is stable under ambient conditions for at least several months. Upon heating in an inert atmosphere decomposition only starts at 440 8C, which is remarkable for an Eu II containing compound. Measurements of the magnetic susceptibilities (m eff = 7.76 m B )a sw ell as 151 Eu Mçssbauer spectroscopy data confirm that exclusivelyE u 2 + cations exist in EuADC. Below room temperature EuADCs hows an egative thermal( volume) expansion (NTE) with a V = À9.4(12) 10 À6 K À1 . This value is slightly larger than that found in SrADC (a V = À4.7(13) 10 À6 K À1 ). [12] This finding is in conflict with established models of NTE, [17] as in both compounds similar M II ÀO bonds are found. However,aprecise determination of lattice parameters is difficult forthis class of compounds, as due to its pseudo-cubic structuresastrong overlap of reflections occurs. Therefore, further experiments with ah igh-resolution set-up using laboratory X-ray sources are under way,a sa cetylenedicarboxylates tend to decompose in high-flux synchrotron beams after long exposure times. According to diffuse reflectance spectra, EuADC exhibits a direct optical band gap of E g = (2.64 AE 0.01) eV thus explaining the orange colour of the solid. Surprisingly,n op hotoluminescence under UV irradiation even down to 10 Kw as observed. In order to investigate the influence of concentration quenching (Eu-Eu distances:4 .43531(4) )a nd in order to make the Eu II -based electronic states more localized in real space, we have startedt os ynthesize solid solutions Eu x Sr 1Àx ADC. Results on this solidsolution will be published in due course.

Experimental Section
Synthesis of EuADC:E uBr 2 (Sigma Aldrich, 99.9 %) and H 2 ADC (Sigma Aldrich, 95 %) were used as purchased. K 2 ADC was obtained by grinding 0.0981 gK (CH 3 COO) (1 mmol) with 0.0571 gH 2 ADC (0.5 mmol) in an agate mortar,u ntil no smell of acetic acid was noticed anymore. The product was finally heated for several hours at 150 8Ci na nArstream. Its purity was checked by XRPD. [43] Twos yntheses were developed according to procedures (1) and (2). Procedure 1): In ag lovebox, 0.312 gE uBr 2 (1 mmol) and 0.190 g K 2 ADC (1 mmol) were transferred to aS chlenk tube and mixed. At aS chlenk line, approx. 7mLd egassed H 2 Ow ere added. After complete dissolution of the solids the initially colourless solution turned slowly to yellow and ay ellow-orange precipitate started to form. The reaction mixture was stirred for approx. 24 ha nd the resulting precipitate was filtered off under inert conditions (Argon atmosphere). The product was washed with degassed water several times to remove the KBr by-product as completely as possible. But even after repeated washing cycles am inor amount of KBr (approx. 3.5 weight %) remained in the product, which was finally dried in adynamic vacuum.
Procedure 2): In ag lovebox, 0.312 gE uBr 2 (1 mmol) was placed in aS chlenk tube and at aS chlenk line, approx. 5mLd egassed H 2 O were added. In as econd Schlenk tube, 0.114 gH 2 ADC (1 mmol) were weighed in. After evacuating and flushing the Schlenk tube with argon three times, approx. 3mLd egassed water were added. After complete dissolution of both solids the H 2 ADC solution was transferred to the EuBr 2 solution via as yringe. The reaction mixture was stirred for approx. 24 ha nd the resulting orange precipitate was filtered off under inert conditions (argon atmosphere) and dried in ad ynamic vacuum. XRPD investigations revealed that a single-phase product had formed. C 4 EuO 4 (264.00): Calcd:C , 18.20 %; Found:C ,1 8.40 %, H, 0%,N ,0 %. Additionally,I Ra nd Raman spectra (Figures S11a nd S12, Supporting Information) showed the expected signals for carboxylate groups and CÀC triple bonds and compared very well with the spectrum obtained for SrADC thus confirming the successful synthesis of EuADC.
Powder X-ray diffraction:T oc heck the purity of the samples XRPD data were recorded with aH uber G670 diffractometer (Ge(220) monochromator,i mage plate detector,r oom temperature, MoKa 1 radiation), ca. 60 min per scan. Samples were sealed in glass capillaries (Ø = 0.3 mm) under inert conditions (argon filled glovebox) prior to all measurements.
Synchrotron powder diffraction:H igh-resolution synchrotron powder diffraction data was recorded at the Swiss Norwegian BeamLine (SNBL, BM01B) [44] at the European Synchrotron (ESRF, Grenoble/France). The wavelength was calibrated with aS is tandard NIST 640c to 0.504477 .T he diffractometer is equipped with six counting channels, delivering six complete patterns collected with as mall 1.18 offset in 2q. AS i(111)a nalyser crystal is mounted in front of each NaI scintillator/photomultiplier detector.D ata was collected at 295 Kw ith steps of 0.0028 (2q)a nd 100 ms integration time per data point leading to ar ecording time of 22 min per scan (48 2q 308). For the final pattern 5scans were added. Data from all detectors and scans was averaged and added to one pattern with local software.
As after long exposure times in the high flux beam of the ESRF a decomposition of EuADC was observed, indicated by ad arkening of the material, temperature dependent diffraction data was collected at beamline BL9 of the DELTA synchrotron radiation facility, Dortmund, [45] which has as ignificantly lower photon flux. The measurement was performed at selected temperatures between 293 and 100 K( N 2 cryostreamer) with aw avelength of l = 0.826566 using aP ILATUS100K detector (78 2q 308,a pprox. 60 min for each scan). To ensure ar eproducibility of the obtained lattice parameters and exclude possible effects of decomposition in the synchrotron beam, patterns were recorded upon cooling (293, 250, 200, 150, and 100 K) and heating (170, 230, and 293 K).
For all experiments EuADC was filled in glass capillaries (Ø = 0.7 mm) and sealed under an argon atmosphere. The capillaries were mounted on spinning goniometers.
Analysis and refinement of powder diffraction data:T he WinX-Pow software package [46] was used for raw data handling and visual inspection of the data. AR ietveld refinement was conducted with GSAS [47] using the high-quality diffraction pattern of EuADC obtained at the ESRF at 295 K. The known crystal structure of SrADC [12] was used as as tarting model for the refinement. Subsequently six background parameters (shifted Chebyshev function), scale, zero shift, lattice parameters (a, c)a nd seven profile parameters (pseudo-Voigt function including four parameters to account for the anisotropic peak broadening) were refined. In the final refinement cycles four positional and four isotropic temperature factors (U iso )w ere introduced leading to 25 variables in total. Including three soft constraints (C1ÀC1 = 1.20(5) ,C 1 ÀC2 = 1.45(5) , C2ÀO1 = 1.25(5) )asmooth and slowly converging refinement was obtained. The resulting fit is shown in Figure 2a nd selected details of the crystal structure, the measurement and the refinement are summarized in Ta ble 1, fractional atomic coordinates and selected interatomic distances are given in Ta ble S1 (Supporting Information). Diamond [48] was used for the visualization of the crystal structure of EuADC ( Figure 3) and Gnuplot 4.6 [49] for the plot of the refinement. To determine the unit cell parameters of the data obtained at the DELTA synchrotron upon cooling (293 to 100 K) and heating back to room temperature precisely,L eB ail fits in Jana2006 [50] were performed using the following five variables in all refinements:t etragonal lattice parameters a and c,z ero shift, and profile parameters GW and LY (pseudo-Voigt). The background was subtracted manually before each refinement. The lattice parameters obtained from these Le Bails fits are summarized in Ta ble S1 (Supporting Information) and the unit cell volumes are plotted in Figure 7. Plots of the Le Bail fits are shown in Figures S3-S10 (all Supporting Information). They were visualized using Gnuplot 4.6. [49] Elemental analysis:E lemental analysis of carbon, hydrogen, and nitrogen was conducted with aHEKAtech GmbH EuroEA 3000 Analyser.
IR spectroscopy:I Rs pectra were recorded with aP erkinElmer Spectrum 400 IR spectrometer (ATR module, diamond crystal). The resulting spectrum is given as Figure S11, Supporting Information.
Raman spectroscopy:R aman spectra of solid EuADC and SrADC ( Figure S12, Supporting Information) were recorded at room temperature on aB ruker MultiRam Raman spectrometer (40 mW; Nd:YAG laser,g ermanium detector) in the spectral range 4000-0cm À1 .T he samples were filled in 1mmc apillaries and sealed under an argon atmosphere.
Measurement of magnetic susceptibilities:T he magnetic susceptibilities of ap ressed powder sample of EuADC (2.41 mg) were measured in aS QUID magnetometer (MPMS, Quantum Design) applying an external field of 100 mT.T he diamagnetic contribution of EuADC was calculated according to the procedures described in the literature. [51] As shown in Figure 4, the inverse susceptibilities follow astraight line suggesting pure paramagnetic behaviour.
Mçssbauer spectroscopy:T he 21.53 keV transition of 151 Eu of a 151 Sm:EuF 3 source with an activity of 50 MBq (0.91 %ofthe total activity; I = 7/2 to I = 5/2 transition) was used for the Mçssbauer spectroscopic experiments, which were conducted in transmission geometry.T he measurements were carried out in ac ontinuous flow cryostat system (Janis Research Co LLC) at 6, 78, and 293 K ( Figure 5). The temperature was controlled by ar esistance thermometer (AE 0.5 Ka ccuracy). The EuADC sample was enclosed in a small PVC container at at hickness corresponding to about 20 mg Mçssbauer active element cm À2 .T he spectra were fitted with the Normos-90 software package. [52] DSC/TGA:D SC/TGA measurements on EuADC were performed with aM ettler To ledo TGA/DSC 1S tar e (Al 2 O 3 crucible;A rgon stream with 30 mL min À1 ;h eating rate 10 8Cmin À1 ). As ample of 2.6961 mg was weighed out and handled under inert conditions (glovebox). The residue obtained after heating to 1000 8Cw as investigated by XRPD ( Figure S15, Supporting Information).
Reflectance spectrum:D iffuser eflectance spectra were acquired with aC ary 5000 UV/vis spectrometer (Agilent) that detects reflectance between 200 and 2500 nm. The reflectance spectrum was corrected for the background with aS pectralon reference sample.
Luminescence spectroscopy:R oom temperature and low temperature (10 K) photoluminescence excitation and emission spectra of EuADC were recorded with aH oriba Jobin Yvon FluoroMax-3 FL3-22 spectrometer equipped with a4 50 WX el amp, double Czerny-Turner monochromators in both the excitation and emission compartment and ap hotomultiplier tube sensitive to the visible range (R928P,H amamatsu). Spectra at 10 Kw ere acquired with al iquid He closed-cycle cryostat (Janis Research Co LLC) attached to an external temperature control unit (Lake Shore). For luminescence measurements, the powdered sample was sealed in pre-evacuated spectroscopically pure quartz ampoules.
If not noted otherwise, diagrams and plots are visualized with Origin 8.5.0. [53]