Pressure dependence of the structure of liquid copper halides

Abstract

X-ray diffraction of liquid CuI, CuBr and CuCl has been measured up to 19 GPa using synchrotron radiations. Static structure factor S(Q) and pair distribution function g(r) were obtained. For liquid CuI, CuBr and CuCl, S(Q) and g(r) change their shapes continuously with increasing pressure, indicating anisotropic compression of the local structures. The pressure dependence of the peak position ratio r(2)/r(1) of g(r) shows that Cu atoms are located in a tetrahedral site at low pressures and then in an octahedral site at high pressures in these liquid copper halides. At higher pressures, the local structure of liquid CuCl is similar to the intermediate structure between NaCl and CsCl structures. These results are compared with the pressure-induced structural transformation in the crystalline phase.

The structure of molten CuCl, CuI and their mixtures as investigated by using neutron diffraction

“…In the β-phase from which CuCl melts at ambient pressure, the Cu + ions occupy tetrahedral holes in an hcp sublattice of chloride ions [11]. In the liquid, the Cu-Cl coordination number nCl Cu = 3.0(7)-3.4 [28,29] which is reported to increase with pressure to ≈3.7 at 1.2 GPa and to ≈5.6 at 18.5 GPa [43]. A comparison is also made in figure 5 between the G (r ) functions derived from both sets of g αβ (r ) and the function measured in the present work for the liquid at a lower temperature of 733 K. The results show that application of the Bartlett modification function leads to a relative broadening of G (r ).…”

“…The discrepancy between the nearest-neighbour Cu-X distance and the sum of the ionic radii is therefore larger for CuI compared with CuCl, in keeping with the smaller ionicity f i of CuI (0.692 cf 0.746 on the Phillips scale [19]). There is some indication, from the total pair distribution function measured by x-ray diffraction, that the Cu-I coordination number for the liquid gradually increases to a value ≈6 when the pressure is increased beyond 6 GPa [43].…”

“…The structure and other properties of molten CuCl and CuI [31][32][33][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52] have been extensively studied by experiment, theory and computer simulation, thus providing a benchmark for work on their mixtures. In particular, the full set of partial structure factors for molten CuCl was measured by Page and Mika [23] in an early use of the method of isotopic substitution in neutron diffraction and the results were subsequently improved upon by Eisenberg et al [28].…”

Evolution of Short‐Range Order of Amorphous GeTe Upon Structural Relaxation Obtained by TEM Diffractometry and RMC Methods

“…S ( q 2 )/ S ( q 1 ) relates the peak heights of the first two maxima in the structure function from which the predominant structural motifs in a disordered material can be concluded [ 13 ] : $$$$\begin{equation} \frac{S(q_2)}{S(q_1)} {\begin{cases} <1 \quad \Rightarrow \mathrm{octahedral}\\ >1 \quad \Rightarrow \mathrm{tetrahedral} \end{cases}} \end{equation}$$$$The quantity r 2 / r 1 is defined by the ratio between the average second nearest‐neighbor to first nearest‐neighbor distance in G ( r ). Using geometric arguments this ratio can be used to judge whether the atomic arrangements are predominantly octahedral or tetrahedral [ 13,52,53 ] : $$$$\begin{equation} \frac{r_2}{r_1}= {\begin{cases} \sqrt {2}&\approx 1.41 \quad \Rightarrow \mathrm{octahedral}\\ \dfrac{4}{\sqrt {6}}&\approx 1.63 \quad \Rightarrow \mathrm{tetrahedral} \end{cases}} \end{equation}$$$$…”

“…The quantity r 2 /r 1 is defined by the ratio between the average second nearest-neighbor to first nearest-neighbor distance in G(r). Using geometric arguments this ratio can be used to judge whether the atomic arrangements are predominantly octahedral or tetrahedral [13,52,53] :…”