Insights into electronic and optical properties of AGdS2 (A = Li, Na, K, Rb and Cs) ternary gadolinium sulfides
Introduction
The ternary rare-earth sulfides ALnS2 (A = alkali metals, Ln = rare earth elements (RE)) are materials worthy of attention considering from the perspective of both the theory and technology, due to their excellent physical properties in cathode luminescence, electroluminescence and photoluminescence applications as phosphors or scintillators [[1], [2], [3], [4], [5]]. In recent years, it can be found that the solid-state lighting phosphors have pivotal roles in white light-emitting diode applications, and luminescence behaviors of rare-earth-doped ternary sulfides were further studied [[5], [6], [7], [8]]. These ternary rare earth metal sulfides were known as semiconductors, luminescent emitting and energy storage materials as early as the 1960s, and their structures were determined by the method of X-ray powder measurement [[9], [10], [11], [12]]. ALnS2 sulfides can mostly crystallize in two kinds of crystal structures based on different Ln3+/A1+ ionic radius ratio [[13], [14], [15]], which are the cubic NaCl-type structure with Fmm space group corresponding to a tiny ratio [16,17] and the trigonal α-NaFeO2 type structure with Rm space group corresponding to the larger ratio [18], respectively. Therefore, to some extent, it is possible to start from these features above to change their physical and chemical properties by modifying the crystal structure of these sulfides and thus achieve the purpose of modulating their precursor properties.
Moreover, as an important branch of metal sulfides, the ternary rare-earth (RE) sulfides can be used in nonlinear optical (NLO) devices owing to their variety of physical properties such as magnetic, low-temperature superconductivity, thermoelectricity, photoelectricity [19]. Among these ternary sulfides, the alkali metals gadolinium sulfides AGdS2 (A = Na, K, Rb and Cs) have unique characteristics of excellent magnetic and optical properties and can be applied to refractory materials, lasers, IR windows, phosphors, and solid-state lightings [20,21]. Recently, considerable literature has grown up around the theme of AGdS2, one of which conveys that AGdS2 with excellent transmittance in a wide wavelength range (>430 nm) can be used in infrared window materials [20]. Currently, the investigation on AGdS2 (A = Na, K, Rb and Cs) sulfides is primarily focused on the assessment of structural and optical properties. It has been confirmed that the emission intensity of KGdS2 is about 40 times higher than that of the Bi4Ge3O12 (BGO) standard [5]. Compared with oxide-based materials, RE3+ doped RbGdS2 has numerous inherent benefits of X-ray phosphors, such as its effective atomic number (Zeff = 52) is significantly increased, the energy transfer from the host is more efficient, and the band gap is lower [6]. However, the content on physical performances of AGdS2 (A = Li, Na, K, Rb and Cs) sulfides such as electronic and optical properties calculated had been largely under-researched.
As is known, the research on photovoltaic and photochemical devices is a manifestation of the fact that the most abundant renewable energy is solar energy [22]. These materials should have the ability to absorb light, because they need to go through the necessary procedures to energy conversion from light to other forms such as chemical energy or electrical energy, including light absorption, exaction dissociation, and charge carrier diffusion [23]. Therefore, the optical properties of these materials play the vital roles in the comprehension of their physical properties. Furthermore, materials are regularly required to have an optimal band gap between 1.4 and 3 eV (visible spectrum) when applied to photovoltaics or photochemistry [[24], [25], [26], [27]], and direct or indirect energy gaps have significantly different effects on the type and nature of the electron jumps [28,29]. Besides, the systematic study of electronic properties is essential, because to some extent the electronic structure is fundamental to the optical properties. As is known, the first-principles calculations based on density function theory (DFT) are very beneficial for theoretically predicting physical properties, and have been extensively used in the theoretical studies of solid physical properties [[30], [31], [32], [33], [34], [35], [36], [37]]. In this work, the structural, electronic and optical properties of AGdS2 (A = Na, K, Rb and Cs) are determined systematically using the first-principles calculations. We hope that these predictions of AGdS2 (A = Na, K, Rb and Cs) ternary gadolinium sulfides can provide the theoretical basis for future synthesis and application of these sulfides.
Section snippets
Computational methods
The first-principles calculation based on DFT [38] was executed to explore the electronic and optical properties of AGdS2 (A = Li, Na, K, Rb and Cs) ternary gadolinium sulfides using the CASTEP code [39] in this work. The valence electrons-ionic core interaction was described by the ultra-soft pseudo-potentials (USPPs). The exchange-correlation energy was characterized by the generalized gradient approximation (GGA) within the spin-polarized local density approximation (LSDA) [40]. The
Structural properties and phase stability
The crystal structures of AGdS2 (A = Li, Na, K, Rb and Cs) sulfides are shown in Fig. 1. AGdS2 sulfides crystallize in the α-NaFeO2 layered type structure with trigonal Rm space group. Here, A atoms situate at 3a (0, 0, 0) site, Gd atoms locate at 3b (0, 0, 0.5) site, and S atoms are in 6c (0, 0, z) site (where z represents 0.2497, 0.2574, 0.2346, 0.2309, and 0.2274 for LiGdS2, NaGdS2, KGdS2, RbGdS2, and CsGdS2, respectively). This type of crystal structure is normally mentioned as the “O3”
Conclusions
In this work, the structural, electronic and optical properties for AGdS2 (A = Li, Na, K, Rb and Cs) ternary gadolinium sulfides were systematically investigated using the first-principles calculation. The structural parameters optimized are basically consistent with the reported literature data. Based on the predicted formation enthalpy, the phase stability of AGdS2 can be ranked as LiGdS2 > NaGdS2 > KGdS2 > RbGdS2 > CsGdS2. Strong covalent Gd–S bonds originated from the Gd-d and S-p
CRediT authorship contribution statement
Ying Wu: Investigation, Data curation, Writing – original draft. Xinyu Wang: Visualization. Yong Wang: Validation. Yonghua Duan: Methodology, Supervision. Mingjun Peng: Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Yunnan Ten Thousand Talents Plan Young & Elite Talents Project under Grant no. YNWR-QNBJ-2018-044, and the National Natural Science Foundation of China under Grant no. 51761023.
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