Structure of molten lanthanum and cerium tri-halides by the method of isomorphic substitution in neutron diffraction

Abstract

The total structure factors of the molten trivalent metal halides MX3, where M3+ denotes La3+ or Ce3+ and X- denotes C1-, Br- or I-, have been measured by using neutron diffraction. Difference function methods were then applied on assuming that the LaX3 and CeX3 melts for a given halide ion are isomorphic. The results which follow from this assumption show that the first sharp diffraction peak in the measured total structure factors arises from cation correlations and its movement to lower scattering vector values with increasing anion size is consistent with an enhanced separation in real space of cation centred polyhedra. On melting the MX3 salts exhibit a decrease in the coordination number of both the cations and anions. In the liquid state the M-X coordination environment is asymmetric with M-Cl, M-Br and M-I nearest-neighbour distances of 2.93(2) Å, 3.01(2) Å, 3.18(2) Å and M-Cl, M-Br and M-I coordination numbers of 8.2(2), 7.4(2), 6.7(2) respectively. The Cl-Cl, Br-Br and I-I nearest-neighbour distances are 3.58(3) Å, 3.76(3) Å, 4.13(2) Å respectively and there is a significant penetration of the X-X partial pair distribution function into the first peak of the M-X partial pair distribution function for all three anions. The Cl-Cl, Br-Br and I-I coordination numbers are 9.2(2), 8.7(2) and 8.2(2) respectively if the M-M coordination number is two.

Short-range structure of molten CeCl3 and NdCl3 determined by XAFS

“…This is presumably because the spectra analysed by using stronger damping factors create larger uncertainties in the coordination numbers, while this procedure helps in smoothing the diffraction data. The light rare earth ions are surrounded by more than six chlorides in pure molten chlorides as suggested by the recent neutron diffraction isomorphic substitution experiments on molten LaCl 3 [4,5]. Also, the molecular dynamics simulations with polarisable ion model have been found recently to satisfactorily reproduce the experimental diffraction pattern obtained without using any damping factor; the results showing a coordination number >6 in the liquid phase [6,7].…”

“…Although conscientious efforts were made to follow all the experimental procedures and useful information obtained for future studies, the inter-ionic distance in the liquid phase was rather underestimated in comparison with the value derived from diffraction techniques [3][4][5] and the other XAFS study [10]. In the present work, our newly designed quartz cells were well adapted to obtain highly resolved spectra.…”

“…Recently, some other approaches, such as neutron diffraction and molecular dynamics simulations, have been used to elucidate their structure. These studies, in contradiction to the earlier works, reveal that the lighter rare earth ions are coordinated by more than six chlorides [4][5][6][7]. To resolve these contradictions, we have performed extended X-ray absorption fine structure (EXAFS) experiments that allow us to investigate the local structure only around the Xray absorption atom.…”

Local structure of molten LaCl3 analyzed by X-ray diffraction and La–LIII absorption-edge XAFS technique

“…The decrease in coordination number for La-Cl pair from RT to 1173 K is thought to be due to the increase in molar volume and to be well interpreted by that on melting, for instance, 19.1% [29]. As pointed out in introduction section, Wasse and Salmon [10,11] reported that the CN of the first La-Cl correlation in molten LaCl 3 was 8.2. In contrast, the corresponding CN in solid LaCl 3 was 9, as described in [26].…”

“…However, another structural image has been recently proposed in neutron diffraction (ND) and molecular dynamics (MD) works. Wasse and Salmon [10,11] reported from systematic ND studies of molten rare-earth trichlorides that the CN of the first M-Cl correlation was not always fixed at 6; for example, that of molten LaCl 3 was 8.2. Hutchinson et al [12,13] obtained MD simulation results of some rare-earth trichloride melts by using a polarizable ionic model (PIM), which reproduced well the ND results by Wasse and Salmon.…”