One-dimensional chain melting in incommensurate potassium

Abstract

Between 19 and 54 GPa, potassium has a complex composite incommensurate host-guest structure which undergoes two intraphase transitions over this pressure range. The temperature dependence of these host-guest phases is further complicated by the onset of an order-disorder transition in their guest chains. Here, we report single-crystal, quasi-single-crystal, and powder synchrotron x-ray diffraction measurements of this order-disorder phenomenon in incommensurate potassium to 47 GPa and 750 K. The so-called chain melting transition is clearly visible over a 22 GPa pressure range, and there are significant changes in the slope of the phase boundary which divides the ordered and disordered phases, one of which results from the intraphase transitions in the guest structure.

Structural and electronic properties of the alkali metal incommensurate phases

“…The guest lattice of K-III has been observed by McBride et al to lose long range order between chains upon heating [26]. There is a transition temperature above which correlations between the guest atom chains in adjacent channels are lost.…”

“…Above 25 GPa in calculations (32 GPa in experiment [26]), the host structure experiences a significant stiffening along the c axis. Comparing the c H /a H axial ratio in Fig.…”

“…As a result, the crystal is biperiodic along the c direction and can be described crystallographically using four-dimensional superspace groups [22]. Various explanations on what makes * a.hermann@ed.ac.uk these incommensurate structures stable have been put forward, including electron localization, s-d electron transfer or hybridization [15], a preference for 9-10 bonded neighbors [18,23], Fermi surface-Brillouin zone effects [24,25], and the high entropy, which leads to the so-called "chain melting" [26].…”

Ultrafast X-Ray Diffraction Studies of the Phase Transitions and Equation of State of Scandium Shock Compressed to 82 GPa

“…This is a (hk0m) "guest-only" peak, and arises from scattering from the chains of guest atoms only [30]. If these chains were disordered, as we have observed in HG-Rb at 300 K [31], and in HG-K at high temperatures [32], then this, and other, (hk0m) guest-only peaks would be extremely weak and not visible. A Rietveld refinement of the 51.1 GPa profile using a disordered HG-model is shown in Figure 3, and is excellent, accounting for all observed features.…”

Cluster melting: new, limiting, and liminal phenomena

“…All of these studies point to a clear outstanding issue in the study of melting of nanostructures: the exploration of the thermal stability of 2D materials. Looking further afield, experimental studies on the nature of melting in realistic 1D materials have been even more limited to date [108]. It is to be imagined that theoretical studies of realistic systems, employing modern first-principles BOMD methods, may lead to the realisation of a wide range of new and liminal phenomena in this domain.…”