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The borate Ag16B4O10 was synthesized at high temperature and at elevated oxygen pressures [Kovalevskiy et al. (2020). Chem. Sci.11, 962–969]. Its structure consists of [B4O10]8− polyanions (isostructural to P4O10) embedded in an Ag matrix. According to the standard valences Ag+, B3+ and O2−, the formula has an excess of eight e− which the above authors proposed were located, pairwise, in four Ag4 tetrahedra within the silver substructure. That conclusion was based on the semiconducting and diamagnetic properties, as well as the very small 'attractors' of the Electron Localization Function (ELF) found at the centre of these Ag4 tetrahedra. However, a large overlap of the projected density of states (DOS) of silver and oxygen indicated possible dispersion interactions between both atomic species. In this article, an alternative description is proposed based on the extended Zintl–Klemm concept. The anion [B4O10]8− can be formulated as Ψ‐[N4O10] P4O10, if it is assumed that the eight e− are transferred to the four B atoms, so converting them into Ψ‐N, this then makes sense of its similarity with P4O10, [N4(CH2)6], adamantane and tetraisopropyladamantane. When the Ag atoms connect to the O atoms, they arrange as the H atoms do in hexamethylenetetramine (HMTA). If the two lone pairs of each of the bridging O atoms in Ψ‐[N4O10] are equated to the C—H bonds in HMTA, then, this same equivalence exists between the C—H bonds and the O—Ag bonds in the compound Ag16B4O10. The 24 Ag atoms surrounding each [B4O10]8− group prolong the sphalerite structure of the borate anion by means of Ag—O bonds which also fit the sphalerite structure formed of AgO. The eight excess electrons might then be distributed between the Ag and the O atoms, so making sense of the mixing of the Ag and O states. The Ag atoms bonded to the O atoms of the [B4O10]8− groups form a coat that interconnects the borate anions through Ag—O bonds. To establish the validity of this new proposal, the study needs to be extended to the compound Ag3B5O9. [ABSTRACT FROM AUTHOR]

This article examines the comparison between the classical formulations used to describe silicates and that derived from the application of the extended Zintl-Klemm concept (EZKC). The ionic strength, I, for 25 silicate lattices is calculated taking into account both formulations, and the results show that, in every single one of the examples, the ionic strength of the Zintl polyanion is higher than that of the classical model which assigns a formal charge of 4+ for silicon. Our earlier study, firstly applied to the germanate (NH4)2Ge[6][Ge[4]6O15] [Vegas & Jenkins (2017). Acta Cryst. B 73, 94-100] and to the polyanion [Ge[4]6O15]6− equivalent to the pseudo-As2O5 derived from it, explained satisfactorily the charge transfer that takes place in the Zintl compounds. The value of I = ½∑ nizi2 for the Zintl polyanion was greater than for the compound as formulated in the classical way. In that article, a meaningful relationship was found between the electron transfers as defined by the EZKC and the ionic strength I of the anion [Ge[4]6O15]6− ≡ Ψ-As2O5. Because the ionic strength, I, of a lattice is directly proportional to the lattice potential energy, UPOT, the higher the I the greater the UPOT; thus it is harder to break up the lattice into its constituent ions and hence the lattice itself is more stable, giving support to the idea that the application of the EZKC and the resulting electron shifts yields structures which are inherently thermodynamically more stable than the starting configuration. [ABSTRACT FROM AUTHOR]

Taking into account new experimental data [Barkalov et al. (2016). Solid State Sci. 61, 220-224] on the pressure-induced Ni2In phase of Li2S, at 30 GPa, three concepts related to high-pressure phase transitions are reviewed here. This paper firstly reviews evidence that chemical oxidation (by inclusion of oxygen atoms) can produce a similar effect to the application of physical high pressure and temperature, in an effect labelled as the oxidation-pressure concept. Secondly, the pressure-induced Ni2In phase of Li2S is the final phase in the double transition antifluorite → anticotunnite → Ni2In, as is observed in other alkali metal sulfides. This new phase for Li2S could be expected after knowledge of the high-pressure Cmcm phase of Li2SO4, which is a distortion of the hexagonal I-Na2SO4 phase, both having M2S subarrays of the Ni2In-type. Thirdly, in order to clarify these links, a simple methodology is proposed for gauging the level of increased stability (by defining a stability enhancement ratio, S) when the extended Zintl-Klemm concept (EZKC) has been applied. The method uses relative values of the lattice potential energies estimated for Li2S and for the pseudo-lattice Ψ-BeS derived by applying the EZKC to Li2S, after which, Li2S can be reformulated as Li+[LiS]− ≡ Li+[Ψ-BeS]. [ABSTRACT FROM AUTHOR]

The structure of ((NH4)2Ge7O15 recently described as being a microporous material containing rings, in which GeO6 octahedra coexist with GeO4 tetrahedra, is re-examined in the light of the Extended Zintl-Klemm Concept as applied to cations in oxides. The Ge[6] atoms together with the NH4+ groups act as true cations, transferring their 6 valence electrons to the acceptor Ge2O5 moiety, so converting it into the [Ge6O15]6-≡3(Ψ-As2O5) ion (where Ψ refers to a pseudo-lattice) and yielding threefold connectivity. The tetrahedral Ge network shows similarities with the Sb2O3 analogue. At the same time, the Ge[6] atoms are connected to other Ge[4] atoms forming blocks that are part of a rutile-type GeO2 structure. Such an analysis shows that both substructures (the Zintl polyanion and the rutile fragments) must be satisfied simultaneously as has already been illustrated in previous articles which considered stuffed-bixbyites [Vegas et al. (2009). Acta Cryst. B65, 11-21] as well as the compound FeLiPO4 [Vegas (2011). Struct. Bond. 138, 67-91]. This new insight conforms well to previous (differential thermal analysis) DTA-TGA (thermogravimetric analysis) experiments [Cascales et al. (1998). Angew. Chem. Int. Ed. 37, 129-131], which show endothermic loss of NH3 and H2O to give rise to the metastable structure Ge7O14, which further collapses to the rutile-type GeO2 structure. We analyze the stability change in terms of ionic strength, I, and so provide a means of rationalizing the driving force behind this concept capable of explaining the atomic arrangements found in these types of crystal structures. Although the concept was formulated in 2003, later than the publication of the germanate structure, it was not used or else ignored by colleagues who solved this crystal structure. [ABSTRACT FROM AUTHOR]

The structures of ternary oxides and chalcogenides of alkali metals are dissected in light of the extended Zintl-Klemm concept. This model, which has been successfully extended to other compounds different to the Zintl phases, assumes that crystal structures can be better understood if the cation substructures are contemplated as Zintl polyanions. This implies the occurrence of charge transfer between cations, even if they are of the same kind. In this article, the charge transfer between cations is even more illustrative because the two alkali atoms have different electronegativity, so that the less electropositive alkali metal and the O/S atom always form skeletons characteristic of the group 14 elements. Thus, partial structures of the zincblende-, wurtzite-, PbO- and SrAl2-type are found in the oxides/sulfides. In this work, such an interpretation of the structures remains at a topological level. The analysis also shows that this interpretation is complementary to the model developed by Andersson and Hyde which contemplates the structures as the intergrowth of structural slabs of more simple compounds. [ABSTRACT FROM AUTHOR]

The structures of the dimorphic FeLi[PO4] phosphate are analysed on the basis of their cation arrays. At ambient conditions, the compound, known as the mineral triphylite, is olivine type (Pnma) (CaMg[SiO4]) and transforms into the olivine-related NaCaVO4-type (Cmcm) structure, at high temperature and high pressure. This phase transition takes place with an exchange in the positions of the Li and Fe atoms, both filling octahedral voids. This feature, impossible to explain with the traditional model of cation-centred anion polyhedra, can be understood in the light of the extended Zintl-Klemm concept (EZKC) which contemplates as possible the electron transfer even between "cations". A detailed dissection of both structures leads to the conclusion that the observed phase transition triphylite → 3-LiFe[PO4] is controlled by the [FeP] cation subarrays which undergo a PbO-type → NiAs-type transition at high pressure. In addition to this relevant feature, it is shown that the [FeP] subarray, PbO type in triphylite, exits in the delithiated phase FePO4. The EZKC allows that LiFe[PO4] can be written with the pseudo- formula Li+[Ψ-FeSO4], so that the [FeP] subarray is converted into Ψ-[FeS], whose structure coincides with that of real FeS. The significant result is that the structure of FeS persists in the pseudoarrays Ψ-[FeS] of both Fe[PO4] and LiFe[PO4]. The "autopsy" also reveals that several substructures, formed by different pairs of atoms, co-exist with those of the PbO type and NiAs type, in triphylite and β-LiFe[PO4], respectively. Fragments of metallic lithium, layers of metallic iron and the rocksalt LiCl are some of the structures that can be revealed through the dissection process. The conclusion is that the formation of any structure implies that several substructures need to be satisfied simultaneously. These multiple substructures act as if they were resonance structures which cooperate to the stabilization of the whole network. [ABSTRACT FROM AUTHOR]

Abstract: We present two new approaches to describe the structures of the tin oxide fluorides Sn2OF2, Sn2OF5 and Sn4OF6, formerly described in terms of linked, cation-centered anion polyhedra. One approach considers the structures as cation arrays with inserted anions. The arrangement of the tin atoms in Sn2OF2 corresponds to that of the tin atoms in the rutile-like structure of SnO2 which, in turn, can be related to the structure of the high pressure phase γ-Sn. The anions in Sn2OF2 take positions like the O atoms in SnO2, but one fourth of them remain vacant. Sn2OF5 contains layers that match layers which can be cut out of the SnO2 structure parallel to (); compared to SnO2, the layers are mutually shifted, but within the layers the distribution of the tin atoms is alike. Sn4OF6 contains parallel columns that correspond to columnar fragments taken out of SnO2 parallel to b. These columns correspond to a composition [Sn3OF3]−; they are interconnected by [Sn2F6]2+ groups. The similarity of the tin atom arrangement in a [Sn3OF3]− column to that in SnO2 helps to recognize this. The other approach is to derive the structures of the oxide fluorides from the SnO2 structure by symmetry reduction, i.e., with the aid of group–subgroup relations between their space groups. It takes three steps of symmetry reduction from , the space group of SnO2, to the space group of Sn2OF2, . All atomic positions occupied in Sn2OF2 can be derived from positions of SnO2, but one fourth of the anion positions remain vacant. Group–subgroup relations reveal only a partial agreement for Sn2OF5 (within the layers), but the Sn atoms between the layers cannot be related to positions in SnO2. The structure of Sn4OF6 cannot be derived group theoretically from SnO2 because of the interfering [Sn2F6]2+ groups between the [Sn3OF3]− columns. [Copyright &y& Elsevier]