Preparation and Crystal Structure of a New Lithium Vanadium Fluoride Li2VF6 with Trirutile-type Structure Suliman Nakhal, Dominik Weber and Martin Lerch Institut fur¨ Chemie, Technische Universitat¨ Berlin, Straße des 17. Juni 135, D-10623 Berlin, Germany Reprint requests to Dr. Suliman Nakhal. Fax: +49 30 314 79656. E-mail: [email protected] Z. Naturforsch. 2013, 68b, 121 – 126 / DOI: 10.5560/ZNB.2013-2303 Received November 22, 2012 A new lithium vanadium fluoride Li2VF6 was prepared by reacting lithium fluoride LiF with ◦ vanadium tetrafluoride VF4 in a monel capsule at 500 C. The crystal structure has been deter- mined by means of powder X-ray diffraction. Trirutile-type dilithium hexafluorovanadate(IV) crys- tallizes in the tetragonal space group P42=mnm with lattice parameters a = 459.99(1), b = 459:99(1), c = 896.64(2) pm. The presence of a Jahn-Teller effect is discussed. Key words: Lithium Metal Fluoride, Synthesis, Crystal Structure Introduction as compared to the corresponding oxides for the same redox pair [6]. Numerous ternary fluorides of the formula type For lithium vanadium fluorides the following phases A2MF6 (A = alkali metal, M = main group element or have been reported in literature: cryolite type-related (III) transition metal) with larger alkali ions are known. The orthorhombic a- and monoclinic b-Li3V F6 [7], crystal chemistry of these compounds, often crystal- (II=III) LiV2 F6 (tetragonal, a mixed-valence trirutile (V) lizing in several modifications, has been discussed for structure) [8], and LiV F6 (rhombohedral, LiSbF6 more than 60 years [1–3]. There were only few re- type) [9]. In this contribution we report on the synthe- ports on compounds containing the small alkali ion sis and the crystal structure of the new lithium vana- (IV) lithium. For most of the oxidation states of d-transition dium fluoride Li2V F6. metal ions the ratio of the ionic radii rM=rF is within the range 0.41 – 0.73 [4]. Most of the reported com- Experimental Section pounds with A2MF6 composition crystallize in the fol- lowing structure types or are closely related to them: For the synthesis of Li2VF6 710 mg vanadium tetraflu- Li2ZrF6, Na2SiF6, trirutile. Differences arise from the oride powder (ABCR, 95%) and 290 mg lithium fluoride type of atom sharing between the coordination octa- (Alfa Aesar, 99:9%), which were used as starting materi- ◦ hedra: a) Li ZrF type (trigonal, space group P31¯ m): als, were dried under vacuum at 250 C for 24 h. The mix- 2 6 ◦ I IV ture of LiF and VF4 (2:1 molar ratio) was heated at 500 C solely corner-sharing between A F6 and M F6 octa- hedra. b) Na SiF type (trigonal, space group P321): for 12 h in a monel capsule and then slowly cooled to am- 2 6 bient temperature. The powder was chemically characterized SiF units share three edges with NaF octahedra. c) 6 6 using a Leco EF-TC 300 N =O analyzer (hot gas extrac- P =mnm 2 2 trirutile type (tetragonal, 42 ) and related struc- tion) for oxygen content determination. Detection of remain- tures (e. g. CuSb2O6 type, monoclinic, spacegroup ing capsule material was carried out by X-ray fluorescence P21=n)[5]: only two common edges. analysis (PANalytical Axios PW4400=24 X-ray fluorescence It should be mentioned that ternary lithium metal spectrometer with an Rh tube and a wavelength dispersive fluorides are of interest as cathodes for lithium ion bat- detector). A PANalytical X’Pert PRO MPD diffractome- teries. The voltage of batteries using transition metal ter (CuKa radiation, Bragg-Brentano (q-q) geometry) with fluoride-based cathode masses is expected to be higher a PIXcel detector was used for the powder XRD measure- © 2013 Verlag der Zeitschrift fur¨ Naturforschung, Tubingen¨ · http://znaturforsch.com 122 S. Nakhal et al. · The Lithium Vanadium Fluoride Li2VF6 ments at ambient temperature, the program package FULL- isotypic to Li2CrF6. This hexafluorochromate(IV) was PROF 2006 [10, 11] for Rietveld refinements. Peak profiles structurally described for the first time by Siebert and were fitted with a pseudo-Voigt function. Hoppe [12] and reported to be of the Na2SnF6 type, a monoclinically distorted trirutile derivative. Later on Results and Discussion the crystal structure of Na2SnF6 was in the focus of discussion. Bournonville et al. pointed out the relation- Following the instructions described in the experi- ships between the monoclinic Na2SnF6 and the tetrag- mental section, a greenish-yellow powder can be pre- onal trirutile type [13], showing that they are identical pared. X-Ray fluorescence measurements give no indi- within the limits of a probable error. Some years later cation for Cu or Ni (capsule materials) to be present Benner and Hoppe [14] presented a structure redeter- in the obtained product. In addition, no significant mination of Na2SnF6 preferring the tetragonal triru- amount of oxygen was detected. Analyzing the pow- tile type. Recently also Li2CrF6 was reported to be der X-ray patterns depicted in Fig.1 , a yet unknown tetragonal [15]. A more detailed discussion of these phase together with b-Li3VF6 is observed. The crys- problems together with a deep insight into the crystal tal structure of the new phase was determined using chemistry of the rutile type and its derivatives, includ- conventional powder X-ray techniques and found to be ing the trirutile type, was presented by Baur [16]. Re- Table 1. Refined parameters for Li2VF6 at ambient tempera- ture (comparison between the monoclinic ‘Na2SnF6’ and the tetragonal trirutile model). Structure type ‘Na2SnF6’ Trirutile Space group P21=c (no. 14) P42=mnm (no. 136) Crystal system monoclinic tetragonal Mr 389.94 a, pm 459.91(2) 459.99(1) b, pm 459.76(3) 459.99(1) c, pm 1007.43(4) 896.64(2) b, deg 117.16(4) 90 V, pm3 189.54(2) × 106 189.72(1) × 106 Refined parameters 32 23 Z 2 2 2q range, deg 5 – 90 5 – 90 Rwp 0.0453 0.0473 RBragg 0.0481 0.0501 Rexp 0.0340 0.0340 S 1.33 1.39 Fig. 1. Powder X-ray diagram of Li2VF6 with the results of the Rietveld refinements (markers: Li2VF6, top; b-Li3VF6, bottom). Upper image: monoclinic model, below: tetragonal Fig. 2 (color online). VF6 polyhedron with the determined trirutile model. bond lengths (pm). S. Nakhal et al. · The Lithium Vanadium Fluoride Li2VF6 123 Table 2. Refined structural parameters for Li2VF6 at ambient temperature (comparison between the monoclinic ‘Na2SnF6’ (top) and the tetragonal trirutile (bottom) model). 2 Atom Wyckoff site x y z Biso (A˚ ) V 2a 0 0 0 0.66(9) Li 4e 0.340a −0.028a 0.336a 0.85 F1 4e 0.2878(5) 0.2787(4) −0.0011(2) 0.8(2) F2 4e −0.0153(6) 0.1998(2) 0.0011(2) 1.3(2) F3 4e 0.3304(3) 0.765(6) 0.1521(4) 1.9(2) a Not refined, data taken from Li2CrF6 [12]. 2 Atom Wyckoff site x y z Biso (A˚ ) V 2b 0 0 0.5 0.42(5) Li 4e 0 0 0.1646a 0.7 F1 8 j 0.1979(3) 0.1979(3) 0.3429(2) 0.41(7) F2 4 f −0.2868(4) 0.2868(4) 0.5 0.72(9) a Not refined, data taken from Li2CrF6 [15]. Fig. 3 (color online). Unit cell of trirutile-type Li2VF6, outlined by green lines, together with the VF6 polyhedra. 124 S. Nakhal et al. · The Lithium Vanadium Fluoride Li2VF6 specting these contributions, Rietveld refinements of with a tripled c cell parameter and an ordered arrange- the structural parameters of the new dilithium hexaflu- ment of the cations (same space group type), the crys- orovanadate(IV) were performed using the tetragonal tal structure of dilithium hexafluorovanadate(IV) can trirutile as well as the monoclinic ‘Na2SnF6’ model. be described in the following way: vanadium occu- The results of the Rietveld refinements are presented pies the 2b position, lithium the 4e, and fluorine the in Fig.1 as well as in Tables1 and2 . An amount positions 4 f and 8 j (see Table2 ). The crystal struc- of ≈ 41 wt-% b-Li3VF6 as a second phase is ob- ture is built up from isolated slightly compressed VF6 served. This may be explained by partial decomposi- octahedra (V–F: 2 × 186.6(2) pm, 4 × 190.8(2) pm, tion of VF4 and significant amounts of VF3 in the start- Fig.2 ) connected by lithium atoms also coordinated ing material. In our refinement procedure the atomic by six fluorine atoms (see Figs.3 ,4 ). The polyhedra positions of lithium were not refined but taken from share two opposing edges leading to straight chains Li2CrF6 [12, 15]. Comparing the results of both mod- along the tetragonal c axis, each of them surrounded by els it is evident that the monoclinic description leads to four other chains sharing common vertices. The aver- slightly better R values. Nevertheless, respecting the age bond length between vanadium and the surround- limitations of powder methods and the presence of ing anions, dV−F, is 189 pm, which is slightly longer such large amounts of a second phase with low sym- compared to the reported V–F bond length in VF4 metry (monoclinic) the use of the tetragonal trirutile (dV−F = 185 pm) [17]. To answer the question whether model seemed to be more reasonable. In the following the compression of the VF6 octahedra is mainly caused we discuss the crystal structure of Li2VF6 in the light by next nearest neighbors connecting the polyhedra of the structure type with higher symmetry. to the above-described framework or by a relatively In respect to the well-known trirutile type, which weak Jahn-Teller effect (keep in mind that V4+ has can be understood as superstructure of the rutile type a d1 configuration), a short look at the crystal struc- Fig.