Orthorhombic Pnma Structure Research Articles

Neutron diffraction study of the crystal structure and structural phase transition of La 0.7Ca 0.3−xSr xCrO 3 (0≤ x≤0.3) : The relationship between thermodynamic behavior and crystal structure changes at the phase transition

The crystal structure of the La 0.7Ca 0.3− x Sr x CrO 3 series, including the compositional and temperature dependence of the structural parameters, has been studied by variable temperature neutron diffraction measurements. The extent of the distortions from the ideal cubic perovskite structure has been evaluated quantitatively using the average bond lengths and the mean volumes of the [CrO 6] octahedron and [(La/Ca/Sr)O 12] polyhedron, and has been shown to decrease with increase of Sr content or temperature. At the structural phase transition from the orthorhombic ( Pnma) structure to the rhombohedral ( R 3 ¯ c ) one, the volume of the [CrO 6] octahedron decreases whereas that of the [(La/Ca/Sr)O 12] polyhedron shows little difference, resulting in an overall decrease in the level of distortion. The change in the degree of distortion at the phase transition decreases with increase of Sr content, in agreement with the smaller variation of the enthalpy and volume for the specimens with higher Sr content.

Fabrication of 0-3 type manganite/insulator composites and manipulation of their magnetotransport properties

In order to promote the technological applications of perovskite manganites, a great fundamental interest has been devoted to tailoring and/or enhancing their magnetotransport properties. Design and fabrication of manganite-based nanocomposites offer great potential to tailor the magnetotransport properties. In this work, we illustrate the promising concept of 0-3 type manganite/insulator composites (where manganite nanoparticles are uniformly and discretely embedded in a three-dimensional-connectivity insulator matrix) through the example of the La0.67Ca0.33MnO3 (LCMO)/MgO system. We present a promising strategy, which is based on the creation of core (LCMO)–shell (MgO) composite powders, for the synthesis of 0-3 type LCMO/MgO composites. A modified polyacrylamide gel method has been developed to prepare the core-shell structured LCMO/MgO composite powders. Besides its ability to create well-defined core-shell composite structures, the present gel method also allows the production of nanopowders with uniform particle size and in spherical shape. In our (1−x)LCMO/xMgO composite system, the lattice of LCMO is found to expand with rising MgO concentration x, yielding a bulk tensile strain. The increase in the tensile strain with x yields to a structural phase transition in the LCMO phase from an orthorhombic Pnma structure (x≤0.2) to a rhombohedral R3¯c structure (x≥0.3), and this structural transition leads to a relaxation of the strain. The strain effects induced by the MgO second phase exert a great influence on the ferromagnetic transition temperature TC. The composite system exhibits a percolative behavior in the conductivity, and the metal-insulator transition temperature TM-I decreases with x and is finally suppressed as the MgO content increases up to x=0.3. An enhancement in magnetoresistance (MR) is observed at low temperatures for the composites, and the low-field sensitivity of MR gets enhanced with the increase in MgO content. This work provides a promising method for manipulating the magnetotransport properties of manganites by composites with a proper insulator phase in a 0-3 connectivity fashion.

EuI2, a low-temperature europium(II) iodide phase

Light-yellow europium(II) diiodide, prepared by the low-temperature reaction of europium and ammonium iodide in liquid ammonia at 200 K and characterized by high-resolution X-ray powder diffraction, represents a new phase for EuI2 that adopts an orthorhombic Pnma structure with all three atoms lying on 4c positions (.m.). It is isotypic with SrI2(IV). Temperature-dependent X-ray measurements performed to investigate the thermal stability of the new phase show that it decomposes irreversibly to amorphous material around 673 K. Total-energy density-functional calculations using the generalized gradient approximation suggest this to be the ground-state structure of EuI2.

Magnetic Refrigeration Properties of ${\hbox{La}}_{0.8}\hbox{Ca}_{0.2}\hbox{Mn}_{0.99}{^{57}\hbox{Fe}_{0.01}}\hbox{O}_{3}$

The La0.8Ca0.2Mn0.99 57Fe0.01O3 sample was prepared by sol-gel method. The crystallographic and magnetic properties of La0.8Ca0.2Mn0.99 57Fe0.01O3 have been studied using X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and Mossbauer spectroscopy measurements. The crystal structure of La0.8Ca0.2Mn0.99 57Fe0.01O3 was found to be orthorhombic (Pnma) structure with lattice constants ao=5.486 Aring, bo=7.761Aring, and co=5.510 Aring at room temperature. The Curie temperature (Tc) is determined to be 160 K by zero field cooled (ZFC) magnetization curve under 5 kOe applied field. The maximum value of magnetic entropy changes, |DeltaSM| is 1.25 J/kgldrK at 157 K under 13 kOe applied field. Mossbauer spectrum at 4.2 K was fitted with two independent magnetic components of the magnetic hyperfine fields Hf,1=526 kOe and Hf,2=501 kOe. Mossbauer spectra show that the two well-resolved sextets show nearly equal intensities between them without any external fields. This unusual phenomenon provides direct evidence of the two-phase character of the metallic state in the mixed valence state of La0.8Ca0.2Mn0.99 57Fe0.01O3 powder.

Effect of high pressure on multiferroicBiFeO3

We report experimental evidence for pressure instabilities in the model multiferroic BiFeO3 and namely reveal two structural phase transitions around 3 GPa and 10 GPa by using diffraction and far-infrared spectroscopy at a synchrotron source. The intermediate phase from 3 to 9 GPa crystallizes in a monoclinic space group, with octahedra tilts and small cation displacements. When the pressure is further increased the cation displacements (and thus the polar character) of BiFeO3 is suppressed above 10 GPa. The above 10 GPa observed non-polar orthorhombic Pnma structure is in agreement with recent theoretical ab-initio prediction, while the intermediate monoclinic phase was not predicted theoretically.

Sintering Behavior and Phase Characterisation of Composite Perovskite/Fluorite Ceramics for Intermediate Temperature SOFCs and Oxygen Separation Membranes

AbstractThe sintering behavior and structural changes of fluorite Gd0.2Ce0.8O2-δ(GDC) with one of three perovskites: LaMnO3 (LMO), (La0.7Sr0.3)0.98MnO3 (LSM) and La0.6Sr0.4Fe0.8Co0.2O3 (LSFC) composite ceramics were studied. Sintering was carried out for two hours at five different temperatures: 1100°C, 1200°C, 1300°C, 1400°C, and 1500°C. The highest sinterability has been found in LSFC + GDC. LSM + GDC appear to undergo two-step sintering mechanisms. Of all types of ceramics, only sintered LMO experiences a phase change, from rhombohedral R C to orthorhombic Pnma structure, with respect to its powder phase at sintering temperatures ≥1200 °C. The transition is notably suppressed when LMO is part of an LMO + GDC composite. GDC in a fluorite and/perovskite composite, when sintered, undergoes a temperature-dependent expansion in its unit cell that is not observed in pure GDC ceramics. This structural change will impact the function of composite ceramics as a fuel cell cathode, or oxygen separation membranes.

Structure of the intermediate phase of PbTe at high pressure

The evolution of PbTe with pressure has been reexamined using synchrotron x-ray diffraction. The phase transition at 6 GPa is not to the GeS (B16) or TlI (B33) type structures, as previously reported, but to an orthorhombic Pnma structure, with cell parameters a=8.157(1), b=4.492(1), and c=6.294(1)A at 6.7 GPa. This structure corresponds to a distortion of the low-pressure NaCl structure with a coordination intermediate between the sixfold B1 (NaCl) and the eightfold B2 (CsCl) structure. We discuss the stability of this new structure with respect to other proposed phases using numerical methods. These results may modify the admitted paths of phase transitions between the B1 and B2 structures.

The magnetic structure of Nd5Sn4

Nd 5 Sn 4 adopts the orthorhombic Pnma structure, and orders antiferromagnetically below 36(3) K. The neutron diffraction pattern at 4 K reveals that the magnetic space group is Pn′m′a′, a complex canted-antiferromagnetic structure. The Nd 4c magnetic mode is AxCz, and the Nd 8d magnetic mode is Ax−Gy−Cz−. The Nd magnetic moments are 1.76(8), 2.44(12), and 2.63(10)μB at 4c, 8d1, and 8d2 sites, respectively. These results are in agreement with our Mössbauer spectroscopy and magnetic measurements.

Pressure induced phase transitions in AmCm alloy

AbstractWe have studied the crystal structure of the AmCm binary alloy under high pressure by means of first-principles self-consistent total-energy calculations using the generalized gradient approximation (GGA) for the density functional theory (DFT). The virtual crystal approximation (VCA) is used for the description of the alloy system. In the present study, we investigated the double hexagonal (P63/mmc) structure, the face centered cubic (Fm3m) structure, the face-centered orthorhombic (Fddd) structure and the primitive orthorhombic (Pnma) structure for the AmCm alloy. Antiferromagnetic calculations have been compared with ferromagnetic calculations for all these phases. Our results are in general good agreement with recent experiment performed by Lindbaum et al. [J. Phys.: Condens. Matter. 15, S2297 (2003)].

The effect of a cation radii on structural, magnetic and electrical properties of doped manganites La 0.6− xPr xSr 0.4MnO 3

Structural, magnetic and transport properties of La 0.6− x Pr x Sr 0.4MnO 3 with x=0.0, 0.03, 0.06, 0.18, 0.3, 0.42, 0.54 and 0.6 are studied. The system exhibits a rhombohedrally distorted (R 3 ̄ c) perovskite structure for x⩽0.3. A rhombohedral–orthorhombic ( Pnma) structure transition is detected in the doping range from x=0.42 to 0.6. The structure refinement by Rietveld analysis of the X-ray powder diffraction data shows that the average distance Mn–O increases in the rhombohedral phases and decreases in the orthorhombic phases. Results show that the Curie temperature decreases from 374 to 310 K when 〈 r A 〉 varies from 1.254 to 1.231 Å. Electrical measurements show that all samples exhibit a metallic to semiconducting transition with increasing temperature. Meanwhile, the size of the resistivity ρ increases near T C. This phenomenon is interpreted as a gradual bending of the Mn–O–Mn bond angle, with decreasing 〈 r A 〉, which causes the narrowing of the electronic bandwidth and the effect of the A-site variance σ 2.

High temperature structural studies of SrRhO 3

High resolution X-ray powder diffraction studies have shown SrRhO 3 to transform from an orthorhombic Pnma structure at room temperature through an intermediate Imma phase to a tetragonal I4/ mcm structure near 800 °C. The orthorhombic Imma phase exists over a very limited temperature range, of less than 20°. The diffraction data suggests the Pnma to Imma transition is continuous and demonstrates that the Imma– I4/ mcm transition is first order.

Neutron diffraction and X-ray absorption study of CaMn0.6Ru0.4O3

Abstract Neutron diffraction and X-ray absorption measurements were carried out for a perovskite ferromagnet, CaMn0.6Ru0.4O3. While a previous X-ray diffraction had shown an orthorhombic Pnma structure, the present neutron diffraction at room temperature provided a better result for monoclinic P21/n. The analysis for the pattern at 4 K was consistent with the ferro- and antiferromagnetic phase separation at low temperatures. An X-ray absorption spectrum at the Ru–K edge showed that the valence of Ru is higher than 4+, which supports the proposed valency effect.

The magnetoresistance of (Tb 1− x Sm x) 0.6Sr 0.4MnO 3+ γ manganites

(Tb 1− x Sm x ) 0.6Sr 0.4MnO 3 γ compounds were obtained by sol–gel method, by using oxides and acetates. The compounds were sintered in O 2 atmosphere at 1200 °C. The samples contain only a perovskite phase, with orthorhombic (Pnma) structure. Dependence of the tolerance factor and of the variance vs. chemical composition of the samples was observed. An increase of Tb concentration leads to a sudden decrease of the specific magnetization of the samples, but has little influence on the Curie temperature. A typical magnetoresistance behaviour (a maximum of the resistivity vs. T) was observed for the sample without Tb. The samples containing Tb have a decrease of the resistivity ( ρ) vs. the intensity of the applied magnetic field and with the increase of the temperature (d ρ/d T <0, typical for a semiconductor). An increase of the activation energy of the charge transport takes place with the increase of the Tb concentration in the samples.

Structural phase transitions and stress accommodation in(La0.67Ca0.33MnO3)1−x:(MgO)xcomposite films

Composite $({\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}{)}_{1\ensuremath{-}x}:(\mathrm{MgO}{)}_{x}$ films were prepared by metalorganic aerosol deposition on a (100)MgO substrate for different concentrations of the (MgO) phase $(0&lt;~x&lt;~0.8).$ At $x\ensuremath{\approx}0.3$ a percolation threshold in conductivity is reached, at which an infinite insulating MgO cluster forms around the ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ grains. This yields a drastic increase of the electrical resistance for films with $x&gt;0.3.$ The film structure is characterized by x-ray diffraction and transmission electron microscopy. The local structure of the ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ within the film depends on the MgO concentration which grows epitaxially along the domain boundaries. A different structural phase transition from the orthorhombic Pnma structure to an unusual rhombohedral $R3\ifmmode\bar\else\textasciimacron\fi{}c$ structure at the percolation threshold $x\ensuremath{\approx}0.3$ is found for ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}.$ A three-dimensional stress accommodation in thick films through a phase transition is suggested.

Band gap in epitaxial NaCl-structure CrN(001) layers

B1–NaCl-structure CrN(001) layers were grown on MgO(001) at 600 °C by ultrahigh vacuum reactive magnetron sputter deposition in pure N2 discharges. X-ray diffraction analyses establish the epitaxial relationship as cube-on-cube, (001)CrN∥(001)MgO with [100]CrN∥[100]MgO, while temperature-dependent measurements show that the previously reported phase transition to the orthorhombic Pnma structure is, due to epitaxial constraints, absent in our layers. The resistivity increases with decreasing temperature, from 0.028 Ω cm at 400 K to 271 Ω cm at 20 K, indicating semiconducting behavior with hopping conduction. Optical absorption is low (α&amp;lt;2×104 cm−1) for photon energies below 0.7 eV and increases steeply at higher energies. In situ ultraviolet photoelectron spectra indicate that the density of states vanishes at the Fermi level. The overall results provide evidence for CrN exhibiting a Mott–Hubbard type band gap.

Electron rich manganites Pr 0.4A 0.6MnO 3: influence of A cations upon structure, magnetism and conductivity

The role of the A-site average cationic size 〈r A〉 upon the structure, the magnetic and resistive properties of Pr 0.4Ca 0.6MnO 3 has been studied by substituting strontium and barium for calcium. At room temperature, the electron microscopy study and refinements of the X-ray diffraction data demonstrate a progressive transformation from an orthorhombic Pnma structure to a cubic Pm3m one, through a tetragonal I4/mcm cell as 〈r A〉 increases. At low temperature (92K), the electron diffraction study reveals a more complex situation depending on 〈r A〉. Three types of charge-ordered (CO) phases with modulated structures are observed for the smaller 〈r A〉 values. For the largest 〈r A〉 values, two phases (without CO) are observed: a monoclinic distorted structure (involving an orthorhombic Fmmm supercell) and a cubic Pm3m structure. Accordingly, the resistivity ρ ( T) and magnetization M ( T) curves exhibit the characteristic signature of CO below T CO, i.e. ρ increases and M decreases. In contrast, the CO is suppressed by increasing 〈r A〉 but ρ shows a slight decrease, which is reminiscent of the A-type antiferromagnetism. This suggests a change of orbital filling, from d z 2 to d x 2− y 2 for the e g electron, as 〈r A〉 is increased.

Perovskite rare-earth nickelates in the thin-film epitaxial state

We have succeeded in the preparation of thin films of rare-earth nickelates RNiO3 (R=Pr, Nd, Sm, and Gd) under reduced oxygen pressure &amp;lt;0.02 bar by metalorganic chemical-vapor deposition owing to their epitaxial stabilization on perovskite substrates. The film–substrate lattice mismatch is critical for the epitaxial stabilization of RNiO3 phases. Increase of the lattice mismatch or film thickness results in the deposition of rare-earth oxides and NiO instead of RNiO3. The epitaxial films of nickelates were strained and consisted of 90° domains with the orthorhombic Pnma structure. The transport properties of the strained films on LaAlO3 were similar to those of the bulk material of the same composition under applied pressure of 9 kbar but they were different from the properties of the bulk material under ambient pressure. The result implies that transport properties of RNiO3 films with sharp metal-to-insulator transition can be effectively tuned by the control of the lattice strain.

A re-examination of the relationship between lattice strain, octahedral tilt angle and octahedral strain in rhombohedral perovskites

A new expression is proposed for the relationship between lattice strain 90°− αpc ( αpc: pseudo-cubic angle) and mean BO6 octahedral tilt angle &lt;ω&gt; in rhombohedral perovskites ABO3. It is derived from volumetric arguments, leading to a cubic equation which incorporates lattice strain 90° −αpc and octahedral elongation explicitly. Numerical solutions of this equation are derived for equally spaced values of octahedral strain, giving rise to a set of parametric curves which relate ω to 90° − αpc for different values of η. These curves can be represented as polynomials of the fourth degree, thereby enabling their routine use in the analysis of rhombohedral perovskite structures. It is anticipated that these parametric curves will supersede earlier work [Megaw &amp; Darlington (1975). Acta Cryst. A31, 161–173], in which an analytical expression was derived linking 90° − αpc, ω and octahedral strain for positive lattice strains only. By comparison, the relationship proposed here accommodates both negative and positive lattice strains. Correlations between values of &lt;ω&gt;, η, 90° − αpc and space-group symmetry are found, with an analysis of known rhombohedral and orthorhombic Pnma structures revealing the importance of cationic charges in determining symmetry. Since the polyhedral volume ratio VA/VB may be quantitatively related to &lt;ω&gt;, allowed values of tilt angle, lattice strain and octahedral elongation may be inferred for a given composition, which has characteristic values of VA and VB .

Lithium insertion into Fe 2( MO 4) 3 frameworks: Comparison of M = W with M = Mo

Room-temperature lithium insertion into the title framework structures can be represented as Fe 3+ 2( Mo 6+ O 4) 3 → 2n- BuLi Li 2 Fe 2+ 2( Mo 6+ O 4) 3 → 6n- BuLi [ Li + 8 Fe 2+ 2( Mo 4+ O 4) 3] Li + 2y Mo 4+ y Fe 2+ 2 O 2+3y+ Li + 8-2y Mo 4+ 3−y O 10−3y with 0 < y < 1 and Fe 3+ 2( W 6+ O 4) 3 → 2n- BuLi Li 2 Fe 2+ 2( W 6+ O 4) 3 → 10n- BuLi [ Li + 12 Fe 0 2( W 4+ O 4) 3]6 Li 2 O+2 Fe+3 WO 2 Electrochemical insertion/extraction is reversible for Li x Fe 2(WO 4) 3, 0 ≤ x ≤ 2.0, as in the molybdate system, a two-phase region extending over the range 0 < x ≤ 1.7. Lithium insertion suppresses the ferroelastic deformation of the unlithiated framework, but is otherwise topotactic for 0 ≤ x ≤ 2.0. Chemical delithiation to x ≈ 0.7 is possible for the two-phase product [Li 8Fe 2(MoO 4) 3]. The Li 2Fe 2( MO 4) 3 orthorhombic ( Pbcn) phases are metastable; they transform to a stable orthorhombic ( Pnma) structure at 537 K for M = Mo, 579 K for M = W. Li 2Fe 2(WO 4) 3 disproportionates to FeWO 4 and Li 2WO 4 above 723 K.