Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm ... - ACS Publications

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Superconductivity in Perovskite Ba1−xLnx(Bi0.20Pb0.80)O3−δ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) Meng Zhang,† Mahammad Asim Farid,‡ Yan Wang,‡ Jinglin Xie,‡ Jinling Geng,‡ Hao Zhang,‡ Junliang Sun,‡ Guobao Li,*,‡ Fuhui Liao,‡ and Jianhua Lin*,‡ †

Department of Chemistry, School of Science, Beijing Jiaotong University, Beijing 100044, People’s Republic of China Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China

‡

ABSTRACT: Solid solutions Ba1−xLnx(Bi0.20Pb0.80)O3−δ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; 0.00 ≤ x ≲ 0.15) have been prepared under 850 °C. They all crystallize in space group P1 at room temperature. XPS data indicate that the valences are 5+ and 3+ for bismuth, 4+ and 2+ for lead, and 3+ or 4+ for lanthanide. Some of them are superconductors. The superconductive transition temperature Tczero decreases or remains constant with an increase of Ln in the sample when Ln = La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu. However, Tczero first decreases, then increases, and finally decreases when Ln = Ce, Pr, which is due to the corresponding sample changes from hole-doped to electron-doped superconductors with an increase of Ce or Pr in the sample.

1. INTRODUCTION BaBiO3 is a double-perovskite semiconductor with two crystallographically independent sites of Bi in the unit cell for Bi3+ and Bi5+ ,respectively (can be noted as Ba2Bi3+Bi5+O6).1−10 After an element is doped at the A (Ba) or/and B (Bi) site, the band gap will be modified and superconductivity can be induced. For example, BaPb1−xBixO3 (0.05 < x < 0.30),11 Ba1−xKxBiO3 (0.30 < x < 0.45),12 and (Na0.25K0.45)Ba3Bi4O1213 are superconductors. These superconductors are usually denoted as bismuth-based superconductors. The atom at the B site is six-coordinated to form a BO6 octahedron, and the octahedrons are linked to each other by corner-sharing oxygen atoms with the atoms at the A site in the cavities formed by eight octahedrons.1 The symmetry of these compounds may be cubic,12,13 tetragonal,7,14,15 orthorhombic,14 or triclinic16 because of the distortion of BO6 octahedron. The doping of K or Na at the A site or of Pb at the B site decreases the electrons in the 6S orbit in comparison to BaBiO3, where there is one electron per 6S orbit. Therefore, these reported superconductors belong to the hole-doped system. The inverse case is the electron-doped system. The hole-doped, electrondoped cuprate-based or iron-based superconductors are wellknown.17,18 However, there has been no electron-doped bismuth-based superconductor reported until now. This is a challenge. In order to obtain the electron-doped bismuth-based superconductor, +3 (or +4) ion should be doped into the A site. Recently Y3+ has been doped into the Ba site of BaBi0.20Pb0.80O3 to form the solid solution Ba1−xYxBi0.20Pb0.80O3, which shows superconductivity below 10 K.19 In this series the electrons at the 6S orbit are still less than that of BaBiO3. The reason is that when x > 0.04 © XXXX American Chemical Society

Ba1−xYxBi0.20Pb0.80O3 does not show superconductivity and when x < 0.04 the electrons at the 6S orbit are less in comparison to BaBiO3. Therefore, it is necessary to try other +3 ions. In this paper, a study on the series Ba1−xLnx(Bi0.20Pb0.80)O3−δ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) is presented. An electron-doped superconductor is found in Ba1−xLnx(Bi0.20Pb0.80)O3−δ with Ln = Ce, Pr.

2. EXPERIMENTAL SECTION Samples with the nominal formula Ba1−xLnx(Bi0.20Pb0.80)O3−δ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.15 and 0.25, denoted Ln1, Ln2, Ln3, Ln4, Ln5, Ln6, Ln7, Ln8, Ln9, Ln10, Ln11, Ln12, and Ln13, respectively) have been synthesized. The raw materials were BaCO3 (AR), La2O3 (99.99%), CeO2 (99.99%), Pr6O11 (99.99%), Nd2O3 (99.99%), Sm2O3 (99.99%), Eu2O3 (99.99%), Gd 2 O 3 (99.99%), Tb 4 O 7 (99.99%), Dy 2 O 3 (99.99%), Ho 2 O 3 (99.99%), Er2O3 (99.99%), Tm2O3 (99.99%), Yb2O3 (99.99%), Lu2O3 (99.99%), Bi2O3 (AR), and PbO (AR). The oven-dried regents were homogenized by about 30 min of grinding for a total 10 g of the mixtures with an agate mortar and a pestle. The mixtures were sintered first at 810 °C for 12 h to avoid possible loss of lead oxide. Then the reacted powders were pressed into pellets under 30 MPa and sintered at 830 °C for 12 h. The sintered mass was again crushed, pulverized, and pressed into cylindrical pellets, which then underwent three 12 h heat treatments at 850 °C followed by a furnace cooling every time with intermediate grinding and then pressing into pellets under 30 MPa. All of the treatments were done in air. Powder X-ray diffraction Received: October 23, 2017

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DOI: 10.1021/acs.inorgchem.7b02693 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. X-ray diffraction data (a) and volume of unit cell of the main phase (b) for Ba1−xHox(Bi0.20Pb0.80)O3−δ and SAED patterns of Ba0.96Ho0.04Bi0.20Pb0.80O3−δ (c−e) along different directions. The indexes shown in (a) and (c)−(e) are based on the triclinic cell a ≈ 6.059 Å, b ≈ 6.053 Å, c ≈ 6.079 Å, α ≈ 59.86°, β ≈ 59.84°, and γ ≈ 59.97°. Because a ≈ b ≈ c and α ≈ β ≈ γ, the index abc (such as 123) includes bac (213), bca (231), acb (132), cab (312), and cba (321). The asterisk in (a) indicates the diffraction peak from PbO (Pbcm with a ≈ 5.900 Å, b ≈ 5.494 Å, and c ≈ 4.767 Å).

Figure 2. Volume of the unit cell of the main phase for Ba1−xLnx(Bi0.20Pb0.80)O3−δ: (a) Ln = La, Ce, Pr; (b) Ln = La, Nd, Sm, Eu, Gd; (c) Ln = Gd, Tb, Dy, Ho, Er; (d) Ln = Er, Tm, Yb, Lu, Y. (PXRD) data were collected on a PANalytical x’Pert3 powder diffractometer with Cu Kα (λ1 = 0.15405 nm and λ2 = 0.15443 nm)

radiation (2θ range 5−120° for 2 h; step 0.0131°) at 40 kV and 40 mA at room temperature. The X-ray diffraction data were analyzed using B


Article

Inorganic Chemistry Table 1. Maximum xM Values of the Solid Solution Ba1−xLnx(Bi0.2Pb0.8)O3−δ Ln xM

xM

La

Ce

Pr

Nd

Sm

Eu

Gd

>0.25

0.171(6)

0.159(6)

0.148(6) Ln

0.126(6)

0.116(6)

0.110(6)

Tb

Dy

Ho

Er

Tm

Yb

Lu

0.116(6)

0.125(6)

0.133(6)

0.141(6)

0.146(6)

0.151(6)

0.156(6)

Pb2+. Then, for Ln = La, 2(rLa3+ − rBa2+) + (rBi3+ − rBi5+ + rPb2+ − rPb4+)/2 is less than zero (here rLa3+ = 1.36 Å (12-coordinated) ,rBa2+ = 1.61 Å (12-coordinated), rBi3+ = 1.03 Å (6-coordinated), rBi5+ = 0.76 Å (6-coordinated), rPb2+ = 1.18 Å (6-coordinated), rPb4+ = 0.775 Å (6-coordinated)). Therefore, with an increase of La3+ in the solid solution the volume of the unit cell of Ba1−xLax(Bi0.20Pb0.80)O3−δ decreases. Meanwhile, when one Ln4+ is doped into the Ba2+ site, one Bi5+ (or Pb4+) should be changed to Bi3+ (or Pb2+). This is the case for Ln = Ce, Pr. For Ln = Ce, (rCe2+ − rBa2+) + (rBi3+ − rBi5+ + rPb2+ − rPb4+)/2 is still less than zero (here rCe4+ = 1.14 Å (12-coordinated)). This may mean that the volume of Ba1−xCex(Bi0.20Pb0.80)O3−δ should also decrease with an increase of Ce in the sample. However, this does not agree with the experimental data. Then one may argue that the contribution to the lattice parameters is different for the radii of atoms at the A site and the B site of the perovskite compound ABO3. For the sake of simplicity, let us consider the cubic perovskite with the arrangement of atoms shown in Figure 3. Typically the atoms

GSAS software.20,21 Selected area electron diffraction (SAED) measurements were carried out on a JEM2100 instrument with an accelerating voltage of 200 kV. The X-ray photoelectron spectroscopy (XPS) patterns were obtained through a UK Kratos Axis Ultra spectrometer with an Al Kα or Mg Kα X-ray source operated at 15 kV and 15 mA. The chamber pressure was less than 5.0 × 10−9 Torr. Electron binding energies were calibrated against the C 1s emission at Eb = 284.8 eV. The resistivities of the samples were investigated with a cryogenic Physical Property Measurement System (PPMS) from 2 to 20 K.

3. RESULTS AND DISCUSSION 3.1. Structure and Solid Solution of Ba1−xLnx(Bi0.20Pb0.80)O3−δ. X-ray diffraction patterns of Ba1−xLnx(Bi0.20Pb0.80)O3−δ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) are similar to that reported for BaBi 0.20 Pb 0.80 O 3− δ , 16 , 19 as shown in Figure 1a for Ba1−xHox(Bi0.20Pb0.80)O3−δ, with several weak peaks due to impurities for some samples marked with an asterisk. Indexes are shown for the diffraction peaks of the main phase. SAED patterns of Ba1−xLnx(Bi0.20Pb0.80)O3−δ have been checked. The P1 space group of BaBi0.20Pb0.80O3−δ is confirmed to be suitable to describe the structure of Ba1−xLnx(Bi0.20Pb0.80)O3−δ. Typical data are shown in Figure 1c−e for Ba0.96Ho0.04Bi0.20Pb0.80O3−δ. Then the X-ray diffraction data were refined with the structure of BaBi0.20Pb0.80O3−δ by the Rietveld method using GSAS software to obtain the volume of the unit cell of the main phase for each sample. The corresponding data are shown in Figure 1b for Ba1−xHox(Bi0.2Pb0.8)O3−δ alone and in Figure 2 for all other Ba1−xLnx(Bi0.2Pb0.8)O3−δ species (Ln = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). The volume of the unit cell decreases or increases linearly with an increase of x in the sample when x is less than the maximum xM, which agrees well with Vegard’s law:22,23 Vx = (1 − x)V0 + xV1

Figure 3. Atom arrangement of cubic perovskite.

at the A site could not touch each other. They may contact the oxygen atoms. Then the lattice parameter a can be equal to α√2(rA + rO), where α is a coefficient and rA and rO are the radii of the atoms at A and O site. In another way, a can be equal to 2β(rB + rO), where β is a coefficient and rB is the radius of the atoms at B site. This may mean that the contribution of the change of rB to the change in the lattice parameter a is about √2 times the contribution of the change of rA. In this case, (rCe4+ − rBa2+) + √2/2(rBi3 − rBi5+ + rPb2+ − rPb4+) is larger than zero. Meanwhile, 2(rLa3+ − rBa2+) + √2/2(rBi3+ − rBi5+ + rPb2+ − rPb4+) is less than zero. Therefore, one may understand the different tendencies of the volume of the unit cell for Ba1−xLnx(Bi0.20Pb0.80)O3−δ between Ln = Ce, Pr and Ln = La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y shown in Figure 2. 3.2. Valence of Bi, Pb, and Ln in Ba1−xLnx(Bi0.2Pb0.8)O3−δ. As discussed above, the valence of the element in the sample has an important effect on the lattice parameters of the corresponding compound. Therefore, it is very useful to know the valence of the elements in the studied samples. Below are

(1)

where V x , V 0 , and V 1 are the unit cell volumes of Ba1−xLnx(Bi0.2Pb0.8)O3−δ, Ba(Bi0.2Pb0.8)O3−δ, and the supposed Ln(Bi0.2Pb0.8)O3−δ and x is the composition variable as given by Ln/(Ln + Ba). According to the data shown in Figure 2, xM can be obtained for each Ln, and these values are given in Table 1. The X-ray diffraction patterns for impurity can be observed as marked in Figure 1a by an asterisk (*) when x is larger than xM and become very weak or disappear when x is less than xM. As shown in Figure 2, the volume of the unit cell of Ba1−xLnx(Bi0.20Pb0.80)O3−δ increases for Ln = Ce, Pr and decreases for Ln = La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y with an increase of Ln in the sample. This may be due to the fact that the valence is 4+ for Ce or Pr and 3+ for all others in the samples. In order to balance the charge, when two Ln3+ are doped into the Ba2+ site, one Bi5+ (or Pb4+) should change to Bi3+ (or Pb2+) if the amount of oxygen is not changed. For the sake of simplicity, it is assumed that half of Bi5+ will be changed to Bi3+ and half of Pb4+ will be changed to C


Article

Inorganic Chemistry

Figure 4. XPS data for Bi 4f7/2 in BaBiO3 (a) and Ba2NdBiO6 (b) and of Pb 4f7/2 in BaPbO3−δ (c).

Figure 5. Binding energies of Bi (a) and Pb (b) for Ba1−xHox(Bi0.2Pb0.8)O3−δ and of Bi (c) and Pb (d) for Ba0.96Ln0.04(Bi0.2Pb0.8)O3−δ.

the existence of an oxygen vacancy24 caused by the synthesis of the sample under air instead of high-pressure O2.25 However, in this case the ratio of Pb2+ and Pb4+ is just 0.2:0.8, which does not agree well with the ratio of the two peaks of around 1:2 as shown in Figure 4c. Then the chemical titration of the present sample was performed, which indicated Pb2+:Pb4+ ≈ 1:2. Therefore, the peaks around 137 and 138 eV can be attributed to Pb 4f7/2 for Pb2+ and Pb4+ respectively. 3.2.2. Valence of Bi and Pb in Ba1−xHox(Bi0.2Pb0.8)O3−δ and Ba0.98Ln0.02(Bi0.2Pb0.8)O3−δ. In order to know the valence of Bi and Pb in Ba1−xLnx(Bi0.2Pb0.8)O3−δ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), the simple way is to obtain the XPS data for the entire series Ba1−xLnx(Bi0.2Pb0.8)O3−δ. This is too boring and not necessary. In fact, the data for the two series Ba1−xHox(Bi0.2Pb0.8)O3−δ (Ho can be changed to other Ln atoms) and Ba0.96Ln0.04(Bi0.2Pb0.8)O3−δ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) may be enough. As shown in Figure 5a, the XPS spectra of Bi for Ba(Bi0.2Pb0.8)O3−δ is similar to that for BaBiO3. Two peaks are

detailed data on the valence of Bi, Pb, and Ln in Ba1−xLnx(Bi0.2Pb0.8)O3−δ. 3.2.1. Valence of Bi and Pb in BaBiO3 and BaPbO3. There are many arguments for the analysis of XPS data of Bi and Pb in oxides. For clarity, the XPS data of Bi in BaBiO3 and Pb in BaPbO3 are discussed first. At present, it is believed that Bi3+ and Bi5+ coexist in BaBiO3 on the basis of the difference between Bi3+−O and Bi5+−O distances found by neutron diffraction.2 Therefore, the broad XPS spectra of Bi 4f7/2 and 4f5/2 for BaBiO3 should be divided into two parts for Bi3+ and Bi5+, respectively, as shown in Figure 4a. For comparison, the single peak of Bi 4f7/2 for Bi5+ in Ba2NdBiO6 is shown in Figure 4b, which is centered at 159 eV. Then, the peak centered at 158 eV in Figure 4a is assigned to Bi 4f7/2 for Bi3+. The XPS spectra of Pb 4f7/2 for BaPbO3−δ is also very broad and can be divided into two peaks as shown in Figure 4c. They can be attributed to Pb2+ and Pb4+, respectively, at the supposition that the sample may be just BaPbO2.80 because of D


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Inorganic Chemistry found for Bi 4f7/2 and 4f5/2 corresponding to Bi3+ and Bi5+, respectively. With an increase in Ho3+, the peak corresponding to Bi3+ becomes small. This means that with the increase of Ho3+ in the sample the amount of Bi3+ decreases. More oxygen vacancies or more Pb4+ ions should be induced to balance the compound. Two peaks are also found for 4f7/2 and 4f5/2 in the XPS data of Pb of the series Ba1−xHox(Bi0.2Pb0.8)O3−δ, which have been attributed to the coexistence of Pb2+ and Pb4+ in the samples. The shape of the envelope of the two peaks changed randomly, which may mean that the ratio of Pb2+:Pb4+ varies randomly. Then the value of δ should vary from one sample to another sample to balance the compound. The XPS data of Bi and Pb for Ba0.96Ln0.04(Bi0.2Pb0.8)O3−δ (Ln = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Tm, Yb, Lu) are shown in Figure 5c,d, respectively. They are very similar to those for Ba1−xHox(Bi0.2Pb0.8)O3−δ. For Ln = Ce, Pr, Gd, Tb, the peak corresponding to Bi3+ is merged into the peak corresponding to Bi5+ and no obvious peak for Bi3+ could be seen. However, the XPS data for Pb indicate that less Pb2+ appears in the corresponding sample. In addition, for Ln = La, Nd, Sm, Dy, Er, Eu, Tm, Yb, Lu, the peak for Bi3+ can be obviously found with more Pb2+ appearing in the corresponding samples. 3.2.3. Valence of Ln in Ba0.96Ln0.04(Bi0.2Pb0.8)O3−δ. In section 3.1, in order to explain the change in the volume of the unit cell for Ba1−xLnx(Bi0.2Pb0.8)O3−δ, the valences of Ce and Pr in the samples are suggested to be 4+ and the valences of La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb are 3+. To confirm this suggestion, the XPS data of Ln for Ba0.96Ln0.04(Bi0.2Pb0.8)O3−δ have been checked; some of these data are shown in Figure 6. Two peaks are found for 3d5/2 of La in Ba0.96La0.04(Bi0.2Pb0.8)O3−δ. The corresponding binding energies are 834.23 and 837.93 eV, respectively. The splitting of 3d5/2 for La is due to a transfer of an electron from oxygen ligands to La 4f (initially empty)26 with a separation of 3.5 eV. The binding energies of La for our sample are lower than those reported for La2O327 but are very similar to those reported for an La3+-containing perovskite compound.28−30 Therefore, the valence of La in Ba0.96La0.04(Bi0.2Pb0.8)O3−δ is 3+. The XPS data for Ce3+ and Ce4+ have been extensively discussed.31−35 The Ce 3d spectra for Ce3+ and Ce4+ are both composed of two multiplets (v and u) corresponding to the spin−orbit split 3d5/2 and 3d3/2 core holes. The spin−orbit splittings are about 18.2 and 18.6 eV, respectively. Each spin− orbit component is dominated by two features for Ce3+ corresponding to Ce 3d94f2 O 2p5 (v0 ≈ 881 eV, u0 ≈ 899 eV) and Ce 3d94f1 O 2p6 (v1 ≈ 885 eV, u1 ≈ 903 eV) states or three features for Ce4+ corresponding to Ce 3d94f2 O 2p4 (v ≈ 883 eV, u ≈ 901 eV), Ce 3d94f1 O 2p5 (v2 ≈ 888 eV, u2 ≈ 907 eV), and Ce 3d94f0 O 2p6 (v3 ≈ 898 eV, u4 ≈ 917 eV) states. As shown in Figure 6b, when an Al anode is used to obtain the XPS data, the peaks around 888 and 900 eV can be found without the peak around 917 eV. This may mean that Ce4+ does not appear in Ba0.96Ce0.04(Bi0.2Pb0.8)O3−δ. However, similar peaks are also found in Ba0.96La0.04(Bi0.2Pb0.8)O3−δ, which means that these peaks are not contributed by Ce. In fact, these peaks are found to be the MNN Auger spectra of Ba2+. In order to move away the MNN Auger spectra of Ba2+, an Mg anode was used. In this case, weak complex spectra are obtained as shown in Figure 6c for the sample Ce5. The typical XPS data for Ce4+ and Ce3+ are also shown in Figure 6c for the sake of comparison. It is found that the obtained spectra are

Figure 6. Selective binding energies for Ba0.96La0.04(Bi0.2Pb0.8)O3−δ (a), for Ba0.96Ce0.04-(Bi0.2Pb0.8)O3−δ and Ba0.96La0.04(Bi0.2Pb0.8)O3−δ (b), for Ba0.96Ce0.04(Bi0.2Pb0.8)O3−δ and Ba0.96Pr0.04(Bi0.2Pb0.8)O3−δ (c), for B a 1 − x P r x ( B i 0 . 2 P b 0 . 8 ) O 3 − δ ( x = 0 . 04 , 0 .0 9 , 0 .1 5 ) a n d Ba0.96Ce0.04(Bi0.2Pb0.8)O3−δ (d), for Ba0.96Nd0.04(Bi0.2Pb0.8)O3−δ (e), and for Ba0.96Gd0.04-(Bi0.2Pb0.8)O3−δ (f). The data for (a), (b), and (d− (f) were obtained using an Al anode. The data for (c) were obtained using an Mg anode.

mainly due to Ce4+ with the addition of Pb 4s. The spectrum for Pb 4s also appears in the XPS data obtained for the sample Pr5 under the same conditions. Similarly, the Pr 3d spectra for Pr3+ and Pr4+ are also composed of two multiplets corresponding to the spin−orbit split 3d5/2 and 3d3/2 core holes. The spin−orbit component is dominated by three features for Pr4+. They are Pr 3d94f3 O 2p4 (c ≈ 930 eV, c1 ≈ 952 eV), Pr 3d94f2 O 2p5 (b ≈ 936 eV, b1 ≈ 955 eV), and Pr 3d94f1 O 2p6 (a ≈ 947 eV, a1 ≈ 968 eV).36−38 The spin−orbit component of Pr3+ is very complex,39 which is dominated by at least two features. They are Pr 3d94f3 O 2p5 (s ≈ 929 eV, s′ ≈ 949 eV) and Pr 3d94f2 O 2p6 (m ≈ 933, m′ ≈ 953 eV). However, one peak around m′ (∼960 eV) could not be well assigned.39 The main peak for 3d5/2 of Pr3+ is around 933 eV with a satellite at around 929 eV,40 which is very close to 936 and 930 eV for Pr4+. The main difference between the XPS data for Pr4+ and Pr3+ is the small peak at around 968 eV, which only appears in the XPS data for Pr4+. Figure 6d shows the XPS data for Ba1−xPrx(Bi0.2Pb0.8)O3−δ (x = 0.04, 0.09, 0.15) and Ba0.96Ce0.04(Bi0.2Pb0.8)O3−δ in the range between 920 and 980 eV. With an increase of Pr in the sample, the Pr 3d spectra become more and more clear. The Bi 4s spectra appearing in this range become more and more weak. The strong peaks at around 970 eV is mainly due to the KLL Auger spectrum of oxygen, which also appears in the XPS data E


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Inorganic Chemistry

0.04 and 0.07; one inset shows how the Tzeroc (the maximum temperature when the resistance of the sample is zero) is chosen, and the other shows under what condition the Tzeroc value for a sample is not reported. The composition-dependent Tzeroc values for Ba1−xHox(Bi0.2Pb0.8)O3−δ are shown in Figure 7. Tzeroc for Ho1−Ho6 decreases linearly with an increase of Ho in the samples, as shown by the solid line. Tzeroc for Ho7 is far lower than that expected by this linear relationship. Further, Tzeroc is not found for Ho8 until the temperature is lowered to 2.3 K. This may means that there is a difference between Ho7, Ho8 and Ho1−Ho6. However, one can use the dashed line to fit the obtained Tzeroc with a large error. In this case, Tzeroc for Ho8 is expected to be lower than 2.3 K. A linear relationship between the Tzeroc value and the amount of Ln in the samples is also found for Ba1−xLnx(Bi0.2Pb0.8)O3−δ when Ln = La (see Figure 8a). It seems normal that the linear relationship is maintained until x = 0.25, because the solid solution limit xM for Ba1−xLax(Bi0.2Pb0.8)O3−δ may exceed 0.25, as shown in Figure 2a. For Ba1−xCex(Bi0.2Pb0.8)O3−δ, a linear relationship can also be drawn for the obtained Tzeroc value and the corresponding amount of Ce in the sample, as shown by the dashed line in Figure 8b. However, the error bar seems slightly large for the Tzeroc of Ba1−xCex(Bi0.2Pb0.8)O3−δ with x = 0.04 (Ce5). In addition, the zero resistivity of Ba1−xCex(Bi0.2Pb0.8)O3−δ for x = 0.05, 0.06, 0.07 could not be found until 2 K. However, the samples Ba1−xCex(Bi0.2Pb0.8)O3−δ with x = 0.08, 0.09, and 0.10 show superconductivity around 3 K. From what is known about many examples of hole-doped and electron-doped cupratebased or iron-based superconductors,17,18 this phenomenon can be attributed to the fact that Ba1−xCex(Bi0.2Pb0.8)O3−δ changes from a hole-doped superconductor to an electron-doped superconductor with a nonsuperconductor intermediate region, as indicated by the solid line in Figure 8b. As is known, Ba(Bi0.2Pb0.8)O3−δ is a hole-doped superconductor, where the

for Ba0.96Ce0.04(Bi0.2Pb0.8)O3−δ and agrees well with the reported data by Shlyakhtina.41 A small peak appears at around 975 eV for Ba1−xPrx(Bi0.2Pb0.8)O3−δ but does not appear for Ba0.96Ce0.04(Bi0.2Pb0.8)O3−δ. In addition, the intensity of this peak increases with the increase of Pr in the sample. Therefore, it is attributed to the peak at around 968 eV for Pr4+. That is to say, the Pr in Ba1−xPrx(Bi0.2Pb0.8)O3−δ may be Pr4+ or a mixture of Pr4+ and Pr3+. The present data could not exclude the existence of Pr3+ in the sample. The XPS data for Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb in the samples Ba0.96Ln0.04(Bi0.2Pb0.8)O3−δ confirm that the valence of these lanthanides is 3+. Typical data are shown in Figure 6 for Ln = Nd, Gd.42 3.3. Superconductivity of Ba1−xLnx(Bi0.2Pb0.8)O3−δ. The temperature-dependent resistivity of the studied samples has been measured between 2 and 20 K. Typical data are shown in the insets of Figure 7 for Ba1−xHox(Bi0.2Pb0.8)O3−δ with x =

Figure 7. Composition-dependent Tzeroc values for Ba1−xHox(Bi0.2Pb0.8)O3−δ. Insets give the temperature-dependent resistance of Ba1−xHox(Bi0.2Pb0.8)O3−δ for x = 0.04, 0.07.

Figure 8. Composition-dependent Tzeroc for Ba1−xLnx(Bi0.2Pb0.8)O3−δ. F


Article

Inorganic Chemistry

room temperature. The volume of the unit cell of Ba1−xLnx(Bi0.20Pb0.80)O3−δ increases for Ln = Ce, Pr and decreases for Ln = La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y with an increase of Ln in the sample. This is due to the fact that the valence is mainly 4+ for Ce and Pr and 3+ for the others. This agrees well with the XPS data. In addition, XPS data indicate that the valences are 5+ and 3+ for bismuth and 4+ and 2+ for lead in this solid solution. The temperaturedependent resistivity of the studied samples show that superconductivity can be observed for Ba1−xLnx(Bi0.20Pb0.80)O3−δ. In most cases, the transition temperature Tczero decreases with an increase of Ln in the sample. However, for Ba1−xCex(Bi0.20Pb0.80)O3−δ and Ba1−xPrx(Bi0.20Pb0.80)O3−δ Tczero first decreases, then increases, and finally decreases with an increase of Ce or Pr in the sample. This is attributed to the fact that, with an increase of Ce or Pr in the sample, the superconductors Ba1−xCex(Bi0.20Pb0.80)O3−δ and Ba1−xPrx(Bi0.20Pb0.80)O3−δ change from hole-doped to electron-doped.

6s orbital is occupied by less than 1 electron per orbital. With an increase of Ce in the Ba site, more electrons occupy the 6s orbital to reach an occupation of the 6s orbital of close to 1. In this case, a nonsuperconductor is obtained. With a further increase of Ce, the occupation of the 6s orbital is larger than 1, and an electron-doped superconductor may appear. In the present case, the valence of Bi is a mixture of 3+ and 5+. For the sake of simplicity, the valence of Bi is assumed to be an equal mixture of 3+ and 5+. Then each Bi will contribute 1 electron for the 6s orbital. If the valence of Pb is 4+, each Pb contributes zero electron for the 6s orbital. As mentioned in the section above, the valence of Ce is 4+. Each doped Ce will contribute two electrons to the 6s orbital. Therefore, the nonsuperconductor area should appear around x = 0.40 for Ba1−xCex(Bi0.2Pb0.8)O3−δ with about 1 electron in the 6s orbital. However, the experimental result suggested that the nonsuperconductor area is around x = 0.06. The reason may be that the valence of Pb in Ba(Bi0.2Pb0.8)O3−δ may be similar to that for BaPbO3−δ, where Pb2+:Pb4+ ≈ 1:2. In this case, for Ba1−xCex(Bi0.2Pb0.8)O3−δ with x = 0.06, the electron in the 6s orbital may be 2 × 0.06 + 0.20 + 0.80 × 0.33 × 2 ≈ 0.85. This is nearly 1. Therefore, the nonsuperconductor area around x = 0.06 for Ba1−xCex(Bi0.2Pb0.8)O3−δ is acceptable. For Ba1−xPrx(Bi0.2Pb0.8)O3−δ, three “linear” relationships can be drawn for the obtained Tzeroc value and the corresponding amount of Pr in the sample by the dashed line shown in Figure 8c. The first relationship belongs to the hole-doped superconductor area. The second and the third relationships belong to the electron-doped superconductor area, as described by the curved line shown in Figure 8c. The nonsuperconductor area is around x = 0.05. This is very similar to the case for Ba1−xCex(Bi0.2Pb0.8)O3−δ. This seems reasonable because the valence state of Pr in Ba1−xPrx(Bi0.2Pb0.8)O3−δ is mainly 4+. For Ba1−xNdx(Bi0.2Pb0.8)O3−δ, the Tzeroc value first decreases linearly with an increase of Nd in the sample and then vibrates around a certain value, which is shown in Figure 8d. Similar behavior is also found for Ba1−xSmx(Bi0.2Pb0.8)O3−δ and Ba1−xEux(Bi0.2Pb0.8)O3−δ as shown in Figure 8e. In addition, for Ba1−xLnx(Bi0.2Pb0.8)O3−δ with Ln = Gd, Tb, Dy, Er, Tm, Yb, Lu, only a linear decrease is found, as shown in Figure 8e,f. As mentioned in the above section, the valence of Ln (=Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) is 3+. Each Ln3+ just contributes one electron to the 6s orbital. Then Ba1−xLnx(Bi0.2Pb0.8)O3−δ (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) may just show hole-doped superconductivity because xM ≈ 0.15. For the sample around xM, the electron in the 6s orbital may be 1 × 0.15 + 0.20 + 0.80 × 0.33 × 2 ≈ 0.88. This is just around the nonsuperconductor region. Therefore, the electron-doped superconductivity is not obvious for Ba1−xLnx(Bi0.2Pb0.8)O3−δ with Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho , Er , Tm , Y b, L u. Sim ila r t o t h e c as e f o r Ba 1 − x Ho x (Bi 0 . 2 Pb 0 . 8 )O 3 − δ , the T z e r o c value of one Ba1−xHox(Bi0.2Pb0.8)O3−δ with Ln = Tb, Dy, Er, Tm, Yb, Lu is far below the linear relationship. More studies are needed.

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AUTHOR INFORMATION

Corresponding Authors

*G.L.: e-mail, [email protected]; tel, (8610)62750342; fax, (8610)62753541. *J.L.: e-mail, [email protected]; tel, (8610)62750342; fax, (8610)62753541. ORCID

Junliang Sun: 0000-0003-4074-0962 Guobao Li: 0000-0003-3061-193X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21771007) and the National Training Program of Innovation and Entrepreneurship for Undergraduates.

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REFERENCES

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4. CONCLUSION Samples with the nominal formula Ba1−xLnx(Bi0.20Pb0.80)O3−δ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.15, 0.25) have been synthesized by a traditional solid state reaction at 850 °C. The solid solution Ba1−xLnx(Bi0.20Pb0.80)O3−δ forms when 0.00 ≤ x ≤ xM; here xM is around 0.15 and varies from La to Lu. They crystallize in space group P1 at G


Article

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Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm ... - ACS Publications

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