https://nova.newcastle.edu.au/vital/access/ /manager/Index en-au 5 https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:22787 Wed 11 Apr 2018 12:02:05 AEST ]]> https://nova.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://nova.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://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:51702 Fri 15 Sep 2023 13:41:02 AEST ]]>