Multinuclear NMR as a tool for studying local order and ...

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Multinuclear NMR as a tool for studying local orderand dynamics in CH3NH3PbX3 (X = Cl, Br, I) hybrid

perovskitesClaire Roiland, Gaelle Trippé-Allard, Kaoula Jemli, Bruno Alonso,

Jean-Claude Ameline, Regis Gautier, Thierry Bataille, Laurent Le Polles,Emmanuelle Deleporte, Jacky Even, et al.

To cite this version:Claire Roiland, Gaelle Trippé-Allard, Kaoula Jemli, Bruno Alonso, Jean-Claude Ameline, et al.. Mult-inuclear NMR as a tool for studying local order and dynamics in CH3NH3PbX3 (X = Cl, Br, I) hybridperovskites. Physical Chemistry Chemical Physics, Royal Society of Chemistry, 2016, Physical chem-istry of hybrid perovskite solar cells, 18 (39), pp.27133-27142. �10.1039/C6CP02947G�. �hal-01354947�

MultinuclearNMRasatoolforstudyinglocalorderanddynamicsinCH3NH3PbX3(X=Cl,Br,I)hybridperovskitesClaireRoiland,aGaelleTrippé-Allard,bKhaoulaJemli,bBrunoAlonso,cJean-ClaudeAmeline,dRégisGautier,aThierryBataille,aLaurentLePollès,a,†EmmanuelleDeleporte,bJackyEven,e,†andClaudineKatana,†

We report on 207Pb, 79Br, 14N, 1H, 13C and 2H NMR experiments for studying the local order and dynamics in hybridperovskitelattices.207PbNMRexperimentsatroomtemperatureonaseriesofMAPbX3compounds(MA=CH3NH3

+;X=Cl,Brand I) showedthat the isotropic 207PbNMRshift is stronglydependenton thenatureof thehalogen ions.Therefore207PbNMRappearstobeaverypromisingtoolforthecharacterisationoflocalorderinmixedhalogenhybridperovskites.207Pb NMR on MAPbBr2I served as a proof of concept. Proton,

13C and 14N NMR experiments confirmed the resultspreviouslyreportedintheliterature.LowtemperaturedeuteriumNMRmeasurements,downto25K,werecarriedouttoinvestigate the structural phase transitions of MAPbBr3. Spectral lineshapes allow following the successive phasetransitionsofMAPbBr3. Finally, quadrupolarNMR lineshapes recorded in theorthorhombicphasewere comparedwithsimulated spectra, using DFT calculated electric field gradients (EFG). Computed data do not take into account anytemperature effect. Thus, the discrepancy between the calculated and experimental EFG evidences the fact that MAcationsarestillsubjecttosignificantdynamics,evenat25K.

IntroductionHalideperovskitesolarcellsproposedrecently1-9havereachedinafewyearsrecordphotoconversionefficienciesmatchingin2016 (22.1%) thebest existing thin film technologies, namelyCdTe (22.1%) and CIGS (22.3%).10 Perovskite cellphotoconversion efficiencies are coming closer to the one ofsilicon (25%),10butnumerous issues (upscaling, resistancetomoistureandlightsoaking,hysteresis…)havetobeovercomebefore reaching a possible industrialization. Chemicalengineering afforded by the hybrid perovskitematerialsmayprovide a variety of applications such as tandem solar cells,light emitters or detectors. Ongoing perovskite celldevelopments toward higher solar cell efficiencies are basedon complex alloys involving both mixings of cations such asmethyl ammonium (MA+), formamidinium (FA+) and Cs+, andhalide anions.7-9 Most important experimental results wereobtained in the initialperiodofperovskite solar cell research(2007-2014) for MAPbX3 where X = I, Br, Cl.

1-6 Prior to that,MAPbX3 bulk materials were the subject of quite a fewfundamental studies, including nuclear magnetic resonance(NMR) spectroscopy,11,12 dielectric and millimetre wavemeasurements,13,14 calorimetry,15 optical characterizationstechniques16,17aswellasX-raydiffraction.18,19The isostructuralMAPbX3hybridperovskiteshaveaprimitivecubicstructureathightemperatureandundergoacubic(Pm-3m) to tetragonal (I4/mcm) antiferrodistorsive phasetransition at Tc~327K, 237K and 179K for X=I, Br and Cl,

13respectively.Giventhesymmetryoftheorganiccation,MAarenecessarily disordered in both the cubic (α-phase) andtetragonal (β-phase) phases. At lower temperatures (162K,149-154K and 173K, respectively)13 the tetragonal phasetransformstoanorthorhombic(Pnma;γ-phase)system.Solar cells using MAPbBr3 have been demonstrated, but thebulkmaterial has not an optimum electronic band gap for asinglejunctioncellcomparedtotheonesbasedonMAPbI3.

20-

22 Nevertheless, MAPbBr3 may lead to a number of possible

applicationsbasedonelectronic band gap tuning for tandemcellapplicationsorinthecompositionofalloyswhichstabilizesthe halide perovskite structure.23-28Moreover,MAPbBr3 bulkmaterial has other interesting intrinsic optoelectronicproperties,29-33 which are enhanced in colloidalnanostructures.34-36 A number of operating light emittingdevices based on MAPbBr3 have indeed beendemonstrated.29,37-41 Recent progresses in halide perovskitesynthesis and crystal growth allowed going deeper into theunderstanding of the bulk properties of MAPbX3 materials,especiallyforthebromidecompound.42-51Forinstance,recentdiffractionandNMRinvestigationsgavebetterinsightintothestructuralandthedynamicalpropertiesofMAPbBr3.

50-51A complex dynamical picture has progressively emerged forMAPbX3 bulk materials, which combine highly anharmoniclattice vibrations and stochastic MA reorientations, at hightemperature. These properties significantly influence theoptoelectronicandthermalproperties.52-60ThefreezingoftheMA cation dynamics at low temperature has recently beeninterpreted as a transition from a plastic crystal phase to anorientational glass.60 Although this theoretical predictionagreeswith recent experimental data,61 further experimentalinvestigationsarerequired.Compared to the numerous diffraction studies devoted toMAPbX3, local structural information using resonancetechniquesisstillscarce.11,12,51Meanwhile,solid-stateNMRisatoolabletoprobedynamicsinthesolidstateinseveralrangesoffrequency.DiverseNMRapproachesareavailabletoprobesuch a dynamics. For instance, T1 relaxation measurementsallow probing the dynamics in the Larmor frequency range(typically 10 MHz to 1 GHz). In a very different frequencyrange,NMRexchange experiments allow to evidencemotionfrom the Hz to the kHz. Dynamics can also induce lineshapemodifications by partially or fully averaging first orderanisotropies (chemical shift or first order quadrupolarinteraction)orsecondorderquadrupolaranisotropies. Inthatcase,NMRexperimentsaresensitivetomotionatfrequencies

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between a kHz and a few hundreds of kHz. The dynamiclineshape modifications are sensitive both to the frequencyandthegeometryofthemotion.Most of the nuclei present in hybrid perovskites can beprobed; however they exhibit different spectroscopicproperties. 1H is a spin-½ nucleus presenting strong dipolarinteractions in a solid without dynamics. Deuterium (I=1,Natural Abundance (N.A) = 0.015%) can be employed ondeuterium-enriched samples. It usually leads to spectradominatedby firstorderquadrupolar interactions. 13C (I=1/2)hasalownaturalabundance(NA=1.1%).Inthesolidstate,13Cnuclei usually present strong dipolar interactions with thesurrounding protons. In a solid without fast dynamics, thisallows to record 13C spectra by using CPMAS (CrossPolarization Magic Angle Spinning) type experiments. Direct13C acquisition can be an option in the case of a solidpresenting fast dynamics. Nitrogen can also be addressedeitherby15N(I=1/2,N.A.=0.37%)or14N(I=1,N.A.=99.63%)NMR.The isotropic chemical shiftof 207Pb (I=1/2,N.A.=22.1%)coversabroadrange,ofmorethan5000ppm,andpresentsusually largechemicalshiftanisotropies.Onthehalogenside,all thehalogennuclei canpotentially beprobedbyNMR.62,63However,bothchlorineandiodineareverychallengingnucleifor NMR spectroscopy due to their low gyromagnetic ratiosand/orhighquadrupolarmoments.79Br(I=3/2,N.A.=50.54%)hasarelativelylargequadrupolarmomentbutseemstobethebest halogen nucleus to probe hybrid perovskite materials.Amongtheseprobes,Wasylishenandco-workershavealreadyinvestigated 2H and 14N,11,12 whereas Baikie and co-workersfocusedon1Hand13C.51Inthepresentwork,wereportonNMRrecordedonMAPbX3(X=I,Br,Cl)powders.Thisincludes207Pb,79Br,14N,1H,13Cand2HNMRexperiments. Room temperature 1H, 13C and 14N areperformed usingMagic Angle Spinning (MAS) NMR. 14N wasprobed between 233 and 333K. Static deuterium NMRinvestigationswerecarriedoutatvarioustemperatures,downto25K.Resultsareinfairagreementwithavailabledatafromthe literature. Moreover, we show that both 207Pb and 79BrNMRareofparticularrelevancetostudylocalenvironmentsinhybridperovskites.Last, temperaturedependant follow-upofdeuterium NMR on MAPbBr3 is shown to provide furtherinsightonthedynamicsofthemolecularmoieties,i.e.theMAcations.

ExperimentalSyntheticprocedures

General procedure for synthesis of methylammonium-d3tribromide (deuteriumenrichment).Hydrobromic acid (47% - 4mL) anddeuteriumoxide (4mL) are stirredduringonehour.Thesolutioniscooledto2°Cbeforeaddingmethylamine2.0Min methanol (5mL, 2 mmol). The mixture is stirred during 2hours at 2°C, before being concentrated in vacuum. Thepowder isrinsedwithdiethyletheranddried inovenat50°Cduring3hours.

General procedure for synthesis of methylammonium-d3 leadtribromide.Toacolourlesssolutionofleadbromide(367mg,1

mmol) inDMF (10mL),methylbromide (whitepowder -115mg, 1 mmol) is added. The solution remains colourless. Theagitation is extended during 15 mn before evaporation ofsolvent. The resulting orange powder is successively washedwithacetoneanddiethylether.Theorangepowderisdriedat60°Cduring1hour.Yield:97%(469mg).

General procedure for synthesis of methylammonium leadtriiodide / CH3NH3Pbl3. To a colourless solution of lead iodide(922mg,2mmol)inGBL(10mL),methyliodide(whitepowder- 318 mg, 2 mmol) is added. The yellow solution is stirredduring5mnbeforeevaporationofsolvent.Theresultingblackpowderissuccessivelywashedwithacetoneanddiethylether.Theorangepowder is dried at 60°Cduring1hour. Yield: 85%(1.05g).

General procedure for synthesis of methylammonium leadtribromide / CH3NH3PbBr3. To a colourless solution of leadbromide (734mg, 2mmol) in DMF (20mL),methyl bromide(white powder - 224 mg, 2 mmol) is added. The solutionremains colourless. The agitation is extended during 5 mnbeforeevaporationofsolvent.Theresultingorangepowderissuccessively washed with acetone and diethyl ether. Theorangepowder isdriedat60°Cduring1hour.Yield:82%(790mg).

General procedure for synthesis of methylammonium leadtrichloride / CH3NH3PbCl3. To a colourless solution of leadchloride(973mg,3.5mmol)inDMSO(30mL),methylchloride(whitepowder – 263.5mg, 3.5mmol) is added. The solutionremainscolourless.Theagitationisextendedduring5mnaftercompletedsolubilisation,andthenthesolventisremoved.Theresulting white powder is successively washed with acetoneand diethyl ether. The white powder is dried at 60°C during1hour.Yield:84%(1.04g).

General procedure for synthesis of methylammonium leaddibromide iodide/CH3NH3PbIBr2.Toacolourless solutionof leadbromide (367mg, 1mmol) in DMF (10mL),methyl iodide (whitepowder -159mg,1mmol) isadded.Thesolutionbecomesyellowinstantaneously. The agitation is extended during 5 mn beforeevaporation of the solvent. The resulting maroon powder issuccessively washed with acetone and diethyl ether. Themaroonpowderisdriedat60°Cduring1hour.Yield:53%(280mg).

NMRspectroscopy

Roomtemperaturesolid-stateNMRfor 1H, 13C, 207Pb, 79Br.MagicAngleSpinning(MAS)solid-stateNMRexperimentswereperformedon a BrukerAvance III 600SB spectrometer (magnetic field : 14T)operatingatLarmorfrequenciesof600.1MHzfor1H,150.9MHzfor13C, 125.5MHz for 207Pb and150.3MHz for 79Br. Solid-stateNMRspectrawererecordedusinga3.2mmMAS(MagicAngleSpinning)double resonanceprobehead, andMAS frequencieswere set to 5kHz for 1Hand13Cand22kHz for 207Pband79Br.Longitudinalandtransverse relaxation times, T1 and T

*2, were measured using a

saturation/recovery sequence and a 2D spin-echo, respectively.Thosemeasurementsmadeuscertainthatusingarecycledelaysetto1sislongenoughtoensureafullrelaxationofthespinssystem.1Hand207Pbwereacquiredusinga90°singlepulsecorrespondingtoaradiofrequenciesof114kHzand58kHz,respectively.Toavoid

Pre-print of Phys. Chem. Chem. Phys., 2016,18, 27133-27142 DOI:10.1039/C6CP02947G DOI: 10.1039/C6CP02947G

lineshape distortion due to the quadrupolar effect, 79Br spectrumwasacquiredusingasinglepulsecorrespondingtoaflipangleof6°.Crosspolarisation (CP) 13C spectrawereperformusinga 1H to 13Ctransferdelayset to2ms.Aprotondecouplingpulse (46kHz)wasappliedduringtheacquisition.Static 207PbNMRexperimentswereperformed on a Bruker Avance I 300 WB spectrometer (7T)operating at Larmor frequency (62.7 MHz) using a 4 mm MASdoubleresonanceprobehead.Inordertoavoidbaselinedistortion,staticNMRspectraweremeasuredusingaspinechosequence.Theechodelayissetto6.7µs,90°pulseto2.2µsandrecycledelayto1s.Whenever possible, static experimentswere preferred insteadofMAS,toavoidpossibledegradationofthesamplecausedbythelightbeamusedtodetecttherotationalspeedoftherotorand/orthesampleheatingdueto frictionsoccurring in fast spinningMASrotors. In addition, 1H spectra were conducted regularly to checkthe quality of the samples. 1H and 13C chemical shifts werereferencedtoTMS. 207Pband79BrchemicalshiftswerereferencedtoPb(NO)3andasolution0.01MofNaBrinD2O,respectively.Fitswereperformedusingthedmfitsoftware76.

Roomtemperaturesolid-stateNMRfor14N.14NNMRspectrawereperformedona600Varianspectrometer(14T)operatingatLarmorfrequency of 43.3 MHz using a 9.5 mm rotors. Single pulse MASspectrawereacquiredusinga30° flipangle,arecycledelaysetat1sandaMAS frequency setat3.5kHz.Temperatureexperimentsweredonefrom233Ktoroomtemperature.Foreachtemperature,the longitudinal relaxation time T1 was measured using asaturation/recoverysequence.14NchemicalshiftswerereferencedtoNH4Cl.

Variable temperature solid-state NMR: 2H. 2H low temperatureNMR experiments were performed on a Bruker Avance I 300WBspectrometer (magnetic field:7T)operatingat Larmor frequenciesof 46MHz for 2H. To allow low temperaturemeasurements, theshims were removed and replaced by an oxford cryostat.Deuteratedhybridperovskitewaspackedina6mmTeflontubeandplaced inahomemadevery lowtemperatureprobe (upto4K). 2Hspectra were recorded under static condition using a solid echosequence and VOCS.79 The 90° pulse corresponds to a RF field of27.5 kHzand theechodelaywas set to150µs. Temperaturewasset using an Oxford temperature regulating system using liquidnitrogenascryogenicfluid.

Computational details.Calculations on CH3NH3PbBr3were carriedout using the CASTEP 6.0 DFT code that explicitly describes thecrystalline structure of the compounds using periodic boundaryconditions.80,81 Theorthorhombic crystal structureofCH3NH3PbBr3determined by Swainson and coworkers was considered.18 OnlyHydrogen atoms were free to relax. The exchange-correlationinteraction was described within the generalized gradientapproximation (GGA) of Perdew, Burke, and Ernzerhof.82 All ultra-soft pseudopotentials (US-PP) were generated using theOTF_ultrasoft pseudo-potential generator included in CASTEP 6.0.ThePAWformalismwasusedtocalculatetheEFGtensorsfromthepseudodensity.83,84 The EFG tensor is traceless; i.e. its eigenvalues(Vxx, Vyy, and Vzz) obey Vxx + Vyy + Vzz = 0.We used the followingconventions for the quadrupolar coupling constant CQ and theasymmetryparameterηQ:CQ=eQVzz/handηQ=(Vxx−Vyy)/Vzzwith|Vzz|≥|Vxx|≥|Vyy|.AquadrupolarmomentQfor2Hequalto2.86

10-29m2wasused.AllcalculationswereproventoconvergeinNMRvalueswithacutoffenergyof800eV. TheMonkhorst−Packk-pointgrid density used was densified until convergence.85 RelativisticeffectswereincludedforallelementsduringtheUS-PPgenerationbysolvingthescalarrelativisticequationofKoellingandHarmon.86Non-linearcorecorrectionshavebeenappliedtoallatoms.87

X-ray powder diffraction. The pattern of CH3NH3PbBr2I wascollected at room temperature using a PANalytical Empyreandiffractometer, with the Cu Kα radiation (Kα1 = 1.5406 Å, Kα2 =1.5444 Å) selected with the Bragg-Brentano HD device (flatmultilayerX-raymirror)fromPANalytical.

Resultsanddiscussion79BrNMR

Chlorine, bromine and iodine are all quadrupolar nuclei(I>1/2).Amongallthethreehalogennucleiprobed,significantNMR signalwasonlyobtained for 79Br inMAPbBr3.Wewerenotabletorecordany127INMRspectrafromMAPbI3.Thelateris in the tetragonal phase at room temperature andpresumably presents a large quadrupolar interaction,preventingitsacquisitionbystandardNMRtechniques.

Figure1.79BrMASNMRspectrumobtainedat14TonMAPbBr3.

The79Brspectrum issketched inFigure1. Itpresentsasinglenarrow resonance located at 49.5 ppm. This spectrum doesnotexhibitanyfeatureofaquadrupolarinteractionconfirmingthehighsymmetryofthebrominesiteandthecubicstructureofMAPbBr3 at room temperature. In the caseof compoundswith lowersymmetrycrystal structures, thehighquadrupolarmoment of 79Br often gives rise to highly complex NMRlineshapes.Inthepresentwork,wedidnotinvestigatefurther79Br NMR inMAPbBr3. However, we suggest the use of highfield 79Br NMR (18T or more) experiments to reduce thesecond order quadrupolar linewidth. In addition, 79/81Br NQRexperiments64may be relevant to investigate distributions ofbromineinmixedhalideperovskites.65

207PbNMR207Pb isaspin-½nucleuspresentinga largerangeofchemicalshift (at least 5000 ppm) and usually large chemical shiftanisotropies. It has already been employed for thecharacterisation of inorganicmaterials,63,66-68 but not in lead-basedhybridperovskitestothebestofourknowledge.

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Figure2. 207PbNMRspectraobtainedat7T forMAPbCl3 (a),MAPbBr3 (b) andMAPbI3(c).

Therearemajorinterestsinusing207PbNMRasaprobeinthepresent context of hybrid perovskites. Even though thematerials used for practical applications contain variouschemicalsubstitutionsontheorganicandhalogenpartsofthestructure, lead is still massively used as a metal centre.Therefore 207Pb NMR spectroscopy would be central toevaluate locally the structural effect of the varioussubstitutions.Furthermore,theleadionsareplayingakeyrolein the density of states at the Fermi level since VB isdominantlycontrolledbyacombinationofleads-orbitalsandhalide p-orbitals, and the CB relies mainly on the lead p-orbitals. Consequently the evolution of lead NMR chemicalshiftisdirectlyrelatedtothebandstructureattheFermilevel.

Static 207Pb spectra are sketched in Figure 2. 207Pb NMRappearstobehighlysensitivetothechemicalcompositionofMAPbX3 compounds.Noteworthy, the 207Pb spectrumof PbI2has recently been reported.63 From the spectrum shown inFigure2,wecanconcludeontheabsenceofanyPbI2impurityin our MAPbI3 powders. Static and MAS (Figure S1) spectraexhibit single lines consistent with a single lead site in theperovskitestructure.

One can observe a general trend in the isotropic chemicalshifts and in the spectra linewidth: going from chlorine tobromine and iodine, the isotropic shifts are -644 ppm, 365ppmand1430ppm,respectively;thecorrespondinglinewidths(FWHM)amountto3.7,15.3and19.8kHz,respectively.Thus,theisotropicshiftvaluesandthelinewidthofthePb2+ionsareincreasingwhilethehalogeninitsvicinityisgettingheavier.

AsimilartrendisobservedfortherelaxationtimesT1(Table1).On the contrary, the T2 transverse relaxation times are quitecomparable forMAPbCl3andMAPbBr3,while their linewidthsare completelydifferent (3.7 kHz and15.3 kHz, respectively).TheseT2valuesconfirmthatthelinewidthisnotonlyduetoarelaxationeffect.

Table 1 207Pb Longitudinal and transverse relaxation times, noted as T1 and T*2,respectively, measured for MAPbX3 compounds. T1 values are obtained using asaturation/recovery sequence underMAS. T*2 aremeasured according to a 2D spin-echounderMAS. T*2 forMAPbI3 is too short toproducemeasurable results underrotorsynchronisedconditions.

T1(ms) T*2(ms)

MAPbCl3 900 75

MAPbBr3 185 70

MAPbI3 50 --

Further investigations at different magnetic fields (𝐁) andtemperatures(T)areneededtorationalizethesefindings.Forinstance, such experiments may provide indications ofwhetherornotadistributionofchemicalshifts(𝐁)orchangesofthechemicalshifttensorrelatedtothedynamics(T)areatwork. Moreover, these results suggest that 207Pb NMRspectroscopyshouldprovideanappropriatetooltoinvestigatemixedcompositions.

Figure3. 207PbNMRspectraobtainedat7T forMAPbCl3 (a),MAPbBr3 (b) andMAPbI3(c).

Inordertoprovideaproofofconcept,wefurtherinvestigateamixedhalidesolidsolution,namelyapowderofMAPbBr2I.The207Pb NMR spectrum shown Figure 3 exhibits an NMR signalwithtwocomponents,thefirstoneat795ppmandasecondoneat347ppm.The latercorresponds to thepositionof thesignal obtained for the MAPbBr3 powder. This raises thequestionofwhetherthe I/Brdistribution ishomogeneous.Tofind out whether the MAPbBr2I powder undergoes phaseseparation, we subsequently performed an X-Ray diffraction(XRD) analysis. Indexing the XRD powder diffraction pattern(Figure S2) clearly evidences that MAPbBr2I is a phase pureproduct,withacubicunitcellparameterof6.0095(7)Å.Thus,it belongs to a solid solution, with no indication of thepresenceoftheMAPbI3andMAPbBr3poles.

Two factsmust be kept inmind for the interpretationof the207PbNMRspectrumofMAPbBr3. Firstly,wemay stressherethat the spectrometer used to record the 207Pb spectrum ofthe MAPbBr2I powder has a filament lamp left oncontinuously. Thus, possible phase separation (MAPbI3 and

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MAPbBr3)duringtheNMRacquisitionfollowedbyareturntoasolidsolutionMAPbIBr2singlephasecannotfullyberuledout.Secondly, thesignificantly lowerT2 relaxation timeofMAPbI3as compared toMAPbBr3 (Table 1) hampers anyquantitativeanalysisof the 207Pb spectrum.As amatterof fact, theHahnecho experiment will overestimate the intensities of thespeciespresentingthelargestT2relaxation.

Despite these limitations thatmay be circumvented by usingalternativeequipment,thisresultshowsthecomplementarityfor the structural studies performed by XRD and solid stateNMR.Thediffractionmeasurementrevealsalong-rangeorderaveragedoutamongthesamplevolume,leadingtoanaverageunit cell.On the contrary, the 207PbNMRexperiment gives amuchmorelocaldescription.Theinterpretationwecanmakeofthetwocontributionsobservedinthe207PbNMRspectrumof MAPbBr2I is that itallowstodistinguishbetweenthe leadatomsconnectedtosixbromine ionsandthoseconnectedtoboth iodineandbromine.Clearly, thedetailedunderstandingofthelocalstructureofsolidsolutions,includingtheirpossibledegradation under irradiation, requires complementaryexperimentalandtheoreticalinvestigations,whicharebeyondthescopeof thepresentwork.However,weestablishedthat207PbNMR isan important tool to study the stabilityand thelocalorderingofmixedhalidehybridperovskites.

LightnucleiNMRresults1H NMR experiments were carried out at 14 T under MASconditions. Spectra are shown in Figure 4. For the threeMAPbX3compounds,spectraexhibittwosignalscorrespondingto NH3 (about 6.5ppm) and CH3 (about 3.5 ppm) protons.Theseresultsareconsistentwiththosecarriedoutat9.4TbyBaikie and coworkers.51 Moreover, the increase of theapparentlinewidthisconnectedtotheincreaseofthehalogenmass.Several spinningsidebands (SSb)areobserved foreachmember of the MAPbX3 series. Since the proton residualdipolar coupling cannot be neglected in this case, the SSbpatterncannotbefitted inordertoextractthechemicalshifttensor, as it is commonly done for slow MAS spectra.69Nevertheless,theSSbpatternisidenticalforthethreeMAPbX3compounds and does not appear discriminant for structuralphaseidentification.

Figure 4. 1H MAS NMR spectra obtained at 14 T under MAS condition forMAPbCl3 (a),MAPbBr3 (b)andMAPbI3 (c).Spinningsidebandsaremarkedwithasterisks.Insert:13CCPMASspectraobtainedonMAPbBr3.

13CCPMASNMRanalysis(insertinFigure4)isconsistentwiththepresenceofmethylgroups.The isotropicchemicalshift isalmostidentical(about31ppm)foralltheMAPbX3series.Theweak signal observed by CPMAS was recorded using arelatively long CP experiment (14h). The difficulty to employtheCPexperiment isprobablyduetoasmall residualdipolarcoupling indicative of a fast dynamics of the MA cations atroomtemperature.

Despite its high natural abundance, 14N is a relativelychallenging nucleus with an integer spin, a high quadrupolarcouplingandalowgyromagneticratio(γ)thatpresentsusuallyverybroadNMR signal. Some recentdevelopmentsopen theway to improved acquisition and understanding of 14N NMRsignals,particularly in thecaseof solid-statecompounds thathaveinterestingstructuraldynamics.70-71

We performed 14N MAS NMR experiments on MAPbCl3 atdifferenttemperatures, from233Kto303K.TheexperimentalspectraaresketchedinFigure5.Inthistemperaturerange,thespectrumpresentsasingleisotropicresonance(arrow)locatedat 7ppmanda SSbpatternwith a Lorentzian shapeanda 4kHzlinewidth.Whendecreasingthetemperature,theisotropicresonanceseems tobecomeslightlybroader (insertofFigure5).

Figure 5 14N MQS NMR spectra obtained on MAPbCl3 from 233K to roomtemperature.Insert:focusontheisotropicresonance.

Table2:14NT1valuesmeasuredonMAPbCl3usingasaturation/recoverypulsesequenceaccordingtothesampletemperature

T(K) T1(ms)

333 280293(RT) 230283 171273 146253 122233 83

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Concomitantly, the longitudinal relaxation time decreasessignificantly, from280ms to 83ms (Table 2). This is a directsignature of the dynamics of the organic cation in the cubicphase. Assuming isotropic motion, namely an Arrheniusbehaviour,wouldallowtoextractthecorrespondingactivationenergy. However, consistently with earlier findings, weobservedanon-ArrheniusbehaviourwhichhindertheuseofasimpleBPPmodelforthedataanalysis.11

Moreover, the isotropic lineshapes are consistent with anisotropic tumblingof theMAcations,at leastdownto233K.This is consistentwith the lower transition temperature (T <179K)13,15 ofMAPbCl3, as compared toMAPbI3 andMAPbBr3,forthestructuralphasetransitionfromthecubicPm-3mphasetoanorderedlowtemperaturephase.

Besides, these results obtained under magic angle spinningconditions are consistent with former static experiments byWasylishen and co-workers.11,12 They suggest that 14N NMRshould provide an appropriate tool to further investigate thelowtemperatureorderingexpectedintheMAPbX3phases.

Temperature-dependant2HNMR

Deuterium NMR lineshape modulations is a well-establishedtool to investigate the dynamics of molecules in the solidstate72-75.TogetinsightinthedynamicalbehaviouroftheMAcations, we use the dynamical modulation by the first orderquadrupolar interaction on a partially deuterated MAPbBr3powder.ThesedeuteriumNMRexperimentswerecarriedoutfrom room temperaturedown to25K.Corresponding spectraareshownFigures6-8.Figure 6. 2H static NMR spectra recorded on CH3ND3PbBr3 from roomtemperaturetoliquidHerangetemperature(downto25K).

In the cubic phase down to 233 K, the deuterium NMRlineshapes are fully isotropic, as already reported in theliterature.11,12Theseisotropiclineshapesrecordedinthecubicphaseareconsistentwithanoverallorientationinapotential

with cubic symmetry. In other words, it is consistent with adynamicalstochastictumblingoftheC-Naxisandconcomitantfastrotationalmotionofthemethylandammoniumrotors.Figure7.Zoomonthe2HstaticNMRspectrashownoffigure5.Comparisonofthelineshapesrecordedinthecubicandtetragonalphases.

Figure7highlights thechanges inspectral lineshapebetweenthe cubic and the tetragonal phase. A small first orderquadrupolar splitting arises that increases as temperaturediminishes. This temperature evolution of the quadrupolarsplitting can be associated with a reduction of the isotropicmotion of the principal axis of themethylammonium cation.Using the Dmfit software,76 we extract the quadrupolarcoupling constant (CQ), which amounts to 4.1 kHz in thetetragonal phase at 190K, and a vanishing asymmetryparameter(η)(Figure8).

Figure8.2HstaticNMRspectra(bluelines)andcorrespondingfittingresults(redlines)inthethreedifferentcrystallographicstructures(cubicatRT,tetragonalatT=190KandorthorhombicatT=50K)ofMAPbBr3.

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A further temperature decrease allows investigating theorthorhombic phase, between 148K and 25K, using a heliumcryostat. At 25K, the deuterium T1 seems to increasesignificantly making difficult the acquisition of spectra withgood signal to noise ratio. The spectral lineshapes between120K and 25K (Figure 6) clearly evidence much largerquadrupolar splitting than that observed in the tetragonalphase. However, this apparent quadrupolar splitting remainsunchangedintheorthorhombicphase.Figure9.Comparisonbetween theexperimental 2H lineshapeobtainedat50K(grey)onMAPbBr3andthesimulatedlineshapesaccordingtotheDFTcomputedEFG.

Togetabetterinsight,wecanrefertoDFTcalculationsoftheelectric field gradient (EFG). They allow evaluating the staticcontribution of the crystal packing on the quadrupolarlineshape. According to available crystallographic data,18 twodifferent deuterium sites are predicted in the orthorhombicPnma phase.However, the theoretical calculations yield verysimilarCQvaluesforthetwosites,188kHz(multiplicity1)and190 kHz, as well as very low η values (0.02). The spectrallineshapes deduced from the computed EFGs and measuredexperimentallyarecomparedinFigure9.Theobviousdiscrepancybetweenthecomputedstaticsplittingand the experimental results, which is much larger than theonegenerallyacceptedbetweenexperimentalandtheoreticalvalues,88reflectsthepartialmotionalnarrowingoftheEFG.Itranges from ~188-190KHz to about 51KHz (Figure 9). Thisdiscrepancy reveals the dramatic effect of themethylammoniumdynamics at low temperature.Meanwhile,the apparent quadrupolar splitting remains constant in theorthorhombic phase (Figure 6). These data suggest a fixedorientation of the CN axis. As the apparent splitting remainsmuch smaller than the splitting calculated by DFT in theabsenceofdynamics,ourresultsfurthersuggestthepresenceof a fast rotational dynamics of the terminal methyl andammoniumgroups.For a more complete understanding of the dynamicalbehaviour of the MA cation across the different structuralphase transitions, one may refer to complementaryexperimental and theoretical findings. Unfortunately, direct

experimental investigationsof theMAcations inMAPbBr3byneutron scattering are scarce.50 Detailed studies by inelasticincoherent neutron scattering are available for MAPbI3.

77,78Two quasi-elastic responses related to both MA tumblingdynamics and cation rotation around the C-N axis weredescribed. The fast dynamics around the C–N axis at hightemperaturewasreportedtoslowdownat lowtemperature,but remaining active in the low temperature orthorhombicphase.78Thisobservationisinlinewiththespectralnarrowingobserved in the present work, when compared to the staticcomputations(Figure9).Inaddition,thecomplexdeuteriumNMRlineshapes(Figures6-8) are compatible with previous theoretical predictions andrecent experimental observations of an orientational glassbehaviour inMAPbI3.

60,61Suchanorientationalglassystateatlow temperature is expected to be related to a disorderedfreezingoftheMAtumblingmotions,i.e.theC-Naxis.AstaticandrandomdistributionofC-NaxisorientationsmaystronglyperturbtherotationalmotionsaroundtheC-Naxis leadingtoa distribution of local dynamics. This issue deserves furtherstudy, for example, by performing solid-state NMR ondeuteratedsinglecrystals.

ConclusionsThis paper reports on extensive high-resolution solid-stateNMRstudiesofMAPbX3powders,X=I,BrandCl.LightnucleiNMR analysis based on 1H, 13C and 14N are consistent withavailabledatafromtheliterature.The1HmagicanglespinningspectraofallthreeMAPbX3compoundsarealmostidentical.Itis thus not discriminant for structural phase identification.Sincethecrosspolarisationtransferisrelativelyinefficientanddue to its lownatural abundance, 13Cdoesnot seem tobearelevantnucleustoprobehybridperovskitestructures.Onthecontrary,resultsobtainedfor14NNMRinthecubicphasearepromising.Combinedtotherecentdevelopmentson14NNMRsignal acquisition and understanding, this nucleus mayrepresent an opportunity for further exploration of hybridperovskites comprising at least one nitrogen atom, which iscommonly found in the ammonium moiety of the organiccation.Wefurthershowthatamongthehalogenatoms,79BrmaybethemostrelevanttostudylocaldistortionsanddistributionsofhalogensinmixedhalideperovskitesusinghighfieldNMR(18Tormore)orNQRspectroscopy.Besides,we demonstrate that 207Pb is highly sensitive to thenatureofthehalogenatominMAPbX3compounds,withlargedifferences in chemical shifts and linewidths. Our resultsstrongly suggest that 207PbNMR spectroscopy is an excellentprobe for lead-based hybrid perovskites and will beparticularlyuseful inthecontextof the investigationsofsolidsolutions,especiallythosebasedonamixofhalogenanions.Finally,temperature-dependantdeuteriumNMRconductedonMAPbBr3 samples proved to be efficient to investigate thestructural phase transitions, the progressive freezing of thedynamics of principle axis of the MA cations and, fromcomparison to DFT computed EFG, the remaining fastrotational dynamics of the terminal methyl and ammonium

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moieties. Although consistent with the existence of anorientational glassy behaviour at very low temperature,60more research isneeded to confirm this trend, suchas solid-stateNMRorneutron,RamanandBrillouinscatteringonbothhydrogenatedanddeuteratedsinglecrystals.

AcknowledgementsThis project has received funding from the EuropeanUnion’sHorizon 2020 research and innovation programmeunder thegrant agreement No 687008. The information and views setout in this publication are those of the authors and do notnecessarily reflect theofficialopinionof theEuropeanUnion.Neither the European Union institutions and bodies nor anypersonactingontheirbehalfmaybeheldresponsiblefortheusewhichmaybemadeoftheinformationcontainedtherein.Work at ISCR and FOTON was performed using grant fromCellule Energie du CNRS (SOLHYBTRANS Project) and theUniversity of Rennes 1 (Action Incitative, Défis ScientifiquesEmergents 2015). J. E. thanks the Fondation d’entreprisesBanque Populaire de l’Ouest for its financial support (GrantPEROPHOT 2015). Rennes Métropole and Région Betagne(FEDER) are acknowledged for funding the X-raydiffractometer.

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