S. Hull et al., CRYSTAL-STRUCTURE AND SUPERIONIC CONDUCTIVITY OF PBF2 DOPED WITH KF, Journal of physics. Condensed matter (Print), 10(38), 1998, pp. 8429-8446
The crystal structure and ionic conductivity of polycrystalline sample
s of (PbF2)(1-x)-(KF)(x) with 0 less than or equal to x less than or e
qual to 0.333 are investigated using neutron diffraction and impedance
spectroscopy techniques. The maximum solid solubility of KF in the fl
uorite structured beta-PbF2 is found to be x = 0.013(6). However, the
introduction of KC and the associated charge compensating anion vacanc
ies has a dramatic effect on the ambient temperature ionic conductivit
y, which increases by a factor 5.6(2) x 10(3) for an x = 0.01 sample a
t 350 K. At higher dopant levels the ambient temperature conductivity
falls steadily in the range 0.05 less than or equal to x less than or
equal to 0.333 due to the presence of an increasing volume fraction of
relatively poorly conducting additional phases. apparently comprising
orthorhombic alpha-PbF2 and a further phase of probable composition P
bKF3. However, the ionic conductivity of these samples show an abrupt
increase at T = 520(5) K, as the two minority phases react to form a s
ingle, superionic phase, Within this high-temperature modification the
anions are dynamically disordered over the tetrahedral and, to a less
er extent, the octahedral interstices created by a body centred cubic
(bcc) cation sublattice formed by Pb2+ and K+. This phase is, therefor
e, an example of an anion conducting bcc superionic and its structure
is 'anti-' to that adopted by the archetypal superionic phase alpha-Ag
I. A plausible model describing the structural relationship between th
ese two antitypes, including the short-range order between mobile ions
within the bcc-(Pb1-xKx)F2-x phase, is given.