Core-level spectra of metallic lanthanides: Holmium (Ho)

The rare earth metal holmium (Ho) is the 11th element in the lanthanide series, with the ground-state electronic configuration [Xe] 4f¹¹ 6s². Given the reactive nature of metallic lanthanides, they have not been as widely studied as their compounds, with the majority of published data mostly limited to non-monochromatic sources (Ref. 1). Like dysprosium, its neighboring element, holmium exhibits multiplet splitting (Ref. 2) arising from the coupling of the core-hole generated in the low-lying 4d orbital with the partly filled 4f orbital yielding a final state of the form 4d⁹4fⁿ. Despite being a relatively uncommon element presented to most surface analysts, holmium and its compounds find use in diverse fields including optical spectrometer calibration (Ref. 3) and data storage (Ref. 4).

Given its electropositive nature, metallic holmium slowly oxidizes in air to form a native oxide and can react with water to form holmium(III) hydroxide. Such affinity extends also to the halides, forming the respective trihalide. As previously noted (Refs. 2 and 5), maintaining a clean surface for lanthanides to record core-level spectra is difficult. Within this paper, the spectra for clean Ho are presented, which were obtained by light argon sputtering (30 s) between acquisitions. The often-unreported Ho 3d peaks are also presented. It is hoped the reported binding energies, modified sensitivity factors, where applicable, and enhanced resolution for the multiplet spitting over those in Refs. 1, 6, and 7 are an aid to improve any analysis on such materials in the future.

Specimen: Holmium, Ho

CAS Registry #: 7440-60-0

Specimen Characteristics: Homogeneous; solid; polycrystalline; conductor; metal; other

Chemical Name: Holmium

Source: Alfa Aesar

Composition: Ho

Form: Solid dendrite, approximately 40 × 20 mm²

Structure: Ho

History and Significance: Distilled dendrites 99.9% (REO) grade material obtained from Alfa Aesar.

As Received Condition: Received as distilled metallic dendrites of varying sizes.

Analyzed Region: Elliptical region approximately 400 × 600 μm within the approximate center of the etched area.

Ex Situ Preparation/Mounting: Given its malleability, a suitable sized dendrite was sandwiched between aluminum foil and pressed flat using a small sample press to a pressure of 1 tonne. The flattened sample was subsequently polished using isopropyl alcohol and SiC paper (grit size 7 μm) to obtain a visibly smooth and shiny surface. After further rinsing with alcohol and drying under nitrogen, the sample was attached to a metallic sample plate using copper clips before insertion into the spectrometer load lock.

In Situ Preparation: Argon ion sputtering

Charge Control: None

Temp. During Analysis: 298 K

Pressure During Analysis: 3.31 × 10⁻⁷ Pa

Pre-analysis Beam Exposure: 30 s

Manufacturer and Model: Thermo Fisher Scientific K-Alpha⁺

Analyzer Type: Spherical sector

Detector: Multichannel resistive plate

Number of Detector Elements: 128

Analyzer Mode: Constant pass energy

Throughput (T = E^N): Calculated from a polynomial fit to a plot of log[peak area/(PE*XSF)] versus log(KE/PE), where PE is the pass energy, KE is the kinetic energy, and XSF is the relative sensitivity factor (Ref. 8).

Excitation Source Window: No window

Excitation Source: Al K_α monochromatic

Source Energy: 1486.6 eV

Source Strength: 72 W

Source Beam Size: 600 × 400 μm²

Signal Mode: Multichannel direct

Incident Angle: 60°

Source-to-Analyzer Angle: 60°

Emission Angle: 0°

Specimen Azimuthal Angle: 0°

Acceptance Angle from Analyzer Axis: 0°

Analyzer Angular Acceptance Width: 30° × 30 °

Manufacturer and Model: Thermo Scientific MAGCIS

Energy: 4000 eV

Current: 6 mA

Current Measurement Method: Faraday cup

Sputtering Species and Charge: Ar⁺

Spot Size (unrastered): 50 μm

Raster Size: 2000 × 1000 μm²

Incident Angle: 58°

Polar Angle: 58°

Azimuthal Angle: 90°

Comment: The ion gun was used to clean the as-introduced sample for 600 s. Prior to recording of each region, the sample was sputtered for a further 30 s to remove any adsorbed species arising from the chamber background gases.

Energy Scale Correction: None as the sample is conductive and in direct contact with the sample holder.

Recommended Energy Scale Shift: 0

Peak Shape and Background Method: A Shirley background is used for all regions, with the exception of Ho 5p and 4s where a linear background is more appropriate. Recommended background start and end points are ±0.2 eV.

Ho 4f: 2.1–14.5 eV

Ho 5p_3/2: 19.0–28.0 eV

Ho 5p: 19.0–35.0 eV

Ho 5s: 44.5–65.5 eV

Ho 4d: 157.2–197.5 eV

Ho 4p: Overlap with the Auger structure, not amenable for quantification with Al radiation.

Ho 4s: 426.4–448.8 eV

Ho 3d_5/2: 1346.8–1365.6 eV

Ho 3d_3/2: 1386.8–1402.0 eV

Quantitation Method: Data analysis was performed using CasaXPS performed in CasaXPS V2.3.26 rev1.2Y, using a Shirley background unless otherwise specified. Electron escape depth correction was performed using the TPP-2M equation within CasaXPS and peak areas corrected using Scofield sensitivity factors. Modified sensitivity factors were used for Ho 5p and 4d levels as noted in the comments on the spectral features table.

This work was performed through the support of the EPSRC National Facility for x-ray photoelectron spectroscopy (“HarwellXPS,” Nos. EP/Y023587/1, EP/Y023609/1, EP/Y023536/1, EP/Y023552/1, and EP/Y023544/1).

The author has no conflict to disclose.

David J. Morgan: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are provided within the article and its supplementary material.