Chemical vapour transport experiments1,2 are convenient for synthesis, crystallisation and purification of transition metal phosphides3,4,5. New experimental techniques (e. g. a special type of thermo balance6) have led to improved performance of transport experiments and allow a detailed thermochemical evaluation. Thus, in one experiment migration rates for more than one condensed phase, deposited masses and the deposition sequence of condensed phases at the sink, can be determined even without opening an ampoule. These experimental observations can be reproduced whith our computer program CVTRANS. From the calculations detailed thermochemical insight about into transport process dominating equilibria is gained. Predictions on the transport behaviour of a particular solid can be made.
The Program. CVTRANS is WINDOWS based. It calculates the composition of the condensed and gas phase under equilibrium conditions by minimizing the free enthalpy of a system according to a procedure described and programmed (SOLGASMIX) by Eriksson7. Limitations and problems of that program (e. g. calculation of saturation pressures; handling of systems with very low partial pressures; selecting condensed phases at equilibrium according to the phase rule) have been overcome by several additional program routines. Further improvement and stability of the iteration process has been achieved by allowing higher internal numerical precision.
Compositions of the condensed and gas phase of source and sink region of a transport experiment are related to each other in CVTRANS by application of the Cooperative Transport Model introduced by Schweizer and Gruehn8. The mass transfer between source and sink (deposition rates) is calculated using the expression given by Schäfer1 in combination with his concept of the solubility of a solid in the gas phase9. This approach includes mass flow by diffusion and Stefan-flow as well.
Transport of Phosphides. Thermochemical calculations using CVTRANS have led to a detailed understanding of heterogenous equilibria responsible for the transport of transition metal monophosphides. For many transport systems MP/I2 formation of condensed transition metal iodides besides the phosphide has been observed either by non-steady state transport (e. g. CoP/I210) or as a result of a sequential series of steady-states (e. g. CrP/I24). The stability of the phosphide can be related to the observed amount of iodide in the condensed phase. Our calculations show, that increasing phosphorus pressures in transport experiments with phosphides lead to PI3 as actual transport agent instead of the originally added iodine (transport of phosphorus-rich phosphides, e. g. CuP2 and Cu2P711). Due to low P2(4) coexistence pressures and the low stability of gaseous phosphorus iodides HgBr2 should be used as transport agent for metal-rich phosphides like Ni3P12 and Fe2P13.
A summary of the transport behaviour of transition metal phosphides will be given on the poster together with a computer presentation of CVTRANS, showing various examples related to the transport of transition metal phosphides.
[1] H. Schäfer, Chemical Vapour Transport Reactions, Verlag Chemie, 1962. 2 R. Gruehn, R. Glaum, Angew. Chem., 2000, 112, 706. 3 R. Glaum, R. Gruehn, Z. anorg. allg. Chem. 1989, 568, 73. 4 R. Glaum, R. Gruehn, Z. anorg. allg. Chem. 1989, 573, 24. 5 J. Martin, R. Gruehn, Solid State Ionics 1990, 43, 19. 6 V. Plies, Th. Kohlmann, R. Gruehn, Z. anorg. allg. Chem. 1989, 568, 62. 7 G. Eriksson, Acta Chem. Scand. 1971, 25, 2561. 8 R. Gruehn, H.-J. Schweizer, Angew. Chem. 1983, 95 80. 9 H. Schäfer, Z. anorg. allg. Chem. 1973, 400 242. 10 A. Schmidt, R. Glaum, Z. anorg. allg. Chem. 1995, 621, 1693. 11 D. Özalp, Ph. D. Thesis, Univ. of Gießen, 1993. 12 M. Blum, planned Ph. D. Thesis, Univ. of Bonn, 2004. 13 K. Czekay, Staatexamensarbeit, Univ. of Gießen, 1999.