Mechanisms of Tenebrescence and Persistent Luminescence in Synthetic Hackmanite Na₈Al₆Si₆O₂₄(Cl,S)₂

Abstract

Synthetic hackmanites, Na8Al6Si6O24(Cl,S)2, showing efficient purple tenebrescence and blue/white persistent luminescence were studied using different spectroscopic techniques to obtain a quantified view on the storage and release of optical energy in these materials. The persistent luminescence emitter was identified as impurity Ti(3+) originating from the precursor materials used in the synthesis, and the energy storage for persistent luminescence was postulated to take place in oxygen vacancies within the aluminosilicate framework. Tenebrescence, on the other hand, was observed to function within the Na4(Cl,S) entities located in the cavities of the aluminosilicate framework. The mechanism of persistent luminescence and tenebrescence in hackmanite is presented for the first time.

Synthesis and Features of Luminescent Bromo- and Iodohectorite Nanoclay Materials

“…It can thus be assumed that a titanium-related center is responsible for the blue emission in Br-Hec and I-Hec. According to previous literature, this blue emission can be attributed to TiO 6 entities as reported for Benitoite (BaTiSi 3 O 9 ) [43] or to Ti 3+ -V O (oxygen vacancy) pairs [23,24,47,48]. Moreover, the doping of the X-Hec materials with Ti 3+ (x Ti = 0.1 mol %) increases the intensity of the main emission ( Figure A6), without significant changes to its band position confirming titanium as the luminescent center.…”

“…The multisite origin for emission is also witnessed in the decrease of emission band width [42] with increasing excitation wavelength. Similar blue-green emission has previously been observed for other silicates such as fluorohectorite [23], chlorohectorite [24], montmorillonite [43], kaolinite [44], pyrophyllite [45], topaz (Al2SiO4(F,OH)2) [46], synthetic hackmanite (Na8Al6Si6O24(Cl,S)2) [47,48], benitoite (BaTiSi3O9) [49], and SiO2 [50,51]. The literature presents mainly two different explanations for the origin of this blue-green emission.…”

“…One proposition categorizes this emission as photoelectric response as well as inner center and recombination type of luminescence [52], attributing the blue emission to triplet-singlet transition in twofold coordinated Si centers [53], due to oxygen deficit and irradiation effects [50]. The second proposition associates the emission with luminescent centers containing trivalent titanium paired with an oxide vacancy, which may exist either as matrix element or as an impurity [23,24,[46][47][48][49].…”

Lanthanide and Heavy Metal Free Long White Persistent Luminescence from Ti Doped Li–Hackmanite: A Versatile, Low‐Cost Material

“…Among them, Ti 3+ has many unique properties such as broad absorption and emission bands as well as color tuning from blue, yellow, to red and even near‐infrared (NIR) depending on the chemical environment enabling white emission. Moreover, the Ti 3+ emission is highly efficient: the emission coming from both undoped zirconia and undoped synthetic hackmanite Na 8 Al 6 Si 6 O 24 (Cl,S) 2 has been attributed to the Ti 3+ present as impurities in the order of 100 ppm. Both these materials are capable of showing cold white PeL even with this few ppm of Ti 3+ , but the latter seems to have a very good trap structure, because its persistence can go beyond 1 h .…”

“…Moreover, the Ti 3+ emission is highly efficient: the emission coming from both undoped zirconia and undoped synthetic hackmanite Na 8 Al 6 Si 6 O 24 (Cl,S) 2 has been attributed to the Ti 3+ present as impurities in the order of 100 ppm. Both these materials are capable of showing cold white PeL even with this few ppm of Ti 3+ , but the latter seems to have a very good trap structure, because its persistence can go beyond 1 h . Hackmanite with Ti 3+ doping was thus used as the starting point for obtaining efficient white PeL.…”