Topochemical fluorination of some Ruddlesden-Popper phases
<p>This thesis describes the synthesis and the characterization of some n = 2 Ruddlesden-Popper oxide phases (A<sub>3</sub>B<sub>2</sub>O<sub>7</sub>; A = La, Y, Sr or Ca; B = Cr, Ni or Mn) and their topochemically fluorinated phases.</p> <p>La&l...
| Tác giả chính: | |
|---|---|
| Tác giả khác: | |
| Định dạng: | Luận văn |
| Được phát hành: |
2017
|
| _version_ | 1826317325531348992 |
|---|---|
| author | Zhang, R |
| author2 | Hayward, M |
| author_facet | Hayward, M Zhang, R |
| author_sort | Zhang, R |
| collection | OXFORD |
| description | <p>This thesis describes the synthesis and the characterization of some n = 2 Ruddlesden-Popper oxide phases (A<sub>3</sub>B<sub>2</sub>O<sub>7</sub>; A = La, Y, Sr or Ca; B = Cr, Ni or Mn) and their topochemically fluorinated phases.</p> <p>La<sub>2</sub>SrCr<sub>2</sub>O<sub>7</sub> adopts an <em>A2/a</em> (a<sup>-</sup>a<sup>-</sup>c<sup>-</sup>/a<sup>-</sup>a<sup>-</sup>c<sup>-</sup>) distorted structure with antiferromagnetic order below TN ~ 260 K. DFT calculations performed on La2SrCr2O7 suggest that this highly distorted A2/a (a<sup>-</sup>a<sup>-</sup>c<sup>-</sup>/a<sup>-</sup>a<sup>-</sup>c<sup>-</sup>) structure is stabilized by energy gains from the charge equalization and the improvement of chromium bonding environments associated with the unusual Sr/La distribution exhibited by this phase (the 12-coordinate A-sites are occupied by 82(2)% La<sup>3+</sup> and 18(2)% Sr<sup>2+</sup>).</p> <p>Topochemical fluorination of La<sub>2</sub>SrCr<sub>2</sub>O<sub>7</sub> (Cr3+) yields La<sub>2</sub>SrCr<sub>2</sub>O<sub>7</sub>F<sub>2</sub> (Cr4+) adopting the A2/a (a<sup>-</sup>a<sup>-</sup>c<sup>-</sup>/a<sup>-</sup>a<sup>-</sup>c<sup>-</sup>) distortion with antiferromagnetic order below T<sub>N</sub> ~ 140 K. The fluorination leads a great compression of the perovskite layer within the Ruddlesden-Popper structure, enhancing the (a<sup>-</sup>a<sup>-</sup>c<sup>-</sup>/a<sup>-</sup>a<sup>-</sup>c<sup>-</sup>) distortion. As a result La<sub>2</sub>SrCr<sub>2</sub>O<sub>7</sub>F<sub>2</sub> lies further from the crossover for insulating to metallic behaviour than many other Cr<sup>4+</sup> oxide phases.</p> <p>The A-site doping in La<sub>2</sub>SrCr<sub>2</sub>O<sub>7</sub> for the preparation of La<sub>2-x</sub>Sr<sub>1+x</sub>Cr<sub>2</sub>O<sub>7</sub> (x = 0.2, 0.3) and La<sub>2</sub>Sr<sub>0.5</sub>Ca<sub>0.5</sub>Cr<sub>2</sub>O<sub>7</sub> leads to no obvious change in structural framework and the antiferromagnetic behaviour. La<sub>2-x</sub>Sr<sub>1+x</sub>Cr<sub>2</sub>O<sub>7</sub>F<sub>2</sub> (x = 0.2, 0.3) behaves similarly to La<sub>2</sub>SrCr<sub>2</sub>O<sub>7</sub>F<sub>2</sub>. </p> <p>Topochemical fluorination of La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub> yields La<sub>3</sub>Ni<sub>2</sub>O<sub>5.5</sub>F<sub>3.5</sub> with no obvious magnetic order. The fluorination leads to an enhancement of the structural distortion exhibited by the oxide starting material, driving a change from an <em>Amam</em>, a<sup>-</sup>a<sup>-</sup>c<sup>0</sup>/a<sup>-</sup>a<sup>-</sup>c<sup>0</sup> lattice distortion to a Pnam a-a-c+/a-a-(c+) lattice distortion in the fluorinated phase. The introduction of a cooperative octahedral tilt around the crystallographic z-axis leads to a local breaking of the inversion symmetry of the host lattice on fluorination, although the global inversion symmetry of the lattice is retained due to an alternation in the z-tilt direction in adjacent perovskite sheets.</p> <p>The substitution of Sr<sup>2+</sup> with Ca<sup>2+</sup> cations in YSr<sub>2</sub>Mn<sub>2</sub>O<sub>7</sub> led to an increasing distortion of the metal-oxide framework, resulting in the formation of an orthorhombic symmetry in space group <em>Pnnm</em> adopted by YSr<sub>0.5</sub>Ca<sub>1.5</sub>Mn<sub>2</sub>O<sub>7</sub>. This novel <em>Pnnm</em> (a-b-c0/b-a-c0, a ≠ b) distortion can be seen as an intermediate in structural change from <em>P4<sub>2</sub>/mnm</em> (a-b0c0/b0a-c0) to <em>Amam</em> (a<sup>-</sup>a<sup>-</sup>c<sup>0</sup>/a<sup>-</sup>a<sup>-</sup>c<sup>0</sup>).</p> <p>The topochemical fluorination of YSr<sub>2</sub>Mn<sub>2</sub>O<sub>7</sub> yields YSr<sub>2</sub>Mn<sub>2</sub>O<sub>5.5</sub>F<sub>3.5</sub> with <em>P4<sub>2</sub>/mnm</em> (a-b0c0/b0a-c0) distortion. The fluorination YSr2Mn2O7 does not lead to a structural change as observed in that of La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub>. This preference of retaining relatively high symmetry is driven by the A-cation ordering over the 9- and 12-coordinate A-sites in YSr<sub>2</sub>Mn<sub>2</sub>O<sub>5.5</sub>F<sub>3.5</sub>.</p> |
| first_indexed | 2024-03-07T02:30:59Z |
| format | Thesis |
| id | oxford-uuid:a739b00c-9f83-4605-b9a4-cf276f3ad187 |
| institution | University of Oxford |
| last_indexed | 2025-02-19T04:38:16Z |
| publishDate | 2017 |
| record_format | dspace |
| spelling | oxford-uuid:a739b00c-9f83-4605-b9a4-cf276f3ad1872025-02-06T08:10:23ZTopochemical fluorination of some Ruddlesden-Popper phasesThesishttp://purl.org/coar/resource_type/c_db06uuid:a739b00c-9f83-4605-b9a4-cf276f3ad187ORA Deposit2017Zhang, RHayward, M<p>This thesis describes the synthesis and the characterization of some n = 2 Ruddlesden-Popper oxide phases (A<sub>3</sub>B<sub>2</sub>O<sub>7</sub>; A = La, Y, Sr or Ca; B = Cr, Ni or Mn) and their topochemically fluorinated phases.</p> <p>La<sub>2</sub>SrCr<sub>2</sub>O<sub>7</sub> adopts an <em>A2/a</em> (a<sup>-</sup>a<sup>-</sup>c<sup>-</sup>/a<sup>-</sup>a<sup>-</sup>c<sup>-</sup>) distorted structure with antiferromagnetic order below TN ~ 260 K. DFT calculations performed on La2SrCr2O7 suggest that this highly distorted A2/a (a<sup>-</sup>a<sup>-</sup>c<sup>-</sup>/a<sup>-</sup>a<sup>-</sup>c<sup>-</sup>) structure is stabilized by energy gains from the charge equalization and the improvement of chromium bonding environments associated with the unusual Sr/La distribution exhibited by this phase (the 12-coordinate A-sites are occupied by 82(2)% La<sup>3+</sup> and 18(2)% Sr<sup>2+</sup>).</p> <p>Topochemical fluorination of La<sub>2</sub>SrCr<sub>2</sub>O<sub>7</sub> (Cr3+) yields La<sub>2</sub>SrCr<sub>2</sub>O<sub>7</sub>F<sub>2</sub> (Cr4+) adopting the A2/a (a<sup>-</sup>a<sup>-</sup>c<sup>-</sup>/a<sup>-</sup>a<sup>-</sup>c<sup>-</sup>) distortion with antiferromagnetic order below T<sub>N</sub> ~ 140 K. The fluorination leads a great compression of the perovskite layer within the Ruddlesden-Popper structure, enhancing the (a<sup>-</sup>a<sup>-</sup>c<sup>-</sup>/a<sup>-</sup>a<sup>-</sup>c<sup>-</sup>) distortion. As a result La<sub>2</sub>SrCr<sub>2</sub>O<sub>7</sub>F<sub>2</sub> lies further from the crossover for insulating to metallic behaviour than many other Cr<sup>4+</sup> oxide phases.</p> <p>The A-site doping in La<sub>2</sub>SrCr<sub>2</sub>O<sub>7</sub> for the preparation of La<sub>2-x</sub>Sr<sub>1+x</sub>Cr<sub>2</sub>O<sub>7</sub> (x = 0.2, 0.3) and La<sub>2</sub>Sr<sub>0.5</sub>Ca<sub>0.5</sub>Cr<sub>2</sub>O<sub>7</sub> leads to no obvious change in structural framework and the antiferromagnetic behaviour. La<sub>2-x</sub>Sr<sub>1+x</sub>Cr<sub>2</sub>O<sub>7</sub>F<sub>2</sub> (x = 0.2, 0.3) behaves similarly to La<sub>2</sub>SrCr<sub>2</sub>O<sub>7</sub>F<sub>2</sub>. </p> <p>Topochemical fluorination of La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub> yields La<sub>3</sub>Ni<sub>2</sub>O<sub>5.5</sub>F<sub>3.5</sub> with no obvious magnetic order. The fluorination leads to an enhancement of the structural distortion exhibited by the oxide starting material, driving a change from an <em>Amam</em>, a<sup>-</sup>a<sup>-</sup>c<sup>0</sup>/a<sup>-</sup>a<sup>-</sup>c<sup>0</sup> lattice distortion to a Pnam a-a-c+/a-a-(c+) lattice distortion in the fluorinated phase. The introduction of a cooperative octahedral tilt around the crystallographic z-axis leads to a local breaking of the inversion symmetry of the host lattice on fluorination, although the global inversion symmetry of the lattice is retained due to an alternation in the z-tilt direction in adjacent perovskite sheets.</p> <p>The substitution of Sr<sup>2+</sup> with Ca<sup>2+</sup> cations in YSr<sub>2</sub>Mn<sub>2</sub>O<sub>7</sub> led to an increasing distortion of the metal-oxide framework, resulting in the formation of an orthorhombic symmetry in space group <em>Pnnm</em> adopted by YSr<sub>0.5</sub>Ca<sub>1.5</sub>Mn<sub>2</sub>O<sub>7</sub>. This novel <em>Pnnm</em> (a-b-c0/b-a-c0, a ≠ b) distortion can be seen as an intermediate in structural change from <em>P4<sub>2</sub>/mnm</em> (a-b0c0/b0a-c0) to <em>Amam</em> (a<sup>-</sup>a<sup>-</sup>c<sup>0</sup>/a<sup>-</sup>a<sup>-</sup>c<sup>0</sup>).</p> <p>The topochemical fluorination of YSr<sub>2</sub>Mn<sub>2</sub>O<sub>7</sub> yields YSr<sub>2</sub>Mn<sub>2</sub>O<sub>5.5</sub>F<sub>3.5</sub> with <em>P4<sub>2</sub>/mnm</em> (a-b0c0/b0a-c0) distortion. The fluorination YSr2Mn2O7 does not lead to a structural change as observed in that of La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub>. This preference of retaining relatively high symmetry is driven by the A-cation ordering over the 9- and 12-coordinate A-sites in YSr<sub>2</sub>Mn<sub>2</sub>O<sub>5.5</sub>F<sub>3.5</sub>.</p> |
| spellingShingle | Zhang, R Topochemical fluorination of some Ruddlesden-Popper phases |
| title | Topochemical fluorination of some Ruddlesden-Popper phases |
| title_full | Topochemical fluorination of some Ruddlesden-Popper phases |
| title_fullStr | Topochemical fluorination of some Ruddlesden-Popper phases |
| title_full_unstemmed | Topochemical fluorination of some Ruddlesden-Popper phases |
| title_short | Topochemical fluorination of some Ruddlesden-Popper phases |
| title_sort | topochemical fluorination of some ruddlesden popper phases |
| work_keys_str_mv | AT zhangr topochemicalfluorinationofsomeruddlesdenpopperphases |