About

Lars Ehm is an Associate Professor and the Director of the Mineral Physics Institute at the Department of Geosciences at Stony Brook University. He holds a diploma and a Dr. rer. nat. degree from Christian-Albrechts-University Kiel, obtained in 2000 and 2003 respectively. His research interests focus on the structure-property relationships of Earth materials under extreme conditions. Ehm has a background that includes postdoctoral fellowships at the Geophysical Laboratory of the Carnegie Institution of Science and at the Mineral Physics Institute at Stony Brook University, where he later became a research faculty member before his current appointment. His work involves investigating the behavior of Earth materials at high pressures and temperatures, contributing to the understanding of phase transitions, amorphization, and other phenomena relevant to geosciences and planetary science.

Research topics

• Materials science
• Physics
• Optics
• Chemistry
• Condensed matter physics
• Atomic physics
• Nanotechnology
• Metallurgy
• Crystallography
• Thermodynamics
• Chemical physics

Selected publications

• Obscuration of Crystalline Ca‐Sulfate in XRD and Raman Data When Coated by Amorphous Ferric Sulfate: Implications for the Amorphous Components at Gale Crater

  Journal of Geophysical Research Planets · 2026-02-01

  articleOpen accessSenior author

  Abstract Throughout Gale crater on Mars, the Curiosity rover has found high abundances (15–73 wt%) of X‐ray amorphous materials in the rocks and sediments. The composition of this amorphous fraction is primarily calculated by subtracting crystalline abundances measured by the Chemistry & Mineralogy X‐Ray Diffractometer (CheMin) from bulk elemental abundances from the Alpha Particle X‐Ray Spectrometer (APXS). If any crystalline phase was underrepresented in the CheMin data, the elemental components of that phase would be overestimated in the amorphous fraction. This study examines the possibility of underrepresented crystalline phases in X‐ray diffraction data using mixtures of amorphous ferric sulfate (AFS) and crystalline Ca‐sulfate. Two compositionally equivalent mixtures were made with different morphologies: first, a simple mixture of AFS and Ca‐sulfate grains and second, Ca‐sulfate grains with AFS coatings. Each mixture was characterized with Raman spectroscopy, and two X‐ray diffractometers (XRDs): a Bragg‐Brentano instrument with CuKα radiation and a Debye‐Scherrer instrument with CoKα radiation. Raman peaks from Ca‐sulfate dominate both mixtures, but are dampened when AFS coats the Ca‐sulfate grains. In XRD data, AFS coatings cause an overestimation of the amorphous percentage, with a difference between the known and refined amorphous abundances of 29–34 wt%, compared to 2–2.8 wt% for the uncoated mixtures. The effects of the coatings were slightly amplified with the Debye‐Scherrer XRD, primarily due to the increased scattering and absorption of the CoKα radiation. This has implications for interpreting the XRD data of mixed amorphous‐crystalline samples on Mars, as any Fe‐rich amorphous coating may cause an overestimation of the amorphous abundance.

  PublisherOA PDFDOI

• EuXFEL #3063 experiment on iron and iron alloys under planetary cores conditions

  Zenodo (CERN European Organization for Nuclear Research) · 2025-12-16

  datasetOpen access

  Community proposal to study iron and iron alloy phases at high pressure and temperature conditions at the European XFEL. Experiment EuXFEL #3063. September 2022. The archive holds The list of samples The list of runs Data from sample screening at PETRA III XFEL beam intensity information for all runs Calibration information for X-ray diffraction data at the EuXFEL Raw data from the corresponding publications. Information on the experiment, data analysis procedure, and corresponding metrology are published in Ginestet et al, 2006 (see below). If used, please cite this archive as well as the corresponding publication: Ginestet et al, Metrology for femtosecond pulsed x-ray heating in diamond anvil cell experiments at the European XFEL: Revisiting the iron phase diagram up to 150 GPa, J. Appl. Phys. 139, 000000 (2026); doi: 10.1063/5.0303953

  PublisherDOI

• Remote Determination of Martian Chloride Salt Abundances

  Journal of Geophysical Research Planets · 2025-03-01 · 1 citations

  articleOpen access

  Abstract Chloride salt‐bearing deposits are widely distributed across the southern highlands of Mars. Because chloride salts are highly water‐soluble, these deposits may be representative of the last significant period of stable liquid water at the Martian surface. Therefore, these deposits are key to understanding the fate and evolution of surface waters on Mars. However, little consensus exists about the formation conditions of these deposits, and their origins remain enigmatic. This is due in part because remote spectroscopic detection and quantification of many anhydrous chlorides is hampered by a lack of easily discernible diagnostic absorption features. To address this issue, we present a novel Hapke radiative transfer model‐based method to estimate hydration states and salt abundances of Martian chloride salt‐bearing deposits using visible/near‐infrared (VNIR) reflectance spectra. VNIR laboratory spectra are used to derive water abundances of analog chloride‐bearing materials, establishing an experimental basis for application of these methods to Mars. These methods are then applied to orbital Compact Reconnaissance Imaging Spectrometer for Mars data to create maps of the hydration state and modeled salt abundance of chloride‐bearing deposits. When overlain onto high resolution 3D digital terrain models, these methods produce the highest resolution site‐specific salt abundance maps currently available, enhancing our understanding of chloride deposit geologic context. As an example, deposits in the Terra Sirenum region are observed to have higher estimated salt abundances than previously recognized, exhibiting spatial variations in both abundance and surface morphology.

  PublisherOA PDFDOI

• EuXFEL #3063 experiment on iron and iron alloys under planetary cores conditions

  Zenodo (CERN European Organization for Nuclear Research) · 2025-12-16

  datasetOpen access

  Community proposal to study iron and iron alloy phases at high pressure and temperature conditions at the European XFEL. Experiment EuXFEL #3063. September 2022. The archive holds The list of samples The list of runs Data from sample screening at PETRA III XFEL beam intensity information for all runs Calibration information for X-ray diffraction data at the EuXFEL Raw data from the corresponding publications. Information on the experiment, data analysis procedure, and corresponding metrology are published in Ginestet et al, 2006 (see below). If used, please cite this archive as well as the corresponding publication: Ginestet et al, Metrology for femtosecond pulsed x-ray heating in diamond anvil cell experiments at the European XFEL: Revisiting the iron phase diagram up to 150 GPa, J. Appl. Phys. 139, 000000 (2026); doi: 10.1063/5.0303953

  PublisherDOI

• Observation of Body-Centered Cubic Iron above 200 Gigapascals

  ArXiv.org · 2025-05-21 · 1 citations

  preprintOpen access

  The crystallographic structure of iron under extreme conditions is a key benchmark for cutting-edge experimental and numerical methods. Moreover, it plays a crucial role in understanding planetary cores, as it significantly influences the interpretation of observational data and, consequently, insights into their internal structure and dynamics. However, even the structure of pure solid iron under the Earth's core conditions remains uncertain, with the commonly expected hexagonal close-packed structure energetically competitive with various cubic lattices. In this study, iron was compressed in a diamond anvil cell to above 200 GPa, and dynamically probed near the melting point using MHz frequency X-ray pulses from the European X-ray Free Electron Laser. The emergence of an additional diffraction line at high temperatures suggests the formation of an entropically stabilized bcc structure. Rapid heating and cooling cycles captured intermediate phases, offering new insights into iron's phase transformation paths. The appearance of the bcc phase near melting at extreme pressures challenges current understanding of the iron phase diagram under Earth's core conditions.

  PublisherOA PDFDOI

• Highly Anisotropic to Isotropic Nature and Ultralow Out-of-Plane Lattice Thermal Conductivity of Layered PbClF-Type Materials

  Inorganic Chemistry · 2024-02-12 · 1 citations

  article

  Materials with an extreme lattice thermal conductivity (κl) are indispensable for thermal energy management applications. Layered materials provide an avenue for designing such functional materials due to their intrinsic bonding heterogeneity. Therefore, a microscopic understanding of the crystal structure, bonding, anharmonic lattice dynamics, and phonon transport properties is critically important for layered materials. Alkaline-earth halofluorides exhibit anisotropy from their layered crystal structure, which is strongly determined by axial bond(s), and it is attributed to the large axial ratio (c/a > 2) for CaBrF, CaIF, and SrIF, in which Br/I acts as a rattler, as evidenced from potential energy curves and phonon density of states. The low axial (c/a) ratio leads to relatively isotropic κl values in the BaXF (X = Cl, Br, I) series. MXF (M = Ca, Sr, Ba) compounds exhibit highly anisotropic (a large phonon transport anisotropy ratio of 10.95 for CaIF) to isotropic (a small phonon transport anisotropy ratio of 1.49 for BaBrF) κl values despite their iso-structure. Moreover, ultralow κl (<1 W/m K) values have been predicted for CaBrF, CaIF, and SrIF in the out-of-plane direction due to weak van der Waals (vdWs) bonding. Overall, this comprehensive study on MXF compounds provides insights into designing low κl layered materials with a large axial ratio by fine-tuning out-of-plane bonding from ionic to vdWs bonding.

  PublisherDOI

• Laser-heated rapid decompression of enstatite (Mg₂Si₂O₆): constraining impact barometry

  Acta Crystallographica Section A Foundations and Advances · 2024-08-26

  articleOpen accessSenior author

  The high velocity bombardment of planetary bodies via asteroid, comet, and meteorite impacts contributes to various processes in planetary genesis, evolution, and habitability [1][2][3].The determination of the peak shock pressures and temperatures is an essential step to derive the original properties of the impactor and target, interpret magnetic signatures, and study the collision history of planetary bodies in our solar system.Numerous classification schemes have been developed in the past to quantify the shock pressure and temperature conditions based on shock induced alterations in the atomic and microstructural level of minerals and rocks.These classifications are very robust for shock stages S1-S4 across a broad set of rocky meteorites.Controversy remains for the classification schemes of highly shocked meteorites in the shock classes S5 (45-55 GPa) and S6 (>60 GPa) due to the fact that these classifications are mainly based on the localized phenomena of melt veins, and the high pressure phases therein.However, the use of high pressure minerals and their equilibrium pressure-temperature stability fields as proxies for shock-thermo-barometry remains controversial, since the shock conditions inferred from the high pressure minerals are significantly lower than those concluded from the S5 and S6 shock classification and the general shock deformation features found in rock forming minerals [4].

  PublisherDOI

• REMOTE DETERMINATION OF MARTIAN CHLORIDE SALT ABUNDANCES.

  2024-06-10

  preprintOpen accessSenior author

  Chloride salt-bearing deposits are widely distributed across the southern highlands of Mars. Because chloride salts are highly water-soluble, these deposits may be representative of the last significant period of stable liquid water at the Martian surface. Therefore, these deposits are key to understanding the fate and evolution of surface waters on Mars. Yet, little consensus exists about the formation conditions of these deposits, and their origins remain enigmatic. This is due in part because remote spectroscopic detection and quantification of many chlorides is hampered by a lack of easily discernible diagnostic absorption features. To address this issue, we present a novel Hapke radiative transfer model (RTM)-based method to estimate hydration states and salt abundances of Martian chloride salt-bearing deposits using visible/near-infrared (VNIR) reflectance spectra. VNIR laboratory spectra are used to derive water abundances of analog chloride-bearing materials, establishing an experimental basis for application of these methods to Mars. These methods are then applied to orbital Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data to create maps of hydration state and modeled salt abundance of chloride-bearing deposits. When overlain onto high resolution 3D digital terrain models (DTMs), these methods produce the highest resolution, site-specific salt abundance maps currently available, enabling new discoveries and understanding of geologic context. As an example, deposits in the Terra Sirenum region are observed to have higher estimated salt abundances than previously recognized, exhibiting spatial variations in both abundance and surface morphology.

  PublisherOA PDFDOI

• MHz free electron laser x-ray diffraction and modeling of pulsed laser heated diamond anvil cell

  Journal of Applied Physics · 2023-09-05 · 5 citations

  articleOpen access

  A new diamond anvil cell experimental approach has been implemented at the European x-ray Free Electron Laser, combining pulsed laser heating with MHz x-ray diffraction. Here, we use this setup to determine liquidus temperatures under extreme conditions, based on the determination of time-resolved crystallization. The focus is on a Fe-Si-O ternary system, relevant for planetary cores. This time-resolved diagnostic is complemented by a finite-element model, reproducing temporal temperature profiles measured experimentally using streaked optical pyrometry. This model calculates the temperature and strain fields by including (i) pressure and temperature dependencies of material properties, and (ii) the heat-induced thermal stress, including feedback effect on material parameter variations. Making our model more realistic, these improvements are critical as they give 7000 K temperature differences compared to previous models. Laser intensities are determined by seeking minimal deviation between measured and modeled temperatures. Combining models and streak optical pyrometry data extends temperature determination below detection limit. The presented approach can be used to infer the liquidus temperature by the appearance of SiO2 diffraction spots. In addition, temperatures obtained by the model agree with crystallization temperatures reported for Fe–Si alloys. Our model reproduces the planetary relevant experimental conditions, providing temperature, pressure, and volume conditions. Those predictions are then used to determine liquidus temperatures at experimental timescales where chemical migration is limited. This synergy of novel time-resolved experiments and finite-element modeling pushes further the interpretation capabilities in diamond anvil cell experiments.

  PublisherOA PDFDOI

• Dynamic optical spectroscopy and pyrometry of static targets under optical and x-ray laser heating at the European XFEL

  Journal of Applied Physics · 2023-08-02 · 9 citations

  articleOpen access

  Experiments accessing extreme conditions at x-ray free electron lasers (XFELs) involve rapidly evolving conditions of temperature. Here, we report time-resolved, direct measurements of temperature using spectral streaked optical pyrometry of x-ray and optical laser-heated states at the High Energy Density instrument of the European XFEL. This collection of typical experiments, coupled with numerical models, outlines the reliability, precision, and meaning of time dependent temperature measurements using optical emission at XFEL sources. Dynamic temperatures above 1500 K are measured continuously from spectrally- and temporally-resolved thermal emission at 450–850 nm, with time resolution down to 10–100 ns for 1–200 μs streak camera windows, using single shot and integrated modes. Targets include zero-pressure foils free-standing in air and in vacuo, and high-pressure samples compressed in diamond anvil cell multi-layer targets. Radiation sources used are 20-fs hard x-ray laser pulses at 17.8 keV, in single pulses or 2.26 MHz pulse trains of up to 30 pulses, and 250-ns infrared laser single pulses. A range of further possibilities for optical measurements of visible light in x-ray laser experiments using streak optical spectroscopy are also explored, including for the study of x-ray induced optical fluorescence, which often appears as background in thermal radiation measurements. We establish several scenarios where combined emissions from multiple sources are observed and discuss their interpretation. Challenges posed by using x-ray lasers as non-invasive probes of the sample state are addressed.

  PublisherOA PDFDOI

Recent grants

• Collaborative Proposal: ATREX - integrated open source data analysis software for mineral and environmental sciences

  NSF · $129k · 2014–2017

Frequent coauthors

• John B. Parise

  Stony Brook University

  97 shared

• F. Marc Michel

  Virginia Tech

  46 shared

• K. Knorr

  46 shared

• Sytle M. Antao

  University of Calgary

  39 shared

• Peter J. Chupas

  Stony Brook University

  35 shared

• G. Morard

  Institut des Sciences de la Terre

  34 shared

• Martin A. A. Schoonen

  Brookhaven National Laboratory

  28 shared

• Daniel R. Strongin

  Temple University

  28 shared

Education

• Other

  Christian-Albrechts-University Kiel

  2000

• Other

  Christian-Albrechts-University Kiel

  2003

• Other

  Geophysical Laboratory, Carnegie Institution of Science

  2005

• Other

  Mineral Physics Institute, Stony Brook University

  2007

• Other

  Mineral Physics Institute, Stony Brook University

  2018