Christopher J. Howard

University of Newcastle, Newcastle, New South Wales, Australia

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Publications (75)150.45 Total impact

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    ABSTRACT: Elastic properties and acoustic dissipation associated with the disorder-order ferroelectric transition in a single crystal metal-organic framework (MOF), [NH4][Zn(HCOO)3], have been investigated using resonant ultrasound spectroscopy (RUS) in the temperature range between 10 K and 300 K. The paraelectric to ferroelectric transition at around 192 K is triggered by the disorder-order transition of ammonium cations within the structure and changes of hydrogen bonding, accompanied by a structural phase transition from the non-polar hexagonal space group P6322 to the polar hexagonal space group P63. The elastic moduli, which are proportional to the square of resonant frequencies, gradually decrease with increasing temperature, and the rate of decrease changes markedly near the transition temperature. The acoustic dissipation tends to gradually increase with increasing temperature, but with a peak near the transition point. This pattern of behaviour can be understood by analogy with phase transitions driven by hydrogen bonding coupled to lattice strain in the mineral lawsonite [CaAl2Si2O7(OH)2•H2O]. The newly formed hydrogen bonds act as braces to stiffen the structure, as opposed to the elastic softening which typically occurs at displacive phase transitions. The acoustic loss mechanism involves dynamical disordering of hydrogen atoms between symmetry related positions and coupling of their motion with local lattice strain.
    CrystEngComm 09/2014; · 3.88 Impact Factor
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    ABSTRACT: Laboratory powder XRD patterns of the perovskite-group mineral lueshite from the type locality (Lueshe, Kivu, DRC) and pure NaNbO3 demonstrate that lueshite does not adopt the same space group (Pbma; #57) as the synthetic compound. The crystal structures of lueshite (2 samples) from Lueshe, Mont Saint-Hilaire (Quebec, Canada) and Sallanlatvi (Kola, Russia) have been determined by single-crystal CCD X-ray diffraction. These room temperature X-ray data for all single-crystal samples can be satisfactorily refined in the orthorhombic space group Pbnm (#62). Cell dimensions, atomic coordinates of the atoms, bond lengths and octahedron tilt angles are given for four crystals. Conventional neutron diffraction patterns for Lueshe lueshite recorded over the temperature range 11-1,000 K confirm that lueshite does not adopt space group Pbma within these temperatures. Neutron diffraction indicates no phase changes on cooling from room temperature to 11 K. None of these neutron diffraction data give satisfactorily refinements but suggest that this is the space group Pbnm. Time-of-flight neutron diffraction patterns for Lueshe lueshite recorded from room temperature to 700 °C demonstrate phase transitions above 550 °C from Cmcm through P4/mbm to Pmoverline{3} m above 650 °C. Cell dimensions and atomic coordinates of the atoms are given for the three high-temperature phases. The room temperature to 400 °C structures cannot be satisfactorily resolved, and it is suggested that the lueshite at room temperature consists of domains of pinned metastable phases with orthorhombic and/or monoclinic structures. However, the sequence of high-temperature phase transitions observed is similar to those determined for synthetic NaTaO3, suggesting that the equilibrated room temperature structure of lueshite is orthorhombic Pbnm.
    01/2014;
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    ABSTRACT: The available data on the structural and magnetic transitions in multiferroic hexagonal YMnO3 have been reviewed, first making use of the computer programs from the group theoretical ISOTROPY software suite to list possible crystal and magnetic structures, then taking into account the capability of neutron diffraction and other physical methods to distinguish them. This leads to a clear view of the transformation sequence, as follows. Hexagonal YMnO3 is paraelectric in P63/mmc at elevated temperatures, and undergoes a single structural transition on cooling through 1250 K to a ferrielectric phase in P63cm that is retained through room temperature. At a much lower temperature, 70 K, there is a magnetic transition from paramagnetic to a triangular antiferromagnetic arrangement, most likely with symmetry P63'cm'. Comment is made on the unusual coupling of ferroelectric and magnetic domains reported to occur in this material, as well as on the so-called `giant magneto-elastic' effect.
    Acta crystallographica Section B, Structural science, crystal engineering and materials. 12/2013; 69(Pt 6):534-540.
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    ABSTRACT: A metal–organic framework perovskite, [(CH2)3NH2][Mn(HCOO)3], exhibits a weakly first order ferroelastic phase transition at ∼272 K, from orthorhombic Pnma to monoclinic 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 ∼10−7 s. 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.
    Acta Materialia. 08/2013; 61(13):4928–4938.
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    ABSTRACT: Hematite, Fe(2)O(3), provides in principle a model system for multiferroic (ferromagnetic/ferroelastic) behavior at low levels of strain coupling. The elastic and anelastic behavior associated with magnetic phase transitions in a natural polycrystalline sample have therefore been studied by resonant ultrasound spectroscopy (RUS) in the temperature range from 11 to 1072 K. Small changes in softening and attenuation are interpreted in terms of weak but significant coupling of symmetry-breaking and non-symmetry-breaking strains with magnetic order parameters in the structural sequence [Formula: see text]. The [Formula: see text] transition at 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 Pb(3)(PO(4))(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(-8) s. The first order [Formula: see text] (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.
    Journal of Physics Condensed Matter 03/2013; 25(11):116006. · 2.22 Impact Factor
  • M. A. Carpenter, E. K. H. Salje, Christopher J. Howard
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    ABSTRACT: The magnetic, elastic, and anelastic behavior of single-crystal KMnF3 have been investigated by superconducting quantum interference device (SQUID) magnetometry and resonant ultrasound spectroscopy (RUS) through the sequence of phase transitions: phase I, Pm3¯m →(Tc1 = 185 K) → phase II, I4/mcm →(Tc2 =TN = 87 K) → phase III, antiferromagnetic, Cmcm → (Tc3 = 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.
    Physical review. B, Condensed matter 06/2012; · 3.77 Impact Factor
  • Christopher J Howard, Michael A Carpenter
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    ABSTRACT: It is evident from the literature that the 122 iron arsenides, XFe(2)As(2) 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(A)mca. 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.
    Acta crystallographica. Section B, Structural science 04/2012; 68(Pt 2):209-12. · 1.80 Impact Factor
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    ABSTRACT: The mixed-valence manganite Sr0.65Pr0.35MnO3 has been prepared and its crystal and magnetic structure investigated between 7 and 1200 K using high-resolution powder neutron diffraction. The structural and lattice parameter data have been used to determine the octahedral tilting and spontaneous strains associated with the structural, electronic, and magnetic phase transitions. At room temperature, the structure is tetragonal and is characterized by cooperative out-of-phase tilts of the MnO6 octahedra about the c axis and a large Jahn–Teller-type distortion due to the presence of Mn3+. The sample exhibits a reversible phase transition from the cubic Pm3̅ m perovskite to a tetragonal I4/mcm structure at 750 K. The Pm3̅ m ↔ I4/mcm phase transition is continuous, and the tetragonal strain, which is dominated by the Jahn–Teller-type distortion of the MnO6 octahedra, exhibits an unusual et0.5 ∝ (Tc − T) temperature dependence. At low temperatures, a C-type antiferromagnetic structure develops with a Neel temperature TN of 250 K. The Mn magnetic moment at 7 K is 2.99(2) μB/Mn. The magnetic ordering introduces additional tetragonal strain, and this strain shows the expected quadratic dependence on the magnetic moment at low temperatures. An increase in the octahedral tilt angle at TN demonstrates an effective coupling between the magnetic ordering process and octahedral tilting.
    Physical review. B, Condensed matter 03/2012; 85(10). · 3.77 Impact Factor
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    Michael A Carpenter, Zhiying Zhang, Christopher J Howard
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    ABSTRACT: The combined structural and antiferromagnetic phase transition in wüstite, Fe(1-x)O, at the Néel temperature, T(N) ~ 195 K, 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(0.99)O the magnetic order parameter, m, varies with temperature as ~m(8) ∝ (T(N) - T), while for Fe(0.94)O 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 ~13 GPa. 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.
    Journal of Physics Condensed Matter 03/2012; 24(15):156002. · 2.22 Impact Factor
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    ABSTRACT: We have studied the crystal structure of mixed-valence Sr0.7Ce0.3MnO3 from 4.2 to 973 K using high-resolution neutron powder diffraction. The crystal structure is tetragonal in space group I4/mcm at 4.2–923 K and cubic in Pm3̅ m at T ≥ 948 K. Lattice parameters and Mn-O bond distances, obtained by Rietveld refinement, have been used to derive the spontaneous strains and MnO6 octahedral distortion, which are interpreted in terms of strain/order parameter coupling using a single Landau free-energy expansion for a Pm3̅ m reference structure with two instabilities (R4+ and Γ3+). Two phase transitions were proposed: an octahedral tilting transition at Tc,φ ∼ 938 K (Pm3̅ m↔ I4/mcm, R4+), and an isosymmetric, electronically driven (Jahn-Teller–like) transition at Tc,JT ∼ 770 K (I4/mcm, R4+ ↔ I4/mcm, R4+ and Γ3+). The nature of the tilting transition appears to be tricritical, while that of the Jahn-Teller–like transition is second order. In addition to the contributions from octahedral tilting and Jahn-Teller–like distortions, there is an excess octahedral distortion at temperatures below 250 K; this is speculated to be associated with an anomaly observed over the temperature range of 275–300 K in the heat-capacity measurements.
    Physical Review B 01/2012; 85(17). · 3.66 Impact Factor
  • Erich H. Kisi, Christopher J. Howard, Jianfeng Zhang
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    ABSTRACT: Precise lattice strains measured using high-resolution neutron diffraction from a solid polycrystalline sample are used to explore the single-crystal elastic constants of α-Al2O3 (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 × 10−12 to −0.47 × 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.
    Journal of Applied Crystallography 02/2011; 44(1). · 3.34 Impact Factor
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    ABSTRACT: We have carried out Ce L- and Mn K-edge x-ray absorption near edge structure (XANES) measurements to experimentally determine the oxidation states of both Ce and Mn in (Sr(1 - x)Ce(x))MnO(3) (x = 0.1-0.4). It was found that although Ce is predominantly 4 + at low doping levels (x = 0.1 and 0.15), the Ce valency decreases with increasing Ce doping (reaching a value of around 3.5 + at x = 0.4). The average Mn oxidation state decreases with the increase of Ce content, with the percentage of Jahn-Teller active Mn(3+) ions increasing from 26% (x = 0.1) to 57% (x = 0.4). Precise structural parameters were also obtained from high resolution neutron diffraction studies for samples with x = 0.1-0.3. The crystal structure remains tetragonal in I4/mcm for x ≤ 0.3. The octahedral tilt angle increases with increasing Ce content, but the distortion of the MnO(6) octahedra is reduced significantly at x ≥ 0.2 due to a transition from long-range ordered Jahn-Teller distortions to incoherent static distortions.
    Journal of Physics Condensed Matter 11/2010; 22(44):445401. · 2.22 Impact Factor
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    ABSTRACT: The elastic and anelastic properties of a polycrystalline sample of Pr0.48Ca0.52MnO3 have been investigated by resonant ultrasound spectroscopy, as a function of temperature (10–1130 K) and magnetic field strength (0–15 T). Marked softening of the shear modulus as the Pnma↔incommensurate phase transition at ∼235 K in zero field is approached from either side is consistent with pseudoproper ferroelastic character, driven by an order parameter with Γ3+ symmetry associated with Jahn-Teller ordering. This is accompanied by an increase in attenuation just below the transition point. The attenuation remains relatively high down to ∼80 K, where there is a distinct Debye peak. It is attributed to coupling of shear strain with the Γ3+ order parameter which, in turn, controls the repeat distance of the incommensurate structure. Kinetic data extracted from the Debye peak suggest that the rate-controlling process could be related to migration of polarons. Elastic softening and stiffening as a function of magnetic field at constant temperatures between 177 and ∼225 K closely resembles the behavior as a function of temperature at 0, 5, and 10 T and is consistent with thermodynamically continuous behavior for the phase transition in both cases. This overall pattern can be rationalized in terms of linear/quadratic coupling between the Γ3+ order parameter and an order parameter with Σ1 or Σ2 symmetry. It is also consistent with a dominant role for spontaneous strains in determining the strength of coupling, evolution of the incommensurate microstructure, and equilibrium evolution of the Jahn-Teller ordered structure through multicomponent order-parameter space.
    Physical review. B, Condensed matter 10/2010; 82(13). · 3.77 Impact Factor
  • Christopher J Howard, Michael A Carpenter
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    ABSTRACT: Computer-based group-theoretical methods are used to enumerate structures arising in A(2)BB'X(6) 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 BX(6) (or B'X(6)) 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 BX(6) (and B'X(6)) 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 Pr(1-x)Ca(x)MnO(3) (x approximately 0.5). The structural hierarchies derived in this work should prove useful in interpreting experimental results.
    Acta crystallographica. Section B, Structural science 02/2010; 66(Pt 1):40-50. · 1.80 Impact Factor
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    ABSTRACT: Structural evolution as a function of temperature through the Pnma↔incommensurate (IC) phase transition in Pr0.48Ca0.52MnO3 perovskite has been analyzed from the perspectives of symmetry and strain. The structure and stability of both phases are shown to depend on combinations of order parameters which have symmetries associated with irreducible representations M3+, R4+, M2+, Γ3+ and Σ2 of space group Pm3̅ m. The physical origin of these can be understood in terms of octahedral tilting, cooperative Jahn-Teller distortions and charge order/Zener polaron ordering. The M2+ order parameter describes the Jahn-Teller ordering scheme which develops in LaMnO3 while the Γ3+ order parameter relates to an ordering scheme in which the unique axes of the distorted octahedra are all aligned in the same direction. Irrep Σ2 contains two components with gradient coupling and provides the symmetry-breaking mechanism by which the IC transition can occur. Each order parameter couples with macroscopic spontaneous strains in a manner that depends strictly on symmetry and this leads to specific interactions between the order parameters through their coupling with common strains. In order to establish the extent and importance of this coupling, symmetry-adapted strains have been extracted from a new set of lattice parameters obtained by high-resolution powder neutron diffraction in the temperature interval 10–1373 K. It is found that the predominant strain of the incommensurate structure (up to ∼2.5%) is a tetragonal shear strain which arises by bilinear coupling with the Γ3+ order parameter. This combination is probably responsible for most of the energy reduction accompanying the Pnma↔IC transition and also gives it some characteristics typical of a pseudoproper ferroelastic transition. Strain coupling promotes mean-field behavior and the evolution of the symmetry-breaking order parameter can be described by a standard Landau tricritical solution, q4∝(Tc−T) with Tc=237±2 K. 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 M2+ scheme below ∼775 K but is replaced by the Γ3+ scheme below Tc. 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 Γ3+ distortions of MnO6 octahedra shows that the nature of the IC structure itself is also determined essentially by geometrical factors and strain.
    Physical review. B, Condensed matter 01/2010; 82(9). · 3.77 Impact Factor
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 01/2010; 27(21).
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    ABSTRACT: The evolution of the crystal structure of rhombohedral PrAlO(3) 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 AlO(6) and polyhedral PrO(12) groups with increasing pressure, is controlled by the relative compressibilities of the AlO(6) octahedra and the PrO(12) site. Because the AlO(6) octahedra are more compressible than the PrO(12) 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 K(0) = 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 [Formula: see text] to Imma transition confirms that electronic interactions stabilize the Imma phase.
    Journal of Physics Condensed Matter 06/2009; 21(23):235403. · 2.22 Impact Factor
  • Michael A Carpenter, Christopher J Howard
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    ABSTRACT: Space groups, order-parameter and strain/order-parameter coupling relationships in ABX3 perovskite structures which combine cooperative Jahn-Teller distortions and octahedral tilting have been investigated from the perspective of group theory using the computer program ISOTROPY. Two common Jahn-Teller ordering schemes are associated with the irreducible representations M2+ and R3+ of the space group Pm3m. A third, less-common ordering scheme is associated with Gamma3+. These combine with tilting instabilities associated with M3+ and R4+ to generate a predicted suite of Jahn-Teller structure types that includes many of the known structures of manganites, vanadates, Cu and Cr halides. Order-parameter coupling and possible phase transitions are described using Landau free-energy expansions, and general expressions for the relationships between symmetry-adapted spontaneous strains and particular order-parameter components are presented. These provide a general formal framework for determining structural evolution across multi-component order-parameter space and for characterizing the influence of tilting instabilities on Jahn-Teller instabilities or of Jahn-Teller ordering on octahedral tilting.
    Acta crystallographica. Section B, Structural science 05/2009; 65(Pt 2):134-46. · 1.80 Impact Factor
  • Michael A Carpenter, Christopher J Howard
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    ABSTRACT: The structural evolution of selected perovskites containing Jahn-Teller cations has been investigated in the light of a formal analysis of symmetry hierarchies for phase transitions driven by octahedral tilting and Jahn-Teller cooperative distortions. General expressions derived from the strain/order-parameter coupling relationships allowed by symmetry are combined with observed changes in lattice parameters to reveal details of order-parameter evolution and coupling. LuVO3, YbVO3, YVO3 and CeVO3 are representative of systems which develop Jahn-Teller ordering schemes associated with irreducible representations M2+ and R3+ of the space group Pm3m. Tilting of their octahedra is associated with M3+ and R4+. The Pnma (M3+ + R4+ tilting) <--> P2(1)/a (M3+ + R4+ tilting, R3+ Jahn-Teller order) transition below room temperature is close to second order in character. Shear strains which depend primarily on tilt angles show little variation, implying that there is only weak coupling between the tilting and Jahn-Teller order parameters. The subsequent P2(1)/a <--> Pnma (M3+ + R4+ tilting, M2+ Jahn-Teller order) is first order in character, and involves either a reduction in the R4+ tilt angle or a change in the strength of tilt/Jahn-Teller order-parameter coupling. In LaMnO3, the isosymmetric Pnma (M3+ + R4+ tilting) <--> Pnma (M3+ + R4+ tilting, M2+ Jahn-Teller order) transition can be described in terms of a classical first-order transition conforming to a 246 Landau expansion with negative fourth-order coefficients. Strain evolution in Ba-doped samples suggests that the transition becomes second order in character and reveals a new strain relaxation mechanism in LaMnO3 which might be understood in terms of local strain heterogeneities due to the disordering of distorted MnO6 octahedra. Transitions in PrAlO3 and La(0.5)Ba(0.5)CoO3 illustrate the transformation behaviour of systems in which the Jahn-Teller ordering scheme is associated with the irreducible representation Gamma3+. Overall, coupled tilting + Jahn-Teller phase transitions in perovskites conform to mean-field behaviour, consistent with the underlying role of strain in promoting long interaction lengths.
    Acta crystallographica. Section B, Structural science 04/2009; 65(Pt 2):147-59. · 1.80 Impact Factor
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    ABSTRACT: We have studied the crystal structures of (Sr(0.8)Ce(0.2))(Mn(1-y)Co(y))O(3) (y = 0 and 0.2) using neutron diffraction. Both (Sr(0.8)Ce(0.2))MnO(3) and (Sr(0.8)Ce(0.2))(Mn(0.8)Co(0.2))O(3) have a tetragonal structure in space group I4/mcm at room temperature, and the octahedral tilt angle around the c-axis is nearly the same. The only significant difference is the shape of the Mn(Co)O(6) octahedron: it is elongated in (Sr(0.8)Ce(0.2))MnO(3) due to the cooperative Jahn-Teller (JT) effect, but essentially regular in (Sr(0.8)Ce(0.2))(Mn(0.8)Co(0.2))O(3) due to the absence of JT-active Mn(3+) ions. With increasing temperature, both compounds undergo a continuous phase transition at around 400 °C to a cubic structure in [Formula: see text], with no indication of a distinct transition in (Sr(0.8)Ce(0.2))MnO(3) from the removal of the static JT distortion. In addition, the temperature dependence of the octahedral tilt angle is very similar in the two samples, implying that the JT distortion has minimal effect on the octahedral tilting and the phase transition to cubic. X-ray absorption near-edge structure (XANES) analysis indicates that the Ce oxidation state is predominantly 4+ in both samples. The electrical conductivity is higher in (Sr(0.8)Ce(0.2))MnO(3) than in (Sr(0.8)Ce(0.2))(Mn(0.8)Co(0.2))O(3) in the temperature range studied (100-900 °C).
    Journal of Physics Condensed Matter 03/2009; 21(12):124218. · 2.22 Impact Factor

Publication Stats

687 Citations
150.45 Total Impact Points

Institutions

  • 2009–2014
    • University of Newcastle
      • School of Engineering
      Newcastle, New South Wales, Australia
  • 2004–2013
    • University of Cambridge
      • Department of Earth Sciences
      Cambridge, ENG, United Kingdom
  • 1994–2012
    • University of Sydney
      • • School of Physics
      • • School of Chemistry
      Sydney, New South Wales, Australia
  • 1991–2010
    • Australian Nuclear Science and Technology Organisation
      • Institute of Materials Engineering
      Kirrawee, New South Wales, Australia
  • 2003
    • Brigham Young University - Provo Main Campus
      • Department of Physics and Astronomy
      Provo, UT, United States