Structural, optical and thermal studies of Eu ions in lithium fluoroborate glasses

Borate glasses doped with trivalent europium were prepared by the conventional melt quenching technique, in the chemical composition of (49.99-x)B₂O₃ + 25Li₂O + 25LiF+xEu₂O₃ by varying the concentration of the rare earth ion in the order 0.01, 0.1, 1, 2 and 3 wt% and their structural, luminescence and thermal behavior have been reported. The XRD and FTIR spectra reveal the glass structure and the functional groups. The UV–VIS, luminescence spectra and lifetime of the Eu³⁺ ions were measured. The local site symmetry around the Eu³⁺ ions were evaluated through the luminescence intensity ratio (R) of the ⁵D₀ → ⁷F₂ to ⁵D₀ → ⁷F₁ transitions. Optical measurements have been carried out to explore the optical properties such as bonding parameters, Judd–Ofelt parameters, stimulated emission cross-section, transition probability, branching ratio, radiative lifetime, etc. The lifetime measurements of the ⁵D₀ level as a function of the concentration of Eu³⁺ ion have been found and is comparable to other reported for Eu³⁺ doped borate, phosphate glasses and higher than that for the tellurite glasses. The thermal properties such as glass transition, crystallization and melting temperatures of the Eu³⁺ glasses were studied through the DSC traces in the temperature range of 30−1200 °C at a heating rate of 10 °C per minute. The change in optical properties with the variation of Eu³⁺ ion concentration have been discussed and compared with similar results.     

A pH-activatable and aniline-substituted photosensitizer for near-infrared cancer theranostics

This work reports a newly designed pH-activatable and aniline-substituted aza-boron-dipyrromethene as a trifunctional photosensitizer to achieve highly selective tumor imaging, efficient photodynamic therapy (PDT) and therapeutic self-monitoring through encapsulation in a cRGD-functionalized nanomicelle. The diethylaminophenyl is introduced in to the structure for pH-activatable near-infrared fluorescence and singlet oxygen (¹O₂) generation, and bromophenyl is imported to increase the ¹O₂ generation efficiency upon pH activation by virtue of its heavy atom effect. After encapsulation, the nanoprobe can target α_vβ₃ integrin-rich tumor cells via cRGD and is activated by physiologically acidic pH for cancer discrimination and PDT. The fascinating advantage of the nanoprobe is near-infrared implementation beyond 800 nm, which significantly improves the imaging sensitivity and increases the penetration depth of the PDT. By monitoring the fluorescence decrease in the tumor region after PDT, the therapeutic efficacy is demonstrated in situ and in real time, which provides a valuable and convenient self-feedback function for PDT efficacy tracking. Therefore, this rationally designed and carefully engineered nanoprobe offers a new paradigm for precise tumor theranostics and may provide novel opportunities for future clinical cancer treatment.     

Photochemical and photophysical properties of lumazine in aqueous solutions

Lumazine (pteridine-2,4(1H,3H)-dione, LU) was investigated for its efficiency of singlet oxygen (¹O₂) production and quenching in aqueous solution. The quantum yield of ¹O₂ production (Φ_Δ) was determined by measurements of the ¹O₂ luminescence in the near-infrared upon continuous excitation of the sensitizer. Values of Φ_Δ are sensitive to the pH and were found to be 0.44 ± 0.01 and 0.080 ± 0.004 in acidic and alkaline media, respectively. The photochemical stability of LU was investigated under different pH conditions, in the presence and in the absence of O₂. The photochemical consumption of LU in aqueous solution at room temperature under irradiation at 350 nm was followed by UV–vis spectrophotometry and HPLC. Values of the quantum yields of LU disappearance are low, indicating that LU is rather photostable under physiological conditions.     

Multifunctional Logic in a Photosensitizer with Triple‐Mode Fluorescent and Photodynamic Activity

Herein we describe a photosensitizer (PS) with the capacity to perform multiple logic operations based on a pyrene‐containing phthalocyanine (Pc) derivative. The system presents three output signals (fluorescence at 377 and 683 nm, and singlet oxygen (¹O₂) production), which are dependent on three inputs: two chemical (concentration of dithiothreitol (DTT) and acidic pH) and one physical (visible light above 530 nm for ¹O₂ sensitization). The multi‐input/multioutput nature of this PS leads to single‐, double‐, and triple‐mode activation pathways of its fluorescent and photodynamic functions, through the interplay of various interrelated AND, ID, and INHIBIT gates. Dual fluorescence emissions are potentially useful for orthogonal optical imaging protocols while ¹O₂ is the main reactive species in photodynamic therapy (PDT). We thus expect that this kind of PS logic system will be of great interest for multimodal cellular imaging and therapeutic applications.     

Luminescence and long-lasting afterglow in Mn<Superscript>2+</Superscript> and Eu<Superscript>3+</Superscript> co-doped ZnO–GeO<Subscript>2</Subscript> glasses and glass ceramics prepared by sol–gel method

To develop new fluorescent and afterglow materials, Mn²⁺ and Eu³⁺ co-doped ZnO–GeO₂ glasses and glass ceramics were prepared by a sol–gel method and their optical properties were investigated by measuring luminescence, excitation and afterglow spectra, and luminescence quantum yield (QY). Under UV irradiation at 254 nm, some glasses and all of the glass ceramics showed green luminescence peaking at 534 nm due to the ⁴T₁ → ⁶A₁ transition of tetrahedrally coordinated Mn²⁺ ions. The strongest luminescence was observed in a glass ceramic of 0.1MnO–0.3Eu₂O₃–25ZnO–75GeO₂ heat treated at 900 °C, with QY of 49.8%. All of the green-luminescent glasses and glass ceramics showed green afterglow, and the afterglow lasting for more than 60 min was obtained in a glass ceramic heat treated at 900 °C. It is considered that the Eu³⁺ ions may behave as electron trapping centers to be associated with the occurrence of the green afterglow due to the Mn²⁺ ions in the co-doped system.     

A Comparison of Dose Metrics to Predict Local Tumor Control for Photofrin‐mediated Photodynamic Therapy

This preclinical study examines light fluence, photodynamic therapy (PDT) dose and “apparent reacted singlet oxygen,” [¹O₂]_rx, to predict local control rate (LCR) for Photofrin‐mediated PDT of radiation‐induced fibrosarcoma (RIF) tumors. Mice bearing RIF tumors were treated with in‐air fluences (50–250 J cm^?2) and in‐air fluence rates (50–150 mW cm^?2) at Photofrin dosages of 5 and 15 mg kg^?1 and a drug‐light interval of 24 h using a 630‐nm, 1‐cm‐diameter collimated laser. A macroscopic model was used to calculate [¹O₂]_rx and PDT dose based on in vivo explicit dosimetry of the drug concentration, light fluence and tissue optical properties. PDT dose and [¹O₂]_rx were defined as a temporal integral of drug concentration and fluence rate, and singlet oxygen concentration consumed divided by the singlet oxygen lifetime, respectively. LCR was stratified for different dose metrics for 74 mice (66 + 8 control). Complete tumor control at 14 days was observed for [¹O₂]_rx ≥ 1.1 mm or PDT dose ≥1200 μm J cm^?2 but cannot be predicted with fluence alone. LCR increases with increasing [¹O₂]_rx and PDT dose but is not well correlated with fluence. Comparing dosimetric quantities, [¹O₂]_rx outperformed both PDT dose and fluence in predicting tumor response and correlating with LCR.     

Defective [Bi2O2]2+ Layers Exhibiting Ultrabroad Near-Infrared Luminescence

Aurivillius phases have been routinely known as excellent ferroelectrics and have rarely been deemed as materials that luminesce in the near-infrared (NIR) region. Herein, it is shown that the Aurivillius phases can demonstrate broadband NIR luminescence that covers telecommunication and biological optical windows. Experimental characterization of the model system Bi_2.14Sr_0.75Ta₂O_9−x, combined with theoretical calculations, help to establish that the NIR luminescence originates from defective [Bi₂O₂]²⁺ layers. Importantly, the generality of this finding is validated based on observations of a rich bank of NIR luminescence characteristics in other Aurivillius phases. This work highlights that incorporating defects into infinitely repeating [Bi₂O₂]²⁺ layers can be used as a powerful tool to space-selectively impart unusual luminescence emitters to Aurivillius-phase ferroelectrics, which not only offers an optical probe for the examination of defect states in ferroelectrics, but also provides possibilities for coupling of the ferroelectric property with NIR luminescence.     

Self-Amplified Photodynamic Therapy through the 1O2-Mediated Internalization of Photosensitizers from a Ppa-Bearing Block Copolymer

Nanocarriers are employed to deliver photosensitizers for photodynamic therapy (PDT) through the enhanced penetration and retention effect, but disadvantages including the premature leakage and non-selective release of photosensitizers still exist. Herein, we report a ¹O₂-responsive block copolymer (POEGMA-b-P(MAA-co-VSPpaMA) to enhance PDT via the controllable release of photosensitizers. Once nanoparticles formed by the block copolymer have accumulated in a tumor and have been taken up by cancer cells, pyropheophorbide a (Ppa) could be controllably released by singlet oxygen (¹O₂) generated by light irradiation, enhancing the photosensitization. This was demonstrated by confocal laser scanning microscopy and in vivo fluorescence imaging. The ¹O₂-responsiveness of POEGMA-b-P(MAA-co-VSPpaMA) block copolymer enabled the realization of self-amplified photodynamic therapy by the regulation of Ppa release using NIR illumination. This may provide a new insight into the design of precise PDT.     

A Targeting Singlet Oxygen Battery for Multidrug-Resistant Bacterial Deep-Tissue Infections

Traditional photodynamic therapy (PDT) is dependent on externally applied light and oxygen, and the depth of penetration of these factors can be insufficient for the treatment of deep infections. The short half-life and short diffusion distance of reactive oxygen species (ROS) also limit the antibacterial efficiency of PDT. Herein, we designed a targeting singlet oxygen delivery system, CARG-Py, for irradiation-free and oxygen-free PDT. This system was converted to the “singlet oxygen battery” CARG-¹O₂ and released singlet oxygen without external irradiation or oxygen. CARG-¹O₂ is composed of pyridones coupled to a targeting peptide that improves the utilization of singlet oxygen in deep multidrug-resistant bacterial infections. CARG-¹O₂ was shown to damage DNA, protein, and membranes by increasing the level of reactive oxygen inside bacteria; the attacking of multiple biomolecular sites caused the death of methicillin-resistant Staphylococcus aureus (MRSA). An in vivo study in a MRSA-infected mouse model of pneumonia demonstrated the potential of CARG-¹O₂ for the efficient treatment of deep infections. This work provides a new strategy to improve traditional PDT for irradiation- and oxygen-free treatment of deep infections while improving convenience of PDT.     

An Iridium Complex as an AIE-active Photosensitizer for Image-guided Photodynamic Therapy

Image-guided photodynamic therapy (PDT) has received growing attention due to its non-invasiveness and precise controllability. However, the PDT efficiency of most photosensitizers are decreased in living system due to the aggregation-caused singlet oxygen (¹O₂) generation decreasing. Herein, we present an Iridium (III) pyridylpyrrole complex (Ir-1) featuring of aggregation-induced emission (AIE) and ¹O₂ generation characteristics for image-guided PDT of cancer. Ir-1 aqueous solution exhibits bright red phosphorescence peaked at 630 nm, large Stokes shift of 227 nm, and good ¹O₂ generation ability. The ¹O₂ generating rate of Ir-1 in EtOH/water mixture solution is 2.3 times higher than that of Rose Bengal. In vitro experimental results revealed that Ir-1 has better biocompatibility and higher phototoxicity compared with clinically used photosensitizers (Rose Bengal and Ce6), suggesting that Ir-1 can serve as a photosensitizer for image-guided PDT of cancer.     

Photophysical Insights of Halogenated Dipyrrolonaphthyridine-Diones as Potential Photodynamic Therapy Agents†

We report the synthesis and photophysical characterization of novel halogenated dipyrrolonaphthyridine-diones (X₂–DPNDs, X = Cl, Br, and I), as candidates for photodynamic therapy (PDT) application. Apart from the heavy atom-induced spin-orbit coupling (SOC) dynamics in the investigated X₂–DPNDs, it was found that the position of the halogen atom (relative to the nitrogen of the pyrrole ring) also influenced the triplet excited state behavior. Interestingly, the faster/efficiency sensitization of ³O₂ to ¹O₂ using X₂–DPND correlates with the rate of triplet population, k_ISC >1.6 × 10⁸ s⁻¹ for I₂–DPND vs k_ISC >2.9 × 10⁹ s⁻¹ for Cl₂–DPND and Br₂–DPND (where τ_ISC = 343 ± 3 ps for I₂–DPND and τ_ISC = 5–6 ns for Cl₂–DPND and Br₂–DPND are the lowest time constants/values for ISC). Furthermore, the heavy atom-induced SOC in Cl₂–DPND and Br₂–DPND did not lead to a reduction of the corresponding fluorescence (ca 75% vs 67% for the parent DPND). The attractive photophysical characteristics of Cl₂/Br₂–DPND put them on the landscape as not only promising PDT agents but also as fluorescence probes. The present study is a stepping stone in the development of novel organic photosystems for synergistic photomedicinal applications.     

Self‐Amplified Photodynamic Therapy through the 1O2‐Mediated Internalization of Photosensitizers from a Ppa‐Bearing Block Copolymer

Nanocarriers are employed to deliver photosensitizers for photodynamic therapy (PDT) through the enhanced penetration and retention effect, but disadvantages including the premature leakage and non‐selective release of photosensitizers still exist. Herein, we report a ¹O₂‐responsive block copolymer (POEGMA‐b‐P(MAA‐co‐VSPpaMA) to enhance PDT via the controllable release of photosensitizers. Once nanoparticles formed by the block copolymer have accumulated in a tumor and have been taken up by cancer cells, pyropheophorbide a (Ppa) could be controllably released by singlet oxygen (¹O₂) generated by light irradiation, enhancing the photosensitization. This was demonstrated by confocal laser scanning microscopy and in vivo fluorescence imaging. The ¹O₂‐responsiveness of POEGMA‐b‐P(MAA‐co‐VSPpaMA) block copolymer enabled the realization of self‐amplified photodynamic therapy by the regulation of Ppa release using NIR illumination. This may provide a new insight into the design of precise PDT.     

Measurement of Intracellular Oxygen Concentration During Photodynamic Therapy in vitro

A technique is introduced that monitors the depletion of intracellular ground state oxygen concentration ([³O₂]) during photodynamic therapy of Mat‐LyLu cell monolayers and cell suspensions. The photosensitizer Pd(II) meso‐tetra(4‐carboxyphenyl)porphine (PdT790) is used to manipulate and indicate intracellular [³O₂] in both of the in vitro models. The Stern–Volmer relationship for PdT790 phosphorescence was characterized in suspensions by flowing nitrogen over the suspension while short pulses of 405 nm light were used to excite the sensitizer. The bleaching of sensitizer and the oxygen consumption rate were also measured during continuous exposure of the cell suspension to the 405 nm laser. Photodynamic therapy (PDT) was conducted in both cell suspensions and in cell monolayers under different treatment conditions while the phosphorescence signal was acquired. The intracellular [³O₂] during PDT was calculated by using the measured Stern–Volmer relationship and correcting for sensitizer photobleaching. In addition, the amount of oxygen that was consumed during the treatments was calculated. It was found that even at large oxygen consumption rates, cells remain well oxygenated during PDT of cell suspensions. For monolayer treatments, it was found that intracellular [³O₂] is rapidly depleted over the course of PDT.     

Light Fluence Rate and Tissue Oxygenation (StO2) Distributions Within the Thoracic Cavity of Patients Receiving Intraoperative Photodynamic Therapy for Malignant Pleural Mesothelioma

The distributions of light and tissue oxygenation (S_tO₂) within the chest cavity were determined for 15 subjects undergoing macroscopic complete resection followed by intraoperative photodynamic therapy (PDT) as part of a clinical trial for the treatment of malignant pleural mesothelioma (MPM). Over the course of light delivery, detectors at each of eight different sites recorded exposure to variable fluence rate. Nevertheless, the treatment-averaged fluence rate was similar among sites, ranging from a median of 40–61 mW cm⁻² during periods of light exposure to a detector. S_tO₂ at each tissue site varied by subject, but posterior mediastinum and posterior sulcus were the most consistently well oxygenated (median S_tO₂ >90%; interquartile ranges ~85–95%). PDT effect on S_tO₂ was characterized as the S_tO₂ ratio (post-PDT S_tO₂/pre-PDT S_tO₂). High S_tO₂ pre-PDT was significantly associated with oxygen depletion (S_tO₂ ratio < 1), although the extent of oxygen depletion was mild (median S_tO₂ ratio of 0.8). Overall, PDT of the thoracic cavity resulted in moderate treatment-averaged fluence rate that was consistent among treated tissue sites, despite instantaneous exposure to high fluence rate. Mild oxygen depletion after PDT was experienced at tissue sites with high pre-PDT S_tO₂, which may suggest the presence of a treatment effect.     

Identification of Ros Produced by Photodynamic Activity of Chlorophyll/Cyclodextrin Inclusion Complexes

Photodynamic therapy (PDT) is a way of treating malignant tumors and hyperproliferative diseases. It is based on the use of photosensitizer, herein the chlorophyll a (chl a), and a light of an appropriate wavelength. The interaction of the photosensitizer (PS) with the light produces reactive oxygen species (ROS), powerful oxidizing agents, which cause critical damage to the tissue. To solubilize chl a in aqueous solution and to obtain it as monomer, we have used cyclodextrins, carriers which are able to interact with the pigment and form the inclusion complex. The aim of this study is to examine which types of ROS are formed by Chl a/cyclodextrin complexes in phosphate buffered solution and cell culture medium, using specific molecules, called primary acceptors, which react selectively with the reactive species. In fact the changes of the absorption and the emission spectra of these molecules after the illumination of the PS provide information on the specific ROS formation. The ¹O₂ formation has been tested using chemical methods based on the use of Uric Acid (UA), 9,10‐diphenilanthracene (DPA) and Singlet oxygen sensor green (SOSG) and by direct detection of Singlet Oxygen (¹O₂) luminescence decay at 1270 nm. Moreover, 2,7‐dichlorofluorescin and ferricytochrome c (Cyt Fe³⁺) have been used to detect the formation of hydrogen peroxide and superoxide radical anion, which reduces Fe³⁺ of the ferricytochrome to Fe²⁺, respectively.     

Nitric oxide and singlet oxygen photo-generation by light irradiation in the phototherapeutic window of a nitrosyl ruthenium conjugated with a phthalocyanine rare earth complex

The photochemical, photophysical and photobiological studies of a mixture containing cis-[Ru(H-dcbpy⁻)₂(Cl)(NO)] (H₂-dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) and Na₄[Tb(TsPc)(acac)] (TsPc = tetrasulfonated phthalocyanines; acac = acetylacetone), a system capable of improving photodynamic therapy (PDT), were accomplished. cis-[Ru(H-dcbpy⁻)₂(Cl)(NO)] was obtained from cis-[Ru(H₂-dcbpy)₂Cl₂]·2H₂O, whereas Na₄[Tb(TsPc)(acac)] was obtained by reacting phthalocyanine with terbium acetylacetonate. The UV–Vis spectrum of cis-[Ru(H-dcbpy⁻)₂(Cl)(NO)] displays a band in the region of 305 nm (λ_max in 0.1 mol L⁻¹ HCl)(π–π*) and a shoulder at 323 nm (MLCT), while the UV–Vis spectrum of Na₄[Tb(TsPc)(acac)] presents the typical phthalocyanine bands at 342 nm (Soret λ_max in H₂O) and 642, 682 (Q bands). The cis-[Ru(H-dcbpy⁻)₂(Cl)(NO)] FTIR spectrum displays a band at 1932 cm⁻¹ (Ru–NO⁺). The cyclic voltammogram of the cis-[Ru(H-dcbpy⁻)₂(Cl)(NO)] complex in aqueous solution presented peaks at E = 0.10 V (NO^+/0) and E = −0.50 V (NO^0/−) versus Ag/AgCl. The NO concentration and ¹O₂ quantum yield for light irradiation in the λ > 550 nm region were measured as [NO] = 1.21 ± 0.14 μmol L⁻¹ and ø_OS = 0.41, respectively. The amount of released NO seems to be dependent on oxygen concentration, once the NO concentration measured in aerated condition was 1.51 ± 0.11 μmol L⁻¹ The photochemical pathway of the cis-[Ru(H-dcbpy⁻)₂(Cl)(NO)]/Na₄[Tb(TsPc)(acac)] mixture could be attributed to a photoinduced electron transfer process. The cytotoxic assays of cis-[Ru(H-dcbpy^-)₂(Cl)(NO)] and of the mixture carried out with B16F10 cells show a decrease in cell viability to 80% in the dark and to 20% under light irradiation. Our results document that the simultaneous production of NO and ¹O₂ could improve PDT and be useful in cancer treatment.     

Effect and Mechanism of a New Photodynamic Therapy with Glycoconjugated Fullerene

When irradiated, fullerene efficiently generates reactive oxygen species (ROS) and is an attractive photosensitizer for photodynamic therapy (PDT). Ideally, photosensitizers for PDT should be water-soluble and tumor-specific. Because cancer cells endocytose glucose more effectively than normal cells, the characteristics of fullerene as a photosensitizer were improved by combining it with glucose. The cytotoxicity of PDT was studied in several cancer cell lines cultured with C₆₀-(Glc)1 (d -glucose residue pendant fullerene) and C₆₀-(6Glc)1 (a maltohexaose residue pendant fullerene) subsequently irradiated with UVA₁. PDT alone induced significant cytotoxicity. In contrast, PDT with the glycoconjugated fullerene exhibited no significant cytotoxicity against normal fibroblasts, indicating that PDT with these compounds targeted cancer cells. To investigate whether the effects of PDT with glycoconjugated fullerene were because of the generation of singlet oxygen (¹O₂), NaN₃ was added to cancer cells during irradiation. NaN₃ extensively blocked PDT-induced apoptosis, suggesting that PDT-induced cell death was a result of the generation of ¹O₂. Finally, to investigate the effect of PDT in vivo, melanoma-bearing mice were injected intratumorally with C₆₀-(Glc)1 and irradiated with UVA₁. PDT with C₆₀-(Glc)1 suppressed tumor growth. These findings indicate that PDT with glycoconjugated fullerene exhibits tumor-specific cytotoxicity both in vivo and in vitro via the induction of ¹O₂.     

Synthesis of Sn nanocluster@carbon dots for photodynamic therapy application

Recently, photodynamic therapy (PDT) has been extensively applied in clinical and coadjuvant treatment of various kinds of tumors. However, the photosensitizer (PS) of PDT still lack of high production of singlet oxygen (¹O₂), low cytotoxicity and high biocompatibility. Herein, we propose a facile method for establishing a new core-shell structured Sn nanocluster@carbon dots (CDs) PS. Firstly, Sn⁴⁺@S-CDs complex is synthesized using the sulfur-doped CDs (S-CDs) and SnCl₄ as raw materials, and subsequently the new PS (Sn nanocluster@CDs) is obtained after vaporization of Sn⁴⁺@S-CDs solution. Remarkably, the obtained Sn nanocluster@CDs show an enhanced fluorescence as well as a higher ¹O₂ quantum yield (QY) than S-CDs. The high ¹O₂ QY (58.3%) irradiated by the LED light (400–700 nm, 40 mW/cm²), induce the reduction of 4T1 cancer cells viability by 25%. More intriguingly, no visible damage happens to healthy cells, with little impact on liver tissue due to renal excretion, both in vitro and in vivo experiments demonstrate that Sn nanocluster@CDs may become a promising PS, owning a high potential for application in PDT.     

Hypocrellin B doped and pH-responsive silica nanoparticles for photodynamic therapy

pH-responsive ¹O₂ photosensitizing systems may serve as selective photodynamic therapy (PDT) agents by targeting the acidic interstitial fluid of many kinds of tumors. In this work, a natural and clinically used photosensitizer (Hypocrellin B, HB) and a pH indicator (Bromocresol Purple, BCP) were co-encapsulated in organically modified silica nanoparticles. BCP successfully regulated the ¹O₂ generation efficiency of HB through the “inner filter” effect, which shows much stronger ¹O₂ generation ability in an acidic than in a basic environment. In vitro experiments also demonstrated that HB-doped nanoparticles are effective in killing tumor cells by PDT.

     

Reinvestigation of the Fe-rich part of the pseudo-binary system SrO–Fe2O3

The phase relations in the Fe-rich part of the pseudo-binary system SrO–Fe₂O₃ (>33 mol% Fe₂O₃) were reinvestigated between 800 and 1500 °C in air. A combination of microscopy, electron probe micro-analysis, powder X-ray diffraction and thermal analysis was used to determine phase relations, crystal structure parameters and phase transition temperatures. M-type hexagonal ferrite SrFe₁₂O₁₉ (85.71 mol% Fe₂O₃) is stable up to 1410 °C. No indication of a significant phase width was found; Sr₄Fe₆O_13±δ appears as a second phase in compositions with <85.71±0.2 mol% Fe₂O₃. Sr₄Fe₆O_13±δ itself is stable between 800 and 1250 °C. Two other hexagonal ferrites were found to exist at high temperatures only: W-type SrFe²⁺₂Fe³⁺₁₆O₂₇ is stable between 1350 and 1440 °C and X-type ferrite Sr₂Fe²⁺₂Fe³⁺₂₈O₄₆ between 1350 and 1420 °C, respectively, which is shown here for the first time. These findings in combination with previously published data were used to derive a corrected phase diagram of the Fe-rich part of the pseudo-binary system SrO–Fe₂O₃.