Structural and thermal properties of some tellurite glasses

Tellurium oxide glasses were prepared by the hammer and anvil technique. The glass systems are (0.85TeO₂ + 0.15Z), where Z = K₂O, TiO₂, V₂O₅, MnO, Fe₂O₃, CoO, NiO or CuO. A second group is a ternary system 0.85TeO₂+(0.15 − x)TiO₂ + xFe₂O₃) with x=0.0, 0.05, 0.1, 0.15 mol. X-ray diffraction, infrared spectroscopy and differential thermal analysis measurements were carried out. The present study showed the different glass-forming groups, the glass transition and crystallization temperatures as well as the crystallization processes.     

Nuclear magnetic resonance studies of alkaline earth phosphosilicate and aluminoborosilicate glasses

The ¹¹B, ²⁷Al, ²⁹Si and ³¹P magic angle spinning (MAS) NMR spectra of MO–P₂O₅, MO–SiO₂–P₂O₅ and MO(M^′₂O)–SiO₂–Al₂O₃–B₂O₃ (M=Mg, Ca, Sr and Ba, M^′=Na) glasses were examined. In binary MO–P₂O₅ (M=Ca and Mg) glasses, the distributions of the phosphate sites, P(Q_n), can be expressed by a theoretical prediction that P₂O₅ reacts quantitatively with MO. In the ternary 0.30MO–0.05SiO₂–0.65P₂O₅ glasses, the 6-coordinated silicon sites were detected, whose population increases in the order of MgOxCaO–0.05SiO₂–(0.95−x)P₂O₅ glasses, its population increases with an increase in f (=([P₂O₅]−[MO]−[B₂O₃]−[Na₂O])/[SiO₂]) and has maximum at f=9. The signal due to the 5-coordinated silicon atoms is also observed when x is smaller than 0.45. When three network-forming oxides such as SiO₂, Al₂O₃ and B₂O₃ coexist, Al₂O₃ reacts preferably with MO. The populations of 4-coordinated boron atoms, N₄, are expressed well with r/(1−r), where r=([Na₂O]−[Al₂O₃])/([Na₂O]−[Al₂O₃]+[B₂O₃]). The correlation of the Raman signal at 1210 and 1350 cm⁻¹ with the NMR signal of Si(Q₆) at −215 ppm is also seen.     

Characterisation of semiconducting V2O5–Bi2O3–TeO2 glasses through ultrasonic measurements

Tellurite containing vanadate (50−x)V₂O₅–xBi₂O₃–50TeO₂ glasses with different bismuth (x=0, 5, 10, 15, 20 and 25 wt%) contents have been prepared by rapid quenching method. Ultrasonic velocities (both longitudinal and shear) and attenuation (for longitudinal waves only) measurements have been made using a transducer operated at the fundamental frequency of 5 MHz in the temperature range from 150 to 480 K. The elastic moduli, Debye temperature, and Poisson’s ratio have been obtained both as a function of temperature and Bi₂O₃ content. The room temperature study on ultrasonic velocities, attenuation, elastic moduli, Poisson’s ratio, Debye temperature and glass transition temperature show the absence of any anomalies with addition of Bi₂O₃ content. The observed results confirm that the addition of Bi₂O₃ modifier changes the rigid formula character of TeO₂ to a matrix of regular TeO₃ and ionic behaviour bonds (NBOs). A monotonic decrease in velocities and elastic moduli, and an increase in attenuation and acoustic loss as a function of temperature in all the glass samples reveal the loose packing structure, which is attributed to the instability of TeO₄ trigonal bipyramid units in the network as temperature increases. It is also inferred that the glasses with low Bi₂O₃ content are more stable than with high Bi₂O₃ content.     

Study of electrical properties of MoO3–Fe2O3–P2O5 and SrO–Fe2O3–P2O5 glasses by impedance spectroscopy. II

The electrical and dielectric properties for three series of MoO₃–Fe₂O₃–P₂O₅ and one series of SrO–Fe₂O₃–P₂O₅ glasses were measured by impedance spectroscopy in the frequency range from 0.01 Hz to 3 MHz and over the temperature range from 303 to 473 K. It was shown in Part I that the MoO₃ is incorporated into phosphate network and the structure/properties are strongly influenced by the overall O/P ratio. The Fe₂O₃ content and Fe(II)/Fe_tot ratio in these glasses have significant effects on the electrical conductivity and dielectric permittivity. With decreasing Fe₂O₃ content in MoO₃–Fe₂O₃–P₂O₅ glasses with O/P at 3.5 the dc conductivity, σ_dc(ω) decreases for two orders of magnitude, which indicates that the conductivity for these glasses depends on Fe₂O₃ and is independent of the MoO₃ content. Also, the dielectric properties such as ^′(ω), ^″(ω) and σ_ac(ω) and their variation with frequency and temperature indicates a decrease in relaxation intensity with increase in the concentration of MoO₃. On the other hand, the dc conductivity for MoO₃–Fe₂O₃–P₂O₅ glasses with O/P > 3.5 increases with the substitution of MoO₃ which has been explained by an increase in the number of non-bridging oxygens and formation of Fe–O–P bonds that are responsible for formation of small polarons. The increase in the dielectric permittivity, ^′(ω) with increasing MoO₃ content is attributed to the increase in the deformation of glass network with increasing bonding defects. For SrO–Fe₂O₃–P₂O₅ glasses the conductivity and dielectric permittivity remained constant with increasing SrO.     

B NMR investigations in xV2O5–B2O3 and xV2O5–B2O3–PbO glasses

11B (I=3/2) MAS NMR in the binary glass system xV₂O₅–B₂O₃ (x=0.053, 0.43) and the ternary glass system xV₂O₅–B₂O₃–PbO (0.1x1.5) has been investigated at room temperature. In the xV₂O₅–B₂O₃ glasses, one NMR line due to BO₃ unit was observed. Meanwhile in the xV₂O₅–B₂O₃–PbO, two NMR lines which arise from BO₃ and BO₄ units were detected, where the appearance of BO₄ units is produced by the presence of PbO. From the computer-simulation of the ¹¹B NMR central transition line (m=−1/2↔1/2), the quadrupole parameters (e²qQ/h and η) for BO₃ units in xV₂O₅–B₂O₃, and those for BO₃ and BO₄ units in xV₂O₅–B₂O₃–PbO were obtained as a function of x. As the V₂O₅ content increases in xV₂O₅–B₂O₃–PbO, the e²qQ/h and η values of the BO₃⁻ associated resonance are found to slightly decrease and increase, respectively. Meanwhile, the e²qQ/h and η values of BO₄⁻ associated resonance in xV₂O₅–B₂O₃–PbO are found to slightly increase and decrease, respectively. By comparing the intensities of the total transitions (m=−3/2↔−1/2,m=−1/2↔1/2, and 1/2↔3/2) for the ¹¹B NMR line of BO₃ and BO₄ units contained in xV₂O₅–B₂O₃–PbO with those of respective standard samples of 0.053V₂O₅–B₂O₃ and NaBH₄, the quantitative fractions of BO₃ and BO₄ in xV₂O₅–B₂O₃–PbO were obtained as a function of x.     

The effect of MgO on the thermodynamics of the Fe/Fe-redox equilibrium and the incorporation of iron in soda-magnesia-aluminosilicate melts

Melts with the basic compositions 10Na₂O · 10MgO · xAl₂O₃ · (80−x)SiO₂ (x=0, 5, 10, 15 and 20), 10Na₂O · xMgO · 10Al₂O₃ · (80−x)SiO₂ (x=5, 10, 15 and 20) and xNa₂O · 10MgO · 10Al₂O₃ · (80−x)SiO₂ (x=5, 10 and 15) all doped with 0.25 mol% Fe₂O₃ were studied using square-wave voltammetry. The temperatures applied were in the range of 1000–1600 °C. The square-wave voltammograms recorded show peaks caused by the reduction of Fe³⁺ to Fe²⁺. The attributed peak potentials measured decreased linearly with decreasing temperatures. Increasing the MgO-concentration led to more negative peak potentials. Introducing alumina in the melt first resulted in less negative peak potentials. If the molar Al₂O₃-concentration is equal to that of Na₂O (=10 mol%) the peak potentials are least negative. Further increase of the Al₂O₃-concentration led to more negative peak potentials. The variation of the Na₂O-concentration led to a maximum in the peak potentials at an Na₂O-concentration of 10 mol%. An empirical formula which allows the calculation of standard potentials from the chemical composition is proposed. Furthermore, a structural explanation for the effect of the chemical composition is given. Especially, the incorporation of Al₂O₃ as AlO₄⁻-tetrahedra at [Al₂O₃] < [Na₂O] and as network modifier at larger concentrations was structurally explained by the similarities of Fe²⁺ and Mg²⁺, with respect to cation radii and metal–oxygen bond lengths.     

Diluted and non-diluted ferric ions in borate glasses studied by electron paramagnetic resonance

Electron paramagnetic resonance (EPR) spectra of lithium borate glass (1 - x)(0.63B₂O₃ · 0.37Li₂O) · xFe₂O₃, with x varying from 0.001 to 0.1, were measured at different microwave frequencies and temperatures. For low Fe³⁺ concentrations (Fe₂O₃ molar contents from 0.001 to 0.01), X-band EPR spectra, consisting of a g_{ef = 4.3} peak accompanied by a shoulder continuing down to g_{ef = 9.7}, are computer simulated on the basis of a ‘rhombic’ spin-Hamiltonian with Zeeman and fine-structure terms. A good fit to the experimental spectra for various Fe₂O₃ contents is observed with the same values of the spin-Hamiltonian parameters and assuming a Lorentzian lineshape and a linewidth increasing linealry with the concentration of Fe³⁺ ions. It is concluded that the spectrum is due to diluted Fe³⁺ ions in a relatively strong crystal field of orthorhombic symmetry, with largely distributed fine-structure parameters. From the concentration dependence of the line width, by extending to glasses a theoretical EPR linewidth expression derived for polycrystalline systems, the minimum distance between diluted Fe³⁺ ions is estimated as 4.9 Å. A diluted state of Fe³⁺ ions in the glass network in this range is also confirmed by the temperature dependence of the g_ef = 4.3 resonance which follows a Curie law. For intermediate concentrations of Fe³⁺ ions (Fe₂O₃ molar contents from 0.01 to 0.1), the width of the g_ef = 4.3 line is proportional to the square root of concentration, still indicating dipolar interactions. On the other hand, the microwave frequency dependence of a broad g_ef ≈ 2 line, which coexists at these concentrations with the g_ef = 4.3 line, shows that the former line is due to pairs or small clusters of exchange-coupled Fe³⁺ ions. The temperature dependence of the g_ef ≈ 2 line intensity in 0.1 mol Fe₂O₃ glass is consistent with a more antiferromagnetic character by comparison with the 0.05 mol Fe₂O₃ glass, which is attributed to an appearance, at higher Fe₂O₃ contents, of iron-containing microclusters not incorporated in the random glass network, with smaller distances between the paramagnetic ions. These microcluster are probably the origin of a new narrow line superposed with the broad g_ef ≈ 2 line in the low-temperature EPR spectra.     

Raman spectroscopic investigations of the effect of the doping metal on the structure of binary tellurium-oxide glasses

Raman studies over the range 10 to 1000 cm⁻¹ have been performed on binary tellurium-oxide glasses (1-x)TeO₂-xMO (M = Pb,Zn or Mg) prepared using a conventional melt technique. The intensities and positions of Raman bands observed in the range above 250 cm⁻¹ were found to depend both on the compound oxide and on the amount of doping. Indeed, the ratio of the intensity of the band around 680 cm⁻¹ (assigned to vibrations of TeO₄ trigonal bipyramids) with respect to that of the component around 750 cm⁻¹ (related to stretching-vibrations in TeO₃₊₁, TeO₃ and MO groups) are affected in different ways for the glass-modifier MgO and for the intermediate glass-formers PbO and ZnO. Concurrently, an explicit dependence on the compound oxide and amount of doping was also observed on the maximum of the boson peak (BP) in the low-frequency region around 40 cm⁻¹. The structural correlation lengths in the glasses, calculated using the model described by Shuker and Gammon, were found to be about 0.50 nm (1-x)TeO₂-xMgO glasses and around 0.65 nm for (1-x)TeO₂-xMO (M = Pb or Zn) glasses. All these results are interpreted in terms of the effect of the metal oxide on the changes induced in the structural arrangements of ¹_χ[TeO₄-TeO₃] chains.     

Li conductivity in lithium niobate: silica glasses

Glasses of composition 65 mol% LiNbO₃:: 35 mol% SiO₂ have been shown to be Li⁺ ion conductors with a conductivity at 200°C > 1 × 10⁻⁵ (η cm)⁻¹ and an activation energy of 0.54 eV. The addition of approximately 0.1 mol% Fe₂O₃ leads to an enhancement of conductivity to ≈10⁻³ (η cm)⁻¹ at 200°C and an activation energy of 0.67 eV. The effect of Fe is shown to be in the control of microstructure in the glass, with Fe₂O₃ concentrations < 1 mol% acting as a grain growth inhibitors and larger concentrations acting as a nucleating agents. A model for this process based on the expected stoichiometry of the melt and the effect of Fe²⁺ and Fe³⁺ in charge compensation is in excellent agreement with experimental data from electron spin resonance.     

The effect of mixed alkaline earths on the diffusivity and the incorporation of iron in 5Na2O · xMgO · (15−x)CaO · yAl2O3 · (80−y)SiO2 melts

Diffusion coefficients of iron were measured in glass melts with the basic compositions 5Na₂O · xMgO · (15−x)CaO · yAl₂O₃ · (80−y)SiO₂ with x=5, 10 and y=0, 5, 7.5, 10 and 15. The melts were doped with 0.25 mol% Fe₂O₃ and studied in the temperature range from 1000 to 1600 °C using square-wave voltammetry. The voltammograms exhibited distinct peaks attributed to the reduction of Fe³⁺ to Fe²⁺, from which peak currents mixed diffusion coefficients of iron were calculated. Diffusion coefficients in all melt compositions which did not show crystallization could be fitted to Arrhenius equation. The diffusivities measured in different melt compositions were related to the same viscosity, i.e. not the same temperature. Increasing the alumina concentration from 5 to 10 mol% resulted in an increase of the viscosity corrected diffusivities. At further increasing alumina concentrations, the diffusivities get smaller again. This can be explained by the stabilizing effect of Na⁺ and Ca²⁺ on FeO⁻₄ and AlO⁻₄-tetrahedra, which strengthens the incorporation of Fe³⁺ into the glass structure.     

Growth and properties of (Pb0.90La0.10)TiO3 thick films prepared by RF magnetron sputtering with a PbO buffer layer

The (Pb_0.90La_0.10)TiO₃ [PLT] thick films (3.0 μm) with a PbO buffer layer were deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by RF magnetron sputtering method. The PLT thick films comprise five periodicities, the layer thicknesses of (Pb_0.90La_0.10)TiO₃ and PbO in one periodicity are fixed. The PbO buffer layer improves the phase purity and electrical properties of the PLT thick films. The microstructure and electrical properties of the PLT thick films with a PbO buffer layer were studied. The PLT thick films with a PbO buffer layer possess good electrical properties with the remnant polarization (P_r=2.40 μC cm⁻²), coercive field (E_c=18.2 kV cm⁻¹), dielectric constant (ε_r=139) and dielectric loss (tan δ=0.0206) at 1 kHz, and pyroelectric coefficient (9.20×10⁻⁹ C cm⁻² K⁻¹). The result shows the PLT thick film with a PbO buffer layer is a good candidate for pyroelectric detector.     

Raman spectroscopic study on the structure of ZnCl2---ZnX2 and ZnCl2---KX (X = Br, I) glasses

Raman spectra have been measured for ZnCl₂---ZnX₂ and ZnCl₂---KX (X = Br, I) glasses to investigate the structure of the glasses with varying composition. The assignment of each band was made, and the change of the spectra with composition was explained in terms of the bridging and non-bridging states of halide ions and the change of the tetrahedral units, ZnX_nCl_4−n²⁻ (n = 0–4), formed in the glasses. As the content of ZnX₂ in ZnCl₂---ZnX₂ glasses increases (20 → 80 mol%), the peak frequency of the Zn---Cl stretching mode increases (238 → 248 cm⁻¹ in X = I glasses, 238 → 259 cm⁻¹ in X = Br glasses) while the Zn---I and Zn---Br stretching frequencies decrease (173 → 120 cm⁻¹ for Zn---I, 196 → 157 cm⁻¹ for Zn---Br). The decrease of the Zn---I and Zn---Br band frequencies was attributed to the increase of the number n of the ZnX_nCl_4−n²⁻ tetrahedra. The increase of the Zn---Cl frequency suggests the existence of the bonding state of Cl⁻ ions which is intermediate between the bridging and the non-bridging states. In ZnCl₂---KX glasses, the Zn---Cl_non-bridging band at about 300 cm⁻¹ was observed in addition to the bands observed in ZnCl₂---ZnX₂ glasses. The addition of KX produces non-bridging anions while the tetrahedral units, ZnX_nCl_4−n²⁻ are also formed.     

Microstructure and growth rate of chemical vapour deposited TiN films in a vertical cold wall reactor

TiN films were grown on SUS304 substrates heated by an induction furnace in a vertical cold wall reactor. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction were used to characterize the microstructures of films obtained at different deposition conditions (temperature, gas flow rate and gas composition). Film structures obtained in the present vertical reactor had the following features compared with those in the tubular reactor: (1) Abnormally grown “star-shaped” crystals were observed on the surfaces of films deposited in the following ranges of total gas flow rate (Q_T), temperature (T) and partial pressures (P): 9.0×10⁻⁶ ≤ Q_T ≤ 1.6×10⁻⁵ m³ s⁻¹, 1223 ≤ T ≤ 1273 K, 0.92 ≤ P_TiCl4 ≤ 6.18 kPa, P_H2 = P_N2. The matrix grains were responsible for (211) preferred orientation. (2) Surface morphologies did not vary so much with P_TiCl4. On the other hand, a drastic change was brought about by adding HCl to the source gas, i.e., plate-shaped crystals dominated and the large “star-shaped” crystals were no longer present. (3) The apparent activation energy for deposition reaction was 230 kJ/mol (1173 ≤ T ≤ 1273 K) and 76.5 kJ/mol (1273 ≤ T ≤ 1373 K) at P_TiCl4 = 2.43 kPa and P_H2 = P_N2 = 49.45 kPa.     

On the mechanism of electrical relaxation in Na-conductive phosphate glasses

The crossover from a frequency independent to a frequency dependent ac response has been studied in glasses with the composition 37.2Na₂O-12.8CaO₅ · 50P₂O₅ and 30Na₂O-5CaO-7.5Al₂O₃-57.5P₂O₅ (mol%) containing 5 × 10⁻³ ≤ mol% Ag₂O ≤ 5 × 10⁻¹. Recently, we have established that in these glasses the diffusion coefficient of guest silver ions varies in space. In this case, as a first approximation, the diffusion coefficient may be considered as a constant within regions whose size is no less than 10 nm across. We assume that the diffusion of sodium ions can be given by the D(r) coefficient with the same spatial dispersion as that of silver ions. It is demonstrated that the frequency dependence of ac response is in fair agreement with the assumption.     

Internal friction in vanadium-phosphate glasses doped with Na2O

The internal friction of _xNa₂O·(0.5−x)V₂O₅·O.5P₂O₅(x = 0.025–0.3) glasses was studied using the low-frequency torsion pendulum technique. The temperature spectrum of internal friction reveals three maxima. Maximum 1, the so-called “electron” maximum, is the same as observed in binary vanadium-phosphate glasses. The origin of maximum 2 can be attributed to ion migration. Maximum 3 appears for glasses containing more than 10 mol.% Na₂O and is probably connected with sodium-proton interactions.     

Electrical conductivities of mixed cation glasses

The electrical conductivities of (1−x) Li₂O · x BaO · 2 SiO₂, (1−x) Na₂O · x MgO ·2 SiO₂, (1−x) Na₂O · x CaO · SiO₂ and (1−x) Na₂O · x BaO · 2SiO₂ glasses were measured at temperature ranging from room temperature to 450°C. The transport numbers for Na⁺ ion in (1−x) Na₂O · x BaO · 2 SiO₂ glasses were measured. It was found that the alkali ion carried a significant part of the current in these glasses except one that had no alkali ions, and the conductivity decreased markedly as the alkali oxide was substituted by an alkaline earth oxide. The results of conductivity measurements combined with the data hitherto reported on mixed alkali glasses led to the proposal that the so-called mixed alkali effect could be explained on the basis of the independent path model in which it is assumed that cations can move only through vacant sites left by those of the same type.     

XAS study of lanthanum coordination environments in glasses of the system K2O---SiO2---La2O3

The La L₁ and L₃ XANES and L₃ EXAFS have been investigated for the series of glasses 10K₂O---50SiO₂---x La₂O₃ (x = 1, 5, 10) and (10 − x)K₂O---40SiO₂−(x/3)La₂O₃ (x = 7.5, 5, 2.5) and model compounds La₂O₃, LaAlO₃, LaPO₄, La₂NiO₄, La₂CuO₄ and La(OH)₃. An edge resonance at 25 eV above the L₁ edge in the glass spectra is concentration-dependent, decreasing in intensity with increasing lanthanum concentration. The 2s → nd forbidden transition increases with La₂O₃ concentration, indicating a reduction in the ‘average’ site symmetry of the first coordination shell of La. Mapping _X(k) space, which is a new and promising technique, was employed to extract bond distance, coordination number and thermal parameters from the EXAFS. By this method, one calculates the complete _X(k) space a function of all physically reasonable values of the adjusted parameters in all possible combinations. The advantage in this method is the assurance of a global minimum. Bond lengths were comparable to those obtained by Fourier transforming the phase corrected EXAFS. The values are 2.42 Å (± 0.03 Å) for La---O. The coordination numbers (N ≤ 7 ± 1.5) were derived by mapping and comparison to the published structures for other La compounds. _X(k) mapping is compared with least-squares fitting the data, and the correlation between the Debye-Waller factor and coordination number is also discussed.     

Influence of chlorine-fluorine substitution on the electrical properties of some fluoride glasses

Study of electrical properties of two series of glasses (ZrF₄)₅₄(BaF₂)₃₀(ThF₄)₅(LaF₃)₃(RbF)_8(1−x)(RbCl)_8x (0 x 1) and (ZrF₄)₅₀(BaF₂)₃₀(ThF₄)₅(LaF₃)₃(RbF)_12(1−x)(RbCl)_12x (0 x 1) demonstrates the existence of a mixed mobile anion effect in these anion-conducting glasses. The effective medium approach has been applied to these materials: a satisfactory fitting is obtained as a function of the Cl/(Cl + F) ratio and temperature; the ionic transport in these glasses takes place along preferential pathways.     

An infrared study of crystallization in sodium-disilicate glasses containing iron oxides

The infrared absorption spectra of sodium-disilicate glasses containing various amounts of Fe₂O₃ ([Na₂O · 2 SiO₂]_1−x [Fe₂O₃]_x, where X = 0.05, 0.1 and 0.2) were investigated in the wavenumber range from 200–2000 cm⁻¹. The addition of Fe₂O₃ to the sodium-disilicate glass does not seem to introduce any new absorption band as compared with the spectrum of a pure sodium-disilicate glass; nevertheless, a general shift of the existing absorption bands toward lower wavenumbers is observed. The amount of shift is, in fact, proportional to the content of Fe₂O₃ in the glass. This observation is consistent with the recently proposed structural model for the bonding of Fe³⁺ ions in the iron-sodium-silicate glass system.

Annealing of 20 mol% iron oxide glasses at 550 and 580°C produced an extra sharp infrared absorption peak at about 610 cm⁻¹ wavenumber. This new peak is believed to be related to the crystallized particles of the glass as concluded from both a scanning electron micrograph and an electron diffraction pattern.     

Ionic transport and defect structure of vitreous beryllium fluoride

Vitreous BeF₂ was prepared by two techniques; (1) remelting of a technical grade material, and (2) vacuum distillation/fluoridation. Infrared spectroscopy studies have established that the first material contains about 0.5 wt.% hydroxyl, predicted to be coherently incorporated into the vitreous network as edge-linked [Be(OH)₄]²⁻ units. The distilled BeF₂ is water-free. The dc electrical conductivity of the remelted BeF₂ was measured as σ = (7.9 × 10³/T) exp(−24500 cal/mol/RT) ω⁻¹ cm⁻¹ and for the distilled BeF₂ as σ = (3.0 × 10⁵/T) exp(−36700 cal/mol/RT ω⁻¹ cm⁻¹ at temperatures to 280°C. Ionic transport studies utilizing a dc electrolysis polarization technique with N₂−F₂ and H₂−HF gas electrodes have demonstrated that the fluorine ion is the transport species. A general model for fluorine transport is proposed based upon a modified anti-Frenkel defect model. The difference in the fluorine transport process for the undistilled grade of BeF₂ is seen as a consequence of the anti-Frenkel defect pair interaction with the [Be(OH)₄[²⁻ groupings.