https://ogma.newcastle.edu.au/vital/access/ /manager/Index en-au 5 https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:5726 Wed 24 Jul 2013 22:56:18 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:28624 Pm̅3m →(T_c₃ = 185 K) → phase II, I4/mcm →(T_c₂ =T_N = 87 K) → phase III, antiferromagnetic, Cmcm → (T_c₃ = 82 K) → phase IV, canted ferromagnet, Pnma. It is concluded that observed changes in the elastic properties can be explained simply in terms of strain/order parameter coupling for the octahedral tilting transitions. There appears to be no evidence in the present data or in data from the literature for coupling between the magnetic order parameter and shear strains. Any coupling between the magnetic and structural transitions is therefore weak, probably occurring only biquadratically through a small common volume strain. The combined data show unambiguously that, for the crystal used, the Néel point and the structural transition at 87 K are coincident. In other crystals, with slightly different stoichiometries and defect contents, this need not be the case, however, and the overlap of transition temperatures in KMnF₃ is essentially accidental. Strong acoustic dissipation at ∼0.1–1 MHz in the stability field of phase II is attributed to the local mobility of transformation twin walls under externally applied stress. A Debye-like loss peak near 130 K is attributed to pinning of at least some twin walls by defects, but relatively high levels of acoustic dissipation below this freezing temperature imply that some of the twin walls remain mobile due to weak pinning or the absence of any pinning. Acoustic losses continue in the stability field of phase III (Cmcm) but diminish substantially in the stability field of phase IV (Pnma), implying quite different twin mobilities in the different structure types. Overlap of the structural and magnetic instabilities in KMnF₃ opens up possibilities for manipulation of ferroelastic twinning by application of a magnetic field and for creation of materials in which the ferroelastic twin walls have quite different magnetic properties from the matrix in which they lie.]]> Wed 11 Apr 2018 15:47:04 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:53732 Wed 10 Jan 2024 11:31:02 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:43114 Tue 13 Sep 2022 14:04:23 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:17700 2O₃ (corundum). The analysis confirms a recent suggestion that, contrary to the long-accepted view, the sign of s₁₄ should be negative. It also indicates that the magnitude of s₁₃ should be adjusted from -0.38 x 10^-12 to -0.47 x 10^-12 Pa^-1. It is found that, micromechanically, the polycrystal responds to stress in a manner very close to the Reuss limit. The results confirm the applicability of the diffraction method, which could prove useful when other techniques give ambiguous results.]]> Thu 29 Aug 2024 11:15:49 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:43310 Thu 15 Sep 2022 14:01:32 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:35735 I lattice. The electronic and soft-mode order parameters have multiple components and are coupled in a linear–quadratic manner as λq_Γq²_Σ. As well as providing comprehensive tables setting out the most important group–subgroup relationships and the order parameters which are responsible for them, examples of NiTi, RuNb, Ti₅₀Ni₅₀−_xFe_x, Ni_2+xMn_1−xGa and Ti₅₀Pd_50−xCr_x are used to illustrate practical relevance of the overall approach. Variations of the elastic constants of these materials can be used to determine which of the multiple order parameters is primarily responsible for the phase transitions that they undergo.]]> Thu 14 Nov 2019 08:59:40 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:8247 Sat 24 Mar 2018 08:40:38 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:8173 Sat 24 Mar 2018 08:36:07 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:8174 Sat 24 Mar 2018 08:36:06 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:8172 Sat 24 Mar 2018 08:36:05 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:11427 ∝(T_c-T) with T_c=237±2K. Octahedral tilting at high temperatures is closely similar to tilting in the Pnma structure of other perovskites, such as SrZrO₃. This is accompanied by a degree of Jahn-Teller ordering on the basis of the M⁺₂ scheme below ~775 K but is replaced by the Γ⁺₃ scheme below T_c. In contrast with the tilting and Jahn-Teller effects, magnetic ordering at the Néel temperature (~180 K) is accompanied by only the slightest volume strain and is not likely to influence the evolution of the other order parameters to any significant extent, therefore. An additional change in the volume strain below ~85 K is perhaps related to changes in magnetic structure at lower temperatures. Line broadening in powder diffraction patterns collected in the temperature interval ~150–260 K appears to be related to the presence of ferroelastic twins arising from octahedral tilting and draws attention to the fact that the Pnma↔IC transition takes place in a material which already contains heterogeneities. Finally, correlation of the repeat distance of the IC structure with Γ⁺₃ distortions of MnO₆ octahedra shows that the nature of the IC structure itself is also determined essentially by geometrical factors and strain.]]> Sat 24 Mar 2018 08:14:41 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:10457 Sat 24 Mar 2018 08:09:13 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:11119 Sat 24 Mar 2018 08:07:48 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:17484 Sat 24 Mar 2018 08:04:11 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:21134 Sat 24 Mar 2018 07:53:52 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:19734 P2₁∣n, and a further transition associated with antiferromagnetic ordering at ⁓8.5 K. The main structural changes, through the phase transition, are orientational ordering of the azetidium groups and associated changes in hydrogen bonding. In marked contrast to conventional improper ferroelastic oxide perovskites, the driving mechanism is associated with the X-point of the cubic Brillouin zone rather than being driven by R- and M-point octahedral tilting. The total ferroelastic shear strain of up to ⁓5% is substantially greater than found for typical oxide perovskites, and highlights the potential of the flexible framework to undergo large relaxations in response to local structural changes. Measurements of elastic and anelastic properties by resonant ultrasound spectroscopy show some of the characteristic features of ferroelastic materials. In particular, acoustic dissipation below the transition point can be understood in terms of mobility of twin walls under the influence of external stress with relaxation times on the order of ⁓10⁻⁷s. Elastic softening as the transition is approached from above is interpreted in terms of coupling between acoustic modes and dynamic local ordering of the azetidium groups. Subsequent stiffening with further temperature reduction is interpreted in terms of classical strain–order parameter coupling at an improper ferroelastic transition which is close to being tricritical. By way of contrast, there are no overt changes in elastic or anelastic properties near 9 K, implying that any coupling of the antiferromagnetic order parameter with strain is weak or negligible.]]> Sat 24 Mar 2018 07:53:46 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:19730 T_N = 946 ± 1 K is an example of a multiferroic transition which has both ferromagnetic (from canting of antiferromagnetically ordered spin moments) and ferroelastic (rhombohedral → monoclinic) character. By analogy with the improper ferroelastic transition in Pb₃(PO₄)₂, W and W' ferroelastic twin walls which are also 60° and 120° magnetic domain walls should develop. These have been tentatively identified from microstructures reported in the literature. The very low attenuation in the stability field of the C2/c structure in the polycrystalline sample used in the present study, in comparison with the strong acoustic dissipation reported for single crystal samples, implies, however, that the individual grains each consist of a single ferroelastic domain or that the twin walls are strongly pinned by grain boundaries. This absence of attenuation allows an intrinsic loss mechanism associated with the transition point to be seen and interpreted in terms of local coupling of shear strains with fluctuations which have relaxation times in the vicinity of ~10⁻⁸s. The first order [formula could not be replicated] (Morin) transition occurs through a temperature interval of coexisting phases but the absence of an acoustic loss peak suggests that the relaxation time for interface motion is short in comparison with the time scale of the applied stress (at ~0.1–1 MHz). Below the Morin transition a pattern of attenuation which resembles that seen below ferroelastic transitions has been found, even though the ideal low temperature structure cannot contain ferroelastic twins. This loss behavior is tentatively ascribed to the presence of local ferromagnetically ordered defect regions which are coupled locally to shear strains.]]> Sat 24 Mar 2018 07:53:45 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:27940 ISOTROPY. For HTB, there is one obvious tilting pattern, leading to a structure in space group P6₃/mmc. This differs from the space group P6₃/mcm frequently quoted from X-ray studies - these studies have in effect detected only displacements of the W cations from the centres of the WO₆ octahedra. The correct space group, taking account of both W ion displacement and the octahedral tilting, is P6₃22 - structures in this space group and matching this description have been reported. A second acceptable tilting pattern has been found, leading to a structure in P6/mmm but on a larger '2 x 2 x 2' unit cell - however, no observations of this structure have been reported. For TTB, a search at the special points of the Brillouin zones revealed only one comparable tilting pattern, in a structure with space-group symmetry I4/m on a '2^1/2 x 2^1/2 by 2' unit cell. Given several literature reports of larger unit cells for TTB, we conducted a limited search along the lines of symmetry and found structures with acceptable tilt patterns in Bbmm on a '2^1/22 x 2^1/2 x 2' unit cell. A non-centrosymmetric version has been reported in niobates, in Bbm2 on the same unit cell.]]> Sat 24 Mar 2018 07:36:10 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:27148 Sat 24 Mar 2018 07:32:59 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:29526 4)₂SrFe(CN)₆·2H₂O. The forbidden tilt pattern we observe arises through coupling to hydration-driven Jahn–Teller-like distortions of the Sr coordination environment. Access to novel distortion mechanisms and the ability to switch these distortions on and off through chemical modification fundamentally expands the toolbox of techniques available for engineering symmetry-breaking processes in solid materials.]]> Sat 24 Mar 2018 07:32:29 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:26919 Sat 24 Mar 2018 07:23:34 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:23365 1-xO, at the Néel temperature, T _N∼195K, is treated in terms of coupling between two macroscopic order parameters related to separate discrete instabilities. These each couple with a rhombohedral shear strain to give an indirect mechanism of linear (structural)quadratic (magnetic) coupling between them. Based on patterns of lattice parameter and magnetic ordering data from the literature, it appears that the overall behaviour matches the general pattern of linear-quadratic coupling for a system with two rather similar instability temperatures. At low pressures, the magnetic instability occurs at a higher temperature than the structural instability but the coupling results in a single phase transition dominated by the influence of the magnetic order parameter. For Fe _0.99O the magnetic order parameter, m, varies with temperature as ∼m ⁸σ(T _NT), while for Fe _0.94O the temperature dependence is of ∼m ⁴ or ∼m ². It is proposed that, with increasing pressure, there is a crossover of instability temperatures such that the structural instability occurs first at pressures above ∼13GPa. This would be expected to give rise to a first-order phase transition, as appears to occur, but magnetic ordering would still occur simultaneously if the coupling is sufficiently strong. Symmetry analysis and comparison with the behaviour of MnO shows that there are a number of different possible magnetically ordered structures which could be stabilized by pressure or non-hydrostatic stress to give topologically rich phase diagrams.]]> Sat 24 Mar 2018 07:16:30 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:23763 2As₂ with X = Ca, Sr, Ba or Eu, undergo one or more phase transitions from a higher-temperature paramagnetic tetragonal structure in grey group I4/mmm1' to an antiferromagnetic structure with magnetic space group C_Amca. Symmetry analysis is used to enumerate the possibilities for the transition (or transitions) between the higher- and lower-symmetry structures, and the results are used as a basis for comment on published experimental results.]]> Sat 24 Mar 2018 07:11:09 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:54762 Mon 11 Mar 2024 15:00:44 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:48095 Fri 24 Feb 2023 15:23:53 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:43387 Fri 16 Sep 2022 09:30:32 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:51864 Fri 10 Nov 2023 15:39:44 AEDT ]]>