The influences of V and Gd dopants on the structures and electrical and magnetic properties of PbPdO₂ thin films

Abstract

PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ thin films with body-centered orthorhombic structure were prepared by PLD technique, respectively. Their structures, magnetic and electrical properties were measured by XRD, SEM, AFM, EDS, XPS and VSM, respectively. The experimental results indicate that the three samples all have the preferred orientation of (002), and room temperature ferromagnetism. From EDS and XPS results, we can deduce that there exist Pb vacancies in the three samples. Meanwhile, the valence states for Pb, Pd, O, Gd and V ions were found to be 2+, 2+, 2−, 3+ and mixed 4+ and 5+, respectively. It was also found that the magnetic moments of PbPdO₂ and PbPd_0.9Gd_0.1O₂ are least and largest, respectively. Moreover, the electrical characteristics analysis indicates that the electrical resistivity is enhanced by V ion substitution, but reduced by Gd ion substitution. In addition, the significant insulator–metal transition temperatures of PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ were found to be about 385 K, 390 K and 430 K, respectively. Finally, according to the experimental facts of Pb vacancies in the three samples, the first-principles calculated models containing Pb vacancies were established. The calculated results explain well the magnetic origin of PbPdO₂, and V and Gd doping roles on its electrical and magnetic properties.

---

Introduction

Materials with a zero or small band gap (such as HgCdTe, HgZnSe, PbPdO₂, graphene, MoS₂ and topological insulators) have attracted increasing attention due to their intriguing physical properties.¹ PbPdO₂ was found to be a zero band gap semiconductor; it is very sensitive to the temperature, pressure and magnetic field.^2–4 Having many novel physical properties, the doped PbPdO₂ materials can hopefully be used in spintronics, electronics, optics, sensors and energy storage.¹ As early as 2005, Ozawa et al. synthesized PbPdO₂ single phase polycrystal samples by solid phase reaction.² The electrical, magnetic and thermoelectric properties were measured. A metal–insulator transition at about 90 K and p-type carriers were observed. In 2008, Wang proposed at first that Co-doped PbPdO₂ is a zero-spin gapless semiconductor (SGS) based on band structure calculations.³ Then, Wang et al. prepared Co-doped PbPdO₂ thin films by pulsed laser deposition (PLD), and studied the effects of current and magnetic field on the resistivity in the thin film.⁴ Both colossal electroresistance (CER) and giant magnetoresistance (GMR) were observed. Wang's pioneering research led to a series of experimental and theoretical studies of PbPdO₂. In the past few years, many researchers followed behind and studied theoretically and experimentally the microstructures, transport and magnetic properties of PbPdO₂ doped with different transition metal elements (such as Mn, Co, Cu, Zn, etc.).^5–13 Su et al. prepared single phase PbPd_0.81Co_0.19O₂ thin films by sol–gel spin coating and oxidation treatment.⁵ It was found that the metal–insulator transition temperature of PbPd_0.81Co_0.19O₂ film is 358 K, which is markedly higher than 90 K. Choo et al. prepared PbPdO₂ and Pb (Pd, Co) O₂ thin films oriented along the (020) direction by PLD, and studied ex situ annealing effects on the surface morphology, electrical and magnetic properties.^6,7 They found that, the electrical conduction was improved for PbPdO₂ film by annealing, which can be well explained by the reduction of lattice imperfections of PdO droplets and Pd or O deficiencies. It is suggested that the hybridization between the Pd 4d and O 2p bands plays key role. Lee et al. studied the physical properties tuned by magnetic and nonmagnetic ion substitutions (Zn and Cu), found that chemical dopants play an important role.⁹ When Pd in PbPdO₂ is substituted by Zn (3d¹⁰), the electrical resistivity decreases and the magnetic properties are not changed to remain diamagnetic. However, by substituting Cu²⁺ (3d⁹) with S = 1/2, the electrical resistivity increases and the magnetization shows paramagnetic behavior.

In this paper, 3d transition element V and rare earth ferromagnetic element Gd were chosen as dopant ions. Why V and Gd dopants are considered? V is also 3d transition metal, and its properties are similar to those of Mn. And Gd is unique rare earth metal with room temperature magnetism. The PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ films were prepared using PLD technique. The structures, ion valence states, magnetic and electrical properties of the three samples were measured. The experimental results indicate that, PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ films have (002) preferred orientation and room temperature ferromagnetism. Both V and Gd dopants can evidently enhance the magnetic moment of PbPdO₂. Moreover, PbPdO₂ and PbPd_0.9V_0.1O₂ have about 390 K insulator–metal transition temperature, while PbPb_0.9Gd_0.1O₂ has a larger insulator–metal transition temperature with 430 K. At last, V dopant has the conductivity of PbPdO₂ increase, but Gd dopant makes the conductivity of PbPdO₂ to decrease. In addition, the calculated results based on density functional theory (DFT) were used to explain the experimental facts.

Experimental and first-principles calculation

At first, PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ nanoparticles were prepared by sol–gel method. Pb(NO₃)₂ (99.99%), Pd(NO₃)₂ (99.99%), V(NO₃)₂ (99.9%) and Gd(NO₃)₃·6H₂O (99.99%) were used as raw materials. The chelating agent and solvent were citric acid monohydrate (C₆H₈O₇·H₂O 99.5%) and deionized water, respectively. In order to supplement the volatilization of Pb in the process of the heating treatment, an additional 5 mol% Pb(NO₃)₂ was used. Secondly, the PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ cylinder bulk sheets were synthesized by using their nanoparticles, respectively. PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ thin films were deposited on (100) oriented MgO single crystal substrate using PLD technique. The substrate temperature was 550 °C. A KrF excimer laser with wavelength of 248 nm was used as the energy source for target ablation. The energy and repetition rate of laser were chosen as about 250 mJ and 1 Hz, and the deposited time was 60 min. The samples were deposited at different O₂ ambient pressure and the initial vacuum condition of the chamber reached 10⁻⁶ torr. After deposition, the samples were ex situ annealed in air at 650 °C.

X-ray diffraction (XRD) patterns of PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ thin films were obtained by Rigaku MiniFlex II (CuK_α, λ = 0.15418 nm). X-ray photoelectron spectra (XPS) were measured by using Thermo Fisher ESCALAB250Xi. The morphologies of the films were observed by Hitachi SU-8010 Scanning Electron Microscopy (SEM) and atomic force microscopy (AFM, Bruker Dimension), and the elements of the samples were analyzed by EDS. The electronic properties were measured by KEITHLEY 4200-SCS and HALL8686 semiconductor characterization system. The magnetic properties were measured by VersaLab (VSM) of Quantum Design.

Our DFT calculations were performed by Vienna ab initio simulation package (VASP)¹⁴ with projected-augmented-wave (PAW) potential.¹⁵ The functional of Perdew–Burke–Ernzerhof generalized gradient approximation (GGA-PBE)^16,17 were used in the all calculations. We choose a PbPdO₂ (002) film with the experimental lattice constants of a = 9.4547 Å, b = 5.4597 Å, c = 4.6605 Å. The adjacent atomic slabs were decoupled by using a vacuum layer of 15 Å. The cutoff energy was set at 550 eV. A 6 × 8 × 4 Monkhorst k-point mesh was adopted for 2 × 2 × 1 PbPdO₂ (002) film supercells. The on-site Hubbard U was included in the strong correlation elements Pd, Gd and V. All the atoms were fully relaxed until the residual forces on each atom are less than 0.02 eV Å⁻¹.

Results and discussion

Fig. 1(a) shows the XRD patterns of PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂, film. In order to distinguish weaker peak, the log intensity as a function of 2θ was present in Fig. 1(b). From Fig. 1(a), it is found that PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ are mainly single-phase with body-centered orthorhombic structure with the standard PDF (No. 38-1357). The diffraction peaks observed at 2θ = 31.52° and 2θ = 38.60° are corresponding to (211) and (002) of PbPdO₂ phase, respectively. PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ films have all (002) preferred direction. V and Gd dopants do not change the crystal structure. Moreover, under log enlargement, four very weak peaks at 2θ ≈ 28.5°, 30.0°, 32.5° and 33.7° were observed in PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ films, which means that there may exist very little impurity phases induced by the V and Gd dopants. By analyzing weak additional peaks, we find that, for PbPdO₂, there does hardly exist the additional peaks; for PbPd_0.9V_0.1O₂, the additional peaks are very weak, and they may correspond PdO phase; for PbPd_0.9Gd_0.1O₂, the additional peaks correspond Gd₂O₃ phase with paramagnetism. When the deposition time is short and the thickness of the sample is thin, there is a lattice mismatch between the substrate and PbPdO₂ thin film. The lattice constant c of PbPdO₂ (002) film is 4.6605 Å, while the lattice constant a of Mg (001) is 4.2112 Å. PbPdO₂ (002) films with c = 4.6605 Å can be grew on the MgO (001) substrate with a = 4.2112 Å because of little lattice mismatch (+9.7%). The detailed discussion can be found in our previous work.¹¹ So, the deposition of the sample on the MgO substrate can lead to the shift of the diffraction peak (111) in the high angle direction of PbPdO₂. Therefore, the diffraction peak at 26.35° corresponds to the (111) diffraction peak of PbPdO₂. It is obvious that PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ films all grow along the (002) crystal face. So far, only PbPdO₂ and PbPd_0.75Co_0.25O₂ films with (112) crystal surface preferred growth were reported by Wang et al.⁴ Here, (112) should be considered to be (002) according to XRD formula.

Fig. 1 XRD patterns of PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂. (a) and (b) present the intensity and log intensity as a function of 2 theta, respectively.

Fig. 2(a)–(f) shows the SEM images and AFM surface morphologies of PbPdO₂, PbPd_0.9V_0.1O₂, PbPd_0.9Gd_0.1O₂ thin films, respectively. Fig. 3(a), (b) and (c) show the EDS spectra of PbPdO₂, PbPd_0.9V_0.1O₂, PbPd_0.9Gd_0.1O₂ thin films, respectively. From Fig. 2(a), it can be found that the PbPdO₂ thin film is composed of nanoparticles with a particle size of about 100 nm and the PbPd_0.9V_0.1O₂ film contains many bar-like structures with width of 100–150 nm in Fig. 2(b). Moreover, Gd doping changes obviously the morphologies of the samples, and a lot of rod-like structures with the length of about 500 nm and width of about 100 nm are observed in Fig. 2(c). Moreover, from the surface morphologies in Fig. 2(d)–(f), the values of the surface roughness (R_q) are 52.4 nm, 92.2 nm and 90.7 nm for PbPdO₂, PbPd_0.9V_0.1O₂, PbPd_0.9Gd_0.1O₂ thin films, which is consistent with the observed results from SEM. The sizes of PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ thin films prepared by PLD technique are larger than those prepared by sol–gel spin coating.^5,12 From the EDS measured results in Fig. 3, it is obtained that the atomic ratio of Pb : Pd is 0.81 : 1.00 for PbPdO₂; Pb : Pd : V atomic ratio is 0.60 : 1.00 : 0.18 for PbPd_0.9V_0.1O₂; the atomic ratio of Pb : Pd : Gd is 0.57 : 1.00 : 0.18 for PbPd_0.9Gd_0.1O₂. It should specially be noticed that the Mg signal in the EDS spectra is from the MgO substrate. From the measured results, we can deduce that there exist Pb vacancies in the three samples, which may be caused by the volatilization of Pb during the heating process. And the percentage of Pb vacancy in the samples also is changed due to the role of different ions. The measured results indicate that the percentage of Pb vacancy in PbPdO₂ is the lowest, and the percentage of Pb vacancy in PbPd_0.9Gd_0.1O₂ is the highest.

Fig. 2 SEM surface images and AFM surface morphologies of PbPdO₂ (a) and (d), PbPd_0.9V_0.1O₂ (b) and (e) and PbPd_0.9Gd_0.1O₂ (c) and (f).

Fig. 3 EDS spectra of PbPdO₂ (a), PbPd_0.9V_0.1O₂ (b) and PbPd_0.9Gd_0.1O₂ (c).

Fig. 4(a), (b) and (c) shows the XPS images of Pb 4f in PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ films, respectively. From the figures, it is found that the XPS spectra of Pb 4f for three films can be divided into two pairs of characteristic peaks corresponding to Pb 4f_5/2 and Pb 4f_7/2, which is similarly to previous studied results.¹³ From Fig. 4(a), two pairs of characteristic peaks are described as following: main peaks (136.55 eV, 141.49 eV), and satellite peaks (137.50 eV, 142.20 eV); from Fig. 4(b), two pairs of characteristic peaks are described as following: main peaks (137.22 eV, 142.07 eV), and satellite peaks (138.05 eV, 142.88 eV); from Fig. 4(c), two pairs of characteristic peaks are described as following: main peaks (136.87 eV, 141.65 eV), and satellite peaks (137.75 eV, 142.50 eV). Clearly, with doping V and Gd, the sites and intensities of two pairs of peaks are changed, which means that doping has important role on ion vacancy and valence. The main peaks can be attributed to the lattice Pb of the samples, and the satellite peaks can be attributed to Pb vacancies.¹¹ Moreover, the areas of main and satellite peaks for Pb 4f_5/2 and Pb 4f_7/2 in Fig. 4 were calculated. The calculated results indicate that the average area ratio of satellite peaks to total spectra (satellite + main peaks) are about 37.65%, 38.28% and 43.7% for Fig. 4(a), (b) and (c), respectively. Thus, it is obtained that Pb vacancies are about 37.65%, 38.28% and 43.7% for PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ films, respectively. The above results are consistent with the results of EDS.

Fig. 4 XPS spectra of Pb in PbPdO₂ (a), PbPd_0.9V_0.1O₂ (b) and PbPd_0.9Gd_0.1O₂ (c).

Fig. 5(a), (b) and (c) shows Pd 3d XPS images for PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ thin films, respectively. From the figure, it is found that, both peaks for Pd 3d are symmetric and centered at around 336.28 eV and 341.59 eV for PbPdO₂, around 336.09 eV and 341.51 eV for PbPd_0.9V_0.1O₂, and around 336.10 eV and 341.51 eV for PbPd_0.9Gd_0.1O₂, respectively. The two peaks stand for Pd 3d_5/2 and Pd 3d_3/2, which can be attributed to the formation of Pd²⁺.^13,18,19

Fig. 5 XPS spectra of Pd of PbPdO₂ (a), PbPd_0.9V_0.1O₂ (b) and PbPd_0.9Gd_0.1O₂ (c).

Fig. 6(a), (b) and (c) shows the XPS spectra of O 1s in PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ films, respectively. From the figure, it can be seen that every O 1s spectrum can be divided into three peaks with one main peak I at about 528.93–529.39 eV and two satellite peaks II and III at about 530.80–531.01 eV and 532.83–533.80 eV. Moreover, the sites of main peak and satellite peaks are evidently influenced by V and Gd dopants. For PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ films, the main peak can be attributed to the lattice O in PbPdO₂, and the satellite peaks can be attributed to the oxygen and hydroxyl absorption on the surface of the sample.^20,21 In particular, the peak III at about 532.83–533.8 eV should be related to the Pb vacancy, which can be explained as follows: since the Pb vacancy is generated, the valence of Pb is changed from +2 to +0, which makes the excess oxygen oxidized to O¹⁻. As a result, O¹⁻ will absorb hydroxyl and CO₂. Thus, the sample surface will be exposed in the atmosphere to absorb the hydroxyl and CO₂.¹¹ Compared with the doped samples, peak area percentage of II and III for the undoped sample is lowest. Thus, the content of Pb vacancies in undoped PbPdO₂ is lowest. It is reasonable to deduce that ion doping can increase surface absorbed oxygen and Pb vacancy. V and Gd dopants increase the content of Pb vacancies. Especially, Gd doping makes PbPdO₂ to produce most Pb vacancies. These results are consistent with the EDS those above.

Fig. 6 XPS spectra of O 1s in PbPdO₂ (a), PbPd_0.9V_0.1O₂ (b) and PbPd_0.9Gd_0.1O₂ (c).

Fig. 7(a) and (b) shows the XPS images of V and Gd in PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ films, respectively. From the Fig. 7(a), it can be seen that, two peaks at 515.70 eV and 516.80 eV should be corresponding to V⁴⁺ 2p_3/2 and V⁵⁺ 2p_3/2, respectively.^22–24 V⁴⁺ and V⁵⁺ ions coexist in the V doping TiO₂ has also been reported.²⁵ At the same time, one can notices that the two peaks of Gd at about of 1186.86 eV and 1220.86 eV should be corresponding to Gd³⁺ 3d_5/2 and Gd³⁺ 3d_3/2.^26,27

Fig. 7 XPS spectrum of (a) V 2p and (b) Gd 3d.

Fig. 8(a), (b) and (c) show the M–H hysteresis loops for PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ at room temperature, respectively. From Fig. 8, it is found that three samples have all ferromagnetism at room temperature. The saturation magnetization of PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ were about 2.0 emu cm⁻³, 5.0 emu cm⁻³ and 8.0 emu cm⁻³, respectively. Obviously, V and Gd dopants improve magnetic moment of PbPdO₂, and enhancing effect of Gd dopant is largest. The source of magnetism is generally related to its defects, the carrier concentration, and the valence of the dopant ions for metal oxides.^28–30 Although, there exist very little impurity PdO and Gd₂O₃ phases in PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂, these impurity phases do not contribute to ferromagnetism. Based on the above EDS and XPS measured results, one can notice that, the Pb vacancy, Pd²⁺ and O¹⁻ exist in PbPdO₂. In theory, the effective spin magnetic moment of Pd²⁺ is zero. The magnetism of PbPdO₂ film should result from O¹⁻. Here, O¹⁻ is produced due to the presence of Pb vacancies, and 2p⁵ in O¹⁻ will lead to 1μ_B effect magnetic moment. Moreover, the valence states of V and Gd ions were confirmed to be V⁴⁺, V⁵⁺ and Gd³⁺ by XPS. For PbPd_0.9V_0.1O₂, V⁴⁺ can contribute 1μ_B magnetic moment and V⁵⁺ makes no contribution. Therefore, the enhanced magnetism results from V doping. By contrast, Gd³⁺ can contribute 7μ_B magnetic moment. Thus, Gd doping increases evidently the magnetism of PbPd_0.9Gd_0.1O₂. Fig. 9 shows the calculated spin DOSs of PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ with Pb vacancy. The calculated results indicate that, the magnetic moment of PbPdO₂ with Pb vacancy is 2.8μ_B, as shown in Fig. 9(a). As seen in Fig. 9(b) and (c), V dopant makes the magnetic moment to enhance to 6.5μ_B, and further Gd doping leads to the largest magnetic moment with 12.3μ_B. The doping role of the above magnetic moments explains well the experimental results in Fig. 8.

Fig. 8 M–H curves at room temperature of PbPdO₂ (a), PbPd_0.9V_0.1O₂ (b) and PbPd_0.9Gd_0.1O₂ (c).

Fig. 9 The calculated spin DOSs of PbPdO₂ (a), PbPd_0.9V_0.1O₂ (b) and PbPd_0.9Gd_0.1O₂ (c).

Fig. 10(a)–(c) shows the magnetization as a function of the temperature under zero-field cooling (ZFC) and field cooling (FC) for PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ thin films. As the temperature is changed from 370 K to 70 K, the ZFC and FC curves gradually separated. As the temperature is larger about 375 K, the FC and ZFC curves tended to coincide, and the crosspoint is named as blocking temperature. The evident deviation between the FC and ZFC curves indicates that superparamagnetism and ferromagnetism can coexist PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ thin films. These results are consistent with the previous report.⁵

Fig. 10 ZFC–FC M–T curves of PbPdO₂ (a), PbPd_0.9V_0.1O₂ (b) and PbPd_0.9Gd_0.1O₂ (c) thin films.

Fig. 11 exhibits the temperature dependence of the DC electrical resistivity (ρ). It can be seen from the figure that the resistivities of PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ at room temperature are 0.179 Ω cm, 0.389 Ω cm and 0.143 Ω cm, respectively. Compared with the bulk resistivity,³ these values become less. In addition, it is found that the resistivity of PbPd_0.9V_0.1O₂ is larger than that of PbPdO₂, and the resistivity of PbPd_0.9Gd_0.1O₂ is smaller than that of PbPdO₂. This indicates that different ion doping has a significant effect on the physical properties of PbPdO₂. To analyze the resistivity change produced by ion doping, the carrier densities and mobilities at room temperature for three films were measured, and the results are as following: for PbPdO₂, the hole-type carrier density and mobility is 5.56 × 10¹⁷ cm⁻³ and 2.22 × 10⁴ cm² V⁻¹ s⁻¹, respectively; for PbPd_0.9V_0.1O₂, they are 1.0 × 10¹⁷ cm⁻³ and 20.2 × 10⁴ cm² V⁻¹ s⁻¹; for PbPd_0.9Gd_0.1O₂, they are 26.9 × 10¹⁷ cm⁻³ and 7.1 × 10⁴ cm² V⁻¹ s⁻¹, respectively. Obviously, the resistivity change produced by ion doping can be explained by difference of the carrier density. To clarify the role of vacancy and dopant on the resistivity, we have also carried out first-principles calculations. From the calculated spin-polarized DOSs of PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ in Fig. 9, one can find that V and Gd dopants have clear role on the band gap. From Fig. 9(a), the presence of Pb vacancies results in a band gap of 0.028 eV in PbPdO₂. After the further doping of Gd and V atoms, we found distinct effects on the band gap: the introduction of Gd³⁺ reduces the band gap to 0.019 eV, while the doping of V^4+,5+ increases the band gap to 0.036 eV. The above calculated results reveal well the variation of electrical resistivity for three samples as shown in Fig. 11.

Fig. 11 Temperature dependence of DC electrical resistivity for PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ thin films. The inset shows the magnified temperature dependence of resistivity of PbPd_0.9Gd_0.1O₂ thin film.

The electron transport in PbPdO₂ is controlled by hole carriers through the Pd–O hybridization, and the doping of Gd ions can make the Pd–O hybrid stronger, while the doping of V ions weaken the Pd–O hybridization. Therefore, ion doping significantly changes the conductivity of the sample. In addition, it is found that there is a significant insulator–metal transition temperature in the samples. The insulator–metal transition temperatures for PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ are respectively about 385 K, 390 K, 430 K, which is much higher than previous experimental reports.^2,4 Significant differences in T_MI may be related to microstructural differences arising from different preparation conditions.

Conclusions

PbPdO₂, PbPd_0.9V_0.1O₂ of PbPd_0.9Gd_0.1O₂ films with the preferred orientation of (002) have been synthesized by PLD technique. The particle size and surface roughness of three films were obtained by SEM and AFM measurements. Moreover, the valence states for Pb, Pd, O, Gd and V ions were found to be 2+, 2+, 2−, 3+ and the mixed 4+ and 5+, respectively. Meanwhile, it is found that three samples have Pb vacancies and room temperature ferromagnetism. The saturation magnetization of PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ were found to be about 2.0 emu cm⁻³, 5.0 emu cm⁻³ and 8.0 emu cm⁻³, respectively. Moreover, the temperature dependence of resistivity indicates that, the electrical resistivity is enhanced by V ion substitution, but being weakened by Gd ion substitution. Significant insulator–metal transition temperatures were also observed. Finally, based on the calculated spin DOSs of PbPdO₂, PbPd_0.9V_0.1O₂ and PbPd_0.9Gd_0.1O₂ with Pb vacancy, the magnetic origin of three samples, and V (Gd) doping roles on the electrical and magnetic properties can be explained well.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Science Foundation of China (61574037, 61404029, 11404058, 11274064); Natural Science Foundations of Fujian Province of China (Grant No. 2017J06001).

Notes and references

1. X.-L. Wang, S. X. Dou and C. Zhang, NPG Asia Mater., 2010, 2(1), 31–38.
2. T. C. Ozawa, T. Taniguchi, Y. Nagata, Y. Noro, T. Naka and A. Matsushita, J. Alloys Compd., 2005, 388, 1–5.
3. X. L. Wang, Phys. Rev. Lett., 2008, 100, 3136–3140.
4. X. L. Wang, G. Peleckis, C. Zhang, H. Kimura and S. X. Dou, Adv. Mater., 2009, 21, 2196–2199.
5. H. L. Su, S. Y. Huang, Y. F. Chiang, J. C. A. Huang, C. C. Kuo, Y. W. Du, Y. C. Wu and R. Z. Zuo, Appl. Phys. Lett., 2011, 99, 102508.
6. S. M. Choo, K. J. Lee, S. M. Park, G. S. Park and M. H. Jung, J. Appl. Phys., 2013, 113, 014904.
7. S. M. Choo, K. J. Lee, S. M. Park, J. B. Yoon, G. S. Park, C.-Y. You and M. H. Jung, Appl. Phys. Lett., 2015, 106, 172404.
8. K. J. Lee, S. M. Choo, J. B. Yoon, K. M. Song, Y. Saiga, C.-Y. You, N. Hur, S. I. Lee, T. Takabatake and M. H. Jung, J. Appl. Phys., 2010, 107, 09C306.
9. K. J. Lee, S. M. Choo and M. H. Jung, Appl. Phys. Lett., 2015, 106, 072406.
10. K. J. Lee, S. M. Choo, Y. Saiga, T. Takabatake and M. H. Jung, J. Appl. Phys., 2011, 109, 07C316.
11. X. Chen, Y. Chen, Y. M. Yang, H. Jia, J. M. Zhang, S. Y. Chen and Z. G. Huang, Ceram. Int., 2017, 43, 10428–10433.
12. J. Liu, C. Mei, P. Y. Chuang, T. T. Song, F. L. Tang, H. L. Su, J. C. A. Huang and Y. C. Wu, Ceram. Int., 2016, 42, 15762–15766.
13. F. L. Tang, J. Liu, C. Mei, S. Y. Huang, T. T. Song, H. L. Su, M. K. Lee, Y. C. Wu and J. C. A. Huang, RSC Adv., 2016, 6, 37522–37529.
14. G. Kresse and J. Hafner, Phys. Rev. B: Condens. Matter Mater. Phys., 1993, 48, 13115–13118; G. Kresse and J. Furthmüller, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54, 11169.
15. P. E. Blöchl, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 50, 17953.
16. G. Kresse and J. Furthmüller, Comput. Mater. Sci., 1996, 6, 15.
17. J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865.
18. K. C. Lee, Y. J. Chiang, Y. C. Lin and F. M. Pan, Sens. Actuators, B, 2016, 226, 457–464.
19. M. Brun, A. Berthet and J. C. Bertolini, J. Electron Spectrosc. Relat. Phenom., 1999, 104, 55–60.
20. H. Q. Zhao, J. Wang, L. X. Zhang, Y. C. Rong, J. Chen, K. Ibrahimb and X. R. Xing, Dalton Trans., 2013, 42, 10358–10364.
21. S. Takatani, H. Miki, K. Kushida-Abdelghafar and K. Torii, J. Appl. Phys., 1999, 85, 7784–7791.
22. J. Xu, C. X. Qin, Y. L. Huang, Y. R. Wang, L. Qin and H. J. Seo, Appl. Surf. Sci., 2017, 396, 1403–1410.
23. R. Larsson, B. Folkesson and G. Schoen, Chem. Scr., 1973, 3, 88.
24. V. I. Nefedov, M. N. Firsov and I. S. Shaplygin, J. Electron Spectrosc. Relat. Phenom., 1982, 26, 65.
25. F. Z. Ren, H. Y. Li, Y. X. Wang and J. J. Yang, Appl. Catal., B, 2015, 176, 160–172.
26. Y. Uwamino, Y. Ishizuka and H. Yamatera, J. Electron Spectrosc. Relat. Phenom., 1984, 34, 69.
27. D. Raiser and J. P. J. Deville, J. Electron Spectrosc. Relat. Phenom., 1991, 57, 91.
28. A. Sundaresan and C. N. R. Rao, Nano Today, 2009, 4(1), 96–106.
29. S. B. Ogale, Adv. Mater., 2010, 22(29), 3125–3155.
30. T. Dietl, Nat. Mater., 2010, 9(12), 965–974.

---