Sodium fluoroaluminates formed in the reaction between aluminum fluoride solution and crystalline sodium fluoride

The reaction of aluminum fluoride solution with crystalline sodium fluoride was investigated. Conditions for the formation of Na₃AlF₆ (cryolite), Na₅Al₃F₁₄ (chiolite) and NaAlF₄.H₂O were established. The hitherto presumed to be unstable NaAlF₄.H₂O was isolated and its X-ray diffraction data as well as thermal behavior were determined. The possibility to convert these compounds one into the other was outlined.     

Calorimetric Investigations of Some Phases in the System Sodium Fluoride - Aluminium Fluoride

Beside the two well-known minerals cryolite, Na₃AlF₆, and chiolite, Na₅Al₃F₁₄, the binary system NaF-AlF₃ also contains a third compound, NaAlF₄, sodium tetrafluoroaluminate. Solid NaAlF₄ has been prepared from its vapour under controlled conditions. The stability of NaAlF₄ has been investigated by differential scanning calorimetry. It is shown that the disproportionation of the compound: 5NaAlF_4(s)=Na₅Al₃F_14(s)+2AlF_3(s) takes place at considerable rate between 700 and 900 K. The enthalpy of this reaction is calculated and found to be -66.9 kJ. Enthalpies of the two solid state transitions α-Na₃AlF₆ → β-Na₃AlF₆ and α-AlF₃ → β-AlF₃ have also been measured and new values are reported. The enthalpy of formation of chiolite, Na₅Al₃F₁₄, at 900 K has been recalculated from enthalpy increment data obtained by drop calorimetry. A value of ΔH₉₀₀ ^o = -7513.6±12.0 kJ mol^-1 has been obtained. This value is in disagreement with the recommended value given in JANAF Thermochemical Tables given at 900 K ΔH_f ^o = -7559.2 kJ mol^-1. This revised version was published online in July 2006 with corrections to the Cover Date.     

NaAlF4: Preparation, crystal structure and thermal stability

The compound NaAlF₄ has been obtained in the form of thin fibrous crystals or fine colorless powder by condensation at 18 °C of vapors arising over chiolite Na₅Al₃F₁₄ or NaCaAlF₆, heated up to 800 °C. Thermal stability has been investigated by the methods of thermal analysis and high temperature X-ray diffraction. When heated in air, NaAlF₄ is stable up to 390-400 °C, then there is an exothermal solid state decay into Na₅Al₃F₁₄(s) and AlF₃(s). At higher temperature Na₅Al₃F₁₄(s) decays into Na₃AlF₆(s) and NaAlF₄(g). The crystal structure (space group Cmcm, a=3.6124(1) Å, b=14.9469(7) Å, c=5.2617(3) Å, V=284.10 Å³) has been determined by X-ray powder diffraction method. In the crystal structure of NaAlF₄ the octahedrons [AlF₆] are joined through vertices and form corrugated layers, sodium ion layers being located between them. The distances between the atoms of Al-F are in the range 1.791-1.814 Å, and those for Na…F are in the range 2.297-2.439 Å. In spite of limited thermal stability of the crystal form, the compound NaAlF₄ is the main component of the gas mixture over solid and molten salts in the ternary system NaF-AlF₃-CaF₂ and participates in chemical transformations between the phases at high temperature.     

Thermal transformation of quaternary compounds in NaF–CaF2–AlF3 system

Details of quaternary compounds formation in the system NaF–CaF₂–AlF₃ are specified. To achieve this aim, the samples of phases NaCaAlF₆ and Na₂Ca₃Al₂F₁₄ have been obtained by high-temperature solid-phase synthesis. Their thermal behavior when heated up to 800 °C has been studied using the methods of high-temperature X-ray diffraction (XRD) and thermal analysis (TA). The system under consideration can be regarded as a quasibinary section CaF₂–NaAlF₄, where at T=745–750 °C invariant equilibrium is implemented with the phases CaF₂–NaCaAlF₆–Na₂Ca₃Al₂F₁₄–(liquid melt)–(NaAlF₄). The peculiarity of the equilibrium is NaAlF₄ metastability at normal pressure. Below the equilibrium temperature the quaternary phase Na₂Ca₃Al₂F₁₄ is stable and NaCaAlF₆ above this temperature. The phase NaCaAlF₆ fixed by rapid quenching from high temperatures and when heated up to 640 °C decomposes, yielding Na₂Ca₃Al₂F₁₄. Further heating in vacuum at temperature up to 740 °C results in decomposition of Na₂Ca₃Al₂F₁₄ into CaF₂ and Na₃AlF₆. The expected reverse transformation of Na₂Ca₃Al₂F₁₄ into NaCaAlF₆ has not been observed under experimental conditions. Transformations in bulk samples reveal direct and reverse transformation of quaternary phases.

Synopsis

Thermal transformation of the quaternary compounds in system (NaF–CaF₂–AlF₃) was investigated using high-temperature X-ray diffraction (XRD) and thermal analysis (TA). In the system the invariant equilibrium is implemented with the phases CaF₂–NaCaAlF₆–Na₂Ca₃Al₂F₁₄–(liquid melt)–(NaAlF₄) at T=745–750 °C.     

The reaction of alf3 solution with aluminium hydroxide and some reactions of the aluminium hydroxyfluoride obtained

AlF₃ solution (150 g/l) reacts with Al(OH)₃ in the m.ratio 2:1 in excess of ca. 115°C to produce Al(OH,F)₃.H₂O with an F/Al at. ratio > 2. At lower temperatures, e.g. 110°C, or at higher reactants ratios, e.g. 3-11, formation of Al(OH,F)₃.H₂O may be accompanied by crystallization of AlF₃-hydrates as AlF₃.3H₂O and/or /gb-AlF₃.H₂O. When crystallization of β - AlF₃.H₂O occurs to a greater extent, Al(OH,F)₃.H₂O may vary in its F/Al at.ratio from ca. 2.5 to 1, during the reaction.Al(OH)F₂.H₂O reacts readily with NaOH, NaF and NH₄F solutions to give sodium and ammonium cryolite. Reactions with NaHF₂ and H₂SiF₆ were unsuccessful, while with AlF₃ solution an increase of the F/Al ratio in the Al basic fluoride used resulted.     

High temperature crystallization of aluminum fluoride. Course,hydrolysis, solid phases.

A study for AlF₃ crystallization from water solution was performed in the temperature range 100 to 200°C.Four solid phases were found to be precipitated, AlF₃.3H₂O (up to ca.120°C, cubic α-AlF₃.H₂O (decomposition of AlF₃.3H₂O in suspension), hexagonal β -AlF₃.H₂O (direct from solution) and the hydroxyfluoride Al(OH,F)₃.H₂O with an F/Al ratio of ca. 2.5 (hydrolysis of AlF₃). The extent of hydrolysis was established as a function of the initial AlF₃ concentration.X-ray diffraction and thermogravimetric data for the monohydrates were given and differences between the two indicated.     

Synthese,Kristallstruktur und thermischer Abbau von Fluoroaluminaten der Zusammensetzung (NH4)[M(H2O)6][AlF6] (M = Zn,Ni), [Zn(H2O)6][AlF5(H2O)] und (PyH)4[Al2F10] · 4 H2O

Synthesis, Crystal Structure and Thermal Behaviour of Fluoroaluminates of the Composition (NH₄)[M(H₂O)₆](AlF₆) (M = Zn, Ni), [Zn(H₂O)₆][AlF₅(H₂O)], and (PyH)₄[Al₂F₁₀] · 4 H₂O Four new fluoroaluminates were obtained from fluoroacidic solutions of respective metal salts. The compounds of zinc ( I a : P2₁/c, a = 12.688(3), b = 6.554(1), c = 12.697(3) Å, β = 95.21(3)°, V = 1051.5(4) ų, Z = 4) and nickel ( I b : P2₁/c, a = 12.685(3), b = 6.517(1), c = 12.664(2)Å, β = 94.55(2)°, V = 1043.6(4) ų, Z = 4) are isotypic and represent a new structure type consisting of two different cations, NH₄⁺ and [M(H₂O)₆]²⁺ and [AlF₆]^3–‐anions. [Zn(H₂O)₆][AlF₅(H₂O)] ( II : C2/m, a = 10.769(2), b = 13.747(3), c = 6.487(1)Å, β = 100.02(3)°, V = 945.7(3) ų, Z = 4) is characterized by a H₂O/F‐disorder in the [AlF₅(H₂O)]‐octahedra in two trans positions. In (PyH)₄[Al₂F₁₀] · 4 H₂O ( III : Cmc2₁, a = 15.035(3), b = 20.098(4), c = 12.750(4) Å, V = 5364(2) ų, Z = 8), bioctahedral [Al₂F₁₀]^4– anions have been found for the first time. The structures are described and discussed in comparison. The new compounds were used as precursors in order to obtain new AlF₃‐phases. However, the thermal decomposition did not result in the formation of any new metastable AlF₃‐phase. Instead, phase mixtures of either α‐AlF₃ and β‐AlF₃ or AlF₃ and MF₂ were obtained.     

The hydrothermal decomposition of aluminum fluoride trihydrate. Kinetics and mechanism

The hydrothermal decomposition of AlF₃.3H₂O in water suspension was investigated by chemical and X-ray analyses at temperatures ranging from 114 to ca. 143°C. It was found that this process runs from a trihydrate to a monohydrate composition according to a zero order rate equation to give subsequently Al(OH,F)₃.H₂O and β -AlF₃.H₂O direct from solution and α -AlF₃.H₂O by conversion of solid trihydrate. Reactions involved in the process are discussed in detail.     

Evidence of 13 hybrid fluoroaluminates in the composition space diagram of the Al(OH)3–tren–HF–ethanol system

Thirteen phases are now evidenced in the composition space diagram of the Al(OH)₃–tren–HF–ethanol system at 190 °C. Solvothermal syntheses are performed under microwave heating. Six new organic–inorganic fluorides crystallise and their structures are determined: (H₃O)·[H₄tren]₂·(AlF₆)₃·6H₂O (P-1, Z = 2), [H₃tren]₂·(AlF₅(H₂O))₃·8H₂O (C2/c, Z = 8), [H₃tren]₄·(AlF₆)₂·(Al₂F₁₁)·(F)·10H₂O (P2₁/n, Z = 2), [H₃tren]₂·(Al₄F₁₈)·3.5H₂O (P6₃, Z = 2), [H₃tren]₂·(Al₄F₁₈) (P-1, Z = 1), and [H₃tren]₄·(Al₈F₃₅)·(OH)·H₂O (P-1, Z = 1). The existence domains are approximately located for all phases. Tren amine is tetraprotonated at high HF concentration, otherwise it is triprotonated. A protonated water cluster, H₃O⁺(H₂O)₆, appears in (H₃O)·[H₄tren]₂·(AlF₆)₃·6H₂O while a new Al₄F₁₈ unit, found in [H₃tren]₂·(Al₄F₁₈), is evidenced; it results from corner and edge sharing association of four AlF₆ octahedra. Finally, the structure of [H₃tren]₄·(Al₈F₃₅)·(OH)·H₂O revealed the largest known fluoroaluminate polyanion, built from eight vertex sharing AlF₆ octahedra, (Al₈F₃₅)¹¹⁻.     

Strukturen und thermische Zersetzung von enH2(H3O)[AlF6] und enH2[AlF5(H2O)]

Structures and Thermal Decomposition of enH₂(H₃O)[AlF₆] and enH₂[AlF₅(H₂O)] By precipitation with ethylene diamine (en) from a hydrofluoric acid solution of aluminium enH₂(H₃O)[AlF₆] is formed. It crystallizes in the orthorhombic space group Pnma, a = 1084.9(1), b = 1079.4(1), c = 682.0(1) pm, R = 0.032. H₃O⁺ cations and [AlF₆]^3– anions are connected via strong H bonds to layers which are further linked to a 3 D network by H bonds from the enH₂²⁺ cations. By recrystallization from water or precipitation from a less acid solution enH₂[AlF₅(H₂O)] is formed, which crystallizes monoclinic in the space group P2₁ (a = 660.0(1), b = 563.5(1), c = 994.4(2) pm, β = 98.44(3)°, R = 0.029). The [AlF₅(H₂O)]^2– anions are linked by strong O–H…F bonding to form ‘einer-double chains' interconnected again via the enH₂²⁺ cations to a 3 D framework. Thermoanalytical investigations show that enH₂[AlF₅(H₂O)], by loss of water above 150 °C, as well as enH₂(H₃O)[AlF₆], by loss of water and HF above 120 °C, transform to enH₂AlF₅. The subsequent decomposition goes over NH₄⁺ containing intermediates towards β-AlF₃. Before decomposition the oxonium fluoroaluminate reveals a reversible phase transition at 99 °C.     

Zum thermischen Verhalten von Aluminiumfluoridhydroxidhydrat AlF2,3(OH)0,7(H2O)

The Thermal Behaviour of Aluminium Fluoridehydroxide Hydrate AlF_2.3(OH)_0.7(H₂O) The thermal decomposition of AlF_2,3(OH)_0,7(H₂O) depends strongly on the partial pressure of the gaseous reaction products and proceeds in three overlapping steps, namely dehydratation, formation of x-ray amorphous Al₂O₃ and metastable β-AlF₃, and formation of α-AlF₃ and α-Al₂O₃. Beside that an exchange of F/OH takes place. The vaporization is mainly determined by the crucible type and pressure conditions, as shown by simultaneous TG-MS measurements too. Main gaseous species are H₂O and HF. The gas complexes HAlF₄, and H₂AlF₅ are observed.     

The missing hydrate AlF3·6H2O [Al(H2O)6]F3: Ionothermal synthesis,crystal structure and characterization of aluminum fluoride hexahydrate

AlF₃ is a strong Lewis acid and several hydrates of it are known, namely the monohydrate, the trihydrate (of which two polymorphs have been described) and the nonohydrate, which forms in the abundance of water, as well as a more complex fluoride of composition Al_0.82□_0.18F_2.46(H₂O)_0.54 whose structure has been related to the ReO₃ type. The monohydrate features edge connected [AlF₆] octahedra, in the tri- and nonahydrate mixed F/O coordination of aluminum is observed. Here we report on a new aluminium fluoride hydrate, AlF₃·6H₂O, which could be obtained via ionothermal synthesis in the ionic liquid n-hexyl-pyridinium tetrafluoroborate. The ionic liquid serves in the synthesis of AlF₃·6H₂O as the reaction partner (fluoride source) and solvent. Overmore it controls the water activity allowing access to the missing AlF₃·6H₂O. Single-crystal X-ray diffraction analysis of AlF₃·6H₂O shows that it crystallizes in the anti-Li₃Bi-type of structure according to F₃[Al(H₂O)₆] (Fm-3m, a = 893.1(2) pm, Z = 4) featuring hexaaqua aluminium(III) cations and isolated fluoride anions. The compound was further characterized by powder X-ray diffraction, TG/DTA, IR analyses.     

Two-dimensional composition diagram of the Al(OH)3-dien-HFaq.-ethanol system: Evidence of a new tetrahedral (Al4F18) polyanion

Six domains appear in the 2D composition diagram of the Al(OH)₃-dien-HF_aq.-ethanol system at 190 °C and [Al³⁺] = 1 mol L⁻¹ under microwave heating. Four organic-inorganic fluorides crystallise: [H₃dien]·(AlF₆) (P2₁/c, Z = 4), [H₃dien]₂·(AlF₅(H₂O))₃·2H₂O (P2₁/n, Z = 4), [H₃dien]·(AlF₆)·2H₂O, which was previously known, and [H₃dien]₂·(Al₄F₁₈) (C2/c, Z = 4). A new (Al₄F₁₈)⁶⁻ polyanion, which results from the tetrahedral association of four AlF₆ octahedra linked by corners, is evidenced in [H₃dien]₂·(Al₄F₁₈).     

The Thermal Dehydration of Magnesium Aluminum Pentafluoride Dihydrate: Crystal Structures of MgAlF5(H2O)2 and MgAlF5

Summary.  Small plate-like single crystals of MgAlF₅(H₂O)₂ have been obtained during hydrothermal treatment (270°C) of microcrystalline material prepared by precipitation of stoichiometric solutions of Al₂(SO₄)₃ ·  18H₂O and Mg(NO₃)₂ · 6H₂O with diluted hydrofluoric acid. The crystal structure of MgAlF₅(H₂O)₂ has been refined from single crystal data (Imma (# 74), Z = 4, a = 7.0637(7), b = 10.1308(10), c = 6.7745(7) ?, 398 structure factors, 33 parameters, R(F² > σ(F ²)) = 0.0245, wR(F² all) = 0.0525). Main features of the inverse weberite type structure are infinite chains of trans-bridged [AlF₆] octahedra which are connected via common fluorine atoms by isolated [MgF₄(H₂O)₂] octahedra. MgAlF₅(H₂O)₂ dehydrates at temperatures above 300°C to give MgAlF₅. XRPD analysis of this phase has revealed isotypism with FeAlF₅. The crystal structure of MgAlF₅ (Immm (# 71), Z = 2, a = 7.268(1), b = 6.123(2), c = 3.543(1) ?) is built of infinite chains of edge-sharing [MgF₆] octahedra and chains of corner-sharing [AlF₆] octahedra along [001]. Upon further heating to temperatures above 500°C, MgAlF₅ decomposes to MgF₂ and α − AlF₃. Received January 15, 2001. Accepted February 12, 2001     

The Thermal Dehydration of Magnesium Aluminum Pentafluoride Dihydrate: Crystal Structures of MgAlF5(H2O)2 and MgAlF5

 Small plate-like single crystals of MgAlF₅(H₂O)₂ have been obtained during hydrothermal treatment (270°C) of microcrystalline material prepared by precipitation of stoichiometric solutions of Al₂(SO₄)₃ ·  18H₂O and Mg(NO₃)₂ · 6H₂O with diluted hydrofluoric acid. The crystal structure of MgAlF₅(H₂O)₂ has been refined from single crystal data (Imma (# 74), Z = 4, a = 7.0637(7), b = 10.1308(10), c = 6.7745(7) ?, 398 structure factors, 33 parameters, R(F² > σ(F ²)) = 0.0245, wR(F² all) = 0.0525). Main features of the inverse weberite type structure are infinite chains of trans-bridged [AlF₆] octahedra which are connected via common fluorine atoms by isolated [MgF₄(H₂O)₂] octahedra. MgAlF₅(H₂O)₂ dehydrates at temperatures above 300°C to give MgAlF₅. XRPD analysis of this phase has revealed isotypism with FeAlF₅. The crystal structure of MgAlF₅ (Immm (# 71), Z = 2, a = 7.268(1), b = 6.123(2), c = 3.543(1) ?) is built of infinite chains of edge-sharing [MgF₆] octahedra and chains of corner-sharing [AlF₆] octahedra along [001]. Upon further heating to temperatures above 500°C, MgAlF₅ decomposes to MgF₂ and α − AlF₃.     

SmAlF5 — ein neues SmII-fluoroaluminat mit Al2F10-Oktaederdoppeln und [AlF2/2F4/1]-Ketten

SmAlF₅ — a New Samarium(II) Fluoroaluminate with Al₂F₁₀ Bioctahedra and [AlF_2/2F_4/1] Chains . SmAlF₅ has been obtained as orange-red transparent single crystals while heating mixtures of SmF₃, Sm-powder and AlF₃ (2:1:3) in a niobium crucible under Ar after 7?10 d at about 750°C. SmAlF₅ crystallises in I 4/m (Nr. 87) with a=1 414.4(4), c=722.2(3) pm and Z=8 (CAD4, 4 340 I_O, R_w=1.7%). The crystal structure of SmAlF₅ is isotypic to BaTiF₅. Characteristic building units are linear chains of trans-corner sharing AlF₆ octahedra, which are connected via corners to two further AlF₆ octahedra. Isolated Al₂F₁₀ octahedra lie disordered between such chains. The Sm atoms connect the AlF₆ octahedra to a three-dimensional network. Measurements of the magnetic susceptibility show the temperature dependence typically found for Sm²⁺. The Madelung part of the lattice energy has been calculated and is discussed.     

Na5AlF2(PO4)2: Darstellung,Kristallstruktur und Ionenleitfähigkeit

Na₅AlF₂(PO₄)₂: Synthesis, Crystal Structure and Ionic Conductivity Two different procedures (precipitation from aqueous solution and solid state reaction) for the synthesis of hitherto unknown Na₅AlF₂(PO₄)₂ were optimized. The crystal structure was determined using diffractometer data (P3 , a = b = 10.483(1), c = 6.607(1) Å, MoKα, 1080 independent reflections, R_w = 0.025). PO₄-tetrahedra and AlO₄F₂-“octahedra” are connected via common vertices forming a twodimensionally extended heteropolyanion. Sodium is located in interconnected spacings of the [AlF₂(PO₄)₂]-part of the structure. Ionic conductivity as expected because of these structural features was affirmed experimentally.     

(H3O)2NaAl3F12, isostructural with A2NaAl3F12 (A = K+, Rb+, Cs+) fluorides having HTB‐type sheets

The title compound, (H₃O)₂NaAl₃F₁₂ [dihydronium sodium trialuminum(III) dodecafluoride], was obtained by solvothermal synthesis from the reaction of aluminium hydroxide, sodium hydroxide, 1,2,4‐triazole and aqueous HF in ethanol at 463 K for 48 h. The structure consists of AlF₆ octahedra organized in [AlF₄⁻]_n HTB‐type sheets (HTB is hexagonal tungsten bronze) separated by H₃O⁺ and Na⁺ cations.     

Preparation, Thermal Behaviour, and Crystal Structure of MgAl2F8(H2O)2

 Single crystals of MgAl₂F₈(H₂O)₂ have been obtained under hydrothermal conditions (250°C, 14 d) from a starting mixture of AlF₃ and MgAlF₅(H₂O)₂ in a 5% (w/w) HF solution. The crystal structure has been determined and refined from single crystal data (Fmmm (#69), Z = 4, a = 7.2691(7), b = 7.0954(16), c = 12.452(2) ?, 281 structure factors, 27 parameters, R(F ² > 2σ (F ²)) = 0.0282, wR(F ² all) = 0.0885). The obtained crystals were systematically twinned according to (010/100/001) as twinning matrix, reflecting the pseudo-tetragonal metric. The crystal structure is composed of perowskite-type layers built of corner sharing AlF₆ octahedra with an overall composition of AlF₄ ⁻ which are connected via common fluorine atoms of [MgF_4/2(H₂O)_2/1] octahedra. Group-subgroup relations of MgAl₂F₈(H₂O)₂ to WO₃(H₂O)_0.33 and to other M(II)M(III)₂ F₈(H₂O)₂ structures are briefly discussed. Above 570°C, MgAl₂F₈(H₂O)₂ decomposes under elimination of water into α-AlF₃, β-AlF₃, and MgF₂.     

Preparation,Thermal Behaviour,and Crystal Structure of MgAl<Subscript>2</Subscript>F<Subscript>8</Subscript>(H<Subscript>2</Subscript>O)<Subscript>2</Subscript>

Summary.  Single crystals of MgAl₂F₈(H₂O)₂ have been obtained under hydrothermal conditions (250°C, 14 d) from a starting mixture of AlF₃ and MgAlF₅(H₂O)₂ in a 5% (w/w) HF solution. The crystal structure has been determined and refined from single crystal data (Fmmm (#69), Z = 4, a = 7.2691(7), b = 7.0954(16), c = 12.452(2) ?, 281 structure factors, 27 parameters, R(F ² > 2σ (F ²)) = 0.0282, wR(F ² all) = 0.0885). The obtained crystals were systematically twinned according to (010/100/001) as twinning matrix, reflecting the pseudo-tetragonal metric. The crystal structure is composed of perowskite-type layers built of corner sharing AlF₆ octahedra with an overall composition of AlF₄ ⁻ which are connected via common fluorine atoms of [MgF_4/2(H₂O)_2/1] octahedra. Group-subgroup relations of MgAl₂F₈(H₂O)₂ to WO₃(H₂O)_0.33 and to other M(II)M(III)₂ F₈(H₂O)₂ structures are briefly discussed. Above 570°C, MgAl₂F₈(H₂O)₂ decomposes under elimination of water into α-AlF₃, β-AlF₃, and MgF₂. Received October 29, 2001. Accepted (revised) December 6, 2001