gamma_l-Pt_5Zn_(21)--a reappraisal of a gamma-brass type complex alloy phase

摘要

The constitution of the gamma-phase field in the Pt-Zn system was reinvestigated. The samples were prepared from the elements in sealed silica tubes at 1320 K followed by slow cooling to 970 K. Three structurally distinct gamma-phases were identified by means of single crystal X-ray diffraction. The gamma_1- and gamma'_1-phase represent two structurally intimately related 2a X 2a X 2a superstructures of the ordinary gamma -Pt_2Zn_(10.75). Three crystal structures were refined from single crystal intensity data: gamma_1-Pt_(5.12)Zn_(20.47)(F4-(bar)3m, a = 1809.1(1) pm, cF416-6.6, R_F = 0.043), gamma_1-Pt_(4.72)Zn_(20.92) (F4-(bar)3m, a = 1812.8(2) pm, cF416-5.8, R_F = 0.034), and gamma'_-Pt_(4.25)Zn_(20.34) (F4-(bar)3m, a = 1814.0(1) pm, cF416-22.7, R_F = 0.054). The structures are discussed in terms of four symmetrically inequivalent 26 atom clusters each comprising inner tetrahedral (IT), outer tetrahedral (OT), octahedral (OH) and cuboctahedral (CO) positions. The homogeneity range of the gamma_1-phase extends between 18.4 and 20.0 at. percent Pt, a range which encompasses the composition Pt_5Zn_(21). Two symmetrically independent, ordered Pt_4Zn_(22) clusters form the compositionally invariant part of the structure. Two complementary, partly disordered Pt_(6+-delta)Zn_(20-delta), clusters account for the homogeneity range of the phase. They vary in composition due to mutual Pt/Zn substitution, vacancies at Zn sites, and partial positional disorder. The closely related phase gamma'_1-Pt_(4.25)Zn_(20.34) is separated from the gamma_-phase by a small miscibility gap which is indicated by isopiestic measurements and a discontinuous jump of the vacancy concentration. The melting point of the phases increases with rise of the Pt content: 1136(2) K (gamma), 1238(5) K(gamma'_1) and 1251(3) K (gamma_1). The magnetic susceptibility is dominated by the core diamagnetism of the constituents. chi_(mol) was found to vary between -4.7 X 10~(-10) and -3.1 X 10~(-10) m~3 mol~(-1).