Electronic and optical behaviour of anisotropic Nd₂NiMnO₆ double perovskite: A first principle study

Highlights

• The manuscript highlights the below mentioned novel ideas in the research field of materials science.

  • Band structure and DOS studies suggest the semiconducting nature of material.

  • The material exhibits anisotropic behavior in case of dielectric properties as well as susceptibility.

  • High value of static dielectric constant suggests the use of the material in low frequency oriented devices.

Abstract

Double perovskites are interesting in the field of material sciences because of their multifunctional properties. Different researches have been done and several investigations have been carried out to explore these compounds. Here in this paper, we have explored one of the less investigated compounds i.e., Nd₂NiMnO₆. Although few experimental studies have been made and some results have been pointed out, still we are going to explore some properties of Nd₂NiMnO₆ using Density Functional Approach. Band structure and density of state (DOS) studies of the material in spin up and spin down channels disclose the semiconducting behavior of Nd₂NiMnO₆. The variation of dielectric constant with frequency and the variation of susceptibility with wavelength have also been presented with the explainable trends. The results also reveal the anisotropic nature in dielectric function and susceptibility of the compound.

Introduction

Perovskites ${\text{ABO}}_3$ and their derivatives double perovskites A₂B´B´´O₆ are of great interest because they show a wide spectrum of properties and features like magnetism, ferromagnetism, magnetoresistance, magnetocoupling, magnetocapacitance, magnetocaloric effect, etc [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]]. In order to understand the physical properties of any compound the positions of the ions are important. Likewise, in double perovskites A₂B´B´´O₆ the B´ and B´ cations determine and give interpretations to their properties [11,16]. The stability of structure of double perovskites is determined by tolerance factor which is defined by the equation [17,18].

$$\text{t}=\frac{{\text{R}}_{\text{A}}+{\text{R}}_{\text{o}}}{\sqrt{2}\left\{\left(\frac{{\text{R}}_{{\text{B}}^{\text{′}}}+{\text{R}}_{{\text{B}}^{\text{″}}}}{2}\right)+{\text{ R}}_{\text{o}}\right\}}$$

where R_A, R_B′, R_B″ and R_o are ionic radii of A, B´, B´´ and O ions respectively.

The most common compound investigated in the family of these compounds is La₂NiMnO₆. When La is replaced by any other element, it will cause a change in ionic radii and hence change in properties [19]. The study of structural and dielectric properties are of utmost use for manufacturing electronic devices [18]. There are three types of B-cation sublattice in double perovskites [16]. Of the three Nd₂NiMnO₆ (NNMO) has been found to follow rock salt ordering [20] where Nd³⁺ ions are situated at interstitial sites formed by octahedra of Ni²⁺O₆ and Mn⁴⁺O₆. This is depicted by Yang [21].

As far as experimental results are concerned, the NNMO compound can be synthesized by either of the two modes of synthesis. These two modes of synthesis are solid state route [22] and Sol gel route [23]. Synthesis conditions are important for structure and disorder in the NNMO [24]. The study of XRD pattern shows that the compound can be refined either by orthorhombic Pbnm space group or monoclinic P2₁/n [24]. This is confirmed by following mean:

• Tolerance factor, for NNMO is less than 0.9, so possibility is monoclinic or orthorhombic [25].

• The existence of ordered arrangement of Nd²⁺ and Mn⁴⁺ suggest the monoclinic symmetry while as disordered arrangement in which oxidation state of both Ni and Mn is 3+ involves orthorhombic symmetry [26]. Since the study show the ordered alignment of Nd²⁺ and Mn⁴⁺ in NNMO so it can be conclude that NNMO has monoclinic symmetry [27]. The XPS study of NNMO also gives clue about the Mn cationic state [26].

The lattice parameters of NNMO have been determined by many researchers [21,24], however Booth et al. [28], where repeated grinding and heat treatment is ensured for achieving ordering determined the parameters as: a = 5.4145(9)Å, b = 5.4842(1) Å, c = 7.6742(1) Å, β = 90.011(2)⁰, and V = 277.883(9) ų. The TEM Study has confirmed the uniform particle size of the NNMO ~100 nm [22].

Talking about the dielectric properties of NNMO, we obtain, from the dependence of dielectric constant on temperature at different frequencies, that NNMO compound is associated with a giant dielectric relaxor phenomenon [25]. Dielectric relaxation is the process in which the maximum of the graph representing dielectric constant versus temperature, shifts towards increasing temperature with rise in frequency. Such a property is found in relaxor ferroelectrics which obey modified Curie-Weiss Law [25]. This relaxation behavior is defined by two equations defining two parameters [26].

$$△{\text{T}}_{\text{diff }}={\text{T}}_{0.9}{\epsilon }_{\text{max}}\left(100\text{ Hz}\right)-\text{T}{\epsilon }_{\text{max}}\left(100\text{ Hz}\right)$$

$$△{\text{T}}_{\text{relax}}=\text{T}{\epsilon }_{\text{max}}\left(100\text{ KHz}\right)-\text{T}{\epsilon }_{\text{max}}\left(100\text{ Hz}\right)$$

where T_0.9ε_max (100 Hz) temperature at 90 % of maximum of dielectric constant towards high temperature side. Tε_max(100 kHz) is the temperature at maximum dielectric constant when frequency is 100 kHz and Tε_max (100 Hz) the temperature at maximum dielectric constant when frequency is 100 Hz.

The frequency dispersion is also defined and described by Vogel-Fulcher equation [26] that is defined by:

$$\text{f}={\text{f}}_0\text{ exp}\left\{-\frac{{\text{E}}_{\text{a}}}{\text{k}\left({\text{T}}_{\text{m}}-{\text{T}}_{\text{f}}\right)}\right\}$$

where f₀ is Debye frequency, E_a is the activation energy, k is Boltzmann constant, T_m is temperature corresponding to maximum dielectric constant and T_f is temperature corresponding to static freezing.

As far as magnetic properties are concerned, they have been discussed in literature at a very vast domain. NNMO is associated with multiple magnetic transitions. It undergoes paramagnetic to ferromagnetic state below 200 K, shows a spin-glass behavior at around 100 K, a downturn in magnetization when temperature is decreased below 50 K [27] and also undergoes magnetic transition below 6 K to a ferrimagnetic state. Antisite disorders also influence the magnetic properties of this compound by reducing the long range order [23].

Density functional theory(DFT) [29] has been emerged as the most curious tool in the explanation of various properties of matter and can be regarded as theoretical microscope. With the Density functional theory, we can approach beyond the experimental limits and abilities. Taking into account the various importance of these compounds we have presented the band structure, density of state, dielectric properties and susceptibility of NNMO that have been obtained using density functional approach. It should be a noted point that up to our knowledge from the literature survey, these results pertaining to band structure for spin up and spin down channel, and tensor components of dielectric constant and susceptibility have not been reported to distinguish our results. However, we have tried to compare our results with the experimental data which is available in literature of NNMO and with other compounds in this family of double perovskites (R₂NiMnO₆; R = rare earth). The various outcomes on the structural and dielectric properties have been made and reported.