**1. Introduction**

Existing studies on the family of manganites perovskites, RMnO3 as a parent of mixed valence perovskites, where R is made up of rare earth elements, such as La, Pr and Nd, were found a few decades ago due to their exotic behavior in electrical and magnetic properties [1,2]. Manganites perovskites were the favorite amongs<sup>t</sup> researchers compared with other families of oxides because of their various possible substitutions that may change their crystal structure and properties [3]. Since the rediscovery of colossal magnetoresistance (CMR), many researchers have been focusing on the doped compound manganites due to their fascinating basis in physics and significance in technological importance, such as magnetic sensors [4]. Few theories have surfaced to explain the CMR mechanisms, such as double exchange (DE), superexchange (SE), Jahn-Teller (JT) effect, polaronic effects and so on.

Primarily, undoped NdMnO3 is an antiferromagnetic (AFM) insulator with Néel temperature 67.2 K due to the interplay between the ferromagnetic (FM) DE and AFM SE interactions present in the compound [5]. Chatterji et al. confirmed that NdMnO3 has an A-type AFM structure of the Mn magnetic sublattice below Néel temperature, TN ≈ 81.7 K [6]. When doped with divalent cations on the Nd sites, for example in Nd1−<sup>x</sup>CaxMnO3, FM semiconducting behavior was observed with x ≤ 0.25. However, when x > 0.80, the compound exhibited AFM semiconducting at low temperatures [7]. For Nd0.5Ca0.5MnO3, the compound undergoes a charge ordering (CO) transition at TCO = 250 K and the AFM ordering transition at TN = 160 K, which is consistent with the study done by Wu et al. [7,8]. In this case, both authors suggested that the presence of CO was due to the JT distortion, resulting in the distortion of MnO6 octahedral and localization of charge carrier, by decreasing its mobility, which seemed to weaken the DE interaction [7–9]. Meanwhile, when NdMnO3 was doped with monovalent cations such as Na<sup>+</sup> and K<sup>+</sup>, it showed an FM insulator behavior and CO transition was observed for Nd1−<sup>x</sup>NaxMnO3 at −250 K when x > 0.1 [5,10]. However, a lack of knowledge still exists on Nd series doped with trivalent cations such as Nd1−<sup>x</sup>BixMnO3.

Previous studies reported that although Bi3<sup>+</sup> (1.24 Å) and La3<sup>+</sup> (1.22 Å) have almost similar ionic radii, both systems exhibit di fferent structures and properties [11]. LaMnO3 exhibited an AFM insulator [12]*,* while BiMnO3 exhibited an FM insulator with TC of 105 K–110 K [11,13,14]. Recent studies on bismuth-doped manganites, such as La0.7−<sup>x</sup>BixAg0.3MnO3 [12], La1−<sup>x</sup>BixMnO3 [13] and La0.67−<sup>x</sup>BixSr0.33MnO3 [15]*,* showed a decreased metal–insulator transition temperature, TMI, with an increase of Bi content. The author suggested that the behavior of the latter was related to possible hybridization of Bi 6s<sup>2</sup> lone pairs and eg electron of Mn3+, which could have a ffected the localization of carriers and weakened the DE interaction. Moreover, Bi substitution may also reduce the Mn–O–Mn angle, which can block the movement on eg electron from Mn3<sup>+</sup> to Mn4+, which decreases the mobility of conduction electron and weaken the DE process [2,12].

Several works were reported on Bi substitution, especially in La-based system, but a lack of study on Bi substitution in Nd-based system still exists, especially for optical and magnetic properties. In this study, we focus on reporting the structural, optical and magnetic properties of Nd1−<sup>x</sup>BixMnO3 to investigate the unique role of Bi in the system.

#### **2. Materials and Methods**

Polycrystalline samples for Nd1−<sup>x</sup>BixMnO3 (x = 0, 0.25 and 0.50) were synthesized by using conventional solid state reaction method. High purity (≥99.9%) neodynium oxide (Nd2O3), bismuth oxide (Bi2O3) and manganese oxide (Mn2O3) powders were weighed in stoichiometric ratio using electronic balance and mixed together. Agate mortar and pestle were used to grind the powders for about 2 h before calcination in air at 700 ◦C for 24 h, repeating twice in order to eliminate volatile foreign particles. Finally, the obtained powders were then pressed into pellet under a pressure of 5 tons and were sintered at 1100 ◦C for 24 h.

The structure and phase purity of the samples were observed by using the X–ray di ffraction technique (X'pert PRO MPD) at room temperature with Cu–Kα (λ = 1.5418 Å) radiation by using a PANanalytical di ffractometer. The XRD data were analyzed using the Rietveld refinement method using several software programs, such as General Structure Analysis System (GSAS) and Graphical User Interfaces (EXP–GUI) for the refinement and CMPR to convert data from ASC file to DAT file. Fourier transform infrared spectroscopy (FTIR) results were recorded using FTIR–Raman Drift Nicolet 6700 in the range of 450–2000 cm<sup>−</sup><sup>1</sup> to directly probe with the functional group present in perovskite samples. Samples were thoroughly mixed in KBr before the characterization. LEO model 982 Gemini Field Emission Scanning Electron Microscopy (FESEM) with energy-dispersive spectroscopy (EDX) were used to determine the surface morphology and homogeneity of the perovskite samples. AC susceptibility ( χ') measurements were carried out using a Stanford research model SR-7265 lock-in amplifier and were performed in the temperature range from 30 K to 300 K.
