Cation exchange chemistry as a route to induce noncentrosymmetry in layered perovskite oxides
<p>This work is focused on the synthesis and characterisation of n = 2 Layered Perovskite Oxides.</p> <p>KNdM2O7 (M = Ta, Nb) were synthesized by a two-step cation exchange reaction from RbNdM2O7 via NaNdM2O7. The K-phases adopt a polar Im2m structure. A competition between the re...
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| Other Authors: | |
| Format: | Thesis |
| Language: | English |
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2022
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| _version_ | 1797107498851762176 |
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| author | Mallick, S |
| author2 | Hayward, M |
| author_facet | Hayward, M Mallick, S |
| author_sort | Mallick, S |
| collection | OXFORD |
| description | <p>This work is focused on the synthesis and characterisation of n = 2 Layered Perovskite Oxides.</p>
<p>KNdM2O7 (M = Ta, Nb) were synthesized by a two-step cation exchange reaction from RbNdM2O7 via NaNdM2O7. The K-phases adopt a polar Im2m structure. A competition between the requirement of bonding optimization of the K+ and Nd3+-cation sites is demonstrated.</p>
<p>A’BiNb2O7 (A’ = Rb, Li, Na, K) were synthesized and their structures were determined. All four compounds adopt noncentrosymmetric crystal structures. Comparison with the spherical Nd-analogues demonstrates that although the Rb and Li phases adopt polar structures described in the same space group, the presence of the asymmetric Bi3+ enhances the polarity in these structurers. In the NaBiNb2O7 phase, the noncentrosymmetry is generated from the 6s2 electronic configuration of the Bi3+. The KBiNb2O7 phase exhibits frequent “axis switch” stacking faults in which the (0, ½, z) stacking changes to (½, 0, z) stacking and vice versa.</p>
<p>Members of the solid solution with composition Li2SrxCa1-xTa2O7 were synthesized and demonstrated that a structural transformation takes place from hybrid improper A21am structure to proper Pna21 structure with Ca-substitution. Some of the room temperature A21am phases in this series of compounds were shown to have a phase transition to the Pna21 structure at low temperature. Similar phase transition behaviour was observed for the Li2SrNb2O7 phase. The polarity in the Pna21 phase is driven by the SOJT effect of the 4d0-Nb5+ cation. Substitution of 4d-Nb by 5d-Ta in the Li2SrNb2O7 structure lowers the SOJT effect which lowers the stability region of the Pna21 phase.</p>
<p>The lifting of inversion symmetry in the polar structures of Li2La(TaTi)O7 and Na2La(TaTi)O7 occurs via two different mechanisms: a trilinear coupled hybrid-improper mechanism for the Li-phase and a more conventional SOJT-driven mechanism for the Na-phase. The operation of these two different mechanisms in materials with a common La(TaTi)O7 core suggests they compete against each other, and further suggests this competition is also occurring in other hybrid-improper ferroelectric phases which contain SOJT active cations.</p>
<p>MnCaTa2O7 was synthesized by a topochemical cation exchange reaction from Li2CaTa2O7. MnCaTa2O7 adopts a polar P21nm structure where Mn2+ cations are ordered in chequerboard arrangement. Mn2+ magnetic moments order antiferromagnetically below 56 K. Further cation exchange reactions were performed to synthesize MCaTa2O7 (M = Fe, Co, Zn). Successful synthesis of these phases indicates that the cation exchange chemistry could be used to incorporate divalent transition metal cations within the distorted double perovskite frameworks of n = 2 Ruddlesden-Popper phases.</p> |
| first_indexed | 2024-03-07T07:17:00Z |
| format | Thesis |
| id | oxford-uuid:8b632a9a-572e-4c51-810b-397f7ca5b33c |
| institution | University of Oxford |
| language | English |
| last_indexed | 2024-03-07T07:17:00Z |
| publishDate | 2022 |
| record_format | dspace |
| spelling | oxford-uuid:8b632a9a-572e-4c51-810b-397f7ca5b33c2022-08-19T10:43:29ZCation exchange chemistry as a route to induce noncentrosymmetry in layered perovskite oxidesThesishttp://purl.org/coar/resource_type/c_db06uuid:8b632a9a-572e-4c51-810b-397f7ca5b33cPerovskite materialsChemistry, InorganicEnglishHyrax Deposit2022Mallick, SHayward, M<p>This work is focused on the synthesis and characterisation of n = 2 Layered Perovskite Oxides.</p> <p>KNdM2O7 (M = Ta, Nb) were synthesized by a two-step cation exchange reaction from RbNdM2O7 via NaNdM2O7. The K-phases adopt a polar Im2m structure. A competition between the requirement of bonding optimization of the K+ and Nd3+-cation sites is demonstrated.</p> <p>A’BiNb2O7 (A’ = Rb, Li, Na, K) were synthesized and their structures were determined. All four compounds adopt noncentrosymmetric crystal structures. Comparison with the spherical Nd-analogues demonstrates that although the Rb and Li phases adopt polar structures described in the same space group, the presence of the asymmetric Bi3+ enhances the polarity in these structurers. In the NaBiNb2O7 phase, the noncentrosymmetry is generated from the 6s2 electronic configuration of the Bi3+. The KBiNb2O7 phase exhibits frequent “axis switch” stacking faults in which the (0, ½, z) stacking changes to (½, 0, z) stacking and vice versa.</p> <p>Members of the solid solution with composition Li2SrxCa1-xTa2O7 were synthesized and demonstrated that a structural transformation takes place from hybrid improper A21am structure to proper Pna21 structure with Ca-substitution. Some of the room temperature A21am phases in this series of compounds were shown to have a phase transition to the Pna21 structure at low temperature. Similar phase transition behaviour was observed for the Li2SrNb2O7 phase. The polarity in the Pna21 phase is driven by the SOJT effect of the 4d0-Nb5+ cation. Substitution of 4d-Nb by 5d-Ta in the Li2SrNb2O7 structure lowers the SOJT effect which lowers the stability region of the Pna21 phase.</p> <p>The lifting of inversion symmetry in the polar structures of Li2La(TaTi)O7 and Na2La(TaTi)O7 occurs via two different mechanisms: a trilinear coupled hybrid-improper mechanism for the Li-phase and a more conventional SOJT-driven mechanism for the Na-phase. The operation of these two different mechanisms in materials with a common La(TaTi)O7 core suggests they compete against each other, and further suggests this competition is also occurring in other hybrid-improper ferroelectric phases which contain SOJT active cations.</p> <p>MnCaTa2O7 was synthesized by a topochemical cation exchange reaction from Li2CaTa2O7. MnCaTa2O7 adopts a polar P21nm structure where Mn2+ cations are ordered in chequerboard arrangement. Mn2+ magnetic moments order antiferromagnetically below 56 K. Further cation exchange reactions were performed to synthesize MCaTa2O7 (M = Fe, Co, Zn). Successful synthesis of these phases indicates that the cation exchange chemistry could be used to incorporate divalent transition metal cations within the distorted double perovskite frameworks of n = 2 Ruddlesden-Popper phases.</p> |
| spellingShingle | Perovskite materials Chemistry, Inorganic Mallick, S Cation exchange chemistry as a route to induce noncentrosymmetry in layered perovskite oxides |
| title | Cation exchange chemistry as a route to induce noncentrosymmetry in layered perovskite oxides |
| title_full | Cation exchange chemistry as a route to induce noncentrosymmetry in layered perovskite oxides |
| title_fullStr | Cation exchange chemistry as a route to induce noncentrosymmetry in layered perovskite oxides |
| title_full_unstemmed | Cation exchange chemistry as a route to induce noncentrosymmetry in layered perovskite oxides |
| title_short | Cation exchange chemistry as a route to induce noncentrosymmetry in layered perovskite oxides |
| title_sort | cation exchange chemistry as a route to induce noncentrosymmetry in layered perovskite oxides |
| topic | Perovskite materials Chemistry, Inorganic |
| work_keys_str_mv | AT mallicks cationexchangechemistryasaroutetoinducenoncentrosymmetryinlayeredperovskiteoxides |