Optical Properties of Pure Hydrated and Dehydrated BaCl 2

The results of the reflection measurements on un-doped barium chloride are shown in Figure 4.12a. A freshly sintered sample shows nearly no absorption in the near infrared, while a sample that was exposed to normal ambient for a week shows strong absorption between 1400 and 1600 nm.

Anhydrous BaCl2 is hygroscopic, which means that under normal conditions it tends to attract water molecules from the surroundings. A stable form of hydrated barium chlo-ride is the dihydrate (BaCl₂:2H₂O), which can be prepared reproducibly from aqueous solution [153] or by controlled back hydration [154]. Hydration by exposing anhydrous BaCl₂ to humid surroundings can lead to incomplete hydration with products such as mono and half-hydrates or even unknown water contents (BaCl₂:xH₂O)[154]. Hydrated barium chloride can be dehydrated by heating to temperatures between 80 and 180^◦C or higher [149].

It is highly likely that the absorptions occurring around 1500 nm are due to the combina-tion of the symmetric (νsymm) and asymmetric (νasymm) vibration mode of two crystallo-graphically non-equivalent water molecules in hydrated BaCl₂. Explicit measurements of combination ν_symm+ν_asymm were not found in the literature for BaCl₂, but for the very similar compound NaBr2:2H2O the absorption at 1440 nm was clearly assigned to the (ν_symm+ν_asymm)-band [155]. For the single vibrations in BaCl₂:2H₂O several experimen-tal figures were found in the literature. A very detailed description of the vibrations of these water molecules is given by Kondyurin et al. [156], where also correction of misin-terpretation in earlier publications are made. The wavenumbers of the vibrations found experimentally by Kondyurin et al. (and relevant for our purposes) are given in Table 4.6, together with the location of the combination of the two modes (νsymm+νasymm) es-timated from this data. As can be seen in the Figure 4.12a, these absorptions do not exactly match the main absorption found in our experiments (also listed in Table 4.6).

The numbers given by Kondyurin et al. are for dihydrates of BaCl2. Lutz et al. [154]

1400 1450 1500 1550 1600

heated at 180°C for several hours

sintered at 1100°C for 1h under N₂atmosphere J5

Absorption[%]

Wavelength [nm]

Figure 4.11: Changing of the infrared absorption spectrum of up-converter sample J5 after heating at 180^◦C for several hours and sintering at 1100^◦C for one hour under inert gas atmosphere. Due to the sintering the spectrum becomes more structured. The absorption properties of J5 resemble after the heat treatment more the properties of the group A1, than group A2, to which J5 was originally assigned.

showed that for mono- and half-hydrates a shift to higher wavenumbers takes place, which equals a shift to shorter wavelengths (see also Table 4.6).

In this publication no distinction between crystallographically inequivalent water molecule positions is mentioned. The main absorptions found experimentally within this thesis are located close to the positions proposed in literature and listed in Table 4.6.

Molecule ν_symm ν_asymm

Table 4.6: Wavenumbers of the symmetricν_symm and asymmetricν_asymmvibrations of the two crystallographically non-equivalent water molecules in the barium chloride dihydrate BaCl₂:2H₂O and in the half- and mono-hydrate. The positions of the infrared absorption for the combination of both vibrationsν_symm+ν_asymm are calculated from these data. For comparison the wavelengths obtained experimentally in this thesis are listed.

1400 1450 1500 1550 1600 1650

1400 1450 1500 1550 1600 1650

0

Figure 4.12: Comparison of the absorption of hydrated and freshly sintered BaCl₂. The strong absorption of the hydrated sample is due to the excitation of vibrational modes of water molecules attracted by barium chloride and erbium chloride respectively.

Optical Properties of Pure Hydrated and Dehydrated ErCl₃ Similarly to bar-ium chloride, erbbar-ium chloride is also hygroscopic with a stable hydration with 6 water molecules per erbium chloride unit (ErCl₃:6H₂O). The dehydration is reported to start at 95^◦C and anhydrous ErCl₃ forms at 175^◦C [157].

A comparison of the reflection spectra of sintered (dehydrated) and hydrated erbium chlo-ride (the latter has an unknown hydration state, ErCl₃:xH₂O) is shown in Figure 4.12b.

In the freshly sintered sample, the absorption lines of the erbium ion are largely isolated and several isolated peaks occur. In the hydrated sample the spectrum is broadened, leading to an absorption ranging from 1400 to 1650 nm with the peak at 1509 nm.

The absorption lines of erbium in dehydrated erbium chloride involve a wavelength range comparable to dehydrated erbium doped barium chloride, but shifted to longer wave-lengths. This suggests that the very structured broad absorption found in the sintered up-converter samples is due to absorption within the erbium ion, where not every ab-sorption actually leads to up-conversion (since no excitation of spectral response is found at these wavelengths). The absorption lines as depicted in Figure 4.12b represent the Stark splitting in ErCl₃, which differ in position and shape from the spectrum found for BaCl2:Er³⁺ due to the different crystal field influencing the erbium ion. For compari-son in Figure 4.13 the absorption of hydrated⁸ pure barium chloride and hydrated and sintered erbium chloride, the up-converter and the infrared spectral response of the two up-converters J2 and J10 are shown.

8Concerning the term hydrated see Section 4.2. In this case the exact hydration state is not clear. This sample was exposed to surrounding air, where it tends to attract water molecules in unknown amounts.

1400 1450 1500 1550 1600 1650 1400 1450 1500 1550 1600 1650

Spectralresponseandabsorptionina.u.

Wavelength [nm]

absorption pure BaCl₂: xH₂O absorption pure ErCl₃: xH₂O absorption pure sintered ErCl₃ absorption J10

spectral response J10

Spectralresponseandabsorption[a.u.]

Wavelength [nm]

absorption pure BaCl₂: xH₂O absorption pure ErCl₃: xH₂O absorption pure sintered ErCl₃ absorption J2

spectral response J2

Figure 4.13: Comparison of the excitation spectrum (infrared spectral response) and the absorption of the samples J10 and J2 with the absorption of un-doped hydrated barium chloride and hydrated and dehydrated erbium chloride.

Raman Measurements A further proof of the presence of crystal water in the BaCl₂ -based up-converter samples was given by Raman measurements⁹. The spectrum of pure BaCl2 exposed to atmosphere at room temperature for several days is shown in Figure 4.14. The vertical lines assign the positions of the water vibrations reported by Jain et al. [153]. The roman numerals assign the crystallographic type of the affected water molecule, the abbreviations r, w and t stand for the different vibrational modes of the H₂O molecule, rocking, wagging and twisting respectively. As can be seen there is a good agreement in the positions obtained experimentally in this thesis with the results obtained by Jain. The internal modes, ν_i, of H₂O (i=1...symmetric vibration, i=2...symmetric deformation, i=3...asymmetric vibration) and their combination occur at wavenumbers larger than 1500 cm⁻¹. All vibrations of the chlorine and barium ions are expected to be at wavenumbers smaller than 150 cm⁻¹. This would include the translatory modes of Ba⁺⁺ between 33 and 75 cm⁻¹ and Cl⁻ between 96 and 145 cm⁻¹ [153].

9These measurements were taken with a Mk1 Renishaw Raman Microscope at the University of New South Wales. The measurement setup allowed only measurements of wavenumbers higher than about 150 cm⁻¹.

200 400 600 800 0.0

0.5 1.0 1.5 2.0

554±2

2^ν

w(II) νr(I)

ν

t(I) ν

w(I) νr(II) νw(II) νt(II)

190 790

714

407 554 611

νtrans(H

2O)

490

251

165 191±2 203±2 715±2

484±2

403±2

250±2

203

BaCl₂:xH₂O

Intensity[a.u.]

Wavenumber [cm^-1]

Figure 4.14: Raman spectrum of pure hydrated BaCl2. The vertical lines assign the location of the Raman peaks as assigned by Jain et al. [153]. For each peak the mode of vibration (w...wagging, r...rocking or t...twisting mode of the H₂O molecule) and the affected crystallographic type of water molecule (I or II) is given.