Pr 4f Research Articles

Effect of Pr substitution on the electronic structure of (La 1− xPr x) 0.85Zr 0.15MnO 3

We measured Mn L 3,2-, K-, O K- and Pr M 5,4-edges X-ray absorption near-edge structure (XANES) spectra of (La 1− x Pr x ) 0.85Zr 0.15MnO 3 (denoted as LPZMO) with x = 0, 0.05, 0.1, 0.15 and 0.2 to study their electronic structures and the effect of Pr substitution. An analysis of the Mn L 3,2- and the K-edges spectra suggests that LPZMO is an electron-doped system. The average charge state of Mn ion is found to increase with the Pr substitution at low concentration as inferred from the enhancement of the unoccupied ↑spin e g subband in the Mn L 3,2-edge XANES spectrum. The Mn L 3,2-, O K- and Pr M 5,4-edge XANES results revealed changes in the O 2p–Mn 3d and O 2p–Pr 4f hybridized states due to the substitution of La ions by smaller Pr ions.

Induction of superconductivity in Y 0.4Pr 0.6Ba 2− xSr xCu 3O 7 system with increasing Sr substitution

Samples of Y 0.4Pr 0.6Ba 2− x Sr x Cu 3O 7 (0.0 ⩽ x ⩽ 1.0) have been synthesized by a solid-state reaction route. The samples with x ⩽ 0.60 crystallize in an orthorhombic structure (with orthorhombic distortion decreasing with increasing x), while the samples with x = 0.80 and 1.0 crystallize in a pseudo tetragonal structure. Resistance ( R) measurements as a function of temperature ( T) show that x = 0.0 sample is highly semiconducting with R 2 K / R 300 K ratio of ∼65. This ratio decreases to only ∼23 and 5 for x = 0.20 and x = 0.4 samples, respectively. Further, the x = 0.60 sample shows onset of a broad superconducting transition temperature ( T c onset ) at around 20 K without achieving the T c R = 0 state down to 2 K. The T c R = 0 state is observed in x = 0.80 and 1.00 samples at around 5 and 14 K, respectively. Thermoelectric power ( S), also exhibits T c S ∼ 0 state at around 11 K and 16 K, respectively for x = 0.8 and 1.00 samples. Thermoelectric power at room temperature is positive and decreases with increasing x, indicating enhanced number of mobile holes. These results demonstrate that substitution of Sr at Ba site systematically induces metallicity and eventually superconductivity in Y 0.4Pr 0.6Ba 2− x Sr x Cu 3O 7 system. The results are explained on the basis of reduced Pr(4f) orbital hybridization with O(2p) in superconducting Cu–O 2 planes, resulting in de-localization of the mobile carriers. The role of decreased Pr/Ba intermixing disorder is also considered.

Magnetic and transport properties in La 1− xPr xCoO 3 single crystals

We have investigated the magnetic and electronic transport properties in the single crystals of La 1− x Pr x CoO 3 system. The signs of the thermoelectric power and the Hall coefficient change from positive to negative by the 10% Pr-doping in LaCoO 3, and they change from negative to positive by 10% La-doping in PrCoO 3. These results imply that the sign of the carrier of the system changes through the hybridization between Pr(4f) and O(2p π ) orbitals depending on the Pr concentration.

Two ferromagnetic transitions in Pr 0.8Ca 0.2MnO 3

The magnetic and structural properties of a single crystal of Pr 0.8Ca 0.2MnO 3 have been investigated by neutron powder diffraction and bulk magnetization as a function of temperature. Two ferromagnetic (FM) transitions near 140 and 70 K, which are ascribed to the ordering of Mn and Pr spins, respectively, are clearly evidenced by both measurements. The low temperature FM transition is largely dependent upon a strong exchange coupling between the localized Pr 4f and the neighboring itinerant Mn 3d spins. Near 140 K distinctive changes occur in lattice parameters, whereas no anomalous variation develops near 70 K.

RESONANT PHOTOEMISSION STUDY OF RFe4P12 (R = La, Ce, Pr)

We have measured the 4d–4f high-resolution resonant photoemission spectra of RFe 4 P 12 (R = La, Ce, Pr), PrSn 3 and PrAl 2. The Ce and Pr 4f spectra show the spectral features corresponding to a strongly hybridized system. However, the [Formula: see text] and the 3H4 peaks located just below the Fermi energy in the Ce 4f and Pr 4f spectra, respectively, are observed as a shoulder instead of the prominent peak expected for the strongly hybridized or high-Kondo temperature systems.

Charge density studies utilizing powder diffraction and MEM. Exploring of high Tc superconductors, C60 superconductors and manganites

Abstract The recent progress of the accurate charge density studies by the Maximum Entropy Method(MEM) utilizing X-ray powder diffraction is reviewed with some examples. Results for PrBCO (PrBa2Cu3O7-δ), YBCO (YBa2Cu3O7-δ), C60 superconductors (Rb2CsC60, K2RbC60, Na2RbC60) and the layered manganite, NdSr2Mn2O7, which is well known as colossal magnetoresistive (CMR)-related material, are given. For non-super conductor, PrBCO, it is found in the MEM charge density that there exist directional robes of the charge density from Pr atom toward the O atoms in the CuO2 conduction planes. On the other hand, for a very well known high Tc super conductor, YBCO, appreciable charge densities in the interatomic region around the Y atom is not recognized in the MEM charge density. The distinct difference of the charge densities between PrBCO and YBCO presents clear experimental evidence of the hybridization between Pr(4f)–O(2pπ) orbitals which supports the idea that the hole trapping by the hybridizes states suppresses the superconductivity in PrBCO. The MEM charge densities of the fullerene superconductors, Rb2CsC60, K2RbC60 and Na2RbC60, show distinct structural differences from that of non-superconductors, C60 and Li2CsC60, reflecting the superconducting properties. And the charge deficiencies of the doped metal atoms, which should be associated with charge transfer from the metal atoms to the C60 molecule seems to have strong correlation to the superconducting transition temperature, Tc. As the bigger the charge charge trnsfer, the higher the Tc. In the accurate MEM charge density of antiferromagnetic manganite, NdSr2Mn2O7,the direct imaging of spontaneous ordering of the dx 2-y 2 orbital indicating anisotropic exchange couplings between the local-spins on the Mn sites, which causes an unique layered-type spin order. The theoretical background of the MEM is also mentioned in some detail.

Magnetic Order in Perovskite Pr1−xBaxCoO3 Studied by Magnetic Circular Dichroism (MCD) Spectroscopy

Magnetic properties have been studied for the perovskite Pr1−xBaxCoO3 (0≤x≤0.5). Ferromagnetic order is observed below ∼190 K for x≥0.2 in dc susceptibility–temperature curves. Magnetic circular dichroism spectra were measured for a ferromagnetic phase at Co–L2,3, Pr–M4,5, Ba–M4,5, and O–K edges. It is revealed that the Co ion has both spin and orbital moments. It is also observed that the Ba 4f orbitals have a localized moment and exhibit ferromagnetic order at low temperatures, though Ba2+ is expected to be almost only in a nonmagnetic 4f0 state. Magnetic order is observed also for Pr 4f and O 2p.

Photoemission spectroscopy of half-metallic manganese compounds: perovskite versus Heusler

We have investigated the electronic structures of the Mn-based half-metallic ferromagnetic systems of Pr 1− x Sr x MnO 3 (PSMO) and PtMnSb Heusler compound, using photoemission spectroscopy. The partial spectral weight (PSW) distributions of Mn 3d, Pr 4f, and Pt 5d electrons have been determined and compared to the LSDA band structure calculations. The overall features of the Mn 3d PSWs of PtMnSb and PSMO are found to be very similar, except for the region near E F. The reduced spectral intensity at E F in the Mn 3d PSW of PSMO suggests the localized nature of the Mn e g electrons in PSMO due to the lattice polaron formation.

Structural phase transitions and the effect on Pr anomalous ordering for the PrBa 2− xLa xCu 3O 7+ y system

Complex crystal symmetry variation is observed for the oxygenated PrBa 2− x La x Cu 3O 7+ y system (0⩽ x⩽1, −0.12⩽ y⩽0.39). Powder X-ray Rietveld analysis indicates that the crystal structure changes from the PrBa 2Cu 3O 7 or CuBa 2PrCu 2O 7 1212C-type (C for chain) orthorhombic O(I)-phase (space group Pmmm) to 1212-type tetragonal T-phase (P4/mmm) around x=0.45, and then to a new 1212-type of orthorhombic O(II)-phase (Cmmm) around x=0.65. However, regardless of the structural change, anomalous Pr ordering temperature T N(Pr) decreases monotonically from 18 K for PrBa 2Cu 3O 6.88 to 2.8 K for PrBa 1.3La 0.7Cu 3O 7.24. The increasing Pr–O bond length with decreasing T N indicates that Pr ordering is closely correlated with the wave function overlap between Pr–4f orbital and O–2p π orbital in the CuO 2 bi-layers.

Electronic structure of PrBa2Cu3O7 and YBa2Cu3O7: A local spin density approximation with the on-site Coulomb interaction study

Using the self-consistent linear muffin-tin orbitals and the atomic sphere approximation method, the band structure calculations for PrBa2Cu3O7 and YBa2Cu3O7 are performed in the regime of the local spin density approximation with the on-site Coulomb interaction (LSDA+U). The strong electron-electron correlation effects are considered for both the Pr 4f and Cu 3d states. Comparing with the LSDA calculations, the LSDA+U calculations show that, while the essential feature of energy band of YBa2Cu3O7 is preserved, the electronic structure of PrBa2Cu3O7 is quite different. For the bands arisen from the CuO2 planes, a gap with a value of 0.5 eV is opened, indicating the insulating or semiconducting character of the CuO2 planes. The pdσ bands associated with the CuO chains are across the Fermi level, revealing the metallic character of the CuO chains.

Hole states in Eu0.9 − xPrxCa0.1BaSrCu3O7 − δ studied by X-ray absorption spectroscopy

The hole states located on different oxygen sites have been investigated in Eu0.9 – xPrxCa0.1BaSrCu3O7 – δ with increasing Pr3+ substitution by high-resolution oxygen K-edge and copper L-edge X-ray absorption spectra. The results reveal that the hole states are depleted systematically by the substitution and in turn lead to a reduction in Tc. However, the rate at which the depletion occurs is lower in Eu0.9 – xPrxCa0.1BaSrCu3O7 – δ compared to that in Y1 – xPrxBa2Cu3O7 – δ and Dy1 – xPrxBa2Cu3O7 – δ. This is due to the partial substitution of Ba2+ by Sr2+ ions and R (rare earth element) by Ca2+ ions in the present case. The depletion of hole states gives evidence in support of the hole-depletion models based on Pr 4f–O 2pπ hybridization. The possible reasons for the anomalous behavior of Pr3+ ions in RBa2Cu3O7 – δ are also discussed.

Study on the crystal and electronic structure of ceramics

(PrBCO) is unique in the ( R = rare earth) series because it is not superconducting. In fact, for , drops monotonically with Pr concentration, with going to zero at 55% Pr. There have been many studies of this material with the hope that an explanation for the lack of superconductivity in PrBCO might help explain why is superconducting. Unfortunately, to date, none of the proposed explanations has been totally satisfactory and without objection due to the considerable controversy of the results obtained by different experiments. In this paper, a series of samples of (YPrBCO) have been studied by x-ray diffraction, Raman spectroscopy and XPS. The results reveal that the Pr dopant does not change the orthorhombic symmetry of the samples. The peak positions at 502 and of the Raman spectra become shifted for different Pr concentration, which suggests a change in oxygen content upon substitution of Pr ion for Y ion. There are two valences of Pr ions in YPrBCO. The valence of Pr is changed with change in Pr content x. The relative content of and is closely related to the total Pr content x. The valence of Pr is close to 3+ for small x, and is increased as x increases. The valence change of the Pr ion would directly result in the increase of oxygen content. A ligand hole localized on the Pr-O(2)/O(3) band would remove a hole from the conduction band and yield a formal site. The strong hybridization between Pr 4f electrons and conduction holes in the planes induces a localization of mobile holes causing the suppression of superconductivity in the YPrBCO system. In addition, the existence of some extra oxygen atoms around Pr will also affect the distortion of the planes and possibly assist in the suppression of the superconductivity.

Impact of Pr on structural, superconducting and magnetic properties of Y 1− xPr xBaSrCu 3O 7

We study the structural, superconducting and magnetic properties of the Y 1− x Pr x BaSrCu 3O 7 system with x=0.0, 0.20, 0.25, 0.30, 0.40, 0.50, 0.60, 0.75 and 1.0, from X-ray diffraction, AC and DC magnetic susceptibility measurements. X-ray diffraction results reveal that while Pr substitutes isostructurally until x=0.50 in Y 1− x Pr x BaSrCu 3O 7, for x=0.75 and 1.0 few small intensity un-reacted lines are observed in the spectrum. The orthorhombic distortion decreases with an increase in x. Both x=0.75 and 1.0 samples are tetragonal. The c lattice parameter of the substituted samples increases with x, indicating probable substitution of comparatively bigger ionic radii Pr 3+/4+ ion at Y 3+ in Y 1− x Pr x BaSrCu 3O 7 system. The AC susceptibility results showed that the transition temperature T c of the Y 1− x Pr x BaSrCu 3O 7 system decreases monotonically with an increase in x. Both x=0.75 and 1.0 samples do not show a superconducting transition down to 4.2 K. The critical concentration of Pr to completely suppress superconductivity in Y 1− x Pr x BaSrCu 3O 7 is higher (0.75) than that reported (0.55) for Y 1− x Pr x Ba 2Cu 3O 7. DC magnetic susceptibility measurements done on PrBaSrCu 3O 7 and PrBa 2Cu 3O 7 samples at 0.5 T revealed that while the Pr ordering temperature seems to be 12 K for the former, the same is 17 K for the latter. These results indicate that the ordering temperature of Pr moments in RE 1− x Pr x : 123-type systems, which decides the Pr 4f hybridisation with the Cu–O conduction band has a direct connection to the suppression of superconductivity. The lower T N of Pr explains the less destructive effect of the same on the superconductivity of the parent undoped system.

X-ray absorption spectroscopy of detwinnedPrxY1−xBa2Cu3O7−ysingle crystals:Electronic structure and hole distribution

Substituting Y in orthorhombic (Y,R)${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ by any rare-earth element R has generally little effect on the superconducting properties. For R=Pr, however, superconductivity is completely suppressed. To understand this effect we have studied the unoccupied electronic structure of ${\mathrm{Pr}}_{\mathrm{x}}$${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{y}}$ (x=0.0,f0.4,f0.8) using polarization-dependent O 1s near-edge x-ray absorption spectroscopy of detwinned single crystals. We identify the hole states in the ${\mathrm{CuO}}_{2}$ planes and the ${\mathrm{CuO}}_{3}$ chains and give estimates of the relative contributions of the O ${2\mathrm{p}}_{\mathrm{x}}$, O ${2\mathrm{p}}_{\mathrm{y}}$, and O ${2\mathrm{p}}_{\mathrm{z}}$ orbitals to these states. Along with the comparison of oxygen-rich (y\ensuremath{\approx}0.1) to the oxygen-depleted materials (y\ensuremath{\approx}0.9), this allows a test of the current theoretical explanations for the Pr-induced suppression of superconductivity. While we can rule out models involving hole filling or charge transfer between the planes and the chains, our data are consistent with approaches based on Pr 4f--O ${2\mathrm{p}}_{\mathrm{\ensuremath{\pi}}}$ hybridization.

Magnetic properties of Pb2Sr2PrCu3O8

The magnetic properties of Pb2Sr2PrCu3O8 were determined using X-ray absorption, inelastic neutron spectroscopy and magnetic susceptibility measurements. X-ray absorption on the Pr L3-edge strongly indicates a trivalent oxidation state. Inelastic neutron scattering results are modeled assuming a 3H4 ground-state multiplet split by a crystalline electric field potential similar to PrBa2Cu3Ox. This potential correctly predicts the value of the Curie–Weiss moment, 2.72μB, obtained from magnetic susceptibility measurements, and so explains the reduction from the free-ion value. The very broad magnetic response found in the inelastic neutron scattering experiments indicates a strong interaction between the Pr 4f electrons and the CuO2 bands.

Pr and Cu magnetism in (Pr1.5Ce0.5)Sr2Cu2MO10−δst(M=Nb, Ta): Correlations with a suppression of superconductivity

The magnetic properties of nonsuperconducting (${\mathrm{Pr}}_{1.5}$ ${\mathrm{Ce}}_{0.5}$ )${\mathrm{Sr}}_{2}$ ${\mathrm{Cu}}_{2}$ ${\mathrm{MO}}_{10\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ with M=Nb, Ta are characterized with dc magnetization, specific-heat, and neutron-diffraction experiments. Data for (${\mathrm{Pr}}_{1.5}$ ${\mathrm{Ce}}_{0.5}$ )${\mathrm{Sr}}_{2}$ ${\mathrm{Cu}}_{2}$ ${\mathrm{NbO}}_{10\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ reveal complex Cu magnetism marked by antiferromagnetic order below 200 K, spin structure transitions at 130 and 57 K, both collinear and noncollinear antiferromagnetic spin structures, and weak ferromagnetic behavior below 130 K. The data also indicate an anomalous ordering of the Pr spins near 10 K, a large linear contribution to the low-temperature specific heat, and a Pr 4f crystal-field ground state similar to that found in ${\mathrm{PrBa}}_{2}$ ${\mathrm{Cu}}_{3}$ ${\mathrm{O}}_{7}$ . Furthermore, there is evidence that the weak ferromagnetic behavior couples to the Pr ordering near 10 K. Identical Pr magnetism and similar Cu magnetism are found in (${\mathrm{Pr}}_{1.5}$ ${\mathrm{Ce}}_{0.5}$ )${\mathrm{Sr}}_{2}$ ${\mathrm{Cu}}_{2}$ ${\mathrm{TaO}}_{10\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ , deoxygenated (${\mathrm{Pr}}_{1.5}$ ${\mathrm{Ce}}_{0.5}$ )${\mathrm{Sr}}_{2}$ ${\mathrm{Cu}}_{2}$ ${\mathrm{NbO}}_{10\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ , and deoxygenated (${\mathrm{Pr}}_{1.5}$ ${\mathrm{Ce}}_{0.5}$ )${\mathrm{Sr}}_{2}$ ${\mathrm{Cu}}_{2}$ ${\mathrm{TaO}}_{10\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ . These results indicate that superconductivity is suppressed in these compounds in the same phenomenological manner as in ${\mathrm{PrBa}}_{2}$ ${\mathrm{Cu}}_{3}$ ${\mathrm{O}}_{7}$ . We interpret this as evidence that superconductivity is suppressed by the same mechanism in both structures and propose that a general correlation exists between anomalous Pr magnetism and a lack of superconductivity in these Pr-based high-${\mathrm{T}}_{\mathrm{C}}$ cuprates. The significance of these results and analyses to understanding and modeling the suppression of superconductivity by Pr in high-${\mathrm{T}}_{\mathrm{C}}$ cuprates is discussed.

Oxidation state and superconducting properties of Y1−xPrxSr2Cu2.85Re0.15O7+δ

Superconductivity exists in orthorhombic Y1−xPrxSr2Cu2.85Re0.15O7+δ up to a critical concentration (xcr) of 0.65. A progressive decrease in Tc occurs as x increases from 0 to 0.65. A further increase in x leads to a tetragonal transformation and as a consequence the Tc vanishes; however, the orthorhombicity of these Sr-based compounds is much lower than that observed for the Ba analog, Y1−xPrxBa2Cu3O7−δ and, hence, the Tc. On the one hand, crystal chemistry correlations indicate that the Pr ion is in trivalent state while on the other hand, the stabilizing cation, viz., Re, is in hexavalent state which accounts for the excess oxygen (>7.0) in the system. The high xcr value of the Sr series compared to the Ba series (xcr=0.55) is attributed to the much reduced orbital overlap of the trivalent Pr(4f ) state with the Cu(3dx2−y2)–O(2p) conduction band, via hole localization and/or pair breaking, and is not due to the much discussed hole filling by tetravalent Pr.

Hole delocalization by isovalent Sr substitution in Nd 0.7Pr 0.3Ba 2− kSr kCu 3O 7

Thermopower measurements on the system Nd 0.7Pr 0.3Ba 2− k Sr k Cu 3O 7 show that the insulator-superconductor transition for k≥0.2 is accompanied by hole delocalization. Arguments are presented to show that Sr substitution reduces the hybridization between Pr 4f and O 2pπ-orbitals and increases the hole density to sustain superconductivity.

Superconductivity in Y0.6Pr0.4Ba2-xSrxCu3O7- delta : The role of apical oxygen in hybridization.

The structural and electric properties of ${\mathrm{Y}}_{0.6}$${\mathrm{Pr}}_{0.4}$${\mathrm{Ba}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ have been investigated as a function of x. X-ray powder-diffraction patterns indicate that a solid solution forms with x extending to about 1.4. It is found that the substitution of Sr for Ba results in a contraction in the c axis at the same rate as in ${\mathrm{YBa}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. The oxygen content and ${\mathit{T}}_{\mathit{c}}$ are found to remain essentially unchanged with increasing Sr. Our analysis suggests that the contraction in c arises mainly from the shift of the apical oxygen and from a shift of Ba(Sr) towards the ${\mathrm{CuO}}_{2}$ planes. The apparent shift of the apical oxygen does not seem to influence the hybridization between the Pr 4f electrons and the valence band of the ${\mathrm{CuO}}_{2}$ planes significantly. The hybridization is more sensitive to the bond length between the Pr and the oxygen located in the ${\mathrm{CuO}}_{2}$ planes. ${\mathit{T}}_{\mathit{c}}$ in this system appears to be controlled both by the shift of the apical oxygen and by the hybridization.

Mechanism of suppression of superconductivity in (Pr xY 1−x)Ba 2Cu 4O 8

The effect of Pr on the superconductivity in (Pr xY 1−x)Ba 2Cu 4O 8 were studied and compared with that in (Pr xY 1−x)Ba 2Cu 3O 7. Transition temperatures of the (Pr xY 1−x)Ba 2Cu 4O 8 compounds decrease with increasing Pr content x, although the rate of decrease is slower than that of (Pr xY 1−x)Ba 2Cu 3O 7. Thus the Pr has a weaker effect to the superconductivity suppression in (Pr xY 1−x)Ba 2Cu 4O 8 than in (Pr xY 1−x)Ba 2Cu 3O 7. From the analysis of X-ray absorption spectra(XAS) near O K-edge in (Pr xY 1−x)Ba 2Cu 4O 8 and in (Pr xY 1−x)Ba 2Cu 3O 7, the effect of Pr substitution on the O 2p electronic state is found to be weaker for (Pr xY 1−x)Ba 2Cu 4O 8, which is parallel to the effect on superconductivity. These results were well explained with the hole localization model based on the hybridization of Pr 4f and O 2p orbitals.