An
ionothermal synthesis study of transition metal phosphates using the
ionic liquid 1-butyl-4-methylpyridinium hexafluorophosphate [C4mpy][PF6] yielded four new, different open framework
manganese compounds, that is, K2Mn3(HPO4)2(PO3F)F2 (1), (NH4)2Mn3(HPO4)2(PO3F)F2 (2), KMn3(H2PO4)(HPO4)2F2 (3), and (NH4)Mn3(H2PO4)(PO3F)2F2 (4). The obtained products not only feature new framework topologies
unprecedented in the family of phosphates but also interesting properties
as the transition metal gives rise to both luminescent (rendering
them potential nonrare earth containing red emitting phosphors) and
unconventional magnetic properties governed by geometric frustrations.
Aside from the structural analysis (powder and single-crystal X-ray
diffraction, infrared spectroscopy), a variety of characterization
methods (photoluminescence spectroscopy and magnetic measurements)
were applied to study the material’s properties. Single crystal
X-ray studies reveal that 1 (P21/c with a = 5.501(1), b = 14.203(3), c = 16.905(4) Å, β
= 108.65(3)°, V = 1251.4 Å3,
and Z = 4) and 2 (P2
1
/c with a = 5.587(1), b = 14.507(3), c =
17.364(3) Å, β = 108.75(3)°, V =
1332.6(5) Å3, and Z = 4) feature
S-shaped 18-ring channels along [100], which are formed by trimer-Mn3O9F2 chains parallel to [100] and interconnecting
PO3(OH) and PO3F tetrahedra. The structure of
compounds 3 (C2/c with a = 20.307(4), b = 7.635(1), c = 7.834(2) Å, β = 103.26(3)°, V = 1182.2(4) Å3, and Z = 4) and 4 (C2/c with a = 20.402(4), b = 7.673(1), c =
7.845(2) Å, β = 103.56(3)°, V = 1193.8(4)
Å3, and Z = 4) are characterized
by layers, which are built of Mn3O8F4 octahedra trimers, with Kagomé topology parallel to the bc plane featuring 3,6-ring channels. The layers are stacked
according to a sequence of AA
i
along the a axis. Taking into account the [P(2)O3(OH)/P(2)O3F] tetrahedra, the Kagomé layers are replenished to
a Mn3O2(HPO4)/Mn3O2(PO3F) composition, which are interlinked by [P(1)O2(OH)2] forming 10-ring channels parallel to [001].
Charge compensation of the macroanions is achieved by K+ (1 and 3) and (NH4)+ (2 and 4) cations. At room temperature,
compounds 1–4 demonstrate a reddish
orange emission ascribed to the spin-forbidden 4T1g(4G) → 6A1g(6S)
transition of the Mn2+ ions. Upon lowering the temperature
to 77 K, the emission of each compound is red-shifted and becomes
pure red. Compounds 1 and 2 contain spin
trimers with a presumable doubled ground state. The intertrimer magnetic
coupling is relatively weak, and small ferrimagnetic domains are possible
in 1. The magnetic behavior of 3 and 4 can be considered as antiferromagnetic. This can be understood
as their staircase Kagomé lattices are distorted, meaning that
the intrinsic geometrical frustration is lifted.