Readme file for data relating to Anion compositional diversity reduces bonding dimensionality in the van der Waals semiconductor Bi4O4SeCl2: The Bi4O4SeCl2 composition can be prepared as a single phase powder. Bi4O4SeCl2 was synthesized from a stoichiometric mixture of BiOCl, Bi, Bi2O3 and Se powder sealed in an evacuated quartz ampoule and heated at 800 °C for 12 hours, followed by regrinding and a second identical heating step. Small single crystals were isolated from the powder for X-ray diffraction measurements, affording a cell of a = 3.8995(8) Å, c = 26.968(5) Å in space group I4/mmm which allowed complete indexing and Pawley refinement (Supplementary Fig.1) of the bulk powder pattern to confirm phase purity. WDX measurements, using BiOCl and Bi2O2Se as standards, revealed a composition of Bi3.9(3)O4.0(3)SeCl1.9(2), which is within error of the expected nominal composition Bi4O4SeCl2. Multiple crystallites of the sample were measured, showing a narrow compositional range (Supplementary Fig. 2). The material showed no sign of degradation on standing in laboratory air for one week, submerged in water for 24 hours or when exposed to simulated solar radiation for 24 hours. The growth of single crystals is described in the Supplementary Information section. Disorder and Entropy Stabilization The stability of (Bi2O2Cl2-xSex)(Bi2O2Se1-xClx) as compared to its building blocks was calculated by dividing the chemical reaction into two steps, i.e., separating (1) Bi2O2Se + 2 BiOCl = Bi2O2Cl2-xSexBi2O2Se1-xClx into (2) Bi2O2Se + 2 BiOCl = Bi4O4Cl2Se + (3) Bi4O4Cl2Se = Bi2O2Cl2-xSexBi2O2Se1-xClx The enthalpy changes of steps 2 and 3 were used to calculate the total energy change with VASP23. The enthalpy change for step 2 was calculated to be +0.46meV/atom. The cut-off energy of plane wave basis set was set to 550 eV. The van der Waals interaction was included with the optB86b-vdW functional24. In order to consider the positional randomness of Cl and Se atoms in Bi2O2Cl2-xSex·Bi2O2Se1-xClx, 4 × 4 × 1 supercells with 352 atoms were built based on the optimized structure of perfect Bi4O4Cl2Se. Eight random configurations, wherein random Cl and Se atoms in each layer were chosen as defect sites with a random number generating Python algorithm (with the constraint that each layer has the same overall defect concentration), were built for Bi2O2Cl2-xSex·Bi2O2Se1-xClx with x ranging from 0 to 1. Bi4O4Cl2Se, Bi2O2Se, and BiOCl unit cells were optimized with Monkhost-Pack k mesh of 7 × 7 × 1, 9 × 9 × 4, and 9 × 9 × 5 respectively. Supercells were optimized with ? point-only calculations. As there are 4 Cl atomic layers and 2 Se atomic layers in the unit cell, the configurational entropy was calculated as ?S = {-4NClkB[xSeln(xSe) + (1 - xSe) ln(1 - xSe)] – 2NSekB[xClln(xCl) + (1 - xCl) ln(1 - xCl)]}/Ntotal where NCl is the number of Cl sites in the Cl atomic layer, NSe is the number of Se sites in Se atomic layer, Ntotal is the total number of atoms in the supercell, and xCl = 2 xSe = x. Electronic Structure Calculations All calculations were performed using periodic, plane-wave based, density functional theory as implemented in VASP23. Core electrons were treated using the projector augmented wave approach25. Structural optimization was performed using the meta-GGA functional SCAN+rVV1026 which includes non-local correlation to better describe van der Waals interactions. Improved values of the band gaps were obtained using single-point calculations with the HSE06 hybrid functional27 with spin-orbit coupling (HSE06+SOC) on reduced k-point grids (Supplementary Table 7) and a plane-wave cut-off energy of 400 eV. Full computational details can be found in the Supplementary Information. Optical and X-ray Photoelectron Spectroscopy Diffuse reflectance measurements were taken on powdered samples of BiOCl, Bi2O2Se and Bi4O4SeCl2 using a Cary 5000 UV-Vis-NIR Spectrometer. Transmission experiments were performed on Scotch tape-cleaved samples, such that the crystals were sufficiently thin, on a Bruker Vertex 70V Fourier transform infrared spectrometer, using a Mid infrared source, KBr beamsplitter, and deuterated L-alanine doped triglycene (DLaTGS) pyroelectric detector. X-ray photoelectron spectroscopy measurements were conducted using a monochromatic Al-ka SPECS (h? = 1486.7 eV) X-ray source operated at 250W. Full details can be found in the Supplementary Information. Transport Measurements The magneto-transport properties were measured on a single crystal using the van der Pauw method. Four indium contacts were soldered on the edge of the crystal with the current and voltage lead configured for measuring both the longitudinal and transverse magnetoresistance between -10 T to 10 T using the ETO option of the MPMS Dynacool system with an AC excitation of 3 mA. The antisymmetric part of the transverse magnetoresistance was used to extract the Hall resistance and the symmetric part of the longitudinal magnetoresistance was used to extract the conventional magnetoresistance. Exfoliation Bi4O4Cl2Se was exfoliated using the well-established ‘Scotch tape’ method.28 Crystals of the material were placed onto a strip of Scotch tape and a new strip of Scotch tape was pressed onto the crystals and removed. This process was repeated on each subsequent new piece of Scotch tape until the material was difficult to see by eye, approximately 8 times. The final piece of Scotch tape containing thin flakes of material was pressed firmly onto a fragment of native oxide Si wafer ranging from 12?cm x 1-2 cm, and cleaned by sonication in propan-2-ol and dried in a stream of nitrogen. The Scotch tape was slowly removed from the surface of the Si to transfer the material. An optical microscope inspection confirmed the transfer of larger flakes of material. AFM was used to determine the thickness of the flakes. Atomic Force Microscopy AFM measurements were performed using an Agilent 5600LS Atomic Force Microscope using tapping mode and Keysight Technologies PicoView control software v1.20.3. Image processing was performed using Gwyddion SPM visualisation and analysis software v2.39. Data is deposited as .mi files which can be opened using the open access Gwyddion software and as .txt matrix file. XPS mapping data is deposited as .csv matrix files. Raman spectroscopy Raman spectroscopy was performed on a Renishaw InViaTM Qontor® confocal Raman microscope using a 532 nm excitation source and 100 x objective lens. Raman mapping images of the Scotch tape exfoliated flakes on Si wafers were obtained at 1 ?m intervals with 1000 accumulations per pixel for a large area scan and 2000 accumulations for a smaller area scan. Data was processed using Renishaw WiRE software. Mapping data is formatted as text file containing the full spectrum for each pixel.