A Review on Recent Advancements in Rare-Earth based Double Perovskite Compounds

Abstract

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.

Keywords

Perovskite compounds ;  Dielectrics; Metal elements ;  photo detector

Introduction

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 [5]. 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 [6]. 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].

Figure 1: Structure of double perovskite material from single perovskite unit material.

Some of A₂BB'O₆ have been studied to show metal-insulator transition, such as Sr₂CoTiO₆ has a transition temperature of about 700 K [10]. 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 (Ru_1-xCux) O₆ with high Curie temperature (Tc), shows superconductivity [13]. 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 [20]. 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) [1614], Ferro Magnetic (FM) [17], spin-glass behavior [18] and ferrimagnetism [19]. 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.

Literature Review

Recent studies demonstrated that these compounds show multi ferocity, such as in Pb₂FeTiO₆ [24]. 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 [25]. Halide-based double perovskite material can be used for optoelectronic applications like X-ray detector, photo detector, light-emitting diode, photocatalyst and solar cell [26]. 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 [34]. 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 [38]. 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. [39] 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 [46] 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 [46] 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.

Figure 2: XRD result of La₂CrMnO₆. Note:

Figure 3: A. FC and ZFC of La₂CrMnO₆ at 2 K–400 K. Inset is inverse susceptibility and temperature. Note: B. Resistivity versus temperature curve without applying a magnetic field; C. Magnetization (M) versus applied magnetic field (H).

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 [49].

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 [50]. 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 [74]. 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.

Figure 4: A. XRD pattern of LNMO synthesized in ethanol media; B. XRD pattern of LNMO synthesized in water media; C. XRD pattern of refinement of LNMO Nanoparticles.

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.

Figure 5: Magnetization curve at a constant temperature at 10 K and temperature varying magnetization curve.

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 [86] 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.

Figure 6: XRD pattern of LVMO sample.

Figure 7: A. Magnetization curve at const. temperature at 5 K; B. FC and ZFC magnetization with varying temperatures.

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 [89]. 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].

Figure 8: XRD pattern of Nd₂NiMnO₆ where A. shows P21/n.

Figure 9: Curve of Magnetization versus temperature and applied magnetic field. Note:

Discussion

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.

Table 1. Reported structure and magnetic behavior of A₂XYO₆ with different synthesis methods.

Conclusion

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.

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