Light-Induced Degradation in Red-Emitting ZnSiF₆·6H₂O:Mn⁴⁺Hydrate Phosphor

Abstract

Photo-induced degradation behaviors of the red-emitting ZnSiF 6 • 6H 2 O:Mn 4+ hydrate phosphor are studied using X-ray diffraction measurement, photoluminescence (PL) analysis, PL excitation spectroscopy, PL decay analysis, diffuse reflectance spectroscopy, and electron spin resonance (ESR) measurement. The ZnSiF 6 • 6H 2 O:Mn 4+ hydrate phosphor exhibits remarkable degradation in the PL intensity under Xe lamp exposure for a few minutes. Coherent laser beams also induce degradation and its degree is in the order of Ar + (488 nm) > He−Cd (325 nm) > He−Ne laser (632.8 nm). The degradation mechanism is found to be due to change in the valence state of manganese ions from Mn 4+ to Mn 5+ by the photooxidation (Mn 4+ → Mn 5+ ) or disproportionation reaction (2Mn 4+ → Mn 3+ + Mn 5+ ). This is supported by the facts that Mn 4+ ion has larger absorbance at 488 nm than at 325 or 632.8 nm and that the Mn 5+ -related absorption structures appear in the diffuse reflectance spectra at wavelengths below ∼700 nm. The ESR measurement also confirms the decreased Mn 4+ spin density in the sample exposed with Xe lamp. A comparative discussion is given on the PL properties of the two different hydrate phosphors, ZnSiF 6 • 6H 2 O:Mn 4+ and K 2 SnF 6 • H 2 O:Mn 4+ .

Synthesis and photoluminescence spectroscopy of BaGeF6:Mn4+ red phosphor

“…We demonstrated a simple method for synthesizing Mn 4+ -activated A 2 BF 6 red phosphors (A = monovalent element, B = quatervalent element), such as K 2 SiF 6 :Mn 4+ and K 2 TiF 6 :Mn 4+ , from aqueous HF/permanganate solutions [1]. More recently, we synthesized BaSiF 6 :Mn 4+ [2], BaTiF 6 :Mn 4+ [3], and ZnSiF 6 Á6H 2 O:Mn 4+ red phosphors [4,5]. These phosphors exhibit efficient red emission under blue or UV excitation and may thus meet efficacy and color-quality targets for future solid-state-lighting devices [6].…”

Photo-induced degradation and thermal decomposition in ZnSnF6·6H2O:Mn4+ red-emitting phosphor

“…Recently, we synthesized some hexafluorometallate red phosphors in a family of A I 2 BF 6 :Mn 4+ or A II BF 6 :Mn 4+ (A I = K, Na, Cs, NH 4 ; A II = Zn, Ba; B = Si, Ge, Sn, Ti) and reported their phosphor properties in detail [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. These phosphors exhibit efficient red emission under blue or UV excitation and may thus meet efficacy and color-quality targets for future white-LED devices.…”

“…These phosphors exhibit efficient red emission under blue or UV excitation and may thus meet efficacy and color-quality targets for future white-LED devices. Note that several phosphors can be present in a form of hydrate (K 2 SnF 6 ÁH 2 O, ZnSiF 6 Á6H 2 O, and ZnGeF 6 Á6H 2 O) with exhibiting a unique instability against vacuum evacuation, immersion in methanol, near UV-visible light irradiation, or thermal annealing [10,11,[18][19][20].…”

“…To the best of our knowledge, any ZnSnF 6 hydrate material has been reported up to date. It was reported that ZnSiF 6 Á6H 2 O and ZnGeF 6 Á6H 2 O hexahydrates can be stably formed and activated by Mn 4+ ions as red phosphors [18][19][20]. These hexahydrate phosphors are pyrolytically decomposed into metal oxide/fluoride (GeO 2 , ZnF 2 ) above 130-200°C and, therefore, show no photoluminescence (PL) emission anymore.…”

“…The present study demonstrates that ZnSnF 6 Á6H 2 O hexahydrate material can be stably formed and activated by Mn 4+ ions. In the case of ZnSiF 6 Á6H 2 O:Mn 4+ and ZnGeF 6 Á6H 2 O:Mn 4+ [18][19][20], thermal-annealing-induced dehydration was never observed; however, ZnSnF 6 Á6H 2 O:Mn 4+ shows dehydration after thermal annealing above $100°C (ZnSnF 6 Á6H 2 O:Mn 4+ ? ZnSnF 6 :Mn 4+ ).…”