Effect of Tb<Superscript>3+</Superscript> ion substitution on structural,optical and magnetic properties of CdS nanoparticles

Cd_1?xTb_xS (x?=?0.00, 0.05, 0.10, 0.15) nanoparticles were synthesized by hydrothermal technique. The effect of Tb-doping on structural, optical and magnetic properties were studied. The X-ray diffraction (XRD) confirmed wurtzite phase without any impurity. HR-TEM micrographs revealed the particle size of ~?18 nm with spherical morphology. Band gap calculated from UV–Visible absorption spectra showed an initial decrease from 2.49 eV (x?=?0) to 2.45 eV (x?=?0.10). Further, increasing Tb-doping to x?=?0.15 band gap increased to 2.54 eV. Photoluminescence emission spectra showed peaks at 430 and 490 nm for x?=?0 and 0.05 which corresponds to blue emission. Further increase in Tb doping, peaks found to be quenched. Peak at 530 nm is due to green emission and shifted to higher wavelength with Tb-doping. Magnetic analysis confirmed ferromagnetism in both undoped and Tb-doped nanoparticles. Saturation magnetization found to decrease upto x?=?0.10 and increased for x?=?0.15.     

Mechanochemical synthesis,crystal structure and ion conduction in the Gd<Subscript>2</Subscript>Hf<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Ti<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>7</Subscript> system

This work describes the mechanochemical synthesis, structural characterization and electrical properties of an interesting group of novel ionic conductors, with general formula Gd₂(Hf_2?x Ti _x )₂O₇. Different compositions in this system (x = 0, 0.4, 0.8, 1.2, 1.6 and 2) were obtained at room temperature, via a mechanochemical reaction between the corresponding elemental oxides, and characterized by using XRD, Raman spectroscopy and SEM. The XRD structural analysis by the Rietveld method revealed that all the Hf-containing compositions show a disordered fluorite-like structure instead of the expected pyrochlore-like atomic ordering, and the cation size mismatch criteria for pyrochlore stability. Increasing Ti content promotes a phase transformation to the pyrochlore structure with post-milling thermal treatments, which takes place in all samples on annealing at 1200 °C, except for Gd₂Hf₂O₇. These results were confirmed by Raman spectroscopy, which also suggests that the x = 0.4 sample has the highest degree of oxygen disorder in the system and that this disorder decreases with increasing Ti⁴⁺ content. Finally, all samples show the pyrochlore structure on firing at 1500 °C. Activation energies E _dc for oxygen migration were determined by using impedance spectroscopy and found to be within the ~0.9–1.2 eV range, whereas conductivity σ _dc values at 700 °C vary from 1.12 × 10^?6 to 2.75 × 10^?4 S cm^?1, with decreasing conductivity as Ti⁴⁺ content increases.     

Magnetic and Mössbauer Studies of Ba(Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript> Single Crystals

Magnetization and ⁵⁷Fe Mössbauer effect spectroscopy (MS) studies of Ba(Fe_1?x Ni _x )₂As₂ single crystals (x=0 to 0.054) at temperatures (5 K to 300 K) have been performed. Magnetic measurements show that for BaFe₂As₂ the magnetic moment decreases below T _N=136 K. T _N is suppressed monotonically by Ni doping. On the other hand, for higher x values the magnetic moment increases below T _N. Unexpectedly for x=0.024 (T _N=67 K), the virgin zero-field-cooled (ZFC) curve is higher than that of field-cooled (FC) one below 48 K. The magnetic MS spectra of this sample are composed of a superposition of two subspectra, corresponding to commensurate and incommensurate field distributions. The average magnetic hyperfine field H _eff decreases with T and becomes zero at 80 K. For higher x values, the samples become superconducting at T _C=15.5 and 19 K for x=0.046 and 0.054, respectively. For both samples below T _C, the FC curves are positive (the paramagnetic Meissner effect) up to applied field of H～15 Oe and the susceptibility is inversely proportional to H. The MS spectra below and above T _C are almost identical, indicating that the MS parameters are not sensitive enough to detect the superconducting state. The peculiar phenomena observed are attributed to disorder induced by the presence of Ni atoms in the Fe sublattice.     

Effect of annealing on exchange bias in Mg<Subscript>0.2</Subscript>Fe<Subscript>x</Subscript>Cr<Subscript>1.8−x</Subscript>O<Subscript>3</Subscript> nano oxides

Magnesium-iron chromium oxides (Mg_0.2Cr_1.8?x Fe _x O₃ with x varying from 0.3 to 0.9) produced by hydrothermal process in a stirred pressure reactor from pure metal chlorides have been annealed at 700 °C. Single phase corundum structure and nanophase structure of the as-synthesized samples were confirmed by X-ray diffraction (XRD). Instead of the correlation between H _EB and D _XRD observed at T _A = 600 °C, we find significant changes. The H _EB increases with decreasing particle size reaches a maximum at ~43 nm (x = 0.5) then decreases.     

Investigations on the structure,defects, electrical and magnetic properties of Ni-substituted BiFeO<Subscript>3</Subscript> ceramics

Polycrystalline BiFe_1?x Ni _x O₃ (x = 0.00–0.30) ceramics were synthesized by solid state reaction method followed by rapid liquid phase sintering. The effect of Ni substitution on the structure, defects, electrical and magnetic properties of BiFeO₃ ceramics was investigated. X-ray diffraction measurements reveal that Ni-substituted samples exhibit distorted rhombohedral perovskite structure as that of unsubstituted BiFeO₃, and the impurity phases such as Bi₂Fe₄O₉ decrease due to Ni substitution. The changes in the phonon frequencies in Raman spectra reveal the lattice distortion in Ni-substituted samples which are in agreement with the XRD analysis. Positron annihilation lifetime measurements reveal that cation vacancy-type defects exist in all samples, the vacancy concentration increases with increasing Ni content from 0.00 to 0.30. The enhancement in leakage current and dielectric properties is observed in Ni-substituted samples due to the suppression of impurity phases and Fe²⁺. Magnetic measurements indicate that the Ni-substituted BiFeO₃ samples exhibit weak ferromagnetism. The magnetic properties are found to increase with increasing Ni concentration due to the internal structural distortion, cation vacancy, the ferromagnetic exchange between the neighboring Fe³⁺ and Ni²⁺ ions and the change in Fe–O–Fe bond angle.     

Microwave dielectric properties of Sr<Subscript>0.7</Subscript>Ce<Subscript>0.2</Subscript>TiO<Subscript>3</Subscript>–Sr(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> ceramics

xSr_0.7Ce_0.2TiO₃–(1???x)Sr(Mg_1/3Nb_2/3)O₃ ceramics, referred to xSCT–(1???x)SMN, were successfully produced by conventional solid-state sintered technology. The compounds, belonging to perovskites with a secondary phase of CeO₂, can be detected even with x down to 0.1 of SCT composition. The overall trend for grain growth illustrates the increase with increasing SCT doping level. The Raman peak at 825 cm^?1 splits into two peaks and causes red shift phenomenon. XPS spectra indicate that Ti and Nb ions exist respectively in tetravalence and pentavalence, and Ce ions exist in trivalence and tetravalence. Dielectrics constant (ε _r ) of SCT–SMN ceramics gradually increases with increasing theoretical dielectric polarizabilities. A wider width of the 825 cm^?1 for FWHM of A_1g mode Raman peaks suggests to a lower Q?×?f value. The increasing tolerance factor in agreement with temperature coefficient of resonant frequency (τ _f ), denotes that the rise of perovskite symmetry. The 0.1SCT–0.9SMN ceramic sintered at 1450?°C for 4 h illustrates excellent microwave dielectric properties with ε _r ?~?35.4, Q?×?f?~?11282 GHz and τ _f ?~?1.7 ppm/°C. Activation energies of 0.1SCT–0.9SMN ceramic at 100, 300 and 500 V, are ~0.436, 0.427 and 0.331 eV, respectively, indicative of a decreased trend with external electric field.     

Good electrical performances and impedance analysis of (1 − <Emphasis Type="Italic">x</Emphasis>)KNN–<Emphasis Type="Italic">x</Emphasis>BMM lead-free ceramics

(1 ? x)(K_0.5Na_0.5)NbO₃–xBi(Mg_0.75Mo_0.25)O₃ [(1 ? x)KNN–xBMM] (x?=?0.005, 0.01, 0.02) ceramics were prepared via a solid-state reaction method. X-ray diffraction patterns (XRD) and Raman spectrum showed that a solid solution was formed between the BMM and KNN, which improved the electrical properties of KNN. With increasing the BMM content, the grain firstly increased and then decreased. When x?=?0.01, the ceramics exhibited the optimized microstructure, indicating that there exits an optimal doping component. Temperature dependence of relative permittivity also increases firstly and then decreases. The relative permittivity (ε_r) of ~?1418 in stabilization zone, ε_max?~?4861 at the Curie temperature T _C ~ 394 °C, good temperature stability ?ε/ε_123 °C?≤?±?15% from 123 °C to 348 °C, and the dielectric loss tanδ?≤?0.036 from 109 to 348 °C were obtained for 0.99KNN-0.01BMM ceramics. Conductivity behavior of the (1 ? x)KNN–xBMM was investigated as a function of temperature from 420 to 520 °C and frequency from 40 to 106 Hz, showing that the basic mechanisms of conduction and relaxation processes were thermally activated, and oxygen vacancies were the possible ionic charge transport carriers at higher temperatures.     

Ferromagnetism in Fe-doped BaTiO<Subscript>3</Subscript> Ceramics

Polycrystalline samples of BaTi_1?xFe_xO₃ (x = 0.00–0.30) are prepared by solid-state reaction method and their structural and magnetic properties are studied. Detailed investigation of XRD patterns reveal the coexistence of tetragonal (space group P4mm) and hexagonal phases (space group P6 ₃/mmc) for x ≥ 0.1. Magnetic measurements reveal room-temperature ferromagnetism in x = 0.15–0.3 samples, and their ferromagnetic transition temperature increases from 397 K for x = 0.15 to 464 K for x = 0.3. The initial magnetization curves for x = 0.15–0.3 are analyzed in terms of bound magnetic polaron (BMP) model. The analysis of susceptibility data in the paramagnetic region by Curie-Weiss law confirms the ferromagnetic transition for x ≥ 0.15 and the effective magnetic moment systematically increases with increase in Fe concentration.     

Stoichiometry Analysis and Normal-State Properties of SmBa2Cu3−x Ru x O7−δ Superconducting Phase

The electrical resistivity of normal and superconducting states for SmBa₂Cu_3?x Ru _x O_7?δ (Sm-123) phase with 0.00 ≤ x ≤ 0.50, prepared by the conventional solid-state reaction technique, was studied. X-ray powder diffraction (XRD), scanning electron microscope (SEM), particle-induced X-ray emission (PIXE), Rutherford backscattering spectroscopy (RBS) and electrical resistivity measurements were performed in order to investigate the effect of Ru⁴⁺ ions substitution in Sm-123 phase. Both the phase formation and superconducting transition temperature T _c enhance up to x = 0.05. For x > 0.05, suppression of both the phase formation and T _c is observed and the superconductivity is completely destroyed around x = 0.50. The normal-state electrical resistivity was analyzed by the two- and three-dimensional variable range hopping (2D-VRH and 3D-VRH) and Coulomb gap CG. The dominant mechanism for Sm-123 phase is CG with x ≤ 0.20 while is 3D-VRH for x ≥ 0.30.     

The magnetic properties of BiY<Subscript>2</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript> nanoparticles doped with Cr ions

BiY₂Cr _x Fe_5?x O₁₂ (x = 0, 0.05, 0.1, 0.2, 0.3) nanocrystals were synthesized by using a sol-gel method. Samples were characterized by the powder X-ray diffraction (XRD), the thermal gravity analysis (TGA) and the differential thermal analysis (DTA), the vibrating sample magnetometer(VSM) and Mössbauer spectrums. The average sizes of the particles were determined by the Scherrer’s formula. The special Ms and Mössbauer spectra of BiY₂Cr _x Fe_5?x O₁₂ nanocrystals are researched at room temperature. It is seen that the special Mss of samples are initially increased with increasing Cr³⁺ content (x < 0.1), and decreased with increasing content of Cr³⁺ ions (x > 0.1).     

Anisotropic ferromagnetism in Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript> nanostructure arrays

Periodic arrays of Fe _x Sn_1?x O₂ nanostructures were fabricated by glancing angle sputter deposition onto self-assembled close-packed arrays of 200-nm-diameter polystyrene microspheres. After annealing at 873 K for 3 h, all the films were crystallized to rutile SnO₂ and maintained good thermal stability in the morphology. Compared with Fe _x Sn_1?x O₂ flat films, arrays of Fe _x Sn_1?x O₂ nanostructures possessed larger saturation magnetic moment and exhibited both perpendicular and in-plane magnetic anisotropy, resulting from the anisotropic morphology of Fe _x Sn_1?x O₂ nanostructures. The EPR signal originating from the oxygen vacancies significantly varied with the Fe concentration and reached the strongest at x = 0.059, which is consistent with the saturation magnetization. It demonstrates that the oxygen vacancies are an important factor for the ferromagnetism of Fe _x Sn_1?x O₂ films.     

Microwave dielectric properties of ZnO–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>–<Emphasis Type="Italic">x</Emphasis>TiO<Subscript>2</Subscript> ceramics prepared by reaction-sintering process

The ZnO–Nb₂O₅–xTiO₂ (1 ≤ x ≤ 2) ceramics were fabricated by reaction-sintering process, and the effects of TiO₂ content and sintering temperature on the crystal structure and microwave dielectric properties of the ceramics were investigated. The XRD patterns of the ceramics showed that ZnTiNb₂O₈ single phase was formed as x ≤ 1.6 and second phase Zn_0.17Nb_0.33Ti_0.5O₂ appeared at x ≥ 1.8. With the increase of TiO₂ content and sintering temperature, the amount of the second phase Zn_0.17Nb_0.33Ti_0.5O₂ increased, resulting in the increase of dielectric constant, decrease of Q × f value, and the temperature coefficient of resonant frequency (τ _f ) shifted to a positive value. The optimum microwave dielectric properties were obtained for ZnO–Nb₂O₅–2TiO₂ ceramics sintered at 1075 °C for 5 h: ε _r  = 45.3, Q × f = 23,500 GHz, τ _f  = +4.5 ppm/°C.     

Enhanced magnetoelectric coupling in La-modified Bi<Subscript>5</Subscript>Co<Subscript>0.5</Subscript>Fe<Subscript>0.5</Subscript>Ti<Subscript>3</Subscript>O<Subscript>15</Subscript> multiferroic ceramics

Multiferroic properties of La-modified four-layered perovskite Bi_5?x La _x Fe_0.5Co_0.5Ti₃O₁₅ (0 ≤ x ≤ 1) ceramics were investigated, by analyzing the magnetodielectric effect, magneto-polarization response and magnetoelectric conversion. X-ray diffraction indicated the formation of pure Aurivillius ceramics, and Raman spectroscopy revealed the Bi ions displacement and the crystal structure variation. The enhancement of ferromagnetic and ferroelectric properties was observed in Bi_5?x La _x Fe_0.5Co_0.5Ti₃O₁₅ after La modification. The evidence for enhanced ME coupling was determined by magnetic field-induced marked variations in the dielectric constant and polarization. A maximum ME coefficient of 1.15 mV/cm·Oe was achieved in Bi_4.25La_0.75Fe_0.5Co_0.5Ti₃O₁₅ ceramic, which provides the possible promise for novel magnetoelectric device application.     

Doping mechanisms and electrical properties of bismuth tantalate fluorites

Phase-pure bismuth tantalate fluorites were successfully prepared via conventional solid-state method at 900 °C in 24–48 h. The subsolidus solution was proposed with the general formula of Bi_3+x Ta_1?x O_7?x (0 ≤ x ≤ 0.184), wherein the formation mechanism involved a one-to-one replacement of Ta⁵⁺ cation by Bi³⁺ cation within ~4.6 mol% difference. These samples crystallised in a cubic symmetry, space group Fm-3 m with lattice constants, a = b = c in the range 5.4477(± 0.0037)–5.4580(± 0.0039) Å. A slight increment in the unit cell was discernible with increasing Bi₂O₃ content, and this may attribute to the incorporation of relatively larger Bi³⁺ cation in the host structure. The linear correlation between lattice parameter and composition variable showed that the Vegard’s law was obeyed. Both TGA and DTA analyses showed Bi_3+x Ta_1?x O_7?x samples to be thermally stable as neither phase transition nor weight loss was observed within ~28–1000 °C. The AC impedance study of Bi₃TaO₇ samples was performed over the frequency range 5–13 MHz. At intermediate temperatures, ~350–850 °C, Bi_3+x Ta_1?x O_7?x solid solution was a modest oxide ion conductor with conductivity, ~10^?6–10^?3 S cm^?1; the activation energy was in the range 0.98–1.08 eV.     

Effects of LiNbO<Subscript>3</Subscript> doping on the microstructures and electrical properties of BiScO<Subscript>3</Subscript>–PbTiO<Subscript>3</Subscript> piezoelectric system

Piezoelectric ceramics xLiNbO₃–yBiScO₃–(1?x?y)PbTiO₃ (LN–BS–PT, 0.00?≤?x?≤?0.10, 0.30?≤?y?≤?0.36) were synthesized and their phase diagram and morphotropic phase boundary between rhombohedral and tetragonal phases have been confirmed. The optimal properties were found at the composition of 0.03LN–0.36BS–0.61PT with piezoelectric coefficient d₃₃^* value of 702 pm/V, d₃₃ of 551 pC/N, planar electromechanical coupling factor k_p of 0.51, remnant polarization P_r of 46.5 µC/cm², Curie temperature T_c of 337 °C, and a large strain of 0.351% at an electric field of 50 kV/cm and frequency of 2 Hz with a low strain hysteresis of 5.9%. The Curie temperature of the ternary system presents a linear relationship with LiNbO₃ and BiScO₃ contents. The optimization of these electric properties was probably ascribed to the enhancement in domain walls and the improving mobility of domain switching due to LiNbO₃ doping.     

Magnetization reversal and tunable exchange bias behavior in Mn-substituted NiCr<Subscript>2</Subscript>O<Subscript>4</Subscript>

Single phase samples of Ni(Cr_1?xMn _x )₂O₄ (x = 0–0.50) were synthesized by using sol–gel route. Investigation of structural, magnetic, exchange bias and magnetization reversal properties was carried out in the bulk samples of Ni(Cr_1?xMn _x )₂O₄. Rietveld refinement of the X-ray diffraction patterns recorded at room temperature reveals the tetragonal structure for x = 0 sample with I4₁/amd space group and cubic structure for x ≥ 0.05 samples with \( {\text{Fd}\bar{3}\text{m}} \) space group. Magnetization measurements show that all samples exhibit ferrimagnetic behavior, and the transition temperature (T_C) is found to increase from 73 K for x = 0 to 138 K for x = 0.50. Mn substitution induces magnetization reversal behavior especially for 30 at% of Mn in NiCr₂O₄ system with a magnetic compensation temperature of 45 K. This magnetization reversal is explained in terms of different site occupation of Mn ions and the different temperature dependence of the magnetic moments of different sublattices. Study of exchange bias behavior in x = 0.10 and 0.30 samples reveals that they exhibit negative and tunable positive and negative exchange bias behavior, respectively. The magnitudes of maximum exchange bias field of these samples are found to be 640 and 5306 Oe, respectively. Exchange bias in x = 0.10 sample originates from the anisotropic exchange interaction between the ferrimagnetic and the antiferromagnetic components of magnetic moment. The tunable exchange bias behavior in x = 0.30 sample is explained in terms of change in domination of one sublattice moment over the other as the temperature is varied.     

Very high thermal stability with excellent dielectric,and non-ohmics properties of Mg-doped CaCu<Subscript>3</Subscript>Ti<Subscript>4.2</Subscript>O<Subscript>12</Subscript> ceramics

In this work, the nominal CaCu_3?xMg_xTi_4.2O₁₂ (0.00, 0.05 and 0.10) ceramics were prepared by sintering pellets of their precursor powders obtained by a polymer pyrolysis solution method at 1100 °C for different sintering time of 8 and 12 h. Very low loss tangent (tanδ)?<?0.009–0.014 and giant dielectric constant (ε′) ～?1.1?×?10⁴–1.8?×?10⁴ with excellent temperature coefficient (Δε′) less than ±?15% in a temperature range of ??60 to 210 °C were achieved. These excellent performances suggested a potent application of the ceramics for high temperature X8R and X9R capacitors. It was found that tanδ values decreased with increasing Mg²⁺ dopants due to the increase of grain boundary resistance (R_gb) caused by the very high density of grain, resulting from the substitution of small ionic radius Mg²⁺ dopants in the structure. In addition, CaCu_3?xMg_xTi_4.2O₁₂ ceramics displayed non-linear characteristics with the significant enhancements of a non-linear coefficient (α) and a breakdown field (E_b) due to Mg²⁺doping. The high values of ε′ (14012), α (13.64) and E_b (5977.02 V/cm) with very low tanδ value (0.009) were obtained in a CaCu_2.90Mg_0.10Ti_4.2O₁₂ ceramic sintered at 1100 °C for 8 h.     

Effect of Co partial substitution on the valence state of Ru in the Gd<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ru<Subscript>2</Subscript>O<Subscript>7</Subscript> pyrochlore

Polycrystalline samples of Gd_2?xCo _x Ru₂O₇ with x = 0.0, 0.1 and 0.4 were synthesized by the molten salt method. The samples were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrical resistivity measurements. Rietveld refinements of the XRD patterns and XPS measurements showed that the Co²⁺ ion replaces Gd³⁺ sites. As a result, the lattice parameter a and the Ru–O bond length decrease; then, the Ru–O–Ru bond angle increases. Those changes induce a charge compensation which was detected by XPS measurements. The analysis of these results shows that the Ru 3d_5/2 core level could be fitted assuming the contribution of two different chemical states of the Ru. The Ru 3d_5/2 core level is localized at 280.7 and 281.6 eV, which corresponds to Ru⁴⁺ and Ru⁵⁺. The valence band XPS spectra show an increase in Co 3d states at the Fermi level as the Co content increases, which contribute to the decrease in the electrical resistivity.     

Effect of Al Substitution in Structural and Magnetic Properties of MnCr<Subscript>2</Subscript>O<Subscript>4</Subscript>

Single-phase samples of Mn(Cr_1?x Al _x )₂O₄ (x = 0 – 0.30) with cubic spinel structure were prepared and the lattice constant is found to decrease from a = 8.4396 Å for x = 0 to a = 8.3801 Å for x = 0.30. The substitution of Al at Cr site is confirmed from the blue shift of Raman modes. Magnetization measurements and analysis show all the prepared samples exhibit ferrimagnetic transition with transition temperature in the range of 46 K for x = 0 to 33 K for x = 0.30. The saturation magnetization (M _s ) and the estimated anisotropy constant (K) show an anomalous behavior up to x = 0.10 and beyond that they decrease monotonously. They are explained by considering different site preferences of Al ³⁺ ions as the doping concentration is increased. The theoretical and experimental effective magnetic moment of the samples is found to be comparable and it decreases with increase in Al concentration.     

Ultralow thermal conductivity of cerium-doped Nd<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> over a wide doping range

In this paper, we report an ultralow thermal conductivity and a high-temperature phase stability of the (Nd_1?x Ce _x )₂Zr₂O_7+x system over the temperature range from room temperature to 1600 °C and over a wide composition range (0.2 ≤ x ≤ 0.8), and the (Nd_1?x Ce _x )₂Zr₂O_7+x system is therefore considered a strong candidate material for the fabrication of next-generation high-temperature thermal barrier coatings. The observed thermal conductivities (0.65–1.0 W/mK) are about 60–40% lower than those of undoped Nd₂Zr₂O₇ over the same temperature range (100–700 °C) and indicate a glass-like behavior. For comparison, the variation in the thermal conductivity with the temperature of the (Gd_1?x Ce _x )₂Zr₂O_7+x system with similar point defects was also measured, and the observed behavior was almost the same as that of undoped Gd₂Zr₂O₇ and was mostly determined by phonon–phonon scattering (λ ∝ 1/T). The effect of point defect scattering and strong phonon scattering sources (rattlers) on the thermal conductivity is also discussed in this paper. The results of this study suggest that the ultralow thermal conductivity of (Nd_1?x Ce _x )₂Zr₂O_7+x can be attributed to the presence of rattlers because of the large difference between the ionic radii of the Nd³⁺ and Ce⁴⁺ ions.