Structural behaviour of alkaline sulfides under compression: High-pressure experimental study on Cs2S

From Simple to Complex: Design of Inorganic Crystal Structures with a Topologically Extended Zintl-Klemm Concept

We present a novel approach to the generation of new crystalline phases, which is based on a combination of the topological description of crystal structures as a periodic net and the extended Zintl-Klemm concept, which establishes the structural relations between chemically and structurally simpler and more complex inorganic compounds. With this approach, we have explored the structural similarities between all known binary sulfides, selenides, and the corresponding simple sulfates and selenates and have theoretically revealed seven new high-pressure phases in the last two groups of compounds. Using density functional theory methods, we have studied the thermodynamic and mechanical stability of the new phases, have adjusted the transition pathways in the sulfate and selenate systems, and have revealed new structural correlations of the baric polymorphism in these systems. The advantages of the topological approach compared to conventional methods of modeling crystal structures are discussed and illustrated.

A First-Principles Exploration of NaxSy Binary Phases at 1 atm and Under Pressure

Interest in Na-S compounds stems from their use in battery materials at 1 atm, as well as the potential for superconductivity under pressure. Evolutionary structure searches coupled with Density Functional Theory calculations were employed to predict stable and low-lying metastable phases of sodium poor and sodium rich sulfides at 1 atm and within 100–200 GPa. At ambient pressures, four new stable or metastable phases with unbranched sulfur motifs were predicted: Na2S3 with C 2 / c and Imm2 symmetry, C 2 -Na2S5 and C 2 -Na2S8. Van der Waals interactions were shown to affect the energy ordering of various polymorphs. At high pressure, several novel phases that contained a wide variety of zero-, one-, and two-dimensional sulfur motifs were predicted, and their electronic structures and bonding were analyzed. At 200 GPa, P 4 / m m m -Na2S8 was predicted to become superconducting below 15.5 K, which is close to results previously obtained for the β -Po phase of elemental sulfur. The structures of the most stable M3S and M4S, M = Na, phases differed from those previously reported for compounds with M = H, Li, K.

Gold(i) sulfide: unusual bonding and an unexpected computational challenge in a simple solid

We report the experimental high-pressure crystal structure and equation of state of gold(i) sulfide (Au2S) determined using diamond-anvil cell synchrotron X-ray diffraction. Our data shows that Au2S has a simple cubic structure with six atoms in the unit cell (four Au in linear, and two S in tetrahedral, coordination), no internal degrees of freedom, and relatively low bulk modulus. Despite its structural simplicity, Au2S displays very unusual chemical bonding. The very similar and relatively high electronegativities of Au and S rule out any significant metallic or ionic character. Using a simple valence bond (Lewis) model, we argue that the Au2S crystal possesses two different types of covalent bonds: dative and shared. These bonds are distributed in such a way that each Au atom engages in one bond of each kind. The multiple arrangements in space of dative and shared bonds are degenerate, and the multiplicity of configurations imparts the system with multireference character, which is highly unusual for an extended solid. The other striking feature of this system is that common computational (DFT) methods fail quite spectacularly to describe it, with 20% and 400% errors in the equilibrium volume and bulk modulus, respectively. We explain this by the poor treatment of static correlation in common density-functional approximations. The fact that the solid is structurally very simple, yet presents unique chemical bonding and is unmodelable using current DFT methods, makes it an interesting case study and a computational challenge.

Crystal structure of the high-temperature form of the trisulfide Cs2S3 and the (3+1)D modulated structure of the telluride K37Te28

The high-temperature polymorph of the trisulfide Cs2S3, which has been synthesized from Cs2S2 and elemental sulfur, crystallizes in a new structure type (monoclinic, space group P21/c, a=999.97(4), b=1029.30(5), c=2642.07(12) pm, β=90.083(2)°, Z=16, R1=0.0324). The structure contains four crystallographically independent angled S 3 2 − ${\rm{S}}_3^{2 - }$ trisulfide ions with S–S distances of 205.7–208.3 pm. The distorted b.c.c. packing of the anions and their insertion in the five-membered rings of 3.53+3.5.3.5. (1:1) Cs nets are similarly found in the r.t. form (Cmc21, K2S3-type structure) and the two polymorphs differ mainly in the orientation of the S3 groups. The second title compound, K37Te28, was synthesized from stoichiometric melts of the elements. It forms a complex (3+1)D modulated tetragonal structure (space group I41/amd (00σ 3)s0s0, q=(0, 0, 0.5143), a=1923.22(2), c=2626.66(4) pm, Z=4, R1all=0.0837). According to K37Te28=K37[Te(1X)]8[Te(2X)2]6[Te(3X)8] the structure contains three different types of Te anions: The two crystallographically different isolated telluride anions [Te(1X)]2− are coordinated by 9/10 K+ cations. Three [Te(2X)2]2− dumbbells (d Te-Te=277.9/286.4 pm) are arranged to ‘hexamers’. The Te(31) and Te(32) atoms are located in columns of face-sharing K square antiprisms. Their z position modulation, which is accompanied by a smaller shift of the surrounding K+ cations, results in the decomposition of the [Te(3X)8]2 chain in a sequence |:Te3–Te2–Te2–Te3–Te2–Te2–Te2:| of dumbbells Te2 2− (d Te–Te=304 pm) and hypervalent linear trimers Te3 4− (d Te–Te=325 pm).

On the role of the alkali cations on methanol thiolation

The electronegativity effect of the alkali cations on the formation of methanethiol by reaction of methanol and H₂S was studied with K⁺, Rb⁺, and Cs⁺ supported on γ-Al₂O₃.

Crystal behavior of potassium bromate under compression

We report on high-pressure angle-dispersive X-ray diffraction data up to 15 GPa and ab initio total-energy calculations up to 242 GPa for KBrO3. No phase transition was found below 15 Pa in contrast to previously reported data. Its experimental bulk modulus in the quasi-hydrostatic regime is B0 = 18.8 (9) GPa with a bulk modulus pressure derivative B'0 = 8.2 (4). However, according to our ab initio calculations, KBrO3 significantly reduces its rhombohedral distortion via small cooperative movements of the atoms and the structure progressively approaches the cubic symmetry, where the KBr subarray would adopt a topology similar to that of the corresponding B2-type bromide. This rearrangement of atoms is directly related to the Buerger's mechanism of the B1-B2 phase transition for halides, confirming that cations (second neighbors) do not arrange in an arbitrary way. The O atoms forming the [BrO3] pyramidal units move smoothly with pressure to the center of the [K8] cube faces, where electron localization function calculations present their maxima in other B2-type compounds, eventually adopting the perovskite-type structure at P ≃ 152 GPa. Our data on KBrO3 has been compared with chemically substituted isostructural halates, providing new insights on the compressibility of this family of compounds.

Structural phase transitions on AgCuS stromeyerite mineral under compression

The structural behavior of mineral Stromeyerite, AgCuS, has been studied by means of angle-dispersive X-ray diffraction measurements up to 13 GPa and ab initio total-energy calculations. Two high-pressure phase transitions are found at 1.4 and 5.7 GPa, from the initial distorted Ni(2)In-type phase (AuRbS-type, RP, space group Cmc2(1)) through an anti-PbClF-type phase (HP1, space group P4/nmm) to a monoclinic distortion of this latter phase (HP2, space group P2(1)/m). The collapse of the metal-metal interatomic distances at the RP-HP1 transition suggests a stronger metallic behavior of the high-pressure phase. The compressibility of the lattice parameters and the equation of state of the first pressure-induced phase have been experimentally determined. First-principles calculations present an overall agreement with the experimental results in terms of the high-pressure sequence and provide chemical insight into the AgCuS behavior under hydrostatic pressure.

Unique thermodynamic relationships for Δf H o and Δf G o for crystalline inorganic salts. I. Predicting the possible existence and synthesis of Na2SO2 and Na2SeO2

The concept that equates oxidation and pressure has been successfully utilized in explaining the structural changes observed in the M 2S subnets of M 2SO x (x = 3, 4) compounds (M = Na, K) when compared with the structures (room- and high-pressure phases) of their parent M 2S `alloy' [Martínez-Cruz et al. (1994), J. Solid State Chem. 110, 397–398; Vegas (2000), Crystallogr. Rev. 7, 189–286; Vegas et al. (2002), Solid State Sci. 4, 1077–1081]. These structural changes suggest that if M 2SO2 would exist, its cation array might well have an anti-CaF2 structure. On the other hand, in an analysis of the existing thermodynamic data for M 2S, M 2SO3 and M 2SO4 we have identified, and report, a series of unique linear relationships between the known Δf H o and Δf G o values of the alkali metal (M) sulfide (x = 0) and their oxyanion salts M 2SO x (x = 3 and 4), and the similarly between M 2S2 disulfide (x = 0) and disulfur oxyanion salts M 2S2O x (x = 3, 4, 5, 6 and 7) and the number of O atoms in their anions x. These linear relationships appear to be unique to sulfur compounds and their inherent simplicity permits us to interpolate thermochemical data (Δf H o) for as yet unprepared compounds, M 2SO (x = 1) and M 2SO2 (x = 2). The excellent linearity indicates the reliability of the interpolated data. Making use of the volume-based thermodynamics, VBT [Jenkins et al. (1999), Inorg. Chem. 38, 3609–3620], the values of the absolute entropies were estimated and from them, the standard Δf S o values, and then the Δf G o values of the salts. A tentative proposal is made for the synthesis of Na2SO2 which involves bubbling SO2 through a solution of sodium in liquid ammonia. For this attractive thermodynamic route, we estimate ΔG o to be approximately −500 kJ mol−1. However, examination of the stability of Na2SO2 raises doubts and Na2SeO2 emerges as a more attractive target material. Its synthesis is likely to be easier and it is stable to disproportionation into Na2S and Na2SeO4. Like Na2SO2, this compound is predicted to have an anti-CaF2 Na2Se subnet.