The three-dimensional atomic configuration of amorphous ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ and GeTe were derived by reverse Monte Carlo simulation with synchrotron-radiation x-ray diffraction data. The authors found that amorphous ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ can be regarded as “even-numbered ring structure,” because the ring statistics is dominated by four- and six-fold rings analogous to the crystal phase. On the other hand, the formation of Ge–Ge homopolar bonds in amorphous GeTe constructs both odd- and even-numbered rings. They believe that the unusual ring statistics of amorphous ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ is the key for the fast crystallization speed of the material.

Two-dimensional crystals, especially graphene and transition metal dichalcogenides (TMDs), are attracting growing interests because they provide an ideal platform for novel and unconventional electronic band structures derived by thinning. The thinning may also affect collective phenomena of electrons in interacting electron systems and can lead to exotic states beyond the simple band picture. Here, we report the systematic control of charge-density-wave (CDW) transitions by changing thickness, cooling rate and gate voltage in nano-thick crystals of 1T-type tantalum disulfide (1T-TaS2). Particularly the clear cooling rate dependence, which has never been observed in bulk crystals, revealed the nearly-commensurate CDW state in nano-thick crystals is a super-cooled state. The present results demonstrate that, in the two-dimensional crystals with nanometer thickness, the first-order phase transitions are susceptible to various perturbations, suggestive of potential functions of electronic phase control.

Epitaxial $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_3\left(\mathrm{PZT}\right)$ films, $1.5\text{–}2.0\mu \mathrm{m}$ in thickness, with a $\mathrm{Zr}∕(\mathrm{Zr}+\mathrm{Ti})$ ratio ranging from 0.20 to 0.75 were grown on ${\left(100\right)}_c\text{-},{\left(110\right)}_c\text{-}$ , and ${\left(111\right)}_c$ -oriented $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3∕∕\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrates at 600 °C by metal-organic chemical vapor deposition (MOCVD). The effects the $\mathrm{Zr}∕(\mathrm{Zr}+\mathrm{Ti})$ ratio had on the crystal structure, dielectric and ferroelectric properties, and piezoelectric response of these films with different crystal orientations were systematically investigated. We ascertained from x-ray-diffraction reciprocal-space-mapping analysis that ${\left(001\right)}_T\text{-}∕{\left(100\right)}_T\text{-}∕{\left(100\right)}_R\text{-},{\left(101\right)}_T\text{-}∕{\left(110\right)}_T\text{-}∕{\left(110\right)}_R\text{-}∕{\left(10\overline{1}\right)}_R\text{-}$ , and ${\left(111\right)}_T\text{-}∕{\left(111\right)}_R\text{-}∕{\left(11\overline{1}\right)}_R$ -oriented films had epitaxially grown on the respective ${\left(100\right)}_c\text{-},{\left(110\right)}_c\text{-}$ , and ${\left(111\right)}_c\text{-}$ oriented $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3∕∕\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrates. The constituent phase changed from a tetragonal single phase, a mixture phase of a tetragonal and rhombohedral, to a rhombohedral single phase with increasing $\mathrm{Zr}∕(\mathrm{Zr}+\mathrm{Ti})$ ratio irrespective of the orientation of the substrates. However, the range of the $\mathrm{Zr}∕(\mathrm{Zr}+\mathrm{Ti})$ ratio of the film with the mixture phase differed depending on crystal orientation. This suggests that the stress relaxation process applied from the substrates changed due to crystal orientation. The relative dielectric constant was maximum for films with the mixture phase regardless of the crystal orientation. Remanent polarization was also maximum for these films on the ${\left(111\right)}_c\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3∕∕\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrates, while it was minimum on the ${\left(100\right)}_c\text{-}$ and ${\left(110\right)}_c\text{-}$ oriented $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3∕∕\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrates. Films with two phases coexisting had larger electric-field-induced strain than films with a single tetragonal or rhombohedral phase for the (111)-oriented films, but there were no remarkable changes in the (100)- and (110)-oriented films. Small ac signal measurements suggested that domain-wall motions easily occurred in the (111)-oriented films with the mixture phase compared with other orientations. These results indicated that the larger field-induced strain of the (111)-oriented PZT films consisting of a mixture of tetragonal and rhombohedral phases largely contributed to extrinsic factors such as domain-wall motions and phase transformation due to the applied electric field.

A high-precision diffractometer has been developed for the structure analysis of a submicrometre-scale single grain of a powder sample at the SPring-8 BL40XU undulator beamline. The key design concept is the combination of a stable focused synchrotron radiation beam and the precise axis control of the diffractometer, which allows accurate diffraction intensity data of a submicrometre-scale single powder grain to be measured. The phase zone plate was designed to create a high-flux focused synchrotron radiation beam. A low-eccentric goniometer and high-precision sample positioning stages were adopted to ensure the alignment of a micrometre-scale focused synchrotron radiation beam onto the submicrometre-scale single powder grain. In order to verify the diffractometer performance, the diffraction pattern data of several powder grains of BaTiO(3), of dimensions approximately 600 x 600 x 300 nm, were measured. By identifying the diffraction data set of one single powder grain, the crystal structure was successfully determined with a reliable factor of 5.24%.

Phase change materials involve the rapid and reversible transition between nanoscale amorphous $\left(a\text{−}\right)$ and crystalline $\left(c\text{−}\right)$ spots in a polycrystalline film and play major roles in the multimedia world, including nonvolatile computer memory. The materials of choice are alloys of Ge, Sb, and Te, e.g., ${\text{Ge}}_2{\text{Sb}}_2{\text{Te}}_5$ (GST) in digital versatile disk–random access memory. There has been much speculation about the structure of $a\text{−}$ GST, but no model has yet received general acceptance. Here we optimize the structure by combining the results of density-functional calculations with high-energy x-ray diffraction data and x-ray photoelectron spectroscopy (XPS) measurements of the electronic densities of states to determine the structure. The structure agrees very well with available experimental data, including the differences in the XPS data between the amorphous and crystalline structures.