ABSTRACT
The atomistic mechanisms of Li(+) ion mobility/conductivity in Li(7-x)PS(6-x)I(x) argyrodites are explored from both experimental and theoretical viewpoints. Ionic conductivity in the title compound is associated with a solid-solid phase transition, which was characterised by low-temperature differential scanning calorimetry, (7)Li and (127)I NMR investigations, impedance measurements and molecular dynamics simulations. The NMR signals of both isotopes are dominated by anisotropic interactions at low temperatures. A significant narrowing of the NMR signal indicates a motional averaging of the anisotropic interactions above 177+/-2 K. The activation energy to ionic conductivity was assessed from both impedance spectroscopy and molecular dynamics simulations. The latter revealed that a series of interstitial sites become accessible to the Li(+) ions, whilst the remaining ions stay at their respective sites in the argyrodite lattice. The interstitial positions each correspond to the centres of tetrahedra of S/I atoms, and differ only in terms of their common corners, edges, or faces with adjacent PS(4) tetrahedra. From connectivity analyses and free-energy rankings, a specific tetrahedron is identified as the key restriction to ionic conductivity, and is clearly differentiated from local mobility, which follows a different mechanism with much lower activation energy. Interpolation of the lattice parameters as derived from X-ray diffraction experiments indicates a homogeneity range for Li(7-x)PS(6-x)I(x) with 0.97 < or = x < or = 1.00. Within this range, molecular dynamics simulations predict Li(+) conductivity at ambient conditions to vary considerably.
ABSTRACT
Li(7)PS(6) and Li(7)PSe(6) belong to a class of new solids that exhibit high Li(+) mobility. A series of quaternary solid solutions Li(7)PS(6-x)Se(x) (0 < or = x < or = 6) were characterised by X-ray crystallography and magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. The high-temperature (HT) modifications were studied by single-crystal investigations (both F43m, Z=4, Li(7)PS(6): a=9.993(1) A, Li(7)PSe(6): a=10.475(1) A) and show the typical argyrodite structures with strongly disordered Li atoms. HT-Li(7)PS(6) and HT-Li(7)PSe(6) transform reversibly into low-temperature (LT) modifications with ordered Li atoms. X-ray powder diagrams show the structures of LT-Li(7)PS(6) and LT-Li(7)PSe(6) to be closely related to orthorhombic LT-alpha-Cu(7)PSe(6). Single crystals of the LT modifications are not available due to multiple twinning and formation of antiphase domains. The gradual substitution of S by Se shows characteristic site preferences closely connected to the functionalities of the different types of chalcogen atoms (S, Se). High-resolution solid-state (31)P NMR is a powerful method to differentiate quantitatively between the distinct (PS(4-n)Se(n))(3-) local environments. Their population distribution differs significantly from a statistical scenario, revealing a pronounced preference for P-S over P-Se bonding. This preference, shown for the series of LT samples, can be quantified in terms of an equilibrium constant specifying the melt reaction Se(P)+S(2-) <==>S(P)+Se(2-), prior to crystallisation. The (77)Se MAS-NMR spectra reveal that the chalcogen distributions in the second and third coordination sphere of the P atoms are essentially statistical. The number of crystallographically independent Li atoms in both LT modifications was analysed by means of (6)Li{(7)Li} cross polarisation magic angle spinning (CPMAS).
ABSTRACT
Crystal chemical data of high- (HT) and low-temperature (LT) modifications of lithium argyrodites with the compositions Li(7)PCh(6) (Ch=S, Se), Li(6)PCh(5)X (X=Cl, Br, I), Li(6)AsS(5)Br, and Li(6)AsCh(5)I (Ch=S, Se) based on single-crystal, powder X-ray (113 K
ABSTRACT
PdBr2Se6 and PdCl2Se8 are two new compounds with cyclic Se6 coordinated to PdBr2 molecules and one-dimensional helical Sex chains coordinated to PdCl2 molecules. PdBr2Se6 is a black solid with a crystal structure similar, but not equal, to PdCl2Se6. It crystallizes in the space group P1 with the lattice constants a = 4.3946(8) A, b = 7.605(1) A, c = 7.992(2) A, alpha = 66.15(2) degrees , beta = 86.44(2) degrees , gamma = 80.90(2) degrees , and Z = 1 and can be handled in air like the deep red PdCl2Se8 which crystallizes in the orthorhombic space group Pbca with the lattice constants a = 9.609(2) A, b = 8.958(2) A, c = 13.799(3) A, and Z = 4. In PdBr2Se6, two cyclic Se6 molecules (chair conformation) are directly coordinated to Pd atoms, forming Pd(Se6)2Br2 groups. These are connected to one-dimensional chains via trans-standing Se atoms. In PdCl2Se8, the selenium substructure consists of helical chains with every fifth Se atom directly coordinated to the Pd atom of a PdCl2 group. Each PdCl2 group on the other hand connects two neighboring Sex helices. The type of Sex helix found for this compound is unique and differs from all other ones reported up to now including elemental alpha-Se. A reproducible twinning observed for PdBr2Se6 crystals in the course of the X-ray single-crystal investigations is checked by transmission electron microscopy in connection with details of the atomic arrangement. The Raman spectra of PdBr2Se6 and PdCl2Se8 are compared to Raman data of elemental Se modifications and give significant support for the Se6 and helical Sex to be neutral molecules. A discussion of the results of thermal analyses gives clear evidence that cyclic Se6 and helical Sex are considerably stabilized by bonding to the PdX2 molecules because the melting temperatures of the composite materials are significantly higher than the ones of the respective elemental modifications.
