ANOMALOUS TEMPERATURE-DEPENDENCE OF THE STRUCTURE OF TLAU(CN)(2) INVESTIGATED BY HIGH-RESOLUTION NEUTRON POWDER DIFFRACTION

By means of high-resolution neutron powder diffraction (sample-limited resolution delta d/d greater than or equal to 10(-3), d = lattice spa cing) the temperature dependence of the orthorhombic structure of the pseudosymmetric layer compound TlAu(CN)(2) was investigated in the ran ge from 5 to 300 K in connection with optical measurements. No phase t ransition occurs, but similar to KAu(CN)(2), the longest lattice param eter b increases anomalously with decreasing temperature, from 30.460 Angstrom at 300 K to 30.851 Angstrom at 5 K. When the [Au(CN)(2)](-) i ons are constrained to be linear, the b-lattice parameter increase is found to be due to the reduction of the angle between the [Au(3)(CN)(2 )](-) ion and the b-axis, from 50.7 degrees at 300 K to 47.6 degrees a t 5 K. Apart from these essential changes the results confirm the stru cture model of Blom et al., corresponding to space group Pbcn as deriv ed previously from a single-crystal neutron diffraction measurement at room temperature. Short interatomic distances Au-Au approaching 3 Ang strom at 5 K indicate significant Au-Au overlap, causing (with Au-TI c ovalent overlap) the particular yellow color of TlAu(CN)(2).