The Structure of Yb³⁺ Aquo Ion and Chloro Complexes in Aqueous Solutions at Up to 500 °C and 270 MPa

Abstract

We report here on X-ray absorption fine structure (XAFS) measurements used to determine the structure of the Yb 3+ ion in aqueous solutions over a range of temperatures from 25 to 500 °C and pressures up to 270 MPa. Fluorescence Yb L 3 -edge spectra were collected separately from nitrate (0.006m Yb/0.16m HNO 3 ) and chloride (0.006 m YbCl 3 /0.017 m HCl) aqueous solutions within a hydrothermal diamond anvil cell. The Yb-O distance of the Yb 3+ aquo ion in the nitrate solution exhibits a uniform reduction at a rate of 0.02 Å/100 °C, whereas the number of oxygens decreases from 8.3 ( 0.6 to 4.8 ( 0.7, in going from 25 to 500 °C. No evidence for nitrate complexes was found from measurements made on this solution. The Yb 3+ is found to persist as an aquo ion up to 150 °C in the chloride aqueous solution. In the 300-500 °C range, chloro complexes are found to occur in the solution, most likely of the type Yb(H 2 O) δ-n Cl n +3-n (δ ≈ 7). The Yb-Cl distance of the chloro ytterbium(III) complexes is found to decrease uniformly at a rate of about 0.02 Å/100 °C, whereas the number of chlorines increases from 0.5 ( 0.3 to 1.8 ( 0.2 in the 300-500 °C temperature range. Conversely, the Yb-O distance undergoes a lower uniform reduction at a rate of 0.007 Å/100 °C, whereas the number of oxygens decreases from 8.3 ( 0.5 to 5.1 ( 0.3 in going from 25 to 500 °C in the same solution.

Hydrothermal transport, deposition, and fractionation of the REE: Experimental data and thermodynamic calculations

“…Fig. 10a shows that in situ XAS studies of RCl 3 solutions (R = La 3+ , Nd 3+ , Gd 3+ , Yb 3+ ) as a function of temperature (25-500°C, P to 2.6 kbar) confirm these trends for chloride complexes (Mayanovic et al, 2002a(Mayanovic et al, , 2002b(Mayanovic et al, , 2007(Mayanovic et al, , 2009a. These in situ spectroscopic studies also demonstrate a general decrease in coordination number with increasing temperature, from~8 to 9 at 25°C (note the relatively large errors compared to Persson et al (2008), which reflects the use of the hydrothermal diamond anvil cell at higher temperature) to 6 to 7 at 500°C (Fig.…”

“…Since 1998, a number of studies have investigated the behavior of simple hydrated REE ions (Mayanovic et al, 2002a(Mayanovic et al, , 2002b(Mayanovic et al, , 2007(Mayanovic et al, , 2009a(Mayanovic et al, , 2009bRagnarsdottir et al, 1998). These studies, however, have focused on the hydration structure and co-ordination of these ions, and have not attempted to revise the values recommended by Shock (1997) and Shock and Helgeson (1988).…”

“…As a result, the stability of REE chloride complexes at elevated temperature has been subjected to much greater scrutiny than REE complexes involving other ligands. The hydrothermal speciation of the REE in chloride-bearing solutions has been investigated using a variety of experimental methods, including solubility (Gammons et al, 1996(Gammons et al, , 2002Migdisov et al, 2009), in situ UV-Vis spectroscopy Williams-Jones, 2002, 2006;Stepanchikova and Kolonin, 2005;, and in situ EXAFS studies (Mayanovic et al, 2002a(Mayanovic et al, , 2002b(Mayanovic et al, , 2007(Mayanovic et al, , 2009a(Mayanovic et al, , 2009b. The studies of Gammons et al (2002) and Migdisov et al (2009) involved simultaneous determination of the stability of a large number of REE in chloride-bearing solutions, whereas the other studies were devoted to single elements.…”

“…2C) chloride species of Nd predominate except in highly dilute solutions, which are not considered in the figure. Mayanovic et al (2002aMayanovic et al ( , 2007Mayanovic et al ( , 2009aMayanovic et al ( , 2009b determined the number of chloride ions bond to REE using in situ XAS. The data reported in Table 3 were used to calculate average chlorination numbers values that are compared with the XAS data in Table 4.…”

Hydration of gas-phase ytterbium ion complexes studied by experiment and theory

“…A study on dilute (*10 -2 M) ytterbium (III) chloride solutions by Mayanovic et al showed that under high pressure and temperature (500°C and 270 MPa), YbCl 2 ? Á(H 2 O) 5 is the predominant species [22]. Using X-ray absorption fine structure spectroscopy (XAFS), they determined that all five water molecules were directly coordinated to the Yb metal center.…”