IVB Metal Research Articles

The effects of refractory elements on the properties of quaternary high entropy carbides—A first-principles and experiment study

In this work, the effects of refractory transition metal elements on the properties of quaternary equimolar ratio high entropy carbides composed of ⅣB, ⅤB and ⅥB transition metals and C element were studied by first-principles calculations and experiments.The calculation results show that all 75 kinds of high entropy carbides were thermodynamically and mechanically stable and can be formed spontaneously. Among them, Hf can promote the synthesis of high entropy carbides, while Cr has the opposite effect. In addition, single phase solid solution structures can be formed for all 73 HECs except (TiZrHfCr)C and (ZrHfVCr)C. Thereafter, the mechanical properties of these 75 quaternary HECs were compared. The calculation results show that W, Mo, and Ta have a positive effect on improving the elastic modulus, fracture toughness, hardness and melting point of high entropy carbides, while Cr has the opposite effect. The calculation results of phonon spectra show that the six high entropy carbides represented by (TiHfNbTa)C, (TiVNbTa)C, (TiZrNbTa)C, (TiZrNbCr)C, (TiZrNbMo)C and (TiZrNbW)C have lattice dynamic stability. (TiHfNbTa)C, (TiVNbTa)C, (TiZrNbTa)C, (TiZrNbMo)C and (TiZrNbW)C face-centered cubic high entropy carbides were prepared by spark plasma sintering and the microstructure and properties were tested. The experimental results are basically consistent with the calculated results.

Propane dehydrogenation catalysis of group IIIB and IVB metal hydrides.

Catalytic propane dehydrogenation (PDH) has mainly been studied using metal- and metal oxide-based catalysts. Studies on dehydrogenation catalysis by metal hydrides, however, have rarely been reported. In this study, PDH reactions using group IIIB and IVB metal hydride catalysts were investigated under relatively low-temperature conditions of 450 °C. Lanthanum hydride exhibited the lowest activation energy for dehydrogenation and the highest propylene yield. Based on kinetics studies, a comparison between the reported calculation results and isotope experiments, the hydrogen vacancies of metal hydrides were involved in low-temperature PDH reactions.

Synthesis of Group IIB and IVB Metal Complexes of 3‐Nitroaniline for Photocatalytic Degradation of Organic Dyes

AbstractIn the current research, new metal complexes have been synthesized by using the greener synthetic approach in which 3‐Nitroaniline reacts with the elements of group‐IIB and group‐IVB. According to this method, ligand and metals were dissolved in 1,4‐dioxane and stirred at room temperature for a specific time period. The method for the preparation of metal‐base complexes is quite simple and cost‐effective. To identify the synthesized compounds various physio‐chemical analytical techniques were used such as Melting point, FT‐Raman Spectroscopy, FT‐IR Spectroscopy, Thermogravimetric analysis, and Scanning electron microscope (SEM) that provide significant data about the general structural characteristics of synthesized compounds. The percentage degradation efficiency of synthetic dyes was obtained as 57 %, 66 %, and 97 % using the synthesized compounds as photocatalysts. A statistical tool, RSM was used for the optimization of parameters (Concentration of Dye, Dose of Catalyst, and contact time).

Design of high-temperature superconductors at moderate pressures by alloying AlH3 or GaH3

Since the discovery of hydride superconductors, a significant challenge has been to reduce the pressure required for their stabilization. In this context, we propose that alloying could be an effective strategy to achieve this. We focus on a series of alloyed hydrides with the AMH6 composition, which can be made via alloying A15 AH3 (A = Al or Ga) with M (M = a group ⅢB or IVB metal), and study their behavior under pressure. Seven of them are predicted to maintain the A15-type structure, similar to AH3 under pressure, providing a platform for studying the effects of alloying on the stability and superconductivity of AH3. Among these, the A15-type phases of AlZrH6 and AlHfH6 are found to be thermodynamically stable in the pressure ranges of 40–150 and 30–181 GPa, respectively. Furthermore, they remain dynamically stable at even lower pressures, as low as 13 GPa for AlZrH6 and 6 GPa for AlHfH6. These pressures are significantly lower than that required for stabilizing A15 AlH3. Additionally, the introduction of Zr or Hf increases the electronic density of states at the Fermi level compared with AlH3. This enhancement leads to higher critical temperatures (Tc) of 75 and 76 K for AlZrH6 and AlHfH6 at 20 and 10 GPa, respectively. In the case of GaMH6 alloys, where M represents Sc, Ti, Zr, or Hf, these metals reinforce the stability of the A15-type structure and reduce the lowest thermodynamically stable pressure for GaH3 from 160 GPa to 116, 95, 80, and 85 GPa, respectively. Particularly noteworthy are the A15-type GaMH6 alloys, which remain dynamically stable at low pressures of 97, 28, 5, and 6 GPa, simultaneously exhibiting high Tc of 88, 39, 70, and 49 K at 100, 35, 10, and 10 GPa, respectively. Overall, these findings enrich the family of A15-type superconductors and provide insights for the future exploration of high-temperature hydride superconductors that can be stabilized at lower pressures.

Plasmonic Titanium Nitride Nanohole Arrays for Refractometric Sensing.

Group IVB metal nitrides have attracted great interest as alternative plasmonic materials. Among them, titanium nitride (TiN) stands out due to the ease of deposition and relative abundance of Ti compared to those of Zr and Hf metals. Even though they do not have Au or Ag-like plasmonic characteristics, they offer many advantages, from high mechanical stability to refractory behavior and complementary metal oxide semiconductor-compatible fabrication to tunable electrical/optical properties. In this study, we utilized reactive RF magnetron sputtering to deposit plasmonic TiN thin films. The flow rate and ratio of Ar/N2 and oxygen scavenging methods were optimized to improve the plasmonic performance of TiN thin films. The stoichiometry and structure of the TiN thin films were thoroughly investigated to assess the viability of the optimized operation procedures. To assess the plasmonic performance of TiN thin films, periodic nanohole arrays were perforated on TiN thin films by using electron beam lithography and reactive ion etching methods. The resulting TiN periodic nanohole array with varying periods was investigated by using a custom microspectroscopy setup for both reflection and transmission characteristics in various media to underline the efficacy of TiN for refractometric sensing.

Unlocking AlN Piezoelectric Performance with Earth‐Abundant Dopants

AbstractThe increasing demand for high‐performance piezoelectric materials and toxicity and thermal stability issues of the widely used lead zirconate titanates (PZT) have spurred a search for better alternatives in electronic devices. In comparison to PZT, group III nitrides such as aluminum nitride (AlN), are only weakly piezoelectric, but doping AlN with scandium (Sc) improves the piezoelectric response by nearly 500%. Relative to PZT, doped‐AlN piezoelectric materials are advantageous because they are far more compatible with complementary metal–oxide–semiconductor (CMOS) materials, and they maintain both piezoelectric and thermodynamic stability up to very high temperatures. Unfortunately, rare‐earth metals are notoriously expensive, and fabricating stable films with rare‐earth dopants is also challenging, limiting their use in industrial applications. In this work, ab initio calculations are combined with targeted fabrication and experimentation to identify alternative earth‐abundant dopants for AlN from the periodic table d‐block. Amongst the 23 elements screened, it is found that group IVB metals, titanium, zirconium, and hafnium induce large piezoelectric enhancements comparable to Sc. This improvement is traced to shifts in the atomic sublattice structure and changes in the local charge states. In demonstrating a highly accessible and affordable path for technological adaptation of AlN‐based piezoelectrics, this work provides the foundation for sustainable, next‐generation electronics.

Insights into Lewis/Brønsted acidity of metal chlorides and solvent effect of alcohols for synthesis of γ-valerolactone by combining molecular dynamics simulations and experiments

The synthesis of biofuel γ-valerolactone (GVL) from furfural is a multistep process that entails the regulation of Lewis/Brønsted (L/B) acidity of catalyst and the hydrogen donation ability of alcohols. In this study, experiments and molecular dynamics (MD) simulations were combined to get insights into the role of catalysts and solvents during the conversion of furfural to GVL. The capacities of various metal chlorides (divalent to hexavalent metal elements) and alcohols (primary and secondary alcohols) were comparatively investigated. The results showed that chlorides of Group IVB metals (ZrCl4 and HfCl4) exhibited excellent catalytic performance and achieved GVL yields of 61.88 % and 68.13 %, respectively. By preliminary statistical assessment, the intervals in which high isopropyl levulinate (IPL) yields and high GVL yields were found corresponded to different L/B acidity (3.5 < N < 4.8, −2 < Z < 1 and N < 5.7, Z > 1.5) (Z: Lewis acid strength. N: number of molecules in the shell). Secondary alcohols displayed the lower reduction potential but the higher interaction energy with reactants than that of primary alcohols. The distribution of Hf4+ around the reactants was tendentious, whereas the distribution of C3H7OH2+ was more homogeneous. The investigation of reaction mechanism proposed new understandings of the reacted sites of reactants and the hydrogen transfer of solvent to reactants. The hypothesis that IPL was capable of hydrogen/deuterium (H/D) exchange was developed.

Two-dimensional MSi2N4 monolayers and van der Waals heterostructures: Promising spintronic properties and band alignments

A class of septuple-atomic-layer two-dimensional (2D) materials, $\mathrm{Mo}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ and $\mathrm{W}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$, was fabricated recently, and the three constituent atoms may be replaced by other elements in the Periodic Table, thus forming a large family. In the current work, using density functional theory calculations, we systematically investigate 18 $M{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ (where $M$ denotes groups IV-B, V-B, and VI-B transition metals) monolayers in both H- and T-phases for their stabilities and their electronic, magnetic, and spintronic properties, and we highlight the spintronic properties and their van der Waals heterostructures. The electronic and magnetic properties of these 2D monolayers are correlated with the $d$-levels splitting (from crystal field and exchange field) and electron filling. Spin-dependent band alignment of $M{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ heterostructures shows that most of them are type II heterostructures for both spin channels. In addition, the valence-band edge of seven $M{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ monolayers resides at the Brillouin zone center ($\mathrm{\ensuremath{\Gamma}}$-point), which may form momentum-matched heterostructures by stacking with other 2D semiconductors. The ferromagnetic semiconductors (H-$\mathrm{V}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$, H-$\mathrm{Nb}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$, and H-$\mathrm{Ta}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$) and the heterostructures composed of them (e.g., H-$\mathrm{Mo}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$/H-$\mathrm{V}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ or H-$\mathrm{W}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$/H-$\mathrm{V}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$) can be a half-semiconductor or half-metal. These results, in addition to the literature reporting high Curie temperatures, indicate that $M{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ and their heterostructures are promising for room-temperature spintronics and optoelectronics.

The role of entropy and enthalpy in high entropy carbides

The thermodynamic stability of equiatomic mixed carbides, commonly referred to as high entropy carbides (HECs), is investigated via the CALculation of PHAse Diagrams (CALPHAD) approach for mixed carbides consisting of the group IVB and VB transition metal carbides as well as tungsten carbide. The Gibbs free energy of the B1-structured mixed carbides is computed using the compound energy formalism while that of the Bh-structured mixed carbides is evaluated using a point-defect model. The required thermodynamic data for the CALPHAD approach are obtained from density functional theory calculations and the Debye-Grüneisen model. The lower temperature limit at which the HECs mix in thermodynamic equilibrium is determined via numerical and analytical approaches. We find that enthalpy of mixing is at least as important as configurational mixing entropy in these mixed transition metal carbide compounds. The lower limit temperature where an equiatomic solid solution is present is largely independent of the number of components with the only exception being solutions containing tungsten carbide, where a weak temperature dependence is noted. Furthermore, the only equiatomic solid solutions that are thermodynamically stable below approximately 1000 K are those stabilized by enthalpy alone, indicating that many currently fabricated HECs are not at equilibrium at room temperature. Collectively, our results demonstrate that the formation of these carbides is controlled by the competition between entropy and enthalpy, or enthalpy alone, and thus these materials should be referred to as multi-principal component carbides since the former terminology can be misleading.

Understanding the role of anharmonic phonons in diffusion of bcc metals

Diffusion in the high-temperature bcc phase of IIIB-IVB metals such as Zr, Ti, and their alloys is observed to be orders of magnitude higher than bcc metals of group VB-VIB, including Cr, Mo, and W. The underlying reason for this higher diffusion is still poorly understood. To explain this observation, we compare the first-principles-calculated parameters of monovacancy-mediated diffusion between bcc Ti, Zr, and dilute Zr- Sn alloys and bcc Cr, Mo, and W. Our results indicate that strongly anharmonic vibrations promote both the vacancy concentration and the diffusive jump rate in bcc IVB metals and can explain their markedly faster diffusion compared to bcc VIB metals. Additionally, we provide an efficient approach to calculate diffusive jump rates according to the transition state theory (TST). The use of standard harmonic TST is impractical in bcc IIIB/IVB metals due to the existence of ill-defined harmonic phonons, and most studies use classical or ab initio molecular dynamics for direct simulation of diffusive jumps. Here, instead, we use a stochastically sampled temperature-dependent phonon analysis within the transition state theory to study diffusive jumps without the need of direct molecular dynamics simulations. We validate our first-principles diffusion coefficient predictions with available experimental measurements and explain the underlying reasons for the promotion of diffusion in bcc IVB metals/alloys compared to bcc VIB metals.

Stability and mechanical properties of single-phase quinary high-entropy metal carbides: First-principles theory and thermodynamics

We have employed thermodynamics and first-principles density-functional calculations to investigate the structural stability and mechanical properties of fifty-six quinary high-entropy metal carbides composed of carbon and Groups IVB, VB, and VIB refractory transition metals, Ti, Zr, Hf, V, Nb, Ta, Mo, and W, thirty-eight of which have not yet been synthesized. To determine the stability of the quinary high-entropy metal carbides, we have constructed a three-dimensional phase diagram in terms of the average melting point, mixing enthalpy, mixing entropy, and lattice size difference, from which we predict that it is feasible to synthesize 38 new high-entropy metal carbides. We have further found that all the 56 metal carbides would have unique mechanical properties of high hardness and high fracture toughness. In addition, our study suggests that the brittleness of high-entropy metal carbides steadily decreases with the increase of the valence electron concentration.

Phase diagram and mechanical properties of fifteen quaternary high-entropy metal diborides: First-principles calculations and thermodynamics

First-principles calculations and thermodynamic theory of mixing entropy and enthalpy are employed to study the phase stability, mechanical properties, and melting points of 15 existing and hypothetical quaternary high-entropy metal diborides (HEMB2s) composed of boron and six group IVB and VB refractory transition metals. A phase diagram in terms of a structural parameter, δ (the lattice size difference), and two thermodynamic parameters, ΔHmix (the mixing enthalpy) and Ω (the ratio of the entropy and enthalpy terms) is constructed. The phase diagram shows that all the 15 metal diborides satisfy the established Ω-δ criterion (i.e. Ω&amp;gt;1 and δ&amp;lt;6.6%), suggesting that they can be formed as single-phase HEMB2s. While five of these equiatomic four-metal diborides were experimentally synthesized already, the remaining ten single-phase HEMB2s are predicted by this work. Each of the 15 quaternary HEMB2s is found to have high Vickers hardness and high fracture toughness, together with an ultrahigh melting point.

Natural Amino Acid L‐Phenylalanine Coordinated Zirconocene Complex as Bifunctional Catalyst for the Synthesis of 1,5‐Benzothiazepines

AbstractTaken the synthesis of 1,5‐benzothiazepines via the condensation between o‐aminothiophenol with ynones as a paradigm, novel mode of IVB metal based bifunctional catalysis in Lewis acidic‐Brønsted basic (LA‐BB) mode, as well as the effective evidence for how the two catalytic sites interact with substrates has been realized and provided respectively. Specifically, the bifunctional catalyst Cp2Zr[OCOCH(NH2)CH2C6H5]2 was formed through in situ coordination between Cp2ZrCl2 and natural amino acid L‐phenylalanine, which was elucidated by 1H NMR experiment and ESI‐MS analysis. Further control experiment reveals that the excellent catalytic efficiency can attribute to coexistence of Zr (IV) center and Brønsted basic NH2 group contained amino acid ligand. More importantly, with the aid of in situ ESI‐MS, the first catalyst‐substrate adduct for IVB metallocene (IV) catalyst was detected, indicating the Lewis acidic catalytic mode of Zr (IV) center on O atom of ynone. Meanwhile, DFT studies indicated that the true catalytic species presumably be zirconocene (IV) bearing both a bidentate and mono‐dentate carboxylate ligand.

Structure and optical properties of ZrxHf1-xO2 films deposited by pulsed laser co-ablation of Zr and Hf targets with the assistance of oxygen plasma

Group IVB metal oxides have demonstrated many potential applications in a variety fields owing to their excellent optical, mechanical, electrical, chemical and thermal properties. In this work, ternary oxides ZrxHf1-xO2 films with variable compositions were deposited by pulsed laser co-ablation of a Zr target and a Hf target in an oxygen plasma generated by electron cyclotron resonance microwave discharge of O2 gas. The oxygen plasma provided an environment containing a high concentration of reactive oxygen species for synthesizing oxides with the Zr and Hf species ablated from the Zr and Hf targets. The structure of the deposited films was characterized and the optical properties were evaluated together with the examination of the effects of post-deposition annealing on the structure and optical properties. The ternary ZrxHf1-xO2 films are much alike with binary ZrO2 and HfO2 films in structure and optical properties. They have a monoclinic structure and are highly transparent in a wide spectral region from mid-ultraviolet to mid-infrared. The ultraviolet absorption edge and optical band gap vary slightly with the pulse energies of the laser beams ablating the targets. Annealing in N2 resulted in the improvement in the film structure.

Synthesis and characterization of (HfMoTiWZr)C high entropy carbide ceramics

(HfMoTiWZr)C high entropy carbides (HEC) were prepared from the commercial carbide powders of IVB (Hf, Ti, Zr) and VIB (Mo, W) group metals of the periodic table via high energy ball milling (HEBM) and spark plasma sintering (SPS). Metal carbide powders (HfC, TiC, ZrC, Mo2C and WC) were HEBM’d for 3 h in a vibratory ball mill, and then SPS’d at different temperatures (1800, 1900, 2000 and 2100 °C). The HEBM’d powders and SPS’d ceramics were characterized in composition, density and microstructure using an X-ray diffractometer (XRD), a scanning electron microscope/energy dispersive spectrometer (SEM/EDS), a particle size analyzer and a pycnometer. Also, microhardness and sliding wear tests were conducted on the SPS’d ceramics. Based on the performed characterization, a single-phase FCC structure was observed at all sintering temperatures indicating a high entropy carbide ceramic, and they all have high hardness and wear resistance values.

Comparative Analysis of the Phase Interaction in Plasma Surfaced NiBSi Overlays with IVB and VIB Transition Metal Carbides.

Important applications of transition metal carbides (TMCs) are as wear resistant composite layers deposited by plasma transferred arc welding (PTAW) and laser methods. Growing interest in them has also been observed in additive manufacturing and in HEA technology (bulk composite materials and layers), and in the area of energy conversion and storage. This paper presents the results of comparative studies on interfacial interactions in the NiBSi−TMCs system for two border IVB and VIB TM groups of the periodic table. Model (wettability and spreadability) and application experiments (testing of the PTAW-obtained carbide particle−matrix boundaries) were performed. Fe from partially melted steel substrates is active in the liquid NiBSi−TMCs system. It was revealed that the interaction of TMCs with the liquid NiBSi matrix tends to increase with the group number, and from the top to bottom inside individual groups. Particles of IVB TMCs are decomposed by penetration of the liquid along the grain boundaries, whereas those of VIB are decomposed by solubility in the matrix and secondary crystallization. No transition zones formed at the interfacial boundaries of the matrix−IVB group TMCs, unlike in the case of the VIB group. The experimental results are discussed using the data on the TMC electronic structure and the physicochemical properties.

Breaking the linear scaling relations in MXene catalysts for efficient CO2 reduction

Electrocatalytic carbon dioxide reduction reaction (CO2RR) toward value-added fuels has attracted increasing attention in carbon-neutral and energy-production fields, but the catalytic efficiency is seriously hindered by the robust linear scaling relations between adsorption energies of intermediates. Herein, we have extensively investigated the effect of a series of group ⅣB, ⅤB, and ⅥB transition metal (TM) atoms substitution for middle Mo in Mo3C2 MXene on the catalytic performance of CO2RR. Our results suggest that the captured CO2 can be selectively reduced to methane (CH4) on both Mo3C2 and Mo2TMC2 bimetal MXenes. We highlight that TM substitution can significantly reduce the limiting potential (UL) of CO2RR from −0.651 V (Mo3C2) to −0.350 V (Mo2TiC2) by decreasing the Gibbs energy difference of rate-determining step (OCH2O* + H++e-= HOCH2O*). The modulation mechanism is illuminated that TM substitution in Mo3C2 MXene gives rise to the upshift of d-band center of Mo atoms, which selectively tunes the adsorption strength of OCH2O* and HOCH2O*, resulting in breaking their linear scaling relations. Further analyses on electron localization function (ELF) visualize the TM substitution induced stronger surface localization lone electrons, which endows the surface Mo with promoted chemical activity. The dynamical stability of Mo2TiC2 has been well verified by phonon dispersion curves and ab initio molecular dynamics (AIMD) simulations, suggesting the robust stability of Mo2TiC2 as an electrocatalyst for CO2RR. Our findings pave the way of MXenes for CO2 capture and pioneer the application of Mo2TiC2 as a novel and efficient catalyst for CO2 to CH4.

Vacancy-cluster and off-lattice metal-atom diffusion mechanisms in transition metal carbides

Using ab initio simulations, we report potential metal atom diffusion mechanisms in the group IVB and VB transition metal carbides. By computing the metal vacancy formation energies of vacancy clusters, we find that a metal vacancy surrounded by six carbon vacancies is the lowest metal vacancy formation energy structure for the group IVB carbides while the lowest energy for the group VB carbides is a metal vacancy surrounded by two carbon vacancies. The vacancy cluster mechanisms reveal activation energies that are consistent with experiments in both TiC (group IVB) and TaC (group VB). We also report that an off-lattice diffusion mechanism, that is only energetically favorable in the group IVB transition metal carbides, has a lower formation energy than the regular vacancy cluster mechanism. This new mechanism shows lower formation energies for a given carbon vacancy concentration which indicates this off-lattice mechanism might be the most dominant metal-atom diffusion mechanism among the existing mechanisms proposed for the group IVB metal carbides.

Phase stability, mechanical properties and melting points of high-entropy quaternary metal carbides from first-principles

With a combination of first-principles calculations and thermodynamics formalism of configurational mixing entropy, we have constructed three-dimensional phase diagram in terms of thermodynamic and structural parameters including the configurational mixing entropy and enthalpy, the temperature of the melting point, and the lattice constant difference of the constitute carbides for fifteen equiatomic quaternary high-entropy metal carbide (HEMC) ceramics of group IVB and VB refractory metals (RM = Ti, Zr, Hf, V, Nb, and Ta). We further predicted nine new HEMCs and provided an explanation for the existence of six experimentally realized quaternary HEMCs. In addition, our calculations of the melting points and mechanical properties show that the HEMCs have the unique properties of high hardness, high fracture toughness, and ultrahigh melting points. The computational procedure involved in this work may be used to design new high-entropy ceramics for specific applications.

Phonon-assisted diffusion in bcc phase of titanium and zirconium from first principles

Diffusion is the underlying mechanism for many complicated materials phenomena, and understanding it is basic to the discovery of novel materials with desired physical and mechanical properties. Certain groups of solid phases, such as the bcc phase of IIIB and IVB metals and their alloys, which are only stable when they reach high enough temperatures and experience anharmonic vibration entropic effects, exhibit "anomalously fast diffusion". However, the underlying reason for the observed extraordinary fast diffusion is poorly understood and due to the existence of harmonic vibration instabilities in these phases the standard models fail to predict their diffusivity. Here, we indicate that the anharmonic phonon-phonon coupling effects can accurately describe the anomalously large macroscopic diffusion coefficients in the bcc phase of IVB metals, and therefore yield a new understanding of the underlying mechanism for diffusion in these phases. We utilize temperature-dependent phonon analysis by combining ab initio molecular dynamics with lattice dynamics calculations to provide a new approach to use the transition state theory beyond the harmonic approximation. We validate the diffusivity predictions for the bcc phase of titanium and zirconium with available experimental measurements, while we show that predictions based on harmonic transition state theory severely underestimates diffusivity in these phases.