Intersubsystem chemical bonds in the misfit layer compounds (LaS)(1.13)TaS(2) and (LaS)(1.14)NbS(2).

Abstract

The modulated structures of incommensurate composite crystals (La(0.912)S)(1.13)TaS(2) at room temperature and of (La(0.949)S)(1.14)NbS(2) at T = 115 K are refined against high-resolution X-ray data. The compounds are isostructural with superspace group F'm2m(alpha,0,0)00s. For (LaS)(1.13)TaS(2), lattice parameters of the first subsystem TaS(2) were obtained as a = 3.2922 (1), b = 5.7776 (2) and c = 23.013 (2) A. For the second subsystem LaS, the same b and c parameters were found, but a = 5.8090 (8) A. Refinements led to a final structure model with R = 0.036 for 4767 observed unique reflections (R = 0.023 for 2147 main reflections, R = 0.099 for 1554 first-order satellites and R = 0.112 for 1042 second-order satellites). The final model includes modulation parameters up to the second-order harmonics for the displacements of the atoms, for the occupational parameters and for the temperature parameters. A clear correlation is found between the relative positions of the subsystems, the displacement modulation, the occupational modulation and the modulation of the temperature parameters. The analysis shows that the variations in environments are resolved by correlated variations in the temperature factors. For (LaS)(1.14)NbS(2), lattice parameters at T = 115 K of the NbS(2) subsystem were obtained as a = 3.3065 (4), b = 5.7960 (5) and c = 22.956 (3) A. For the LaS subsystem, the same values for b and c were obtained, but a = 5.7983 (7) A. Refinements led to a final structure model with R = 0.048 for 5909 observed unique reflections (R = 0.034 for 2528 main reflections, R = 0.092 for 2171 first-order satellites and R = 0.113 for 1103 second-order satellites). The final structure model is similar to that of (LaS)(1.13)TaS(2). In particular, it is found that the values of the modulation parameters are almost equal and it is concluded that the modulations are independent of the temperature and the replacement of Ta with Nb, and thus represent a general mechanism of resolving the strain between the mutually incommensurate layers.