Polymorphism of H 2 SeO 3 , NaHSeO 4 and Na 5 H 3 (SeO 4 ) 4 (H 2 O) 2 , and Re‐refinement of the Crystal Structure of Te 2 O 4 (OH) 2

New modifications (denoted as β ‐) of the compounds H 2 SeO 3 , NaHSeO 4 , and Na 5 H 3 (SeO 4 ) 4 (H 2 O) 2 were isolated from aqueous solutions and their crystal structures determined from single crystal data. β ‐H 2 SeO 3 crystallizes in the space group P 1 with lattice parameters of a = 4.6794(4) Å, b = 5.0988(4) Å, c = 5.7743(5) Å, α = 102.252(6)°, β = 91.594(6)°, γ = 98.054(6)°, and consists of two unique trigonal‐pyramidal SeO(OH) 2 units that are held together by O–H ··· O hydrogen bonds. β ‐NaHSeO 4 crystallizes in the space group P $\bar{1}$ with lattice parameters of a = 5.0814(2) Å, b = 5.6345(3) Å, c = 6.5125(3) Å, α = 88.352(3)°, β = 73.163(3)°, γ = 82.027(3)°, and is made up of distorted NaO 6 octahedra and tetrahedral SeO 3 (OH) – anions, with the hydrogen atom statistically disordered between two positions. β ‐Na 5 H 3 (SeO 4 ) 4 (H 2 O) 2 crystallizes in the space group I 2/ c with lattice parameters of a = 7.0472(15) Å, b = 13.194(3) Å, c = 17.845(4) Å, β = 101.345(9)°. The asymmetric unit contains three Na + sites (one situated on a twofold rotation axis and each with a distorted octahedral coordination by oxygen atoms), one water molecule, one SeO 3 (OH) – anion, and half of an [SeO 4 ··· H ··· O 4 Se] unit with a very short interpolyhedral O ··· O distance of 2.447 (4) Å. The crystal structures of the β ‐modifications are comparatively discussed with those of the corresponding known α ‐modifications. Moreover, the crystal structure of Te 2 O 4 (OH) 2 was reinvestigated [ Pna 2 1 , a = 12.0512(8) Å, b = 8.0267(5) Å, c = 4.7268(3) Å], allowing the localization of the hydrogen atoms and hence a more detailed view of the hydrogen bonding system.