Acta Materialia 207 (2021) 116684 Contents lists available at ScienceDirect Acta Materialia journal homepage: www.elsevier.com/locate/actamat Enhancing the Néel temperature in 3 d /5 d R 2 NiIrO 6 (R = La, Pr and Nd) double perovskites by reducing the R 3 + ionic radii P. Kayser a , b , ∗, A. Muñoz c , J.L. Martínez a , F. Fauth d , M.T. Fernández-Díaz e , J.A. Alonso a , ∗ a Instituto de Ciencia de Materiales de Madrid, C.S.I.C., Cantoblanco, E-28049 Madrid, Spain b Centre for Science at Extreme Conditions and School of Chemistry, The University of Edinburgh, Edinburgh EH9 3JZ, U.K c Dpto. de Física Aplicada, EPS, Universidad Carlos III, Avda. Universidad 30, Leganés, 28911 Madrid, Spain d CELLS – ALBA Synchrotron, 08290, Cerdanyola del Valles, Barcelona, Spain e Institut Laue Langevin, BP 156X, Grenoble F-38042, France a r t i c l e i n f o Article history: Received 28 June 2020 Revised 11 January 2021 Accepted 18 January 2021 Available online 21 January 2021 Keywords: Iridium Double perovskites 5d transition metals Neutron powder diffraction La 2 NiIrO 6 Pr 2 NiIrO 6 Nd 2 NiIrO 6 a b s t r a c t Double perovskites containing Ir 4 + were synthesised by a citrate technique, followed by an annealing treatment in air at 1100 °C. The crystal structure of the three compounds, with formula R 2 NiIrO 6 (R = La, Pr and Nd), were determined using a combined refinement against neutron powder diffraction (NPD) and synchrotron x-ray powder diffraction (SXRPD) data sets. At room temperature, all the samples were indexed in the space group P2 1 /n and the monoclinic symmetry remains in the 300 to 1273 K tempera- ture range. Magnetization measurements suggest competitive antiferromagnetic and ferromagnetic inter- actions, with an unexpected increment of the ordering temperature (T N ) along the series. The magnetic structures of all the samples were defined with the propagation vector k = 0; the Ni 2 + and Ir 4 + moment arrangement, different for each compound, shows a strong dependence on the nature of the rare-earth ion. © 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 1 a t m t s t o i q ½ i r a 5 A d o c a t n e t t d R 1 w i p a f h 1 . Introduction Iridium-based perovskites have shown to exhibit interesting nd unexpected physical properties due to their particular elec- ronic structures, arising from the spin-orbit coupling effect. In the ore spatially extended 5d orbitals, the Coulomb interactions are ypically weaker and its energy value is comparable to the corre- ponding spin-orbit coupling energy scale, leading to a modifica- ion of the electronic structure. In the perovskite structure, the 5d- rbitals of the metals located in an octahedral coordination split nto e g and t 2g levels due to the crystal electric field and, subse- uently, the t 2g triplet develops into the levels with J eff= 3/2 and due to the spin-orbit coupling effect. Since the ground state of ridium perovskites seems to be strongly modifiable, this offers a ich opportunity to tailor the physical properties of these materi- ls.[ 1 , 2 ] Rock-salt ordered double perovskites A 2 BB’O 6 containing 3d and d (Ir, Re, Os) metals over the B-site have recently attracted a great ∗ Corresponding authors. E-mail addresses: paula.kayser@ed.ac.uk (P. Kayser), ja.alonso@icmm.csic.es (J.A. lonso). t b b d ttps://doi.org/10.1016/j.actamat.2021.116684 359-6454/© 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. eal of attention. [3] There are many examples of 3d/5d double per- vskites with outstanding properties, such as as half-metals with olossal magnetoresistance (Sr 2 FeReO 6 ) [ 4 , 5 ] or magnetic order far bove room temperature (Sr 2 CrOsO 6 , Sr 2 CrReO 6 ) [ 6 , 7 ]; however, here are just a few examples containing iridium. In the present paper, we aim to investigate the crystal and mag- etic structure of three members of the R 2 NiIrO 6 family (R = rare arths). In these oxides, containing Ni 2 + and Ir 4 + over the B-site, he magnetic ordering temperature varies from 75 K (La 2 NiIrO 6 ) o 207 K (Lu 2 NiIrO 6 ) [8–14] . The common behaviour observed in ouble perovskites to accommodate the effect of decreasing the 3 + size in the structure is to reduce the tilting angle Ni-O-Ir from 80 degrees, leading to more distorted structures. In general, this ould worsen the orbital overlapping and therefore, the magnetic nteractions. In this particular system, we have observed the op- osite trend. The enhancement of the magnetic ordering temper- ture with the reduction of the A-site cation size is understood rom the magnetic structure features, unveiling the magnetic in- eractions predominant in this system and how they are affected y changes in the crystal structure. This is a comprehensive study, y state-of the art techniques like neutron or synchrotron x-ray iffraction, including aspects that have been never been addressed, https://doi.org/10.1016/j.actamat.2021.116684 http://www.ScienceDirect.com http://www.elsevier.com/locate/actamat http://crossmark.crossref.org/dialog/?doi=10.1016/j.actamat.2021.116684&domain=pdf mailto:paula.kayser@ed.ac.uk mailto:ja.alonso@icmm.csic.es https://doi.org/10.1016/j.actamat.2021.116684 P. Kayser, A. Muñoz, J.L. Martínez et al. Acta Materialia 207 (2021) 116684 l a m 2 p o t 9 ( t d s a t t O f p t fi b t t c t S p t c i p h s a t p t ( l t t i p t b i Q ( r ( 0 fi a d v 3 a Fig. 1. Combined SXRPD-NPD Rietveld refinement profiles for (a) La 2 NiIrO 6 , (b) Pr 2 NiIrO 6 and (c) Nd 2 NiIrO 6 at room temperature. The crosses are the experimental data, the black solid line is the calculated profile, the first series of Bragg reflec- tions correspond to the main perovskite phase ( P2 1 /n ), and the second one on the Nd 2 NiIrO 6 pattern corresponds to vanadium from the sample container. ike the high-temperature structural evolution until almost 10 0 0 °C nd the description of the microscopic arrangements of the low T agnetic structures. . Experimental section Polycrystalline samples of R 2 NiIrO 6 (R = La, Pr, Nd) were pre- ared by using the citrate-nitrate route. Stoichiometric amounts f La 2 O 3 (pre-treated at 900 °C to remove carbonates and wa- er) Pr 6 O 11 , Nd 2 O 3 (Alfa Aldrich, 99.9%) and Ni(NO 3 ) 3 .6H 2 O (Merck, 9.5%) were dissolved in 200 ml of citric acid aqueous solution 10% w/w); 1 ml of HNO 3 (68% vol) was added in order to facilitate he dissolution of the rare-earth oxides. IrO 2 (Strem, 99%) was not issolved and remained in suspension under constant stirring. The olution was evaporated, resulting in an organic resin that includes homogeneous distribution of all the cations and, subsequently, he resin was dried at 140 °C. To eliminate the organic residues and he nitrates, a thermal treatment at 600 °C during 12 h was applied. nce the precursors were prepared, they were annealed at 1100 °C or 12 h in air. The initial characterization of the samples and the reaction rogress was followed after each treatment by X-ray diffrac- ion (XRD) until single-phase perovskite materials were identi- ed. A Bruker-AXS D8 diffractometer (40 kV, 30 mA), controlled y DIFFRACPLUS software, in Bragg −Brentano reflection geome- ry with Cu K α radiation ( λ = 1.5418 Å) and a position sensi- ive detector (PSD) were utilized. A detailed investigation of the rystal structure was carried out by means of high-angular resolu- ion synchrotron x-ray diffraction and powder neutron diffraction. ynchrotron x-ray powder diffraction (SXRPD) experiments were erformed at the BL04-MSPD beamline of ALBA synchrotron and he patterns were collected in high- angular resolution mode (so- alled MAD set-up, Multicrystal Analyser Detectors), selecting an ncident beam with 38 keV radiation ( λ= 0.3251 Å) [15] . The sam- les were contained in 0.7 mm diameter quartz capillaries. Using igh-energy synchrotron x-rays allowed collecting data on such ab- orbing materials in transmission mode. SXRPD data were recorded t 295 (RT), 673, 1073 and 1263 K to study potential phase transi- ions. Neutron powder diffraction (NPD) experiments were accom- lished at room temperature and 3 K in the high-angular resolu- ion D2B instrument ( λ = 1.594 Å) of the Institut Laue- Langevin ILL) in Grenoble (France). The low-temperature data were col- ected to investigate the magnetic structure of the samples. All he structures were refined by the Rietveld method [16] using he Fullprof refinement software [17] . No regions were excluded n the refinement. To generate the line shape of the diffraction eaks, a pseudo-Voigt function was chosen. The following parame- ers were refined in the final analysis: scale factor, zero-point error, ackground coefficients, lattice parameters, positional coordinates, sotropic atomic displacements and magnetic moments. The magnetic properties were analysed in a commercial uantum-Design superconducting quantum interference device SQUID) magnetometer in the range of 5–300 K. Direct cur- ent magnetic susceptibility was measured under zero-field cooled ZFC) and field-cooled (FC) conditions with a magnetic field of .1 T. Isothermal magnetization curves were obtained for magnetic elds going from −5 T to 5 T at 4 and 300 K. The Scanning Electron Microscopy study was carried out in Hitachi instrument, model TM-10 0 0, coupled to an energy- ispersive X-ray spectrometer (EDX), working with an acceleration oltage of 15 kV and 60 s of acquisition time. . Results and discussion Room- temperature crystal structure . The samples were obtained s pure black and well-crystallised powders. The SXRPD and NPD 2 P. Kayser, A. Muñoz, J.L. Martínez et al. Acta Materialia 207 (2021) 116684 Table 1 Unit-cell, positional and thermal parameters for R 2 NiIrO 6 defined in the P2 1 /n (no. 14) space group, from SXRPD-NPD combined refinement at RT. Chemical composition (in weight) for the metal elements from EDX is also included, with the nominal values in parentheses. La 2 NiIrO 6 Pr 2 NiIrO 6 Nd 2 NiIrO 6 a ( ̊A) 5.56657(7) 5.47637(9) 5.44394(11) b ( ̊A) 5.63139(7) 5.67944(10) 5.68510(15) c ( ̊A) 7.89310(12) 7.80462(16) 7.77770(20) β ( °) 90.036(4) 90.022(6) 90.027(8) V ( ̊A 3 ) 247.429(6) 242.745(8) 240.71(1) R 4 e (x y z) x -0.0088(6) -0.0127(6) -0.0142(5) y 0.45560(15) 0.44250(20) 0.43843(20) z 0.2500(3) 0.2501(3) 0.2502(3) B ( ̊A 2 ) 0.67(2) 0.62(4) 0.86(3) Ni 2 d (0 0 ½) Occ (Ni/Ir) 2b 0.962(6)/0.038(6) 0.908(8)/0.092(8) 0.870(8)/0.128(8) B ( ̊A 2 ) 0.481(3) 1.51(9) 1.51(9) Ir 2 a (0 0 0) Occ (Ir/Ni) 2a 0.962(6)/0.038(6) 0.908(8)/0.092(8) 0.870(8)/0.128(8) B ( ̊A 2 ) 0.13(2) 0.13(3) 0.12(4) O1 4 e (x y z) x 0.0834(9) 0.0886(12) 0.0977(10) y 0.0180(8) 0.0286(14) 0.0279(10) z 0.2497(19) 0.248(2) 0.246(2) B ( ̊A 2 ) 0.99(7) 1.88(11) 1.59(9) O2 4 e (x y z) x 0.2061(16) 0.1960(15) 0.1930(17) y 0.2800(13) 0.2856(14) 0.3057(15) z -0.0483(13) -0.0392(11) -0.0382(11) B ( ̊A 2 ) 0.71(16) 0.55(16) 0.63(16) O3 4 e (x y z) x 0.2865(15) 0.292(2) 0.299(2) y 0.8008(13) 0.8034(19) 0.7928(19) z -0.0385(12) -0.0569(15) -0.0584(13) B ( ̊A 2 ) 0.21(15) 1.7(2) 1.4(2) Reliability Factors χ2 2.29 1.66 1.66 Rp (%) 2.87 2.40 2.36 Rwp (%) 3.59 3.02 3.05 Rexp (%) 2.37 2.35 2.37 R I (%) 2.59 2.67 2.54 Chemical analysis (EDX) R 53.2 (52.54) 54.8 (52.90) 54.7 (53.48) Ni 12.0 (11.10) 11.5 (11.02) 12.2 (10.88) Ir 34.8 (36.35) 33.7 (36.08) 33.1 (35.64) p P o t ½ 4 r p d n r e v i s i ( p a a s a t t r e d a p l O t N s r u a d ( atterns were successfully indexed in the monoclinic space group 2 1 /n . The unit-cell parameters are related to a 0 (ideal cubic per- vskite a 0 ≈ 3.8 Å) as a ≈ √ 2 a 0 , b ≈ √ 2 a 0 and c ≈ 2 a 0 , β≈ 90 °. In his model, R atoms are located at 4 e (x y z) positions, Ni at 2 d (0 0 ) positions, Ir at 2 c (0 0 0) and the three types of oxygen atoms at e (x y z) sites. SXRPD patterns displayed sharp and well-defined eflections characteristic of a perovskite-like structure including su- erstructure reflections that confirm the existence of rock-salt or- ering of Ni 2 + and Ir 4 + . Based on the Anderson et al criterion [18] , either the ionic radii difference of the cations, r(Ni 2 + ) = 0.69 ̊A and (Ir 4 + ) = 0.625 Å, nor the difference in the charge is big enough to stablish full 1:1 B-site ordering. However, the difference in co- alency between Ir-O bonds with the less covalent Ni-O bonds, s sufficient to drive a long-range cationic ordering [19] . The re- ults from EDX analysis showed well-defined peaks correspond- ng to R, Ni and Ir; the determined weight % of these elements Table 1 ) is in reasonable agreement with the nominal values (in arentheses). Fig. 1 shows the plots after a combined Rietveld refinement gainst SXRPD and NPD data. High-angular resolution SXRPD data 3 re helpful to unequivocally assign the correct symmetry to the tructure, whereas NPD provides essential information about the tomic coordinates of the anions. For this family of compounds, his combined method is especially appropriate to refine the crys- al structure. On one side, perovskite structures with non- optimal elationship between the size of A- and B- cations undergo a coop- rative tilting of the BO 6 octahedral network, which implies small isplacements of the anions. Thus, NPD is one of the most suit- ble techniques to determine the atomic coordinates and the dis- lacement factors of the oxygen atoms, which is especially chal- enging in compounds containing heavy metals such as iridium. n the other side, the presence of Ni and Ir over the B-sites of he perovskites, with extremely similar neutron scattering lengths, i (1.03 fm) and Ir (1.06 fm), prevents the refinement of the anti- ite disorder in the structure from NPD data, and then, SXRPD is equired. Table 1 summarizes the obtained structural parameters: nit-cell parameters, volume and atomic positions. Bond distances nd selected angles as well as the valence of the cations were etermined with the program Bonds-Str in the Fullprof package Table 2 ). P. Kayser, A. Muñoz, J.L. Martínez et al. Acta Materialia 207 (2021) 116684 Table 2 Main interatomic bond distances ( ̊A) and selected angles (deg) for R 2 NiIrO 6 , de- termined from the combined refinement from SXRPD-NPD data at RT. The bond- valence sums (BVS) values have also been included for Ni and Ir. La 2 NiIrO 6 Pr 2 NiIrO 6 Nd 2 NiIrO 6 R O polyhedra R-O1 2.518(5) 2.458(8) 2.422(6) R-O1 2.421(6) 2.420(8) 2.419(6) R-O2 2.827(10) 2.702(9) 2.722(9) R-O2 2.446(9) 2.485(9) 2.422(9) R-O2 2.622(9) 2.669(9) 2.644(9) R-O3 2.704(9) 2.596(12) 2.564(11) R-O3 2.449(9) 2.363(12) 2.383(11) R-O3 2.740(9) 2.815(12) 2.815(11) < R -O > 8 short 2.591(9) 2.560(9) 2.548(8) NiO 6 polyhedra Ni-O1 (x2) 2.034(15) 2.037(15) 2.059(15) Ni-O2 (x2) 2.102(9) 2.135(8) 2.087(9) Ni-O3 (x2) 2.099(8) 2.136(11) 2.069(11) < Ni-O > 2.08(1) 2.10(3) 2.07(1) � 10 4 2.28 4.90 0.32 IrO 6 polyhedra Ir-O1 (x2) 2.028(15) 2.006(15) 1.998(15) Ir-O2 (x2) 1.995(8) 1.991(8) 2.077(9) Ir-O3 (x2) 1.989(8) 2.048(11) 2.118(11) < Ir-O > 2.00(1) 2.01 (3) 2.06(1) � 10 4 2.02 1.43 5.88 Ni-O1-Ir 152.6(6) 149.7(6) 146.9(6) φ 13.7 15.1 16.5 Ni-O2-Ir 152.8(3) 153.5(3) 149.8(4) φ 13.6 13.2 15.1 Ni-O3-Ir 153.9(3) 147.4(4) 147.5(4) φ 13.0 16.3 16.3 O1-Ni-O2 88.7(7) 89.1(8) 87.2(8) O1-Ni-O3 88.2(6) 90.0(9) 90.0(8) O2-Ni-O3 89.9(5) 89.0(6) 86.1(7) O1-Ir-O2 89.3(7) 86.7(8) 85.6(8) O1-Ir-O3 89.3(7) 87.1(8) 88.5(8) O2-Ir-O3 86.8(5) 87.6(7) 89.4(7) BVS Ni 1.91(2) 1.80(2) 1.94(2) Ir 4.20(5) 4.06(5) 3.60(5) c s d A f g f c f s 7 s h p u c H s s v s i c c Fig. 2. Variation of the unit-cell parameters and volume (inset) for R 2 NiIrO 6 per- ovskites (R = La, Pr and Nd) with the ionic radius of R 3 + . t R o a f s b e d a d v 2 - s a r u V R c I i f s s w S w t s t s a p t w 1 M During the refinement, the occupancies of Ni and Ir sites were onstrained as occ(Ni) + occ(Ir) = 1. At room temperature, all the amples show certain degree of disorder over the B-site. Anti-site isorder occurs if some Ni is located at Ir positions, and vice versa. s long as the tolerance factor, defined as t = ( r A + r O ) √ 2 ( r B + r O ) deviates rom the ideal value of 1, the percentage of anti-site degree under- oes an increment: 4% (La), 9% (Pr) and 13% (Nd). The occupancy actors of oxygen atoms have also been refined, and the results onfirm a full oxygen stoichiometry for all the samples. The present unit-cell parameters are in good agreement with ormerly reported values. La 2 NiIrO 6 was initially described using a implified pseudocubic perovskite structure of unit-cell parameter .918 Å [10] but more recent works on powder (R = La [11] ) and ingle crystals (R = Pr and Nd [12] ) reported monoclinic symmetry, ence concurring with our results. The variation of the unit-cell arameters and volume are represented in Fig. 2 . An increment of nit-cell volume with the ionic size is shown in the inset of Fig. 2 , orresponding to a volume decrease along the lanthanides series. owever, while the a - and c - axes exhibit a decrease along the eries, the corresponding b -axis slightly elongates and then starts hortening from a critical R 3 + size. [8] In Lu 2 NiIrO 6 , the reported b alue is 5.63533(2) Å [8] , which is virtually equal to the corre- ponding one for R = La. This effect has been previously observed n similar perovskites, such as RVO 3 [20] or R 2 NiRuO 6 [21] . It is as- ribed to the tilting system adopted by this family of compounds, orresponding to the a −a −b + in the Glazer’s notation, [22] where 4 he structural distortion driven by the reduction of the size of the 3 + causes minimal modifications of the b value. In addition, it is bserved that the value c / √ 2 lies between a and b, which is char- cteristic of the so-called O structure, and it represents a common eature in the perovskites where the primary distorting effect is teric. The local coordination of the three different cations has also een investigated. The A-site of the perovskite, occupied by rare arths (La 3 + , Pr 3 + , Nd 3 + ), is highly distorted and the effective coor- ination environment is reduced to a RO 8 polyhedron, rather than RO 12 oxygen coordination. In the octahedral sites, the average istances < Ni-O > and < Ir-O > compare well with the calculated alues obtained from the sums of Shannon [23] effective radii: of .09 Å for VI Ni 2 + (0.690 Å)- VI O 2- and 2.025 Å for VI Ir 4 + (0.625 Å) VI O 2 . It is observed that the average bond distances scale with the ize or R 3 + , while the Ni-O and Ir-O bond lengths remain constant long the series and thus, the drop of the unit-cell volume is di- ectly related to the contraction of the RO 8 polyhedra. The valences of the cations and anions, S ij ̧ were estimated sing Sij = exp[(R 0 −R ij )/B], corresponding to the Brown’s Bond- alence Model (BVS), and the following parameters: B = 0.37, 0 (Ni 2 + ) = 1.654, and R 0 (Ir 4 + ) = 1.87, were employed for the cal- ulation. [24] In all of the samples, the oxidation state of Ni and r are reasonably close to 2 + and 4 + , respectively ( Table 2 ). It is nteresting the trend of the valence of Ir to progressively decrease rom La to Nd perovskites, which can be understood as the ba- icity of the rare-earth ion decreases and, therefore, the ability to tabilize a high oxidation state of this transition metal ion is also eaker. Variable- temperature crystal structure. Fig. 3 shows a portion of XRPD patterns acquired at 673, 1073 and 1263 K (RT patterns ere included for the sake of comparison) for R = La and Pr, aiming o study the thermal evolution of the crystal structure. A first vi- ual analysis of the data suggested that there is a structural phase ransition towards higher symmetries, such as tetragonal or cubic, ince the peak splitting seems to vanish upon heating. However, careful examination of the patterns revealed that, in this tem- erature range, all the reflections still remain. In order to identify he correct tilting system and space group, some of the reflections ere indexed in the double-edge cubic cell (a = 7.9649(1) Å), at 260 K. Subsequently, they were assigned to the corresponding R-, - and X-point modes [25] , which are associated with the rock- P. Kayser, A. Muñoz, J.L. Martínez et al. Acta Materialia 207 (2021) 116684 Fig. 3. Portions of SXRPD for R 2 NiIrO 6 perovskites (R = La and Pr) at 298, 673, 1073 and 1263 K (top) and the SXRPD pattern for R = La highlighting superlattice reflections associated with cooperative tilting of the octahedra (bottom). s o i X a s p c R o m d b 1 a d p s s F t c c i u s i v o alt ordering or/and out-of-phase octahedral tilting, the in-phase ctahedral tilting and to the coupling of the out-of-phase and the n-phase octahedral tilting, respectively ( Fig. 3 ). The relevant point in this analysis is the presence of M- and -points. The prevalence of these diffraction peaks at high temper- ture suggests the same a-a-c + tilt system corresponding to the pace group P2 1 /n [ 22 , 25 ]. It is worth commenting that the coupling of the in- and out- hase octahedral tilting provokes a displacement of the A-site ations, leading to a reduction of the coordination environment of 3 + cations, from RO 12 to RO 8 , as observed in the bond- distances btained from the refinements ( Table 2 ). As shown in Fig. 3 , the symmetry of the Pr 2 NiIrO 6 structure re- ains virtually unchanged with the temperature: only a small re- uction of the peak splitting is observed. While La 2 NiIrO 6 seems to e about undergoing a phase transition at temperatures just above 263 K, in the Pr 2 NiIrO 6 compound no signs of phase transitions 5 re observed. The smaller ionic radii of Pr 3 + gives rise to a more istorted structure at room temperature and requires a higher tem- erature range to evolve into a higher symmetry structure. Based on the discussion above, all the variable-temperature tructures were refined using the room temperature model in the pace group P2 1 /n . The goodness of the refinements is shown in igure S1. Tables S1 and S2 summarise the refined parameters. The thermal evolution of the lattice parameters shows the same ype of behaviour than those previously observed with the R 3 + ation size. a and c unit-cell parameters evolve as expected, in- reasing while the temperature rises; however, the variation of b s negligible, with a very slight decrement. Overall, the unit cell ndergoes an expansion with the temperature, as it can be ob- erved in the evolution of the volume (inset of Fig. 4 ), which s strongly anisotropic. Another common feature is the observed alue of c / √ 2, which lies between a and b along the whole range f temperatures. P. Kayser, A. Muñoz, J.L. Martínez et al. Acta Materialia 207 (2021) 116684 Fig. 4. Thermal evolution of the unit-cell parameters and volume (inset) for R 2 NiIrO 6 perovskites (R = La and Pr). The error bars are smaller than the size of the symbols. Fig. 5. Temperature dependence of the dc magnetic susceptibility for the R 2 NiIrO 6 (R = La, Pr, Nd) perovskites, measured under a 0.1 T magnetic field. The inset shows the reciprocal ZFC susceptibility versus temperature. 6 P. Kayser, A. Muñoz, J.L. Martínez et al. Acta Materialia 207 (2021) 116684 Fig. 6. Magnetization versus magnetic field isotherms of R 2 NiIrO 6 (R = La, Pr, Nd) at 300 K and 4 K measured under a magnetic field ranging from −5 to 5 T. i t 7 Z m e i t a h i a o o c W t t m c I μ a 4 v c t t s r t f n P c t R A d t B a h i p a O b d r g Magnetic properties. dc-susceptibility curves ( Fig. 5 ), measured n both ZFC (zero-field cooled) and FC (field cooled) modes, suggest he existence of antiferromagnetic order at the Néel temperature of 5 K (La), 103 K (Pr) and 121 K (Nd). The divergence between the FC and FC curves is ascribed to magnetic irreversibilities, due to agnetic frustration or canting of the spins that allows the pres- nce of weak ferromagnetic components of the magnetic moment n an antiferromagnetic matrix. [10] Another feature observed in he susceptibility curves is the presence of the so-called param- gnetic tail at 30 K in the Pr 2 NiIrO 6 and Nd 2 NiIrO 6 perovskites. It as been previously observed in other Ir-based perovskites [3] , and t is attributed to imperfections in the long-range ordering of the ntiferromagnetic material, which may leave some isolated areas f non-ordered atoms that follow the usual thermal dependence f paramagnetic substances. Above the magnetic ordering temperature, the reciprocal sus- eptibility follows the Curie- Weiss law χ = C/( T − θW ). The eiss temperature and the effective magnetic moments were ob- ained from the fitting. The latter parameter was estimated from he Curie constant as 2.84 √ C. For R = La, Pr and Nd, the deter- ined values are 3.85, 6.22 and 6.33 μB /f.u. and the theoreti- al ones, considering spin-only magnetic moments for Ni 2 + and r 4 + ions, are 3.30, 5.96 and 6.10 μB , respectively, estimated as eff= (2μ(R 3 + ) 2 + μ(Ni 2 + ) 2 + μ(Ir 4 + ) 2 ) 1/2 . The J-values for the Pr 3 + nd Nd 3 + cations have been used in this calculation ( 3 H 4 for Pr 3 + ; I 9/2 for Nd 3 + ; yielding 3.58 and 3.62 μB , respectively). The higher alue of the effective magnetic moments compared to the theoreti- 7 al ones is ascribed to a small orbital contribution of Ni 2 + and Ir 4 + o the magnetic moment. All the samples display negative Weiss emperature ( θW = -51 K, -36 K and -37 K for La, Pr and Nd, re- pectively), confirming that the dominant interactions are antifer- omagnetic in nature. From those values, we can estimate the frus- ration index, defined as f frus = | θW |/T N [26] . Since geometrically rustrated materials exhibit f frus > 10 we can affirm that these are on-frustrated materials, with f frus = 0.68, 0.34 and 0.30 for R = La, r and Nd, respectively. It is worth commenting the unusual behaviour observed con- erning the evolution of the antiferromagnetic (AFM) ordering emperature, T N , which evolves from 75 K for R = La to 121 K for = Nd. Normally, an opposite trend is observed in many series of BO 3 perovskites containing rare-earths at the A position. The re- uction in size of R 3 + ions, according to the lanthanide contrac- ion, implies a decrease of the tolerance factor, involving narrower -O-B angles and weaker superexchange AFM interactions, which re mediated through the B-O-B chemical bonds. The observed be- aviour in the present series may be a consequence of compet- tive ferromagnetic and antiferromagnetic interactions. This com- lex magnetic behaviour is intermediate between an ideal collinear rrangement Ir-O-Ni of 180 ° (ferromagnetic), or an arrangement Ir- -O-Ir and Ni-O-O-Ni of 90 ° (antiferromagnetic). Since the Ir-O-Ni ond angle is ~ 150 °, the antiparallel coupling is presumed to be ominant over ferromagnetic one.[ 9 , 27 , 28 ] In this scenario, as the are-earth ion radius decreases from La 3 + to Nd 3 + , the Ir-O-Ni an- le deviates more from the ideal value (180 °) and therefore, the P. Kayser, A. Muñoz, J.L. Martínez et al. Acta Materialia 207 (2021) 116684 Fig. 7. Observed (red circles), calculated (solid black line), and difference (blue line) NPD Rietveld profiles for R 2 NiIrO 6 at 3 K (La and Pr) and 5 K (Nd). The first series of Bragg reflections corresponds to the main perovskite phase and the second one to the magnetic structure. p a t 5 l m a m a B a c a p t t t t s t c t d ( i i e T s s e v n m o l c c r m r p t i h c m a s s w d c t L t i r t r e 8 ossible ferromagnetic interactions are weakening. This results in predominance of the antiferromagnetic interactions, which leads o an increase in the temperature of the magnetic order ( T N ). M/H isotherm curves were measured at 300 K and 4 K between T and -5 T, as shown in Fig. 6 . The R = La compound shows a inear behaviour at both temperatures, confirming that antiferro- agnetic interactions are predominant. For R = Pr and Nd, there is noticeable hysteresis loop at 4 K with a non- saturated value of agnetization of 2.36 and 0.62 μB /f.u. Magnetic structure. In the neutron diffraction patterns acquired t 3 K and 5 K for La 2 NiIrO 6 and Nd 2 NiIrO 6 , respectively, new ragg reflections forbidden by the monoclinic space group P2 1 /n re observed, in particular the reflections (1,0,0) and (0,1,0). In the ase of Pr 2 NiIrO 6 , a notable increase of the intensity is observed t 3 K for the (0,1,1) and (1,0,1) reflections. For the three com- ounds, all these Bragg reflections are magnetic in origin, implying hat the magnetic order is characterized by the propagation vec- or k = (0,0,0). The possible magnetic structures compatible with he space group P2 1 /n have been obtained by using the representa- ion analysis technique described by Bertaut [29] . In this case, the olutions have been determined by using one of the programs of he Fullprof suite package [17] . For k = (0,0,0) the small group G k oincides with the space group P2 1 /n whose irreducible represen- ations are given in Table S3. The R (La, Pr, Nd) atoms occupy the 4 e site and they are enoted as 1 (x,y,z), 2 (-x + 1/2,y + 1/2,-z + 1/2), 3 (-x,-y,-z) and 4 x + 1/2,-y + 1/2,z + 1/2). The notation for the Ni atoms at the 2 b site s 5 (0,0,1/2) and 6 (1/2,1/2,0) and for the Ir atoms at the 2 a site s 7 (0,0,0) and 8 (1/2,1/2,1/2). The basis vectors corresponding to ach irreducible representation are shown in Table 3 . According to able 3 , only the irreducible representations 1 and 3 present ba- is vectors for all the sites. In principle, the magnetic transition ob- erved at T N for all the compounds is a second order one, so it is xpected that all the magnetic atoms order according to the basis ectors belonging to the same irreducible representations. For Nd 2 NiIrO 6 , after checking the different solutions, the mag- etic structure that shows the best agreement with the experi- ental NPD data acquired at 5 K, corresponds to the basis vectors f the irreducible representations 1 . The refined parameters are isted in Table 4 . The experimental and calculated NPD patterns are ompared in Fig. 7 . For the Nd atoms, the coupling along the a and axis is antiferromagnetic, whereas the magnetic moments are fer- omagnetically coupled along the b -axis. The Ni atoms are ferro- agnetically coupled along the b -axis and the Ir atoms are antifer- omagnetically coupled along the c -axis. This scenario implies the resence of both antiferromagnetic and ferromagnetic exchange in- eractions, what is in good agreement with the results observed n the magnetic measurements, since the susceptibility curves ex- ibit irreversibilities between FC and ZFC curves, characteristic of ompetitive interactions, with a negative Weiss temperature and a agnetic hysteresis observed in the magnetization curve measured t 5 K. A plot of the magnetic structure obtained by the VESTA oftware [30] is displayed in Fig. 8 . Let us point out that in the usceptibility curve of Fig. 5 , an anomaly is observed around 50 K, hat would be associated with the appearance of the magnetic or- ering of the Nd moments. The Nd atoms exhibit a ferromagnetic omponent along the b -axis, what would explain the increase of he susceptibility at low temperatures. Regarding the magnetic structure of the Pr 2 NiIrO 6 and a 2 NiIrO 6 perovskites, in both cases the best agreement is also ob- ained for the magnetic structure given by the basis vectors of the rreducible representation 1 . In both compounds, the magnetic eflections are weak and the fitting factor R Mag is high. A view of he magnetic structures is given in Fig. 8 b and c. According to the esults displayed in Table 4 for Pr 2 NiIrO 6 , both Pr and Ir atoms only xhibit antiferromagnetic interactions. However, Ni atoms are fer- P. Kayser, A. Muñoz, J.L. Martínez et al. Acta Materialia 207 (2021) 116684 Table 3 Basis vectors associated with each of the irreducible representations R(Nd, Pr) Ni Ir 1 2 3 4 5 6 7 8 1 [ 100 ] [ 010 ] [ 001 ] [ ̄1 00 ] [ 010 ] [ 00 ̄1 ] [ 100 ] [ 010 ] [ 001 ] [ ̄1 00 ] [ 010 ] [ 00 ̄1 ] [ 100 ] [ 010 ] [ 001 ] [ ̄1 00 ] [ 010 ] [ 00 ̄1 ] [ 100 ] [ 010 ] [ 001 ] [ ̄1 00 ] [ 010 ] [ 00 ̄1 ] 2 [ 100 ] [ 010 ] [ 001 ] [ ̄1 00 ] [ 010 ] [ 00 ̄1 ] [ ̄1 00 ] [ 0 ̄1 0 ] [ 00 ̄1 ] [ 100 ] [ 0 ̄1 0 ] [ 001 ] 3 [ 100 ] [ 010 ] [ 001 ] [ 100 ] [ 0 ̄1 0 ] [ 001 ] [ 100 ] [ 010 ] [ 001 ] [ 100 ] [ 0 ̄1 0 ] [ 001 ] [ 100 ] [ 010 ] [ 001 ] [ 100 ] [ 0 ̄1 0 ] [ 001 ] [ 100 ] [ 010 ] [ 001 ] [ 100 ] [ 0 ̄1 0 ] [ 001 ] 4 [ 100 ] [ 010 ] [ 001 ] [ 100 ] [ 0 ̄1 0 ] [ 001 ] [ ̄1 00 ] [ 0 ̄1 0 ] [ 00 ̄1 ] [ ̄1 00 ] [ 010 ] [ 00 ̄1 ] Table 4 Results of the fitting of the magnetic structures from the NPD patterns acquired for Nd 2 NiIrO 6 (T = 5 K), Pr 2 NiIrO 6 (T = 3 K) and La 2 NiIrO 6 (T = 3 K). Atom Nd 2 NiIrO 6 Pr 2 NiIrO 6 La 2 NiIrO 6 R m x , m y , m z (μB ) 0.98(10), 0.4(2), 0.38(14) 0.93(13), 0, 0 - | m | (μB ) 1.1(3) 0.93(13) - Ni m x , m y , m z (μB ) 0, 1.4(2), 0 0, 2.30(12), 0 1.25(12), 0, 0.8(4) | m | (μB ) 1.4(2) 2.30(12) 1.5(4) Ir m x , m y , m z (μB ) 0, 0, 1.6(2) 1.3(2), 0, 1.6(2) 0.29(13), 0, -0.6(5) | m | (μB ) 1.6(2) 2.1(6) 0.7(5) R Bragg (%) 9.8 11.3 2.7 R Mag (%) 16.0 23.1 33.3 χ 2 2.8 1.6 3.4 Fig. 8. Schematic view of the magnetic structures of (a) Nd 2 NiIrO 6 , (b) Pr 2 NiIrO 6 and (c) La 2 NiIrO 6 below T N . r n h w t K ( b L T r o o w o s b s s a o a i c a t I m v m t o a 4 f l d c c s m c t t t o c d omagnetically coupled along the b -axis. For La 2 NiIrO 6 , the mag- etic moments are oriented in the ac plane, and both atoms ex- ibit an antiferromagnetic structure, what is in good agreement ith the magnetic measurements, as in this case a magnetic hys- eresis is not observed in the magnetization curve measured at 5 . The fact that Ir moment is much smaller in the La compound 0.7(5) μB vs 1.6(2) and 2.1(6) μB for Nd and Pr, respectively) can e explained by a higher covalent character of the Ir-O bonds in a 2 NiIrO 6 , induced by the more basic character of La vs Nd and Pr. hat means a higher delocalization of the valence electrons and a eduction of the magnetic moment. It is noteworthy mentioning the difference in the arrangement f the magnetic moments for Ni and Ir in the three members f the R 2 NiIrO 6 family. The rotation of the magnetic moments ith respect to the intrinsic magnetic structure of this double per- vskites is triggered by the difference in their magnetic anisotropy, trongly influenced by the nature of the rare earth. This feature has een previously reported in similar families of doubles perovskites uch as R 2 NiMnO 6 [31] and R 2 NiRuO 6 [21] . As it is observed, the pin structure of La 2 NiIrO 6 (La 3 + is paramagnetic), is considered s the intrinsic magnetic structure for these series of double per- vskites. Here, the magnetic moments are contained within the c plane, and both Ni and Ir magnetic sublattices experience an ndependent AFM ordering compatible with k = 0, with a non- ollinear arrangement between Ni and Ir moments. For Pr 2 NiIrO 6 nd Nd 2 NiIrO 6 the Ni spins are ferromagnetically coupled, parallel 9 o the b direction, while Ir moments are still within the ac plane. n particular, the Nd perovskite exhibits a collinear AFM arrange- ent of Ir moments along the c axis. The knowledge of such a ariety of microscopic arrangements is essential to understand the agnetic properties described in Figs. 5 and 6 , including the hys- eresis cycles observed for Pr and Nd, arising from the FM coupling f Ni spins, and the presence of a strong divergence between ZFC nd FC curves. . Conclusions In summary, we have prepared three member of the R 2 NiIrO 6 amily (R = La, Pr and Nd); the samples are pure and well crystal- ized. The outcome of the refinement, combining SXRPD and NPD ata, results in B-cation ordered double perovskites with small per- entage of antisite of the Ni 2 + and Ir 5 + cations that slightly in- reases along the series. The evolution of the unit-cell parameters cales with the size of the rare-earth ion, as well as their ther- al evolution in the 300- 1263 K temperature range. The mono- linic symmetry and the P2 1 /n space group persists in the men- ioned temperature interval. This has been confirmed by analysing he superlattice reflections. Magnetization measurements confirm he existence of antiferromagnetic order at the Néel temperature f 75 K (La), 103 K (Pr) and 121 K (Nd) and the uncommon in- rease of the magnetic ordering temperature when the size of R 3 + ecreases is ascribed to the competition of ferromagnetic and an- P. Kayser, A. Muñoz, J.L. Martínez et al. Acta Materialia 207 (2021) 116684 t t a m a D c i A o E s a S f R [ [ [ [ [ [ [ [ [ [ [ [ [ iferromagnetic interactions. While the magnetic structures of the hree samples are defined by the same propagation vector, k = 0, substantial difference in the arrangement of the magnetic mo- ents for Ni and Ir is observed, which is explained in terms of the nisotropy of the rare-earth ion. eclaration of Competing Interest The authors declare that they have no known competing finan- ial interests or personal relationships that could have appeared to nfluence the work reported in this paper. cknowledgements We thank the financial support of the Spanish Ministry of Econ- my and Competitiveness to the project MAT2017-84496-R and of PSRC . 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http://refhub.elsevier.com/S1359-6454(21)00064-1/sbref0031 http://refhub.elsevier.com/S1359-6454(21)00064-1/sbref0031 http://refhub.elsevier.com/S1359-6454(21)00064-1/sbref0031 Enhancing the Néel temperature in 3d/5d R2NiIrO6 (RLa, Pr and Nd) double perovskites by reducing the R3+ ionic radii 1 Introduction 2 Experimental section 3 Results and discussion 4 Conclusions Declaration of Competing Interest Acknowledgements Supplementary materials References