Bond-valence Theory Research Articles

New dielectric material systems of SrxNd2(1–x)/3TiO3 perovskites-like at microwave frequencies

Perovskite compounds, SrxNd2(1−x)/3TiO3 (SNTx) with x ranging from 0.05 to 0.2, were prepared by the conventional solid–state reaction method. X-ray diffraction patterns illustrated that high density and single phase products were obtained for all samples, indicating that a small level of Sr addition could substantially stabilise the structure and maintain a single phase for Nd2/3TiO3 ceramics. But there was also an orthorhombic space groups transition from Pnma to P4/mmm when x was above 0.18. In addition, the dielectric constant (εr) of SNTx ceramics had improved from ∼85.6 to ∼112 with increasing x value due to the larger ionic polarizability of Sr2+ than that of Nd3+. Notably, the product of dielectric Q value and resonant frequency (Q × f) of ∼295–650 at 3.07–3.32 GHz was strongly dependent on the grain size and the cation deficient in the present systems. Either increasing grain size or decreasing A-site vacancies led to an increase in the Q × f value. Besides, a temperature coefficient of resonant frequency (τf) of the SNTx ceramics sintered at their optimum densification temperature gradually increases from 191.7 ppm/°C (x = 0.05) to 221.3 ppm/°C (x = 0.2), which results from the lower rattling effect and higher symmetry in BO6 octahedra of perovskite structure according to bond valence theory and tolerance factor (t).

On understanding the structure and composition of crystals

Minerals come with complex chemical formulas and a variety of crystal structures, but although many different minerals are known, their number is far exceeded by the possible structures and compositions that apparently do not occur in nature. What is it that selects the particular combinations of compositions and structures that we observe? This is the question addressed in a recent paper in this journal by Frank Hawthorne (2015). He answers this question using bond valence theory that he carefully distinguishes from what he calls bond valence curves, more usually known as the bond valence sum (BVS) model widely used for the validation of new structure determinations.

Toward theoretical mineralogy: A bond-topological approach

The patterns of linkage of chemical bonds in space contain significant energetic information that may be used as the basis of a theoretical approach to the structure and chemical composition of minerals. This approach combines aspects of graph theory, bond-valence theory, and the moments approach to the electronic-energy density-of-states to interpret topological aspects of crystal structures, and allows consideration of many issues of crystal structure, mineral composition, and mineral behavior that are not addressed by established theoretical methods. The chemical composition of a mineral is controlled by the weak interaction between the structural unit and the interstitial complex. The principle of correspondence of Lewis acidity-basicity asserts that stable structures will form when the Lewisbase strength of the structural unit closely matches the Lewis-acid strength of the interstitial complex. This principle allows analysis of the factors that control the chemical compositions and aspects of the structural arrangements of minerals, and provides a mechanism to understand the relations between structure, the speciation of its constituents in aqueous solution, and its mechanism of crystallization. (H2O) groups in the structural unit limit the polymerization of the structural unit in one or more directions, controlling the polymerization of the structural unit. This is a major cause of structural diversity in oxygen-based minerals, and accounts for the systematic distribution in mineral species from the core to the surface of the Earth.

Structure Refinement and Dielectric Properties of Bismuth-Based Pyrochlores Containing Titanium

Bismuth-based pyrochlore dielectrics with the formula (Bi1.5Zn0.5)(Ti1.5M0.5)O7 (M = Nb, Ta and Sb) have been synthesized to investigate the influence of M between the dielectric properties and crystal structure. The XRD patterns show that all of the three samples give single phase. The refined data by GSAS program and the bond valence theory were used to analyze the differences in dielectric properties with occupation of different ions in B site. The dielectric constant of BZTS is the smallest in the BZTM, which is due to the polarizability and the contribution of BO6octahedra which has weak correlation when Sb placed into the center of the octahedra would also result in small αε. When it comes to the moderate temperature range, oxygen vacancies migration was thermally activated as the carriers mechanism, and the different activation energies is related to the association and the disassociation of massive complex defects. The Rietveld refinement data pointed that with the respectively change of B ions, more oxygen vacancies and free oxygen ions are provided by structural defects to participate in the conduction which can lead to the σ increase gradually.

Bond valence at mixed occupancy sites. I. Regular polyhedra.

Bond valence sum calculations at mixed occupancy sites show the occurrence of systematic errors leading to apparent violations of the Valence Sum Rule (bond valence theory) in regular and unstrained bonding environments. The systematic deviation of the bond valence from the expected value is observed in the long-range structure, and is discussed from geometric and algebraic viewpoints. In the valence-length diagram, such a deviation arises from discrepancies between the intersection points of the long-range bond valences and the theoretical bond valences with the valence-length curves of involved cations. Three factors cause systematic errors in the bond valences: difference in atomic valences, bond valence parameters Ri (the length of a bond of unit valence) and bond valence parameters bi (the bond softness) between the involved cations over the same crystallographic site. One important consequence strictly related to the systematic errors is that they lead to erroneous bond strain values for mixed occupancy sites indicating underbonding or overbonding that actually does not exist.

The site occupation and valence of Mn ions in the crystal lattice of Sr4Al14O25 and its deep red emission for high color-rendering white light-emitting diodes

The synthesis and component of red phosphor, Sr4Al14O25: Mn, were optimized for application in white light-emitting diodes. The microstructure and morphology were investigated by the X-ray diffraction and scanning electron microscopy. Different valences of Mn ions in Sr4Al14O25 were discriminated using the electron paramagnetic resonance and X-ray absorption near-edge structure spectroscopy techniques. The bond-valence theory was used to analyze the effective valences of Sr2+ and Al3+ in Sr4Al14O25. As a result, the strong covalence of Al3+ in the AlO4 tetrahedron other than in the AlO6 octahedron is disclosed. The deep red emission is attributed to Mn4+ occupying the center of AlO6 octahedron. The mechanism of energy transfer is mainly through dipole–dipole interaction, revealed by the analyses of critical distance and concentration quench. A high color rendering white LED prototype with color-rendering index up to Ra 93.23 packaged by using the red phosphor demonstrates its applicability.

Tetrahedrally coordinated Co2+ in oxides and silicates: Effect of local environment on optical properties

The Co 2+ ion in fourfold coordination provides d-d electronic transitions with the strongest optical density among oxides and silicates. For this reason, it is widely used in pigments and dyes to get blue shades detectable down to a very low cobalt concentration. Such a low-detection limit turns the Co 2+ ion into a suitable probe to disclose the local ligand environment in a wide range of materials by means of optical spectroscopy. Even if extensively studied in organometallic complexes, an in-depth investigation of optical properties of Co 2+ in tetrahedral coordination into oxidic structures is limited to some case-study in minerals and synthetic analogs (spinel, zincite, gahnite, willemite, calcium cobalt selenite). The present study represents an attempt to outline crystal structural (long-range metal–oxygen distances, O–T–O bond angles, and distortion parameters by XRD) and optical parameters (10 Dq , Racah B and C , band splitting by EAS) in 13 samples of oxides and silicates providing a wide set of different local fourfold coordination around Co 2+ added as a dopant. Subtle variations of crystal field strength and interelectronic repulsion can be appreciated in gahnite, Ca-Sr-hardystonite, Ca-Sr-Ba-akermanite, willemite, Ba 2 MgSi 2 O 7 melilite-related (where Co 2+ substitutes Mg 2+ or Zn 2+ by 0.25–0.3 apfu) as well as in gehlenite and fresnoite (where Co 2+ substitutes Al 3+ and Ti 4+ , respectively, by 0.2 apfu due to charge mismatch). Results are compared with literature data about hibonite, spinel s.s. , staurolite, yttrium garnets, and zincite. Spectral interpretation is not straightforward owing to the occurrence of different Co 2+ bands: spin-allowed and spin-forbidden electronic transitions, two- or threefold split due to both lowering of point symmetry at the tetrahedron and spin-orbit coupling plus presumably vibronic transitions. Optical spectra vary significantly even for apparently small changes in the long-range CoO 4 arrangement as measured by XRD. The expected relationship between 10 Dq and the mean Co–O distance is fulfilled, but the accommodation into small AlO 4 sites in gehlenite (YAG and hibonite) implies a significant structural relaxation around the Co 2+ ion. The threefold splitting of the spin-allowed 4 T 1 (F) and 4 T 1 (P) bands can be related to the angular distortion of the CoO 4 tetrahedra. Overall, changes of spectral features of tetrahedrally coordinated Co 2+ can be attributed to different local arrangement of ligands with an effect correlated to the second nearest neighbors by the bond valence theory. This was disclosed contrasting 10 Dq with the ratio of the observed and ideal bond valence sum for the polyhedra sharing oxygen with the Co-centered tetrahedron.

Mean bond-length variation in crystal structures: a bond-valence approach.

The distortion theorem of the bond-valence theory predicts that the mean bond length 〈D〉 increases with increasing deviation of the individual bond lengths from their mean value according to the equation 〈D〉 = (D' + ΔD), where D' is the length found in a polyhedron having equivalent bonds and ΔD is the bond distortion. For a given atom, D' is expected to be similar from one structure to another, whereas 〈D〉 should vary as a function of ΔD. However, in several crystal structures 〈D〉 significantly varies without any relevant contribution from ΔD. In accordance with bond-valence theory, 〈D〉 variation is described here by a new equation: 〈D〉 = (DRU + ΔDtop + ΔDiso + ΔDaniso + ΔDelec), where DRU is a constant related to the type of cation and coordination environment, ΔDtop is the topological distortion related to the way the atoms are linked, ΔDiso is an isotropic effect of compression (or stretching) in the bonds produced by steric strain and represents the same increase (or decrease) in all the bond lengths in the coordination sphere, ΔDaniso is the distortion produced by compression and stretching of bonds in the same coordination sphere, ΔDelec is the distortion produced by electronic effects. If present, ΔDelec can be combined with ΔDaniso because they lead to the same kind of distortions in line with the distortion theorem. Each D-index, in the new equation, corresponds to an algebraic expression containing experimental and theoretical bond valences. On the basis of this study, the ΔD index defined in bond valence theory is a result of both the bond topology and the distortion theorem (ΔD = ΔDtop + ΔDaniso + ΔDelec), and D' is a result of the compression, or stretching, of bonds (D' = DRU + ΔDiso). The deficiencies present in the bond-valence theory in explaining mean bond-length variations can therefore be overcome, and the observed variations of 〈D〉 in crystal structures can be described by a self-consistent model.

Static positional disorder in ulvospinel: A single-crystal neutron diffraction study

A single-crystal neutron diffraction study of a synthetic ulvospinel sample of composition Fe 3+ 0.40 Fe 2+ 1.80 Ti 0.80 O 4 was performed to investigate the static positional disorder at the octahedrally coordinated M site. Anisotropic structural refinement was performed in the space group Fd 3 m against neutron Laue diffraction data collected at 298 K from two millimetric-sized crystals. Initial structure refinements were conducted with Fe and Ti sharing the M site (at 1/2, 1/2, 1/2), and their partial site occupancy was refined. The tetrahedrally coordinated T site (at 1/8, 1/8, 1/8) was modeled as fully occupied by Fe. For both crystals, the final R 1 index was about 3% for 9 refined parameters and 129 unique reflections, with no significant residuals. As the atomic displacement factors obtained were anomalously high, according to the previous experimental findings, F obs - and ( F obs – F cal )-difference Fourier maps of the nuclear density were generated. Fourier maps showed a significant minimum located out-of-center of the M site, and indicating a displacement of the Ti 4+ from the center of the octahedron. A further test refinement was successfully conducted with two mutually exclusive sites: M Ti out-of-center (at 0.49, 0.49, 0.49) and M Fe on the center (at 1/2, 1/2, 1/2). The resulting displacement of Ti from the octahedral center appears to be shorter than 0.15 A. Using bond-valence theory, the out-of-center distortion of M Ti 4+ is interpreted as a result of intrinsic distortions in the ulvospinel structure. The potential implication of the octahedral distortion on the behavior of ulvospinel at non-ambient conditions is discussed.

A critical comment on Ertl et al. (2012): "Limitations of Fe2+ and Mn2+ site occupancy in tourmaline: Evidence from Fe2+- and Mn2+-rich tourmaline"

In this paper we have presented a detailed response to Ertl et al. (2012a) who, in a paper in volume 97 (year 2012), pages 1402–1416, this journal, claim evidence for limitations of Fe2+ and Mn2+ occupancy at the Z site of the tourmaline structure. They also propose a model by which the distance of tourmaline varies as a function of its and bond lengths. We have examined their conclusions and find that a different distribution of cations over the Y and Z sites gives better agreement with the extensive experimental information available. In fact, on the basis of crystal-structure refinements, Mossbauer spectroscopy, optical absorption spectroscopy, bond-valence theory, ionic radius concept and literature, the occurrence of Fe2+ at the Z site of tourmaline is well supported. Conversely, existing experimental data does not provide indisputable evidence for the occurrence of Mn2+ at the Z site. Despite this, there is no evidence for inductive effects of YMn2+ on , and the proposed effects must be regarded as speculative. Statistical analysis shows that the average value is 1.906(2) A, which is consistent with the observed values of at the 99% confidence limit (within 3σ) in tourmalines with the Z site fully occupied by Al. Consequently, the proposed inductive effect of and on can be ruled out.

A bond-topological approach to theoretical mineralogy: crystal structure, chemical composition and chemical reactions

Here, I describe a theoretical approach to the structure and chemical composition of minerals based on their bond topology. This approach allows consideration of many aspects of minerals and mineral behaviour that cannot be addressed by current theoretical methods. It consists of combining the bond topology of the structure with aspects of graph theory and bond-valence theory (both long range and short range), and using the moments approach to the electronic energy density-of-states to interpret topological aspects of crystal structures. The structure hierarchy hypothesis states that higher bond-valence polyhedra polymerize to form the (usually anionic) structural unit, the excess charge of which is balanced by the interstitial complex (usually consisting of large low-valence cations and (H2O) groups). This hypothesis may be justified within the framework of bond topology and bond-valence theory, and may be used to hierarchically classify oxysalt minerals. It is the weak interaction between the structural unit and the interstitial complex that controls the stability of the structural arrangement. The principle of correspondence of Lewis acidity–basicity states that stable structures will form when the Lewis-acid strength of the interstitial complex closely matches the Lewis-base strength of the structural unit, and allows us to examine the factors that control the chemical composition and aspects of the structural arrangements of minerals. It also provides a connection between a structure, the speciation of its constituents in aqueous solution and its mechanism of crystallization. The moments approach to the electronic energy density-of-states provides a link between the bond topology of a structure and its thermodynamic properties, as indicated by correlations between average anion coordination number and reduced enthalpy of formation from the oxides for [6]Mg [4] Si n O(m+2n) and MgSO4(H2O) n .

Short-range order in tourmaline: a vibrational spectroscopic approach to elbaite

Polarized Fourier-transform infrared and Raman spectra were acquired on an elbaite sample previously characterized by electron- and ion microprobe analysis, X-ray diffraction and structure refinement. Spectra from the two vibrational spectroscopy techniques reveal a close similarity in the OH-stretching region, with three main absorption bands strongly polarized in the c-axis direction. By means of bond-valence theory arguments, the observed OH bands are interpreted and assigned to specific local cation arrangements around the O1 (≡W) and O3 (≡V) anion sites. In combination with the relatively simple composition of the studied sample, bond-valence constraints are used to identify stable anion-cation arrangements, which permit the occurrence of short-range ordering to be assessed. Evidence for nearly complete short-range order at the O1 site, with the stable arrangements Y(LiAlAl)0.6–W(OH)0.6 and Y(LiLiAl)0.4–W(F)0.4, are presented. These two local arrangements can be further expanded to obtain the larger ordered clusters [W(OH)–Y(LiAl2)–V(OH)3–Z(Al)6]0.6 and [W(F)–Y(Li2Al)–V(OH)3–Z(Al)6]0.4.

The role of H2O in controlling bond topology:I. The[6]Mg(SO4)(H2O)n(n= 0–11) structures

AbstractHere, we examine the role of (H2O) in controlling polymerization of structural units in simple hydrated oxysalt compounds of the form Mg(SO4)(H2O)n. As the number of (H2O) groups in the structure increases, the number ofM—M(M= Mg) linkages decreases in a 1 : 1 ratio with the (increasing) number of (H2O) groups until noM—Mlinkages are left, and then the number ofM—T(T= S) linkages decreases with the (increasing) number of (H2O) groups until noM—Tlinkages are left (atn= 6). The change in bond topology is monotonic as a function of (H2O) content except forn= 5 where, instead of replacing anM—Tlinkage, the fifth (H2O) group is held in the structure by hydrogen bonds only. The valence-sum rule of bond-valence theory constrains the anion-coordination numbers to [2], [3] and [4] in these structures. The handshaking lemma of graph theory allows us to derive an equation relating the coordination numbers of the cations (Mg = [6], S6+= [4], H = [2]) to the coordination numbers of the anions. There are four integer solutions to this equation, and the Mg(SO4)(H2O)nstructures correspond to two of these solutions. Structures are known forn= 0–2.5, 4–7, 11; considering the variations in connectivity and coordination number as a function ofn, a possible structural arrangement is proposed for Mg(SO4)(H2O)3.

Contribution of molecular dynamics simulations and ab initiocalculations to the interpretation of Mg K-edge experimental XANES inK2O–MgO–3SiO2 glass

The structural environment of Mg in a K-bearing silicate glass of compositionK2MgSi3O8 is investigated by x-ray absorption near-edge structure (XANES) spectroscopy at the MgK-edge. XANES calculations are performed using a plane-wave electronic structure code,and a structural model obtained by classical molecular dynamics coupled to ab initiorelaxation. Bond valence theory is used to validate plausible environments within thestructural models. Comparison between the experimental and calculated spectra enables usto conclude that the Mg atoms are located in distorted tetrahedral sites. Sitedistortions are found to be correlated to the theoretical shift of the XANES edgeposition.

On Oxygen Deficiency in Nanocrystallites La1-xSrxCoO3

The crystal lattice and the bond valence structure of perovskite system La1-xSrxCoO3 are reported. The atomic valences for the given perovskites were determined on the basis of bond-valence theory. For the studied compounds the failure of distortion theorem was observed, and the non-stoichiometry of oxygen was estimated. The model for oxygen distribution is then suggested. The relationship between the grain diameter and oxygen content is discussed. [DOI: 10.1380/ejssnt.2011.469]

45Sc Spectroscopy of Solids: Interpretation of Quadrupole Interaction Parameters and Chemical Shifts

The aims of the present study is to describe for the first time the 45Sc MAS NMR spectra of X2-Sc2SiO5 and C-Sc2Si2O7, to combine the spectroscopic information with the structures published from diffraction data, and to propose a rational interpretation of the chemical shifts and quadrupolar parameters. For that purposed, we have correlated the experimental quadrupole coupling parameters of 45Sc determined for a number of scandium compounds to those found by a simple electrostatic calculation and we have found that the isotropic chemical shift of the 45Sc is linearly correlated to the shift parameter, calculated by bond-valence theory. We also show that a simple point charge calculation can approximate the electric field gradient to a sufficiently good approximation that it provides a valuable mean to assign the NMR spectra.

CRYSTAL STRUCTURE AND MOSSBAUER SPECTROSCOPY OF TSCHERMAKITE FROM THE RUBY LOCALITY AT FISKENAESSET, GREENLAND

The crystal structure of tschermakite from Fiskenaesset, Greenland, (K 0.05 Na 0.38 ) ∑0.43 (Ca 1.80 Na 0.05 Fe 2+ 0.15 ) ∑2.00 (Mg 3.65 Fe 2+ 0.18 Mn 0.03 Fe 3+ 0.09 Al 0.81 Cr 0.19 Ti 0.05 ) ∑5.00 (Si 6.43 Al 1.57 ) ∑8.00 O 22 [(OH) 1.95 F 0.04 Cl 0.01 ] ∑2.00 , a 9.8059(5), b 17.9721(8), c 5.3012(2) A, β 105.063(1)°, V 902.15 A 3 , space group C 2/ m , Z = 2, has been refined to an R 1 index of 3.2% using Mo K α single-crystal X-ray diffraction. The ratio Fe 3+ /Fe tot was determined by Mossbauer spectroscopy. The unit formula (calculated from the results of electron-microprobe analysis), the refined site-scattering values, observed average bond-lengths, and Mossbauer spectroscopy were used to assign site populations. Tetrahedrally coordinated Al occurs at both the T (1) and T (2) sites, but is strongly ordered at T (1), and [6] Al occurs at both the M (2) and M (3) sites, but is strongly ordered at M (2). Magnesium shows the site preference M (1) = M (2) > M (3), and higher-valence transition metals (Ti, Fe 3+ and Cr 3+ ) are ordered at M (2). The formula calculated from the electron-microprobe analysis showed an excess of C -group cations of 0.15 apfu , exactly in accord with the amount of Fe 2+ at M (4) indicated by Mossbauer spectroscopy. At the A site, Na was split between the A (2) and A ( m ) sites, and K was assigned to the A ( m ) site. We derived the short-range arrangements involving the M (4), O(3) and A sites, and calculated their relative fractions. As with other complicated monoclinic amphiboles, short-range order is common and dictated by the valence-sum rule of localized bond-valence theory.

Crystal chemistry of the magnetite-ulvospinel series

Spinel single crystals of 19 compositions along the magnetite-ulvöspinel join were synthesized by use of a flux-growth method. To obtain quantitative site populations, the crystals were analyzed by single-crystal X-ray diffraction, electron-microprobe techniques, and Mössbauer spectroscopy. All results were processed by using an optimization model. The unit-cell parameter, oxygen fractional coordinate, and tetrahedral bond length increase with increasing ulvöspinel component, whereas the octahedral bond length decreases marginally. These changes result in sigmoidal crystal-chemical relationships consistent with cation substitutions in fully occupied sites. As a first approximation, the Akimoto model T (Fe 3+ 1-X Fe 2+ x )M(Fe 2+ Fe 3+ 1-X Ti X )O 4 describes the cation substitutions. Deviations from this model can be explained by an electron exchange reaction T Fe 2+ + M Fe 3+ = T Fe 3+ + M Fe 2+ , which causes M Fe 2+ ≠ 1 and T Fe 2+ /Ti ≠ 1. The resultant S-shaped trends may be related to a directional change in the electron exchange reaction at Ti ≈ 0.7 apfu. In general, variations in structural parameters over the whole compositional range can be split into two contributions: (1) a linear variation due to the T Fe 3+ + M Fe 3+ = T Fe 2+ + M Ti 4+ chemical substitution and (2) non-linear variations caused by the internal electron exchange reaction. In accordance with bond-valence theory, strained bonds ascribable to steric effects characterize the structure of magnetite-ulvöspinel crystals. To relax the bonds and thereby minimize the internal strain under retained spinel space group symmetry, the electron exchange reaction occurs.

Hydrogen bond connectivity in jennite from ab initio simulations

The protonation scheme and the hydrogen bond connectivity in the structure of jennite were investigated by ab initio molecular dynamics simulations. The calculated statistics of hydrogen bonds at ambient conditions is consistent with the protonation scheme proposed by Bonaccorsi et al. (2004) based on the bond valence theory. The protons in the system are associated with the 2Ca–OH linkage and H 2O molecules. The dangling Si–O bond on the bridging tetrahedra is de-protonated. The proton dynamics revealed in the molecular dynamic simulations explains the apparent discrepancies in the NMR and X-ray diffraction studies of jennite.

Disordering of Fe2+ over octahedrally coordinated sites of tourmaline

The partitioning of iron among octahedrally coordinated sites in tourmaline, and its stereochemical consequences, were investigated in a Fe-rich dravite in a skarn rock from Uto, Sweden. A multi-analytical approach using structure refinement (SREF), electron microprobe analysis (EMPA), and Mossbauer spectroscopy (MS) established the chemical and structural nature of the tourmaline. A structural formula obtained by optimization procedures indicates disordering of Al, Mg, and Fe 2+ over the Y and Z sites, and ordering of Fe 3+ at the Y site. Two Fe-rich tourmalines from the literature, reexamined with the optimizing site assignment procedure, appear to have iron partitioning comparable to that of the Uto tourmaline with Fe 2+ disordered over the octahedral sites. This is best explained by disordered Fe 2+ distributions that minimize the strain state of the Y -O bonds and provide a shielding effect reducing Y - Z repulsion. This is consistent with predictions from bond-valence theory and Pauling’s rules. An indication of Z -site occupancy by Fe 2+ in tourmaline may be signaled by a significant correlation between Z -O> and the c lattice parameter ( r 2 = 0.96). The c value for a very Fe 2+ -rich tourmaline and an ideal end-member schorl, with Fe 2+ and Al ordered at Y and Z (respectively), yielded Z -O> values larger than 1.907 A (the likely bond length for Z Al-O>). These large Z -O> lengths indicate that Fe 2+ occurs at the Z site. The hypothesis of a dragging effect from Y -O> to explain lengthening of Z Al-O> is not supported by experimental evidence.