Structural field of K₂Al₂B₂O₇-family crystals

Abstract

The known crystal structures of borate compounds with general composition A I 2 M III 2 B 2 O 7 (A I : Na, K, Cs, Rb) have been considered. The structures in space groups P6 3 /m, P-31c, P321 and P2 1 /c have been found depending on cation combination. Criterion k = (r(M III )+r(B)+2r(O))/r(A I ) has been formulated and it controls the formation of trigonal K 2 Al 2 B 2 O 7 (KABO) type structure. The existence of noncentrosymmetric KABO-type borates is possible over the range of k = 2.7 -2.06. This range is promising for searching new borate crystals and solid solutions with pronounced nonlinear optical properties.

Blue-Emitting K2Al2B2O7:Eu2+ Phosphor with High Thermal Stability and High Color Purity for Near-UV-Pumped White Light-Emitting Diodes

“…Rare earth ions doped borate compounds as the luminescence materials have been widely investigated, as a result of their low synthetic temperature and good physical and chemical stability. − As for the potassium aluminum borate K 2 Al 2 B 2 O 7 (KAB), the powder of KAB was preliminarily studied by Kaduk and Satek at first, and then the crystal of KAB was found to be a new UV nonlinear optical crystal. , Therefore, the crystal growth of KAB and its detail structure were intensively studied due to its easy crystal growth, good chemical stability, and mechanical properties and high UV transparency. − Recently, the optical properties of Dy 3+ doped KAB for n-UV excited w-LEDs were reported by Palaspagar et al To the best of our knowledge, the luminescence properties of Eu 2+ doped KAB (KAB:Eu 2+ ) and its potential application in w-LEDs have not yet been reported in the literature.…”

Beryllium‐Free β‐Rb2Al2B2O7 as a Possible Deep‐Ultraviolet Nonlinear Optical Material Replacement for KBe2BO3F2

“…These isostructural materials are, however, centrosymmetric (CS) . The centricity of the A 2 Al 2 B 2 O 7 (A=K, Rb, Cs) materials is thought to be correlated with the size of the alkali cation . Centrosymmetry is observed as the size of the alkali cation increases.…”

“…Centrosymmetry is observed as the size of the alkali cation increases. As α‐RABO is CS, it was suggested that NCS β‐RABO would not exist …”

“…β‐RABO is isostructural to KABO (Figure ) . For A 2 Al 2 B 2 O 7 (A=K, Rb, Cs), the building units of corner‐sharing AlO 4 5− tetrahedra have a tendency to rotate to accommodate larger alkaline cations, resulting in the evolution from NCS (S.G. P 321) to CS (S.G. P 2 1 / c ) structures (Figure ) . The effective atomic radius of the Rb + cation (1.66 Å) is between K + (1.51 Å) and Cs + (1.85 Å) cations, which may explain why only RABO is capable of exhibiting the two, i.e., NCS β‐RABO and CS α‐RABO polymorphs …”

“…We also compare the energetics of the family of aluminum borates as a function of alkali cation size in order to determine their stability with DFT calculations. CS α‐RABO and NCS β‐RABO exist as a pair, but only CS CABO (Cs 2 Al 2 B 2 O 7 ) and NCS KABO have been synthesized . Notably, α‐RABO and CABO have the same packing motif but different crystal structures.…”