Bond Valence Analysis Research Articles

Crystal structure, chemical bonds and microwave dielectric properties of ultra-low loss Li2Mg2GaTi2O8F ceramics

Ceramics of Li2Mg2GaTi2O8F (LMGTOF) were prepared by the traditional solid-state method. XRD and TEM results confirmed that a single phase of Li2Mg2GaTi2O8F, with an Fd-3m space group and a cubic spinel structure, was formed by the ceramics. It was shown by the results that the lattice parameters of the ceramics at 1050 °C were measured as a = b = c = 8.36576 Å, with a volume of V = 585.485549 Å3. Furthermore, it was observed through the analysis of the Raman peak at 718 cm−1 that an inverse relationship was exhibited between the Raman shift and εr. Similarly, an opposing trend was also demonstrated between the Q × f value and FWHM. By bond valence analysis, it was shown that Mg2+, Ga3+, and Ti4+ ions were in a “rattling” state, and the large positive deviation between εcorr and εtheo was attributed to the strong “rattling” effect induced by the cations. At 1050 °C, the best microwave dielectric properties (εr = 14.58, Q × f = 76,689 GHz, and τf = −52 ppm/°C) were exhibited by LMGTOF ceramics.

Raman spectroscopy, chemical bonding, and microwave dielectric properties of Li4AlSbO6 ceramics

A novel Li4AlSbO6 (LASO) microwave dielectric ceramic with a monoclinic structure and space group C12/c1 was successfully prepared using traditional solid-state methods. When the sintering temperature was 1200 °C, it had excellent microwave dielectric properties, with εr = 8.2, Q × f = 22,098 GHz, and τf = −15.53 ppm/°C. Raman spectroscopy analysis showed that the FWHM of the strong peak at 691 cm−1 reflected the variation trend of Q × f value and was opposite to the variation trend of Q × f value. Additionally, the augmentation in the Q × f value can be attributed to a higher relative density. As per the bond valence analysis, εr is closely connected to the rattling and compressing effects of A-site cations, and the higher εtheo than εr in Li4AlSbO6 can be attributed to the strong compression effect induced by Li+. Additionally, τf may be influenced by both the Sb–O bond and relative density.

Crystal structure, chemical bonding and infrared reflectivity of microwave dielectric SrIn2O4 ceramics

In this work, the orthorhombic structured SrIn2O4 ceramics with a space group Pnam were synthesized by a solid-state reaction method. A high relative density (96.5%) coupled with excellent microwave dielectric properties (εr ~ 12.3, Q × f = 96,900 GHz, τf ~ -61.6 ppm/°C) were obtained as sintered 1300 °C for 4 h. The bond valence analysis demonstrates that the large sized cation Sr2+ exhibits a compressed state, while the In3+ exhibits a weaken rattling effect. The P-V-L chemical bond theory analysis indicates that the In-O bonds play a key role in affecting the dielectric loss. The thermally conductivity activation energy Edc (0.94 eV) of SrIn2O4 was obtained by the dielectric spectroscopy, indicating that the Edc was contributed to the double ionized oxygen vacancies. Furthermore, the intrinsic dielectric properties (εr ~ 11.2, Q × f = 148,900 GHz) of SrIn2O4 were obtained by infrared reflectivity spectroscopy.

Crystal structure, bond characterization and lattice energy of B-site 1:3 ordering inverse spinel Li1.25Ga0.25Ti1.5O4 ceramic

In this work, inverse spinel structured Li1.25Ga0.25Ti1.5O4 ceramics with a space group of P4332 were synthesized by a solid-state reaction method. XRD, SAED, HRTEM, and Raman spectra analysis demonstrated that the Li1.25Ga0.25Ti1.5O4 ceramic was formed a A-site 1:3 disordered and B-site 1:3 ordered spinel structured. A high relative density (98.7%) with excellent microwave dielectric properties of εr = 23.8, Q×f = 93,800 GHz, and τf = –21.9 ppm/oC were obtained in 1110 °C for 4 h. Bond valence analysis demonstrated that the average cations of Li1.25Ga0.25Ti1.5O4 were in the rattling state, and the polarizability of Ti4+ was underestimated. The P–V-L chemical bond theory and lattice energy analysis results indicated that B–O bonds were the main factor to affect the εr and Q × f, and the under-bonding Ti4+and Ga3+ has a significant effect on the τf. All the results show that the Li1.25Ga0.25Ti1.5O4 ceramic might be a potential candidate for 5 G communication technology.

High-Pressure Synthesis and Magnetic Property of a Novel Ferrimagnetic Quadruple Perovskite YMn<sub>3</sub>Co<sub>4</sub>O<sub>12</sub>

A novel quadruple perovskite oxide YMn3Co4O12 has been synthesized in high-pressure and high-temperature conditions of 16 GPa and 1473 K, respectively. Rietveld refinement of synchrotron X-ray powder diffraction data reveals that this oxide crystallizes in a cubic quadruple perovskite structure in a space group of Im3 (No. 204) with a 1:3-type atomic ordering of Y and Mn at A-sites, and the bond valence analysis suggests the valence state of YMn3+3Co3+4O12. A ferromagnetic-like transition is observed at ~125 K and the magnetization value reaches up to ~3 µB per formula unit at 5 K. The low-temperature magnetic state can be interpreted as ferrimagnetism with antiparallel alignment of A'-site Mn3+ (S = 2) and B-site Co3+ (high-spin, S = 2), which is in contrast to the antiferromagnetism in other A'-Mn-containing quadruple perovskite oxides. This finding proposes that the coexistence of A’-Mn3+ and B-Co3+ leading to ferrimagnetic state is realized in the specific structure with distinct crystallographic sites.

Exploring the Ln–O bonding nature and charge characteristics in monazite in relation to microwave dielectric properties

AbstractThe microstructure design is important for regulating the microwave dielectric properties of materials. However, in‐depth studies on the frequency temperature stability and related micromechanism remain poorly understood. The work investigates the correlation among the sintering behavior, crystal structure, bonding nature, and microwave dielectric properties of LnPO4 (Ln = Eu, Pr) ceramics by combining first‐principles calculations and experimental perspective. The high density (ρ > 97%) and large grains associated with lattice expansion benefit the optimum dielectric properties: εr = 11.24, Q×f = 61,138 GHz @ 13.311 GHz, and τf = −30.3 ppm/°C for EuPO4 sintered at 1500°C (εr = 11.35, Q×f = 63,496 GHz @ 13.042 GHz and τf = −39.5 ppm/°C for PrPO4 sintered at 1525°C). Bond valence analysis shows a rattling effect in [EuO9] due to a smaller ionic radius. The effect induces an abnormally large polarization, effectively shifting the negative τf toward near‐zero values. The electron localization functions, charge transfer, and bonding nature are discussed by density functional theory calculations, which illustrate stronger charge transfer and ionicity between Eu and O. This observation effectively predicts and validates the nonharmonic lattice vibrations and abnormally large polarization obtained from Raman spectrums and Rietveld refinement. These findings systematically clarify the optimized effect and micromechanism of lanthanides on the dielectric properties of monazite ceramics.

Metal Template Synthesis of ‘Broken’ Aromatic Preorganized Terdentate Hosts for the Recognition of Lanthanide Tris‐β‐Diketonate Guests

AbstractThe binding of the terdentate precursor 2,2′‐(4‐methyl‐3,5‐divinylpyridine‐2,6‐diyl)bis(1‐allyl‐5‐bromo‐1H‐benzo[d]imidazole) (1) to the lanthanide container [Ln(hfac)3] (Ln=La, Eu, Gd, Y, Er; H‐hfac=1,1,1,5,5,5‐hexafluoropentane‐2,4‐dione) ensures the cis‐cis orientation of the two adjacent α,α′‐diimine units that is required for the successful intramolecular Grubb ring‐closing metathesis generating the target rigid 6‐methyl‐9,11‐dihydro‐1H,3H‐2λ2,10λ2‐pyrido[2,3‐c:6,5‐c′]bis(azepine) scaffold decorated with two terminal 5‐bromo‐1H‐benzo[d]imidazole in ligand L7. The bond valence analysis of the crystal structures of the associated nine‐coordinate adducts [L7Ln(hfac)3] (Ln=La, Eu, Gd, Er, Y) reveals a satisfying match between the rigid terdentate cavity and the size of the bound lanthanide metal, with a pronounced preference for the largest lanthanum cation. Thermodynamic studies in dichloromethane confirm the formation of [L7Ln(hfac)3] adducts with unprecedented stabilities due to the removal of the energy penalty associated with trans‐trans to cis‐cis reorganization. The introduction of saturated methylene groups within the polyaromatic ligand backbone breaks extended aromatic delocalization and clears the visible part of the electromagnetic spectrum from emission arising from low‐energy ligand‐based excited states.

Versatility of Tellurium in Heteroanionic Ln2O2Te (Ln = La, Ce, Pr) and Tellurate Ln2TeO6 (Ln = La, Pr).

Heteroanionic compounds continue to gain interest in materials design because the expanded composition space provides opportunities to discover new phases and tune physical properties. Among heteroanionic materials, oxytellurides comprised of oxygen and tellurium anions are relatively underexplored despite the significant role of tellurium in emerging technologies. Herein, we present synthetic strategies toward oxytelluride Ln2O2Te (Ln = La-Pr), whose layered structure features square nets of Te2- anions. Upon heating in H2 or air, we find a reversible phase transition between the oxytelluride and tellurate Ln2TeO6 (Ln = La, Pr), wherein Te is octahedrally coordinated and a 6+ oxidation state is corroborated by bond valence analysis. We use X-ray diffraction along with thermogravimetric analyses to confirm the presence of oxytelluride and tellurate phases and emphasize key structural distinctions. In contrast, we find that Ce2O2Te decomposes to form CeO2 and demonstrate the instability of Ce2O2Te in ambient conditions by timelapse X-ray diffraction and diffuse-reflectance spectroscopy experiments. Band gaps of Ln2O2Te (Ln = La-Pr) were estimated from diffuse-reflectance spectroscopy in the semiconducting range ∼2.1-2.7 eV, while band gaps for La2TeO6 and Pr2TeO6 were much larger at ∼4.3 and ∼3.7 eV, respectively.

The bond valence model as a prospective approach: examination of the crystal structures of copper chalcogenides with Cu bond valence excess.

Bond valence analysis has been applied to various copper chalcogenides with copper valence excess, i.e. where the formal valence of copper exceeds 1. This approach always reveals a copper bond valence excess relative to the unit value, correlated to an equivalent ligand bond valence deficit. In stoichiometric chalcogenides, this corresponds to one ligand electron in excess per formula unit relative to the valence equilibrium considering only CuI. This ligand electron in excess is 50/50 shared between all or part of the Cu-atom positions, and all or part of the ligand-atom positions. In Cu3Se2, only one of the two Cu positions is involved in this sharing. It would indicate a special type of multicentre bonding (`one-electron co-operative bonding'). Calculated and ideal structural formulae according to this bond valence distribution are presented. At the crystal structure scale, Cu-ligand bonds implying the single electron in excess form one-, two- or three-dimensional subnetworks. Bond valence distribution according to two two-dimensional subnets is detailed in covellite, CuS. This bond valence description is a formal crystal-chemical representation of the metallic conductivity of holes (mixing between Cu 3d bands and ligand p bands), according to published electronic band structures. Bond valence analysis is a useful and very simple prospective approach in the search for new compounds with targeted specific physical properties.

Structure characteristics and enhanced microwave dielectric properties of Ba(Mg1/3Nb2/3)O3–Mg2SiO4 composite ceramics with a tunable τf

The performance of microwave communication devices widely used in 5G network is strongly determined by the properties of dielectric ceramics used in the fabrication, which are required to have a suitable dielectric constant (εr), high Q × f value and near-zero temperature coefficient of resonant frequency (τf). In this work, (1-x)Ba(Mg1/3Nb2/3)O3–xMg2SiO4 (x = 0.1, 0.2, 0.3, 0.4, 0.5) composite ceramics meeting the application requirements have been developed and investigated. Ba(Mg1/3Nb2/3)O3 and Mg2SiO4 could coexist without any unexpected phases due to the different crystal structures and coordination relationship. The sintering behavior and the structural changes of Ba(Mg1/3Nb2/3)O3 were specifically relevant to the microwave dielectric properties. Introducing appropriate Mg2SiO4 content promoted the sintering of Ba(Mg1/3Nb2/3)O3 and the maximum Q × f value of 110,000 GHz was obtained at the sample with x = 0.3 sintered at the optimized temperature of 1420 °C. Due to the low dielectric loss and the opposite sign τf value of Mg2SiO4, the composite ceramic successfully adjusted the τf value to near zero while maintaining a high Q × f value. However, the experimentally measured values of εr and τf were significantly different from the theoretically calculated values due to the “rattling” effect of B-site cations and the enhanced symmetry of oxygen octahedra in Ba(Mg1/3Nb2/3)O3. These structural changes were confirmed by the results of bond valence analysis and Raman vibration spectra. The 0.6Ba(Mg1/3Nb2/3)O3–0.4Mg2SiO4 ceramic sintered at 1420 °C exhibits competitive microwave dielectric properties: εr = 18.9, Q × f = 91,000 GHz (at 8.016 GHz) and τf = 2.6 ppm/°C, which indicates it a promising application prospect of microwave communication.

The redefinition of gunterite, Na4Ca[V10O28]·20H2O

ABSTRACT Gunterite was originally assigned the ideal formula Na4[H2V10O28]·22H2O. More detailed bond-valence analysis brought into question the presence of a protonated decavanadate anion, which led to the reexamination of the mineral. Infrared spectroscopy confirmed the absence of NH4. Reinterpretation of the original crystal structure data and new electron-probe microanalyses support the redefinition of gunterite as having the ideal formula Na4Ca[V10O28]·20H2O. This redefinition has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association.

Hydrogen bond network and bond valence analysis on uranyl sulfate compounds with organic-based interstitial cations

Seven new uranyl sulfate compounds with organic charge-balancing cations have been synthesized and structurally characterized. The structural unit topologies of the two chains and five sheets have been previously reported in uranyl sulfate crystal chemistry, although they were synthesized using different organic molecules. With the inclusion of six of these structures to the 48 previously published uranyl sulfate compounds, a total of 54 known uranyl sulfate compounds with organic charge-balancing cations were compiled and analyzed. A graphical approach was used to compare the structural unit topologies, and a bond valence approach was used to quantify the hydrogen bond networks that exist between the interstitial cationic and solvent species to the uranyl sulfate anionic structural units. This analysis helped elucidate which oxygen atoms in the structural unit receive hydrogen bonds and how the organic cations stabilize the overall crystal structures in this subclass of U(VI) materials.

Identification of Active Sites in Pt–Co Bimetallic Catalysts for CO Oxidation

Identifying the active sites of catalysts under working conditions is crucial for precise understanding of reaction mechanisms and rational design of catalysts. However, the nature of active sites of bimetallic catalysts for CO oxidation is still a subject of debate. Herein, we employ in situ X-ray absorption and infrared spectroscopy to monitor the realistic structures of active centers in a bimetallic Pt–Co/Al2O3 catalyst during CO oxidation. This catalyst brings 100% CO conversion at room temperature and 30-fold higher turnover frequency than monometallic Pt/Al2O3 catalysts. The in situ studies reveal that under the CO oxidation condition, a fraction of Co atoms are segregated from the PtCo alloy into the surface CoO species that decorates the remaining PtCo nanoparticles through direct Pt–O–Co interfacial bonds. The bond-valence analysis unravels one dangling Co–O coordination per Co2+ in the surface CoO, which acts as the active sites for O2 activation. The synergy between the CoO species and the PtCo alloy brings the superior catalytic activity. We also show that the directly connected Pt–O–Co interface is more beneficial to the catalytic performance than the unconnected Pt–CoO interface and provides a promising strategy toward the design of advanced catalysts for the redox reaction.

Tetrafluoroborate-Monofluorophosphate (NH4)3[PO3F][BF4]: First Member of Oxyfluoride with B-F and P-F Bonds.

Inspired by the strategy of fluorine introduction in borates and phosphates, the inorganic oxyfluoride (NH4)3[PO3F][BF4] with B-F and P-F bonds has been characterized as the first fluoroborate-fluorophosphate. The International Union of Pure and Applied Chemistry (IUPAC) name for (NH4)3[PO3F][BF4] should be ammonium tetrafluoroborate-monofluorophosphate according to the structure characteristics. The existence and coordination of fluorine in (NH4)3[PO3F][BF4] were confirmed by several approaches, including single-crystal structure analysis; bond valence analysis; and X-ray energy dispersive, infrared spectrum, and also nuclear magnetic resonance spectroscopy. This work is of great significance to enrich the solid-state chemistry of borates and phosphates and also open a new branch of mixed anion compound with fluoroborate-fluorophosphates.

X-ray diffraction Rietveld analysis and Bond Valence analysis of nano titania containing oxygen vacancies synthesized via sol-gel route

Nano titanium dioxide (TiO2) synthesized by sol-gel route was calcined at different temperatures, the powder calcined at 673 K indicates biphasic mixture of brookite and anatase. On the other hand the powder calcined at 773 K exhibited three phase mixture of anatase, brookite and rutile phases. The microstrain present in the multiphasic powder was studied by the X-ray diffraction Williamson - Hall method. Detailed Rietveld analysis with line profile analysis combined with spherical harmonics indicated the anisotropic microstrain in the calcined powders. The strain was found to reduce with calcination temperature. The anisotropic microstrain is correlated to oxygen vacancies, which influence the crystal structure parameters of the phases in terms of their TiO6 octahederal bond length/bond angle values. The bond valence sum analysis of the powders confirms for the oxygen vacancy in the titania powders.

In Situ Structural Study of Sb(V) Adsorption on Hematite (11̅02) Using X-ray Surface Scattering.

The binding mechanism of Sb(V) on a single-crystal hematite (11̅02) surface was studied using crystal truncation rod X-ray diffraction (CTR) under in situ conditions. The best-fit CTR model indicates Sb(V) adsorbs at the surface as an inner-sphere complex forming a tridentate binding geometry with the nearest Sb-Fe distance of 3.09(4) Å and an average Sb-O bond length of 2.08(5) Å. In this binding geometry, Sb is bound at both edge-sharing and corner-sharing sites of the surface Fe-O octahedral units. The chemical plausibility of the proposed structure was further verified by bond valence analysis, which also deduced a protonation scheme for surface O groups. The stoichiometry of the surface reaction predicts the release of one OH- group at pH 5.5.

High air-stability and superior lithium ion conduction of Li3+3xP1-xZnxS4-xOx by aliovalent substitution of ZnO for all-solid-state lithium batteries

A series of new solid electrolytes of Li3+3xP1-xZnxS4-xOx (x = 0.01, 0.02, 0.03, 0.04, 0.05, 0.06) are synthesized successfully via Zn, O co-doping the Li3PS4 glass-ceramic for the first time. The result shows that Li3PS4 aliovalent substitution of 2 mol% ZnO (Li3.06P0.98Zn0.02S3.98O0.02) presents the highest conductivity of 1.12×10−3 S cm−1 at room temperature, which is twice that of the pristine Li3PS4. Besides, Li3.06P0.98Zn0.02S3.98O0.02 exhibits excellent stability against humid air, lithium metal and chlorobenzene solvent. The mechanisms of the enhancement of conductivity and air-stability are well understood by conducting first-principles density functional theory (DFT) calculation and Bond-Valence (BV) analysis, and the results well demonstrate that the conductivity and air-stability of Li3PS4 could be improved via Zn, O dual-doping, in which partial P5+ could be substituted by Zn2+, and a part of S2- could be replaced by O2-. Finally, the all-solid-state lithium battery (ASSLB) with bi-layer electrolytes of LiCoO2/Li10GeP2S12/Li3.06P0.98Zn0.02S3.98O0.02/Li is assembled, and it delivers an initial discharge capacity of 139.1 mAh g−1 at 0.1 C and a capacity retention of 81.0% after 100 cycles at room temperature. This work combines systematical experimental characterizations and sufficient theoretical calculations to develop a new promising sulfide electrolyte with superior lithium ion conductivity and high air-stability for ASSLBs application.

The crystal-structure determination and redefinition of eztlite, Pb22+Fe33+(Te4+O3)3(SO4)O2Cl

ABSTRACTThe crystal structure of eztlite has been determined using single-crystal synchrotron X-ray diffraction and supported using electron microprobe analysis and powder diffraction. Eztlite, a secondary tellurium mineral from the Moctezuma mine, Mexico, is monoclinic, space groupCm, witha= 11.466(2) Å,b= 19.775(4) Å,c= 10.497(2) Å, β = 102.62(3)° andV= 2322.6(9) Å3. The chemical formula of eztlite has been revised to${\rm Pb}_{\rm 2}^{2 +} {\rm Fe}_3^{3 +} $(Te4+O3)3(SO4)O2Cl from that stated previously as${\rm Fe}_6^{3 +} {\rm Pb}_{\rm 2}^{2 +} $(Te4+O3)3(Te6+O6)(OH)10·nH2O. This change has been accepted by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association, Proposal 18-A. Eztlite was reported originally to be a mixed-valence Te oxysalt; however the crystal structure, bond-valence analysis and charge balance considerations clearly show that all Te is tetravalent. Eztlite contains a unique combination of elements and is only the second Te oxysalt to contain both sulfate and chloride. The crystal structure of eztlite contains mitridatite-like layers, with a repeating triangular nonameric [${\rm Fe}_9^{3 +} $O36]45–arrangement formed by nine edge-sharing Fe3+O6octahedra, decorated by four trigonal pyramidal Te4+O3groups, compared to PO4or AsO4tetrahedra in mitridatite-type minerals. In eztlite, all four tellurite groups associated with one nonamer are orientated with the lone pair of the Te atoms pointing in the same direction, whereas in mitridatite the central tetrahedron is orientated in the opposite direction to the others. In mitridatite-type structures, interlayer connections are formed exclusivelyviaCa2+and water molecules, whereas the eztlite interlayer contains Pb2+, sulfate tetrahedra and Cl–. Interlayer connectivity in eztlite is achieved primarily by connectionsviathe long bonds of Pbφ8and Pbφ9groups to sulfate tetrahedra and to Cl–. Secondary connectivity isviaTe–O and Te–Cl bonds.

In situ structural study of the surface complexation of lead(II) on the chemically mechanically polished hematite ([formula omitted]) surface

A structural study of the surface complexation of Pb(II) on the (11¯02) surface of hematite was undertaken using crystal truncation rod (CTR) X-ray diffraction measurements under in situ conditions. The sorbed Pb was found to form inner sphere (IS) complexes at two types of edge-sharing sites on the half layer termination of the hematite (11¯02) surface. The best fit model contains Pb in distorted trigonal pyramids with an average PbO bond length of 2.27(4) Å and two characteristic Pb-Fe distances of 3.19(1) Å and 3.59(1) Å. In addition, a site coverage model was developed to simulate coverage as a function of sorbate-sorbate distance. The simulation results suggest a plausible Pb-Pb distance of 5.42 Å, which is slightly larger than the diameter of Pb’s first hydration shell. This relates the best fit surface coverage of 0.59(4) Pb per unit cell at monolayer saturation to steric constraints as well as electrostatic repulsion imposed by the hydrated Pb complex. Based on the structural results we propose a stoichiometry of the surface complexation reaction of Pb(II) on the hematite (11¯02) surface and use bond valence analysis to assign the protonation schemes of surface oxygens. Surface reaction stoichiometry suggests that the proton release in the course of surface complexation occurs from the Pb-bound surface O atoms at pH 5.5.

The crystal structure of cesbronite, Cu3TeO4(OH)4: a novel sheet tellurate topology

The crystal structure of cesbronite has been determined using single-crystal X-ray diffraction and supported by electron-microprobe analysis, powder diffraction and Raman spectroscopy. Cesbronite is orthorhombic, space group Cmcm, with a = 2.93172 (16), b = 11.8414 (6), c = 8.6047 (4) Å and V = 298.72 (3) Å3. The chemical formula of cesbronite has been revised to CuII 3TeVIO4(OH)4 from CuII 5(TeIVO3)2(OH)6·2H2O. This change has been accepted by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association, Proposal 17-C. The previously reported oxidation state of tellurium has been shown to be incorrect; the crystal structure, bond valence studies and charge balance clearly show tellurium to be hexavalent. The crystal structure of cesbronite is formed from corrugated sheets of edge-sharing CuO6 and (Cu0.5Te0.5)O6 octahedra. The structure determined here is an average structure that has underlying ordering of Cu and Te at one of the two metal sites, designated as M, which has an occupancy Cu0.5Te0.5. This averaging probably arises from an absence of correlation between adjacent polyhedral sheets, as there are two different hydrogen-bonding configurations linking sheets that are related by a ½a offset. Randomised stacking of these two configurations results in the superposition of Cu and Te and leads to the Cu0.5Te0.5 occupancy of the M site in the average structure. Bond-valence analysis is used to choose the most probable Cu/Te ordering scheme and also to identify protonation sites (OH). The chosen ordering scheme and its associated OH sites are shown to be consistent with the revised chemical formula.