Chemical reactions and phase stabilities in the Si-Te system at high pressures were explored using in situ angle-dispersive synchrotron powder diffraction in a large-volume multianvil press together with density functional theory-based calculations. Cubic and rhombohedrally distorted clathrates, with the general formula Te8@(Si38Te8) and wide compositional range, preceded by a hexagonal phase with the composition Si0.14Te, are formed for different mixtures of Si and Te as starting materials. Si0.14Te, with the structural formula Te2(Te0.74Si0.26)3(Te0.94Si0.06)3, is the very first chalcogenide with the Mn5Si3-type structure. Silicon sesquitelluride α-Si2Te3 decomposes into a mixture of phases that includes the clathrate and hexagonal phases at high pressures and high temperatures. The higher the pressure, the lower the temperature for the two phases to occur. Regardless of the starting compositions, only the clathrate is quenched to atmospheric conditions, while the hexagonal phase amorphizes on decompression. The rhombohedral clathrates Te8@(Si38Te8) form on quenching of the cubic phases to ambient conditions. There is a high degree of interchangeability of Si and Te not only in the clathrates but also in the Mn5Si3-type structure. The theoretical calculations of enthalpies indicate that the reported decomposition of α-Si2Te3 is energetically favorable over its transformation to another polymorph of the A2X3 type at extreme conditions.
Chemical bonding in main-group IV chalcogenides is an intensely discussed topic, easily understandable because of their remarkable physical properties that predestine these solid-state materials for their widespread use in, for instance, thermoelectrics and phase-change memory applications. The atomistic origin of their unusual property portfolio remains somewhat unclear, however, even though different and sometimes conflicting chemical-bonding concepts have been introduced in the recent years. Here, it is proposed that projecting phononic force-constant tensors for pairs of atoms along differing directions and ranges provide a suitable and quantitative descriptor of the bonding nature for these materials. In combination with orbital-based quantitative measures of covalency such as crystal orbital Hamilton populations (COHP), it is concluded that the well-established many-center and even n-center bonding is an appropriate picture of the underlying quantum-chemical bonding mechanism, supporting the recent proposal of hyperbonded phase-change materials.
α-Sb2O3 (senarmontite), ß-Sb2O3 (valentinite), and α-TeO2 (paratellurite) are compounds with pronounced stereochemically active Sb and Te lone pairs. The vibrational and lattice properties of each have been previously studied but often lead to incomplete or unreliable results due to modes being inactive in infrared or Raman spectroscopy. Here, we present a study of the relationship between bonding and lattice dynamics of these compounds. Mössbauer spectroscopy is used to study the structure of Sb in α-Sb2O3 and ß-Sb2O3, whereas the vibrational modes of Sb and Te for each oxide are investigated using nuclear inelastic scattering, and further information on O vibrational modes is obtained using inelastic neutron scattering. Additionally, vibrational frequencies obtained by density functional theory (DFT) calculations are compared with experimental results in order to assess the validity of the utilized functional. Good agreement was found between DFT-calculated and experimental density of phonon states with a 7% scaling factor. The Sb-O-Sb wagging mode of α-Sb2O3 whose frequency was not clear in most previous studies is experimentally observed for the first time at â¼340 cm-1. Softer lattice vibrational modes occur in orthorhombic ß-Sb2O3 compared to cubic α-Sb2O3, indicating that the antimony bonds are weakened upon transforming from the molecular α phase to the layer-chained ß structure. The resulting vibrational entropy increase of 0.45 ± 0.1 kB/Sb2O3 at 880 K accounts for about half of the α-ß transition entropy. The comparison of experimental and theoretical approaches presented here provides a detailed picture of the lattice dynamics in these oxides beyond the zone center and shows that the accuracy of DFT is sufficient for future calculations of similar material structures.
We report the crystal structure of Ba(CN3H4)2 as synthesized from liquid ammonia. Structure solution based on X-ray diffraction data suffers from a severe pseudo-tetragonal problem due to extreme scattering contrast, so the true monoclinic symmetry is detectable only from neutron powder diffraction patterns, and structure solution and refinement was greatly aided by density-functional theory. The symmetry lowering is due to slight deviations of the guanidinate anion from the mirror plane in space group P 4 â¾ b2, a necessity of hydrogen bonding. At 300â K, barium guanidinate crystallizes in P21/c with a=6.26439(2)â Å, b=16.58527(5)â Å, c=6.25960(2)â Å, and a monoclinic angle of ß=90.000(1)°. To improve the data-to-parameter ratio, anisotropic displacement parameters from first-principles theory were incorporated in the neutron refinement. Given the correct structural model, the positional parameters of the heavy atoms were also refinable from X-ray diffraction of a twinned crystal. The two independent guanidinate anions adopt the all-trans- and the anti-shape. The Ba cation is coordinated by eight imino nitrogens in a square antiprism with Ba-N contacts between 2.81 and 3.04â Å. The IR and Raman spectra of barium guanidinate were compared with DFT-calculated phonon spectra to identify the vibrational modes.
Due to the weak oxidative force of N2 , nitrides are only typically formed with the less electronegative metals. Meeting this challenge, we here present Pb2 Si5 N8 , the first nitridosilicate containing highly electron-affine cations of a metal from the right side of the Zintl border. By using advanced synchrotron X-ray diffraction, the crystal structure was determined from a tiny single crystal of 1×3×3â µm3 in size, revealing a significantly different bonding situation compared to all other nitridosilicates known so far. Indeed, DFT calculations confirm distinct amounts of covalency not only between Pb and N but also between formal Pb2+ cations. Thus, unprecedented cationic Pb2 dumbbells with a stretching vibration at 117â cm-1 were found in Pb2 Si5 N8 , the first representative of a crystallographically elucidated lead nitride, stabilized by high amounts of covalency.
We explore the thermodynamic properties of the layered copper(II) carbodiimide CuNCN by heat-capacity measurements and investigate the corresponding thermal atomic motions by means of neutron powder diffraction as well as inelastic neutron scattering. The experiments are complemented by a combination of density-functional calculations, phonon analysis and analytic theory. The existence of a soft flexural mode-bending of the layers, characteristic for the material structure-is established in the phonon spectrum of CuNCN by giving characteristic temperature-dependent contributions to the heat capacity and atomic displacement parameters. The agreement with the neutron data allows us to extract a residual-on top of the lattice-presumably spinon contribution to the heat capacity [Formula: see text], speaking in favor of the spin-liquid picture of the electronic phases of CuNCN.
We present density-functional theory calculations of the lattice dynamics of bismuth telluride, yielding force constants, mean-square displacements and partial densities of phonon states which corroborate and complement previous nuclear inelastic scattering experiments. From these data, we derive an element- and energy-resolved view of the vibrational anharmonicity, quantified by the macroscopic Grüneisen parameter γ which results in 1.56. Finally, we calculate thermochemical properties in the quasiharmonic approximation, especially the heat capacity at constant pressure and the enthalpy of formation for bismuth telluride; the latter arrives at ΔHf (Bi2Te3) = -102 kJ mol(-1) at 298 K.
We present a comprehensive survey of electronic and lattice-dynamical properties of crystalline antimony telluride (Sb2Te3). In a first step, the electronic structure and chemical bonding have been investigated, followed by calculations of the atomic force constants, phonon dispersion relationships and densities of states. Then, (macroscopic) physical properties of Sb2Te3 have been computed, namely, the atomic thermal displacement parameters, the Grüneisen parameter γ, the volume expansion of the lattice, and finally the bulk modulus B. We compare theoretical results from three popular and economic density-functional theory (DFT) approaches: the local density approximation (LDA), the generalized gradient approximation (GGA), and a posteriori dispersion corrections to the latter. Despite its simplicity, the LDA shows excellent performance for all properties investigated-including the Grüneisen parameter, which only the LDA is able to recover with confidence. In the absence of computationally more demanding hybrid DFT methods, the LDA seems to be a good choice for further lattice dynamical studies of Sb2Te3 and related layered telluride materials.
Antimony selenide (antimonselite, Sb2Se3) is a versatile functional material with emerging applications in solar cells. It also provides an intriguing prototype to study different modes of bonding in solid chalcogenides, all within one crystal structure. In this study, we unravel the complex bonding nature of crystalline Sb2Se3 by using an orbital-based descriptor (the crystal orbital Hamilton population, COHP) and by analysing phonon properties and interatomic force constants. We find particularly interesting behaviour for the medium-range Sb···Se contacts, which still contribute significant stabilisation but are much softer than the "traditional" covalent bonds. These results have implications for the assembly of Sb2Se3 nanostructures, and bond-projected force constants appear as a useful microscopic descriptor for investigating a larger number of chalcogenide functional materials in the future.
A hitherto unknown sodium magnesium plumbide, Na2MgPb, was synthesized by heating the constituent elements. Na2MgPb crystallizes in a hexagonal unit cell with the Li2CuAs-type structure (P63/mmc, Z = 2, a = 5.110(2) Å, c = 10.171(4) Å at 293 K). The compound furthermore displays polymorphism: high-temperature powder XRD measurements revealed that hexagonal Na2MgPb (dubbed the "α" phase) transforms to another hexagonal phase (ß) which is existent at 493-553 K, and the ß phase changes to a cubic structure (γ) at 533-633 K further. The molar volume of γ-Na2MgPb is approximately 9% and 13% smaller than the molar volumes of the α phase and the ß phase, respectively (at 543 K). The electrical resistivity of Na2MgPb is 0.39 mΩ at 300 K; it rises with increasing temperature from 300 to 491 K, and then drops at 491 and 523 K. These abrupt changes in resistivity may be attributed to the α â ß and ß â γ phase transitions, respectively. To gain further insight into the structure of cubic γ-Na2MgPb, putative models with regular Heusler-type (Cu2MnAl-type) and inverse Heusler-type (Li2AgSb-type) arrangements were probed using first-principles computations based on density functional theory (DFT). These computations indicate that, for the cubic γ phase, an inverse Heusler-type structure is distinctly more stable than the alternative regular Heusler type (at 0 K); beyond that, ab initio thermochemistry was successfully used to verify the stability ordering (α-Na2MgPb being favorable at low temperature, γ-Na2MgPb at high temperature), albeit the theoretically predicted transition temperature of 900 K which is higher than observed in experiment.
Dinitrogen (N2) ligation is a common and well-characterized structural motif in bioinorganic synthesis. In solid-state chemistry, on the other hand, homonuclear dinitrogen entities as structural building units proved existence only very recently. High-pressure/high-temperature (HP/HT) syntheses have afforded a number of binary diazenides and pernitrides with [N2](2) and [N2](4) ions, respectively. Here, we report on the HP/HT synthesis of the first ternary diazenide. Li2Ca3[N2]3 (space group Pmma, no. 51, a = 4.7747(1), b = 13.9792(4), c = 8.0718(4) Å, Z = 4, wRp = 0.08109) was synthesized by controlled thermal decomposition of a stoichiometric mixture of lithium azide and calcium azide in a multianvil device under a pressure of 9 GPa at 1023 K. Powder X-ray diffraction analysis reveals strongly elongated NN bond lengths of dNN = 1.34(2)1.35(3) Å exceeding those of previously known, binary diazenides. In fact, the refined NN distances in Li2Ca3[N2]3 would rather suggest the presence of [N2](3·) radical ions. Also, characteristic features of the NN stretching vibration occur at lower wavenumbers (12601020 cm(1)) than in the binary phases, and these assignments are supported by first-principles phonon calculations. Ultimately, the true character of the N2 entity in Li2Ca3[N2]3 is probed by a variety of complementary techniques, including electron diffraction, electron spin resonance spectroscopy (ESR), magnetic and electric conductivity measurements, as well as density-functional theory calculations (DFT). Unequivocally, the title compound is shown to be metallic containing diazenide [N2](2) units according to the formula (Li(+))2(Ca(2+))3([N2](2))3·(e())2.
Germanium dioxide (GeO2 ) takes two forms at ambient pressure: a thermodynamically stable rutile-type structure and a high-temperature quartz-type polymorph. Here, we investigate the phase stability at finite temperatures by ab initio phonon and thermochemical computations. We use gradient-corrected density-functional theory (PBE-GGA) and pay particular attention to the modeling of the "semicore" germanium 3d orbitals (ascribing them either to the core or to the valence region). The phase transition is predicted correctly in both cases, and computed heat capacities and entropies are in excellent agreement with thermochemical database values. Nonetheless, the computed formation energies of α-quartz-type GeO2 (and, consequently, the predicted transition temperatures) differ significantly depending on theoretical method. Remarkably, the simpler and cheaper computational approach produces seemingly better results, not worse. In our opinion, GeO2 is a nice test case that illustrates both possibilities and limitations of modern ab initio thermochemistry.
