Structure and luminescence of K2TaF7 and K2NbF7

doi:10.1016/0022-4596(87)90333-1

Journal of Solid State Chemistry

Volume 67, Issue 1, March 1987, Pages 21-25

https://doi.org/10.1016/0022-4596(87)90333-1 Get rights and content

Abstract

The crystal structure of K₂TaF₇ has been refined using single-crystal X-ray diffraction data. It is monoclinic with lattice parameters a = 5.8559(6), b = 12.708(1), and c = 85125(9) Å and β = 90.17° in the space group $P 2_{1} c$ . The structure is composed of TaF₇ units that are interconnected by potassium ions. TaF₇ polyhedra may be described as monocapped trigonal prisms with the capping atom located on one of the rectangular faces. Potassium atoms are 9-coordinated and may be viewed as distorted monocapped square prisms. The previously unreported luminescence properties of K₂TaF₇ and K₂NbF₇ are reported and discussed. In addition to the intrinsic emission, impurity luminescence due to OH⁻ and Eu²⁺ were observed.

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All the geometries were fully optimized without imaginary frequency and considered as closed-shell singlet structures. The crystal structure of K2TaF7 was redetermined by X-ray diffraction, Raman and IR analyses which belonged to the trigonal crystal system with the P21/C space group [26–28]. For this crystal, K2TaF7 is isomorphous with the same unit cell dimensions (a = 5.99, b = 12.93, and c = 9.05 Å), and its crystal packing structure was viewed along the YOZ plane shown in Fig. 1.
Combined with Raman spectroscopy and theoretical calculations, the tantalum fluoride and oxyfluoride ions in molten FLiNaK were studied. Firstly, the Raman spectra of K₂TaF₇ were recorded from 298 to 973 K and no structural evolution was observed. The coexistence of TaF₇^2– and TaF₈^3– anions was confirmed in K₂TaF₇ and molten FLiNaK-K₂TaF₇ mixtures. With the addition of Li₂O (3 mol%), the terminal oxo ligands TaOF₅^2– and TaOF₆^3– were formed at the expense of TaF₇^2– and TaF₈^3– with the release of free F^– ions, respectively. When the Li₂O content increased to 13 mol%, two new oxyfluorides TaO₂F₂^– and TaO₂F₃^2– were observed that contained two non-bridging oxygens. As the content of Li₂O further increased to 40 mol%, insoluble solid tantalum oxide species like β-Ta₂O₅ was found, which indicates the transformation of intermediate oxyfluorides formed by the reaction of TaF₇^2– and TaF₈^3– fluorides with O^2– in molten FLiNaK to oxides.
Calorimetric, dilatometric and DTA under pressure studies of the phase transitions in elpasolite (NH<inf>4</inf>)<inf>2</inf>KZrF<inf>7</inf>
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For example, the substitution of the central atom or monovalent cations in six-coordinated compounds A2A’MF6 (A, A’: Cs, Rb, K, NH4; M: Lu, Sc, Fe, Ga, Al) [4,6,8–13] and (NH4)2MF6·NH4F (M: Si, Ge, Ti, Sn, Pb) [14–23] does not change the cubic Fm-3 m symmetry of the initial phase but leads to strong change in the succession of structural distortions associated with the nonferroelectric transformations and their parameters (degree of the structural disordering, temperature, entropy, sensitivity to external pressure, etc.). Other situation was observed in fluorides with seven-coordinated anionic polyhedra which can be formed as a monocapped trigonal prism (A2MF7; A: Rb, NH4, K; M: Ta, Nb) [24–30] or a pentagonal bipyramid (A3MF7; A: K, NH4; M: Hf, Zr) [31–36]. The variation of cations and central atom in the former compounds is accompanied by strong change in the symmetry and structural disorder in the initial and distorted phases.
Heat capacity, thermal expansion, and sensitivity to the hydrostatic pressure of (NH₄)₂KZrF₇ elpasolite are studied in a wide temperature range. The changes in deformation and entropy during successive phase transitions are determined: Δ(ΔV/V) = 3·10⁻⁴; ΔS = 8 J/mol K The temperatures and entropies of phase transitions turned out to be slightly sensitive to pressure changes. An analysis of the entropy of phase transformations was performed in the framework of the model of the cubic phase structure Fm-3 m. In the low temperature phase, an anomalous behavior of thermodynamic properties, which is not characteristic of phase transitions, was observed, accompanied by a significant change in the crystal lattice entropy.
Hydrothermal synthesis and crystal structure of new red phosphors, KNaMF <inf>7</inf> :Mn 4+ (M: Nb, Ta)
2019, Materials Research Bulletin
Single crystals of new alkali niobium and tantalum fluorides, KNaMF₇ (M: Nb, Ta), were prepared by hydrothermal reactions using a perovskite-type oxides, KMO₃ as one of the starting compounds. Their crystal structures were determined from single crystal X-ray diffraction data. These fluorides crystallized in the orthorhombic space group P2₁2₁2₁ (#19) with a = 5.509(2), b = 9.100(4), and c = 11.234(5) Å for KNaNbF₇ and a = 5.517(2), b = 9.093(3), and c = 11.273(4) Å for KNaTaF₇. The crystal structures contained isolated MF₇ polyhedra showing a monocapped trigonal prism geometry and the M⁵⁺ sites could be replaced partially by Mn⁴⁺ ions during the hydrothermal reaction. These Mn⁴⁺ ion-doped fluorides exhibited photoluminescence properties similar to those of red phosphors. The local environment of Mn⁴⁺ ions in these fluorides was confirmed by the X-ray absorption fine structure (XAFS) analysis.
Optical and calorimetric studies of K<inf>2</inf>TaF<inf>7</inf>
2019, Journal of Fluorine Chemistry
Citation Excerpt :
Rietveld refinement was performed by using TOPAS 4.2 [16]. All peaks were indexed by monoclinic cell P21/c with parameters close to previously found for K2TaF7 [10,11]. This crystal structure (Fig. 1) was taken as starting model for Rietveld refinement which was found to be stable revealing low R-factors (Table 1, Fig. 1 a).
Optical and calorimetric experiments on K₂TaF₇ are performed in a wide temperature range. No features were found in the behavior of the birefringence Δn_b(T), the angle of rotation of the indicatrix ϕ(T) and the heat capacity ΔC_p(T) except for those associated with the Pnma↔P2₁/c phase transition. Structural transformation was characterized as strong first order “proper” ferroelastic accompanied by a huge angle ϕ ≈40° and strong pre-transition phenomena in Δn_b(T). Two contributions to the anomalies of the optical properties were found associated with the photoelastic effect and the transition parameter related linearly to the spontaneous deformation. Thermal treatments cause correlated changes in temperature and enthalpy of the phase transition, which leads to the invariance of the large magnitude of the corresponding entropy ΔS = 22 J/mol·K which does not match the model with the absence of structural disorder in the Pnma phase.
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Lack of red-light of current w-LEDs restricts their applications in the field of high color rendering index warm solid-state lighting. Fluoride phosphors doped with Mn⁴⁺ provide an efficient way to resolve the problem. Unfortunately, these excellent Mn⁴⁺ activated fluoride red phosphors are generally synthesized in highly toxic HF solution. In this paper, we report a novel green synthetic route to prepare a non-rare-earth and nonequivalent doping type of K₂NbF₇:Mn⁴⁺ red phosphor with micro-rod-like morphology. The results of XRD and its Rietveld refinement reveal that all the samples possessed only a single phase with monoclinic P21/c structure and the Mn⁴⁺ ions were successfully incorporated into the crystal lattice. The Mn⁴⁺ ions in K₂NbF₇ matrix have a highly distorted octahedral environment, which strengthen the intensity of zero phonon line (ZPL). The strongest absorption and excitation bands appear in blue region of diffuse refection spectra (DRS) and the photoluminescence excitation (PLE) of K₂NbF₇:Mn⁴⁺ sample, which demonstrates the product perfectly match with the most popular GaN-based blue chip. Under blue light illumination, the as-prepared K₂NbF₇:Mn⁴⁺ sample exhibits intense sharp-line red fluorescence (∼628 nm) with internal quantum yield (QY) of 71.3%, an extreme narrow full width at half-maximum (FWHM) of ∼2.8 nm, and an ultra-high-color purity of 99.2%. Impressively, by incorporating K₂NbF₇:Mn⁴⁺ as a red component, a warm w-LED with high CRI of 83.8, low color temperature of 3010 K and high luminous efficiency of 115.91 lm/W is obtained, demonstrating great validity of K₂NbF₇:Mn⁴⁺ red phosphor for white LED devices.
Red emitting K<inf>2</inf>NbF<inf>7</inf>:Mn4+ and K<inf>2</inf>TaF<inf>7</inf>:Mn4+ for warm-white LED applications
2017, Journal of Luminescence
Mn⁴⁺ activated luminescent materials have attracted much attention recently. In particular, alkali metal hexafluorometallates, such as K₂SiF₆:Mn⁴⁺ or K₂TiF₆:Mn⁴⁺, emit light in the red spectral range upon blue excitation and are thus applied onto phosphor converted LEDs (pcLEDs). K₂NbF₇:Mn⁴⁺ and K₂TaF₇:Mn⁴⁺ exhibit efficient red photoluminescence peaking at 627 nm, which can be assigned to the ²E_g → ⁴A_2g intraconfigurational transition of Mn⁴⁺ ([Ar]3d³ configuration) as part of the [MnF₆]^2- octahedra onto the niobium/tantalum site in the respective host structures. Photoluminescence properties, such as temperature dependence of the PL intensity and luminescence decay are presented. Additionally, the band structure of both undoped host materials has been treated with Density Functional Theory (DFT). The theoretical results have been evaluated experimentally with diffuse UV reflectance spectroscopy. Finally, luminous efficacy and color rendering values of simulated warm white emitting pcLEDs comprising a dichromatic phosphor blend employing K₂NbF₇:Mn⁴⁺ or K₂TaF₇:Mn⁴⁺ as the red emitting component are calculated and compared to the performance of warm white emitting pcLEDs comprising different commercial phosphors as red emitting component.

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^∗: Contribution No. 4084.

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Journal of Solid State Chemistry

Abstract

References (13)

J. Solid State Chem.

Structure and Bonding

Mat. Chem. Phys.

J. Luminescence

J. Am. Chem. Soc.

Acta Crystallogr.

Cited by (67)

Raman spectroscopic and theoretical studies on the structure of Ta (V) fluoro and oxofluoro complexes in molten FLiNaK

Calorimetric, dilatometric and DTA under pressure studies of the phase transitions in elpasolite (NH<inf>4</inf>)<inf>2</inf>KZrF<inf>7</inf>

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A green synthetic route to K<inf>2</inf>NbF<inf>7</inf>:Mn<sup>4+</sup> red phosphor for the application in warm white LED devices

Red emitting K<inf>2</inf>NbF<inf>7</inf>:Mn<sup>4+</sup> and K<inf>2</inf>TaF<inf>7</inf>:Mn<sup>4+</sup> for warm-white LED applications