Structural and magnetic characterization of the double perovskites R2NiRuO6 (R = Pr-Er): A neutron diffraction study

Acta Materialia 126 (2017) 114e123

Contents lists available at ScienceDirect

Acta Materialia journal homepage: www.elsevier.com/locate/actamat

Full length article

Structural and magnetic characterization of the double perovskites R2NiRuO6 (R ¼ Pr-Er): A neutron diffraction study ~ oz c, M.T. Ferna ndez-Díaz d P. Kayser a, *, J.A. Alonso b, A. Mun a

School of Chemistry, The University of Sydney, Sydney, New South Wales, 2006, Australia Instituto de Ciencia de Materiales de Madrid, C.S.I.C., Cantoblanco, E-28049, Madrid, Spain c Dpto. de Física Aplicada, EPS, Universidad Carlos III, Avda. Universidad 30, 28911, Legan es, Madrid, Spain d Institut Laue Langevin, BP 156X, Grenoble, F-38042, France b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 August 2016 Received in revised form 11 December 2016 Accepted 12 December 2016

Novel members of the R2NiRuO6 family of double perovskites have been prepared via a nitrate-citrate route, followed by annealing treatments in air in the 1100-1300  C temperature range. The crystal and magnetic structures were characterized from neutron powder diffraction (NPD) data at 300 K and 2 K. All the samples are monoclinic, P21/n space group, with a √2a0√2a02a0 unit-cell, compared to the simple perovskite a0 edge. Ni2þ and Ru4þ ions occupy distinct octahedral positions, with a certain antisite disordering. The magnetic structures are defined by the propagation vector k ¼ 0. The magnetic moments of the Ni2þ cations are antiparallel to the spins of the Ru4þ sublattice; the structure can be described as ferromagnetic [011] layers antiferromagnetically (AFM) coupled to each other. For R ¼ Ho and Er perovskites, the rare-earth moments participate in the magnetic structure: Ho3þ cations are ordered along the direction (010) in alternate planes AFM coupled and the magnetic moments of Er3þ are ferromagnetically ordered along the c axis. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Ruthenium Double perovskite 4d transition metal Neutron powder diffraction La2NiRuO6 Pr2NiRuO6 Nd2NiRuO6 Tb2NiRuO6 Dy2NiRuO6 Y2NiRuO6 Ho2NiRuO6 Er2NiRuO6

1. Introduction In recent years, double perovskites of stoichiometry A2BB’O6 have been widely studied due to their outstanding physical properties as half-metals with colossal magnetoresistance (Sr2FeMoO6, Sr2FeReO6) [1] or magnetic order far above room temperature (Sr2CrOsO6, Sr2CrReO6) [2,3]. Especially, oxides containing ruthenium ions unveiled interesting electronic and magnetic properties that could be used in technological applications. The initial interest on this family of compounds arose from the discovery of unconventional superconductivity in the Sr2RuO4 layered perovskite [4]. Lately, many other compounds containing ruthenium were found extremely attractive, for instance, the solid solutions Sr2-xCaxRuO4 [5] combining the properties of the Ca2RuO4 oxide, a Mott insulator,

* Corresponding author. E-mail address: [email protected] (P. Kayser). http://dx.doi.org/10.1016/j.actamat.2016.12.024 1359-6454/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

with the superconductivity exhibited in Sr2RuO4 is a representative example. These ruthenium-based layered perovskites and the corresponding tridimensional compounds provide an excellent opportunity to study the relationship between superconductivity and magnetism. ARuO3 perovskites show also intriguing properties and may contribute to a better understanding of the magnetic behavior in these systems. The change from high-temperature ferromagnetism in the perovskite SrRuO3 to paramagnetism in the equivalent compound CaRuO3 constitutes an excellent starting point for defining the nature of these phases. Substitution of ruthenium by another transition metal in the ARuO3 simple perovskites results in more complex structures; diverse ruthenium-based perovskites have been recently reported, including doubles perovkites oxides containing Ru and Ti, Cr, Mn, Fe or Ni [6e10] at B-site. Fascinating changes in the magnetic and electrical properties have been observed when Ru is partly replaced. For instance, the Mn doping drives the perovskite physical properties from ferromagnetic and metallic to an insulating-

P. Kayser et al. / Acta Materialia 126 (2017) 114e123

antiferromagnetic behavior [11,12]. Doping with rare earths and Y at the B-site gives rise to the group of compounds with general formula A2LnRuO6 [13]; within this family, the Sr2YRuO6 oxide is particularly appealing, as yttrium cations hamper the Ru-O interactions, leading to a more localized electric and magnetic behavior, which results in an antiferromagnetic and insulating material. La2BRuO6 oxides represent another notable example of ruthenium-based double perovskites. In the following compounds with B¼ Mg, Zn, Co, Ni, Mn and Fe [14], recently reported, interesting results have been found depending on the B cation nature: La2CoRuO6 and La2NiRuO6 are antiferromagnetic semiconductors, while La2MnRuO6 and La2FeRuO6 are ferromagnetic and spin-glass materials, respectively. La2NiRuO6 perovskite has attracted particular attention, thus, its crystal structure [15e21] and magnetic and transport properties [14,22] have been described in a number of publications. Based on the appealing behavior shown by the lanthanum compound, in the present work we report on the synthesis of different novel members of the family R2NiRuO6 (R ¼ Pr, Nd, Tb, Y, Dy, Ho and Er), including a study of the crystallographic and magnetic structures from neutron powder diffraction (NPD) data, complemented with magnetic measurements. In addition, the evolution of the internal structural parameters is discussed in light of the bondvalence model.

115

Er2NiRuO

6

*

Ho2NiRuO

6

*

Y2NiRuO

6

*

Dy2NiRuO

6

Tb2NiRuO

6

2. Experimental section R2NiRuO6 oxides (R ¼ Pr, Nd, Tb, Dy, Y, Ho and Er) were prepared via a nitrate-citrate route. The stoichiometric quantities of analytical grade Pr6O11, Nd2O3, Tb2O3, Dy2O3, Y2O3, Ho2O3, Er2O3, Mn(CO3), and Ni(NO3)2$6H2O were dissolved, under magnetic stirring, in 250 mL of 10% citric-acid aqueous solution containing some drops of HNO3. RuO2 was not dissolved in the solution but remained in suspension. The resulting suspension was stirred and gently heated yielding organic resins that contain a homogeneous distribution of the involved cations. After water evaporation, the resins were dried at 140  C and then heated at 600  C for 12 h (2  C/ min rate) in order to decompose the organic materials and eliminate the nitrates. This treatment gave rise to highly reactive precursor materials that were finally heated in air to obtain pure phases. The different synthesis conditions for all of the samples (temperatures and times) are summarized in Table 1. The reaction products were characterized by x-ray diffraction (XRD) for phase identification and to assess phase purity. The characterization was performed with a Bruker-AXS D8 diffractometer (40 kV, 30 mA), controlled by DRIFFACTPLUS software, in Bragg-Brentano reflection geometry with Cu Ka radiation (l ¼ 1.5418 Å) and a PSD (Position Sensitive Detector). Neutron powder diffractions (NPD) experiments were performed at room temperature in the high-flux D2B instrument of the Institut Laue- Langevin (ILL) in Grenoble (France), in order to

Table 1 Synthesis conditions for R2NiRuO6. R

T, t

Pr Nd Tb Dy Y Ho Er

1100 1100 1100 1100 1300 1300 1300



C C  C  C  C  C  C 

12 h 12 h 12 h þ 1100  C 6 h 12 h þ 1100  C 12 h 2h 2h 2h

Nd2NiRuO

6

Pr2NiRuO

6

20

30

40

2θ (deg)

50

60

Fig. 1. XRD patterns (Cu Ka, l ¼ 1.5406 Å) for R2NiRuO6 (R ¼ Pr-Er) oxides.

study in detail the crystal structures. To determine the magnetic structure of the samples, NPD data were collected at 2 K in the D2B diffractometer for M ¼ Pr, Tb, Dy, Er and Y. Vanadium sample holders were used in all cases; for R ¼ Dy a double-walled sample holder was necessary to minimize the neutron absorption. For R ¼ Ho, low temperature NPD data collected at the D1B diffractometer were analyzed. The program FULLPROF [23] was used to refine the crystal structure by the Rietveld method [24]. A pseudoVoigt function was chosen to generate the line shape of the diffraction peaks. No regions were excluded in the refinement. The following parameters were refined in the final runs: scale factor, background coefficients, zero-point error, pseudo-Voigt corrected for asymmetry parameters, positional coordinates and isotropic displacements. The magnetic properties were studied in a commercial Quantum-Design superconducting quantum interference device (SQUID) magnetometer in the range of 5e300 K. Zero-field cooled (ZFC) and field-cooled (FC) magnetic susceptibility data were measured with a magnetic field of 1000 Oe and the isothermal magnetization curves were obtained for magnetic fields going from 5 T to 5 T at 4 and 300 K.

116

P. Kayser et al. / Acta Materialia 126 (2017) 114e123

3. Results

3.2. Magnetic properties

3.1. Crystal structure

The thermal evolution of the magnetic susceptibility and reciprocal susceptibility of all the R2NiRuO6 perovskites is shown in Fig. 4. In those phases containing paramagnetic earth-rare cations, the magnetic transitions seem to be masked by the rare-earth paramagnetism. However, La2NiRuO6 [14,22] and Y2NiRuO6 oxides, with non-magnetic rare-earth at the A-site, show antiferromagnetic order at 28 and 93 K, respectively, due to the establishment of magnetic interactions between Ni2þ and Ru4þ spins. For R ¼ Nd, Tb, Y, Ho and Er compounds, a divergence between ZFC and FC curves is observed. This behavior is associated with magnetic irreversibilities due to magnetic frustration or to a certain level of magnetic disorder. In addition, this feature may be related to magnetic interactions between R3þ and Ni/Ru cations. The inverse of the susceptibility presents a linear behavior at high temperature for all of the compounds. A fit to a CurieeWeiss law gives negative values of the Weiss temperatures confirming that the dominating magnetic interactions are antiferromagnetic. Table 4 summarizes the magnetic parameters. The Weiss temperature for R ¼ Y is extremely high compared to the other members of the R2NiRuO6 family, probably due to the greater degree of cationic disorder over the B site in this compound. It may give rise to the formation of Ni or Ru clusters and thus, the establishment of shortrange magnetic interactions within them. The effective magnetic moments, estimated from the fit, agree with the calculated values considering the following paramagnetic moments: 2.83 mB (Ni2þ), 2.83 mB (Ru4þ), 3.5 mB (Pr3þ), 3.5 mB (Nd3þ), 9.5 mB (Tb3þ), 10.6 mB (Dy3þ), 10.4 mB (Ho3þ), and 9.5 mB (Er3þ). Fig. 5 illustrates the magnetization isotherms for R ¼ Pr, Nd, Dy, Ho, Y and Er at 5 K. For those compounds with non-magnetic or weak paramagnetic cations over the A-site, the M vs. H curves show an almost linear behavior characteristic of antiferromagnetic materials. In the perovskites with strongly magnetic rare earths (Dy, Er, Ho), the magnetization progressively increases with the field up to 5 T due to the polarization of the rare-earth moments; the obtained saturation magnetizations values are 6.5, 5.0 and 9.0 mB/f.u. respectively. Fig. S3 displays individually the M vs. H curves for each compound.

R2NiRuO6 (R ¼ Pr, Nd, Tb, Dy, Y, Ho and Er) oxides were obtained as well crystallized powder compounds. The XRD patterns, shown in Fig. 1, are characteristic of a perovskite-type structure, exhibiting superstructure peaks corresponding to the Ni2þ/Ru4þ ordering (Fig. S1 Supplementary Information). Test refinements in the orthorhombic Pbnm space group, defining a disordered structure where Ni and Ru ions are distributed at random, do not explain the mentioned superstructure reflections. In all cases the monoclinic P21/n space group, considering the long-range ordering of both octahedral cations, satisfactorily explains the extra peaks (Fig. S1). As the rare-earth radius decreases, the synthesis conditions require higher temperatures to avoid the formation of pyrochlore phases R2Ru2O7. However, the temperature increment entails the formation of trace amounts of oxides R2O3 (marked with * in the diagrams of R ¼ Y, Ho and Er). The analysis of NPD data, collected at RT in the D2B instrument (l ¼ 1.594 Å) (ILL-Grenoble), additionally reaffirmed that R2NiRuO6 (R ¼ Pr, Nd, Tb, Dy, Y, Ho and Er) perovskites crystallize in a monoclinic cell with P21/n space group. The cell parameters are related to a0 (ideal cubic perovskite a0z 3.8 Å) as az √2a0, b z √2a0 and c z 2a0, b z 90 . In this model, R atoms are located at 4e (x y z) positions, Ni at 2d (½ 0 0) positions, Ru at 2c (½ 0 ½) and the three types of oxygen atoms at 4e (x y z) sites. The Rietveld refinement of the crystal structure resulted in a satisfactory agreement between the observed and calculated profiles. A representative NPD diagram corresponding to Dy2NiRuO6 is displayed in Fig. 2. The NPD patterns of the remaining samples are included in Fig. S2. In a further step, the refinement of the level of anti-site disorder between Ni and Ru cations at both B and B0 sites indicates the presence of certain percentage of Ni cations at Ru sites and vice versa. The occupancy factors of oxygen atoms have been also refined, and the results confirm a full oxygen stoichiometry for the all the samples. Table 2 summarizes the most important atomic parameters after the structural refinements, and Table 3 includes the interatomic distances and some selected bond angles for all of the compounds. Fig. 3 shows a view of the R2NiRuO6 doubleperovskite crystal structures, highlighting the long-range ordering of Ni2þ and Ru4þ ions and the tilting of the large NiO6 and small RuO6 octahedra.

Fig. 2. Observed (crosses), calculated (solid line) and difference (bottom) NPD Rietveld profiles for Dy2NiRuO6 at RT, collected at the high flux D2B-ILL diffractometer.

3.3. Magnetic structure The magnetic structures of the R2NiRuO6 compounds (R ¼ Ho, Er and Y) have been determined from neutron powder diffraction (NPD) patterns acquired below the ordering temperature. For Ho2NiRuO6, the NPD pattern was obtained at 2 K using the D1B diffractometer whereas for Er2NiRuO6 and Y2NiRuO6 the NPD patterns were obtained at 5 K on D2B. In all of the patterns it can be observed (Fig. S4) the presence of additional peaks at Q1 ¼ 1.38 and 1.45 Å1 (32 and 34 for R ¼ Ho or 20 and 21 for Y and Er) corresponding to the (011) and (101) reflections, respectively, which indicates that the magnetic moments of the Ni and Ru atoms are rather aligned within the plane ab. For Ho2NiRuO6 and Er2NiRuO6, the rare-earths also contribute to the magnetic structures. In the former compound, the ordering of the Ho3þ magnetic moments was followed in situ by NPD. Fig. S4a presents the thermal evolution of the NPD patterns in the temperature range 2e110 K. At 10 K the intensity of reflection at 2Ɵ ¼ 27 (marked with a red arrow) regularly increases when the temperature decreases, corresponding to the antiferromagnetic coupling of the Ho3þ moments. The Er3þ magnetic moments align ferromagnetically, contributing to the scattering of the (110), (002) and (111) reflections (red arrows). For all the compounds, the analysis of the position of the magnetic reflections indicates that the corresponding magnetic

P. Kayser et al. / Acta Materialia 126 (2017) 114e123

117

Table 2 Unit-cell, positional and thermal parameters for R2NiRuO6 defined in the P21/n (no. 14) space group, from NPD data at RT.

a(Å) b (Å) c (Å) V (Å3) b (deg)

Pr

Nd

Tb

Dy

Y

Ho

Er

5.4788(2) 5.6401(2) 7.7812(2) 240.45(1) 90.02(1)

5.4343(2) 5.6750(3) 7.7345(3) 238.53(1) 89.99(2)

5.2967(1) 5.7020(1) 7.5854(2) 229.092(8) 89.928(5)

5.2762(1) 5.6911(1) 7.5655(1) 227.173(7) 89.917(4)

5.25532(8) 5.68687(8) 7.54259(10) 225.420(5) 90.091(2)

5.25558(8) 5.68781(8) 7.54668(10) 225.590(6) 89.916(3)

5.23994(10) 5.63205(10) 7.5336(13) 223.900(7) 89.883(3)

0.9873(8) 0.0551(4) 0.252(2) 0.70(5)

0.9864(6) 0.0599(3) 0.2546(14) 0.65(1)

0.9778(4) 0.0737(2) 0.2478(10) 0.626(6)

0.9785(2) 0.0756(1) 0.2465(5) 0.04(2)

0.1176(3) 0.4573(3) 0.2468(6) 0.854(3)

0.9753(2) 0.0768(2) 0.2498(6) 0.63(2)

0.9760(3) 0.0777(2) 0.2475(6) 0.702(5)

0.12(9) 0.780(1)/ 0.220(1)

0.48(4) 0.784(8)/ 0.216(8)

0.96(2) 0.860(7) /0.140(7)

0.1(2) 0.69(2)/ 0.31(2)

0.93(3) 0.400(5)/ 0.600(2)

0.43(9) 0.772(7)/ 0.228(7)

0.18(5) 0.616(10)/ 0.384(10)

0.80(12) 0.220(1)/ 0.780(1)

0.56(4) 0.216(8)/ 0.784(8)

0.17(3) 0.140(7)/ 0.860(7)

0.4(3) 0.31(2)/ 0.69(2)

0.27(3) 0.600(5)/ 0.400(5)

0.92(11) 0.228(7)/ 0.772(7)

0.85(3) 0.384(10)/ 0.616(10)

0.0918(4) 0.4762(4) 0.2529(11) 0.65(4)

0.0932(6) 0.4761(6) 0.253(2) 0.73(1)

0.1123(4) 0.4616(4) 0.2549(10) 0.769(8)

0.1141(5) 0.4584(5) 0.2527(12) 0.20(5)

0.1175(3) 0.4573(3) 0.2468(6) 0.661(6)

0.1176(3) 0.4580(3) 0.2534(7) 0.80(3)

0.1188(3) 0.4566(3) 0.2547(6) 0634(7)

0.6990(13) 0.3053(8) 0.0428(8) 0.20(11)

0.6889(14) 0.3089812) 0.0440(10) 0.57(3)

0.6818(12) 0.3088(14) 0.0598(8) 1.29(4)

0.680(1) 0.301(2) 0.0607(10) 0.14(14))

0.6877(7) 0.3075(!0) 0.0582(5) 0.624(13)

0.6835(10) 0.3078(11) 0.0588(6) 0.88(8)

0.6826(9) 0.3028(9) 0.0609(5) 0.59(2)

0.2028(14) 0.2108(10) 0.9487(9) 1.12(14)

0.205(2) 0.2077(13) 0.9456(10) 0.96(4)

0.1910(10) 0.1971(12) 0.9478(7) 0.29(2)

0.1917(14) 0.186(2) 0.9446(10) 0.03(14)

0.1800(8) 0.2926(10) 0.9417(5) 0.770(14)

0.1857(9) 0.1931(11) 0.9438(6) 0.84(8)

0.1841(8) 0.1870(9) 0.9422(6) 0.65(2)

2.75 1.96 3.57 3.33 2.21

2.77 2.56 3.54 1.91 3.84

1.90 1.98 2.43 1.51 2.27

1.70 1.69 2.12 1.59 2.6

2.63 2.22 3.32 1.5 2.15

2.02 2.00 2.56 1.64 1.96

1.98 2.14 2.49 1.35 2.26

R x y z B (Å2) Ni1/Ru1 B (Å2) OccNi1/Ru1 Ni2/Ru2 B (Å2) OccNi2/Ru2 O1 x y z B (Å2) O2 x y z B (Å2) O3 x y z B (Å2) Reliability factors Rp Rexp Rwp

c2 RI

structure is defined by the propagation vector k ¼ 0. It means that the magnetic unit cell coincides with the chemical one. In order to calculate the possible magnetic structure models compatible with the crystallographic structure, the representation analysis technique of the group theory has been used. The procedure described by Bertaut has been followed [25]. The basis vectors associated with each possible magnetic structure has been obtained with the program BasIreps of the Fullprof package [23]. For k ¼ 0, the small group Gk coincides with the P21/n space group. The P21/n space group has 4 irreducible representations, which are shown in Table S1. The notation for the R atoms (Ho, Er and Y) at the 4e site is 1 (x,y,z), 2 (xþ1/2,yþ1/2,zþ1/2), 3 (x,y,z) and 4 (xþ1/2,yþ1/2,zþ1/2). The notation for the Ru atoms at the 2c site is 5 (1/2,0,1/2) and 6 (0,1/2,0). The notation for the Ni atoms at the 2d site is 7 (1/2,0,0) and 8 (0,1/2,1/2). The possible solutions for each one of the irreducible representations of the small group P21/n are given in Table S2. For the rare-earth at the 4e site, there is a possible solution for each irreducible representation; however, for the Ru and Ni atoms at the 2c and 2d sites, respectively, only the irreducible representations G1 and G3 constitute a possible solution. The notation of the magnetic coupling of the different basis vectors for the atoms of the 4e site is given in the last row of Table S2. After checking the different solutions given in Table S2, the best one for all the compounds is that associated with the basis vectors of G3. According to Table S2, the magnetic moments of Ru and Ni can be different, but the best agreement with the experimental data

is obtained if it is assumed that the magnetic moments are the same for both sites. On the other hand, it has resulted that the coupling of the magnetic moments between the Ru and Ni sites is antiferromagnetic along the a-axis, giving rise in all of the cases to a collinear antiferromagnetic arrangement over the B-site spins. The magnetic moments value and the characteristic parameters of the fittings are shown in Table 5. The good agreement between the observed and calculated NPD patterns for Ho2NiRuO6, Er2NiRuO6 and Y2NiRuO6 are shown in Fig. 6 (left panel). A schematic view of the magnetic structures for R ¼ Y, Ho and Er oxides is shown in Fig. 6 (right panel). In all the compounds, the magnetic moments of the cations located at 2c sublattice (Ni2þ) are antiparallel with respect to the spins of the 2d sublattice (Ru4þ). Therefore, the structure can be described as ferromagnetic [011] layers coupled antiferromagnetically with each other. For R ¼ Ho and Er perovskites, the rare-earth moments participate in the magnetic structure: Ho3þ cations are ordered along the direction (010) in alternate planes antiferromagnetically coupled and the magnetic moments of Er3þ are ferromagnetically ordered along the c axis. 3.4. Transport properties The transport properties have been probed for R ¼ Nd, measured in a small pellet (10  3x3 mm3) by using the conventional four-probe technique in the temperature range 50e300 K. Fig. 7 shows the thermal evolution of the resistivity, corresponding

118

P. Kayser et al. / Acta Materialia 126 (2017) 114e123

Table 3 Main interatomic bond distances (Å) and selected angles (deg) for R2NiRuO6 determined from NPD data at RT.

Polyhedra RO8 R-O1 R-O1 R-O1 R-O2 R-O2 R-O2 R-O3 R-O3 R-O3 〈R-O〉8 short Octahedra Ni1/Ru1O6 Ni1/Ru1-O1 (2) Ni1/Ru1-O2 (2) Ni1/Ru1-O2 (2) 〈 Ni1/Ru1-O〉 104 Dd OctahedraRu2/Ni2O6 Ru2/Ni2-O1 (2) Ru2/Ni2-O2 (2) Ru2/Ni2-O2 (2) 104Dd Ni1/Ru1-O1-Ni2/Ru2 (2)

f Ni1/Ru1-O2-Ni2/Ru2 (2)

f Ni1/Ru1-O3-Ni2/Ru2 (2)

f t BVC R Ni Ru

Pr

Nd

Tb

Dy

Y

Ho

Er

2.443(3) 3.204(5) 2.349(5) 2.672(11) 2.361(12) 2.660(14) 2.783(14) 2.403(12) 2.552(11) 2.528(10)

2.432(4) 3.185(5) 2.335(5) 2.695(11) 2.316(11) 2.797(13) 2.405(13) 2.502(11) 2.432(11) 2.511(10)

2.324(3) 3.190(3) 2.263(3) 2.509(8) 2.263(9) 2.688(9) 2.635(9) 2.321(8) 2.512(7) 2.439(7)

2.294(3) 3.197(3) 2.251(3) 2.473(9) 2.293(9) 2.698(9) 2.621(8) 2.262(9) 2.524(9) 2.427(7)

2.2852(20) 3.1964(19) 2.2391(19) 2.469(5) 2.282(6) 2.660(6) 2.640(6) 2.260(6) 2.500(5) 2.417(5)

2.3937(20) 3.1886(19) 2.2438(19) 2.482(6) 2.263(6) 2.660(6) 2.642(6) 2.282(6) 2.484(6) 2.419(5)

2.276(2) 3.192(2) 2.232(2) 2.445(6) 2.281(6) 2.687(6) 2.617(6) 2.237(6) 2.503(5) 2.410(5)

2.035(9) 2.009(6) 2.014(7) 2.019(8) 0.31

2.026(15) 2.037(7) 2.042(8) 2.035(10) 0.11

2.035(7) 2.057(7) 2.040(6) 2.044(7) 0.21

2.019(9) 2.083(9) 2.095(9) 2.066(9) 2.61

1.975(4) 2.022(4) 2.036(6) 2.011(5) 1.68

2.025(5) 2.039(6) 2.044(6) 2.036(6) 0.16

2.033(4) 2.055(5) 2.066(5) 2.051(5) 0.45

1.992(9 2.065(5) 2.055(7) 2.037(7) 2.47 2.028(7) 150.0(4) 15 149.6(2) 15.2 150.1(3) 14.9 0.881

1.981(15) 2.060(7) 2.034(8) 2.025(10) 2.64 2.030(10) 149.7(6) 15.1 147.0(3) 16.5 149.1(3) 15.5 0.875

1.967(7) 2.058(6) 2025(7) 2.019(7) 3.75 2.032(7) 143.0(3) 18.5 142.0(3) 19 146.4(2) 16.8 0.849

1.979(9) 2.013(9) 1.987(8) 1.993(9) 1.54 2.029(9) 142.2(4) 18.9 142.6(3) 18.7 143.9(3) 18.1 0.846

2.023(4) 2.054(5) 2.054(5) 2.044(5) 0.51 2.027(5) 141.20(17) 19.4 143.53(19) 18.7 142.4(2) 18.8 0.843

1.975(5) 2.048(6) 2.0295) 2.017(5) 2.35 2.027(5) 141.3(2) 19.4 142.7(2) 18.7 143.9(2) 18.1 0.844

1.964(4) 2.019(5) 2.014(4) 1.999(4) 1.54 2.025(5) 140.85(17) 19.6 142.72(20) 18.6 142.21(19) 18.9 0.842

3.02 2.24 3.48

2.92 2.14 3.60

2.90 2.090 3.70

2.770 1.98 3.91

2.98 2.09 3.73

3.01 2.14 3.67

2.81 2.05 3.85

to an insulator behavior; at RT the resistivity reaches a value around 200 U cm.

Fig. 3. View of the R2NiRuO6 double perovskite crystal structure, highlighting the long-range ordering of Ni2þ and Ru4þ ions and the tilting of the large NiO6 and small RuO6 octahedra.

4. Discussion XRD and NPD studies show that all the members of the R2NiRuO6 family crystallize with the monoclinic P21/n superstructure at room temperature. The analysis of the internal parameters, obtained from the NPD data, and their evolution along the series is discussed below. The average bond distances in the rather distorted RO8 polyhedra compare well with the corresponding distances estimated from the sums of Shannon [26] effective radii (VIIIR3þ: 1.126, 1.109, 1.04, 1.027, 1.019, 1.015, and 1.004 Å, respectively and VIO2: 1.40 Å): 2.526, 2.509, 2.44, 2.427, 2.419, 2.415 and 2.404 Å. In the octahedral sites, the average distances for Ni-O and Ru-O bonds are affected by the degree of disorder over the B and B0 sites, and their values slightly deviate from those expected: of 2.09 Å for VINi2þ (0.690 Å)-VIO2- and 2.02 Å for VIRu4þ(0.620 Å) -VIO2-. The actual valences of the cations and anions can be estimated using the Brown's Bond-Valence Model (BVS) [27,28], by means of an empirical relationship between the observed bond lengths and the valence of a bond. Table 3 shows the values of the bond valences of Ni and Ru for all of the samples, which are close to the expected values of þ2 and þ4, respectively. For instance, for Tb2NiRuO6, showing a relatively low antisite disordering, the valences for Ni and Ru are þ2.09 and þ 3.70, respectively. The observed deviation should not be ascribed to the antisite disordering, which would result in the opposite effect (lower valence for Ni and higher for Ru) but could indicate a certain charge transfer between both ions. In any case, the differences from the ideal are within the typical scatter, especially when transition metals are involved.

P. Kayser et al. / Acta Materialia 126 (2017) 114e123

119

Fig. 4. Right axis: temperature dependence of the dc magnetic susceptibility for the R2NiRuO6 (R ¼ Pr-Er) perovskites, measured under a 0.1 T magnetic field. Left axis: reciprocal ZFC susceptibility vs. temperature.

120

P. Kayser et al. / Acta Materialia 126 (2017) 114e123

Table 4 Magnetic parameters for R2NiRuO6 from the magnetic susceptibility data. Compound

Ms (mB/f.u.)

qWeiss(K)

meff (mB/f.u.)

mcalc (mB/f.u.)

Pr2NiRuO6 Nd2NiRuO6 Tb2NiRuO6 Dy2NiRuO6 Y2NiRuO6 Ho2NiRuO6 Er2NiRuO6

1.35 2.0 e 6.43 0 4.86 8.9

49 55 19 10 357 21 29

6.61 6.74 13.64 15.65 4.90 10.92 14.65

6.36 6.36 14.02 15.51 4.0 15.24 14.02

Fig. 5. Magnetization versus magnetic field isotherms of R2NiRuO6 (R ¼ Pr-Er) at 5 K measured under a magnetic field ranging from 5 to 5 T. Each curve is individually represented in Fig. S3 (Supplementary Information), for the sake of clarity.

The variation of the unit-cell parameters and the volume across the series are displayed in Fig. 8a. It is observed that the a and c unit-cell parameters regularly decrease with the size of the rareearth cations, while the b parameter slightly increases and then diminishes with R3þ ionic radius. This fact is explained based on the tilting scheme of NiO6 and RuO6 octahedra in the P21/n space group, corresponding to the a-a-bþ in the Glazer's notation [29]. In this tilting system, the structural distortion driven by the reduction of the R3þ, causes minimal modifications of the b value. This behavior is also exhibited in other families of perovskites RMO3, such RVO3 [30] or RCoO3 [31]. It is also worth commenting that, in all cases, the value c/√2 lies between a and b. This is characteristic of the so-

Table 5 Magnetic moments for R2NiRuO6 from NPD at 5 K. Nd2NiRuO6

Y2NiRuO6

Ho2NiRuO6

Er2NiRuO6

R

e

e

Ni

mx ¼ 0.31(3) my ¼ 0 mz ¼ 0 jmj ¼ 0.31(3) mx ¼ 0.31(3) my ¼ 0 mz ¼ 0 jmj ¼ 0.31(3) 2.4 14.8 2.7

mx ¼ 1.26(2) my ¼ 0 mz ¼ 0 jmj ¼ 1.26(2) mx ¼ 1.26(2) my ¼ 0 mz ¼ 0 jmj ¼ 1.26(2) 2.8 13.8 3.1

mx ¼ 0 my ¼ 1.21(2) mz ¼ 0 jmj ¼ 1.21(12) mx ¼ 1.21(2) my ¼ 0 mz ¼ 0 jmj ¼ 1.21(2) mx ¼ 1.21(2) my ¼ 0 mz ¼ 0 jmj ¼ 1.21(2) 1.5 4.6 4.0

mx ¼ 0 my ¼ 0 mz ¼ 1.29(5) jmj ¼ 1.29(5) mx ¼ 1.21(3) my ¼ 0 mz ¼ 0 jmj ¼ 1.21(3 mx ¼ 1.21(3) my ¼ 0 mz ¼ 0 jmj ¼ 1.21(3) 2.2 9.7 1.7

Ru

RI Rmag

c2

called O structure and represents a common feature in the perovskites where the primary distorting effect is steric. Fig. 8b represents the evolution of R-O, Ni-O and Ru-O distances and the tilting angle (f) along the series. The average 〈R-O〉 bond lengths linearly correlate with the radius of the rare-earth ions, following the same trend observed in the size evolution of the unitcell. On the other hand, the R3þ volume does not affect the octahedral size, and both and average distances are virtually unchanged across the series. This suggests that the effect of R3þ contraction is accommodated in the structure by bending the Ru-O-Ni angles of the octahedral network, as shown in Fig. 8b (left axis), with tilting angles between 19 and 15 . This phenomenon is also observed in other system such RNiO3, RFeO3 or RMnO3. Besides comparing the structural parameters, obtained by neutron diffraction, of the entire family R2NiRuO6, it is interesting to analyze the differences between these double perovskites, with Ni and Ru located over the B sites, and the corresponding simple perovskites RNiO3 [32e35] and RRuO3 [36], which represent the end-members of the series. First of the all, it is worth commenting the differences in the synthesis procedure. While the members of the series RNiO3 (R ¼ Pr-Lu), containing trivalent Ni3þ, require extreme synthesis conditions (high O2 pressure and high temperature synthesis), the replacement of Ni by Ru4þ, in order to obtain the double perovskite R2NiRuO6, significantly soften the experimental conditions, as Ni is now incorporated with divalent oxidation state and it can be stabilized in air atmosphere. The structural changes and the nature of the involved cations drive considerable changes in the physical properties upon introducing Ru in the B-site of the RNiO3 perovskites. Regarding the magnetic properties, it is only possible to compare YNiO3 with its counterpart Y2NiRuO6, since the magnetic behavior of the rest of phases is hidden by the strong paramagnetic moment of the rareearth ions, as mentioned before. Both perovskites are antiferromagnetic and exhibit the same monoclinic structure defined in the P21/n space group at RT; the observed magnetic-ordering temperatures for YNiO3 and Y2NiRuO6 are 145 and 93 K, respectively. The lower ordering magnetic temperature showed by the Ru substituted compound is related to a higher monoclinic distortion, as reveals the tolerance factor of these perovskites: t ¼ 0.864 (YNiO3) and 0.843 (Y2NiRuO6). In the latter compound, the size mismatch between the cations over the B site leads into a more tilted octahedral network; the average bond angle Ni-O-Ru is 142.4(2) while the corresponding value for the YNiO3 is 148.3(2) . As a result, the magnetic interactions are weaker in Y2NiRuO6, exhibiting a lower TN. The transport properties of R2NiRuO6 can be justified based on previous results reported for the RNiO3 series. These compounds show a metal-insulator transition (at TMI) associated with a structural phase transition, changing the symmetry from monoclinic to orthorhombic, above TMI. The structural reorganization gives rise to the splitting of the single 4b site in the metallic phase (orthorhombic symmetry) into two individual crystallographic sites, 2c and 2d, in the monoclinic symmetry; it occurs due to a charge disproportionation 2Ni3þ/Ni(3d)þþ Ni(3þd)þ at the metalinsulator temperature, leading to a charge localization and thus, to an insulator behavior. In a similar way, the insulator response of R2NiRuO6 samples, here probed for R ¼ Nd, correspond to localized Ni2þ and Ru4þ charges, hindering the charge delocalization and leading to the observed semiconducting behavior (Fig. 7). 5. Conclusions We have synthesized novel members of the series of double perovskites R2NiRuO6 (R ¼ Pr, Nd, Tb, Y, Dy, Ho and Er) with

P. Kayser et al. / Acta Materialia 126 (2017) 114e123

121

Fig. 6. Left: observed (crosses), calculated (solid line) and difference (bottom) NPD Rietveld profiles for R2NiRuO6 at 2 K, collected at the D1B-ILL (R ¼ Ho) and D2B-ILL (R ¼ Y and Er) diffractometers. The first series of Bragg reflections corresponds to the main perovskite phase; the second one corresponds to the vanadium, used as a sample holder, and the third one to the magnetic structure. Right: Schematic view of the magnetic structures below TC.

substantial degree of long-range structural order between Ni2þ and Ru4þ cations. The samples crystallize in a monoclinic P21/n structure, where large NiO6 octahedra alternate with small RuO6 ones along the three crystallographic axes. The unit-cell parameters, volume, and Ni-O-Ru angles decrease with the size of the rare-earth ions. The magnetic properties are also strongly influenced by the nature of R3þ. For Y2NiMnO6 a collinear antiferromagnetic structure is observed, with Ni2þ and Ru4þ magnetic

moments arranged in an antiparallel arrangement; the structure can be described as ferromagnetic [011] layers antiferromagnetically (AFM) coupled to each other. For R ¼ Ho and Er perovskites, the rare-earth moments participate in the magnetic structures: Ho3þ cations are ordered along the (010) direction in alternate planes, AFM coupled; for R ¼ Er the magnetic moments of Er3þ are ferromagnetically ordered along the c axis. The Nd sample shows a semiconducting behavior, with large resistivity values at

122

P. Kayser et al. / Acta Materialia 126 (2017) 114e123

Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.actamat.2016.12.024. References

Fig. 7. Resistivity vs temperature for Nd2NiRuO6.

Fig. 8. (a)Variation of the unit-cell parameters (left axis) and volume (right axis), (b) RO, Ni-O and Ru-O bond distance (left axis) and tilting angle (ɸ) for R2NiRuO6 perovskites (R ¼ Pr-Er) with the ionic radius of R3þ.

low temperatures as a consequence of the charge localization established between Ni2þ and Ru4þ. Acknowledgments We thank the financial support of the Spanish Ministry of Economy and Competitiveness to the project MAT2013-41099-R. We are grateful to the Institut Laue-Langevin for making all facilities available.

[1] K.I. Kobayashi, T. Kimora, H. Sawada, K. Terakura, Y. Tokura, Room-temperature magnetoresistance in an oxide material with an ordered doubleperovskite structure, Nature 395 (1998) 677e680. [2] K.-W. Lee, W.E. Pickett, Half semimetallic antiferromagnetism in the Sr2CrTO6 system (T¼Os,Ru), Phys. Rev. B 77 (2008) 115101. [3] H. Kato, T. Okuda, Y. Okimoto, Y. Tomioka, Y. Takenoya, A. Ohkubo, M. Kawasaki, Y. Tokura, Metallic ordered double-perovskite Sr2CrReO6 with maximal Curie temperature of 635 K, Appl. Phys. Lett. 81 (2002) 328. [4] Y. Maeno, H. Hashimoto, K. Yoshida, S. Nishizaki, T. Fujita, J.G. Bednorzt, F. Lichtenberg, Superconductivity in a layered perovskite without copper, Nature 372 (1994) 532e534. [5] S. Nakatsuji, Y. Maeno, Quasi-two-dimensional Mott transition system Ca2xSrxRuO4, Phys. Rev. Lett. 84 (2000) 2666. [6] V. Hardy, B. Raveau, R. Retoux, N. Barrier, A. Maignan, Nature of the ferromagnetism induced by nonmagnetic substitutions on the Ru site of CaRuO3, Phys. Rev. B 73 (2006) 94418. [7] A. Maignan, B. Raveau, V. Hardy, N. Barrier, R. Retoux, Ferromagnetism induced by chromium substitution in the CaRuO3 perovskite, Phys. Rev. B 74 (2006) 024410. [8] A.I. Shames, E. Rozenber, C. Marti, A. Maigna, B. Ravea, G. Andre, G. Gorodetsky, Crystallographic structure and magnetic ordering in CaMn1xRuxO3 (x  0.40) manganites: neutron diffraction, ac susceptibility, and electron magnetic resonance studies, Phys. Rev. B 70 (2004) 134433. [9] G. Cao, S. Chikara, X.N. Lin, E. Elhami, V. Durairaj, Itinerant ferromagnetism to insulating antiferromagnetism: a magnetic and transport study of single crystal SrRu1xMnxO3 (0x<0.60), Phys. Rev. B 71 (2005) 035104. [10] M. Yokoyama, C. Satoh, A. Saitou, H. Kawanaka, H. Bando, K. Ohoyama, Y. Nishihara, Antiferromagnetic order in disorder-induced insulating phase of SrRu1xMnxO3 (0.4 x 0.6), J. Phys. Soc. Jpn. 74 (2005) 1706. [11] T. He, R.J. Cava, Disorder-induced ferromagnetism in CaRuO3, Phys. Rev. B 63 (2001) 172403. [12] T. He, R.J. Cava, The effect of Ru-site dopants on the magnetic properties of CaRuO3, J. Phys. Condens. Matter 13 (2001) 8347. [13] Y. Doi, Y. Hinatsu, A. Nakamura, Y. Ishii, Y. Morii, Magnetic and neutron diffraction studies on double perovskites A2LnRuO6 (A ¼ Sr, Ba; Ln ¼ Tm, Yb), J. Matter. Chem. 13 (2003) 1758e1763. [14] R.I. Dass, J.-Q. Yan, J.B. Goodenough, Ruthenium double perovskites: transport and magnetic properties, Phys. Rev. B 69 (2004) 094416. [15] F. Galasso, W. Darby, Preparation of single crystals of complex perovskite ferroelectric and semiconducting compounds, Inorg. Chem. 4 (1965) 71e73. [16] I. Fernandez, R. Greatrex, N.N. Greenwood, A study of the new cubic, ordered perovskites BaLaMRuO6 (M ¼ Mg, Fe, Co, Ni, or Zn) and the related phases €ssbauer spectroscopy and La2MRuO6 (M ¼ Mg, Co, Ni, or Zn) by 99Ru Mo other techniques, J. Solid State Chem. 32 (1980) 97e104. [17] P.A. Seinen, F.P.F. van Berkel, W. AGroen, D.J.W. Ijdo, The ordered perovskite system Ln2NiRuO6, Mater. Res. Bull. 22 (1987) 535e542. [18] P.D. Battle, C.W. Jones, The crystal structure of La2NiRuO6, Mater. Res. Bull. 22 (1987) 1623e1627. [19] K. Yoshii, H. Abe, M. Mizumaki, H. Tanida, N. Kawamura, Structure, magnetism and transport of La2NiRuO6, J. Alloys Compd. 348 (2003) 236e240. [20] M. Gateshki, J.M. Igartua, Y. Brouard, Cation ordering and high-temperature crystal structure of La2NiRuO6, Mater. Res. Bull. 38 (2003) 1661e1668. [21] N. Kamegashira, T. Mori, A. Imamura, Y. Hinatsu, Crystal structure and magnetic property of La2MnRuO6 phase, J. Alloys Compd. 302 (2000) L6eL11. [22] K. Yoshii, N. Ikeda, S. Mori, Y. Yoneda, M. Mizumaki, H. Tanida, N. Kawamura, Trans. Mater. Res. Soc. Jpn. 32 (2007) 51. [23] J. Rodríguez-Carvajal, Recent advances in magnetic structure determination by neutron powder diffraction, Phys. B 192 (1993) 55e69. [24] H.M. Rietveld, A profile refinement method for nuclear and magnetic structures, J. Appl. Crystallogr. 2 (1969) 65e71. [25] E.F. Bertaut, in: G.T. Rado, H. Shull (Eds.), Magnetism, vol. III, Academic Press, New York, 1963 ch. 4. [26] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomie distances in halides and chaleogenides, Acta Crystallogr. A 32 (1976) 751. [27] I.D. Brown, in: M. O'Keefe, A. Navrotsky (Eds.), Structure and Bonding in Crystals, vol. 2, New York: Academic Press, 1981, p. 1. [28] I.D. Brown, Z. Kristallogr, Modeling the structures of La2NiO4 199 (1992) 255e272. [29] A.M. Glazer, The classification of tilted octahedra in perovskites, Acta Cryst. B 28 (1972) 3384e3392. ndez-Díaz, Volution of [30] M.J. Martínez-Lope, J.A. Alonso, M. Retuerto, M.T. Ferna the crystal structure of RVO3 (R ¼ La, Ce, Pr, Nd, Tb, Ho, Er, Tm, Yb, Lu, Y) perovskites from neutron powder diffraction data, Inorg. Chem. 47 (2008) 2634e2640. [31] J.A. Alonso, M.J. Martínez-Lope, C. de la Calle, V. Pomjakushinb, Preparation

P. Kayser et al. / Acta Materialia 126 (2017) 114e123 and structural study from neutron diffraction data of RCoO3 (R ¼ Pr, Tb, Dy, Ho, Er, Tm, Yb, Lu) perovskites, J. Mater. Chem. 16 (2006) 1555e1560. ndez[32] J.A. Alonso, M.J. Martínez-Lope, M.T. Casais, M.A.G. Aranda, M.T. Ferna Díaz, MetalInsulator transitions, structural and microstructural evolution of RNiO3 (R ¼ Sm, Eu, Gd, Dy, Ho, Y) Perovskites: evidence for roomtemperature charge disproportionation in monoclinic HoNiO3 and YNiO3, J. Am. Chem. Soc. 121 (1999) 4754e4762. ~ oz, [33] J.A. Alonso, M.J. Martínez-Lope, M.T. Casais, J.L. García-Mún M.T. Fern andez-Díaz, M.A.G. Aranda, High-temperature structural evolution of RNiO3 (R¼Ho, Y, Er, Lu) perovskites: charge disproportionation and electronic localization, Phys. Rev. B 64 (2001) 094102.

123

[34] M.T. Fern andez-Díaz, J.A. Alonso, M.J. Martínez-Lope, M.T. Casais, J.L. García~ oz, M.A.G. Aranda, Charge disproportionation in RNiO3 perovskites, Phys. Mún B 276e278 (2000) 218e221. [35] Y. Bodenthin, U. Staub, C. Piamonteze, M. García-Fern andez, M.J. MartínezLope, J.A. Alonso, Magnetic and electronic properties of RNiO3 (R ¼ Pr, Nd, Eu, Ho and Y) perovskites studied by resonant soft x-ray magnetic powder diffraction, J. Phys. Condens. Matter 23 (2011) 036002. [36] H. Kobayashi, M. Nagata, R. Kanno, Y. Kawamoto, Structural characterization of the orthorhombic perovskites: [ARuO3 (A ¼ Ca, Sr, La, Pr)], Mater. Res. Bull. 29 (1994) 1271e1280.