Phase Separation and Nanocrystallization in KF–ZnF₂–SiO₂ Glasses: Lessons from Solid-State NMR

Abstract

In the present paper, we studied the structural evolution of the glass matrix and formation of crystalline fluoride phases for 15KF−15ZnF 2 − 70SiO 2 (glass A) and 25KF−25ZnF 2 −50SiO 2 (glass B) glasses upon thermal treatment employing transmission electron microscopy, X-ray diffraction (XRD), and solid-state NMR. The first step marks a phase separation into an SiO 2 rich phase and a phase enriched with Zn, K, and fluoride. As shown by XRD, depending on the nominal compositions, K 2 SiF 6 (for 15KF−15ZnF 2 −70SiO 2 ) and KZnF 3 (for 25KF−25ZnF 2 −50SiO 2 ) are found as the first crystalline phases being formed. Upon longer heat treatment, ZnF 2 is additionally formed in both cases. Surprisingly, a significant amount of SiF 6 2− units is detected employing 29 Si-MAS and 29 Si{ 19 F}CPMAS-NMR spectroscopy even in the base glass, which is completely amorphous according to the X-ray results. The vast majority of Si, however, is found in an exclusive SiO 4 environment as Q 4 , Q 3 , or Q 2 units. The large fraction of nonbridging oxygens per SiO 4 tetrahedron (ca. 0.7 for glass A and 0.65 for glass B) indicates quite large fluorine loss during glass synthesis (approx. 60−80%). Employing dipolar NMR techniques, i.e., 29 Si−{ 19 F}-REDOR (rotational echo double resonance) and 29 Si{ 19 F}CPMAS-NMR (cross polarization magic angle spinning), the presence of Si−F connectivity (in excess of the identified SiF 6 2− units) could be ruled out. The observed differences in the 29 Si− 19 F heteronuclear dipole couplings between the Si-Q 4 and Si Q 3 unitsas determined by REDOR NMRare compatible with the assumption of a core−shell structure with a mixed cation fluoride as the core and an SiO 2 -enriched shell.

2.5–5.5 μm mid‐infrared emission from Ni2+‐doped chalcohalide glass ceramics containing CsPbI3 perovskite nanocrystals

“…This implies that phase separation occurs in the glass preparation process. 27,32 For the glass ceramic, the CsPbI 3 nanoparticles are uniformly distributed in the glassy matrix and the grain size increases to ~20 nm (see Figure 2B), which is consistent with the crystal size estimated from the XRD pattern. Figure 2C shows the high-resolution TEM image of a single CsPbI 3 nanocrystalline grain.…”

Crystallization control in Ni2+‐doped glass‐ceramics for broadband near‐infrared luminesce

“…In the past several years, numerous optical investigations have been conducted on Ni 2+ ‐doped oxyfluoride glass‐ceramics. For example, broadband luminescence at 1550 and 1750 nm has been reported in the Ni 2+ ‐doped oxyfluoride glass‐ceramics embedded with Rb 2 SiF 6 and KZnF 3 nanocrystals . The full width at half maximum (FWHM) of these material candidates is only around 500 nm.…”

“…For example, broadband luminescence at 1550 and 1750 nm has been reported in the Ni 2+ -doped oxyfluoride glass-ceramics embedded with Rb 2 SiF 6 and KZnF 3 nanocrystals. [15][16][17][18] The full width at half maximum (FWHM) of these material candidates is only around 500 nm. Thus, the luminescence bandwidth should be further extended.…”

Structural phase evolved Ni2+‐doped fluoride nanocrystals in KF−ZnF2−SiO2 glass‐ceramics

“…The droplet phase separation occurs in 15KF‐15ZnF 2 ‐70SiO 2 (KZS70) glass with a core‐shell structure which mixed cationic fluorides act as the core and SiO 2 enriched shell. However the interpenetrating phase separation is evolved in 25KF‐25ZnF 2 ‐50SiO 2 (KZS50) glass under thermal annealing 12 . In the past few years, because the KF‐ZnF 2 ‐SiO 2 glass can provide tunable precipitated fluorides and reasonable unique spectroscopic properties by controlled APS, many efforts have been devoted to utilize them as the hosts of REIs or TMIs, making it possible to be employed in photonic applications 13‐15 .…”

“…However the interpenetrating phase separation is evolved in 25KF-25ZnF 2 -50SiO 2 (KZS50) glass under thermal annealing. 12 In the past few years, because the KF-ZnF 2 -SiO 2 glass can provide tunable precipitated fluorides and reasonable unique spectroscopic properties by controlled APS, many efforts have been devoted to utilize them as the hosts of REIs or TMIs, making it possible to be employed in photonic applications. [13][14][15] Hence, such KF−ZnF 2 −SiO 2 oxyfluoride GCs are promising for the photonic applications based on REIs and TMIs.…”