Ternary sulfides ALnS₂:Eu²⁺ (A = Alkaline Metal, Ln = rare-earth element) for lighting: Correlation between the host structure and Eu²⁺ emission maxima

Highlights

• Ternary sulfides ALnS₂ can be used for white LED applications.

• Structural properties of ALnS₂ ternary sulfides were modeled.

• Relations between Eu²⁺ emission maxima and structural properties were found.

• Criteria for getting Eu²⁺ emission in a particular spectral range were identified.

Abstract

Ternary sulfides ALnS₂ (A = Alkaline Metal, Ln = rare-earth element) doped with the Eu²⁺ ions are characterized by a very wide range of tunability of the Eu²⁺ emission maxima with the host’s chemical composition. In the present work, a detailed analysis of the structural properties of these materials was performed. It was shown that their lattice constants can easily be expressed as the linear functions of the ionic radii and electronegativities of the constituting elements. As a further step, the relations between the structural properties of the hosts and optical properties of the Eu²⁺ dopant were analyzed. It was demonstrated that the position of the Eu²⁺ emission maxima is a linear function of the so-called hexagonality ratio c/a, where c and a are the lattice constants of the rhombohedral ternary sulfides. The obtained results can be used as helpful guides for manufacturing Eu²⁺-doped ternary sulfides with desired emission properties with high potential for white LEDs applications.

Introduction

The Alkali-metal-Rare-earth-metal chalcogenides were firstly studied in 1960 s by the group of Ballestracci [1], [2], [3], originally for their structural properties. Similarly, in 1984 Sato and co-workers prepared and structurally characterized a series of the NaLnS₂ sulfides [4]. A flux method was employed to prepare higher quality KLaS₂ crystal [5]. Later on, another work describing these ternary sulfides arose in Germany in 1993 when Bronger and his colleagues managed to synthesize ALnX₂ (A = Alkaline Metal, Ln is a rare-earth element, X  = S, Se, Te) and determined their structural properties [6]. But with a few exceptions, however, no detailed physical characterization of these compounds was provided [7], [8], [9]. Some rare examples can be mentioned here, for example, the magnetic properties of LiLnS₂, LiLnSe₂, NaLnS₂ and NaLnSe₂ (Ln = lanthanides) were reported [10]. Similarly, the magnetic susceptibility of NaCeS₂ between 3.7 and 297 K was measured by the Faraday method [11]. The electrical conductivity and Hall coefficient measured by the van der Pauw method for NaGdS₂ and NaLaS₂ in a temperature range of 17–300 K were studied in Ref. [12]. The electrical conductivity was the highest for NaGdS₂ (7.75 × 10² and 11.2 × 10² S m⁻¹ at 17 and 300 K, respectively). In other works, NaLaS₂ is proposed as an interesting infrared window material or a fast ionic conductor [13], [14], [15]. The anisotropy of the g tensor of Yb³⁺ 4f¹³ ions in NaYbS₂ was investigated [16]. However, the intense interest in the optical properties of these compounds sparkled in 2011 when the scintillation and optical properties of rare-earth doped RbLaS₂ compounds were published by Havlák et al. [17]. Since then the ternary sulfides of a general formula ALnS₂, where A stands for any combination of the alkali metals and Ln for any combination of lanthanides or yttrium, have been experiencing a renaissance, which can be documented by constantly increasing numbers of experimental [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], structural [31], [32], [33] and theoretical works devoted to them [34], [35], [36], [37], [38], [39], [40], [41], [42]. Due to their favorable properties such as elevated density (5.18 g/cm³ for RbLuS₂ [26]) and effective atomic number (61.4 for RbLuS₂ [26]), efficient energy transfer from the host to the activator (the intensity of the Sm^{3+ 4}G_5/2 → ⁶H_5/2 emission under the X-ray excitation exceeds the Bi₄Ge₃O₁₂ standard scintillator by a factor of 40 [22]) and very high light yield exceeding 35,000 ph/MeV (KLuS₂:Eu²⁺ 0.05%) [20], [26], they may be considered as very interesting scintillators emitting in green (KLuS₂:Eu²⁺) [20] or yellow (KLuS₂:Ce³⁺) [23] spectral ranges. Another application of these compounds may be related to the white light emitting diodes (LEDs) – a very rapidly developing direction of modern research in the lighting industry [43], [44].

From this point of view, it should be emphasized that the Eu²⁺ 5d → 4f emission peak in these hosts may vary significantly with the chemical composition and therefore, the resulting emission spectrum can easily be tuned to reach various desired color correlated temperatures (CCT) as well as color rendering indices (CRI) or color quality scales (CQS) needed for the solid state white LED technology. To demonstrate the above-mentioned tuning capacities, we mention here that the Eu²⁺ emission is peaking at 498 nm in RbLuS₂ and at 776 nm in NaGdS₂ [24], at 521 nm for K_0.93Na_0.07LuS₂ and at 631 nm for K_0.16Na_0.84LuS₂ [27], [28] and at 567 nm for K_0.85Na_0.15YS₂, at 600 nm for K_0.88Na_0.12GdS₂ and at 555 nm for K₀.₉₂Na_0.08Lu_0.39Y_0.61S₂ [29]. Therefore, one can immediately see the variability of emission features in these ternary sulfides hosts [24], [27], [28], [29]. Furthermore, a broad absorption band of the Eu²⁺ activator in these hosts is situated at around 400 nm, matching well both near UV (~390 nm) and blue (~450 nm) pumping diodes, which are used for white LED lighting systems, see Ref. [28] for details. Using the Raman spectroscopy, it was found that the KLuS₂ phonon energy is E_ph ≈ 220 cm⁻¹, which is fully comparable to the best low phonon materials investigated nowadays for mid-IR lasers, such as KPb₂Cl₅, YCl₃ and others, see Ref. [30] for more details.

The structural, elastic and electronic properties of KYS₂ and KLaS₂ are examined in Ref. [34] where Becke, 3-parameter, Lee-Yang-Parr hybrid functional has been employed to obtain better accuracy in the electronic structure calculations. The calculated value of the band gap 3.815 eV for KLaS₂ [34] matches really well the experimentally obtained value 3.76 eV [24]. The generalized Gradient Approximation - Perdew-Burke-Ernzerhof (GGA-PBE) and Local Density Approximation (LDA) were also used to estimate the band gap, but it is well known that both LDA and GGA underestimate its value [45]. Similarly, the Vienna ab initio simulation package (VASP) with the projector-augmented wave (PAW) potentials was used to calculate the crystal and electronic structures of RE-doped KLuS₂ compounds [35]. Furthermore, designing promising co-doped KLuS₂:Ln/Ln′ compounds is carried out based on the total energy of KLuS₂:Ln/Ln′ and mutual energy levels of impurities, conduction band minimum and valence band maximum. In another theoretical work [36], the lattice parameters, unit-cell volume, band gap, the single-crystal elastic constants, total and partial density of states, the dielectric function, reflectivity and electron energy loss function of the AEuS₂ (A = Na, K, Rb) were calculated using the first-principle calculations. New magnetic semiconductors based on RbLnSe₂ (Ln = Ce, Pr, Nd, Gd) compounds are proposed on the basis of DFT calculations showing that RbNdSe₂ and RbGdSe₂ can be used in photoresponse applications [37]. Furthermore, if synthesized in the form of thin films they could be of great interest for spintronic applications [37]. The band gap energies of NaYS_2(1–x)Te_2x alloys as a function of Te concentration (results from Heyd-Scuseria-Ernzerhof screened hybrid functional calculations) are presented in Ref. [38] and interestingly, even if the pure compounds NaYS₂ and NaYTe₂ crystallize in the α-NaFeO₂-type trigonal structure with the space group 166 (R3̅m), the alloys NaYS_2(1–x)Te_2x (with x = 0.33 and x = 0.67) have on paper the lower triclinic symmetry P1 (space group 1) and are more attractive for photovoltaic applications [38]. It was shown that KMS₂ (M = Nd, Ho, and Er) are the ground state ferromagnetic compounds with large spin magnetic moments, so these materials can have potential applications in the field of magnetic semiconductors or for photovoltaic materials meant for space technology [39]. Transparent conductive materials are now essential components of many modern technologies as they are used as transparent electrodes for optoelectronic device applications (e.g., flat-panel displays and solar cells). KLaS₂, RbYS₂ and CsLaS₂ sulfides are studied theoretically for this purpose in Ref. [40].

It should be stressed out that the constant rapid development of computational facilities and progress in quantum-chemical calculations along with the advanced machine learning techniques and creation of large databases of functional materials made a profound impact on the smart search for new efficient phosphors. Thus, the Materials Project that is a core program of the Materials Genome Initiative should be mentioned first of all [46]. A critical analysis of the accumulated arrays of different materials-related data has already led to the discoveries of new phosphors. An analysis of 72 Eu²⁺-doped phosphors performed in Ref. [47] allowed to identify 32 descriptors related to certain properties of the studied compounds. By screening 2259 nitrides, a correlation between the width of the Eu²⁺ emission and electronic structures of the hosts was found in Ref. [48]. A full-visible-spectrum (i.e. emitting white light) Sr₂AlSi₂O₆N:Eu²⁺ phosphor was suggested in Ref. [49] after performing extensive data mining and large-scale DFT calculations. One more strontium lithium aluminate phosphor Sr₂LiAlO₄ doped either with Eu²⁺ or Ce³⁺ ions was discovered in Ref. [50] through data mining and DFT-based analysis. Such an important parameter of inorganic phosphors as the thermal stability was assessed by application of the machine learning methods in Refs. [51], [52]. The trends in the centroid shift of the Ce³⁺ 5d energy levels were analyzed in detail in Ref. [53]. The machine learning-based search for the phosphors with enhanced red upconversion luminescence emission was successfully performed in Ref. [54]. A new quaternary sulfide phosphor Ba_2-xLiAlS₄:Eu²⁺ was reported in Ref. [55] after a thorough screening of the Ba-Li-Al-S compositional space supported by the DFT calculations. These examples – which should not be considered as a full and complete literature survey on this subject – indicate importance of research in the field of machine learning and its application to the phosphor materials.

Since the ternary sulfides doped with the Eu²⁺ ions are very attractive phosphor materials with broad emission spectra covering the whole visible spectral range, in the present paper we performed a systematic in-depth study of the structural and optical properties of these materials. As a result, several simple linear correlations between their lattice constants and Eu²⁺ emission maxima positions were found, which allow to predict the lattice constants of novel ternary sulfides from this group of materials and identify the spectral interval corresponding to the maximal emission intensity of the Eu²⁺ ions. These equations possess a high predictive power and can be considered as useful guides for a smart search for new phosphor materials with desired emission properties.

In the next Section 2 we give more information about tunability of the Eu²⁺ emission in the considered sulfides. Then we proceed with the structural analysis (Section 3) and correlation between the structural data on the studied ternary sulfides and emission properties of the Eu²⁺ dopant (Section 4). The paper is concluded with a summary of the obtained results.