Different incommensurate composite crystal structure for Sc-II

Abstract

The long uncertain crystal structure of the high-pressure phase of scandium, Sc-II, is found to have an incommensurate composite structure comprising a body-centered host structure and a C-face-centered guest structure. At 23 GPa, the shortest distance between guest atoms is ϳ2.7 Å, comparable to the average hosthost distance of 2.8 Å. This differs from the structure proposed recently ͓H. Fujihisa et al., Phys. Rev. B 72, 132103 ͑2005͔͒ which has a body-centered guest structure, with anomalously short guest-guest distances.The development of advanced angle-dispersive powder and single-crystal diffraction techniques has led to the discovery of a number of complex incommensurate crystal structures in the elements at high pressures. 1 The group II and V elements, Ba and Sr, 2,3 and As, Sb, and Bi, 4-6 have high-pressure phases comprising an 8-atom body-centered host structure and linear chains of guest atoms lying in channels, along the c axis, through the host framework ͓Fig. 1͑a͔͒. And these guest chains form a simple body-centered ͓Fig. 1͑a͔͒ or C-face-centered structure which is incommensurate with the host along their common c axis. The group I elements K and Rb ͑Refs. 1, 7, and 8͒ also have a composite host-guest structure at high pressure, but with a 16-atom host structure ͓Fig. 1͑b͔͒.Recently, Fujihisa et al. 9 have reported that scandium, the first of the 3d transition metals, also possesses an incommensurate composite structure above 22 GPa ͑Sc-II͒, with the same body-centered host and guest structures as Bi-III and Sb-II ͓as in Fig. 1͑a͔͒. This is an important result in revealing the stability of host-guest composite structures in another group of elements with a quite different electronic configuration, and it suggests that still more such phases may be found. However, this solution has an anomalously short guest-guest distance along the chains, as Fujihisa et al. acknowledge, 9 although the fit to the powder diffraction profiles is good. At 23 GPa, the guest-guest distance ͑d 4 in Ref.9͒ is only 2.285 Å, some 18% shorter than the average hosthost contact distance of 2.79 Å ͑d 1 and d 2 in Ref. 9͒. And this difference increases to 21% at 101 GPa, where the guest-guest spacing is only 1.957 Å. This is quite unlike all the other composite structures found to date ͑Ba-IV, Sr-V, K-III, Rb-IV, As-III, Sb-II, Sb-IV, and Bi-III͒, in which the spacing of the guest atoms differs from the host-host contact distances by no more than 2%. Fujihisa et al. suggest that the short spacing might arise from a difference in the electronic structure of the host and guest atoms. But the magnitude of the effect seems improbably large, noting that the spacing of 1.957 Å at 101 GPa is even smaller than the closest contact distance of 2.052͑2͒ Å in Sc-V at the very much higher pressure of 242 GPa. 10 In this paper we propose a different incommensurate composite solution. It has the same host structure but a guest component that has a larger spacing along the c axis and is C-face-centered rather than body-centered, isostructural w...

High-pressure studies with x-rays using diamond anvil cells

“…19 near here…] A comprehensive review by McMahon and Nelmes [351] categorizes the complex structures into four types: incommensurate composite structures, incommensurate modulated structure, modulated layer structures, and other complex structures. Incommensurate composite structures comprise of two interpenetrating components, a 'host' and a 'guest', which are incommensurate with each other along one or more axes, e.g., in Ba [352], Sr [353], Rb [354], Sb [355,356], and Sc [357]. Incommensurate modulated structure can be viewed as a basic structure with the atoms displaced by a modulation wave, which is incommensurate with the basic unit cells.…”

One-dimensional chain melting in incommensurate potassium

“…These structures, found first in barium (Ba) [1], and then in an abundance of other group 1, 2, 3, and 15 elements [2][3][4][5][6][7][8][9][10][11][12], and which are also predicted to occur in other elements at extreme pressures [13], comprise a tetragonal (or monoclinically distorted) host structure with channels running along the c axis. Located within these channels are one-dimensional (1D) chains of guest atoms, which order into a number of different structures.…”

Prediction of incommensurate crystal structure in Ca at high pressure

“…These calculations also showed that the hcp phase is competitive with the pseudoBa-IV phase and one could expect that elements with the hcp structure because of partial sd-hybridized bonding might undergo a transition to a composite phase at high pressures. Indeed, the Sc-II phase, which was unknown for a long time, has been recently determined as an incommensurate host-guest structure, isostructural to Sr-V phase (9). The structural changes are accompanied also by drastic changes of the transport properties with pressure.…”