Synthesis, Crystal Structure, Optical, Magnetic and Thermal Properties of (NH 4 ) 2 Mn[B 2 P 3 O 11 (OH) 2 ]Cl

(NH 4 ) 2 Mn II [B 2 P 3 O 11 (OH) 2 ]Cl was synthesized as a crystalline colorless powder by reaction of MnCl 2 , (NH 4 ) 2 HPO 4 , H 3 BO 3 , and H 3 PO 4 under hydrothermal conditions at 180 °C. According to X‐ray single‐crystal investigations (NH 4 ) 2 Mn[B 2 P 3 O 11 (OH) 2 ]Cl crystallizes in a new structure type in the monoclinic space group P 2 1 / n (no. 14) [ Z = 4, a = 905.24(3), b = 847.29(3), c = 1652.32(5) pm, β = 92.303(1)°, R 1 = 0.025, wR 2 = 0.063, 2227 data, 229 parameters]. The crystal structure comprises infinite layers of corner‐sharing borate, hydroxo‐borate phosphate, and hydroxo‐phosphate tetrahedra with Mn 2+ , NH 4 + , and Cl – ions in‐between. The loop‐branched (lB) vierer‐single layer reveals an open‐branched (oB) vierer‐single ring as a novel fundamental building unit (FBU) 2 ∞ [B 2 P 3 Φ 13 ] ( Φ = O, OH). The Mn atom is coordinated slightly distorted octahedrally by five oxygen atoms from two different borophosphate layers and one chlorine atom. Amongst others the magnetic properties of the compound are presented confirming that the compound obeys Curie's law.