KTm[B4O6(OH)4] ⋅ 3H2O: A New Member of Borate Family with Mica-like Tetrahedral Layers

Crystals of new borate KTm[B4O6(OH)4] ⋅ 3H2O (sp. gr. Р $$\bar {3}$$ 1m, a = 4.5472(7) A, c = 12.151(3) A) have been obtained under hydrothermal conditions at T = 280°C and P = 100 atm. Their structure contains packets of two polar mica-like tetrahedral layers [B4O6(OH)4]∞∞, connected by TmO6 octahedra. The interpacket space is occupied statistically by K atoms and water molecules. A similar layer is typical of peprossiite and its synthetic analogue NdAl2.07[B4O10]O0.6 and KTa[B4O6(OH)4](OH)2 ⋅ 1.33H2O, which have another sp. gr. P $$\bar {6}$$ 2m and trigonal-prismatic coordination of heavy atoms. The structure of the new compound is compared with two other structures of this family: KGd[B6O10(OH)2] and KHo[B6O10(OH)2]. A previously unknown combination of simple mica-like layers and octahedra in one packet is implemented in the new member of the family. All compounds under consideration are characterized by disorder and statistical occupancy of sites, especially in the interpacket space. The KTm[B4O6(OH)4] ⋅ 3H2O crystals demonstrate strong emission of blue light due to the radiative 4f–4f  transitions of the Tm3+ cation. The most intense transition in the photoluminescence spectrum is 1D2–3F4 at 450 nm.