Polymorphism of IrSn4

摘要

A high-temperature polymorph of IrSn4 (beta-IrSn4) was prepared from the elements by annealing tin-rich melts at temperatures between 800degreesC and 900degreesC and subsequent rapid quenching. The crystal structure of beta-IrSn4 was determined from single-crystal X-ray data (tetragonal, space group I4(1)/acd, Z = 8, a = 6.3096(6) Angstrom, c = 22.770(7) Angstrom) and corresponds to that of MoSn4. In beta-IrSn4, Ir atoms are arranged in layers centering Sn-8 square antiprisms. This is in contrast to the trigonal low-temperature modification (alpha-IrSn4) where homogeneously distributed Ir atoms are coordinated in a bisdisphenoidal fashion by eight Sn atoms. The structure Of beta-IrSn4, however, is very similar to that of the high-pressure modification of IrSn4 (HP-IrSn4), which adopts the PtSn4 structure. Structural stability of the IrSn4 polymorphs was investigated by first-principles calculations. It was found that alpha-IrSn4 represents the ground state of the system. At ambient pressure beta-IrSn4 is 0.027eV per formula unit (2.6lkJ/mol) higher in energy than alpha-IrSn4. At a pressure of about 5 GPa, our calculations reveal a phase transition alpha-IrSn4 --> beta-IrSn4; the latter remains the most stable polymorph up to pressures of at least 10GPa. Thus, reported HP-IrSn4 would correspond to a high-pressure-high-temperature form of IrSn4. However, it is suggested that the structures of HP-IrSn4 and beta-IrSn4 are identical. (C) 2002 Elsevier Science (USA). [References: 23]