Vacancies and electron localization in the incommensurate intergrowth compound (La0.95Se)1.21VSe2

Towards dynamically configured databases for CIFs: the new modulated structures open database at the Bilbao Crystallographic Server

This article presents a web-based framework to build a database without in-depth programming knowledge given a set of CIF dictionaries and a collection of CIFs. The framework consists of two main elements: the public site that displays the information contained in the CIFs in an ordered manner, and the restricted administrative site which defines how that information is stored, processed and, eventually, displayed. Thus, the web application allows users to easily explore, filter and access the data, download the original CIFs, and visualize the structures via JSmol. The modulated structures open database B-IncStrDB, the official International Union of Crystallography repository for this type of material and available through the Bilbao Crystallographic Server, has been re-implemented following the proposed framework.

Modulation doping and charge density wave transition in layered PbSe-VSe2 ferecrystal heterostructures

Controlling charge carrier concentrations remains a major challenge in the application of quasi-two-dimensional materials. A promising approach is the modulation doping of transport channels via charge transfer from neighboring layers in stacked heterostructures. Ferecrystals, which are metastable layered structures created from artificial elemental precursors, are a perfect model system to investigate modulation doping, as they offer unparalleled freedom in the combination of different constituents and variable layering sequences. In this work, differently stacked combinations of rock-salt structured PbSe and VSe₂ were investigated using X-ray photoelectron spectroscopy. The PbSe layers act as electron donors in all heterostructures, with about 0.1 to 0.3 donated electrons per VSe₂ unit cell. While they initially retain their inherent semiconducting behavior, they themselves become metallic when combined with a larger number of VSe₂ layers, as evidenced by a change of the XPS core level lineshape. Additional analysis of the valence band structure was performed for selected stacking orders at different sample temperatures to investigate a predicted charge density wave (CDW) transition. While there appear to be hints of a gap opening, the data so far is inconclusive and the application of spatially resolved techniques such as scanning tunneling microscopy is encouraged for further studies.

Confined linear molecules inside an aperiodic supramolecular crystal: the sequence of superspace phases in n-hexadecane/urea

High-resolution studies of the host-guest inclusion compound n-hexadecane/urea are reported at atmospheric pressure, using both cold neutrons and x-ray diffraction. This intergrowth crystal presents a misfit parameter, defined by the ratio c(h)/c(g) (c(host)/c(guest)), which is temperature independent and irrational (γ = 0.486 ± 0.002) from 300 to 30 K. Three different structural phases are reported for this aperiodic crystal over this temperature range. The crystallographic superspaces are of rank 4 in phases I and II, whereas phase III is associated with an increase in rank to 5, with a supplementary misfit parameter (δ = 0.058 ± 0.002) that is constant throughout this phase. The superspace group of phase I is hexagonal P6(1)22(00γ) down to T(c1) = 149.5 ± 0.5 K; phase II, which persists down to T(c2) = 127.8 ± 0.5 K is orthorhombic P2(1)2(1)2(1)(00γ), and phase III is orthorhombic P2(1)2(1)2(1)(00γ)(00δ).

Crystal structure determination of the TiSe2-based misfit layer compound (LaSe)1.20(TiSe2)2

The new inorganic misfit layer compound (LaSe)1.20(TiSe2)2 was prepared by high-temperature reaction of the elements. It can be regarded as an intercalate of TiSe2 with LaSe double layers inserted into every second van der Waals gap of the TiSe2 sandwiches (stage-2 intercalate). The structure, determined by single crystal X-ray diffraction on the basis of (3 + 1)-dimensional superspace, is described by two interpenetrating, incommensurately modulated subsystems. The first subsystem comprises of paired TiSe2 sandwiches. Its basic space group is C[unk](0.5957(4), −0.0004(10), −0.189(4)) and the lattice parameters are a 11 = 3.556(2) Å, a 12 = 6.120(2) Å, a 13 = 17.876(10) Å, α 1 = 86.71(3)°, β 1 = 92.93(3)°, and γ 1 = 90.03(3)°. The second subsystem comprises of LaSe double layers with basic space group C[unk](1.679(1), 0.001(2), 0.317(6)) and unit cell dimensions a 21 = 5.969(2) Å, a 22 = 6.118(2) Å, a 23 = 17.859(9) Å, α 2 = 86.69(3)°, β 2 = 89.33(4)°, and γ 2 = 90.01(3)°. The two subsystems have the common ([unk],[unk]) plane. Refinement on 1602 reflections with I > 2.5σ(I) converged to R = 0.126 (wR = 0.155) for the average structure. Stacking disorder of TiSe2 is found corresponding to displacements of the sandwiches along the [unk]21 axis; in the final structure model 9.9(4)% of TiSe2 is rigidly shifted by ½ [unk]11.