Stoichiometric Valence and Structural Valence—Two Different Sides of the Same Coin: “Bonding Power”

Abstract

Recent studies use the term valence to describe two distinct aspects of the phenomenon bonding power of an atom. Measured in valence units, one valence term, the classical chemical valence, has integer values and is derived solely from the composition of a compound. The second one, used mainly by solid-state physicists and crystallographers, has non-integer values. It is determined from structure data, which are derived from diffraction experiments, spectroscopy, or quantum-chemical calculations. To distinguish clearly between these two types of valencies, the descriptive terms stoichiometric valence and structural valence and the respective symbols (stoich)V and (struct)V should be used. For the majority of crystalline structures, values of (stoich)V and (struct)V, both measured in valence units, differ by less than 5%. However, for p-block atoms with one lone electron pair, differences between (stoich)V and (struct)V of up to 30% have been reported.

A crystal chemical approach to superconductivity. I. A bond-valence sum analysis of inorganic compounds

“…It has recently been made clear that in the literature the term valence is used for two different properties [10].…”

“…Table 1), have been used to describe the stoich V and struct V values, respectively [10]. According to Pauling [11], struct V i of an atom i is distributed among the bonds to its n surrounding neighbours as…”

Structural analysis of the microporous semiconductor K-SBC-1 during its reversible sorption of water

“…It was also shown that water readsorbed in K-SBC-1 following a heat treatment at 300 1C can be much more readily removed from the structure than the water present prior to such a heat treatment [3]. For the structurally related cetineite family [4][5][6], it was recently reported that two different crystallographic positions of water guest molecules are present in the {Sb 12 O 18 } tubular unit [7]. The K;Se version of the cetineite exhibits sorption properties that can be exploited for detecting noble gases but requires evacuation at 10 À5 mbar to desorb water and to make accessible the porosity of the {Sb 12 O 18 } tube [7].…”

Nonstoichiometry and bond character in unconventional superconductors

“…Typically, the valence values of the atoms are given as integers þ1, þ2, and þ3 for Cu, þ3 and þ5 for Bi, and þ2 and þ3 for Pu in the superconducting alloys PuCoGa 5 and PuRhGa 5 [5], etc. This indicates that the atoms are considered to have their classical valence that is commonly used in the chemical literature and which is called stoichiometric valence, stoich V [6]. Although it had been reported that FeSe [7,8] and FeSe 1Àx Te x [9,10], when both have a TE : CH 2 ratio of 1 : 1 and are superconducting at low temperatures, other studies gave contradicting results.…”