Microscopic surface structure of liquid alkali metals

Abstract

We report an x-ray scattering study of the microscopic structure of the surface of a liquid alkali metal. The bulk liquid structure factor of the eutectic K 67 Na 33 alloy is characteristic of an ideal mixture, and so shares the properties of an elemental liquid alkali metal. Analysis of off-specular diffuse scattering and specular x-ray reflectivity shows that the surface roughness of the K-Na alloy follows simple capillary wave behavior with a surface structure factor indicative of surface-induced layering. Comparison of the low-angle tail of the K 67 Na 33 surface structure factor with the one measured for liquid Ga and In previously suggests that layering is less pronounced in alkali metals. Controlled exposure of the liquid to H 2 and O 2 gas does not affect the surface structure, indicating that oxide and hydride are not stable at the liquid surface under these experimental conditions.The structure of the free surface of a liquid metal ͑LM͒ is fundamentally different from that of a dielectric liquid, due to the strong coupling between conduction electrons and ion cores. 1 At the liquid-vapor interface of a LM, the Coulomb interaction between the free electron Fermi gas and the classical gas of charged ions acts like an effective hard wall and forces the ions into ordered layers parallel to the surface. The existence of surface-induced layering in LM's has been verified unambiguously by experiment for liquid Hg, 2 Ga, 3 and In. 4 Layering is also present in liquid binary alloys, even though it may be suppressed by surface segregation 5 or the formation of surface phases. 6,7 Even though surface layering appears to occur in LM's in general, comparison of the detailed surface structure reveals qualitative differences between different LM's. 4,8 In fact, very little is understood about how the surface-induced layering predicted for an ideal LM is affected by the details of the more complicated electronic structure exhibited by the polyvalent metals studied so far. One important goal therefore is to study the surface structure of those LM's that show the most ideal electronic structure in the bulk phase, characterized by itinerant conduction electrons which are only weakly perturbed by a small ionic pseudopotential. These LM's are referred to as nearly-free-electron type liquid metals. 9 In a previous study we have compared the surface structure of liquid In, which is considered to be a nearly-free-electron type liquid metal, with the surface structure of liquid Ga, which displays a considerable degree of covalency in the bulk. 4 However, even though liquid In has less tendency toward covalent bonding than Ga, it is trivalent, and the most simple models do not adequately describe its electronic structure or metallic properties. 11 The liquid metals closest to nearly-free-electron behavior are the monovalent alkali metals, which have a half-filled s band and an almost spherical Fermi surface. 10 The metallic properties of alkali metals are explained by the most simple solid state models such as the Drude theory...

Interplay between the ionic and electronic density profiles in liquid metal surfaces

“…Mv, 64.70.Fx, 71.15.Pd X-ray reflectivity measurements on the surface of liquid metals and alloys, along with other techniques like diffuse scattering or grazing incidence diffraction, have shown the existence of layering in the ionic density profile. 1,2,3,4,5,6,7,8 Its origin is yet not clear and several reasons have been mooted. Rice and coworkers 9,10,11 have pointed that the reason for surface layering in metals is the interconnection between the ionic and electronic densities and that the abrupt decay of the electron density at the surface induces an effective wall against which the ions, behaving like hard-spheres, stack.…”

In-plane structure and ordering at liquid sodium surfaces and interfaces from ab initio molecular dynamics

“…This behavior does not generally occur at free surfaces of liquid dielectrics -though it is well known that atomic layers form at the solid-liquid interfaces of both dielectric and metallic substances 38 due to geometrical confinement. Surface-induced layering at free liquid metal surfaces was predicted theoretically 7,8,9,10,16,17,18,37 , and subsequently confirmed experimentally 20,26,29,31,32,33,34,35,39,46,47 for various metals and alloys by x-ray reflectivity…”

Processes in phase boundary region during liquid phase growth

“…Growth of materials with the same chemical composition but with different crystalline structures (for example polytypic and/or polymorphic modifications of SiC, grown on SiC or ZnS-on ZnS and etc. ), showed that in a liquid phase growth process a new polytypic or polymorphic modification arises very seldom [14,15]. With the aim to realize a parallel growth of two polytypic modifications of 4H-SiC and 6H-SiC on 6H-SiC, we have established that in the most experiments the layers reproduce the substrate's modification.…”

“…• C (the monotectic temperature) between the Bi monolayer, situated on the surface, and Ga-rich bulk resides a 30 0 A thick Bi-rich wetting film [12][13][14][15]. In the present article a different point of view concerning the structure of the phase boundary region and the processes, carried out there, is presented.…”

“…Into the liquid phase bulk at a distance of about two atomic diameters the electrons density reduces to the value of the liquid bulk. Investigating the binary alloys of liquid metals Ga-Bi [12][13][14][15] one reveals at the surface the availability of a Bi monolayer. For T > T mono = 222…”