ABSTRACT
Semiconductors are key materials in modern electronics and are widely used to build, for instance, transistors in integrated circuits as well as thermoelectric materials for energy conversion, and there is a tremendous interest in the development and improvement of novel materials and technologies to increase the performance of electronic devices and thermoelectrics. Tetramorphic Ag(10)Te(4)Br(3) is a semiconductor capable of switching its electrical properties by a simple change of temperature. The combination of high silver mobility, a small non-stoichiometry range and an internal redox process in the tellurium substructure causes a thermopower drop of 1,400 microV K(-1), in addition to a thermal diffusivity in the range of organic polymers. The capability to reversibly switch semiconducting properties from ionic to electronic conduction in one single compound simply by virtue of temperature enables novel electronic devices such as semiconductor switches.
ABSTRACT
Single crystals of disilver(I) monofluorophosphate(V), Ag2PO3F (1), were obtained by slow evaporation of a diluted aqueous Ag2PO3F solution. Compound 1 adopts a new structure type and crystallizes in the monoclinic space group C2/c with eight formula units and lattice parameters of a = 9.2456(8) A, b = 5.5854(5) A, c = 14.7840(13) A, and beta = 90.178(2) degrees. The crystal structure of 1 [R(F2 > 2sigma(F2) = 0.0268, wR(F2 all) = 0.0665] is composed of three crystallographically independent Ag+ cations and PO3F2- anions as single building units. The oxygen environment around each of the Ag+ cations is different, with one Ag+ in distorted octahedral (d(Ag-O) = 2.553 A), one in nearly rectangular (d(Ag-O) = 2.445 A), and one in distorted tetrahedral (d(Ag-O) = 2.399 A) coordination. Additional Ag-F contacts to more remote F atoms located at distances >2.80 A augment the coordination polyhedra for the two latter Ag+ cations. The monofluorophosphate anion deviates slightly from C3v symmetry and exhibits the characteristic differences in bond lengths, with a mean of 1.510 A for the P-O bonds and one considerably longer P-F bond of 1.575(2) A. Compound 1 was further characterized by vibrational spectroscopy (Raman and IR) and solid-state 19F, 31P, and 109Ag MAS NMR spectroscopy. The value for the isotropic one-bond P-F coupling constant in 1 is 1JPF = -1045 Hz. Thermal analysis (TG, DSC) revealed a reversible phase transition at 308 degrees C, which is very close to the decomposition range of 1. Under release of POF3, Ag4P2O7 and Ag3PO4 are the thermal decomposition products at temperatures above 450 degrees C.
