Structure of amorphous Ag/Ge/S alloys: experimentally constrained density functional study.

Density functional/molecular dynamics simulations have been performed to determine structural and other properties of amorphous Ag/Ge/S and Ge/S alloys. In the former, the calculations have been combined with experimental data (x-ray and neutron diffraction, extended x-ray absorption fine structure). Ag/Ge/As alloys have high ionic conductivity and are among the most promising candidates for future memristor technology. We find excellent agreement between the experimental results and large-scale (500 atoms) simulations in Ag/Ge/S, and we compare and contrast the structures of Ge/S and Ag/Ge/S. The calculated electronic structures, vibrational densities of states, ionic mobilities, and cavity distributions of the amorphous materials are discussed and compared with data on crystalline phases where available. The high mobility of Ag in solid state electrolyte applications is related to the presence of cavities and can occur via jumps to a neighbouring vacant site.

Structural singularities in Ge(x)Te(100-x) films.

Structural and calorimetric investigation of Ge(x)Te(100-x) films over wide range of concentration 10 < x < 50 led to evidence two structural singularities at x ∼ 22 at. % and x ∼ 33-35 at. %. Analysis of bond distribution, bond variability, and glass thermal stability led to conclude to the origin of the first singularity being the flexible/rigid transition proposed in the framework of rigidity model and the origin of the second one being the disappearance of the undercooled region resulting in amorphous materials with statistical distributions of bonds. While the first singularity signs the onset of the Ge-Ge homopolar bonds, the second is related to compositions where enhanced Ge-Ge correlations at intermediate lengthscales (7.7 Å) are observed. These two threshold compositions correspond to recently reported resistance drift threshold compositions, an important support for models pointing the breaking of homopolar Ge-Ge bonds as the main phenomenon behind the ageing of phase change materials.

Acidic properties of aqueous phosphoric acid solutions: a microscopic view.

We report on new neutron and x-ray diffraction data on D2O:D3PO4 solutions at two concentrations, 1:1 and 3:1. The experimental datasets were modelled simultaneously by the reverse Monte Carlo (RMC) method. From the resulting models, partial radial distribution functions (prdf) and coordination numbers were obtained. The acidity was found to decrease with increasing D3PO4 concentration. The ratio of dissociated acidic protons was estimated by dedicated simulation runs using average coordination number constraints. It was found that in the saturated solution the ratio of dissociated protons cannot exceed 20%.

Silver environment and covalent network rearrangement in GeS3-Ag glasses.

The structure of Ag-doped GeS3 glasses (0, 15, 20, 25 at.% Ag) was investigated by diffraction techniques and extended x-ray absorption fine structure measurements. Structural models were obtained by fitting the experimental datasets simultaneously by the reverse Monte Carlo simulation technique. It is observed that Ge has mostly S neighbours in GeS3, but Ge-Ge bonds appear already at 15% Ag content. Sulfur has ~2 S/Ge neighbours over the whole concentration range, while the S-Ag coordination number increases with increasing Ag content. Ag-Ag pairs can already be found at 15% Ag. The Ag-S mean coordination number changes from 2.17 ± 0.2 to 2.86 ± 0.2 between 15% and 25% Ag content. Unlike the As-S network in AsS2-25Ag glass, the Ge-S network is not fragmented upon Ag-doping of GeS3 glass.

Short range order and stability of amorphous Ge(x)Te(100-x) alloys (12 ≤ x ≤ 44.6).

Amorphous Ge(x)Te(100-x) alloys were obtained over a broad composition range (12 ≤ x ≤ 44.6) by thermal co-evaporation. Their structure was investigated by x-ray diffraction and extended x-ray absorption fine structure measurements. Experimental datasets were fitted simultaneously by the reverse Monte Carlo simulation technique. It is concluded that Te is mostly twofold coordinated and the majority of Ge atoms have four neighbours. The number of Ge-Ge and Te-Te bonds evolves monotonically with composition. Ge-Ge bonding can be observed already at x = 24 while Te-Te bonds can be found even in Ge44.6Te55.4. The models obtained by simulation show that the structure of compositions with x > 24 should be considered as a random covalent network but there is chemical ordering for x ≤ 24, exactly in the composition range where glasses can be obtained from the melt by fast quenching. The composition dependences of some physical properties also point to the connection between chemical short range order and the stability of the amorphous phase: while the glass transition temperature and microhardness increase monotonically with the composition, the thermal stability of the amorphous films goes through a maximum around x = 20-24.

Structural investigations of Ge5As(x)Se(95-x) and Ge15As(x)Se(85-x) glasses using x-ray diffraction and extended x-ray fine structure spectroscopy.

The structure of Ge(5)As(x)Se(95-x) (x = 10, 20, 30, 38 at.%) and Ge(15)As(x)Se(85-x) (x = 10, 25, 34 at.%) glasses has been investigated by high-energy x-ray diffraction and extended x-ray absorption fine structure measurements. The experimental datasets have been modelled using the reverse Monte Carlo simulation technique. The model atomic configurations have been analysed in detail. It has been found that the homonuclear Ge-Ge, As-As, Se-Se and heteronuclear Ge-As bonds play an important role in the structure formation of the Ge-As-Se glasses. The total number of these bonds decreases quite slowly with the mean coordination number similarly to the nonlinear refractive index.

Structure of GeSe4-In and GeSe5-In glasses.

(Ge(0.2)Se(0.8))(100-x)In(x) and (Ge(0.17)Se(0.83))(100-x)In(x) (x = 0, 5, 10, 15 at.%) chalcogenide glasses have been studied with high-energy x-ray diffraction, neutron diffraction and extended x-ray absorption spectroscopy at Ge, Se and In K-edges. The experimental data were modelled simultaneously with the reverse Monte Carlo simulation method. GeSe(4/2) tetrahedra are shown to be the main structural units in the binary and ternary glasses investigated. Indium bonds to the excess Se atoms in the ternary Ge-Se-In glasses. While the majority of In atoms have three Se neighbours, some In atoms may be tetrahedrally coordinated by Se.

On the atomic structure of Zr60Cu20Fe20 metallic glass.

The structure of Zr(60)Cu(20)Fe(20) metallic glass has been studied with high-energy x-ray diffraction, neutron diffraction and extended x-ray absorption spectroscopy and modelled with the reverse Monte Carlo simulation technique. It is found that Cu and Fe atoms prefer Zr as a nearest neighbour. The mean interatomic distance between Cu/Fe and Zr atoms in the glass is remarkably shorter than the sum of the respective atomic radii. The coordination numbers for Cu/Fe-Cu/Fe pairs are very close to each other, suggesting a regular distribution of Cu and Fe atoms in the Zr(60)Cu(20)Fe(20) metallic glass.

Modelling the atomic structure of Al92U8 metallic glass.

The local atomic structure of the glassy Al(92)U(8) alloy was modelled by the reverse Monte Carlo (RMC) method, fitting x-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) signals. The final structural model was analysed by means of partial pair correlation functions, coordination number distributions and Voronoi tessellation. In our study we found that the most probable atomic separations between Al-Al and U-Al pairs in the glassy Al(92)U(8) alloy are 2.7 Å and 3.1 Å with coordination numbers 11.7 and 17.1, respectively. The Voronoi analysis did not support evidence of the existence of well-defined building blocks directly embedded in the amorphous matrix. The dense-random-packing model seems to be adequate for describing the connection between solvent and solute atoms.

Nanometer range correlations between molecular orientations in liquids of molecules with perfect tetrahedral shape: CCl(4), SiCl(4), GeCl(4), and SnCl(4).

Neutron and x-ray weighted total scattering structure factors of liquid carbon, silicon, germanium, and tin tetrachlorides, CCl(4), SiCl(4), GeCl(4), and SnCl(4), have been interpreted by means of reverse Monte Carlo modeling. For each material the two sets of diffraction data were modeled simultaneously, thus providing sets of particle coordinates that were consistent with two experimental structure factors within errors. From these particle configurations, partial radial distribution functions, as well as correlation functions characterizing mutual orientations of molecules as a function of distance between molecular centers were calculated. Via comparison with reference systems, obtained by hard sphere Monte Carlo simulations, we demonstrate that orientational correlations characterizing these liquids are much longer ranged than expected, particularly in carbon tetrachloride.

Atomic structure of As(2)S(3)-Ag chalcogenide glasses.

(As(0.4)S(0.6))(100-x)Ag(x) glasses (x = 0, 4, 8, 12 at.%) have been studied with high-energy x-ray diffraction, neutron diffraction and extended x-ray absorption spectroscopy at As and Ag K-edges. The experimental data were modelled simultaneously with the reverse Monte Carlo simulation method. Analysis of the partial pair correlation functions and coordination numbers extracted from the model atomic configurations revealed that silver preferentially bonds to sulfur in the As(2)S(3)-Ag ternary glasses, which results in the formation of homoatomic As-As bonds. Upon the addition of Ag, a small proportion of Ag-As bonds (N(AgAs)≈0.3) are formed in all three ternary compositions, while the direct Ag-Ag bonds (N(AgAg)≈ 0.4) appear only in the glass with the highest Ag content (12 at.%). Similar to the g- As(2)S(3) binary, the mean coordination number of arsenic is close to three, and that of sulfur is close to two, in the As(2)S(3)-Ag ternary glasses. The first sharp diffraction peak on the total structure factors of As(2)S(3) binary and (As(0.4)S(0.6))(100-x)Ag(x) ternary glasses is related to the As-As and As-S correlations.

Structure of As(x)Te(100-x) (20

A systematic and detailed investigation of the structure of As(x)Te(100-x) glasses (20

Network structure of 0.7SiO(2)-0.3Na(2)O glass from neutron and x-ray diffraction and RMC modelling.

The structure of 0.7SiO(2)-0.3Na(2)O glass was investigated by means of neutron and high-energy x-ray diffraction. The maximum momentum transfer was 35 and 23.5 Å(-1) for the two experiments. The two datasets were modelled simultaneously by the reverse Monte Carlo simulation technique. By using reasonable constraints it was possible to separate the six partial pair correlation functions. Nearest neighbour distances, coordination numbers and bond angle distributions have been revealed. It was found that 63% of the O atoms are in the bridging position. The Na-O distance is 2.29 Å and the coordination number is 2.5. The Na-Na nearest neighbour distance is 2.6 Å, a value significantly smaller than previously reported. Neighbouring sodium ions tend to be located at the same oxygen atom. The average Si-O ring size is 7.6.

Atomic structure of As(25)Si(40)Te(35) glass.

Glassy As(25)Si(40)Te(35) has been studied by x-ray and neutron diffraction as well as x-ray absorption spectroscopy (EXAFS) at As and Te K-edges. Simultaneous modelling of the four independent measurements by means of the reverse Monte Carlo (RMC) simulation technique allowed the separation of partial pair distribution functions and estimation of the corresponding coordination numbers. It is shown that the atomic structure of As(25)Si(40)Te(35) glass can be presented as a three-dimensional network of twofold coordinated Te, threefold coordinated As and fourfold coordinated Si atoms.

'Wrong bonds' in sputtered amorphous Ge(2)Sb(2)Te(5).

The structure of sputtered amorphous Ge(2)Sb(2)Te(5) was investigated by high energy x-ray diffraction, neutron diffraction and Ge-, Sb- and Te K-edge EXAFS measurements. The five datasets were modelled simultaneously in the framework of the reverse Monte Carlo simulation technique. It was found that apart from Te-Sb and Te-Ge bonds existing in the crystalline phases, Ge-Ge and Sb-Ge bonding is also significant in sputtered amorphous Ge(2)Sb(2)Te(5). According to our results, all components obey the '8-N' rule.

Structure of GeO(2)-P(2)O(5) glasses studied by x-ray and neutron diffraction.

The structures of three xGeO(2)-(1- x)P(2)O(5) glasses, where x = 0.98,0.88, and 0.81, have been studied by neutron and x-ray diffraction experiments that yield well resolved P-O and Ge-O bond distances. The Ge-O coordination number (N(GeO)) increased from 4.0 ± 0.2 to 4.5 ± 0.2 with the decrease in x from 0.98 to 0.81. The increase in N(GeO) is consistent with a structural model that assumes that all oxygen form Ge-O-Ge and P-O-Ge linkages between Ge polyhedra and P tetrahedra and that new GeO(5) or GeO(6) polyhedra are formed with isolated PO(4) units when P(2)O(5) is added to GeO(2). The bond valencies in the P-O bonds of the PO(4) tetrahedra are greater than unity and are balanced in P-O-Ge bridges with underbonded Ge-O links in the GeO(5) or GeO(6) polyhedra. Mixed site connections are expected for the GeO(5) (or GeO(6)) and PO(4) units in glasses with relatively low (<20 mol%) P(2)O(5) content due to the overwhelming fraction of GeO(4) tetrahedra. The structural changes are compared with those reported for alkali germanate glasses. Several features indicate different characteristics for the compositional dependence of N(GeO) for the GeO(2)-P(2)O(5) and alkali germanate glasses. However, the distributions of the first-neighbour Ge-O distances are found to be nearly identical for the GeO(2)-P(2)O(5) and K(2)O-GeO(2) glasses of equimolar K(2)O and P(2)O(5) content.