X-Ray Diffraction Study of the Electron Density and Anharmonicity in K₂PtCl₆

Abstract

X-ray diffraction data in heavy-atom compounds may be sensitive to anharmonic atomic displacements, since the large core electron densities result in appreciable scattering amplitudes at large reciprocal distances. Since bonding electron densities may also exhibit sharp features affecting high-order reflections, they may be difficult to distinguish from anharmonic effects. We have analyzed an accurate room-temperature single-crystal X-ray data set of K 2 PtCl 6 using least-squares anharmonic displacement and charge density formalisms. The Hirshfeld charge density formalism, which has successfully been applied to many light-atom structures, fails to parametrize satisfactorily the data, whereas the electron densities at K and CI are easily accounted for by an anharmonic Gram-Charlier expansion to 4th order. Densities around Pt are parametrized only by a combination of anharmonicity and charge density formalisms. If economical parametrizations of the experimental data are preferred to more complicated ones, anharmonicity may be conjectured to play an important role while the main bonding feature consists of a preferential occupation of the 5d-orbitals of Pt with t 2g symmetry.

Electron density – structure relationships in some perovskite-type compounds

“…In both cases electron depletion along the K-Pd vector is stronger than that along a vector at right angles. The features around the K atom for the isomorphic K2PtCl6 compound in corresponding Ap maps obtained by Restori & Schwarzenbach (1993) closely resemble those around K in K2SiF6, shown in Fig 4. The heights of the difference-density peaks for the K2PdCI6 and K2SiF6 structures differ, perhaps because the approach to extinction correction differs in the two studies. When a least-squares extinction correction similar to that used by Restori & Schwarzenbach (1993) is applied to the K2PdCI6 data of Hester et al (1993a), positive features similar to those observed for the other compounds appear.…”

“…The features around the K atom for the isomorphic K2PtCl6 compound in corresponding Ap maps obtained by Restori & Schwarzenbach (1993) closely resemble those around K in K2SiF6, shown in Fig 4. The heights of the difference-density peaks for the K2PdCI6 and K2SiF6 structures differ, perhaps because the approach to extinction correction differs in the two studies. When a least-squares extinction correction similar to that used by Restori & Schwarzenbach (1993) is applied to the K2PdCI6 data of Hester et al (1993a), positive features similar to those observed for the other compounds appear. A change in the scale factor affects the strong low-angle reflections significantly, but in so far as the local topography is derived from the bulk of the reflections which, being weaker, are affected less strongly by the scale factor, the topographies correspond closely.…”

“…The possibility that anharmonic atomic vibration explains these features must be considered, especially as Restori & Schwarzenbach (1993) refined anharmonic vibration parameters for the K atom, rendering /I/9 in that vicinity essentially fiat. That is consistent with the hypothesis that the sharper /119 features near the K nuclei are due to anharmonicity, but does not eliminate electron redistribution as an alternative.…”

Anharmonic MotionvsChemical Bonding: on the Interpretation of Electron Densities Determined by X-ray Diffraction

“…Model (i) provided the most economical parametrization but this is per se not sufficient to prove the presence of anharmonic motions. The corresponding difference electron-density maps (Restori & Schwarzenbach, 1993, 1995 show a prominent octopolar density centered on K with maxima of "-'0.5 e ~-3 peaking on the tetrahedral (111) directions at ~0.4.~. from the atom center.…”

“…However, anharmonic motions of the metal atom may result in similar features since they are expected to have larger amplitudes in directions towards secondnearest neighbors and smaller amplitudes towards nearest neighbors. The electron density in K2PtC16 (Restori & Schwarzenbach, 1993) presents another example that is at the origin of the present work: it may be parametrized in terms of both anharmonic and multipolar functions since 3p shows compact features close to the atomic centers.…”