Thermal expansion of pyrolusite, β-MnO2; a synchrotron X-ray diffraction study 2019 Brendan J. Kennedy.pdf

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JournalofPhysicsandChemistryofSolids
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Thermalexpansionofpyrolusite,β-MnO2;asynchrotronX-raydiffraction
T
study

SchoolofChemistry,TheUniversityofSydney,Sydney,NSW,2006,Australia
ARTICLEINFOABSTRACT
Keywords:HighresolutionsynchrotronX-raypowderdiffraction(S-XRD)methodshavebeenusedtostudythetemperature
Thermalexpansiondependenceofthestructureofβ-MnO2betweenroomtemperatureand400°-XRD
Pyrolusitedataprovidedaccurateandpreciseuntilcellparametersfromwhichthethermalexpansioncoefficients(TEC)
Rutile−6−1−6−1.
weredetermined,αa=×10Kandαc=×10KThedifferenceintheanisotropyintheTEC
Manganesedioxide
andthetemperaturedependenceofthedistortionofMnO6andTiO2suggestsMneMninteractionsacrossthe
sharededgearesignificant.
-sensitizedsolarcells[11].Evidentlyestablishingthe
temperaturedependenceofthestructureofrutiletypeoxidessuchasβ
Manganesedioxide(MnO2)existsinseveralcrystallographicforms,--MnO2
α,β,γ,δandistheactivecomponentinanumberoftechnicallyandappearstobetheworkofDoseandDonnewhoemployedconventional
economicallyimportantmaterials,includingbatteryelectrodes[1],X-raydiffractionmethods[12].Thepresentworkhasexploitedthe
supercapacitors[2],catalysis[3]andmolecularsieves[4].Thisdi-accuracyandprecisionprovidedbyRietveldanalysisofsynchrotron
versityofapplicationsreflectsthedifferenttopologiesadoptedbythepowderdiffractiondata,whichhasyieldedprecisioninthestructural
manganesecationsinthevariousformsthatvaryfrom1Dtunnelsparametersforβ-MnO2morethananorderofmagnitudebetterthan
-thosereportedbyDoseandDonne[12].Theseresultsarecompared
ties,andhenceapplications,ofMnO2arelargelydependentonthewiththoseofpublishedstudiesofthethermalexpansionbehaviourof
crystalstructureconvolutedwithfactorsincludingthesizeandmor-TiO2.
phologyoftheparticles.
Pyrolusiteorβ-
dioxidepolymorph[5].Thestructureofβ-MnO2wasfirstdetermined
byStrunzin1943[6],andsubsequentlyBaurreportedasingle-crystalApolycrystallinesampleofβ-MnO2waspreparedbythedecom-
X-raydiffractionstudy[7].In1992Bolzanetal.[8]usedRietveldpositionofMn(NO3)2at390°-raydiffrac-
analysisofpowderneutrondiffractiondatatoprovideapreciseandtionpatternrevealedtheformationofahighlycrystallinerutile-type
accuratestructuralmodelforβ--MnO2phase,
hasatetragonalstructureandisisostructuralwithrutile[9].-raydif-
structureconsistsofchainsofedge-sharingMnO6octahedrathatsharefractionpatternswerecollectedonthehighresolutionpowderdif-
cornerswithneighbouringchainstoformaframeworkstructurecon-fractometeratbeamline10oftheAustralianSynchrotron[13].The
taining1--(111)mono-
Manyoftheapplicationsinvolvingrutiletypeoxidesoccurabovechromatorsandtheprecisewavelengthwasestablishedtobe
ambienttemperature,forexampleTi1-2xCrxNbxO2[10](2)Å
demonstratedtodisplaystructuralandredoxstability,makingitausing16Mythen-II,microstripdetectormodules,covering∼80°in2θ.
candidateforuseathightemperaturesinsolidoxidefuelcellsandTiO2Datawerecollectedfor300sateachoftwodetectorpositionstoavoid
E-mailaddress:brendan.******@.
/
Received15August2018;Receivedinrevisedform18September2018;Accepted20September2018
Availableonline10October2018
0022-3697/©.
(2019)131–134
−6−1−6−1
anisotropywithαc=×10K<αa=×-
luesoftheTECaresimilartothatreportedbyDoseandDonne[12]
−6−1−6−1
αc=×10Kandαa=×10K,intheirlowerresolution
.[16],whousedsynchrotronX-ray
diffractiontostudythethermalexpansionofTiO2,reportedαc>αa,
×10−×10−6K−
theunitcellparametershowsslightdeviationfromlinearityandthiswas
V=V+Vcoths
fittedusingtheexpression012(T),whereθsisthesaturation
temperatureforthethermalexpansion[17].Ofmoreinterestisthe
thermalevolutionintheratiooftheunitcellparametersc/
(c)thec/aratioinitiallydecreases,reachingaminimumvalue
near250°.
Sincethemagnitudeofthec-axiscorrespondstotheshortestMneMn
separationacrossthesharededgethissuggestsachangeintheMneMn
interactionsmaybeoccurring.
TheRietveldrefinementsagainsttheS-XRDprofilesyieldedprecise
valuesforthevariablepositionalparameteroftheoxygenatomsatthe
4f(x,x,0)(d)thisisweaklytemperature
,calculatedanddifferenceRietveldprofilesfor2MnOatroomdependent,correspondingtoasmallincreaseinthelengthofthetwo
temperatureobtainedusingsynchrotronpowderdiffraction(λ=(2)
Å).MneObondsisessentiallyindependentoftemperature,presumablyto
retainoptimalbondingfortheMncations.
TherearetwoshortOeOdistancesintherutilestructure,namely
Table1acrossthesharededgeandalongthec-
Refinedstructuralparametersfor2MnOatroomtemperaturerefinedinspaceshorttominimisetheMneMnrepulsionacrossthesharededge.
groupP42/=(4),SugiyamaandTakeuchiintroducedtheconceptoftheshrinkageindex
c=(27)Åvolume=(9)Å-l
definedasRs=l0wherelisthelengthofthesharededgeataparti-
.
culartemperatureandl0istheedgelengthofaregularoctahedron
2
NameSitexyzUi/Ue*100(Å)havingthesamevolumeastheMnO6octahedronatthesametem-
perature[18].AsfoundbySugiyamaandTakeuchiforTiO2,thedis-
*
tanceacrossthesharededgeinMnO2isobservedtoincreaseasthe
(13)(13)(17)
U11U22U33U12U13=U23temperatureisincreased,howeverwhereastheshrinkageindexwas
(7)(7)(12)−(19)-
[18]suggestedthatinTiO2
thereductionintheshrinkageindex(Rs)withincreasingtemperature
actstominimisetheelectrostaticrepulsionofthetwoTicationsacross

achievedusinganOxfordCyberstarhot-.
wasrampedfrom25to400°Cat5°/minanddiffractionpatternswereInβ-MnO2Rsactuallyincreaseswithincreasingtemperatureimplying
measuredevery15°±3°thattheinteractionsbetweentheMncationsacrossthesharededge
below100°Candbetterthan1°-MnO2isa
usingtheGSAS/EXPGUIsuiteofprogramsutilisingpeakshape4tonarrow(∼)bandgapsemiconductoranditispossiblethatin-
accountforasmallamountofmicrostrainrevealedduringtherefine-creasingtemperatureactstoincreasefavourableoverlapofthepartially
ments[14,15].filledd-bandsandthatthisattractiveinteractionoutweighsthether-

issignificantlylarger(∼3eV)andsincethed-bandsareunoccupied
[19].
Inconclusionthetemperaturedependenceofthestructureofsyn-
-raydiffractionprofile,andtheticPyrolusite,β-MnO2,hasbeenestablishedusinghighresolution
thecorrespondingRietveldfit,forapolycrystallinesampleof2β-MnOSynchrotronX-
refinedinthetetragonalspacegroupP42/-MnO2exhibitsanisotropicthermalexpansionofthetetragonalunit
°
valuesareingoodagreementwiththeresultsoftheearlierstudiesoftheoxygenatomsatthe4f(x,x,0)sitesincreaseslinearlywithtem-
Bolzanetal.[8]andBaur[7]asisthevalueofthesinglevariableperature,resultinginareductionintheshrinkageindex,-
positionalparameter,thatoftheoxygenatomsatthe4f(x,x,0)sitestulatedthatthechangeintheoccupancyofthed-bandscoupledwith
wherex=(13)-Minteractionsalongthec-axisisresponsiblefor
ThetemperaturedependenceoftheunitcellparametersandunitcellthereversalofboththeanisotropyoftheTECbetweenTiO2andβ-
.

(a400a25)
i=
coefficients(TEC)between25and400°C,definedasa25Twhererutiletypedioxideswithdifferingoccupanciesofthed-bands.
iistheunitcelldirectionandΔTthechangeintemperature,revealslight
132
(2019)131–134
(a)theunitcellparameters(b)unitcellvolume(c)thec/aratio(d)thepositionalparameteroftheoxygenatom,(e)the
MneObonddistancesand(f)theO-Odistanceacrossthesharededge,andtheratioofthistotheidealO-
V=V+Vcoths
(T),whereθsisthesaturationtemperatureforthethermal
expansion.
Acknowledgmentselectrochemicalcapacitanceproperties,(11)(2008)
4406–4417.
[3],Manganeseoxideminerals:crystalstructuresandeconomicanden-
Thisworkwas,inpart,performedatthepowderdifractionbeam-vironmentalsignifcance,(7)(1999)3447–3454.
lineattheAustralianSynchrotronwiththeassistanceofDrQinfenGu.[4],,,-Mower,,,
,,Hydrothermalsynthesisofstructure-andshape-controlled
,
(9)(2006)1247–1253.
[5],,Thermodynamicpropertiesofmanganeseoxides,.
(7)(1996)1761–1768.
[6],Articleonthepyrolusitproblem,Naturwissenschaften31(1943)89–91.
Supplementarydatarelatedtothisarticlecanbefoundathttps://[7],Rutile-,Acta
/-(JUL15)(1976)2200–2204.
[8],,,,Powderneutron-difractionstudy
ofpyrolusite,Β-MnO2,(6)(1993)939–944.
References[9],,,,Structuralstudiesofrutile-type
metaldioxides,-(1997)373–380.
[10],-Vazquez,,,Structure,con-
[1],,Structuralandelectrochemicalpropertiesoftheprotonductivity,andthermalexpansionstudiesofredoxstablerutileniobiumchromium
gamma-MnO2system,(1)(1995)1–,(15)(2009)
[2],,EfectofcrystallographicstructureofMnO2onits
133
(2019)131–134
3549–3561.[16],,,Thermalexpansionofanataseandrutile
[11],,,Comparisonofdye-sensitizedrutile-andbetween300and575KusingsynchrotronpowderX-raydifraction,PowderDifr.
anatase-basedTiO2solarcells,(38)(2000)8989–(4)(2012)352–357.
[12],,Thermalexpansionofmanganesedioxideusinghigh-[17],,,,,,
temperatureinsituX-raydifraction,(2013)1283–