Thermophysical properties of Gd2Zr2O7 powders ... - Repositorio INEEL

Journal of Alloys and Compounds 649 (2015) 1145e1150

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Thermophysical properties of Gd2Zr2O7 powders prepared by mechanical milling: Effect of homovalent Gd3þ substitution n a, *, O.J. Dura b, M.R. Díaz-Guille n c, E. Bauer d, M.A. Lo pez de la Torre b, J.A. Díaz-Guille A.F. Fuentes e n de Estudios de Posgrado e Investigacio n, Instituto Tecnolo gico de Saltillo, 25280 Saltillo, Coahuila, Mexico Divisio GFMA, Departamento de Física Aplicada, Escuela T ecnica Superior de Ingenieros Industriales, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain c Instituto de Investigaciones El ectricas, 62490 Cuernavaca, Morelos, Mexico d Institute of Solid State Physics, Vienna University of Technology, Wien A-1040, Austria e Cinvestav Unidad Saltillo, Apartado Postal 663, 25000 Saltillo, Coahuila, Mexico a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 April 2015 Received in revised form 3 July 2015 Accepted 17 July 2015 Available online 20 July 2015

This contribution analyzes the thermophysical properties of Gd1.6Ln0.4Zr2O7 (Ln ¼ La3þ, Nd3þ, Sm3þ, Dy3þ and Er3þ) ceramics synthesized at room temperature, by mechanically milling stoichiometric mixtures of high purity oxides. Regardless of chemical composition, powders milled for 27 h show XRD patterns similar to fluorite-type materials. Post-milling thermal treatments at 1500 C, facilitates the evolution to the ordered pyrochlore derivative for Gd2Zr2O7, and the La3þ-, Nd3þ-, and Sm3þ-containing materials. By contrast, samples containing the smaller lanthanides (Dy3þ or Er3þ), maintain the fluorite structure. Thermal conductivity of the as-prepared samples was obtained as a function of temperature, from thermal diffusivity, heat capacity and density values, using sintered pellets. We found that doping has an important effect in lowering Gd2Zr2O7 thermal conductivity, with final values ranging from 1.22 to 1.94 W m1 K1; Nd3þ- and Er3þ-containing samples represent an optimum combination of defects and disordering of oxygen vacancies that generate the lowest conductivity values of all samples tested. © 2015 Elsevier B.V. All rights reserved.

Keywords: Mechanochemical processing Heat conduction Order-disorder effects X-ray diffraction Point defects Pyrochlore

1. Introduction Lanthanide zirconates with the Ln2Zr2O7 general stoichiometry (Ln ¼ LaeLu and Y) have attracted considerable attention over the past few years because of their high thermochemical stability and structural flexibility, susceptible to alteration via processing and/or doping. Moreover, the series exhibit a wide range of chemical and physical properties of scientific, and technological interest [1e4]. Thus, some members of the series display significant oxygen ion conductivity at high temperatures, and are attractive component materials for electrochemical devices such as solid oxide fuel cells, and electrochemical pumps [5e8]. Lanthanide zirconates exhibit also very low thermal conductivities, and are under consideration as thermally insulating coatings (TBC's), to protect metallic components of gas turbines and diesel engines [9e13]. Depending on the size of the lanthanide ion, Ln2Zr2O7 oxides adopt either, the

gico de Saltillo, V.Carranza 2400 Col. * Corresponding author. Instituto Tecnolo gico, 25280 Saltillo, Coahuila, Mexico. Tecnolo n). E-mail address: [email protected] (J.A. Díaz-Guille http://dx.doi.org/10.1016/j.jallcom.2015.07.146 0925-8388/© 2015 Elsevier B.V. All rights reserved.

fluorite structure (Ln ¼ TbeLu and Y) or its ordered derivative, the pyrochlore structure (Ln ¼ La-Gd) [14], with many Ln2 Zr2 O7 Ln02 Zr2 O7 systems showing almost complete miscibility. Moreover, different lanthanide ions might be combined in systems of solid solutions (i.e., Ln2x Ln0x Zr2 O7 ), where the degree of structural disorder is mostly governed by the averaged RLn/RZr size ratio, i.e., the LneLn0 size difference [14e18]. However, processing conditions might have also, a significant effect on disorder. Therefore, these solid solutions are ideal oxide systems to analyze the influence of defects and disorder, on materials properties. Ideal pyrochlore oxides are isometric (S.G. ¼ Fd3m (227); Z ¼ 8), and characterized by the A2B2O6O0 stoichiometry, where A and B are cations of different charge and size. Selecting the origin choice 2 of the space group, the larger cation (A in the previous formula) is located at the Wyckoff 16d position (½,½,½), surrounded by eight oxygen ions (coordination polyhedron ¼ AO6O0 2) forming an axially compressed scalenohedron. The smaller B cation is placed at the 16c site (0,0,0), within a trigonal antiprism with all six oxygen ions at equal distances from the metal atom (coordination polyhedron ¼ BO6). Furthermore, the structure contains two non-

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J.A. Díaz-Guillen et al. / Journal of Alloys and Compounds 649 (2015) 1145e1150

equivalent oxygen ions (O and O0 ) which fully occupy two tetrahedrally coordinated sites, the 48f (x,1/8,1/8) and 8b (3/8,3/8,3/8) Wyckoff positions respectively; the 48f-site is surrounded by two A and two B nearest neighbors, whereas the 8b-site is coordinated by four A metal atoms. A distinctive peculiarity of the pyrochlore structure is the presence of an additional interstitial site in the unit cell, 8a (1/8,1/8,1/8), which is nominally empty in ideal (ordered) structures but easily accessible for anions. This vacant site would complete a fluorite-type anion sublattice, and provides the pyrochlore structure with a remarkable ability to tolerate lattice distortions. Pyrochlore oxides intrinsic concentration of anion vacancies and capacity to accumulate structural defects, afford a large variety of phonon scattering sources for low thermal conductivity. State-of-the-art technology in refractory-oxide coatings is based on a tetragonal metastable zirconia phase t0 -ZrO2 (~7 wt% Y2O3 stabilized ZrO2) [19,20] which offers a rather exceptional combination of properties such as low thermal conductivity (~1.4e2 W m1 K1), high melting point (>2500 C) and high fracture toughness at high temperature. However, metastable t0 ZrO2 shows accelerate sintering activity above 1200 C, and transforms into the more stable cubic and tetragonal zirconia polymorphs; t-ZrO2 evolves further to the monoclinic form on cooling, and the concomitant molar volume expansion has a catastrophic effect on coatings durability. Therefore, intense research efforts are underway to identify alternate materials to replace t0 -ZrO2. Although there are various thermal and mechanical requirements to be considered when evaluating a prospective TBC material, the main metric would be probably, its thermal conductivity (k) at operating temperatures. In this context, Ln2Zr2O7 zirconates are receiving a great deal of attention because of their low k, high melting point (>2300 C) and rather sluggish sintering kinetics [9e13,19,20]. Computer simulations have predicted k values along the series, in the 1.85e2.15 W m1 K1 range, with little dependence on the Ln ion [21]; however, experimental k values (~1.1e1.7 W m1 K1 between 700 and 1200 C) are consistently lower than predictions, and exhibit almost flat dependence on temperature, Ln size and crystal structure (fluorite vs. pyrochlore) [9,10,22]. Even lower k values have been measured in Ln2x Ln0x Zr2 O7 systems [11,12,23,24] though the origin of such phenomenon remains poorly understood. The present research deals with the thermal properties of mechanochemically prepared Gd2Zr2O7 powders, and the influence of Gd homovalent substitutions on heat transport. Compare to other processing techniques, mechanical milling (MM) is simple to implement and yet, capable of providing relatively large batches of many complex oxides in an economically viable manner [25,26]. As reactants are processed under non-equilibrium conditions, uncommon metastable phases are frequently obtained featuring a high concentration of structural defects. Furthermore, mechanically induced defects are generally difficult to relax; therefore, additional processing (e.g. post-milling thermal treatments) offers the possibility of isolating highly defective intermediate states, which are inaccessible for more conventional near-equilibrium processing techniques. 2. Materials and methods Pure Gd2Zr2O7 and five different samples with the general Gd1.6Ln0.4Zr2O7 formulae (Ln ¼ La3þ, Nd3þ, Sm3þ, Dy3þ and Er3þ), were prepared by mechanical milling starting from appropriate mixtures of high-purity oxides (Gd2O3, Ln2O3 and ZrO2; >99%, SigmaeAldrich, Inc.). As most Ln2O3 oxides are highly reactive towards atmospheric H2O and CO2, Ln2O3, starting chemicals were fired overnight at 900 C prior to weighing to decompose existing

hydroxides, carbonates and oxycarbonates, and ensure accurate stoichiometries. In a typical experiment, 15 g of reactants were placed in 125 ml YPSZ (yttria partially stabilized zirconia) containers together with 20 mm diameter YPSZ balls (balls-to-powder mass ratio equal to 10:1) as grinding media; milling was carried out in air, in a Retsch PM400 planetary ball mill (rotating disc speed ¼ 350 rpm) with reversed rotation every 20 min to favor reaction. Phase evolution was analyzed by X-ray diffraction (XRD) using a Philips X'Pert diffractometer, and Ni-filtered Cu-Ka radiation (l ¼ 1.5418 Å). Milling time needed to achieve single phase products was determined by examining at different time intervals, the XRD patterns of two samples selected as representatives of the series, i.e., Gd1.6La0.4Zr2O7 and Gd1.6Er0.4Zr2O7 (largest size mismatch to the Gd3þ host); mechanically induced chemical reactions were considered completed when no traces of the starting reagents were evident by this technique. To minimize any processing effect on the final structural/microstructural characteristics of the target materials, the same milling parameters were used to prepare the whole series. Specimens used for the thermophysical study were fabricated by cold-pressing (~1 GPa) the as-prepared powders into pellets (1 cm diameter, ~3 mm thickness), and sintering at 1500 C for 6 h (heating and cooling rate ¼ 2 C,min1). Fired samples bulk density (r) was measured using the Archimedes principle, and deionized water as the immersing medium. Specific heat capacity values (Cp) were obtained by differential scanning calorimetry (Pt crucibles), using a simultaneous DSC/TGA Netzsch STA 449C Jupiter Thermal Analyser working continuously from room-temperature to 800 C with a scan rate of 20 K min1; measurements reproducibility and accuracy, were checked at regular intervals with a synthetic sapphire standard material. Thermal diffusivities (a) were measured by the laser flash method using an Anter FlasLine™ 3000 instrument. Prior to every measurement, samples were coated at the front and rear faces with a thin layer of colloidal graphite to enhance absorption and emission of the laser beam respectively (temperature range: 100e900 C at 100 C intervals; N2 atmosphere). Coating stability, and measurements reproducibility were confirmed by performing three analyses under identical conditions. To take into account radiative losses occurring in nonadiabatic conditions, the Clarke and Taylor correction was used to calculate a from the experimental data [27,28]. Thermal conductivities were determined from separate measurements of a, Cp and r of each sample, according to the following equation:

k0 ¼ a$r$Cp

(1)

As heat transport is dramatically affected by the presence of porosity, not only by the pores volume fraction but also their morphology and spatial distribution, calculated values were corrected as suggested in Refs. [10,29]:

k0=k ¼ 1

4 f 3

(2)

where ø would be the porosity as inferred by the Archimedes method, and k, the thermal conductivity of a fully dense solid. Sintered pellets were also examined by scanning electron microscopy (SEM) in a Philips XL30 ESEM microscope equipped with an EDAX Inc. energy-dispersive X-ray detector for microanalysis.

3. Results The stability of the pyrochlore crystal structure relies basically on both, A and B absolute size (rA and rB) and their size ratio (rA/rB). Thus, radius ratio constraints for A2B2O6O0 lanthanide titanates and


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Table 1 Size and atomic mass changes produced by replacing Gd in Gd2Zr2O7 with different Ln3þ. rLn and mLn: weighted average ionic radii and atomic mass for lanthanide ions considered in each composition; rLn/rZr: Ln to Zr ionic radii ratio; Dm(%) and DrLn(%): atomic mass and ionic radii changes (%) produced when replacing Gd in Gd2Zr2O7, by different lanthanides. Composition

Averaged rLn

Averaged mLn

rLn/rZr

Crystal structure

Dm (%)

DrLn (%)

Gd1.6La0.4Zr2O7 Gd1.6Nd0.4Zr2O7 Gd1.6Sm0.4Zr2O7 Gd2Zr2O7 Gd1.6Dy0.4Zr2O7 Gd1.6Er0.4Zr2O7

1.074 1.064 1.058 1.053 1.048 1.043

153.5 154.6 155.8 157.2 158.7 159.2

1.492 1.478 1.469 1.462 1.456 1.448

Pyrochlore Pyrochlore Pyrochlore Pyrochlore Fluorite Fluorite

2.35 1.65 0.89 0 0.95 1.27

2.00 1.04 0.47 0 0.47 0.95

Ionic radii used in calculations are those given by Shannon [30].

zirconates (A ¼ Ln3þ and Y3þ; B ¼ Ti4þ and/or Zr4þ) result in pyrochlore formation when 1.46 rA/rB 1.78 [14]. Outside this range, cations size mismatch favors a structural change to, either perovskite related monoclinic layered structures (rA/rB > 1.78), or fluorite structures (rA/rB < 1.46). Accordingly, pyrochlore zirconates show increasing tendency to disorder as the rA/rB decreases leading eventually, to the fluorite structure when rA/rB z 1.46. As shown in Table 1, Gd2Zr2O7 lies precisely in the lower limit of the above mentioned stability field (rGd/rZr ¼ 1.462); correspondingly, two crystal forms of this compound are known with either pyrochlore (PeGd2Zr2O7) or fluorite structure (FeGd2Zr2O7 or alternatively Gd0.5Zr0.5O7/4): whereas the first one is stable at roomtemperature, the latter only exists above ~1550 C. Nevertheless, the enthalpy difference separating both phases is small (e.g. ~10 kJ mol1 at 25 C) [31], and the fully disordered anion deficient fluorite structure FeGd2Zr2O7 has been also stabilized at ambient conditions via processing or by chemical substitutions. As shown in Table 1, replacing Gd3þ in Gd2Zr2O7 by smaller lanthanides (e.g., Dy3þ and Er3þ) decreases the rA/rB size ratio, and the stability of the pyrochlore phase; by contrast, substituting lanthanides larger than Gd3þ (e.g., La3þ, Nd3þ and Sm3þ) yield better ordered pyrochlore oxides. Mechanical milling (MM) has been extensively used in this research group for the preparation of pyrochlore type materials. Regardless the rA/rB size ratio, this processing method favors the formation of highly disordered structures [32,33]; this is also the case for the title Gd2Zr2O7-based series. Fig. 1a shows an XRD study of the evolution of the Ndcontaining mixture selected as representative of the series, with milling time; Fig. 1b and c displays the XRD patterns collected for the whole set of samples under study, after milling for 27 h (b), and the same milled samples, after firing at 1500 C (c). As shown in Fig. 1a, MM produces a gradual change in the XRD pattern of the reaction mixture, with new reflections clearly emerging after 14 h; the characteristic reflections of the elemental oxides (m-ZrO2, CeGd2O3 and A-Nd2O3), all disappeared after 27 h with the resultant XRD pattern resembling that characteristic of a fluoritelike material. Furthermore, irrespective of the replacing Ln3þ cation, all XRD patterns displayed in Fig. 1b are similar, and show no evidence [34] of the superlattice reflections characterizing the long-range ordering of cations, anions and vacancies of pyrochlore oxides (e.g., Miller indexes (111), (311) or (331) lines at ~15 , ~29 and ~38 (2q) respectively). As the intensity of the superstructure diffraction lines decreases with increasing structural disorder, results shown in Fig. 1b confirm that MM promotes the formation of fully disordered, and fluoritelike materials instead of the ordered pyrochlore derivative. Broad reflections are typical of mechanochemically prepared materials, and are due to small domain size and microstrain effects [33]. The small shift of the diffraction lines towards higher or lower angles (2q) related to the pristine Gd2Zr2O7 material (e.g., zooming in 1b at the (311) peak at ~58 ), proves the existence of a mechanically induced chemical reaction on milling: Gd2Zr2O7 lattice parameter decreases when replacing Gd3þ by smaller ions (Dy3þ

and Er3þ), and increases when using larger substituting lanthanides (La3þ, Nd3þ and Sm3þ). As shown in Fig. 1c, exposure of the asprepared samples to high temperatures, facilitates the atomic rearrangement, and pyrochlore ordering. Superstructure reflections are evident for pure Gd2Zr2O7 as well as for Sm, Nd or Ladoped Gd2Zr2O7; whereas, Dy and Er-containing powders maintain the fluorite structure. To evaluate the thermophysical properties of pure and doped Gd2Zr2O7, mechanochemically obtained powders were coldpressed, and sintered at 1500 C for 6 h; Fig. 2 shows the bulk density values of such pellets together with the corresponding theoretical densities determined from the XRD lattice parameters as a reference. In general, bulk densities reach 90e96% of the theoretical values but for the Er-containing sample, which shows much larger porosity. Fig. 3a and b illustrate pellets typical morphology: sintered (unpolished) specimens (e.g. Gd2Zr2O7 and Gd1.6La0.4Zr2O7) consist basically of irregularly shaped grains of clean boundaries, and ~2 mm of maximum size. Fig. 4 shows the Cp and a values measured for the title series. As observed in Fig. 4a, Cp values increase with increasing temperature (inset) though they are almost composition independent: i.e. varying rLn/rZr size ratio have little effect on Cp; in fact, the maximum difference is about 4.5% which is not much larger than the experimental uncertainty estimated in ~3% [35]. Thermal diffusivity dependence on temperature is similar to most polycrystalline materials as shown in Fig. 4b: decreases progressively with increasing temperature and then reach an almost temperature-independent value above 400 C, which has been related with a dominant heat transport mechanism via phonons [36]. However, a shows a strong dependence on chemical composition; Gd substitution decreases a although values measured for the Er-containing sample, are considerably lower than the rest. Finally, k values were calculated according to Equation (1), corrected to fully dense materials using Equation (2), and plotted in Fig. 5 as a function of temperature, and the substituting lanthanide. As shown, k values are all included in the 1.22e1.94 W m1 K1 range, limiting values corresponding to the Er- and Sm-containing samples at 400 and 800 C respectively. These values are lower than those typically reported in literature for bulk t0 -ZrO2 in the same measuring temperature interval (2 W m1 K1), but similar to those found by different research groups for Gd2Zr2O7 (1.3e1.6 W m1 K1) [37e39]; furthermore, they show similar temperature dependence than some others substituted zirconates La2Zr2O7 [23]. Worth mentioning is also a pronounced radiative effect on heat transport at high temperatures, notably from 500 C onwards. 4. Discussion Heat transport in non-metallic and crystalline solids, is mostly carried out by quantized lattice vibrations (phonons) although radiant transfer (photons) might become significant at high temperatures [40]. According to the simple Debye model for heat


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Fig. 2. Comparison between bulk density values determined by the Archimedes method and theoretical densities calculated from XRD data (bottom); % apparent density (top).

phonon group velocity and l is the phonons mean free path (distance travelled by phonons between two consecutive scattering events). In this context, reducing l would be the most effective strategy to suppress k, either by increasing the number of collisions between phonons themselves or between phonons, and lattice imperfections (e.g. point defects, grain boundaries, impurities, etc.).

Fig. 1. (a) XRD study of the evolution of the ZrO2 and Gd2O3-Nd2O3 mixture (2:1 M ratio) with milling time; (b) XRD patterns of the as-prepared powders after milling for 27 h and (c), the same samples after firing at 1500 C. The characteristic Bragg reflections of fluorite and pyrochlore structures are respectively shown in 1b (PDF 80e0471) and 1c (PDF 79e1146). Number in parenthesis are the corresponding Miller indexes. Emerging reflections after 14 h of milling in Fig. 1 (a) are marked with (*).

transfer, k would be given to a good approximation by the expression:

k¼

1 C $n$l 3 V

(3)

where CV is the heat capacity per unit volume, n is the average

Fig. 3. Typical micrographs of the as-prepared powders after firing at 1500 C for 6 h: (a) Gd2Zr2O7 and (b) Gd1.6La0.4Zr2O7.


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Fig. 5. Thermal conductivity dependence with temperature for the as-prepared Gd1.6Ln0.4Zr2O7 powders (Ln ¼ La, Nd, Sm, Gd, Dy and Er).

Fig. 4. (a) Specific heat capacity (Cp) at different temperatures and (b) thermal diffusivity (a) values measured for the title series. The inset in (a) shows Cp evolution with temperature for the Er-containing sample.

In particular, the efficiency of substitutional defects in lowering k, will mostly depend on the change in atomic bonding, and on the mass difference (mass disorder) and the relative ionic size difference between both, defect and host atoms [41]. Available experimental data [11,12,23,24,42e44] and molecular-dynamic simulations [45], have indeed confirmed doping as a suitable mechanism to reduce k in most pyrochlore zirconates. Some experimental values reported in La2Zr2O7-based solid solutions [24], are even close to the glasslike lower limit of thermal conductivity of highly disordered solids [46]. However, the role played by substitutional defects is not fully understood; open questions remain about which factor is more effective (i.e. size mismatch vs. atomic mass difference) or even, if substitutional defects are stronger phonon scatters in ZrO2-based materials, than a non-random distribution of oxygen vacancies. As shown in Table 1 and taking into account the substitutional levels selected in this work, replacing Dy3þ or Sm3þ for Gd3þ in Gd2Zr2O7, produces little “mass disorder” (Dm%) and/or “bond disorder” (DrLn%) since both, host and dopants, have similar size and atomic weights. Accordingly, these cations have a small effect on

Gd2Zr2O7 thermal conductivity; however, lower k is observed for the Dy-doped sample, most likely because this substitution induces a phase transition to a fluorite structure (see Fig. 1c). The situation is somehow more complex for the remaining three lanthanides tested, La3þ, Nd3þ and Er3þ; thus, La substitution in Gd2Zr2O7 decreases k although the reduction is lower than expected, based merely on size and atomic mass differences; the largest effect is observed when using Nd or Er as dopants. An explanation for these results may lie in considering the structural effect of these substitutions on pyrochlore-type Gd2Zr2O7. Thus, for identical dopant content, La3þ and Nd3þ produce pyrochlore structures with better ordered oxygen vacancies (increasing rLn/rZr size ratio and decreasing oxygen population at the 8a-site); whereas, Er3þ has the opposite effect, and increases disorder by inducing a phase transition to a fluoritelike structure. Interaction between phonons is weaker with better ordered oxygen vacancies in the La-containing sample, than with more randomly distributed defects, as in the Nd and Er-doped powders; vacancy ordering compensates the larger La3þ size mismatch and atomic mass difference to Gd3þ, with the net effect being a reduced depressing effect on k. By contrast, the synergistic effect of substitutional defects and disordering of oxygen vacancies in Gd1.6Nd0.4Zr2O7 and Gd1.6Er0.4Zr2O7 yield lower k values than in the remaining samples. Furthermore, “coloring” (i.e., dopants that change the color of the insulating material, and reduce radiation transport in the visible and near infrared regions) has been proposed as a mechanism to reduce radiative heat transport in TBC's [40]. Nd and Er doping colored the Gd2Zr2O7 samples (light blue and pink respectively) increasing powders opaqueness to infrared radiation, further reducing these samples thermal conductivity at high temperatures. 5. Conclusions Lanthanide zirconates Gd1.6Ln0.4Zr2O7 (Ln ¼ La3þ, Nd3þ, Sm3þ, Dy3þ and Er3þ) were successfully synthesized at room temperature, by milling stoichiometric mixtures of high purity oxides at a moderate rotating disc speed; regardless of chemical composition, as-obtained powders show XRD patterns similar to those of fluorite-type materials. Thermal treatments at high temperatures facilitate an evolution to ordered pyrochlore structures for undoped Gd2Zr2O7 and La3þ-, Nd3þ-, and Sm3þ-containing materials;

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whereas, those containing the smaller lanthanides (Dy3þ or Er3þ) maintain the anion deficient fluorite structure even after sintering at 1500 C. Thermophysical properties of the as-prepared samples were measured as a function of temperature; our results show that doping reduces Gd2Zr2O7 thermal conductivity with the lowest experimental values measured for the Nd3þ and Er3þ-containing samples. These compositions seem to present the optimum combination of substitutional defects and disordering of oxygen vacancies of the series to maximize phonon scattering and thus, yield lower conductivity. Acknowledgments Financial support from CONACYT (Grants CB-2011-01-166995 gico Nacional de Mexico and CB-2013-01-221701) and Tecnolo (Grant ITS/DEPI/Septiembre/14/003) is greatly appreciated.

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