Received: 23-March-2022 Manuscript No. JPAP-22-58269-; Editor assigned: 25- March-2022 Pre QC No. JPAP-22-58269(PQ); Reviewed: 08-April-2022, QC No. JPAP-22-58269; Revised: 12-April-2022, Manuscript No. JPAP-22-58269(A) Published: 15-April-2022, DOI:10.4172/2320-2459.10.3.004
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Double Perovskites (DP) materials with the rare-earth based compound is the most and widely studied due to its various fascinating properties such as magnetic, structural, electrical and optical properties. In this article, we comprehensive reviewed the structural as well as magnetic properties of A2XMnO6 (A=La, Nd and X=Cr, Ni, V) double perovskite compounds. It has been found that the Rare-Earth Based Double Perovskite (REBDP) compounds exhibits different structural and magnetic properties by changing the synthesis methods. Moreover, magnetic field and temperature affect the electronic and magneto electric characteristic of REBDP. These changes occur because of different exchange interactions such as super exchange interaction and double exchange interaction. Therefore, this review helps to exaggerate the new properties of these materials by introducing another synthesis method. In addition, a wide window is open where the ions at A site and X site can be varied to explore the new materials which are environment friendly and energy efficient.
Perovskite compounds ; Dielectrics; Metal elements ; photo detector
At present, double perovskite compounds bring so much attention because of their fascinating properties [1-4]. Perovskite materials are skillfully investigated for a considerable range of gripping properties. It is well known that single perovskite materials are very flexible in their structural and compositional property . To extend the properties of perovskite material, the standard way is to substitute the cation place by other cations. Figure 1 predicts that if we merge two units of perovskites, then we obtain A2B2O6 type double perovskites compounds in which half of B site ions is replaced by B'. Due to this substitution and arrangement between B and B' with corner-sharing BO6 and B'O6 octahedral units double perovskite structure A2BB'O6 is acquired. A site cation occupies every hole created by BO6 and B'O6 octahedra, as demonstrated in Figure 1. In A2BB'O6 structure B and B' are transition metal elements and A is alkaline material. Cation at A site is 12-fold oxygen coordinated while B and B' cations are 6-fold oxygen coordinated . When Double Perovskite compounds, electronic properties dominantly affected by cations at B-site. It is not surprising that the variety of properties like metallic, half-metallic, semiconducting, dielectrics, thermoelectric, ferroelectric and superconducting are found in La2NiFeO6, Sr2CoIrO6 and Sr2FeMoO6 compounds [7-9].
Some of A2BB'O6 have been studied to show metal-insulator transition, such as Sr2CoTiO6 has a transition temperature of about 700 K . Half-metallic properties of double perovskite materials are useful in spintronic devices. Some A2BB'O6 ompounds show magneto resistance behavior, even though they were not half-metallic such as La2CoMnO6 [11,12]. Since most of the A2BB'O6 perovskites are insulators that can be studied to enhance the properties for microwave applications. Few layered A2BB'O6 perovskites such as Sr2Y (Ru1-xCux) O6 with high Curie temperature (Tc), shows superconductivity . Low thermal conductivity reported in Sr2CoReO6 which results thermoelectric and photo catalytic properties in the material [14,15]. These materials show a variety of magnetic properties by intr13oducing paramagnetic cations at all three sites. Magnetic characteristics rely on the spatial orbital overlap of different cations. A2BB'O6 comFCC) lattices, which are responsible for AFM in these materials . A2BB'O6 such as R2B'MnO6 where B'=Co, Ni and R stands for rare earth ions shows ferromagnetic supepounds show different magnetic properties such as Anti Ferro Magnetic (AFM) , Ferro Magnetic (FM) , spin-glass behavior  and ferrimagnetism . In A2BB'O6 , B and B' forms Face Centered Cubic (r exchange interaction between two B-site cations that are highly ordered structurally distorted [21-23]. Sr2NiUO6 shows ferromagnetic nature in which cause of magnetic ordering is not only super exchange interactions but also the moving electrons. Double perovskite showing ferrimagnetism can further splits into two groups; first for which Tc>300 K are mainly metallic or half metallic and second for which Tc<200 K are generally insulating. Spin flop or meta-magnetic transitions are also found in Sr2NiReO6, Nd2LiRuO6 and Sr2YIrO6. Reason of this transition is spin reorientation caused by an external magnetic field.
Recent studies demonstrated that these compounds show multi ferocity, such as in Pb2FeTiO6 . The synchronicity of ferroelectric and ferromagnetic behavior at room temperature makes these materials applicable for spintronic devices. Some A2BB'O6 perovskite such as Ba2PrBiO6 are found as electrode materials which are used in solid oxide fuel cells . Halide-based double perovskite material can be used for optoelectronic applications like X-ray detector, photo detector, light-emitting diode, photocatalyst and solar cell . Since the last few decades, much research has been dedicated to double perovskite material [27,28]. In this review article, we have mainly discussed A2BB'O6 type double perovskite materials where A site ions can be rare-earth element (La, Nd, Sr) and B and B' site ions can be transition metal element (Cr, Ni, Mn, V). Our main aim is to exaggerate the engrossing magnetic properties and structural properties of these materials. Its electronic arrangement vacillating from insulating to metallic and half-metallic and magnetic ordering range vary from anti-ferromagnetic to ferromagnetic within a single material as we change the synthesis process of the material [27,33]. Above all, the thorough literature survey so far allows us to predict new environment friendly A2BB'O6 compounds yet to be synthesized and to find the correlation between electrical, magnetic, optical and structural characteristics.
Schematic of La2CrMnO6
Double Perovskite material La2XMnO6 (X=Cr) show magneto dielectric, magneto capacitance, and magneto resistance properties, making them suitable for magneto electric devices . Literature review suggests that La2CrMnO6 (LCMO) differs remarkably in the electrical and magnetic properties using different synthesis conditions. These differences arise due to change in crystal structures, which are related to the electrical and magnetic properties. Hence, synthesis condition directly affects electrical and magnetic properties [35-37]. The previous report predicted that synthesis of LCMO material by ceramic technique had a monoclinic phase and deformation from ideal cubic symmetry . While LCMO synthesized by combustion method, show rhombohedra structure with space group R3c. Then, Palalkal, et al. used the combustion method to synthesize LCMO, where citric acid was used as fuel. The observed structure was orthorhombic with space group Pbnm. Thus, in general researchers reported the structure of La2CrMnO6which was orthorhombic with space group Pbnm.
Magnetic exchange interactions have been studied extensively in LCMO. Sun et al. and Borrozo et al. acquired that the magnetism in LCMO is originated because of double exchange interaction of Cr+3-O-2-Cr+3. Karpinsky et al.  reported that reason of ferromagnetic behavior in LCMO is Mn+3-O-2-Mn+3 super exchange interaction and anti-ferromagnetic behavior is due to negative Cr+3-O-2-Cr+3 interaction [40-45]. Yang synthesized LCMO by solid-state reaction method with precursors as La2O3, MnO2; Cr2O3 mixed them in ethanol for 6h and dried it for 12 h at 353 K.
Structural results of solid state synthesized LCMO  as shown in Figure 2 reveal that the crystal structure of LCMO is orthorhombic with space group Pbnm and lattice parameters are a=5.5224(2)Å, b=5.4800(2)Å, c=7.7680Å. Magnetic nature of LCMO sample prepared via solid state reaction  was done by Magnetic Property Measurement System (MPMS). ZFC and FC magnitudes were accomplished in temperature extent 2 K to 400 K, Yang reported that material showed ferrimagnetic or ferromagnetic transition at a temperature ~118 K, as shown in Figure 3. Further, the magnetic hysteresis loop curve was traced out at various temperatures (2 K, 32 K, 112 K, 152 K, and 212 K) to confirm the actual magnetic behavior of the sample. Yang confirmed ferromagnetic behavior in LCMO.
Schematic of La2NiMnO6
In double perovskites compounds, La2NiMnO6 (LNMO) is largely studied due to its unique and stable ferromagnetic insulating phase. Its Curie temperature (Tc) is nearly about room temperature. LNMO is widely used in many potential devices like sensor, memory device, radio frequency filter and phase shifter [47,48]. Due to various electronic interactions such as Ni+2-O-Mn+4 interactions, this compound is still open for researchers to design multipurpose functional materials. It is found that LNMO shows ferromagnetism through Mn+4-O-Ni+2 interchange interaction .
In contrast, Mn+4-O-Mn+4 and Ni+2-O-Ni+2 interchange interaction leads to antiferromagnetic in LNMO. So, these interactions affect the overall magnetic properties of LNMO . LNMO was synthesized through solid-state reaction at temperature ≥723ºC, found coexistence of monoclinic phase (P21/n) and rhombohedra (R3m) structure [51-60]. According to the research done so far, the ferromagnetism in this compound is because of the Mn+3-O-Ni+3 super exchange interactions. In contrast, another report predicted that the main reason of ferromagnetism is Ni+2-O-Mn+4 super exchange interactions [61-67]. Earlier report suggested that the changes in atmosphere and annealing temperature affect the magnetic properties and structure of LNMO.
In LNMO, the anti-site cation disordering took place because of the resemblance in ionic radii of Mn and Ni cations, which makes this compound magnetically complicated [68-73]. Although synthesis of LNMO at low temperatures was a challenge, Vishwajit Gaikwad synthesized the LNMO nanoparticles via the co-precipitation method. They used two different solvents, ethanol and water . Single LNMO nanoparticles were obtained at claimed temperature 600°C using La(NO3)3.6H2O, MnCl2.4H2O and Ni(NO0)2.6H2O precursors in presence of aqueous ammonia NH4OH as precipitating agent. In order to understand the effect of solvent agent structural and magnetic properties were reported in both water and ethanol as solvent. On further calcination above 1000ºC leads to the construction of the crystalline LNMO monoclinic phase. The result obtained from powder X-ray technique shown in Figure 4, shows that the as synthesized LNMO-E is an amorphous material. Rietveld refined data obtained through XRD of LNMO-W (600ºC, 6 h) are relevant to single phase of monoclinic (P21/n) shown in Figure 4.
The magnetic characteristics of as synthesized material revealed ferromagnetic behavior whose origin is Ni+2-O-Mn+4 super exchange interactions. Although, only one ferromagnetic transition was observed, this suggests that the material is in single-phase as shown in Figure 5.
Schematic of La2VMnO6
The La2VMnO6 (LVMO) compound in double perovskites has excellent potential because this material was theoretically predicted to be half-metallic anti-ferromagnetic. Half metallic materials were first placed in light by De Groot et al. [75-81], which showed metallic performance for one spin direction and insulating or semiconducting for another one. This type of material is applicable in spintronic devices. These materials offer a unique possibility that the state has a significant spin polarization but vanishes macroscopic magnetic moment and may be used as an exotic superconducting state, single spin superconductivity [82-85]. As reported earlier, LVMO synthesized by the art-melting method  shows the cubic structure with ferrimagnetism, which contrasts with earlier predicted AFM. S. Chakraverty et al. synthesized the La2VMnO6 thin-film by Pulsed Laser Deposition (PLD) technique on SrTiO3 (STO) material [87,88]. XRD data reported lattice parameter of LVMO film 3.919 Å as compared to theoretically predicted lattice parameter which is 3.89 Å. The variation observed in lattice parameter was 0.75% which results due to film grown on STO [87,88]. Figure 6 shows the XRD pattern of LVMO film. LVMO film showed potential magnetic properties as shown in Figure 7.
Magnetic properties shown in Figure 7 revealed FC and ZFC measurements. These observations propound that the temperature associated with the peak is of the order of anti-ferromagnetic temperature. Neel temperature calculated from the rise of FC and ZFC was reported at 21 K. Also reported that ground state of the material is ferromagnetic. This result is confirmed by magnetic measurements of the material with Mn+3 at high spin state.
Schematic of Nd2NiMnO6
Double perovskites materials have unique characteristics, i.e., double perovskite materials change their properties when rare earth elements replace A site ion with smaller ionic radii. For example, in LNMO material, La+3 ions are replaced by Nd+3. Many properties of this material vary, and a new class of compounds is obtained. Nowadays many researchers are taking more interest in this type of material due to their various applications such as magneto electric coupling, magneto caloric effect and magneto capacitance . Replacement of La+3 ions by Nd+3 ions causes the variation in Mn-O-Ni bond angles. Super exchange interaction strength is decreased through this replacement [89,90], which shows that magnetic transition temperature shrinkages monotonically as a decrement in ionic radius of ions. In Nd2NiMnO6, two types of magnetic transitions have been reported [91-94].
At high temperature, it shows ferromagnetic transition and at lower temperature magnetic anomaly. The low temperature transition was expected to be originated through Ni+3-O-Mn+3 super exchange interactions. In contrast, super exchange interactions between Mn+4 and Ni+2 get originated from high-temperature ferromagnetic transition at about 195 K. The anti-site disordered-ness in super exchange interaction between Mn+4-O-Mn+4 and Ni+2-O-Ni+2 ions results in the anti-ferromagnetism. Another possible interaction responsible for anti-ferromagnetic evolution is occurred by Ni+3(Mn+3)-O-Ni+3(Mn+3) interaction, and ferromagnetic interaction is occurred by Ni+3-O-Mn+3 interactions [95,96]. These kinds of anti-site disordered-ness arise because of synthesis conditions like annealing temperature and time. Insufficient annealing time can disorder B and B' site ion interaction. Cederv et al. synthesized the sample Nd2NiMnO6 by solid-state reaction method using Nd2O3, NiO and Mn2O3 as precursors. Ball milled in zirconia medium for 4 h at 295 MPa pressure, then sintered at 1223 K, 1373 K, 1523 K and 1623 K for 12 h [97,98]. XRD characterize phase identification and pureness of powder materials at 295 K. XRD pattern of Nd2NiMnO6 is either an orthorhombic Pnma space group or a monoclinic P21/n as shown in Figure 8. Figure 9 shows below FC and ZFC graphs, which show the ferromagnetic interaction below 200 K. Here, Mn+4-O-Ni+2 interactions are reason of ferromagnetic ordering. Low temperature behavior is the same as previously reported. Nearly at 100 K anti-site disorder took place (Ni+2-O-Ni+2 / Mn+4-O-Mn+4 bonds) [99,100].
In addition, its sister compound La2NiMnO6 shows interesting magnetic properties and crystal structure. From the detailed study of this material we got to know that Ni+2-O-Mn+4 electronic interaction is the reason of arising enthralling magnetic properties in this material like ferromagnetism, magneto resistance, magneto capacitance etc.
When La2NiMnO6 nanoparticles reported by co-precipitation method at low temperature with monoclinic phase structure. Its magnetic study shows that the nanoparticles are anti-ferromagnetic in nature. The reason of this behavior is anti-site disordered-ness between Mn-O-Mn (Ni-O-Ni) octahedral i.e., Mn+4-O-2-Mn+4, Ni+2-O-2-Ni+2 super exchange interaction. Another similar family material La2VMnO6 is a half-metallic compound. The theoretically predicted structure of La2VMnO6 is an essential perovskite unit cell of substance. When La2VMnO6 deposited on STO film, its lattice parameter increases, more significant by 0.75%, and magnetic properties are anti-ferromagnetism and ordering of spin magnetic moment acquired with Mn+3 at high spin state. In order to understand the changes in parental properties of double perovskite, a site cation was replaced by Nd and studied thoroughly by researchers. Nd2NiMnO6 synthesized through solid-state reaction method reported monoclinic P21/n crystal structure and the magnetic system is ferromagnetic at high temperature and antiferromagnetic at low temperature.
Reported structure and magnetic behavior of all the above reviewed materials with different synthesis methods is concluded in Table 1.
|Solid state reaction||
|Solid state reaction||
|Ethylene glycol gel method||
|PVA sol gel method||
|La2VMnO6||Theoretical Method (Generalized gradient approximation)||
|Pulsed laser deposition||
|Nd2NiMnO6||Solid-state reaction method||
Table 1. Reported structure and magnetic behavior of A2XYO6 with different synthesis methods.
In this review, we briefly reviewed the current research on double perovskite materials covering mainly structural and magnetic properties. B-site cation largely affects the properties of double perovskite materials. As we have discussed that the substitution at A and B site ions affects the magnetic and structural characteristics. It was also reviewed that different synthesis methods affect magnetic and structural characteristics of double perovskites because change in synthesis method changes the type of exchange interaction. Magnetic nature of pulse laser deposited La2CrMnO6 material suggests that the ground state of La2CrMnO6 is ferrimagnetic. Whereas La2CrMnO6 synthesized by solid-state reaction method, an orthorhombic Pbnm crystal structure is acquired as soft magnetic substance with a minor coercive field. Magnetic behaviour is attributable to Cr+3-O-2-Mn+3 exchange interaction. La2CrMnO6 prepared by sol gel acquires orthorhombic structure with the Pbnm group.