Synthesis, Structure & Properties of CuBiSeCl2: A Chalcohalide Material with Low Thermal Conductivity
Cara J. Hawkins, Jon A. Newnham, Batoul Almoussawi, Nataliya L. Gulay, Samuel L. Goodwin, Marco Zanella, Troy D. Manning, Luke M. Daniels, Matthew S. Dyer, Tim D. Veal, John B. Claridge, Matthew J. Rosseinsky

SUPPLEMENTARY DATA
MAIN TEXT

SAMPLE PREPARATION
Powder samples of CuBiSeCl2 on a 1 g scale were synthesised by grinding stoichiometric amounts of reagents in an agate mortar, pressing the reagent mix into an 8 mm diameter pellet, and loading into a flame-dried quartz ampoule of dimensions 1 cm × 20 cm. The ampoule was evacuated to 1 × 10−3 mbar and sealed under vacuum. The sample was placed horizontally into a high temperature oven. The sample was then heated using a ramp rate of 5 °C / min, fired at 430 °C for 10 hours and cooled to 200 °C using a ramp rate of 0.1 °C / min, before shutting off the oven and allowing to cool to room temperature. Single crystals of CuBiSeCl2 were grown using the same reaction conditions used for powder synthesis, but a constant cooling rate of 1 °C / hour was used between 430 °C and room temperature.

SPARK PLASMA SINTERING TO FORM DENSE PELLETS FOR PROPERTY MEASUREMENTS
Dense pellets (~88% theoretical density) were obtained by Spark Plasma Sintering (SPS) of phase-pure CuBiSeCl2 powder at 300 MPa and 270 °C for 5 min in a 10-3 mbar vacuum using a commercial Thermal Technology LLC DCS10 furnace. Powder samples (∼0.45 g) were pressed in a 10 mm diameter, graphite-foil-lined, tungsten carbide die set (with 6% Co binder). Heating and pressure ramp rates were set to 20 °C / min and 100 MPa / min, respectively. The temperature was monitored through a borehole in the side of the die set using a thermocouple. After pressing, the pellets were lightly polished with SiC paper to remove the graphite foil from the pellet surface.


Powder X-Ray Diffraction Data: CuBiSeCl2_PXRD_CuBruker_5_100_16hrs.xy

Preliminary phase identification was carried out using a Rigaku SmartLab diffractometer with Mo Kα radiation (λ = 0.7107 Å) in Debye-Scherrer geometry. Samples were sealed inside 0.2 mm diameter borosilicate capillaries in an Ar-filled glovebox. 16-hour scans were conducted on a Bruker D8 Advance with monochromatic Cu Kα1 radiation (λ = 1.54056 Å) in Debye-Scherrer geometry to obtain high quality data for structural refinement. A LaB6 internal standard was ground into the CuBiSeCl2 mixture to extract accurate lattice parameters during refinement. 12 hour scans were conducted on an X’Pert Panalytical diffractometer with monochromatic Co Kα1 radiation (λ=1.788965 Å) in Bragg-Brentano geometry to quantify the extent of preferred orientation in pressed pellets of CuBiSeCl2.


Raw XPS Valence Band Data: CuBiSeCl2_VB.txt

Core-level and valence band XPS measurements were collected at HarwellXPS, Didcot, U.K. XPS analysis was performed using a Kratos Axis SUPRA XPS fitted with a monochromatic Al Kα X-ray source (hν = 1486.6 eV), a spherical sector analyser and 3 multichannel resistive plate, 128 channel delay line detectors. All data were recorded at 150 W and a spot size of 700 × 300 μm2. Survey scans were recorded at a pass energy of 160 eV, and high-resolution scans recorded at a pass energy of 20 eV. Electronic charge neutralization was achieved using a magnetic immersion lens. All sample data were recorded at a pressure below 10−8 Torr and temperature of 294 K. Data were analysed using CasaXPS v2.3.19PR1.0. Peaks were fit with a Shirley background prior to component analysis. Mixed Gaussian-Lorentzian lineshapes (GL(50)) were used to fit components. All binding energies were measured with respect to the Fermi edge of an Ag foil reference sample.
The ionisation potential (IP) of CuBiSeCl2 was determined by collecting the secondary electron cut-off (SEC) region using a monochromatic Al Kα SPECS X-ray source with a PSP Vacuum Technology hemispherical electron energy analyser with mean radius of 120 mm. Pass energies of 2 eV for the SEC, 10 eV for core levels and 50 eV for survey scans were used to measure the emitted photoelectrons. Measurements were performed in an ultrahigh vacuum chamber with a base pressure of 2 × 10−10 mbar. Secondary electron cut-off data were recorded at 16 W to prevent overloading the analyser. A 10 V bias was applied to the sample to remove any effects from the material work function. The Cu 2p region was also measured to use as a reference. The spectrometer resolution was determined to be 0.40 eV by fitting a Fermi-Dirac function to the Fermi edge of Ag foil.


UV-Vis Spectroscopy Data: CuBiSeCl2_UV_Vis.csv

Diffuse reflectance of CuBiSeCl2 powder was measured using an Agilent Cary 5000 between 200 and 2500 nm with a step size of 1 nm. Calibration to 100% and 0% reflectance was performed prior to measurement using a PTFE standard and a light trap, respectively. The band gap was determined from a Tauc plot using a method described by Makuła et al.


Resistivity, Thermal Conductivity, Seebeck Coefficient Data: CuBiSeCl2_SinteredPellet_TTO_1.dat

Heat Capacity Data: CuBiSeCl2_SinteredPellet_HC_1.dat


SPS prepared pellets were cut into semicircles with 1.13 mm thickness and 4.9 mm radius using a low-speed, diamond-blade saw for Seebeck coefficient, electronic resistivity, and thermal conductivity measurements. Copper electrodes were attached to the pellet using Ag epoxy and left to dry overnight. The offcuts from the pellets were used for powder diffraction and compositional analysis. The thermal conductivity, electronic conductivity and Seebeck coefficient were measured simultaneously between 2 and 300 K using two probe geometry on a dense pellet of CuBiSeCl2 using the Thermal Transport Option (TTO) on a Quantum Design Physical Properties Measurement System (PPMS).  Heat capacity measurements were performed on a small fragment of dense pellet, with mass 0.0125 g, of CuBiSeCl2 using the Heat Capacity Option (HCO) on the PPMS. The sample was mounted using N grease. An addenda measurement was performed on the sample holder and grease prior to mounting and measuring the pellet fragment. Both the addenda and pellet fragment measurements were performed between 2 and 300 K.



SUPPORTING INFORMATION

For the compositional analysis, data are included both with and without correction factors (CF).
Scanning Electron Microscopy (SEM) was performed on a Tescan S8000. Pelletized powder and single crystal samples were attached to an adhesive carbon tape stuck on an aluminium SEM stub. To reduce charging effects, the samples were coated with a thin layer of carbon. Energy Dispersive X-ray Spectroscopy (EDX) and Wavelength Dispersive X-ray Spectroscopy (WDX) were performed on the same instrument using X-MaxN and Wave detectors from Oxford Instruments. WDX calibration, for the different elements, were obtained by measuring the WDX spectra of appropriate standards. Standard purity was confirmed using X-ray diffraction and electron microscopy. Quantification was performed using Aztec software.



EDX Data (Powder): CuBiSeCl2_Powder_SEMEDX_NoCF; CuBiSeCl2_Powder_SEMEDX_CF.txt 

WDX Data (Powder): CuBiSeCl2_Powder_WDX_CF.txt

EDX Data (Pellet): CuBiSeCl2_Pellet_SEMEDX_NoCF.txt; CuBiSeCl2_Pellet_SEMEDX_CF.txt


XPS Core Levels, Survey Scan & Secondary Electron Cutoff Data: CuBiSeCl2_Bi4f.txt; CuBiSeCl2_Cl2p.txt; CuBiSeCl2_Cl2s.txt; CuBiSeCl2_Cu2p.txt; CuBiSeCl2_Se3d.txt; CuBiSeCl2_SecondaryElectronCutoff.txt; CuBiSeCl2_Survey.txt

Environmental Stability PXRD Data: CuBiSeCl2_AirStability_10mins_10_80.xy; CuBiSeCl2_AirStability_72hours_10_80.xy; CuBiSeCl2_AirStability_1week_10_80.xy; CuBiSeCl2_AirStability_3weeks_10_80.xy; CuBiSeCl2_AirStability_9weeks_10_80.xy; CuBiSeCl2_NoWater_10_80.xy; CuBiSeCl2_Water_10_80.xy

The stability of CuBiSeCl2 in ambient air and in water was determined as part of this study. The water stability of CuBiSeCl2 was determined by mixing CuBiSeCl2 powder with distilled water for 10 minutes and pipetting several drops of the suspension onto a glass slide. The slide was then left under ambient conditions for 24 hours to allow the water to evaporate. PXRD data were measured before and after water exposure. The air stability was determined by sprinkling CuBiSeCl2 powder onto a glass slide, leaving the slide under ambient conditions for several weeks and measuring PXRD data at regular time intervals. 


Preferred Orientation PXRD Data: CuBiSeCl2_Simulated_CoKa1.xy; CuBiSeCl2_Powder_10_80_30mins.xy; CuBiSeCl2_SinteredPellet_10_70_40mins.xy