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Low-energy paths for octahedral tilting in inorganic halide perovskites
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 10, article id 104105Article in journal (Refereed) Published
Abstract [en]

Instabilities relating to cooperative octahedral tilting is common in materials with perovskite structures, in particular in the subclass of halide perovskites. In this work the energetics of octahedral tilting in the inorganic metal halide perovskites CsPbI3 and CsSnI3 are investigated using first-principles density functional theory calculations. Several low-energy paths between symmetry equivalent variants of the stable orthorhombic (Pnma) perovskite variant are identified and investigated. The results are in favor of the presence of dynamic disorder in the octahedral tilting phase transitions of inorganic halide perovskites. In particular, one specific type of path, corresponding to an out-of-phase "tilt switch," is found to have significantly lower energy barrier than the others, which indicates the existence of a temperature range where the dynamic fluctuations of the octahedra follow only this type of path. This could produce a time averaged structure corresponding to the intermediate tetragonal (P4/mbm) structure observed in experiments. Deficiencies of the commonly employed simple one-dimensional "double-well" potentials in describing the dynamics of the octahedra are pointed out and discussed.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC , 2019. Vol. 99, no 10, article id 104105
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:liu:diva-156096DOI: 10.1103/PhysRevB.99.104105ISI: 000461955500001OAI: oai:DiVA.org:liu-156096DiVA, id: diva2:1302143
Note

Funding Agencies|Swedish Research Council (VR) [2014-4750]; Centre in Nano Science and Nano Technology (CeNano) at Linkoping University

Available from: 2019-04-03 Created: 2019-04-03 Last updated: 2020-05-06
In thesis
1. A First-Principles Study of Highly Anharmonic and Dynamically Disordered Solids
Open this publication in new window or tab >>A First-Principles Study of Highly Anharmonic and Dynamically Disordered Solids
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is a first-principles theoretical investigation of solid materials with high degrees of anharmonicity. These are systems where the dynamics of the constituent atoms is too complex to be well-described by a set of uncoupled harmonic oscillators. While theoretical studies of such systems pose a significant challenge, they are under increasing demand due to the prevalence of these sytems in next-generation technological applications. Indeed, very anharmonic systems are ubiquitous in envisioned materials for future solid-state batteries and fuel-cells, thermoelectrics and optoelectronics. In some of these cases, the anharmonicity is a “side-effect” that simply has to be dealt with in order to accurately model certain properties, while in other cases the anharmonicity is the origin of the high-performance of the material.

There are two main parts to the thesis: The first is on materials with perovskite-related structures. This is a very large class of materials, members of which are typically highly anharmonic, not least in relation to a series of complex phase transformations between different structural modifications. In the thesis, I have studied a specific class of such phase-transformations that relate to tilting of the framework of octahedra that make up the structure. The oxide CaMnO3 and a set of inorganic halide perovskites were taken as model systems. The results shed some light on the experimentally observed differences between the local and average atomic structure in these systems. I have further studied Cs2AgBiBr6, a member of the so-called lead-free halide double perovskites. I rationalized its temperature induced phase transformation and found high degrees of anharmonicity and ultra-low thermal conductivity. Finally, I studied the influence of nuclear quantum effects, which are often ignored in computational modelling, on the structure and bonding in the hybrid organic-inorganic lead-halide perovskite, CH3NH3PbI3.

The second part of the thesis deals with theoretical studies of the phase stability of dynamically disordered solids. These are solids which have some sort of time-averaged long-range order, characteristic of a crystalline solid, but where the anharmonicity is so strong that the basic concept of an equilibrium atomic position cannot be statically assigned to all atoms. Examples include certain solids with very fast ionic conduction, so called superionics, and solids with rotating molecular units. This absence of equilibrium atomic positions makes many standard computational techniques to evaluate phase-stability inapplicable. I outline a method to deal with this issue, which is based on a stress-strain thermodynamic integration on a deformation path from an ordered variant to the dynamically disordered phase itself. I apply the method to study the phase stability of the high-temperature δ-phase of Bi2O3, which is the fastest know solid oxide ion conductor, and to Li2C2, whose high temperature cubic phase contains rotating C2 dimers.

The thesis exemplifies the need to go beyond many of the standard approximations used in first-principles computational materials science if accurate theoretical predictions are to be made. This is true, in particular, for many emerging material classes in the field of energy materials.

Abstract [sv]

I den konventionella teoretiska modellen för ett (kristallint) fast material antags varje atom kunna tillordnas en jämviktsposition. Rörelsen av atomerna runt dessa jämviktspositioner antags sedan ofta vara harmoniskt, d.v.s. hyfsat kunna beskrivs i termer av en samling (kvantmekaniska) fjädrar. Denna avhandling behandlar teori- och beräkningsstudier av material där ett eller båda av dessa antaganden inte är giltiga, så kallade anharmoniska material. En nogrann teoretisk behandling av sådana material är ofta ordentligt utmanande.

I takt med en snabb teknologiska utveckling ställs allt mer specifika och stränga krav på de material som behövs för diverse applikationer. Inom flertalet områden dyker då denna typ av komplexa och anharmoniska material upp som potentiella kandidater. Till exempel som fastelektrolyter för batterier och bränsleceller eller som solcellsmaterial. Inom vissa applikationer är denna anharmonicitet en bieffekt som man helt enkelt måste ta hänsyn till för att kunna göra noggranna teoretiska förutsägelser om diverse materialegenskaper, i andra fall är anharmoniciteten själva ursprunget för materialets goda egenskaper.

I den första delen av avhandlingen behandlar jag material som har, eller är nära relaterade till, den så kallade perovskitstrukturen. Detta är en väldigt stor klass av material, och strukturen dyker därför upp inom en mängd olika tillämpningar, inte minst i lovande solcellsmaterial. Dessa material är ofta mycket anharmoniska, vilket tar sig uttryck bland annat i en rad komplexa fastransformationer mellan olika typer av perovskitmodifikationer. I perovskitoxiden CaMnO3, samt i en samling halogenperovskiter, har jag har studerat en specifik typ av fastransformationer som tillkommer på grund av rotationer av de oktaedrar som utgör en del av strukturen. Jag har fortsatt studerat den väldigt kraftiga anharmoniciteten i den så kallade blyfria halogendubbelperovskiten Cs2AgBiBr6, och slutligen har jag studerat hur en kvantmekanisk behandling av atomkärnorna, något som oftast inte görs, påverkar materialegenskaper i CH3NH3PbI3, en så kallad hybrid organisk-inorganisk bly-halogenperovskit, som är ett extremt lovande solcellsmaterial.

I den andra delen av avhandlingen studerar jag dynamiskt oordnade fasta material. I dessa material är atomernas rörelse för komplex för att varje atom ska kunna tilldellas en statisk jämviktsposition. Material i denna klass är, till exempel, lovande som fastelektrolyter i bränsleceller och batterier. Mer specifikt studerar jag en typ av fasövergång, från en ordnad fas till en fas med dynamisk oordning, som ofta sker när dessa material värms upp. Jag introducerar en beräkningsmetod för att utvärdera deras fasstabilitet. Metoden är baserad på en så kallad termodynamisk integration, utförd mellan en ordnad variant och den dynamiskt oordnade fasen själv. Metoden gör det möjligt att beräkna fastransformationstemperaturer i denna typ av material. Jag applicerar metoden på Bi2O3, som i sin δ-fas är det fasta material med högst känd syrejonledningsförmåga, samt på Li2C2 vars kubiska fas innehåller roterande C2 molekyler. Resultaten stämmer bra överens med kända experimentella mätningar.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2020. p. 80
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 2072
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:liu:diva-165554 (URN)10.3384/diss.diva-165554 (DOI)9789179298555 (ISBN)
Public defence
2020-06-05, Online through Zoom (contact sergey.simak@liu.se) and Planck, F Building, Campus Valla, Linköping, 10:15 (English)
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Supervisors
Available from: 2020-05-06 Created: 2020-05-06 Last updated: 2020-05-15Bibliographically approved

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