Nature of the Color of Borates with “Anti-Zeolite” Structure

Abstract

Crystals of the Mn Ba(BO)F phase were grown from a high-temperature solution. This new fluoride borate is built of positively charged [Ba(BO)] blocks, the so-called "anti-zeolite" pattern. Using X-ray single-crystal diffraction, the bulk atomic arrangement in the centrosymmetric tetragonal unit cell in I4/ mcm could be elucidated. Crystals of the (MnF) group-containing solid solution Mn Ba(BO)F are dark brown in color in contrast to the differently colored crystals of (LiF) group-containing "anti-zeolite" LiBa(BO)F ( P4 bc). According to the electron spin resonance and optical spectroscopic investigation, the absorption spectrum of LiBa(BO)F crystals results from the absorption of light by both exciton and free charge carriers and can be tuned by varying the initial composition of the high-temperature solution.

Electrides: a review

“…64 Interestingly, recent work reports anti-zeolite solids that are positively charged borate frameworks that can also host a range of guest counter-anions. 65 Of several materials reported in this work they report the direct observation of free careers in LiBa 12 (BO 3 ) 7 F 4 , which could indicate that this material is another rigid-framework electride, just like C12A7:e À .…”

Optical and Magnetic Properties of Cu-Containing Borates with “Antizeolite” Structure

“…It is highly possible for Ba 3 (BO 3 ) 2– x F 3 x crystals, both undoped and activated with copper, to possess the great number of oxygen anions and electrons, causing intense light absorption in the visible region of the spectrum. We believe that similar to other colored antizeolite crystals, the absorption spectrum of Ba 3 (BO 3 ) 2– x F 3 x :Cu crystals results from the absorption of light by both exciton and free charge carriers . The exciton is formed when the electron of the valence band is transferred to an excited state but remains bound to the hole.…”

“…The angle between the z axis (F 6 –Cu–F 6 ) of the center and the crystallographic axis b is 29° (Figure b). The evaluated values for the lines of the first set ( g z = 2.411) are 63 A = 79 ± 3, 19 A = 40 ± 5 G. Six-seven lines of the second complex ( g y = 2.123) with various width and intensity, located at distance of 59 ± 7 Gs, could be caused by interactions of an unpaired electron (hole) of Cu 2+ with six nonequivalent 19 F nuclear moments ( A y = 59 ± 7 Gs). Besides, on the extreme low-intensive line ( g = 2.077) we observed additional splitting to 4 lines ( 63 A y = 19 ± 3 Gs) which are due to an unpaired electron interacting with own nuclear moment I = 3/2 of 63 Cu.…”

“…The “antizeolite” borates Ba 3 (BO 3 ) 2– x F 3 x , (Li,Na)­Ba 12 (BO 3 ) 7 F 4 , and Mn x Ba 12 (BO 3 ) 8–2 x F 8 x show linear dichroism and hence can be applied for dichroic polarization . According to electron spin resonance and optical spectroscopic investigations, the coloring of antizeolitic borates is understood by the light absorption by both the exciton (electron of the valence band which is transferred to an excited state but remains bound to the hole) and free charge carriers . The light absorption of antizeolitic borates is tunable by varying the packing degree of B-layers.…”

“…The consequent variation with the lowest and the highest Li + concentration in B-layers resulted in deep purple and colorless crystals, respectively. The shift of the absorption edge might be connected with the exciton mechanism …”