Valence changes of manganese and praseodymium in Pr1−xSrxMn1−yInyO3−δ perovskites upon cation substitution as determined with XANES and ELNES

Systematic valence changes in Pr_1−xSr_xMn_1−yIn_yO_3−δ upon cation substitution with Sr²⁺ and In³⁺ have been found using Mn K-edge and Pr L-edge X-ray absorption, and Mn L_II,III and Pr M_IV,V electron energy-loss spectroscopy. The average valence of the praseodymium ions is close to +3.0 and virtually constant over the sample set when the samples also contained manganese ions. Pr_0.5Sr_0.5InO_3−δ showed a distinct increase in the praseodymium valence state. In contrast, the average valence of the manganese ions changed from the trivalent state to intermediate values between +3.0 and +4.0 and approached the tetravalent state depending on the level of substitution. The knowledge of the valence is required to understand the conduction mechanisms in the material due to the small polaron hopping (electronic conductivity) and motion of oxygen ions along the vacancies (ionic conductivity). Addition of strontium and indium led to the formation of oxygen vacancies. A previously assumed intermediate valence of praseodymium as causal factor for the higher oxygen catalytic activity cannot be confirmed with room temperature measurements.     

Structural and magnetic inhomogeneity, phase transitions, magnetoresonance and magnetoresistive properties of La0.6 − x Pr x Sr0.3Mn1.1O3 (x = 0–0.6)

Ceramic samples of manganite perovskites La_{0.6 ? x} Pr _x Sr_0.3Mn_1.1O₃ (x = 0?0.6) have been studied using the X-ray diffraction, resistive, magnetic (χ_ac, ⁵⁵Mn NMR), microscopic, and magnetoresistive methods. It has been found that an increase in the praseodymium concentration x leads to a transition from the rhombohedral R $\bar 3$ c (x = 0–0.3) to orthorhombic Pbnm (x = 0.4–0.6) perovskite structure. It has been shown that the real perovskite structure contains anion and cation vacancies, whose concentrations increase with an increase in the praseodymium concentration x. A decrease in the metal-insulator phase transition temperature T _mi and the ferromagnetic-paramagnetic phase transition temperature T _c with increasing x correlates with an increase in the concentration of vacancies weakening the high-frequency electronic exchange Mn³⁺ ? Mn⁴⁺. For compositions with x = 0 and 0.1, when the lattice contains not only vacancies but also nanostructured clusters with Mn²⁺ in the A-positions, there is an anomalous hysteresis. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra of the compounds has revealed a high-frequency electronic exchange of the ions Mn³⁺ Mn⁴⁺ in the B-positions and a local heterogeneity of their surrounding by other ions (La²⁺, Pr³⁺, Sr²⁺) and vacancies. The phase diagram has demonstrated that there is a strong correlation between the composition, imperfection of the perovskite structure, phase transition temperatures T _mi and T _c , and magnetoresistive properties.     

Donor doping process and white light generation in CaMoO4 powders with multivalence Pr codoping

Both trivalent praseodymium (Pr³⁺) and quadrivalent praseodymium (Pr⁴⁺) were doped in molybdate powders. Visible emission from matrix was enhanced by multivalent Pr codoping. It was proposed that Pr³⁺ ions was donor and supplied quasi-free electron when Pr³⁺ took place the Pr⁴⁺ sites. The result showed that multivalence codoping would be an effective way to enhance emission of CaMoO₄. White light can be generated from Ca_0.98Pr_0.02MoO₄ powder via combination of broadband emissions originated from CaMoO₄ matrix and radiative transition of Pr³⁺. It showed warm white light with T_c of 3450 K that implies promising application in white light emitting diodes (LEDs).     

Electronic and Magnetic States of Pr and Mn in the Pr<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> Films Studied by XANES and XMCD Spectroscopy

The spectral dependences of X-ray absorption near-edge spectroscopy (XANES) and X-ray magnetic circular dichroism (XMCD) and the field dependences of XMCD near the K edge of Mn and the L_2,3 edges of Pr in the Pr_0.8Sr_0.2MnO₃ and Pr_0.6Sr_0.4MnO₃ films at T = 90 K are studied. The spectral dependences point to a mixed valence state of Mn and Pr in the films. It is found that, as compared to XANES, XMCD is more sensitive to the valence state of Pr⁴⁺. The field dependences of XMCD point to ferromagnetic behavior of Mn ions and the Van Vleck paramagnetism of Pr ions, which makes a significant contribution to the total magnetization of the films. It is shown that as the Sr concentration increases, the XMCD intensity at the K edge of Mn increases, which indicates a growth of the total magnetic moment of the film due to an increase in the 4p–3d hybridization.     

Relationships between the structural properties and the superconductivity in Y0:4Pr0:6Ba2-xSrxCu3O7-δ (x = 0;1:0)

The Y_0:4Pr_0:6Ba_2-xSr_xCu₃O_7-δ (x = 0;1:0) samples were prepared and the structural properties were studied by X-ray Rietveld analysis. It has been found that the average Pr-O bond length increases from 2.435Å at x = 0 to 2.457Å at x = 1:0, the Cu(2)-O-Cu(2) angles within the CuO₂ planes become larger, and there is no significant mutual substitution between Pr and Sr ions. The BVS calculation suggests that Sr doping brings about the increase of the hole concentration on CuO₂ planes and the weakening of Pr-O covalent mixing. Our work supports that Pr-O covalent mixing effect is responsible for the superconductivity suppression by Pr.     

Temperature evolution of the crystal structure of Bi1 − x Pr x FeO3 solid solutions

The crystal structure of solid solutions in the Bi_{1 ? x} Pr _x FeO₃ system near the structural transition between the rhombohedral and orthorhombic phases (0.125 ≤ x ≤ 0.15) has been studied. The structural phase transitions induced by changes in the concentration of praseodymium ions and in the temperature have been investigated using X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. It has been established that the sequence of phase transformations in the crystal structure of Bi_{1 ? x} Pr _x FeO₃ solid solutions with variations in the temperature differs significantly from the evolution of the crystal structure of the BiFeO₃ compounds with the substitution of other rare-earth elements for bismuth ions. The regions of the existence of the single-phase structural state and regions of the coexistence of the structural phases have been determined in the investigation of the crystal structure of the Bi_{1 ? x} Pr _x FeO₃ solid solutions. A three-phase structural state has been revealed for the solid solution with x = 0.125 at temperatures near 400°C. The specific features of the structural phase transitions of the compounds in the vicinity of the morphotropic phase boundary have been determined by analyzing the obtained results. It has been found that the solid solutions based on bismuth ferrite demonstrate a significant improvement in their physical properties.     

Radiation stability of M1 − xPrxF2 + x (M = Ca,Sr, Ba) crystals

The radiation stability of the mixed crystals M_1 ? xR_xF_2 + x (M = Ca, Sr, Ba) depends on types of the alkaline-earth and rare-earth ions. Different to Eu- and Ce-containing systems, M_1 ? xPr_xF_2 + x solid solutions have a low radiation resistance, which may be associated with hole trapping on praseodymium ion according to the reaction Pr³⁺ → Pr⁴⁺ which is typical for praseodymium. The coloration efficiency of M_1 ? xPr_xF_2 + x crystals grows in the row Ca → Sr → Ba, which is explained satisfactorily within the model of rare-earth clusters, the structure of which is determined by the ratio of the base alkaline-earth cation to the praseodymium ion radii.     

Investigation on cation distribution and magnetic properties in self-doped manganites La0.6−xSr0.4MnO3−δ

Self-doped manganites with nominal composition La_0.6−xSr_0.4MnO_3−δ (0≤x≤0.175) have been prepared by the sol–gel method. The X-ray diffraction (XRD) patterns and magnetic measurements indicate that the samples have two phases with the ABO₃ perovskite structure being the dominant phase and Mn₃O₄ being the minor phase when the doping level x≥0.05. On the basis of the thermal equilibrium theory of crystal defects, the contents of various ions in the perovskite phases were estimated, in which there are Mn²⁺ ions and no vacancies at A sites. The ion contents have been corrected by Rietveld fitting of the powder samples' X-ray diffraction data. The change tendency of the Curie temperature T_C vs. the Mn⁴⁺ ion content ratio at the B sites of ABO₃ perovskite phase is in accord with the experimental result of the samples La_1−xSr_xMnO₃.     

Effects of compositional phase transitions on luminescence of Sr1−xCaxTiO3:Pr

The mixed-compound of Sr_1−xCa_xTiO₃ has shown several compositional phase transformations. Photoluminescence and excitation spectra of the samples with different x and doped with 0.2% Pr³⁺ were investigated. Changes in the emission spectra were observed in different phases. The blue emission at 491 nm from ³P₀ state was found quite strong in the tetragonal phase, and was thermally quenched in the orthorhombic phases. The intensity of the red luminescence from ¹D₂ increases with increasing content of calcium. The strongest red emission is obtained from CaTiO₃:Pr³⁺. The results are discussed based on the configuration coordinate model and interaction of Pr with the charge transfer exciton state of the Ti complex.     

Co-L3 X-ray absorption near-edge structure analysis of Pr1−xCaxCoO3−δ and Pr1−xSrxCoO3−δ

The valence state of Co ions in Pr_1−xCa_xCoO_3−δ and Pr_1−xSr_xCoO_3−δ has been investigated by an analysis of the Co-L₃ X-ray absorption near-edge structure (XANES) profile. The observed intensity distributions of Co-L₃ XANES change continuously with increasing concentration of alkaline-earth ions. To investigate the origin of this change in the XANES profile, charge transfer multiplet calculations were carried out, which could successfully explain the change in the spectral profile; they also suggest that the valence state of Co ions in Pr_1−xCa_xCoO_3−δ and Pr_1−xSr_xCoO_3−δ is between 3+ and 4+ and increases gradually with the concentration of alkaline-earth ions.     

Structural and magnetic properties in the self-doped perovskite manganites with nominal composition La0.7Sr0.3−xMnO3−δ

Perovskite manganites with nominal composition La_0.7Sr_0.3−xMnO_3−δ (0.00≤x≤0.20) have been prepared by the sol-gel method with the highest heat treatment temperature being 1073 K. The XRD patterns indicate that when the doping level is x≤0.10 the samples have only a single phase, with the R3?c perovskite structure, while for x>0.10, the samples have two phases with the R3?c perovskite being the dominant phase and Mn₃O₄ being the second phase. A quantitative analysis and Rietveld fitting of the X-ray powder diffraction data indicate that on the basis of the thermal equilibrium theory of crystal defects there are Mn²⁺ ions at the A sites and Mn³⁺ plus Mn⁴⁺ ions at the B sites in the ABO₃ perovskite phase. The curves of magnetization versus applied magnetic field at 10 K showed that the magnetic moments of the Mn²⁺ ions at the A sites are antiparallel to those of the Mn³⁺ and Mn⁴⁺ ions at the B sites.     

Spectral-kinetic characteristics of Pr luminescence in LiLuF4 host upon excitation in the UV-VUV range

Spectral-kinetic study of Pr³⁺ luminescence has been performed for LiLuF₄:Pr(0.1 mol%) single crystal upon the excitation within 5-12 eV range at T=8 K. The fine-structure of Pr³⁺ 4f ²→4f 5d excitation spectra is shown for LiLuF₄:Pr(0.1 mol%) to be affected by the efficient absorption transitions of Pr³⁺ ions into 4f 5d involving 4f ¹ core in the ground state. Favourable conditions have been revealed in LiLuF₄:Pr(0.1 mol%) for the transformation of UV-VUV excitation quanta into the visible range. Lightly doped LiLuF₄:Pr crystals are considered as the promising luminescent materials possessing the efficient Pr³⁺³P₀ visible emission upon UV-VUV excitation. The mechanism of energy transfer between Lu³⁺ host ion and Pr³⁺ impurity is discussed.     

Magnesium,a promising dopant for the improvement of the CMR properties of perovskite manganites

The study of the substitution of magnesium for manganese in the type I Pr_0.7(Ca,Sr) _0.3MnO₃ and type II Pr_0.5Sr_0.5MnO₃ manganites has been performed. Remarkable colossal magneto resistance (CMR) properties have been evidenced for the manganites Pr_0.7Ca_0.2Sr_0.1 Mn_{1 x}Mg_xO₃, with x ≤ 0.02, for which R_OT/R_7T resistance ratio values ranging from 10⁴ to 4.10⁵ at 105 K and 70 K respectively were obtained in a magnetic field of 7 T. The study of the type II phases Pr_0.5Sr_0.5M_{1 x}Mg_xO₃, shows their similarity with the trivalent metal doped manganites Pr_0.5Mn_1?x M_xO₃ with M = Al, Ga, In, in contrast to the tetravalent metal doped manganites with M = Ti, Sn. The latter properties are interpreted in terms of two factors, the molar ratio Mn(III): Mn(IV), and the size of the doping cation.     

X-ray photoelectron spectroscopy analysis of (Ln1−xSrx)CoO3−δ (Ln: Pr,Nd and Sm)

The chemical states of the surface of (Ln_0.5Sr_0.5)CoO_3?δ (Ln (lanthanides) = Pr, Nd and Sm) used for cathode materials of intermediate temperature operating solid oxide fuel cells (IT-SOFCs) were investigated by X-ray photoelectron spectroscopy (XPS). Oxygen peaks comprised of lower binding energy (LBE) and higher binding energy (HBE) peaks from (Ln_0.5Sr_0.5)CoO_3?δ and Pr_0.3Sr_0.7CoO_3?δ (PSC37) showed that some merged oxygen peak behavior is a function of the Sr and lanthanide concentrations. By investigating the oxygen peaks, it was determined that more oxygen vacancies were generated on the surface of the cathodes when the lanthanides and Sr were substituted into perovskite oxides. When comparing the binding energies (BEs) of PSC37 with Pr_0.5Sr_0.5CoO_3?δ (PSC55), the LBE and HBE of the Sr peaks both increased when Sr was substituted at the A-site of a perovskite. Surface analysis of the Co peak on the surface of the cathode materials showed that the Co exists mainly as Co³⁺ and partially oxidized to Co⁴⁺ on the cathode materials. The partial existence of Co⁴⁺ can provide some polaron hopping providing electronic conduction for the solid oxide fuel cell.     

Structural and magnetic heterogeneities, phase transitions, 55Mn NMR, and magnetoresistive properties of La0.6Sr0.3 − x Bi x Mn1.1O3

The structural, resistive, magnetic, and magnetoresistive properties of the La_0.6Sr_{0.3 ? x} Bi _x Mn_1.1O₃ ceramics have been studied. The substitution of Bi ions for Sr ions increases the lattice parameter of the rhombohedral perovskite structure, decreases the metal-semiconductor and ferromagnet-paramagnet phase transition temperatures and the peak of the magnetoresistive effect, and increases the resistivity, approaching the system to the ferroelectric state. The ⁵⁵Mn NVR study indicates on the high-frequency Mn³⁺ ? Mn⁴⁺ superexchange and heterogeneity of the valence and magnetic states of manganese due to the nonuniformity of distribution of all ions and defects. The phase diagram has been constructed, which shows a strong correlation between the structural, magnetic, and magnetoresistive properties.     

Variations of ionic binding energies and the distribution of charge carriers in orthorhombic La2−x Sr x CuO4

The significance of heterovalent, substitutional disorder for the distribution of charge carriers in La_2?x Sr _x CuO₄ has been investigated. Disorder is shown to cause strong variations of binding energies of the ions ranging to some eV for Sr contentsx=0.1. Balancing the energy for a hole transport, Cu³⁺+O^2?→Cu²⁺+O^?, and taking binding energy variations into account, the process is realized to become possible without consuming energy for a subset Θ for allx Cu³⁺ in one formula unit of La_2?x Sr _x CuO₄. The functions Θ(x) are presented for hole transports to apex and in-plane oxygens, respectively. The delocalization of charge carriers is interpreted to be caused by valency disorder on metal lattice sites.     

Effect of Sr concentration on microstructure and magnetic properties of (Ba1−xSrx)(Ti0.3Fe0.7)O3 ceramics

Fe-doped (Ba_1−xSr_x)TiO₃ ceramics were prepared by solid-state reaction, and ferromagnetism was realized at room temperature. The microstructure and magnetism were modified by the Sr concentration control (0≤x≤75 at%) at a fixed Fe concentration, and the relevant magnetic exchange mechanism was discussed. All the samples are shown to have a single perovskite structure. When increasing the Sr concentration, the phase structure is transformed from a hexagonal perovskite into a cubic perovskite, with a monotonic decrease in lattice parameters induced by ionic size effect. The room-temperature ferromagnetism is expected to originate from the super-exchange interactions between Fe³⁺ on pentahedral and octahedral Ti sites mediated by the O²⁻ ions. The increase in Sr addition modifies two main influencing factors in magnetic properties: the ratio of pentahedral to octahedral Fe³⁺ and the concentration of oxygen vacancies, leading to a gradually enhanced saturation magnetization. The highest value, obtained for Fe-doped (Ba_0.25Sr_0.75)TiO₃, is an order of magnitude higher than that of the Fe-doped BaTiO₃ system with similar Fe concentration and preparation conditions, which may indicate (Ba_1−xSr_x)TiO₃ as a more suitable matrix material for multiferroic research.     

Neutron diffraction studies of the structure of substituted complex cobalt oxides

The effect of substitution of manganese and niobium for cobalt on the magnetic and crystal structure of doped cobaltites has been studied by the neutron diffraction method. The Pr_0.5Sr_0.5Co_{1 ? x} Mn _x O₃ samples undergo, with increasing x, a series of transitions of the crystal structure from the monoclinic I2/a to orthorhombic Immm phase, and then, to the high-temperature tetragonal I4/mcm phase and low-temperature orthorhombic Fmmm phase. Undoped Pr_0.5Sr_0.5CoO₃ is a ferromagnet, but the magnetic ordering is destroyed with increasing x. At high degrees of substitution of manganese, the A-type antiferromagnetic ordering is observed. Substitution of niobium for cobalt in La_{1 ? x} Sr _x CoO₃ compositions leads to changes in the Co-O bond length and the unit cell volume, which is accompanied by a decrease in the ferromagnetic moment. The Co → Nb substitution prevents the formation of Co⁴⁺ and leads to the stabilization of Co³⁺ in a high-spin state.     

Phase transformations in Pr1–x Sr x CoO3

The elastic properties and crystal structure of the Pr_1?x Sr _x CoO₃ system are studied. Two types of crystal structure transitions are found. For the composition x = 0.5, the monoclinic phase transforms to a rhombohedral one in the high-temperature transition (T ≈ 310 K), while the unit cell symmetry remains monoclinic though the unit cell parameters change drastically in the low-temperature transformation (T ≈ 110 K). It is suggested that the high-temperature transition is caused by the dimensional effect, while the low-temperature transition is associated with the presence of praseodymium ions actively involved in chemical bonding.     

Electronic property and structure of double-doping Y1−2xPrxCaxBa2Cu3O7−δ with 0 ⩽ x ⩽ 0.14

Equal amount Pr and Ca double-doping Y_1?2xPr_xCa_xBa₂Cu₃O_7?δ with 0 ? x ? 0.14 have been investigated by X-ray diffraction, resistivity, and X-ray photoemission spectroscopy (XPS). The deviation of the linearly decreasing of T_c vs. x curve was observed when x < 0.10. The resistivity and the temperature coefficient of resistivity also exhibit abnormal behaviors around x = 0.10. It is revealed that the conductivity behavior of Y_1?2xPr_xCa_xBa₂Cu₃O_7?δ with low Pr content (x < 0.10) is different from that of the relative high Pr content (x > 0.10), which suggests a change of Pr valence with the Pr content. XPS measurement shows that the relative amount of Pr³⁺ and Pr⁴⁺ is closely related to the total Pr content x. The valence of Pr is close to +3 when x < 0.10 and increases towards +4 when x > 0.10, which implies a different mechanism for depression of superconductivity of Pr content x < 0.10 from that of Pr content x > 0.10 in Pr doping Y-123.