La0.9Y0.1H10 Superhydrides: Structure and Superconductivity

• The paper establishes a clear link between micrometer-scale phase distribution and distinct superconducting onsets at approximately 244 K and 220 K.
• It employs synchrotron X-ray diffraction imaging and four-probe DC transport to correlate cubic Fm3̅m and hexagonal P63/mmc clathrate phases under extreme pressures.
• Substituting 10% yttrium in LaH10 results in reduced lattice volumes and broadened superconducting transitions, evidencing chemical pre-compression effects.

Searching arXiv for the primary paper and closely related LaH10 superhydride literature for contextual support. (La$_{0.9}$Y$_{0.1}$)H$_{10}$ is a chemically substituted LaH$_{10}$-type clathrate superhydride in which yttrium partially occupies the rare-earth sublattice and the material exhibits coexisting cubic Fm$\bar{3}$m and hexagonal P6$_3$/mmc clathrate phases over the pressure range from 168 GPa down to 136 GPa. In "X-ray Diffraction and Electrical Transport Imaging of Superconducting Superhydride (La,Y)H$_{10}$" (Marathamkottil et al., 24 Jul 2025), the compound is characterized by synchrotron-based X-ray diffraction imaging (XDI) and four-probe DC transport, with superconductivity confirmed through two distinct resistance onsets: $T_{c1}\approx244\,$K associated with the cubic phase and $T_{c2}\approx220\,$K linked to the hexagonal phase. The system is notable because the structural and transport data are spatially correlated at the micrometer scale, establishing a direct relation between local phase distribution and superconducting response under megabar conditions.

1. Composition, synthesis, and high-pressure environment

The reported material was synthesized from an arc-melted La$_{0.9}$Y$_{0.1}$0 alloy, verified by SEM-EDS, with ammonia borane (NH$_{0.1}$1BH$_{0.1}$2) used as both hydrogen source and pressure medium. The diamond anvil cell employed 65 $_{0.1}$3m-culet diamond anvils, a sample chamber in a cBN gasket, and in-situ Pt-foil contacts of approximately 2 $_{0.1}$4m thickness arranged in a van-der-Pauw four-probe geometry. Pressure calibration was performed from diamond-Raman edge shifts, and laser heating used a modulated Yb-fiber laser with $_{0.1}$5 ms pulses at the metal/NH$_{0.1}$6BH$_{0.1}$7 interface (Marathamkottil et al., 24 Jul 2025).

Two independent runs were described. In Run 1, the sample was compressed to 172 GPa, laser-heated to $_{0.1}$8–$_{0.1}$9 K, and the pressure then relaxed to 168 GPa. In Run 2, the sample was compressed to 158 GPa, laser-heated, pressure relaxed to 153 GPa, and subsequently decompressed stepwise down to 136 GPa for transport measurements. These preparation conditions are central because the observed phase coexistence and the transport response were tracked across precisely this pressure window.

2. Crystal chemistry and clathrate polymorphism

The structural characterization identifies two clathrate polymorphs. The cubic phase is Fm$_{10}$0m, described as “FCC,” and the hexagonal phase is P6$_{10}$1/mmc, described as “HCP.” For the Fm$_{10}$2m clathrate, the lattice parameters are $_{10}$3Å and $_{10}$4Å$_{10}$5 at 168 GPa, $_{10}$6Å and $_{10}$7Å$_{10}$8 at 153 GPa, and $_{10}$9Å and $_{10}$0Å$_{10}$1 at 136 GPa. For the P6$_{10}$2/mmc clathrate, the lattice parameters are $_{10}$3Å, $_{10}$4Å, and $_{10}$5Å$_{10}$6 at 168 GPa, $_{10}$7Å, $_{10}$8Å, and $_{10}$9Å$\bar{3}$0 at 153 GPa, and $\bar{3}$1Å, $\bar{3}$2Å, and $\bar{3}$3Å$\bar{3}$4 at 136 GPa (Marathamkottil et al., 24 Jul 2025).

In the cubic clathrate, La/Y occupy the 8a site and H atoms form the H$\bar{3}$5 cages around each rare-earth atom; the H atoms are associated with the 32f and 48h Wyckoff positions, with occupancy $\bar{3}$6. The reported volume trends follow LaH$\bar{3}$7 but are systematically $\bar{3}$8–$\bar{3}$9 smaller, consistent with 10% Y substitution. No secondary LaH$_3$0 or YH$_3$1 phases were detected, and the mixed clathrates remain single-solid-solutions. A common misconception in mixed-phase hydride transport studies is that multiple transitions necessarily imply extraneous impurity phases; in this case, the reported structural evidence specifically attributes the complexity to coexisting clathrate polymorphs rather than detectable secondary LaH$_3$2 or YH$_3$3 products.

Phase	Pressure	Lattice parameters
Fm$_3$4m	168 GPa	$_3$5Å, $_3$6Å$_3$7
Fm$_3$8m	153 GPa	$_3$9Å, $_{10}$0Å$_{10}$1
Fm$_{10}$2m	136 GPa	$_{10}$3Å, $_{10}$4Å$_{10}$5
P6$_{10}$6/mmc	168 GPa	$_{10}$7Å, $_{10}$8Å, $_{10}$9Å$T_{c1}\approx244\,$0
P6$T_{c1}\approx244\,$1/mmc	153 GPa	$T_{c1}\approx244\,$2Å, $T_{c1}\approx244\,$3Å, $T_{c1}\approx244\,$4Å$T_{c1}\approx244\,$5
P6$T_{c1}\approx244\,$6/mmc	136 GPa	$T_{c1}\approx244\,$7Å, $T_{c1}\approx244\,$8Å, $T_{c1}\approx244\,$9Å$T_{c2}\approx220\,$0

3. Spatially resolved diffraction imaging

The phase distribution was mapped by synchrotron X-ray diffraction imaging via scanning X-ray diffraction microscopy. Measurements were performed at beamlines 13-ID-D (pre-upgrade) and 16-ID-B at the upgraded APS-U. At 16-ID-B, the microbeam diameter was $T_{c2}\approx220\,$1m, and raster grids included 30$T_{c2}\approx220\,$230 $T_{c2}\approx220\,$3m$T_{c2}\approx220\,$4 with 3 $T_{c2}\approx220\,$5m steps, as well as 50$T_{c2}\approx220\,$650 $T_{c2}\approx220\,$7m$T_{c2}\approx220\,$8 and 15$T_{c2}\approx220\,$915 $_{0.9}$0m$_{0.9}$1 scans (Marathamkottil et al., 24 Jul 2025).

The reported XDI workflow consisted of three steps: collect 2D diffraction patterns at each grid point; integrate intensity of phase-unique Bragg peaks, for example (111)$_{0.9}$2 versus (100)$_{0.9}$3, via XDI software; and assemble 2D intensity maps in which red pixels denote Fm$_{0.9}$4m, blue denotes P6$_{0.9}$5/mmc, and gray denotes Pt. The result was micron-scale coexistence of FCC and HCP domains, with $_{0.9}$6 FCC and $_{0.9}$7 HCP coverage at 153 GPa. The central significance of these maps is that they resolve structural inhomogeneity across the sample rather than inferring it indirectly from broadened diffraction or transport alone.

4. Electrical transport and assignment of superconducting transitions

Transport was measured in the van-der-Pauw geometry using four Pt contacts labeled #1–#4 and an excitation current $_{0.9}$8A. The partial resistance was defined as

$_{0.9}$9

To remove Seebeck offsets, the average “four-probe” resistance was defined as

$_{0.1}$00

These measurement definitions are important because the transport response depends on current and voltage permutations, not only on an averaged signal (Marathamkottil et al., 24 Jul 2025).

At 153 GPa in warming data, two distinct superconducting onsets were reported. The first, $_{0.1}$01K, is a sharp drop with $_{0.1}$02 K and is spatially correlated with Fm$_{0.1}$03m-rich domains, for example along paths between #3–#4. The second, $_{0.1}$04K, is broader and two-step and is correlated with P6$_{0.1}$05/mmc-rich regions, for example between #1–#4. Upon decompression to 136 GPa, $_{0.1}$06 shifts down to $_{0.1}$07 K, $_{0.1}$08 shifts to $_{0.1}$09 K, and the total width becomes $_{0.1}$10–$_{0.1}$11K. Resistance profiles collected from different current and voltage permutations showed variations in transition width and onset temperature that correlated with the spatial phase distribution mapped by XDI. This directly supports the phase-specific assignment of the two superconducting transitions.

5. Yttrium substitution, strain, and superconducting response

The role of yttrium substitution is described along three closely related axes: phase stability, lattice compression, and superconducting behavior. At 10 at%, Y extends the coexistence of Fm$_{0.1}$12m and P6$_{0.1}$13/mmc clathrates down to 136 GPa, below the $_{0.1}$14 GPa transition of pure LaH$_{0.1}$15 to R3m or C2/m. The lattice volumes are $_{0.1}$16–$_{0.1}$17 smaller than LaH$_{0.1}$18 at the same pressure, indicating chemical pre-compression. The superconducting onset temperatures are suppressed by $_{0.1}$19–$_{0.1}$20 K relative to pure LaH$_{0.1}$21, for which the comparison value given is $_{0.1}$22K at 188 GPa (Marathamkottil et al., 24 Jul 2025).

The transport broadening is also interpreted in terms of substitution-induced heterogeneity. Broad transitions with $_{0.1}$23–$_{0.1}$24K and two-step behavior arise from microscale phase segregation, enhanced by Y-induced strain gradients. This suggests that partial substitution does not simply shift a homogeneous phase boundary; it also redistributes local structural environments and therefore modifies the connectivity of superconducting paths sampled by different electrode permutations.

For superconductivity modeling, the standard McMillan/Allen–Dynes expression was given, although direct fits were not reported:

$_{0.1}$25

where $_{0.1}$26 is the EPC constant, $_{0.1}$27 the Coulomb pseudopotential, and $_{0.1}$28 a Debye-like temperature. In the present context, the formula serves as a conventional framework for discussing how chemical substitution and structural polymorphism may alter electron-phonon coupling without providing fitted parameters for this specific sample.

6. Broader significance and interpretive boundaries

The principal result is the establishment of a direct, micrometer-scale link between structural domains and local superconducting behavior in a high-$_{0.1}$29 hydride under extreme pressure. The combined use of multi-channel transport and SXDM/XDI under megabar conditions demonstrates that mixed-phase superconductors can be interrogated with spatial specificity rather than treated as laterally uniform media (Marathamkottil et al., 24 Jul 2025).

This has two immediate implications. First, it provides a pathway for rational design of chemically substituted clathrate hydrides, where targeted alloying may tune phase stability and optimize electronic coupling. Second, it offers a template for future studies of heterogeneous superconductors under pressure, because the coexistence of discrete local domains can be linked to distinct transport signatures through contact-resolved measurements. A plausible implication is that, in substituted clathrate hydrides, electrode geometry and current path selection can function as a probe of mesoscale superconducting topology rather than merely a measurement detail.

An additional interpretive boundary concerns phase assignment. The reported data support an unambiguous assignment of the cubic and hexagonal clathrate structures to the two discrete superconducting transitions, but they do not report direct McMillan/Allen–Dynes fits. Accordingly, the work establishes a structural-transport correlation at micron scale while leaving microscopic EPC parameter extraction for future study.