The high-pressure phase of tellurium, Te-III, is found to have an incommensurate monoclinic structure, superspace group I(')2/m(0q0)s0, of a type previously unknown in the elements. Te-III is stable from 4.5(2) to 29.2(7) GPa; the previously reported transition to a distinct Te-IV phase at 10.6 GPa is not observed. The incommensurate wave vector of Te-III is strongly pressure dependent and varies in a strongly nonlinear way. Se-IV is found to be isostructural with Te-III.

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We describe an x-ray diffraction study of liquid Cs at high pressure and temperature conducted in order to characterize the structural changes associated with the complex melting curve and phase transitions observed in the solid phases. At 3.9 GPa we observe a discontinuity in the density of the liquid accompanied by a decrease in the coordination number from about 12 to 8, which marks a change to a nonsimple liquid. The specific volume of liquid Cs, combined with structural analysis of the diffraction data, strongly suggest the existence of dsp(3) electronic hybridization above 3.9 GPa, similar to that reported on compression in the crystalline phase.

The kinetic stability of pure and yttrium-doped tetragonal zirconia (ZrO2) polymorphs prepared via a pathway involving decomposition of pure zirconium and zirconium + yttrium isopropoxide is reported. Following this preparation routine, high surface area, pure, and structurally stable polymorphic modifications of pure and Y-doped tetragonal zirconia are obtained in a fast and reproducible way. Combined analytical high-resolution in situ transmission electron microscopy, high-temperature X-ray diffraction, and chemical and thermogravimetric analyses reveals that the thermal stability of the pure tetragonal ZrO2 structure is very much dominated by kinetic effects. Tetragonal ZrO2 crystallizes at 400 °C from an amorphous ZrO2 precursor state and persists in the further substantial transformation into the thermodynamically more stable monoclinic modification at higher temperatures at fast heating rates. Lower heating rates favor the formation of an increasing amount of monoclinic phase in the product mixture, especially in the temperature region near 600 °C and during/after recooling. If the heat treatment is restricted to 400 °C even under moist conditions, the tetragonal phase is permanently stable, regardless of the heating or cooling rate and, as such, can be used as pure catalyst support. In contrast, the corresponding Y-doped tetragonal ZrO2 phase retains its structure independent of the heating or cooling rate or reaction environment. Pure tetragonal ZrO2 can now be obtained in a structurally stable form, allowing its structural, chemical, or catalytic characterization without in-parallel triggering of unwanted phase transformations, at least if the annealing or reaction temperature is restricted to T ≤ 400 °C.

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...