ISSN:
1089-7550
Source:
AIP Digital Archive
Topics:
Physics
Notes:
Phase transitions have been found in Pb5Al3F19 at Tc = 285 K, Pb5Ti3F19 at 695 K, Pb5V3F19 at 630 K, Pb5Cr3F19 at 545 K (reported previously), Pb5Fe3F19 at 740 K, and Pb5Ga3F19 at 945 K. In striking contrast to the variation of Tc with M atom in Sr5M3F19 and Ba5M3F19 (reported previously), for which materials the phase transition temperature increases with increasing atomic number from M=Ti to M=Cr but with a lower Tc for M=Fe before increasing again, Tc in Pb5M3F19 decreases from M=Ti to M=Cr with a sharp increase for M=Fe before it decreases again. The transition temperature for Pb5Al3F19 is below room temperature, that for isotypic Sr5Al3F19 is not detectable before reaching a dissociation temperature around 1000 K. All phase transitions in the Pb5M3F19 family are first order. Each new material has been fully characterized. Differences between materials with A=Pb, M=Al or Cr, and those with A=Sr, M=Al or Cr have been investigated through the formation of solid solutions. Six separate systems have been studied. Complete solid solutions form between end components with identical A and different M atoms. Partial solid solutions form with different A and different M atoms. Solid solutions do not form with different A and identical M atoms. M atom replacement, hence, appears to be energetically favorable, A atom replacement unfavorable, probably due to the role of the Pb atom lone pair of electrons. Two models outlining this role are presented.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.349589
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