http://nova.newcastle.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:10457 Computer-based group-theoretical methods are used to enumerate structures arising in A₂BB'X₆ perovskites, with either rock-salt or checkerboard ordering of the B and B' cations, under the additional assumption that one of these two cations is Jahn–Teller active and thereby induces a distortion of the BX₆ (or B'X₆) octahedron. The requirement to match the pattern of Jahn–Teller distortions to the cation ordering implies that the corresponding irreducible representations should be associated with the same point in the Brillouin zone. Effects of BX₆ (and B'X₆) octahedral tilting are included in the usual way. Finally, an analysis is presented of more complex models of ordering and distortion as might lead to the doubling of the long axis of the common Pnma perovskite, observed in systems such as Pr₁₋ xCaxMnO₃(x ≃ 0.5). The structural hierarchies derived in this work should prove useful in interpreting experimental results. 2012-03-21T04:20:02.226Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:8172 The evolution of the crystal structure of rhombohedral PrAlO₃ perovskite with pressure has been investigated by single-crystal x-ray diffraction and Raman scattering experiments. The structural evolution as indicated by lattice strains, octahedral tilts, and the distortions of the octahedral AlO₆ and polyhedral PrO₁₂ groups with increasing pressure, is controlled by the relative compressibilities of the AlO₆ octahedra and the PrO₁₂ site. Because the AlO₆ octahedra are more compressible than the PrO₁₂ sites, up to 7.4 GPa the structure evolves towards the high-symmetry cubic phase like any other rhombohedral perovskite. The variation of volume of the rhombohedral phase with pressure can be represented by a third-order Birch–Murnaghan equation of state with bulk modulus K₀ = 193.0(1.2) GPa and K' = 6.6(4). Above 7.4 GPa the evolution towards a cubic phase is interrupted by a phase transition. Observations are consistent with the assignment of Imma symmetry to the high-pressure phase. Comparison with the low-temperature R3̅c to Imma transition confirms that electronic interactions stabilize the Imma phase. 2011-09-13T00:40:55.522Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:1344 Group-theoretical methods are used to analyze perovskite structures where both ferroelectric cation displacements and simple tilting of octahedral units are present. This results in a list of 40 different structures, each with a unique space-group symmetry. The list is compared with that of Aleksandrov & Bartolome [Phase Transit. (2001), 74, 255–335] and a number of differences are found. The group–subgroup relationships between the structures are also determined, along with an indication of those phase transitions that must be first order by Landau theory. 2010-04-27T06:56:04.986Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:3226 The major trans (1) and minor cis (2) isomers of 1,4,8,11-tetraazacyclotetradecane-6,13-dicarboxylate have been characterized as the complexes [Co(1)](ClO₄) and [Co(H2)(OH₂)]Cl(ClO₄)·H₂O. The former crystallized in the C2/c space group and the latter in the P2₁/c space group, with cell parameters a 16.258(7), b 9.050(3), c 15.413(6) Å, β133.29(3)°, and a 9.694(4), b 16.135(1), c 12.973(5) Å, β 93.00(2)°, respectively. Their characterization completes identification of the respective trans and cis isomers for the series of c-pendant macrocycles also including 1,4,8,11-tetraazacyclotetradecane-6-amine-13-carboxylate ((3), (4)) and 1,4,8,11-tetraazacyclotetradecane-6,13-diamine ((5), (6)). The complexes show limited distortion from octahedral geometry with the strain in the presence of the coordinated c-pendant carboxylate significantly reduced compared with that for the c-pendant amine in analogues, a consequence mainly of six-membered as opposed to five-membered chelate rings involving the pendant donor. A comparison of the physical properties for the trans isomers of the octahedral complexes of (1), (3), and (5), which reflect progressively increasing strain, is presented. 2010-04-27T04:53:36.900Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:3221 The unsymmetric mixed-donor Nam,Npy,O-ligand N-trans-(2'-hydroxycyclohexyl)-2-aminomethylpyridine (pyca, 1) was readily prepared by reaction of an epoxide (cyclohexene oxide) with (2-aminomethyl)pyridine. The ligand forms octahedral bis-tridentate complexes, exemplified with isolation of copper(II), nickel(II), zinc(II), and cobalt(III) compounds; in addition, a square-planar palladium(II) complex with the ligand behaving as bidentate was prepared. The bis-tridentate octahedral complexes may exist in six geometric isomeric forms, and molecular mechanics analysis was employed to predict the most stable isomer for copper(II), nickel(II), and cobalt(III) complexes. For the copper(II) and cobalt(III) complexes, the predictions match the X-ray crystal structures of the single isomer isolated in each case. Both the copper(II) complex [Cu . 1₂](ClO₄)₂ and the cobalt(III) complex [Co . 1 . (1–H)](ClO₄)₂ crystallized in the monoclinic space group P2₁/n (no. 14), with a 9.8400(13), b 15.434(2), c 10.0180(13) Å, and β 109.004(2)°, and a 14.162, b 18.583, c 21.596 Å, and β 100.04°, respectively. Both complexes adopt distorted octahedral structures. The copper(II) complex exists as the all-trans-fac isomer with Jahn–Teller elongation of the Cu–O bond [2.3795(19) Å] compared to Cu–Npy [2.040(2) Å] and Cu–Nam [2.024(2) Å] distances. The cobalt(III) complex appears as the cis,cis,trans(Npy) fac isomer, but deprotonation of one of the two alcohol groups leads to all six M–L distances differing: for each of two similar independent complex cations, Co–O [e.g. 1.9123(16), 1.9459(16) Å], Co–Nam [e.g. 1.949(2), 1.968(2) Å], and Co–Npy [e.g. 1.938(2), 1.940(2) Å] vary, with particularly the deprotonated alcohol bond significantly shorter than the protonated bond, despite a strong hydrogen bonding interaction with the protonated alcohol in the partner complex cation in the unit cell. 2010-04-27T04:53:20.040Z ]]>