Ammonium-calcium　mixed-cation　pentaborate　NH4Ca[B5O7(OH)4]·H2O　has　been　synthesized　for　the　first　time.　Single-crystal　X-ray　diffraction　analysis　reveals　that　it　crystallizes　in　the　triclinic　space　group　P-1　with　lattice　parameters　a = 6.7042(3)　Å,　b = 7.1859(3)　Å,　c = 10.7447(4)　Å,　α = 100.304(3),　β = 97.774(3),　γ = 96.972(3).　The　structure　is　characterized　by　1D　boron–oxygen　chains　composed　of　[B5O7(OH)4]3−　polyanions　that　are　further　interspaced　by　Ca2+,　NH4+　cations,　along　with　lattice　water　molecules.　The　phase　purity　and　thermal　stability　of　the　as-synthesized　sample　were　confirmed　by　XPRD　and　TG-DSC　analyses.　Experimental　UV–vis　diffuse　reflectance　spectroscopy　coupled　with　Urbach　fitting　indicates　a　direct　bandgap　of　4.50 eV,　contrasting　with　theoretical　calculations　that　suggest　an　indirect　bandgap　of　5.53 eV.　Additionally,　both　theoretical　computation　and　polarized　light　experimental　test　indicate　that　NH4Ca[B5O7(OH)4]·H2O　exhibits　moderate　birefringence.　©　2024　Wiley-VCH　GmbH.