Readme file for data relating to Computationally-guided discovery of a new compound in the Li-Al-S phase field, Li3AlS3: structure and lithium conductivity

Synthesis

Materials
Li2S (99.98 % pure), LiNO3 (> 99 %), Li2CO3 (> 99 %), 7Li2CO3 (99 % 7Li), Al(OH)3 (reagent grade) were purchased from Sigma Aldrich, while Li2O (99.5 %), urea (99.0 %), Al2S3 (99+%), Al(NO3)3,9·H2O (98 %) were obtained from Alfa Aesar. 

Exploratory synthesis of the compounds in the Li-Al-O-S phase field
For all sulphide-containing materials, precursors and resulting powders were handled in a He-filled glovebox. Compositions belonging to the Li-Al-O-S phase diagram were synthesized from stoichiometric mixtures of Li2O, Al2S3, and, when needed Li2S and pre-synthesized LiAlO2 (according to a procedure from Gao et al.45 cf. Supplementary Information, SI). The precursors were weighed in the appropriate amount in order to obtain a total mass of 300 mg. The powders were then mixed and ground in an agate mortar for 15 min, transferred to an alumina crucible and placed in a quartz tube before sealing under vacuum (10-4 mbar). The tube containing the sample was heated to 800 °C at a ramp rate of 5 °C·min-1, held at 800 °C for 48 hours, and then quenched in water. The resulting powder was then manually ground in order to obtain a fine powder. 

Final synthesis of Li3AlS3
After identifying Li3AlS3 as a new phase through the described synthesis method above, its synthesis was slightly modified in order to improve purity. Li2S (1.4358 g, 31.2 mmol), and Al2S3 (1.5642 g, 10.4 mmol) were weighed according to the stoichiometry 3:1 ratio. The resulting powders were then mixed and ground in an agate mortar for 15 min, transferred in an alumina crucible and placed in a quartz tube before sealing under vacuum (10-4 mbar). The tube containing the sample was heated at 700 °C for 12 hours and then to 800 °C for 12 hours with an intermediate grinding step in between both firings. The heating ramp rates of the furnace was 5 °C·min-1, and cooling was performed by quenching the tube in water. Neutron powder diffraction (NPD) experiments were conducted on 7Li-enriched samples of 7Li3AlS3, using 7Li2S as precursor material which was synthesized according to a method described by Leube et al.46 starting from 7Li2CO3. For consistency of the structural analysis, the 7Li3AlS3 sample was also used for synchrotron diffraction experiments. 
The importance of the quenching step was investigated by either letting the sample cool down by turning off the furnace, and by setting the cooling ramp rate to 1 °C·min¬-¬1. The sample cooled down by turning the furnace off did not show any extra impurity peaks in the X-Ray diffraction pattern, however, the sample cooled with the 1 °C·min-¬1 ramp resulted in the partial decomposition into Li5AlS4 and LiAlS2. This decomposition was also observed when a formerly prepared Li3AlS3 sample was heated slowly to 700 °C at 1 °C·min-¬1, maintained at this temperature for 30 min and quenched to room temperature. We therefore maintained the 5 °C·min-1 fast heating ramp rate and the quenching step for the synthesis of the material. 

Probe Structure Generation and Energy Calculations
All energies were computed using periodic Density Functional Theory (DFT) with the VASP programme.47 The PBE functional was used48 with the projector augmented wave approach to treat core electrons.49 
A probe structure approach was used to sample compositions in the Li+-Al3+-O2?-S2? phase space.36 Crystal structure prediction (CSP) was used to generate a probe structure at each composition. Each probe structure was assumed to have an energy close enough to the global minimum energy structure to assess the thermodynamic stability of a hypothetical compound at that composition against the formation of an assemblage of known phases. 
The CSP was performed using the in-house code ChemDASH (Chemically Directed Atom Swap Hopping). Cells containing hexagonally close packed (hcp) and cubic close packed (ccp) anion lattices hosting O2? and S2? were constructed, and some octahedral and tetrahedral interstitial sites occupied by Li+ and Al3+ cations. The hcp cells contained eight anions, and the ccp cell nine anions, with a correct number of cations to satisfy charge neutrality at each composition. Structures were initialised with a random decoration of the anion and cation sublattices, and their structures optimised. To generate new structures, the positions of some anions were swapped on the anion sublattice, or in the interstitial sites the positions of some cations were either swapped or moved to previously vacant interstitial sites. The new structure was then optimised by relaxation of the atomic positions to the nearest local minimum. At each step a Monte Carlo sampling algorithm was used to accept swaps which lowered the energy of the system or increased it by an amount lower than the Monte Carlo energy threshold. The process was continued until 1000 structures had been generated and the lowest energy structure at each composition taken forward for calculating the stability.
One of the features of ChemDASH is to perform structural optimisations in a number of stages which can use different parameters. This was done when optimising each of the structures generated during the CSP process, with each stage using an increasing level of accuracy. In the first stage of each geometry optimisation ? point-only calculations were used with a plane wave cut-off of 400 eV. By the final stage of each geometry optimisation, a 2×2×2 k-point grid was used with a plane wave cut-off of 600 eV. The cell vectors and atomic positions were optimised until forces fell below 0.02 eV·Å-1.
Once a probe structure had been obtained, its energy was re-calculated at a more accurate level, which was also the level of accuracy used to calculate the energies of previously reported phases. These energies were used to generate the convex hulls of chemical stability. A plane wave cut-off of 700 eV was used with a k-point spacing of 0.15 Å?1. Cell vectors and atomic positions were optimised until forces fell below 0.001 eV·Å-1. The convex hull of chemically stable compositions was generated using pymatgen.50
DFT calculations were also performed on an ordered analogue of the experimentally refined crystal structure of Li3AlS3. The split Li sites were merged onto a single high symmetry site and all sites given full occupancy. The structure was then optimised in VASP using the more accurate parameters detailed above. No imaginary frequency modes were found in phonon calculations, showing that the structure is stable against displacement of ions from their relaxed positions. The computed phonon frequencies are presented in Table S2 of the supplementary information.

Elemental analysis
Elemental analysis of Li3AlS3 was performed by Mikroanalytishes Labor Pascher at Remagen-Bandorf, Germany, after dissolution in a HF/HCl solution at elevated temperature and pressure.

Diffraction

X-Ray diffraction
Synchrotron X-ray diffraction (SXRD) was performed at Diamond Light Source UK, on high resolution beamline I11, at ? = 0.82465 Å. The sample was introduced into a 0.7 mm diameter borosilicate glass capillary to record the pattern in transmission mode [0 ° < 2? < 150 °, and ?(2?) = 0.004 °] using the high resolution multianalyser crystal (MAC) detector. Data in a 3-column format (TOF, counts, error).


Neutron diffraction
Time of flight neutron powder diffraction (NPD) data were collected on Li3AlS3 using the High Resolution Powder Diffractometer (HRPD) instrument at ISIS, UK. Experiments were carried out at ambient temperature on the 7Li enriched sample sealed in thin-walled vanadium cans of diameter 8 mm, sealed with an indium gasket under 1 atm of helium gas. Data in a 3-column format (TOF, counts, error).

NMR spectroscopy
The 6Li Magic Angle Spinning (MAS) NMR spectra were recorded at 9.4 T on a Bruker DSX spectrometer using a 4 mm HXY MAS probe (in double resonance mode) and at 20 T on a Bruker NEO spectrometer using a 3.2 mm HXY MAS probe (in triple resonance mode). The 6Li MAS spectra were obtained at 9.4 T with a pulse length of 3 ?s at a radio-frequency (rf) field amplitude of ?1/2? = 83 kHz and a MAS rate of ?r/2? = 10 kHz and at 20 T with a pulse length of 4.5 ?s at a rf field amplitude of ?1/2? = 56 kHz and a MAS rate of ?r/2? = 20 kHz. The 27Al MAS NMR data were recorded at 9.4 T on a Bruker Avance III HD under MAS at a rate of ?r/2? = 12 kHz using a 4 mm HXY MAS probe (in double resonance mode) and at 20 T on a Bruker NEO spectrometer using a 3.2 mm HXY MAS probe (in triple resonance mode). The 27Al spectra were obtained at 9.4 T with a short pulse angle of 30° of duration 0.33 ?s at a rf amplitude of ?1/2? = 83 kHz and at 20 T with a short pulse angle of 30° of duration 0.55 ?s at a rf amplitude of ?1/2? = 50 kHz. The 27Al triple quantum magic angle spinning (MQMAS)57 was obtained at 9.4 T with a z-filtered sequence58 and using rf field amplitude of ?1/2? = 83 kHz for the excitation and reconversion pulses, and of 4 kHz for the selective 90° pulse. All spectra were collected at room temperature and obtained under quantitative recycle delays of more than 5 times longer than the spin-lattice relaxation times T1 which were measured using the saturation recovery pulse sequence and fitted with a stretch exponential function of the form 1 – exp[-(?/T1)?] (with ? ranging from 0.3 to 1). The 6Li and 27Al shifts were referenced to 10 M LiCl in D2O and 0.1 M Al(NO3)3 in H2O at 0 ppm, respectively. 

AC impedance spectroscopy
A pellet of the Li3AlS3 powder was made by uniaxial pressing ~30 mg of powder in a 5 mm diameter cylindrical steel die at a pressure of 125 MPa, followed by sintering in evacuated quartz tube at 800 ºC for 12 h. A relative density of 80 % was obtained by this method.
A.C. impedance measurements were performed using an impedance analyser (Solartron 1296 dielectric interface coupled with the Solartron 1255B frequency response analyser) in the frequency range from 1 MHz to 100 mHz (with an amplitude of 50 mV). Silver paint (RS silver conducting paint 186-3600), brushed on both sides of the pellet and dried under vacuum at room temperature, was used as ion blocking electrodes. Variable temperature conductivity measurements were carried out under argon (flow rate 50 mL·min-1), using a custom built sample holder, in the temperature range 25-125 °C. The impedance spectra were fitted with an equivalent circuit using the ZView2 program. Data in a 3-column format (Re(Z), -Im(Z), frequency)and the temperature is indicated in the name of the file.









Powder X-ray diffraction:
Synchrotron powder X-ray diffraction patterns collected on Beamline I11 at Diamond Light Source. Data in a 2-column format (2theta, counts). All data were collected at ambient temperature. All data were collected using the wide-angle position sensitive detector, with a Si-calibrated wavelength of 0.82612(1) Angstroms and zero error of -0.0902(1) degrees. For samples containing internal standards Synchrotron powder X-ray diffraction patterns collected on Beamline I11 at Diamond Light Source. Data in a 2-column format (2theta, counts). All data were collected at ambient temperature. All data were collected using the wide-angle position sensitive detector, with a Si-calibrated wavelength of 0.82608(1) Angstroms and zero error of -0.0275(1) degrees

Electron ionised mass spectrometry was conducted at the EPSRC UK National Mass Spectrometry Facility using a Thermo Scientific DSQ-II. Data in two-column format (m/z; Normalised intensity, arbitrary units).

Gas chromatography (GC) to detect any hydrogen emission from the sample was carried out using an Agilent 6890N with He as the carrier gas. Data are in a two-column format: retention time (minutes) and signal (mV)

Raman spectroscopy data, collected using Renishaw inVia Spectrometer with a 785 nm excitation laser. Data in two column format (Raman shift, cm-1; Intensity, arbitrary units).