Crystal Structure, Characterization, and Luminescence Properties of Mn⁴⁺‐Doped K₃Nb₂O₄F₅

Abstract

In this paper, we present the high-pressure/high-temperature synthesis and characterization of the potassium fluoridooxidoniobate K 3 Nb 2 O 4 F 5 . Single-crystal analysis revealed that the phase crystallizes in the trigonal space group R � 3m (hR18) with a = 5.799(2), c = 21.371(4) Å, V = 622.4(4) Å 3 and Z = 3 at T = 299 K exhibiting a structure type closely related to that of Ba 2 RbFe 2 F 9 . The assignment of the anion positions to oxygen and fluorine is based on crystallographic data and BLBS/CHARDI calculations. Further characterization via EDX spectroscopy was carried out, corroborating the ratio of K to Nb. Doping of the title compound with Mn 4 + was achieved in a secondary step using a ball-mill, resulting in a red phosphor material, which was additionally analyzed by luminescence spectroscopy. The emission maximum is located at λ max = 630 nm.

K3Mo2O5.6F3.4 and K3V2O3.3F5.7 – exploring transition metal cation valence and anion distribution in oxyfluorides

“…19 K 3 Nb 2 O 4 F 5 is the only other known A 3 M 2 X 9 representative containing potassium and was first reported by our group early last year. 18 While quite similar to K 3 V 2 O 4 F 5 and K 3 Mo 2 O 4 F 5 at first glance, the oxyfluorides reported in the present investigation were synthesized under high-pressure/high-temperature conditions and crystallize in a different structure type, namely Ba 2 RbFe 2 F 9 . 20 Another common point of contention in oxy-fluorides is the oxidation state of the transition metal cations.…”

“…In contrast, K 3 Nb 2 O 4 F 5 , which shares all the main structural features with the two oxyfluorides presented here, but exhibits a different anion distribution and deflection of the transition metal cation, shows essentially no residual electron density at that position. 18 The main M1 octahedral void in K 3 V 2 O 3.3 F 5.7 can host both V 5+ ( r V 5+ = 0.68 Å) and V 4+ ( r V 4+ = 0.72 Å), making it so only very few atoms find their way into the larger, less adequate M2 octahedral void in between the main layers. For K 3 Mo 2 O 5.6 F 3.4 , the size difference between the two available transition metal cations is more pronounced ( r Mo 6+ = 0.73 Å, r Mo 4+ = 0.79 Å), rendering one octahedral void much more suitable for a cation of a given charge than the other.…”

“…A detailed description of the packing of atoms can be found in our previous publication for K 3 Nb 2 O 4 F 5 , which also crystallizes in the Ba 2 RbFe 2 F 9 structure type. 18…”

“…A detailed description of the packing of atoms can be found in our previous publication for K 3 Nb 2 O 4 F 5 , which also crystallizes in the Ba 2 RbFe 2 F 9 structure type. 18 Fundamentally, the crystal structures consist of corner sharing [VA 6 ] n− or [MoA 6 ] n− octahedra (M1) forming wavelike layers in the ab plane (Fig. 1).…”

“…12 As for potassium variants, three representatives containing only pentavalent transition metal cations have been reported before: K 3 V 2 O 4 F 5 , 14–16 K 3 Mo 2 O 4 F 5 17 and K 3 Nb 2 O 4 F 5 . 18 The structure of K 3 V 2 O 4 F 5 has never been elucidated in detail, and two different datasets are available for K 3 Mo 2 O 4 F 5 : a powder diffraction file (PDF) entry from 1983 and a complete structure determination from 2012. 19 K 3 Nb 2 O 4 F 5 is the only other known A 3 M 2 X 9 representative containing potassium and was first reported by our group early last year.…”