Synthesis and thermal stability of rare earth compounds REF3, REF3·nH2O and (H3O)RE3F10·nH2O (RE = Tb − Lu, Y), obtained from sulphide precursors

doi:10.1016/j.jfluchem.2017.12.001

Journal of Fluorine Chemistry

Volume 207, March 2018, Pages 77-83

https://doi.org/10.1016/j.jfluchem.2017.12.001 Get rights and content

Highlights

•
The REF₃&903·nH₂O and (H₃O)RE₃F₁₀&903·nH₂O compounds were synthesized from RE₂S₃ and HF.
•
The REF₃&903·nH₂O compounds have an amorphous phase.
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After heat treatment the REF₃&903·nH₂O and (H₃O)RE₃F₁₀&903·nH₂O compounds moved into a REF₃ compound of crystal structure as β-YF₃.

Abstract

The nature of the interaction between sulphides of the rare-earth elements (RE) of the yttrium subgroup RE₂S₃ (Dy − Lu, Y) with hydrofluoric acid (concentration of 49%) was studied. Powders of RE₂S₃ compounds were obtained by the reaction between RE₂O₃ (Dy − Lu, Y) and Tb₄O₇ with a stream of H₂S and CS₂ in the 1000–1050 °C temperature range. The reaction between RE₂S₃ and HF was carried out in a glassy carbon crucible in the 21–25 °C temperature range in solution, and the precipitate was washed with H₂O and dried at 90–100 °C. For the hydrates of RE fluorides with decreasing RE radius, the RE content increases the sorption of water for TbF₃·0.5H₂O; DyF₃·0.5H₂O; YF₃·0.5H₂O; HoF₃·0.8H₂O; and ErF₃·0.9H₂O. The powders of REF₃·nH₂O consisted of micro- and nano-sized particles containing the amorphous phase. Water loss occurs in the 50–180 °C temperature range and is accompanied by a natural increase in enthalpy: from 23.6 J/g for TbF₃ · 0.5H₂O to 97.8 J/g for ErF₃·0.9H₂O and from 51.1 J/g for (H₃O)Tm₃F₁₀·1.7H₂O to 64.5 J/g for (H₃O)Lu₃F₁₀·0.9H₂O. Exothermic effects of formation of polycrystalline phase REF₃ were recorded in the 200–300 °C temperature range. The reaction of RE₂S₃ (Tm, Yb, Lu) powders with HF proceeds with the formation of the zeolite type compounds (H₃O)Tm₃F₁₀·1.7H₂O, (H₃O)Yb₃F₁₀·1.0H₂O and (H₃O)Lu₃F₁₀·0.9H₂O. Thermal dissociation occurs with gradual loss of water and HF to form RE fluorides with the β-YF₃ structure as described by

Graphical abstract

Introduction

The unique electronic structure of rare earth elements (RE) leads to the appearance of compounds with different optical, electrical [[1], [2]] and magnetic [3] properties, which have attracted considerable interest from researchers in recent years. Fluorides of rare earth elements are widely used in many fields (optical telecommunications, lasers, new optoelectronic devices, diagnostics and biological markers) [[4], [5], [6], [7]]. In recent decades, a particularly extensive attention was devoted to rare earth fluorides (REF₃) as luminescent materials [8]. Owing to the low energy of wave oscillations, the decrease in the radiating activity of doping substances in the electronic excitation state is minimal [[9], [10]]. As a consequence, a high-luminescence quantum yield can be achieved [5]. Currently, fluorides of rare earth elements are used or tested in optoelectronics, optical fibres and amplifiers, lasers, and medicine. In view of their chemical stability, materials based on REF₃ could potentially be used as bioimaging fluorescent probes [[5], [8]].

Fluorides of rare earth elements La-Nd crystallize in the tysonite (LaF₃) structure type (ST) (trigonal crystal system, space group P3¯c1). Morphotropics changed with decreasing ionic radius [[11], [12]]. For fluorides of Sm-Gd, dimorphism is typically observed with the low-temperature type β-YF₃ (orthorhombic crystal system, space group Pnma) and high-temperature modifications of tysonite [[13], [14]]. The REF₃ (Tb-Ho) retain the modification characteristic of β-YF₃ up to their melting point, whereas for fluorides of Er-Lu, Y, in addition to the basic orthorhombic modification (β-YF₃), the high-temperature structure with cubic α-YF₃ cubic (space group P23) can be obtained [[15], [16]].

REF₃ are obtained by chemical and physics-chemical methods. The most common and available is the method of precipitation from aqueous solutions, referred to as “soft chemistry” [[5], [17], [18]]. Methods of obtaining fluorides can be divided into hydrothermal [[19], [20], [21], [22]], micro-emulsion [23] and microwave [24] thermal decomposition [25] types. Synthesis of nanofluorides of RE is mainly realised by one of the following methods: pyrolysis of fluorine-containing precursors; hydrothermal synthesis; micro-emulsion method; solvo-thermal synthesis; and the sol-gel technique [[26], [27], [28]]. At elevated temperatures, RE fluorides participate in pyrohydrolysis reaction to form oxy-fluorides (REOF) [[29], [30]]. The preparation of REF₃ of the cerium subgroup during the interaction of hydrofluoric acid with RE₂S₃ provides a yield of close to 100%, and does not contain rare-earth oxyfluoride impurities, because the formation of the H₂S gas product shifts the equilibrium towards fluoride formation.

RE₂S₃ compounds exist in form of 4 basic modifications. RE₂S₃ compounds with RE = Ce-Dy have a low-temperature modification of the rhombic system of α-RE₂S₃ ST α-La₂S₃, sp.gr. Pnma and high-temperature cubic modification of γ-RE₂S₃ ST Th₃P₄, sp.gr. I43d. The δ-RE₂S₃ compounds (Re = Y, Ho, Er, Tm) have the monoclinic structure of ST δ-Ho₂S₃, sp.gr. P21/m. For ε-Yb₂S₃ and ε-Lu₂S₃, the hexagonal structure of ST α-Al2O3, sp.gr. R3c is found. In the literature, there is only one report on the extent of the RE₂S_3-xO_x solid solutions, mainly for γ-RE₂S₃: γ-(Sm₂S₃ − Sm₂S_2.94O_0.06) [31], γ-(Gd₂S₃ − Gd₂S_2.97O_0.03) [32].

In the process of hydrothermal synthesis of fluorides of rare-earth elements (Er-Lu, Y), the interaction of lanthanide oxalates with hydrofluoric acid leads to the formation of compounds δ-(H₃O)RE₃F₁₀·nH₂O [33]. A compound with the (H₃O⁺)Y₃F₁₀⁻·nH₂O structure was obtained as the second (secondary) phase by the interaction of a solution of yttrium nitrates and alkaline earth metal (strontium [34], barium [35]) with hydrofluoric acid (47%). The compound is crystallized in a cubic structure and consists of octahedral diamond [RE₆F₃₂]⁻¹⁴ blocks constructed from square REF₈ anti-prisms [36]. The behaviour of water molecules and the cation-exchange properties are observed to be of the zeolite type [37]. Ionic radius of the atom RE is an important factor in the formation of stable fluoride compounds. The connection structure of diamond [38] is formed from rare earth elements rHo³⁺ (coordination number (CN) is 8) = 1.015 Å − rLu³⁺_(CN8) = 0.977 Å, Y: ARE₃F₁₀·nH₂O (A⁺ = K⁺, Rb⁺, NH₄⁺, Cs⁺) [33], (C₂N₂H₁₀)_0.5RE₃F₁₀·nH₂O [22], (C₃N₂H₁₂)_0.5RE₃F₁₀·nH₂O [39].

In this paper, the nature of the interaction of sulphides of rare earth elements RE₂S₃ (Dy − Lu, Y) with hydrofluoric acid and the reaction products are studied, and their crystal chemical, physics-chemical characteristics, and thermal stability are considered.

Section snippets

Materials

We used 99.99 wt.% ultrapure RE₂O₃ and Tb₄O₇ (manufacturer Uralredmed, Verkhnyaya Pyshma, Russia), 99.9 wt.% pure HF and NH₄SCN (manufactured by TEK Systems) and distilled water as our starting material without further purification.

Synthesis of RE₂S₃

Powders of RE sulphides were obtained by reacting powders of Tb₄O₇ and RE₂O₃ (Dy-Lu, Y) oxides with gaseous CS₂ and H₂S. The maximum mass of one batch does not exceed 7 g. The rate of supply of sulphiding gases is 20–25 ml/min [40]. The synthesis temperature is

Characteristics of the precursors RE₂S₃

The RE₂S₃ formation reaction proceeds through the dioxysulfide formation stage, in which the oxide is transformed into the sulphide according to: RE₂O₃ → RE₂O₂S → RE₂S₃. The structure of the RE₂OS₂ compositions (Sm-Yb, Y) is presented in previous works [[42], [43]]. The formation of the RE₂OS₂ compound as a co-product due to the interaction of oxides of RE with gaseous carbon disulphide and hydrogen sulphide was not detected. The impurity phase RE₂O₂S is identified by X-ray diffraction when its

Conclusions

The phase compositions of the products of the reactions of the RE₂S₃ (Tb-Lu, Y) compounds with an aqueous HF solution (49%) were established. The reactions were carried out at 21–25 °C, and the precipitate was decanted and dried at 90–100 °C. Rare earth element fluorides with the compositions TbF₃·0·5H₂O, DyF₃·0·5H₂O, YF₃·0·5H₂O, HoF₃·0·8H₂O, ErF₃·0·9H₂O were obtained. Upon heating, more than 2/3 of crystalline water passes into the gas phase in the 50–170 °C temperature range. This process is

Acknowledgements

The authors express their gratitude to Mrs. Sofia A. Basova for the preparation of this manuscript.

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Synthesis and thermal stability of rare earth compounds REF3, REF3·nH2O and (H3O)RE3F10·nH2O (RE = Tb − Lu, Y), obtained from sulphide precursors

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Synthesis of RE2S3

Characteristics of the precursors RE2S3

Conclusions

Acknowledgements

J. Fluorine Chem.

J. Lumin.

J. Solid State Chem.

J. Solid State Chem.

Chem. Engin. J.

J. Solid State Chem.

Mater. Res. Bull.

J. Fluorine Chem.

Mendeleev Commun.

Mater. Res. Bull.

J. Fluorine Chem.

J. Solid State Chem.

J. Solid State Chem.

J. Solid State Chem.

J. Solid State Chem.

J. Cryst. Growth

Kolk,;1; Systematics in the optical and electronic properties of the binary lanthanide halide, chalcogenide and pnictide compounds: an overview

New J. Phys.

Specific features of anionic transfer in HoF3 crystals at high temperatures

Phys. Solid State

Magnetic properties of lanthanide chalcogenide semiconducting nanoparticles

J. Am. Chem. Soc.

Synthesis and photoluminescence properties of truncated octahedral Eu-doped YF3 submicrocrystals or nanocrystals

J. Phys. Chem. C.

Influence of matrix on the luminescent and structural properties of glycerine-capped, Tb3+-doped fluoride nanocrystals

J. Phys. Chem. C.

Synthesis, characterization, and cytotoxicity in human erythrocytes of multifunctional, magnetic, and luminescent nanocrystalline rare earth fluorides

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Cooperative energy transfer up-conversion and quantum cutting down-conversion in Yb3+:TbF3 nanocrystals embedded glass ceramics

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Cryst. Rep.

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Cryst. Reports

Synthesis and thermal stability of rare earth compounds REF₃, REF₃·nH₂O and (H₃O)RE₃F₁₀·nH₂O (RE = Tb − Lu, Y), obtained from sulphide precursors

Synthesis of RE₂S₃

Characteristics of the precursors RE₂S₃

Specific features of anionic transfer in HoF₃ crystals at high temperatures

Synthesis and photoluminescence properties of truncated octahedral Eu-doped YF₃ submicrocrystals or nanocrystals

Influence of matrix on the luminescent and structural properties of glycerine-capped, Tb³⁺-doped fluoride nanocrystals

Facile synthesis and luminescence properties of uniform and well-dispersed YF₃:Eu³⁺ architectures

Cooperative energy transfer up-conversion and quantum cutting down-conversion in Yb³⁺:TbF₃ nanocrystals embedded glass ceramics