Synthesis, structure and magnetic properties of Ti doped La2MnNiO6 double perovskite 80 D O I: 1 0. 15 82 6/ ch im te ch .2 01 9. 6. 3. 01 Uma Dutta, Ariful Haque, Md. Motin Seikh Chimica Techno Acta. 2019. Vol. 6, No. 3. P. 80–92. ISSN 2409–5613 Uma Dutta, Ariful Haque, Md. Motin Seikh* Department of Chemistry, Visva-Bharati University, Santiniketan-731235, West Bengal, India *E-mail: mdmotin.seikh@visva-bharati.ac.in Synthesis, structure and magnetic properties of Ti doped La2MnNiO6 double perovskite We report sol-gel synthesis, structural characterization and magnetic proper- ties of La2Mn1–xTixNiO6 (0 ≤ x ≤ 1.0). Ti doping removed the biphasic structure of La2MnNiO6 by suppression of rhombohedral structure and all the Ti containing samples crystallized in monoclinic P21 / n symmetry. La2MnNiO6 exhibits multiple magnetic transitions. The high temperature ferromagnetic transition of La2MnNiO6 gradually shifted to lower temperatures with increase in Ti doping. La2TiNiO6 (x = 1.0) does not show any long-range magnetic ordering. The suppression of magnetic transition by Ti doping is ascribed to the destruction of Mn4+ – O – Ni2+ superexchange interaction. However, the signature of ferromagnetic phase persists up to 70 % Ti doping, indicating the robustness of magnetic ordering in La2MnNiO6. These results suggest that the addition of Ti 4+ truncates the ferro- magnetic Mn4+ – O – Ni2+ superexchange path and it likely promotes ferromagnetic cluster formation. The robustness of ferromagnetic state towards Ti substitu- tion compared to the simple perovskite or spinel structure can be attributed to cationic ordering in double perovskite structure. Both the pure and Ti-doped samples exhibit magnetic frustration at lower temperatures due to partial cationic disordering. The absence of long-range ordering in La2TiNiO6, unlike La2TiCoO6 or Pr2TiCoO6, could be related to cationic disordering. Keywords: double perovskite; ferromagnetic; superexchange interaction; magnetic frustration; cationic disorder; magnetic cluster. Received: 20.08.2019. Accepted: 11.09.2019. Published: 15.10.2019. © Uma Dutta, Ariful Haque, Md. Motin Seikh, 2019 Introduction Oxides of transition metals with perovskite structure LnMO3 (where Ln is rare earth elements or alkaline earth and M is transition element) exhibit various ex- otic physical properties related to the cor- relation between spin, charge, lattice and orbital degrees of  freedom [1–3]. These strongly correlated electronic systems show competing electronic and magnetic states. Accordingly, compounds with perovskite structure have most extensively been stud- ied by the physics and chemistry commu- nities due to their magnetic, transport and magnetotransport properties [4]. Presence of second transition metal in the perovskite structure further improves the characteristic 81 features of the so-called double perovskites, Ln2MM’O6. The important features of dou- ble perovskite are cationic ordering, an- tiphase boundaries and multiple exchange interactions [5–7]. The rock salt ordering with alternate occupancy of  octahedra by different metal ions takes place when there is a large size mismatch or the charge difference is  greater than 2 [6, 8, 9]. All these are governed by the local electronic configuration of the transition metal ions. Extensive investigations on double perovs- kites reveal that by virtue of wide M / M’ cationic range they exhibit multifunctional properties like insulating, metallic, ferro- magnetic, magnetodielectric, multiferroic, etc., which are suitable for various techno- logical applications [10–13]. Among the  double perovskites, La2MnNiO6 has attracted special attention due to its striking properties such as mul- tiple structures, multiple magnetic ground states, spin frustration and ferromagnetic insulating behaviour near room tempera- ture with giant magnetodielectricity and magnetoresistance [14–19]. La2MnNiO6 is reported to crystallize in biphasic nature with rhombohedral (R3c) and monoclinic (P21 / n) [14, 15, 20–22] or rhombohedral and orthorhombic (Pbnm) symmetry [17, 23]. The  high temperature synthesized sample is rhombohedral and low tempera- ture one is monoclinic or orthorhombic, whereas for  intermediate temperature range it is biphasic [23]. The phase fraction in the sample is also sensitive to the synthe- sis condition and post annealing treatment [14, 15, 18]. It was reported that the high- temperature phase transforms to  P21 / n phase at low temperature [14]. The mag- netic ordering temperatures are differ- ent for  different phases. The R3c phase shows ferromagnetic ordering at relatively higher temperature (~280  K) compared to  the  P21 / n or  Pbnm phase (~150  K) [16, 17]. The high temperature magnetic phase is governed by Mn4+ – O – Ni2+, where for  low temperature it is  Mn3+ – O – Ni3+ superexchange interactions [15–17, 23, 24]. There is  also report on  ferromag- netic transition at TC~100  K in  partially disordered sample, which was attributed to the Mn3+ – O – Ni3+ interaction [15]. Fur- thermore, there is also appearance of spin glass behaviour in La2MnNiO6 associated with the  competing interaction between ferromagnetic superexchange and disorder induced antiferromagnetic Mn4+ – O – Mn4+ and Ni2+ – O – Ni2+ interactions [15, 18, 25]. This suggests that the cationic disordering suppressed the high temperature ferromag- netic Mn4+ – O – Ni2+ interaction of  per- fectly ordered phase. Another extensively studied compound is  La2MnCoO6 with ferromagnetic TC~230 K [26]. The double perovskites in which both metal ions are magnetic exhibit ferromagnetic ordering. However, for the ordered perovskite with single magnetic ion the long-range mag- netic ordering is  antiferromagnetic and the  interaction is  supersuperexchange with separation of magnetic centres larger than 5  Å [27, 28]. Such interaction also becomes weaker as reflected from the or- dering temperatures. The  observed TN values for La2CoTiO6 and Pr2CoTiO6 are respectively, 15 K and 17 K [28, 29]. Thus, it would be interesting to investigate the fer- romagnetic to antiferromagnetic crossover starting from an  ordered ferromagnetic system by gradual replacing one magnetic ion by a nonmagnetic one. In the present studies we have gradu- ally replaced Mn4+ in La2MnNiO6 by Ti 4+ to better understand the role of Mn4+ in de- termining the  structural and magnetic properties. To the best of our knowledge there is no report in the literature on in- 82 vestigation of  Ti doping in  La2MnNiO6. The ionic radius of Ti4+ (0.605 Å) is larger than that for Mn4+ (0.530 Å) [30]. In addi- tion, there are other aspects of this substi- tution. The replacement of magnetic Mn4+ by nonmagnetic Ti4+ will make magnetic dilution in  system, which in  turn will truncate the ferromagnetic exchange path. Thus, substitution of  Ti4+ in  La2MnNiO6 is  expected to  rapidly destroy the  ferro- magnetic state. Furthermore, by  consid- ering the retention of cationic ordering it will bring antiferromagnetic supersuper- exchange Ni2+ – O – Ti4+ – O – Ni2+ in place of ferromagnetic Ni2+ – O – Mn4+ – O – Ni2+ exchange path. We observed that the fer- romagnetic ground state of  La2MnNiO6 is  very robust and it persists up to  70 % doping of  Ti4+. We did not observe any antiferromagnetic ordering in La2TiNiO6 could be due to cationic disordering. Experimental Polycr ystalline L a 2Mn 1–xTi xNiO 6 (0 ≤ x ≤ 1) samples were prepared by modi- fied sol-gel technique. At  first, stoichio- metric amounts of metal nitrates (La, Ni) and acetate (Mn) were dissolved in 100 ml double distilled water followed by the addi- tion of about 5 ml concentrated nitric acid to  prevent the  hydrolysis of  the  aquated metal ions. In another small beaker stoi- chiometric amount (1:1) of titanium iso- propoxide [Ti{OCH(CH3)2}4], and acetyl acetone were mixed and stirred for about several minutes. These two different so- lutions were mixed in  one single beaker and were stirred for about one hour using a magnetic stirrer to get a clear solution. Citric acid was then added to  the  solu- tion at four times mole ratio of the total metal ions. The  final pH of  the  solution was found to be ~2. The resulting solution was stirred overnight followed by evapora- tion of solvent at 100 °C to obtain the gel. The  obtained gel was dried by  increas- ing the temperature of hotplate to 150 °C to transform the gel into crude precursor. The  crude powders were ground thor- oughly by using an agate mortar-pestle and calcined at 500 °C for 6 h in air. The final calcination was performed at 700 °C for 6 h in air to achieve the pure phase samples. The powder X-ray diffraction (PXRD) patterns were registered with a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å) operating at 40 kV and 40 mA. The PXRD patterns were re- corded in the 2θ range of 10–120° using Lynxeye detector (1D mode) with a step size of  0.02° and a  dwell time of  1s per step. Iodometric titration of the samples confirms the oxygen stoichiometry fixed to ‘‘O6” within the limit of accuracy ±0.05. The  dc magnetization measurements were performed using a superconducting quantum interference device (SQUID) magnetometer with a  variable tempera- ture cryostat (Quantum Design, San Diego, USA). The magnetic ac susceptibility, χac(T) was measured with a  PPMS (Quantum Design, San Diego, USA) with the  fre- quency ranging from 10  Hz to  10  kHz. All the magnetic measurements were per- formed on powder samples putting inside a Teflon capsule. Results and discussion The  PXRD patterns for  all the La2Mn1–xTixNiO6 (x = 0, 0.2, 0.3, 0.5, 0.7 and 1.0) samples recorded in the 2q-range 10–120° are shown in Fig. 1. All the pat- 83 terns are refined by Rietveld method us- ing FullProf suit program [31]. The pattern of the parent compound (x = 0) La2MnNiO6 can only be refined properly by considering a mixed rhombohedral R3c and monoclin- ic P21 / n phases. The biphasic nature com- posed of  rhombohedral and monoclinic phases for La2MnNiO6 sample was reported in the literature by several researchers [14, 15, 18, 32]. However, the  phase fraction depends on  synthesis condition as  well as on post-synthesis annealing treatment. We observed predominant monoclinic phase (80 %) over rhombohedral (20 %) one for the sol-gel synthesized La2MnNiO6 with final heat treatment in air at 700 °C for 6 h. However, all the Ti doped samples can be nicely indexed with monoclinic structure (sp. gr. P21 / n) (Fig. 1). Thus, the Ti dop- ing suppressed the  rhombohedral phase of La2MnNiO6. The detailed structural and refinement parameters are given in Table 1. From the Table, one can see the systematic increase in cell volume with the increase in  Ti4+ content as  expected for  its larger ionic radius compared to that of Mn4+ [30]. We observed that the size of NiO6 octahe- dra are slightly larger than that of MnO6 as reported in the literature [33]. Fig. 2 shows the temperature depend- ent dc-magnetization measured in  both the  zero field-cooled (ZFC) and field- cooled (FC) protocols under an  applied magnetic field of 500 Oe for the tempera- ture range 5–300  K. The  magnetization data of x = 0 parent phase exhibit ferro- magnetic transition (TC~280 K) just below room temperature (Fig. 2, a). One should notice the large thermomagnetic irrevers- ibility between ZFC and FC data branch immediate below TC~280  K, where ZFC data show a  hump. The  ZFC data also Fig. 1. Powder X-ray diffraction patterns of La2Mn1–xTixNiO6 (x = 0, 0.2, 0.3, 0.5, 0.7 and 1.0). The open red circles, black lines, the bottom blue lines and vertical bars represent the experimental data, calculated pattern, difference curve and Bragg position, respectively 84 Ta bl e 1 St ru ct ur al re fin em en t p ar am et er s fo r Ln 2M n 1 -x T i xN iO 6 ( 0 ≤ x ≤ 1) c er am ic s x = 0 x = 0. 2 x = 0. 3 x = 0. 5 x = 0. 7 x = 1 P2 1/ n R 3c a (Å ) 5. 47 1 (4 ) 5. 51 6 (2 ) 5. 48 3 (9 ) 5. 49 0 (8 ) 5. 51 1 (8 ) 5. 52 4 (5 ) 5. 54 2 (6 ) b (Å ) 5. 49 3 (3 ) 5. 51 6 (1 ) 5. 50 6 (4 ) 5. 51 3 (6 ) 5. 52 7 (2 ) 5. 53 8 (6 ) 5. 55 3 (6 ) c (Å ) 7. 74 2 (4 ) 13 .2 37 (7 ) 7. 76 0 (4 ) 7. 76 7 (4 ) 7. 78 3 (7 ) 7. 80 2 (5 ) 7. 82 1 (6 ) V (Å 3 ) 23 2. 72 34 8. 78 23 4. 34 2 23 5. 15 6 23 7. 11 0 23 8. 74 5 24 0. 77 8 β (o ) 89 .3 69 γ = 12 0 89 .5 40 89 .7 19 89 .7 20 90 .1 97 90 .2 44 R B (% ) 10 .4 13 .6 10 .8 11 .9 9. 20 12 .3 R f ( % ) 14 .5 22 23 .4 17 .0 24 .4 26 .8 χ2 2. 97 2. 63 2. 38 2. 28 2. 26 2. 30 B on d le ng th 2× M n— O 1: 1. 93 0  Å 2× M n— O 2: 1. 87 5  Å 2× M n— O 3: 1. 95 6  Å 2× N i— O 1: 1. 99 1  Å 2× N i— O 2: 2. 03 7  Å 2× N i— O 3: 2. 00 7  Å La — O 1: 2 .3 18  Å M n1 — O 1: 1. 89 9  Å N i2 — O 1: 2. 02 9  Å La 1— O 1: 3× 2. 45 3  Å 6× 3. 05 6  Å 3× 2. 77 1  Å ×M n/ T i— O 1: 1. 93 4  Å 2× M n/ T i— O 2: 1. 88 5  Å 2× M n/ T i— O 3: 1. 90 6  Å 2× N i— O 1: 2. 02 3  Å 2× N i— O 2: 2. 04 6  Å 2× N i— O 3: 2. 03 3  Å 2× M n/ T i— O 1: 1. 93 5  Å 2× M n/ T i— O 2: 1. 88 8  Å 2× M n/ T i— O 3: 1. 90 9  Å 2× N i— O 1: 2. 01 9  Å 2× N i— O 2: 2. 04 8  Å 2× N i— O 3: 2. 03 5  Å 2× M n/ T i— O 1: 1. 93 9  Å 2× M n/ T i— O 2: 1. 89 4  Å 2× M n/ T i— O 3: 1. 91 4  Å 2× N i— O 1: 2. 02 3  Å 2× N i— O 2: 2. 05 4  Å 2× N i— O 3: 2. 04 1  Å ×M n/ T i— O 1: 1. 94 1  Å 2× M n/ T i— O 2: 1. 89 9  Å 2× M n/ T i— O 3: 1. 92 0  Å 2× N i— O 1: 2. 03 1  Å 2× N i— O 2: 2. 05 8  Å 2× N i— O 3: 2. 04 4  Å 2× T i— O 1: 1. 94 6  Å 2× T i— O 2: 1. 90 5  Å 2× T i— O 3: 1. 92 6  Å 2× N i— O 1: 2. 03 7  Å 2× N i— O 2: 2. 06 4  Å 2× N i— O 3: 2. 05 0  Å 85 x = 0 x = 0. 2 x = 0. 3 x = 0. 5 x = 0. 7 x = 1 P2 1/ n R 3c La — O 1: 3. 12 1  Å La — O 1: 2. 72 3  Å La — O 1: 2. 82 9  Å La — O 2: 2. 78 7  Å La — O 2: 2. 72 9  Å La — O 2: 3. 01 0  Å La — O 2: 2. 46 3  Å La — O 3: 2. 71 3  Å La — O 3: 2. 45 5  Å La — O 3: 2. 76 7  Å La — O 3: 3. 05 7  Å 2L a— O 1: 2. 33 8  Å La — O 1: 2. 72 1  Å La — O 1: 2. 82 6  Å La — O 1: 3. 14 8  Å La — O 2: 2. 47 6  Å La — O 2: 2. 73 3  Å La — O 2: 2. 79 2  Å La — O 2: 3. 02 4  Å La — O 3: 2. 45 9  Å La — O 3: 2. 72 7  Å La — O 3: 2. 78 0  Å La — O 3: 3. 06 3  Å La — O 1: 2. 34 1  Å La — O 1: 2. 72 5  Å La — O 1: 2. 83 0  Å La — O 1: 3. 15 2  Å La — O 2: 2. 48 0  Å La — O 2: 2. 73 5  Å La — O 2: 2. 79 3  Å La — O 2: 3. 02 9  Å La — O 3: 2. 46 0  Å La — O 3: 2. 73 2  Å La — O 3: 2. 78 4  Å La — O 3: 3. 06 4  Å La — O 1: 2. 34 9  Å La — O 1: 2. 73 2  Å La — O 1: 2. 83 8  Å La — O 1: 3. 16 3  Å La — O 2: 2. 48 7  Å La — O 2: 2. 74 2  Å La — O 2: 2. 80 0  Å La — O 2: 3. 03 7  Å La — O 3: 2. 46 7  Å La — O 3: 2. 73 9  Å La — O 3: 2. 79 1  Å La — O 3: 3. 07 2  Å 2L a— O 1: 2. 35 5  Å La — O 1: 2. 73 7  Å La — O 1: 2. 84 3  Å La — O 1: 3. 17 2  Å La — O 2: 2. 50 0  Å La — O 2: 2. 74 1  Å La — O 2: 2. 79 8  Å La — O 2: 3. 05 2  Å La — O 3: 2. 46 5  Å La — O 3: 2. 75 4  Å La — O 3: 2. 80 5  Å La — O 3: 3. 07 0  Å La — O 1: 2. 36 7  Å La — O 1: 2. 74 5  Å La — O 1: 2. 85 1  Å La — O 1: 3. 18 2  Å La — O 2: 2. 50 8  Å La — O 2: 2. 74 8  Å La — O 2: 2. 80 5  Å La — O 2: 3. 06 1  Å La — O 3: 2. 47 2  Å La — O 3: 2. 76 2  Å La — O 3: 2. 81 3  Å La — O 3: 3. 07 8  Å B on d an gl e (° ) M n— O 1— N i: 15 8. 44 M n— O 2— N i: 16 1. 16 M n— O 3— N i: 16 2. 62 M n— O — N i: 16 2. 67 M n— O 1— N i: 15 8. 31 M n— O 2— N i: 16 2. 61 M n— O 3— N i: 16 1. 20 M n— O 1— N i: 15 8. 3 M n— O 2— N i: 16 2. 6 M n— O 3— N i: 16 1. 21 M n— O 1— N i: 15 8. 26 M n— O 2— N i: 16 2. 62 M n— O 3— N i: 16 1. 22 M n— O 1— N i: 15 8. 27 M n— O 2— N i: 16 2. 57 M n— O 3— N i: 16 1. 27 M n— O 1— N i: 15 8. 25 M n— O 2— N i: 16 2. 57 M n— O 3— N i: 16 1. 28 C on tin ua tio n of ta bl e 86 show a  second broad hump at  low tem- perature centred around 25 K. Below TC, FC data show a plateau followed by a defi- nite slope change below 100 K (Fig. 2, a). Such magnetic behaviour of  the  parent phase is  in  good agreement with the  re- ported data which supports the  preva- lence three magnetic phases in  the  tem- perature window 5–300 K [15–17, 23, 24]. The high temperature TC is associated with the ferromagnetic Mn4+ – O – Ni2+ super- exchange interaction of  cation ordered state. The transition below 100 K ascribed to the Mn3+ – O – Ni3+ superexchange in- teraction and lowest temperature anomaly was attributed to the magnetic frustration arising out of partial cationic disordering [15, 18, 25] For the Ti doped sample with x = 0.2, the high temperature magnetic transition largely shifted (~60 K) to lower tempera- ture at  TC~220  K (Fig.  2, a). However, the  shape of  the  curve remains similar to that of the parent phase, though the ther- momagnetic hysteresis loop shrinks com- pared to the x = 0 sample (Fig. 2, a). How- ever, the upturn in magnetization below 100 K is not much shifted for x = 0.2 sample compared to x = 0. This result indicates that the high temperature magnetic transition associated with Mn4+ – O – Ni2+ superex- change interaction is  largely hampered compared to  the  Mn3+ – O – Ni3+ super- exchange interaction. This could be due to preferential isovalent substitution effect. Thus, it can be suggested that for 20 % Ti doped sample the cationic ordering per- sists. For x = 0.5 the TC value comes down to 150 K and still one can observe the dou- ble humps in  ZFC branch data, upturn in FC data below 50 K as well as the ther- momagnetic irreversibility (Fig.  2, b). It is  worth mentioning that the  parent phase sample with orthorhombic Pbnm or  monoclinic P21 / n structures exhibits ferromagnetic transition at TC~150 K as- cribed to the Mn3+ – O – Ni3+ superexchange interaction [15–17, 24]. However, we be- lieve that the TC~150 K for x = 0.5 sam- ple is not associated with Mn3+ – O – Ni3+ superexchange interaction like in parent phase, rather it is related to the weakening of Mn4+ – O – Ni2+ superexchange interac- tion due to substitution of nonmagnetic Ti4+ in place of Mn4+. The second magnetic phase further shifted below 50 K as indicat- ed by the upturn in FC data in x = 0.5 sam- ple (Fig. 2, b). This indicates that still there is possible cationic ordering up to 50 % Ti doping in La2MnNiO6. On further increase in Ti doping to x = 0.7, there is only one broad hump in  ZFC data around 20  K and FC data show up turn below 100 K (Fig. 2, b). This indicates that the high and Fig. 2. Temperature dependent dc- magnetization of La2Mn1–xTixNiO6: a — for x = 0 and 0.2 and (b) for x = 0.5 and 0.7. Inset in (b) shows the data for La2TiNiO6 87 low temperature transitions as well as low temperature magnetic frustration merged together. For complete substitution of Mn by Ti i.e. x = 1.0 sample does not show any anomaly in  the  magnetization data and ZFC-FC superimposed as  shown in  in- set of Fig. 2, b. This behaviour is typical for a paramagnetic material. This suggests that La2TiNiO6 does not show any kind of long-range ordering. The gradual change in magnetic prop- erties with replacement of magnetic Mn4+ by nonmagnetic Ti4+ in La2MnNiO6 is also supported from the isothermal magnetiza- tion data measured at 5 K under an applied field of ±5 T (Fig. 3). There is also a system- atic decrease in magnetization as expected for magnetic dilution, but all the samples up to x = 0.7 exhibit clear hysteresis loop suggesting the  prevalence of  ferromag- netic component in the system. However, the opening of the loop is small as reported for the parent phase [15, 24]. There is no hysteresis loop for x = 1.0 sample (see inset in  Fig.  3) which practically shows a  lin- ear increase in  magnetization with field as  expected for  a  paramagnet (see inset in Fig. 3). Let us discuss the  observed change in  magnetization in  La2Mn1–xTixNiO6. The  substitution of  Mn4+ by  nonmag- netic Ti4+ will truncate the ferromagnetic Ni2+ – O – Mn4+ – O – Ni2+ superexchange path to Ni2+ – O – Ti4+ – O – Ni2+. This dop- ing not only destroy the  ferromagnetic exchange path, but also results in  weak antiferromagnetic interaction between the  Ni2+ cation. The  weak antiferromag- netic interaction between Ni2+ takes place via super superexchage interaction me- diated through O – Ti4+ – O linker. This type of  antiferromagnetic interaction is  observed in  half doped LaNi0.5Al0.5O3 through Ni – O – Al – O – Ni exchange path [34]. Thus, with increase in Ti dop- ing the ferromagnetism in La2MnNiO6 be- comes more and more weaker as reflected by the change in TC shown in Fig. 4. This result is  very contrasting with the  effect of Ti doping in single perovskite mangan- ite Sm0.55Sr0.45MnO3, where just 4 % of Ti doping leads for disappearing of the ferro- magnetic state [35]. The robustness of fer- romagnetism towards magnetic dilution in double perovskite La2MnNiO6 may be related to the cationic ordering. Most likely the disruption of Ni2+ – O – Mn4+ – O – Ni2+ superexchange path leads to fragmented Fig. 3. Isothermal magnetization of La2Mn1–xTixNiO6 (x = 0, 0.2, 0.5 and 0.7) recorded at 5 K. Inset shows the data for La2TiNiO6 -40 -20 0 20 40 -4 -2 0 2 4 La2Mn1-xTixNiO6 M ag ne tiz at io n (µ β/ f.u ) H (kOe) x = 0 x = 0.2 x = 0.5 x = 0.7 0 20 400.00 0.05 0.10 X=1 M (µ B/f .u ) H ( kOe ) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 50 100 150 200 250 300 La2Mn1-xTixNiO6 T C (K ) x Fig. 4. Variation of ferromagnetic Curie temperature (TC) with x in La2Mn1–xTixNiO6 88 ferromagnetic clusters which are respon- sible for  observed magnetic behaviour of the Ti doped samples even for 70 % dop- ing. This is even higher than the effect of Ti doping in spinel structure. Mn1+xFe2−2xTixO4 has been reported to exhibit ferrimagnet- ic ordering up to 50 % Ti doping [36, 37]. However, one cannot rule out the  cati- onic disordering with the  increase in  Ti doping. La2CoTiO6 and Pr2CoTiO6 have been reported to show long-range antifer- romagnetism at  15 K and 17  K, respec- tively, in cationic ordered state. The anti- ferromagnetic exchange interaction path is Co2+ – O – Ti4+ – O – Co2+ in cationic or- dered samples [28, 29]. However, we did not observe any such long range antifer- romagnetic ordering in La2TiNiO6. The ab- sence of such long range ordering can be related to the cationic disordering. To look at the magnetic features above TC we have fitted the high temperature data with Curie-Weiss law. The calculated ef- fective paramagnetic moments also found to  decrease with the  increase in  Ti dop- ing. The µeff value for the parent phase is 6.30 µB / f.u. which is  slightly larger than the calculated value 5.97 µB / f.u., which has been attributed to the possible formation of ferromagnetic cluster above TC [38, 39]. The  µeff value decreases from 4.8 µB / f.u. for x = 0.2 to 1.72 µB / f.u. for x = 1.0 sample revealing the effect of magnetic dilution. Finally, to confirm the association of low temperature magnetic anomaly with spin glass behaviour as  reported for  the  par- ent phase we have measured the  ac-sus- ceptibility of  x = 0 and x = 0.5 samples at different driving frequencies in the low temperature regions. Fig. 5 shows the real and imaginary parts of the ac-susceptibil- ity data for these two samples. The χ’(T) for x = 0 sample is too broad to uniquely identify the glass transition temperature Tg Fig. 5. Temperature dependent ac-susceptibilities of La2Mn1–xTixNiO6 for x = 0 and 0.5. Panels (a, b) show real part, χ’(T), and (c, d) show imaginary part, χ”(T), at selected frequencies 89 (Fig. 5, a). However, the χ”(T) revealed two frequency dependent peak around 20 K and 50 K (Fig. 5, c), respectively, suggesting the  magnetic frustration. The  multiglass behaviour of La2MnNiO6 has been reported in the literature [18]. This magnetic frus- tration is  associated with the  competing interaction between the antisite disorder induced antiferromagnetic Mn4+ – O – Mn4+ and Ni2+ – O – Ni2+ interactions and ferro- magnetic clusters [18]. The x = 0.5 sample exhibit only one peak around 20 K, which is frequency dependent as revealed from both χ’(T) and χ”(T) data (Figs. 5, b, d). This indicates the  presence of  magnetic frustration in pure and Ti doped samples. Conclusions In  the  present study, we have syn- thesized La2Mn1–xTixNiO6 (0 ≤ x ≤ 1.0) by modified citrate-based sol-gel method. Rietveld analysis of the PXRD patterns re- vealed that the parent phase (x = 0) is bi- phasic in nature composed of rhombohe- dral R3c and monoclinic P21 / n structures, whereas Ti doped samples crystalized in  single phase monoclinic P21 / n struc- ture with the  suppression of  rhombohe- dral phase. The cell volume of the Ti doped samples increased due to larger size of Ti4+ compared to Mn4+ ion. The magnetic meas- urements suggest the  multiple magnetic transition in La2MnNiO6. The high tem- perature ferromagnetic transition with TC~280  K associated with the  cationic ordered ferromagnetic superexchange in- teraction Ni2+ – O – Mn4+ becomes weaker by replacement of Mn4+ by nonmagnetic Ti4+. There is  a  gradual shift in  TC with increase in Ti doping eventually leading to  a  paramagnetic state in  La2TiNiO6. 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