Raman folder:
Raman spectra were collected at room temperature using an Alpha300M+ confocal microscope (Witec Gmbh). A 532 nm laser line was delivered to the microscope via a single-mode optical fiber, with the laser power at the sample not exceeding 25 mW. The Raman signal was collected through a 50× long working distance objective (NA = 0.40) and transmitted through a photonic-crystal fiber to a lens based spectrometer (Witec UHTS 300, f/4 aperture, focal length 300 mm) equipped with a back illuminated Andor iDUS 401 detector, which was thermoelectrically cooled to -60 °C. The spectra were acquired using a 1800 mm–1 grating, resulting in a spectral resolution of 1.2 cm-1. Typical acquisition was done from a 10 x 10 μm2 area with a 5 x 5 sampling. Acquisition time at each sampling point ranged from 8 to 22 s. The resulting 25 spectra were post-processed using the Project FIVE software (Witec Gmbh) be performing background subtraction and cosmic-ray removal and then averaged into a single spectrum. The signal from the sample and the diamond anvil were taken from the same area. The positions of Raman bands were determined using the Fityk 1.3.1 software by fitting the observed bands with pseudo-Voigt profiles.
Description of files (ASCII format):
spectra.dat – normalized spectra obtained upon compression (Raman shift in cm–1 in first column, pressure given in the header of each column
freq.dat – position of Raman bands (in cm–1) obtained by fitting the Raman spectrum with pseudo-Voigt profiles (pressure in GPa in first column)
DFT folder:
Input files and results of Density Functional Theory (DFT) modelling (geometry optimization and Raman spectrum calculations) performed in VASP 6.3.4 code for three polymorphs of K2CuF4 (results for each structure in separate folders). Calculations performed using the DFT+U method. Force-sets used for vibrational properties calculations contained in separate subfolder.
Calculation parameters: 800 eV cutoff of the plane-wave basis set; PAW pseudopotentials; SCF convergence: 10–7 eV per atom; k-point mesh with a 2π × 0.03Å−1 spacing.
Description of files (ASCII format):
POTCAR – PAW pseudopotential file
K2CuF4-[symmetry tag]-[pressure in kbar].vasp – structure optimized at a given pressure
KPOINTS – k-point sampling
FORCE_SETS_[pressure in kbar] – force sets at a given pressure
job_script_geom.sh – script used for geometry optimization (contains INCAR file information)
job_script_J.sh - script used for single point calculations of different spin states (contains INCAR file information)
PXRD folder:
High pressure powder X-ray diffraction (PXRD) measurements were performed at the DanMAX beamline at the MAX IV synchrotron (λ = 0.3542 Å, beam size ∼19 × 15 µm2) with NaCl (in the B1 structure acting as a pressure calibrant.4 Diffractograms were recorded on a Dectris Pilatus3 X 2M CdTe area detector calibrated with a Si standard. All 2D diffractograms were integrated using the Dioptas software.
Description of files (ASCII format):
Si_calibration.poni – detector calibration file
K2CuF4_[pressure in kbar].xy – integrated powder x-ray diffraction profile at a given pressure
