XANES Research Articles

Intracellular biotransformation and disposal mechanisms3d cancer cells.

Magnetosomes are magnetite nanoparticles biosynthesized by magnetotactic bacteria. Given their potential clinical applications for the diagnosis and treatment of cancer, it is essential to understand what becomes of them once they are within the body. With this aim, here we have followed the intracellular long-term fate of magnetosomes in two cell types: cancer cells (A549 cell line), because they are the actual target for the therapeutic activity of the magnetosomes, and macrophages (RAW 264.7 cell line), because of their role at capturing foreign agents. We show that cells dispose of magnetosomes using three mechanisms: splitting them into daughter cells, excreting them to the surrounding environment, and degrading them yielding less or non-magnetic iron products. A deeper insight into the degradation mechanisms by means of time-resolved X-ray absorption near-edge structure (XANES) spectroscopy has allowed us to follow the intracellular biotransformation of magnetosomes by identifying and quantifying the iron species occurring during the process. In both cell types there is a first oxidation of magnetite to maghemite and then, earlier in macrophages than in cancer cells, ferrihydrite starts to appear. Given that ferrihydrite is the iron mineral phase stored in the cores of ferritin proteins, this suggests that cells use the iron released from the degradation of magnetosomes to load ferritin. Comparison of both cellular types evidences that macrophages are more efficient at disposing of magnetosomes than cancer cells, attributed to their role in degrading external debris and in iron homeostasis. This article is protected by copyright. All rights reserved.

X-ray Absorption Fine Structural Study of Atomic Structures and Chemical States of Dopants in 4H-SiC(0001)

The atomic structures and chemical states of active and inactive dopant sites in n-type and p-type 4H-SiC(0001) substrates have been investigated by X-ray absorption near edge structure (XANES) and photoelectron spectroscopy (PES). In N atom-doped n-type 4H-SiC(0001), the PES results indicated that a N–C species was formed near the surface. XANES simulations showed that SiNx and N–C species were attributed to active and inactive dopant states, respectively. Angle-resolved XANES measurements showed that the N–C species acted as an inactive dopant site in N-doped 4H-SiC(0001). In Al-doped p-type 4H-SiC, PES analysis revealed that a single chemical state was present at the Al-doped 4H-SiC. The simulated XANES spectra showed that the Si site in 4H-SiC(0001) was replaced by an Al dopant atom, which was the active dopant site. Furthermore, relaxation of the local structure around the Al dopant in Al-doped 4H-SiC(0001) was observed due to bond stretching.

Evidence for a Pressure-Induced Phase Transition in the Highly Distorted TlNiO3 Nickelate

Rare-earth perovskite nickelates RNiO3 (R stands for Y, rare earths, Tl or Bi) have attracted wide attention in the past few decades because of their unique electronic properties that emerge from the insulator–metal transition (IMT). This transition can be tuned by chemical substitutions or by external variables, like pressure and temperature, but the mechanism of this transition still remains debated. While most previous studies focused on nickelates with partially filled 4f cations on the R site, here we studied the mechanism of the pressure-induced phase transition on a rare Tl3+ nickelate, which comprises fully filled 4f orbitals. We probed the pressure-induced structural and electronic changes taking place at the bulk, medium, and local atomic level at room temperature by using synchrotron X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) as probing techniques, respectively. High-resolution XRD data under ambient conditions show evidence for the monoclinic phase of TlNiO3, as characterized by the charge ordering between Ni3−δ and Ni3+δ in the insulator regime. High-pressure XRD data elucidated an anomaly in the evolution of the lattice parameters at 10.8 GPa, which we assigned to the structural transition P21/n → Pbnm. High-pressure EXAFS data at the Ni K-edge provide a solid proof for a short-order charge disproportionation under near ambient conditions. The Ni–O pair-bond compression curves unveiled an isotropic overlapping Ni t2g6eg1 → O 2p, with an anomaly at 10.8 GPa. The pressure evolution of the bond-distance variance (Debye–Waller exponents) lodged a hardening at this pressure point, suggesting constrained atomic displacements in the average range of ±0.04(5) Å. We propose that this reduced motional freedom could be linked to an electron–phonon coupling, which is the driving force behind the mechanisms of the IMT. We demonstrated that the XANES technique can be applied to ascertain the pressure-induced transition, by tracking the anomalous behavior of the position and width (full width at half-maximum) of the XANES features.

A library of calcium mineral reference spectra recorded by parallel imaging using NEXAFS spectromicroscopy

Calcium minerals are ubiquitous in geology and life chemistry. Understanding the phase and chemical state of calcium minerals is important for numerous processes including materials chemistry, hard tissue biogenesis and geological processes. Photoemission spectroscopies such as near edge X-ray absorption fine structure (NEXAFS) and scanning transmission X-ray microscopy have been instrumental in identifying and characterizing calcium minerals in all these areas. In this work, we have recorded reference spectra for a range of different calcium minerals including a series of calcium carbonates, calcium oxalates and calcium phosphates. While collections of reference spectra for several calcium minerals can be found in the literature, these spectra have been reported in different contexts using a variety of instruments. We, here, report a comprehensive list of references recorded in parallel in a single experiment by imaging an array of calcium minerals using a NEXAFS microscope. We present reference NEXAFS spectra at the calcium L-, carbon K- and oxygen K-edges.

Understanding the dynamic evolution of atomically dispersed Cu catalyst for CO2 electrochemical conversion using integrated XANES analysis and mechanistic studies

Direct electrochemical conversion of CO2 to ethanol (CH3CH2OH) offers a promising strategy to lower CO2 emission while storing energy from renewable electricity. Our recent study reported a carbon-supported atomically dispersed Cu catalyst that achieved the highest reported selectivity for CH3CH2OH formation (91%) at a relatively low potential (-0.6 V), however, the active site structure that is responsible for such high activity and selectivity has yet to be understood. In this paper, we demonstrate a computational investigation combining X-ray absorption near edge structure (XANES) simulations and a mechanistic study via density functional theory (DFT) to understand the catalyst structures of this Cu catalyst during electrocatalysis and the corresponding reaction mechanisms of the key products. An integrated computational and experimental XANES analysis depicted the dynamic evolution of the catalytic site during electrocatalysis. The as-prepared, atomically dispersed Cu catalyst aggregates and forms metallic clusters/nanoparticles under electrochemical condition, which then break down to smaller oxidized clusters after electrocatalysis. The formed Cu clusters/nanoparticles showed distinct catalytic activity and selectivity as a function of particle size based on the mechanistic investigation using DFT, which is consistent with experimental observations for catalyst samples with different Cu loadings. This comprehensive study which combines experimental and computational XANES investigation, mechanistic study via DFT calculations, and experimental performance of the catalysts, provides unprecedented dynamic and mechanistic insights into the supported atomically dispersed metal catalysts for CO2 reduction. Such strategy and details gained can further guide discovery of novel catalyst materials for CO2 electrochemical reduction.

Uncovering the 3d and 4d Electronic Interactions in Solvated Ru Complexes with 2p3d Resonant Inelastic X-ray Scattering.

The electronic structure and dynamics of ruthenium complexes are widely studied given their use in catalytic and light-harvesting materials. Here we investigate three model Ru complexes, [RuIII(NH3)6]3+, [RuII(bpy)3]2+, and [RuII(CN)6]4-, with L3-edge 2p3d resonant inelastic X-ray scattering (RIXS) to probe unoccupied 4d valence orbitals and occupied 3d orbitals and to gain insight into the interactions between these levels. The 2p3d RIXS maps contain a higher level of spectral information than the L3 X-ray absorption near edge structure (XANES). This study provides a direct measure of the 3d spin-orbit splittings of 4.3, 4.0, and 4.1 eV between the 3d5/2 and 3d3/2 orbitals of the [RuIII(NH3)6]3+, [RuII(bpy)3]2+, and [RuII(CN)6]4- complexes, respectively.

Temporal Ni K-Edge X-ray Absorption Spectroscopy Study Reveals the Kinetics of the Ni Redox Behavior of the Iron-Nickel Oxide Bimetallic OER Catalyst

Operando X-ray absorption spectroscopy (XAS) can be utilized to probe the phase and structural changes of FeNiOx and similar transition metal oxide electrocatalysts during electrocatalytic reactions. However, capturing the temporal changes occurring in the operando chemistry of electrocatalysts has been little studied. In this work, we successfully capture the time-resolved changes at the Ni K-edge for both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions of an aqueous phase synthesized FeNiOx bimetallic nanoparticle electrocatalyst. During a stepped voltage experiment, the temporal change in the Ni K-edge was observed as the applied voltage was stepped from 0.7 to 0.8 V versus the silver/silver chloride reference electrode, and the change observed is associated with the Ni redox transition from the 2+ to 3+/4+ oxidation state. Individual XAS spectra were obtained in 90 s, and a total of 9 scans post voltage step showed a unique transition from a hydroxide to an oxyhydroxide phase. The shift in absorption edge energy position and the changes in spectral shape for individual scans explain the typically observed broadening of the Ni K-edge XANES spectrum during time-averaged operando XAS, and a kinetic analysis revealed a first-order observed rate constant, |kobs|, of 0.00426 s–1 and a half-life (t1/2) of 163 s. Multivariate curve resolution-alternating least squares analysis followed by linear combination fitting analysis for time-averaged versus time-resolved Ni K-edge changes upon voltage step show distinct differences in estimated contributions from Ni2+ oxide/hydroxide phases. Detailed EXAFS modeling shows the phase transition from hydroxide to oxyhydroxide on top of the unchanged metallic core.

The origin of Ti 1s XANES main edge shifts and EXAFS oscillations in the energy storage materials Ti2CT x and Ti3C2T x MXenes

A potential application of two-dimensional (2D) MXenes, such as Ti2CT x and Ti3C2T x , is energy storage devices, such as supercapacitors, batteries, and hydride electrochemical cells, where intercalation of ions between the 2D layers is considered as a charge carrier. Electrochemical cycling investigations in combination with Ti 1s x-ray absorption spectroscopy have therefore been performed with the objective to study oxidation state changes during potential variations. In some of these studies Ti3C2T x has shown main edge shifts in the Ti 1s x-ray absorption near-edge structure. Here we show that these main edge shifts originate from the Ti 4p orbital involvement in the bonding between the surface Ti and the termination species at the fcc-sites. The study further shows that the t 2g –eg crystal field splitting (10Dq) observed in the pre-edge absorption region indicate weaker Ti–C bonds in Ti2CT x and Ti3C2T x compared to TiC and the corresponding MAX phases. The results from this study provide information necessary for improved electronic modeling and subsequently a better description of the materials properties of the MXenes. In general, potential applications, where surface interactions with intercalation elements are important processes, will benefit from the new knowledge presented.

In-Plane Hydrogen Bonds and Out-of-Plane Dipolar Interactions in Self-Assembled Melem Networks

Melem (2,6,10-triamino-s-heptazine) is the building block of melon, a carbon nitride (CN) polymer that is proven to produce H2 from water under visible illumination. With the aim of bringing additional insight into the electronic structure of CN materials, we performed a spectroscopic characterization of gas-phase melem and of a melem-based self-assembled 2D H-bonded layer on Au(111) by means of ultraviolet and X-ray photoemission spectroscopy (UPS, XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. In parallel, we performed density functional theory (DFT) simulations of the same systems to unravel the molecular charge density redistribution caused by the in-plane H-bonds. Comparing the experimental results with the spectroscopic DFT simulations, we can correlate the induced charge accumulation on the Namino atoms to the red-shift of the corresponding N 1s binding energy (BE) and of the Namino 1s → LUMO+n transitions. Moreover, when introducing a supporting Au(111) surface in the computational simulations, we observe a molecule–substrate interaction that almost exclusively involves the out-of-plane molecular orbitals, leaving those engaged in the in-plane H-bonded network rather unperturbed.

Deposition of amorphous carbon nitride thin films using pressure-gradient RF magnetron sputtering and their chemical bonding structures

Amorphous carbon nitride (a-CNx) thin films were deposited through radio frequency (RF) magnetron sputtering under increasing nitrogen gas pressure around a graphite target to provide nitrogen radicals at high concentrations. The basic properties of the films, mainly chemical bonding structures, were evaluated using X-ray photoelectron spectroscopy (XPS), soft X-ray emission spectroscopy (SXES), and near-edge X-ray absorption fine structure (NEXAFS), and compared with other films deposited through the common sputtering mode, i.e., at a uniform gas pressure in the chamber. To increase the gas pressure around the graphite target, the deposition rate was slightly increased, and the chemical bonding structure of the a-CNx films was changed. However, the nitrogen content in the a-CNx films obtained from XPS analysis did not increase when the nitrogen gas pressure around the graphite target was increased. The effect of nitrogen incorporation on the a-CNx films was comparatively more effective at higher RF powers. The intensity ratio of sp3-σ and sp2-σ in the SXES spectra of the films increased with increasing RF power, from 85 W to 130 W, then decreased with a further increase in the RF power. This trend was confirmed by the peaks in the C K-edge of the NEXAFS spectra.

Synthesis, Characterization, and Stability of Two Americium Vanadates, AmVO3 and AmVO4.

In search for chemically stable americium compounds with high power densities for radioisotope sources for space applications, AmVO3 and AmVO4 were prepared by a solid-state reaction. We present here their crystal structure at room temperature solved by powder X-ray diffraction combined with Rietveld refinement. Their thermal and self-irradiation stabilities have been studied. The oxidation states of americium were confirmed by the Am M5 edge high-resolution X-ray absorption near-edge structure (HR-XANES) technique. Such ceramics are investigated as potential power sources for space applications like radioisotope thermoelectric generators, and they have to endure extreme conditions including vacuum, high or low temperatures, and internal irradiation. Thus, their stability under self-irradiation and heat treatment in inert and oxidizing atmospheres was tested and discussed relative to other compounds with a high content of americium.

Platinum nanoparticle modulated titania electronic structure descriptors for selective photocatalytic CO2 conversion

Modulation of the electronic structure of TiO2 nanorod array (NR), which governs the selectivity for the photocatalytic CO2 conversion, is demonstrated by the deposition of Pt nanoparticles (NPs) with various configurations. In-situ X-ray absorption near edge structure (XANES) measurements suggest that the active sites for the photocatalytic reaction are located on the surface of TiO2 NRs, but not on the Pt NPs, because the photocharge transfer from TiO2 NRs to Pt NPs is inefficient. The t2g/eg orbital ratio and dz2/dx2-y2 orbital ratio of TiO2 acquired from Ti L3-edge spectra of the Pt NP/TiO2 NR photocatalysts, which are strongly influenced by the feature of the deposited Pt NPs, are respectively well-correlated to the experimental selectivity of H2 and CH4 for photocatalytic CO2 conversion as well as the density functional theory calculated adsorption energies of H atom and CO molecule on the TiO2 surface. The Pt NPs thus play a crucial role as a promoter rather than as a cocatalyst. Besides the adsorption energies of H and CO on TiO2, the t2g/eg and dz2/dx2-y2 ratios of Ti 3d unoccupied state can be the electronic structure-based descriptors of TiO2 heterostructures for photocatalytic CO2 conversion to distinguish between H2 evolution and CH4 formation.

Versatile tabletop setup for picosecond time-resolved resonant soft-x-ray scattering and spectroscopy.

We present a laser-driven, bright, and broadband (50 to 1500eV) soft-x-ray plasma source with <10ps pulse duration. This source is employed in two complementary, laboratory-scale beamlines for time-resolved, magnetic resonant scattering and spectroscopy, as well as near-edge x-ray absorption fine-structure (NEXAFS) spectroscopy. In both beamlines, dedicated reflection zone plates (RZPs) are used as single optical elements to capture, disperse, and focus the soft x rays, reaching resolving powers up to E/ΔE > 1000, with hybrid RZPs at the NEXAFS beamline retaining a consistent E/ΔE > 500 throughout the full spectral range, allowing for time-efficient data acquisition. We demonstrate the versatility and performance of our setup by a selection of soft-x-ray spectroscopy and scattering experiments, which so far have not been possible on a laboratory scale. Excellent data quality, combined with experimental flexibility, renders our approach a true alternative to large-scale facilities, such as synchrotron-radiation sources and free-electron lasers.

Effect of Re Addition on the Water–Gas Shift Activity of Ni Catalyst Supported by Mixed Oxide Materials for H2 Production

Water–gas shift (WGS) reaction was performed over 5% Ni/CeO2, 5% Ni/Ce-5% Sm-O, 5% Ni/Ce-5% Gd-O, 1% Re 4% Ni/Ce-5% Sm-O and 1% Re 4% Ni/Ce-5% Gd-O catalysts to reduce CO concentration and produce extra hydrogen. CeO2 and M-doped ceria (M = Sm and Gd) were prepared using a combustion method, and then nickel and rhenium were added onto the mixed oxide supports using an impregnation method. The influence of rhenium, samarium and gadolinium on the structural and redox properties of materials that have an effect on their water–gas shift activities was investigated. It was found that the addition of samarium and gadolinium into Ni/CeO2 enhances the surface area, reduces the crystallite size of CeO2, increases oxygen vacancy concentration and improves Ni dispersion on the CeO2 surface. Moreover, the addition of rhenium leads to an increase in the WGS activity of Ni/CeMO (M = Sm and Gd) catalysts. The results indicate that 1% Re 4% Ni/Ce-5% Sm-O presents the greatest WGS activity, with the maximum of 97% carbon monoxide conversion at 350 °C. An increase in the dispersion and surface area of metallic nickel in this catalyst results in the facilitation of the reactant CO adsorption. The result of X-ray absorption near-edge structure (XANES) analysis suggests that Sm and Re in 1% Re 4% Ni/Ce-5% Sm-O catalyst donate some electrons to CeO2, resulting in a decrease in the oxidation state of cerium. The occurrence of more Ce3+ at the CeO2 surface leads to higher oxygen vacancy, which alerts the redox process at the surface, thereby increasing the efficiency of the WGS reaction.

Synthesis of a powerful single copper/tungsten atom oxide photocatalyst dispersed on the surface of a reduced graphene oxide-titanium composite for H2 production and pollutant degradation

The specific activity and selectivity of many catalytic processes have been frequently enhanced, making single-atom catalysis a new frontline in the area of catalysis. In this study, a powerful photocatalyst was synthesized for H2 production and ofloxacin (OFL) degradation, utilizing a catalytically active TiO2-reduced graphene oxide (rGO) support material. A Cu mono-substituted heteropoly acid(HPA) was used to generate surface-dispersed single copper/tungsten atom oxide (SCu/WAO) sites as co-catalysts. Many single atoms were homogeneously deposited on the TiO2-rGO catalyst. A TiO2-SCu/WAO-rGO catalyst was successfully prepared, and the presence of the SCu/WAO species was confirmed using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), extended X-ray absorption fine structure (EXAFS), and X-ray absorption near edge structure (XANES) analyses. The prepared photocatalyst was found to facilitate high rates of H2 production (14.32 mmol/g/h) and a high maximum OFL degradation (98.4%). Additionally, it could effectively substitute current noble metal catalysts as an extremely effective and long-lasting solar radiation-based photocatalyst for renewable H2production and OFL degradation.

Modeling and spectroscopic investigation of U(VI) removal on porous amidoxime-functionalized metal organic framework derived from macromolecular carbohydrate

The investigation of interaction mechanism of U(VI) selective removal on amidoxime-functionalized metal organic framework (i.e., UiO-66(Zr)-AO) derived from macromolecular carbohydrate is conducive to apply metal organic frameworks in actual environmental remediation. The batch experiments showed that UiO-66(Zr)-AO displayed the fast removal rate (equilibrium time of 0.5 h), high adsorption capacity (384.6 mg/g), excellent regeneration performance (<10 % decrease after three cycles) towards U(VI) removal due to the unprecedented chemical stability, large surface area and simple fabrication. U(VI) removal at different pH can be satisfactorily fitted by diffuse layer modeling with cation exchange at low pH and an inner-sphere surface complexation at high pH. The inner-sphere surface complexation was further demonstrated by X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis. These findings revealed that UiO-66(Zr)-AO can be an effective adsorbent to remove the radionuclides from aqueous solution, which is crucial for recycling of uranium resource and decreasing the uranium harm to the environment.

Spatial variability of aerosol iron mineralogy and oxidation states over the Arctic Ocean

The mineralogy and oxidation state of aerosol iron (Fe) play important roles in controlling aerosol Fe solubility and consequent bioavailability in seawater. In this study, the spatial variability of Fe mineralogy and oxidation states in aerosols collected during the US GEOTRACES Western Arctic cruise (GN01) were determined using synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy. Both Fe(II) minerals (biotite, ilmenite) and Fe(III) minerals (ferrihydrite, hematite, Fe(III) phosphate) were found in these samples. However, aerosol Fe mineralogy and solubility observed during this cruise varied spatially and can be grouped into three clusters based on the air masses that affected aerosols collected in different regions: (1) biotite-enriched particles (87 % biotite, 13 % hematite) with the air masses passing over Alaska, showing relatively low Fe solubility (4.0 ± 1.7 %); (2) ferrihydrite-enriched particles (82 % ferrihydrite, 18 % ilmenite) collected in the remote Arctic air, showing relatively high Fe solubility (9.6 ± 3.3 %); (3) the fresh dust derived from North America and Siberia, primarily dominated by hematite (41 % hematite, 25 % Fe(III) phosphate, 20 % biotite, 13 % ferrihydrite), showing relatively low Fe solubility (5.1 ± 3.5). A significant positive correlation was found between Fe oxidation state and Fe fractional solubility, suggesting that long-range transport could modify iron (hydr) oxide such as ferrihydrite through atmospheric processing, influencing aerosol Fe solubility and consequently Fe bioavailability in the remote Arctic Ocean.

Phase evolution in the UO2–CeO2 system under oxidizing and reducing conditions: X-ray diffraction and spectroscopic studies

The solid-solution formation mechanism and defect dynamics in U1-xCexO2±δ solid solutions have been investigated by means of X-ray diffraction (XRD) analysis, Raman spectroscopy, and X-ray absorption near-edge structure (XANES) spectroscopy. Diffraction studies have revealed the formation of single-phase solid solutions with a fluorite-type structure at x ≤ 0.2 and a single-phase fluorite-type structure with broadened diffraction peaks, attributed to inhomogeneity in the O/M ratio, at 0.3 ≤ x ≤ 0.8 under reducing conditions. Oxidizing conditions result in the formation of single-phase solid solutions with fluorite-type structure at x ≥ 0.4. Upon the formation of Ce3+ in solid solutions at x ≤ 0.2 under reducing conditions, charge neutrality is achieved through oxygen vacancy creation and formation of higher oxidation states of uranium through a charge compensation mechanism. Lattice parameter–composition relationships for fluorite-type phases synthesized under oxidizing conditions show a decrease in lattice parameters as compared to their stoichiometric counterparts due to the oxidation of U4+ to U5+/U6+. XANES studies provide direct experimental evidence of the presence of Ce3+ in solid solutions, despite synthesis under oxidizing conditions.

Decoding structure-spectrum relationships with physically organized latent spaces

A new semi-supervised machine learning method for the discovery of structure-spectrum relationships is developed and demonstrated using the specific example of interpreting X-ray absorption near-edge structure (XANES) spectra. This method constructs a one-to-one mapping between individual structure descriptors and spectral trends. Specifically, an adversarial autoencoder is augmented with a novel rank constraint (RankAAE). The RankAAE methodology produces a continuous and interpretable latent space, where each dimension can track an individual structure descriptor. As a part of this process, the model provides a robust and quantitative measure of the structure-spectrum relationship by decoupling intertwined spectral contributions from multiple structural characteristics. This makes it ideal for spectral interpretation and the discovery of new descriptors. The capability of this procedure is showcased by considering five local structure descriptors and a database of over fifty thousand simulated XANES spectra across eight first-row transition metal oxide families. The resulting structure-spectrum relationships not only reproduce known trends in the literature, but also reveal unintuitive ones that are visually indiscernible in large data sets. The results suggest that the RankAAE methodology has great potential to assist researchers to interpret complex scientific data, test physical hypotheses, and reveal new patterns that extend scientific insight.

The Ti-in-zircon thermometer revised: The effect of pressure on the Ti site in zircon

The Ti content of zircon (ZrSiO4) can be used as a geothermometer. Ti could substitute for Si or Zr in zircon, which would result in opposing responses of Ti solubility to (i) the activities of SiO2 (aSiO2quartz/coesite) and TiO2 (aTiO2rutile) and (ii) changes in pressure due to differences in the molar volumes of the components ZrTiO4 and TiSiO4 relative to ZrSiO4. Previous calibrations of the Ti-in-zircon thermometer assumed that Ti occupies the Si-site. X-ray absorption near edge structure (XANES) spectroscopy has confirmed that Ti is tetrahedrally coordinated on the Si-site at atmospheric pressure, however, Ti is predicted to change to the Zr-site at higher pressures. Ti-bearing zircons were prepared at 1400 °C and pressures from 0.0001 to 6.5 GPa using a flux-method that produced crystals up to 50 μm in size. Ti K-edge XANES spectra of zircons prepared at < 4.0 GPa are characterised by an intense pre-edge peak (attributed to the 1s → 3d transition), which is consistent with Ti in tetrahedral coordination on the Si-site. The intensity of this peak is proportional to the amount of Ti on the Si-site, and decreases abruptly > 4.0 GPa (accompanied by a change in the energy and shape of the absorption edge), indicating a change in the coordination of Ti. Ti-O bond lengths determined from extended X-ray absorption fine structure (EXAFS) spectra of 1.788(7) Å at 1.0 GPa and both 1.87(4) Å and 2.21(2) Å at 6.5 GPa are consistent with Ti changing from the Si-site to the larger Zr-site. The proportion of Ti on each site varies systematically with pressure, with equal proportions of Ti on both sites at ∼ 4.4 GPa. Sole occupancy of Ti on the Zr-site is predicted to occur at pressures coinciding with the transition of zircon to reidite (∼ 8.0 GPa). This change in site is consistent with a systematic decrease in the Ti content of these zircons with increasing pressure when aSiO2quartz/coesite and aTiO2rutile = 1. As a result, an updated model for estimating zircon crystallisation temperatures from the Ti content was derived:logTif=5.847-480086T-0.121P-0.005615P3-logaSiO2quartz/coesitef+logaTiO2rutilewhere Ti is in ppm, f is the fraction of Ti on the Si-site given by 1/(1 + 10−(3.37(13) − 0.77(5)P)), T is temperature in K, P is pressure in GPa, and the uncertainties are 1σ. For zircons with independent estimates of temperature, pressure, aSiO2quartz/coesite and aTiO2rutile this new model predicts crystallisation temperatures within 40 °C (or 8 %) of the independent temperature estimates at all pressures. This contrasts with previous models for which predicted temperatures are within certainty of nominal temperatures at ∼ 1.0 GPa, but underestimated (by up to 70 °C) at pressures < 1.0 GPa and underestimated (by up to 300 °C) at higher pressures where the proportion of Ti on the Zr-site should be significant.