Heavy metal fluoride glasses: A tutorial review

Since the first discovery of fluorozirconate glasses in 1974,¹ Heavy Metal Fluoride Glasses (HMFG) have been the subject of numerous studies in the 1980s and 1990s, resulting in more than 10 000 papers, theses, and technical reports.^2–4 While glasses based on beryllium fluoride had been extensively studied in the first half of the 20th century, their structure closely resembles that of fused silica.⁵ In contrast, HMFGs are a completely new class of non-oxide glasses that are entirely different in their chemical composition, their mechanism of glass formation, and their physical properties. For that reason, HMFGs exclude fluoroberylate glasses.

HMFGs emerged from a serendipitous discovery that was made by the Poulain brothers in 1974 at Rennes University in France. Unexpected glass formation was observed in a chemical system based on zirconium fluoride, barium fluoride, and sodium fluoride.¹ From that point on, numerous new glasses were discovered in other multicomponent systems. This eventually led to the three major families of HMFGs that are widely known and in use at present: fluorozirconates, fluoroaluminates, and fluoroindates. These new vitreous materials are free of oxygen and do not contain the classical vitrifiers of industrial glasses, namely, SiO₂, B₂O₃, Al₂O₃, and P₂O₅. They exhibit highly unique and specific optical, physical, and chemical properties that make them so attractive for many applications, and that will be discussed in this tutorial paper.

The optical transmission window of HMFGs stretches continuously from the ultraviolet (UV) part of the electromagnetic spectrum to the mid-infrared (mid-IR). While standard optical fibers made of silica glass are well suited for most applications in the visible and near infrared, fluoride glasses have emerged as the ideal materials for use in the 2–5 μm mid-IR spectral range. While previously mainly used in scientific research, fluoride fibers have recently reached a level of maturity that makes them also attractive for industrial applications. These days, fluoride optical fibers feature extremely low transmission loss values and are commercially available from several sources. In addition, fluoride fibers doped with rare-earth ions are of great interest as active mediums and are used in supercontinuum sources and new fiber lasers. The spectroscopy of rare earths in these glasses has been extensively investigated.^2,6,7 The practical advantages of this vitreous matrix are a large lanthanide solubility, a low phonon energy, and a large interaction length that can be achieved in optical fibers. As a consequence, numerous new emission lines were demonstrated with fluoride glass fiber lasers.⁷ With the availability of powerful and affordable pump diodes, these laboratory prototypes are now being translated into commercial products and are enabling new and exciting applications, especially in the medical field.

Glass is normally obtained by cooling the initial glass forming melt until it becomes a rigid solid. A necessary pre-condition is that the liquid does not crystallize during this cooling step. Apart from BeF₂, no pure fluoride exists in the vitreous state. Only the combination of several fluorides results in melts in which the crystallization rate is low enough to allow glass to be obtained by fast cooling. The first HMFG was reported in 1975 in the ZrF₄–BaF₂–NaF ternary system.¹ In subsequent studies, other glass-forming systems based on ZrF₄ (i.e., fluorozirconates) were identified. Multicomponent glasses based on the AlF₃–MF₂ (with M = Ca, Sr, Ba, and Pb) material group (fluoroaluminates) were also reported in the early 1980s.⁸ Fluoroindate glasses based on InF₃ first appeared, among other more exotic glasses, in patent applications that were filed by two different research groups in 1980.^9,10 However, it took several years before these glasses were extensively studied.^11–13 Taking into account their chemical similarities, fluorohafnate glasses based on HfF₄ are usually classified as fluorozirconates, while fluorogallate glasses based on GaF₃ belong to the group of fluoroindates. Apart from these three groups (fluorozirconates, fluoroaluminates, and fluoroindates), numerous other fluoride glasses have been reported. Those are based on the transition metals V, Cr, Mn, Fe, Co, Ni, Cu,¹⁴ and also on Sc, Zn, Cd, Th, U, Sn, and Bi.^15,16 However, all those various glasses have not led to practical applications to date. Typical compositions (in mol %) of the most important HMFGs are summarized in Tables I–III.

Similar to most inorganic glasses, fluoride glasses are fabricated by cooling a clear melt fast enough to prevent crystallization. The detailed glass preparation process requires several steps, some of which are also common to oxide glasses, including the following well-known processes:

• selecting and mixing of the starting materials,

• melting,

• fining,

• casting,

• cooling, and

• annealing.

However, considering the unique chemical behavior of fluorides, the particular details of these processes are critical for the quality of the final glass. In comparison to oxide glasses, the processing of fluoride glasses is significantly different, and time and temperature adjustments are crucial. Most importantly, fluoride melts are very sensitive to hydrolysis and are furthermore prone to devitrification. In addition, their viscosity is very low at liquidus temperatures. The chemical reactivity of fluorides limits the possible materials that can be used for crucibles and enclosures: platinum, gold, and vitreous carbon are preferred, while silica is excluded.²⁶ 

Great care must be taken in the selection, storage, and handling of the starting materials. Fluoride glass synthesis requires a very low concentration of various impurities, with water being the most critical one. Water is a common impurity that can be found not only in many so-called anhydrous fluorides but also in reportedly non-hygroscopic compounds. Anionic oxygen that may appear as residual oxyfluoride also must be minimized. Other impurities include transition metals and paramagnetic rare earths that can induce optical absorption at various wavelengths. On the other hand, diamagnetic elements such as alkali and earth alkali metals play a less important role.

For the glass preparation, anhydrous fluorides are powdered, mixed, and finally homogenized. They are then heated to complete melting in platinum or vitreous carbon crucibles. The raw glass samples obtained by cooling usually appear grayish, and the glass must be fined to become fully transparent. The fining step consists of heating the melt at a high temperature—that is, above the liquidus temperature—in an oxidizing atmosphere. The viscosity decreases, and the melt is homogenized without stirring. Volatile species are eliminated, and reduced phases that give rise to light scattering are oxidized and dissolved. A clear and homogeneous glass is obtained upon cooling after the fining stage. Finally, the samples are annealed at a temperature close to the glass transition temperature. This annealing stage aims at removing any mechanical stresses induced by non-uniform cooling.

Optical fibers are obtained from glass preforms that are fabricated in an anhydrous atmosphere by pouring a core glass into a cladding tube. This tube may be fabricated by evacuation (“build in casting”) or by rotational casting. The overall process is described in detail in Fig. 1. For single mode fibers, a second drawing step is necessary to obtain a small diameter core.

The most attractive feature of fluoride glasses is their large bandwidth of optical transmission that extends from the ultraviolet part of the electromagnetic spectrum to the mid-infrared, including the visible part of the spectrum. This can be seen in Fig. 2, which shows the transmission curve of 4 mm thick bulk samples made from standard fluoride glasses. The limits of transmission are determined by the chemical composition of the glasses. In the mid-IR, the multi-phonon absorption edge moves toward longer wavelengths according to the following sequence: fluoroaluminates, fluorozirconates, and fluoroindates.

The refractive index also changes with temperature. While the numerical quantity dn/dT is positive for silica glass, it is negative for HMFGs. The dn/dT change in fluoride glasses has been addressed in the early stages of their development, and values range between −8 and −15 × 10⁻⁶ K⁻¹.³² The main and common features of HMFGs are that dn/dT is negative and that the magnitude of the change is smaller as compared to other glasses such as chalcogenides.

The nonlinear refractive index is small in fluoride glasses, similar to fused silica. According to the literature, it is in the range 1−2 × 10⁻²⁰ m²/W,²³ which is substantially lower as compared to heavy metal oxide and chalcogenide glasses.

Numerous studies have investigated the chemical durability of fluorozirconate glasses.^33,34 While HMFGs are resistant to fluorinating reagents, they are sensitive to water attack. Of main concern is their behavior in aqueous solutions. When a piece of glass is immersed in water, it undergoes a slow chemical attack. As a result, the surface is damaged and the pH of the solution drops, i.e., it becomes more acidic. The rate of attack increases rapidly as pH decreases, potentially resulting in a catastrophic runaway process. In contrast, the rate of attack can be kept very low in a slightly alkaline solution with a pH value of 8. Bulk glass is, therefore, stable in a buffered solution at this pH.³⁵ The mechanism of attack is complex and includes several steps: first, fluorine anions are replaced by hydroxyls; then some glass components enter the liquid phase; finally, less soluble fluorides (e.g., BaF₂) precipitate onto the glass surface, creating mechanical stress and surface defects.³⁴ As a consequence, when liquid water stays at the fiber surface for several hours (even when the fiber is coated with an epoxy acrylate resin), the fiber becomes practically unusable.

In practice, one must absolutely avoid any liquid water remaining on the surface of an HMFG. It is important to note, however, that the rate of attack is relatively slow, which allows the use of water for polishing fluoride bulk glasses and preforms.

The durability of a glass depends on its chemical composition, in particular on the relative concentration of fluorides that exhibit significant solubility in water. For example, HMFGs containing sodium are more sensitive to water attacks, while calcium, yttrium, or thorium increase their resistance. When exposed to aqueous solutions, fluoroindates behave very similar to fluorozirconates, while fluoroaluminate glasses are more resistant in some compositions.^21,23

There is no chemical reaction between HMFGs and water vapor below 100 °C. Bulk HMFGs and fibers do not suffer surface degradation when exposed to a humid atmosphere; optical components have been stored for tens of years in an ambient atmosphere without visible damage. However, condensation must be prevented. Deliquescence has never been observed for any current fluoride glasses.

HMFGs are classified as soft glasses in contrast to silica and soda lime glasses. Their microhardness depends on the chemical composition. The reported values are around 250 kg/mm² for ZBLAN and fluoroindate glasses,³² yet can exceed 320 kg/mm² for fluoroaluminates. Ultrasonic measurements are used to determine the three elastic moduli: bulk modulus, shear modulus, and Young’s modulus. A summary of the mechanical properties of ZFG, IFG, and AFG glasses in comparison to fused silica is provided in Table IV.

Theoretical analysis suggests that the intrinsic strength of fluoride glasses should be half that of silica, and fracture toughness has been reported to be in the range from 0.25 to 0.30 MPa m^1/2. In practice, mechanical failure usually originates from extrinsic defects, mostly surface defects. Because the emphasis is mainly on optical fibers, most available data relates to fibers. Depending on drawing conditions, fiber strengths may reach 1600 MPa,^36,37 while typical values rather lie between 500 and 1000 MPa. This is still far below the intrinsic strength value.

The structure of HMFGs has been the subject of numerous studies. In analogy with other glasses, it is assumed that it can be described as an aperiodic network in which modifying cations are inserted. This network is constructed from large coordination polyhedra that are sharing corners and edges. Typical polyhedra are ZrF₈ square antiprisms and dodecahedra, as well as AlF₆ and InF₆ octahedra. The structure of HMFGs does not comply with Zachariasen’s rules that specify that coordination polyhedra should be small—a tetrahedron or triangle—and should share only apexes, not edges.³⁸ Alternatively, HMFG may be described as a disordered packing of fluorine anions in which cations are randomly inserted.³⁹ Both models are equivalent, and the correspondence between them is illustrated in Fig. 4, which shows a bidimensional picture of a fluorozirconate glass.⁴⁰ An additional element of the glass structure is the occurrence of vacancies in the random packing,⁴¹ as shown in Fig. 5. It leads to a “cluster” model and provides a physical meaning of the classical “excess free volume” concept that has been introduced a long time ago.

This picture emphasizes the existence of bidimensional channels that account for ionic conductivity. Despite these vacancies, the structure of HMFGs is highly compact, in stark contrast to silica or germania glasses, in which the vacancy rate is close to 50% as it is impossible to fill the entire space only with tetrahedra sharing apexes.

HMFGs belong to the group of low melting point glasses. Their characteristic and processing temperatures lie below 1000 °C. The glass transition temperature T_g marks the limit between the solid and the molten (liquid) state. In practice, glass components cannot be used beyond this temperature. Most HMFGs are prone to devitrification when heated between the glass transition and melting temperatures. The crystallization temperature T_x is usually measured by thermal analysis (Differential Thermal Analysis, DTA, or Differential Scanning Calorimetry, DSC) at a heating rate of 10 K/min. The thermal stability range is defined as ΔT = T_x − T_g and gives an estimate of glass stability vs devitrification. Table V lists the characteristic temperatures of various HMFGs.

The viscosity η is an important physical parameter in glass technology. It is measured between the glass transition temperature and the liquidus temperature. At T_g, it is close to 10¹³ poises, while it may be lower than 1 poise at the liquidus temperature. The variation, therefore, covers more than ten orders of magnitude. For this reason, it is typically plotted on a logarithmic scale, i.e., Log η = f (T⁻¹). This plot is linear in the case of an Arrhenian behavior that is exceptional for glasses and applies only to pure silica and vitreous BeF₂. While most industrial glasses follow the Vogel–Tammann–Fulcher law, the viscosity of fluoride glasses deviates even further from linearity.^42,43 They, therefore, belong to the group of “fragile glasses” as introduced by Angell.⁴⁴ A typical viscosity/temperature curve for fluorozirconate glasses with different sodium concentrations⁴² is shown in Fig. 6. The viscosity decreases sharply above the glass transition temperature and is very low around the liquidus temperature (less than 1 poise). As a consequence, the temperature must be controlled very accurately for glass transformation, which is of utmost importance for fiber drawing.

Three kinetical parameters are derived from the JMAK relation: n, E, and k₀. The physical meaning of the activation energy E and the frequency constant k₀ is not clear. In multicomponent glasses, the chemical composition of the crystalline phases that are the result of devitrification is generally different from that of the parent glass. The search for stable glass compositions remains empirical, as there is no obvious way to predict glass stability. The so-called “confusion principle” has often been used as a guideline to find glass compositions that are more stable against devitrification. This principle states that glass stability should increase with the number of glass components: a quaternary glass should be more stable than a ternary glass, which is expected to be more stable than a binary one. The standard ZBLAN fluorozirconate glass has been defined this way.

The potential of HMFG fibers for telecommunications has already been pointed out in their early stages of development when it was emphasized that the intrinsic optical losses of these fibers could reach values that are well below those of silica fibers, i.e., as low as 10⁻²–10⁻³ dB/km. This would enable repeater-less long haul links, possibly even transoceanic,^18,48–50 a potential that raised enormous interest among major telecom companies. However, after a few years of intense research, it appeared that the optical losses could not be reduced below the levels achieved in silica fibers, where the theoretical loss limits had been reached. The major problems were identified to be chemical impurities and devitrification. More recently, it was reported that crystallization could be inhibited in microgravity. Several US companies currently investigate this approach.⁵¹ 

Optical amplification is another field of application of HMFG fibers. This applies specifically to the telecommunications O and S bands for which rare earth doped silica fibers do not allow amplification. Fluoride glasses that exhibit low phonon energy make it possible to develop such optical amplifiers using thulium and praseodymium as active elements.

Combining individual telescopes via fiber links significantly increases the achievable accuracy and resolution. The atmospheric transmission window between 2–2.5 μm (known as the astronomical K band) is highly attractive in this respect. The combination of two signals from distinct telescopes results in a broadband interferometric pattern from which images are obtained. This approach requires low loss single mode fibers that can be manufactured only from fluoride glasses. After pioneering achievements at the Kitt Peak Observatory in the US,⁵² the method was used for several instruments: Ohana,⁵³ GRAVITY,^54,55 SPIRou,⁵⁶ and SPIP.⁵⁷ Gravity was used by the 2021 Nobel laureates in physics.

Fiber links are required for many systems operating in the mid-IR, for example, for sensing and remote detection. Fluoride glass fibers are used for wavelengths between 2.4 μm (the limit of silica fibers) and 5 μm. The various fields of application include scientific research, thermal imaging, and laser power delivery, most importantly around 2.9 μm, which coincides with the absorption peak of liquid water. With the emergence of Quantum Cascade Lasers (QCLs) and Interband Cascade Lasers (ICLs) fluoride glass optical fibers that can be coupled to these diodes are in high demand.

Another field of applications is enabled by the excellent UV transparency of fluoride fibers. Compared to silica fibers, HMFG fibers are less sensitive to photodarkening, and this makes them well suited for the delivery of UV light.

Rare earth ions have long been studied for laser emission, and various solid-state laser crystals have been used for this purpose. In comparison to crystals, actively doped optical fibers offer a range of advantages, especially long interaction lengths and improved heat management. Among other parameters, the possibility of laser emission from a particular atomic transition depends on the phonon energy of the host material. While it is rather low in most crystals, it is higher in classical oxide glasses, which reduces the number of available emission lines. Rare earth doped fluoride fibers, on the other hand, combine low phonon energy, low optical losses, and a high rare earth solubility. As a consequence, a large number of new emission lines have been identified. This concerns not only the mid-infrared beyond 2 μm⁵⁸ but also visible lasers either by upconversion or by pumping using blue (GaN) diodes. Continuous-wave (cw) fluoride glass fiber lasers operating at 2.9 μm are now being industrially produced for use in dermatology, where they are replacing more traditional CO₂ lasers as they are more efficient at low power and less painful for the patient.⁵⁹ Those lasers are also very promising for endoscopic surgery.

Multi-watt visible fiber lasers are currently under development. Based on rare earth-doped fluoride fibers, they will find important applications in several medical fields such as ophthalmology, microscopy, imaging, and diagnosis.

Based on nonlinear optical effects, fibers can be used to generate broadband emission that is fundamentally only limited by the transparency window of the fiber material. Such supercontinuum (SC) sources typically consist of a pulsed laser source with high peak power that is injected into a low loss optical fiber. Using silica fibers, SC sources can cover a wide spectrum between 600 and 2500 nm. Based on fluoride fibers, SC sources with a wavelength range between 700 and 5000 nm are available.⁶⁰ These sources are mainly developed for mid-IR spectroscopy but can also be used for infrared countermeasures.

The concentration of fluorescent rare earth ions in glasses can be adjusted with high precision. The low phonon energy of fluoride glasses further enhances their efficiency, which makes them suitable for fluorescence standards in the visible spectrum (green, yellow, and red).

Fluoride glasses have been extensively studied and developed over the last fifty years. Initially, the prospect of ultra-low optical losses that could enable repeater-less long-haul transmission attracted significant interest and funding. Later, fluoride optical fibers were found to be complementary to silica fibers for mid-infrared transmission and fiber lasers.

A large number of multicomponent fluoride glasses have been reported in the past, yet only a small fraction has been further developed for practical applications. Those can be categorized into the three major groups of fluoroaluminates, fluorozirconates, and fluoroindates which are recognized as an original class of glasses with unusual structure, low characteristic temperatures, and specific physical properties. The main interest in these glasses stems from their broad optical transmission that extends from the UV to the mid-infrared spectrum and their extremely low OH contamination.

Most applications of HMFGs are based on optical fibers. Fiber drawing requires glass compositions that are very stable against devitrification. To date, a large variety of fibers have been reported, including multimode, singlemode, polarization maintenance, and double clad. Passive applications for fluoride fibers include sensing, interferometry, and laser power delivery, whereas active applications include rare earth-doped fibers, leading to new fiber lasers in the mid-IR and visible spectral ranges.

In order to be used as materials for optical fibers, HMFGs must withstand the processing conditions and be resistant enough to devitrification. Their extended optical transmission range makes them suitable for passive broadband optical components. Their low phonon energy allows the development of mid-infrared fiber lasers and supercontinuum sources. Numerous sensors and analysis systems based on fluoride fibers will emerge in the coming years. One can expect important applications in the medical field.

The author has no conflicts to disclose.

Marcel Poulain: Writing – original draft (equal).

Data sharing is not applicable to this article, as no new data were created or analyzed in this study.