Hydrothermal synthesis of the perovskite manganites Pr0.5Sr0.5MnO3 and Nd0.5Sr0.5MnO3 and alkali-earth manganese oxides CaMn2O4, 4H-SrMnO3, and 2H-BaMnO3

We report the synthesis of the perovskite manganites Pr_0.5Sr_0.5MnO₃ and Nd_0.5Sr_0.5MnO₃ using mild hydrothermal conditions. Both are formed as polycrystalline powders from solutions of metal salts in aqueous potassium hydroxide at 240 °C, and crystallise as a tetragonal polymorph (space group I4/mcm). Scanning electron microscopy shows both materials to contain cuboid-shaped crystallites several microns in dimension, and the average particle size is verified by light scattering measurements. We also report the first hydrothermal synthesis of 2H-BaMnO₃ and 4H-SrMnO₃, and the first subcritical hydrothermal synthesis of CaMn₂O₄ (marokite). Despite the formation of these alkali-earth manganese oxides at 240 °C, we have been unable to isolate rare-earth manganese oxides LnMnO₃ using similar conditions. We discuss the formation of perovskite manganites in hydrothermal reactions by relating our new results to those manganites already reported to form under hydrothermal conditions, and rationalise the trends seen by considering tolerance factor of the perovskite and the variance of the A-site metal radius.     

Crystal structures and phase transition in the system SrTiO3-La2/3TiO3

The distribution of A-site cations in the perovskite system La_xSr_1−3x/2TiO₃ depends on the concentration of La³⁺ ions and associated vacancies. For small x (x?0.2), the substitutions are expected to be random. For x?0.55, the cations are ordered in such a way that successive layers of A-sites are occupied to greater and lesser degree, and this ordering drives a tetragonal distortion. For x from about 0.3 to 0.5, the X-ray patterns show diffuse peaks indicative of similar ordering, but this is not long-range order and no tetragonal distortion results. The lower temperature structures also exhibit out-of-phase tilting of the TiO₆ octahedra, setting in at temperatures varying linearly with composition from 105 K for x=0, to about 650 K at x=2/3.     

The nature of the orthorhombic to tetragonal phase transition in Sr1−xCaxMnO3

The orthorhombic-tetragonal phase transition in the perovskite series Sr_1−xCa_xMnO₃ 0.4?x?0.6 has been studied by synchrotron X-ray powder diffraction. At room temperature the Ca rich oxides x?0.45 have the orthorhombic Pbnm superstructure whereas Sr_0.6Ca_0.4MnO₃ is two phases with both tetragonal I4/mcm and orthorhombic Pbnm. Analysis of the octahedral tilts suggest the co-existence of these two phases is a consequence of a first-order I4/mcm to Pbnm transition. The evolution of the structure of Sr_0.5Ca_0.5MnO₃ with temperature is also described and this is found to evolve from orthorhombic to tetragonal and ultimately cubic.     

A primitive tetragonal intermediate in the orthorhombic-cubic phase transition of perovskite-type strontium niobate Sr0.92NbO3

The Sr deficient perovskite Sr_0.92NbO₃ was synthesized from Sr₅Nb₄O₁₅ and Nb and its crystal structure was determined using powder neutron diffraction. At room temperature the structure is orthorhombic in space group Pnma with both in-phase and out-of-phase tilting of the NbO₆ octahedra. High temperature measurements have shown that the oxide undergoes a sequence of phase transitions with increasing temperature: Pnma→P4/mbm→Pm3¯m. The intermediate tetragonal phase has only in-phase tilts of the NbO₆ octahedra, rather than the out-of-phase tilts present in the more commonly observed I4/mcm structure, due to initial softening at the M point rather than R point. The tetragonal phase exists only over a very narrow temperature range. The importance of M-M and M-O bonding in controlling the transition temperatures in SrMO₃ perovskites is discussed.     

Crystal structure, magnetic, and dielectric properties of Aurivillius-type Bi3Fe0.5Nb1.5O9

Bi₃Fe_0.5Nb_1.5O₉ was synthesized using conventional solid state techniques and its crystal structure was refined by the Rietveld method using neutron powder diffraction data. The oxide adopts an Aurivillius-type structure with non-centrosymmetric space group symmetry A2₁am (a=5.47016(9) Å, b=5.43492(9) Å, c=25.4232(4) Å), analogous to other Aurivillius compounds that exhibit ferroelectricity. The Fe and Nb cations are disordered on the same crystallographic site. The [(Fe,Nb)O₆] octahedra exhibit tilting and distortion to accommodate the bonding requirements of the Bi cations located in the perovskite double layers. Magnetic measurements indicate non-Curie-Weiss-type paramagnetic behavior from 300 to 6 K. Measurements of dielectric properties and electrical resistivity exhibited changes near 250-260 °C and are suggestive of a ferroelectric transition.     

Rhombohedral-cubic transition in Li0.2Na0.3La0.5TiO3 perovskite

High-temperature behavior of the fast ionic conductor Li_0.2Na_0.3La_0.5TiO₃ has been investigated by neutron powder diffraction between 300 and 1073 K. The Rietveld analysis of diffraction patterns showed around 1000 K a change from rhombohedral () to cubic () symmetry. During the heating, the tilting of octahedra along the [111] direction of the cubic perovskite decreased and the rhombic distortion of oxygen square windows that relates contiguous A-sites of the perovskite was eliminated. The influence of the octahedral tilting on Li mobility is finally discussed.     

Magnetoresistance in La0.5Sr0.5MnO2.5

Resistance measurements indicate the presence of magnetoresistance in the La_0.5Sr_0.5MnO_2.5 brownmillerite related compound. An 80 % of magnetoresistance is found at 75 K. In spite of the partial break‐up occurring at the 3D network of octahedra sharing corners, characteristic of the full oxygen content perovskite phase, the oxygen deficient compound exhibits complex magnetic and electric properties. Such behavior can be explained on the basis of ferromagnetic and metallic clusters randomly distributed at the octahedral layers separated from each other by an insulating antiferromagnetic matrix. AC susceptibility measurements suggest spin glass behavior at low temperature as a consequence of the competition between different magnetic interactions.     

Magnetic properties of La0.5−xLnxSr0.5MnO3 (Ln=Pr, Nd, Gd and Y)

Magnetic and electron transport properties of four series of manganates of the composition La_0.5−xLn_xSr_0.5MnO₃ (Ln=Pr, Nd, Gd and Y) have been investigated to examine how the ferromagnetic metallic nature of the parent La compound changes over to antiferromagnetic insulating behavior, with change in Ln and x due to the associated changes in the A-site cation radius as well as the size disorder. When Ln=Pr and Nd, there is a transition from the tetragonal I4/mcm structure to the orthorhombic Immm and Imma structures at x=0.2 and 0.35, respectively. There is a gradual evolution of the properties from those of La_0.5Sr_0.5MnO₃ to those of Pr_0.5Sr_0.5MnO₃ or Nd_0.5Sr_0.5MnO₃ with increase in x. Thus, when x>0.2 and >0.35, respectively, the Pr- and Nd-substituted manganates show ferromagnetic transitions followed by antiferromagnetic transitions at low temperatures, with the ferromagnetic T_C decreasing with increasing x. The Gd and Y series of compounds are all orthorhombic and show a decrease in T_C with the increase in x, the ferromagnetism disappearing at high x. At a value of x corresponding to the A-site cation radius of Pr_0.5Sr_0.5MnO₃, the Gd and Y series of compounds exhibit ferromagnetism in the 250-300 K region and undergo an antiferromagnetic transition on cooling. The T_C−T_N gap is sensitive to the disorder arising from the size mismatch.     

Preparation, structural study from neutron diffraction data and magnetism of the disordered perovskite Ca(Cr0.5Mo0.5)O3

We report on the preparation and characterization of the Ca(Cr_0.5Mo_0.5)O₃ perovskite, obtained in the search of the hypothetical double perovskite Ca₂CrMoO₆. This material was prepared in polycrystalline form by solid state reaction in H₂/Ar flow. It has been studied by X-ray and neutron powder diffraction (NPD) and magnetic measurements. Ca(Cr_0.5Mo_0.5)O₃ crystallizes in the orthorhombic Pbnm (No. 62) space group, with the unit-cell parameters a=5.4110 (4) Å, b=5.4795 (5) Å, c=7.6938 (6) Å. There is a complete disordering of Cr³⁺ and Mo⁵⁺ over the B-site of the perovskite, and the (Cr,Mo)O₆ octahedra are tilted by 12.4° in order to optimize the Ca-O bond lengths. The magnetic susceptibility is characteristic of a ferrimagnetic behavior, with T_C=125 K, and a small saturation magnetization at T=5 K, of 0.05 μ_B/f.u.     

On the novel double perovskites A2Fe(Mn0.5W0.5)O6 (A= Ca,Sr, Ba). Structural evolution and magnetism from neutron diffraction data

New A₂Fe(Mn_0.5W_0.5)O₆ (A = Ca, Sr, Ba) double perovskite oxides have been prepared by ceramic techniques. X-ray diffraction (XRD) complemented with neutron powder diffraction (NPD) indicate a structural evolution from monoclinic (space group P2₁/n) for A = Ca to cubic (Fm-3m) for A = Sr and finally to hexagonal (P6₃/mmc) for A = Ba as the perovskite tolerance factor increases with the A²⁺ ionic size. The three oxides present different tilting schemes of the FeO₆ and (Mn,W)O₆ octahedra. NPD data also show evidence in all cases of a considerable anti-site disordering, involving the partial occupancy of Fe positions by Mn atoms, and vice-versa. Magnetic susceptibility data show magnetic transitions below 50 K characterized by a strong irreversibility between ZFC and FC susceptibility curves. The A = Ca perovskite shows a G-type magnetic structure, with weak ordered magnetic moments due to the mentioned antisite disordering. Interesting magnetostrictive effects are observed for the Sr perovskite below 10 K.     

Neutron powder diffraction study of phase transitions in Sr2SnO4

The phase transitions in Sr₂SnO₄ at high temperature have been studied using high resolution time-of-flight powder neutron diffraction. The room temperature structure of Sr₂SnO₄ is orthorhombic (Pccn), which can be derived from the tetragonal K₂NiF₄ structure by tilting the SnO₆ octahedra along the tetragonal [100]_T- and [010]_T-axes with non-equal tilts. At the temperature of about 423 K, it transforms to another orthorhombic structure (Bmab) characterized by the SnO₆ octahedral tilt around the [110]_T-axis. At still higher temperatures (∼573 K) the structure was found to be tetragonal K₂NiF₄-type (I4/mmm).     

Composition- and temperature-dependent phase transitions in 1:3 ordered perovskites Ba4−xSrxNaSb3O12

A series of 25 members of the 1:3 ordered perovskite family of the type Ba_4−xSr_xNaSb₃O₁₂ has been synthesized and their structures determined using synchrotron X-ray and neutron powder diffraction techniques. At room temperature the sample Ba₄NaSb₃O₁₂ has a cubic structure in space group with a=8.2821(1) Å, where the Na and Sb cations are ordered in the octahedral sites but there is no tilting of the (Na/Sb)O₆ octahedra. As the average size of the A-site cation decreases, through the progressive replacement of Ba by Sr, tilting of the octahedra is introduced firstly lowering the symmetry to tetragonal in P4/mnc then to orthorhombic in Cmca and ultimately a monoclinic structure in P2₁/n as seen for Sr₄NaSb₃O₁₂ with a=8.0960(2) Å, b=8.0926(2) Å, c=8.1003(1) Å and β=90.016(2)°. The powder neutron diffraction studies show that the orthorhombic and tetragonal phases in Cmca and P4/mnc co-exist at room temperature for samples with x between 1.5 and 2.     

High-resolution neutron powder diffraction study on the structure of Sr2SnO4

From the high-resolution time-of-flight neutron powder diffraction data, the crystal structure of Sr₂SnO₄ at the temperature range between 4 and 300 K has been investigated. The Rietveld refinement has shown that Sr₂SnO₄ belongs to the space group Pccn, which can be derived from the tetragonal K₂NiF₄ structure by tilting the SnO₆ octahedra along the [100]_T- and [010]_T-axis, respectively, with non-equal tilts. The earlier reported first-order phase transition in Sr₂SnO₄, from Bmab to P4₂/ncm, has not been observed.     

Structural studies of the disorder and phase transitions in the double perovskite Sr2YTaO6

The evolution of the crystal structure of the double perovskite Sr₂YTaO₆ from room temperature to 1250 °C has been studied using powder neutron and synchrotron X-ray diffraction. At room temperature Sr₂YTaO₆ crystallises in a monoclinic superstructure with the space group P2₁/n. The tilting of the octahedra evident in the room temperature structure is progressively lost on heating, resulting in a sequence of phase transitions that ultimately yields the cubic structure in space group Fm3?m. The high temperature tetragonal and cubic phases are characterised by strongly anisotropic displacements of the anions. The amount of defects in the crystal structure of Sr₂YTaO₆ is found to be sensitive to the preparative method.     

Synthesis, crystal structure and magnetic properties of the Sr2Al0.78Mn1.22O5.2 anion-deficient layered perovskite

A new layered perovskite Sr₂Al_0.78Mn_1.22O_5.2 has been synthesized by solid state reaction in a sealed evacuated silica tube. The crystal structure has been determined using electron diffraction, high-resolution electron microscopy, and high-angle annular dark field imaging and refined from X-ray powder diffraction data (space group P4/mmm, a=3.89023(5) Å, c=7.8034(1) Å, R_I=0.023, R_P=0.015). The structure is characterized by an alternation of MnO₂ and (Al_0.78Mn_0.22)O_1.2 layers. Oxygen atoms and vacancies, as well as the Al and Mn atoms in the (Al_0.78Mn_0.22)O_1.2 layers are disordered. The local atomic arrangement in these layers is suggested to consist of short fragments of brownmillerite-type tetrahedral chains of corner-sharing AlO₄ tetrahedra interrupted by MnO₆ octahedra, at which the chain fragments rotate over 90°. This results in an averaged tetragonal symmetry. This is confirmed by the valence state of Mn measured by EELS. The relationship between the Sr₂Al_0.78Mn_1.22O_5.2 tetragonal perovskite and the parent Sr₂Al_1.07Mn_0.93O₅ brownmillerite is discussed. Magnetic susceptibility measurements indicate spin glass behavior of Sr₂Al_0.78Mn_1.22O_5.2. The lack of long-range magnetic ordering contrasts with Mn-containing brownmillerites and is likely caused by the frustration of interlayer interactions due to presence of the Mn atoms in the (Al_0.78Mn_0.22)O_1.2 layers.     

Magnesium substitution in Nd0.7Sr0.3MnO3

Magnesium substitution in Nd_0.7Sr_0.3MnO₃ has been studied by neutron powder diffraction. Polycrystalline samples of nominal compositions Nd_0.7Sr_0.3Mn_1−yMg_yO₃ with y=0.0, 0.1, 0.2 and 0.3 were synthesized by the standard solid-state reaction method. Rietveld refinements of the neutron powder diffraction data showed that all samples had distorted perovskite structure of orthorhombic symmetry. Mg initially preferred to substitute for Nd and only at Mg concentration greater than 0.1, a substantial substitution for Mn occurred. Our study also showed that Mg-substitution did not change the crystal structure of Nd_0.7Sr_0.3MnO₃.     

A Fe Mössbauer study of charge ordering and phase separation in the rare-earth manganates, Nd0.5Ca0.5MnO3 and Nd0.5Sr0.5MnO3

Mössbauer studies of 2% ⁵⁷Fe-doped Nd_0.5Ca_0.5MnO₃ and Nd_0.5Sr_0.5MnO₃ have been carried out over the 4.2-300 K range after ensuring that such doping does not change their basic properties. The charge-ordering transition in these manganates is marked by abrupt changes in the quadrupole splitting. In the case of Nd_0.5Ca_0.5MnO₃, two phases manifest themselves on cooling below the charge-ordering transition temperature. The evolution of the spectra as a function of temperature shows that long-range magnetic order does not occur sharply. The observed evolution with temperature is different in the two materials studied. In Nd_0.5Ca_0.5Mn_0.98⁵⁷Fe_0.02O₃, it resembles that of a disordered magnetic material, whereas the temperature dependence of line shape of Nd_0.5Sr_0.5Mn_0.98⁵⁷Fe_0.02O₃ is typical of a superparamagnetically relaxed magnetic system. Although both the manganates show well-resolved magnetic hyperfine spectra at 4.2 K, the lines are slightly broad indicating possible coexistence of phases at low temperatures. A weak paramagnetic signal is also seen in the spectra of both the manganates at 4.2 K.     

Anion vacancy ordering in Sr7Mn4O15−x phases

The synthesis of phases of composition Sr₇Mn₄O_15−x 0<x<3 is reported via the topotactic manipulation of the anion lattice of Sr₇Mn₄O₁₅. In addition the reduced, cation substituted phases BaSr₆Mn₄O₁₃ and Sr₆CaMn₄O₁₃ are described. Structural characterisation using powder X-ray and neutron diffraction data reveals that in all phases anions are selectively removed from sites which link the MnO₆ octahedra in the structure in an apex-sharing manner. An explanation based on the total lattice energy of the anion deficient phases is presented to account for this unusual selectivity.     

Synthesis, structure, and magnetic properties of SrMn1−xGaxO3−δ (x=0-0.5) perovskites

We report the synthesis of SrMn_1−xGa_xO_3−δ perovskite compounds and describe the dependence of their phase stability and structural and physical properties over extended cation and oxygen composition ranges. Using special synthesis techniques derived from thermogravimetric measurements, we have extended the solubility limit of random substitution of Ga³⁺ for Mn in the cubic perovskite phase to x=0.5. In the cubic perovskite phase the maximum oxygen content is close to 3−x/2, which corresponds to 100% Mn⁴⁺. Maximally oxygenated solid solution compounds are found to order antiferromagnetically for x=0-0.4, with the transition temperature linearly decreasing as Ga content increases. Increasing the Ga content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn_0.5Ga_0.5O_2.67(3) below 12 K. These properties are markedly different from the long-range antiferromagnetic order below 180 K observed for the layer-ordered compound Sr₂MnGaO_5.50 with nominally identical chemical composition.