C-type Phase Research Articles

Photometric Analysis of the Nucleus of the Main-belt Comet 2010 LH15

We analyzed the inactive nucleus of the main-belt comet 2010 LH15 at a true anomaly of 262°. Observations using the Gemini North Telescope in 2023 July yielded an absolute R-band magnitude of (18.4 ± 0.2) mag. Assuming a C-type geometric albedo and phase function, we estimated the nucleus radius to be (0.5 ± 0.1) km. The examination of the lightcurve profile did not yield clear evidence for a rotational period shorter than 2 hr.

Decoding the Reactivity Enhancement of Ln2Ce2O7 Compounds (Ln = Yb, Y, Tb, and Gd) for Soot Combustion: The Remarkable Contribution of Variable Valence A-Sites.

The impact of variable valence A-sites on the redox property and reactivity of Ln2Ce2O7 compounds in soot particulate combustion has been investigated. It was observed that Yb2Ce2O7, Y2Ce2O7, and Gd2Ce2O7 formed a rare earth C-type phase, while Tb2Ce2O7 formed a solid solution phase. Both Tb2Ce2O7 and Yb2Ce2O7 possess dual valence state A-sites, resulting in significantly more surface vacancies. Additionally, the advantageous solid solution phase structure of Tb2Ce2O7 leads to even more surface vacancies than Yb2Ce2O7, which is crucial to generate active oxygen sites. Moreover, the introduction of NO into the reaction feed enhances combustion activity by producing active surface monodentate nitrates. A catalyst with higher numbers of surface vacancies exhibits improved NO oxidation ability and better NO2 utilization efficiency. Consequently, the Tb2Ce2O7 compound demonstrates not only the best soot combustion activity, but also an optimal NOx-assistance effect. Therefore, it is concluded that variable valence A-site is the intrinsic factor to improve the reactivity of Ln2Ce2O7 catalysts.

The defects of structural and phase transformations in polycrystalline cerium dioxide under heating in vacuum and in air

Structural changes in cerium dioxide, when heated in vacuum in the range of 25‒1600 °C, in air in the range of 25‒1500 °C, and during successive annealing in the range of 1600‒2100 °C in air, followed by quenching in water, were studied. In the crystal lattice of CeO2‒х, the F → F1 phase transformation in vacuum proceeds in the range of 1100‒1600 °C; additionally, at 1200 °C, X-ray lines of the C-type Ce2O3 phase appear. The thermal expansion coefficient of phases of the fluorite type F and F1 in the range of 25‒1500 °C in air, as well as phases of the fluorite type F, F1 and type C Ce2O3 in the range of 25‒1600 °C are determined in vacuum and their dependence on the change in the oxygen content in the CeO2‒х crystal lattice was found. The kinetic conditions for reduction of cerium dioxide in vacuum and oxidation in air are different. The cubic structure of the fluorite type F CeO2‒х, when the samples are heated in air, is preserved up to 1800 °C with the content of anionic vacancies, at 1900 °C the transformation F → F1 occurs. The formation of loops, edge and screw dislocations in the structure of cerium dioxide grains after annealing of samples in the range of 1900‒2100 °C in air were discovered for the first time. The decomposition of the structure F1 into cerium oxide phases of types F and C proceeds at 2100 °C along the height and boundaries of screw dislocations. It was found that fragments of the C-type phase of cerium oxide are located on loops along the height of screw dislocations, which indicates the movement and evaporation of these fragments. When the samples are oxidized at 1600 °C in air, the black-colored C Ce2O3‒х phase in a gradient of different concentrations moves along certain trajectories to opposite ones. grain boundaries, abuts against dislocation loops, bends them, and oxidizes to phases F1 and F. In the structure of polycrystalline cerium dioxide, when heated in vacuum and in air, certain concentrations of defects control phase transformations.

Tuning the melting point and phase stability of rare-earth oxides to facilitate their crystal growth from the melt

The challenge of growing rare-earth (RE) sesquioxide crystals can be overcome by tailoring their structural stability and melting point via composition engineering. This work contributes to the advancement of the field of crystal growth of high-entropy oxides. A compound with only small REs (Lu,Y,Ho,Yb,Er)2O3 maintains a cubic C-type structure upon cooling from the melt, as observed via in-situ high-temperature neutron diffraction on aerodynamically levitated samples. On the other hand, a compound with a mixture of small and large REs (Lu,Y,Ho,Nd,La)2O3 crystallizes as a mixture of a primary C-type phase with an unstable secondary phase. Crystals of compositions (Lu,Y,Ho,Nd,La)2O3 and (Lu,Y,Gd,Nd,La)2O3 were grown by the micro-pulling-down (mPD) method with a single monoclinic B-type phase, while a powder of (Lu,Y,Ho,Yb,Er)2O3 did not melt at the maximum operating temperature of an iridium-rhenium crucible. The minimization of the melting point of the two grown crystals is attributed to the mismatch in cation sizes. The electron probe microanalysis reveals that the general element segregation behavior in the crystals depends on the composition.

Lattice and magnetic dynamics in the YVO3 Mott insulator studied by neutron scattering and first-principles calculations

The Mott insulator YVO$_{3}$ with $T_{N}$ = 118 K is revisited to explore the role of spin, lattice and orbital correlations across the multiple structural and magnetic transitions observed as a function of temperature. Upon cooling, the crystal structure changes from orthorhombic to monoclinic at 200 K, and back to orthorhombic at 77 K, followed by magnetic transitions. From the paramagnetic high temperature phase, C-type ordering is first observed at 118 K, followed by a G-type spin re-orientation transition at 77 K. The dynamics of the transitions were investigated via inelastic neutron scattering and first principles calculations. An overall good agreement between the neutron data and calculated spectra was observed. From the magnon density of states, the magnetic exchange constants were deduced to be $J_{ab}$ = $J_{c}$ = -5.8 meV in the G-type spin phase, and $J_{ab}$ = -3.8 meV, $J_{c}$ = 7.6 meV at 80 K and $J_{ab}$ = -3.0 meV, $J_{c}$ = 6.0 meV at 100 K in the C-type spin phase. Paramagnetic scattering was observed in the spin ordered phases, well below the C-type transition temperature, that continuously increased above the transition. Fluctuations in the temperature dependence of the phonon density of states were observed between 50 and 80 K as well, coinciding with the G-type to C-type transition. These fluctuations are attributed to optical oxygen modes above 40 meV, from first principles calculations. In contrast, little change in the phonon spectra is observed across $T_{N}$.

Perovskite as a spin current generator

We theoretically show that materials with perovskite-type crystal structures provide a platform for spin current generation, taking advantage of a mechanism requiring neither the spin-orbit coupling nor a ferromagnetic moment, but is based on spin-split band structures in certain kinds of collinear antiferromagnetic states. By investigating a multiband Hubbard model for transition metal compounds by means of the Hartree-Fock approximation and the Boltzmann transport theory, we find that a pure spin current is induced by an electric field applied to a C-type antiferromagnetic metallic phase. The spin current generation originates from a cooperative effect of spatially anisotropic electron transfer integrals owing to the GdFeO$_3$-type lattice distortion, which is ubiquitous in many perovskites, and the collinear spin configuration. We discuss our finding from the symmetry point of view, in comparison with other spin current generator candidates with collinear antiferomagnetism. We also propose several ways to detect the phenomenon in candidate perovskite materials.

Symmetry Origin of the Dzyaloshinskii-Moriya Interaction and Magnetization Reversal in YVO3.

We have investigated magneto-structural phase transitions in polycrystalline YVO3 using high-resolution neutron powder diffraction toward understanding the phenomenon of magnetization reversal. Contrary to earlier reports, our study reveals that both C-type and G-type antiferromagnetic ordering, corresponding to G-type and C-type orbital ordered phases, respectively, occur at the same temperature (TN = 115 K) with the G-type antiferromagnetic phase growing at the expense of the C-type one on cooling. These processes cease at TS ∼ 77 K; however, a minor (∼4%) untransformed C-type phase remains unchanged down to 1.7 K. The symmetry analysis indicates different symmetry origins of the Dzyaloshinskii-Moriya interaction in each phase, which can explain the magnetization reversal observed between TN and TS. We discuss that magnetic phase separation and associated weak ferromagnetism may be the common mechanism underlying the magnetization reversal phenomenon observed in other RVO3 systems (R = rare earth).

Doping effects on the structure and electrical properties of La2Ce2O7 proton conductors

Recent studies have reported that the conductivity of La2Ce2O7 proton conductors is improved by aliovalent doping, despite the high concentration of oxygen vacancies in the pristine material. In this study, we provide new insights into the structural and conducting properties of La2-x AxCe2O7±δ (A = Na+, Ca2+, Sr2+, Ba2+, Y3+ and W6+; x ≤ 0.2). The materials are apparently single phase compounds with a disordered fluorite-type structure by conventional X-ray diffraction; however, the local structure, studied by Raman spectroscopy, reveals a biphasic mixture of fluorite and C-type phases, which depends on dopant type and the sintering temperature. In addition, phase segregations are observed for Ca, Sr and Ba-doped samples by scanning electron microscopy. The different substitutions do not improve the bulk conduction at low temperature, and the conductivity differences between the samples are mainly ascribed to microstructural changes. High sintering temperatures produce a significant reduction of the bulk conductivity, i.e. 1.38 and 1.01 μS cm−1 at 300 °C for samples sintered at 1400 and 1500 °C, respectively. The use of Zn as sintering aid greatly improves the densification of La2Ce2O7 at lower temperature, without affecting negatively the conductivity.

Relationship between the Structure and Transport Properties in the Ce1-xLaxO2-x/2 System.

La-doped CeO2 materials have been widely investigated for potential applications in different high-temperature electrochemical devices, such as fuel cells and ceramic membranes for hydrogen production. However, the crystal structure is still controversial, and different models based on fluorite, pyrochlore, and/or type-C structures have been considered, depending on the lanthanum content and synthesis method used. In this work, an exhaustive structural analysis of the Ce1-xLaxO2-x/2 system (0.2 < x ≤ 0.7) is performed with different techniques. The average crystal structure, studied by conventional X-ray diffraction, could be considered to be a disordered fluorite; however, the local structure, examined by electron diffraction and Raman spectroscopy, reveals a biphasic mixture of fluorite and C-type phases. The thermal and electrical properties demonstrate that the materials with x ≥ 0.4 are oxide ion proton conductors in an oxidizing atmosphere and mixed ionic electronic conductors in a reducing atmosphere. The water uptake and proton conductivity increase gradually with the increase in La content, suggesting that the formation of the C-type phase is responsible for the proton conduction in these materials.

Structural Competition and Reactivity of Rare-Earth Oxide Phases in Y xPr2- xO3 (0.05 ≤ x ≤ 0.80).

We report, for the first time, members of the Y xPr2- xO3 system with non-bixbyite or defect fluorite structures. The synthesis, structure, phase transitions, and high temperature reactivity of the trigonal A-type and monoclinic B-type structures are reported along with those of the cubic C-type phase (bixbyite). Combined powder X-ray and neutron diffraction Rietveld refinements are used to report structural details of all three reported phases. Phase transitions are investigated, showing a clear dependence on average cation size. Using neutron diffraction, phase transitions are followed in situ, revealing that all high temperature phases are quenchable. In-situ powder X-ray diffraction experiments in flowing oxygen allow insights into mechanistic details of redox processes in the reported phases. In contrast to the C-type cubic bixbyite, the trigonal A-type and monoclinic B-type structures do not allow for topotactic oxygen uptake, displaying instead a phase transition to either the bixbyite C-type capable of accommodating additional oxide anions or the direct oxidation to the cubic defect fluorite structure. The findings reported here agree with the accepted lanthanide sesquioxide phase diagrams and provide exceptional control of phases. The work is important for the prediction of structures, and the synthetic control needed for rational design of functional materials.

Influence of composition and oxygen-vacancy ordering on lattice parameter and elastic moduli of Ce1-xGdxO2-x/2: A theoretical study

We study the behaviour of the lattice parameter and elastic moduli of Ce1-xGdxO2-x/2 for the random (fluorite-like) and C-type ordered oxygen-vacancy configurations [Žguns et al., PCCP 20 (2018) 11805-11818]. For the fluorite phase, elastic moduli decrease linearly with Gd concentration. For the C-type phase, the bulk, shear and Young moduli are found to be systematically larger and the lattice parameter smaller than those for disordered fluorite phase. Essentially the linear behaviour of the bulk modulus and lattice parameter depending on the degree of the C-type order is found. Our findings explain the experimentally observed elastic moduli of Ce1-xGdxO2-x/2.

Synthesis and Electrochemical Performance of C-Base-Centered Lepidocrocite-like Titanates for Na-Ion Batteries

Lepidocrocite-like titanate, Na0.9[Ti1.7Li0.3]O4, having a C-base-centered lattice (C-type) is prepared by dehydration of Na0.9[Ti1.7Li0.3]O4·nH2O having a primitive lattice (P-type) and influence of the interlayer water on its electrochemical performance is examined in nonaqueous Na cells. Upon heating up to 350 °C, P-type Na0.9[Ti1.7Li0.3]O4·nH2O transforms into the anhydrous C-type phase as a result of gradual removal of the interlayer water. The C-type Na0.9[Ti1.7Li0.3]O4 delivers reversible capacity of 120 mAh g–1 between 2.0 and 0.1 V vs Na with high Coulombic efficiency at an initial cycle and good capacity retention over 50 cycles. Structural change of the C-type phase during sodiation is investigated by using operando X-ray diffraction measurement.

Phase Transformations in the CeO2-Sm2O3 System: A Multiscale Powder Diffraction Investigation.

The structure evolution in the CeO2-Sm2O3 system is revisited by combining high resolution synchrotron powder diffraction with pair distribution function (PDF) to inquire about local, mesoscopic, and average structure. The CeO2 fluorite structure undergoes two phase transformations by Sm doping, first to a cubic (C-type) and then to a monoclinic (B-type) phase. Whereas the C to B-phase separation occurs completely and on a long-range scale, no miscibility gap is detected between fluorite and C-type phases. The transformation rather occurs by growth of C-type nanodomains embedded in the fluorite matrix, without any long-range phase separation. A side effect of this mechanism is the ordering of the oxygen vacancies, which is detrimental for the application of doped ceria as an electrolyte in fuel cells. The results are discussed in the framework of other Y and Gd dopants, and the relationship between nanostructuring and the above equilibria is also investigated.

Exchange bias training effect in phase separated polycrystalline Sm0.1Ca0.7Sr0.2MnO3

Magnetic properties of antiferromagnetic (AFM) electron doped manganite Sm0.1Ca0.7Sr0.2MnO3 have been investigated, focusing mainly on the exchange bias (EB) effect and associated training effect. The studied compound exhibits the ground state with heterogeneous spin configuration, consisting of the C-type antiferromagnetic phase with the Néel temperature TN-C ≈ 120 K, the G-AFM phase with the Néel temperature TN-G ≈ 60 K, and ferromagnetic-like phase with a very weak spontaneous magnetic moment. Measurements of hysteresis loops have shown that the exchange bias field monotonously decreases with increasing temperature and vanishes above 40 K, while the coercivity disappears only above 70 K. The temperature variation of the exchange bias field has been successfully described by an exponential decay form. The stability of EB has been evaluated in the studies of the training effect, which has been discussed in the frame of the spin relaxation model, elucidating the important role of the AFM domain rearrangement at the interface. The complex phase separation and possible contributions from different interfaces between coexisting magnetic phases to the EB effect have also been discussed.

Phase formation and UV luminescence of Gd3+ doped perovskite-type YScO3

Synthesis of pure and Gd3+doped perovskite-type YScO3 was attempted by a polymerized complex (PC) method and solid state reaction (SSR) method. Crystalline phases and UV luminescence of samples were examined with varying heating temperatures. The perovskite-type single phase was not simply formed in the SSR method, as reported in some literatures, and two cubic C-type phases of starting oxide materials remained forming slightly mixed solid solutions. UV luminescence of Gd3+ doped samples increased with an increase in heating temperatures and volume of the perovskite-type phase. In contrast, a non-crystalline precursor was crystallized to a single C-type phase at 800°C in the PC method forming a completely mixed solid solution. Then, the phase of perovskite-type YScO3 formed at 1200°C and its single phase was obtained at 1400°C. It was revealed that high homogeneousness of cations was essential to generate the single perovskite-phase of YScO3. Because Gd3+ ions were also dissolved into the single C-type phase in Gd3+ doped samples, intense UV luminescence was observed above 800°C in both C-type phase and perovskite-type phase.

Effect of shock compression on optical and structural properties of Eu2O3 and Y2O3:Eu3+ powders

Shock-recovery experiments on Eu2O3 and Y2O3:Eu3+ powders using a metal plate projectile accelerated by a single-stage powder-propellant gun were performed to investigate phase stability and response at high pressures and temperatures. The recovered samples were characterized using powder X-ray diffraction analysis and photoluminescence spectroscopy. The onset of the structural phase transition from the cubic (C-type) to monoclinic (B-type) phase was observed for both Eu2O3 and Y2O3:Eu3+ powders at shock pressures of 8 and 13 GPa, respectively. For Eu2O3, the amount of B-type phase increases with increasing shock pressure up to 23 GPa, whereas for Y2O3:Eu3+, a maximum was reached at 25 GPa followed by a decrease with increasing shock pressure; only the C-type phase was detected in the sample shocked at 51 GPa. The change in the amount of B-type phase indicates stability for the monoclinic phase against shock-induced heat and mechanical deformation. The large range in shock pressure for which the C-type and B-type phases coexist in Eu2O3 and Y2O3:Eu3+ indicates that the pressure-induced phase transition is too sluggish to be completed within the shock duration. The D50→7F2/5D0→7F1 intensity ratios for the shock-recovered Eu2O3 and Y2O3:Eu3+ samples were independent of the shock pressure and the amount of C-type phase in the samples. No relationship was observed between the crystal-field parameter B20 and the amount of C-type phase in both shock-recovered samples. However, with increasing B20 2, the D50→7F2/5D0→7F1 intensity ratio decreased, whereas the D50→7F0/5D0→7F1 intensity ratio increased. These results suggest that shock-induced deformation leads to enhanced J-mixing in both the Eu2 O3 and the Y2O3:Eu3+ samples.

Thickness controlled proximity effects in C-type antiferromagnet/superconductor heterostructure.

Modulation of the superconducting state possessing a C-type antiferromagnetic phase in the Nd0.35Sr0.65MnO3/YBa2Cu3O7 heterostructure is investigated, with the Nd0.35Sr0.65MnO3 thickness (t) varying from 40 to 200 nm. Both the superconducting transition temperature and the upper critical field along the c-axis decrease with increasing t; while the in-plane coherence length increases from 2.0 up to 3.6 nm. Meanwhile, the critical current density exhibits a field-independent behavior, indicating an enhanced flux pinning effect. Furthermore, low-temperature spin canting induces a breakdown and re-entrance of the superconductivity, demonstrating a dynamic completion between the superconducting pairing and the exchange field. An unexpected colossal magnetoresistance is observed below the superconducting re-entrance temperature at t = 200 nm, which is attributed to the dominant influence of the exchange field over the pairing energy.

Size and spatial correlation of defective domains in yttrium-doped CeO2

The size of dopant-rich nanodomains was assessed in four samples of Ce1−μYμO2−μ/2 through systematic pair distribution function (PDF) refinements. Experimental G(r) curves were fitted by different structural models with the aim of finding a description which balanced precise structure parameterization and reasonable number of parameters. The most reliable model was a single Y2O3-like phase, which best accommodated to the close relationship between the fluorite (CeO2-like) and C-type (Y2O3-like) structures. In this model, a refined cation coordinate, x(M2), measured the relative occurrence in the G(r) of the chemical environment of Y and Ce at any value of r. The r-value at which x(M2) vanished, i.e. at which the refined C-type cell becomes a redundant, low-symmetry description of a fluorite cell, was assumed as the size of a C-type domain. Subtle features in G(r) could be attributed to the fluorite or C-type phase up to ~500 Å thanks to the narrow instrumental resolution function of the ID31 beamline (now ID22) at the ESRF, which allows us to get high resolution PDF data.

Thermoelectric properties of A-site substituted Lanthanide Ca0.75R0.25MnO3

We present the electronic structure and thermoelectric properties of doped and intrinsic CaMnO3 investigated by first principles and semi-classic Boltzmann theory. The G-type anti-ferromagnetic phase is most stable among five intrinsic anti-ferromagnetic phases, however, the C-type phase is most stable phase after Lanthanides doping in A-site of CaMnO3. Electrons transfer from Lanthanides to Mn d orbital in 25% Lanthanides doped materials, which result in part of conductive bands are occupied by Mn d electrons. The electron transport changes from semiconductor (intrinsic) to metal (doped), this change can be found in Seebeck coefficient and electrical conductivity as the function of temperature. The electrical conductivity is enhanced by Lanthanides doping, and about 2 orders of magnitudes higher than those of intrinsic CaMnO3. So the Lanthanides doping is useful method to modify thermoelectric properties of CaMnO3.

Unconventional exchange bias effect driven by phase separation in basically antiferromagnetic Sm0.1Ca0.6Sr0.3MnO3

Magnetic investigations, focused on the uncommon behavior of the exchange bias (EB) effect, have been performed for basically antiferromagnetic (AFM) electron doped manganite Sm0.1Ca0.6Sr0.3MnO3. The studied system in the ground state exhibits a heterogeneous spin configuration consisting of the C-type antiferromagnetic phase with the Néel temperature TN−C≈150K, the G-type AFM phase with the Néel temperature TN−G≈70K, and a FM-like phase with very weak spontaneous magnetic moment. The phase separation, into two different AFM phases and a FM-like phase at the temperatures below TN–G, leads to unusual magnetic properties, such as: narrowing of magnetic hysteresis loops in field cooling process, unconventional EB effect associated with spontaneous magnetization at temperatures below TN–C, strong magnetic field dependence of the negative exchange bias at fields below 100Oe turning into practically field independent one for fields above 0.5kOe, significant shift of EB with temperature with a change of the sign from negative at 10K to positive above 40K. The atypical magnetic properties are discussed and related to two different interfaces appearing between coexisting magnetic phases.