title: An ammonium iron(II) pyrophosphate, (NH(4))(2)[Fe(3)(P(2)O(7))(2)(H(2)O)(2)], with a layered structure.
abstract: Diammonium diaquabis(phosphato)triferrate(II), (NH(4))(2)[Fe(3)(P(2)O(7))(2)(H(2)O)(2)], was synthesized under solvo-thermal conditions at 463 K. The crystal structure, isotypic to its Mn and Ni analogues, is built from iron pyrophosphate layers parallel to (100), which are linked by ammonium ions sitting in the inter-layer space via O-H⋯O and N-H⋯O hydrogen bonds. There are two crystallographic Fe sites in the crystal structure, one at a special position (2a, [Formula: see text]), the other at a general position (4e, 1). The former Fe atom on the inversion centre is coordinated by six O atoms, forming an FeO(6) octa-hedron, while the latter is coordinated by five phosphate O atoms and one water mol-ecule, forming an FeO(5)(H(2)O) octa-hedron. Each FeO(6) octa-hedron shares trans edges with two FeO(5)(H(2)O) octa-hedra, forming a linear trimeric unit. These trimers share the lateral edges of FeO(5)(H(2)O) with other trimers, forming a zigzag chain running along [010]. The zigzag chains are further linked by P(2)O(7) groups into a layered structure parallel to (100).
authors.affiliation: Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, Fujian Province,  People's Republic of China.
authors.forename: Biao
authors.initials: B
authors.lastname: Liu
authors.forename: Xin
authors.initials: X
authors.lastname: Zhang
authors.forename: Lei
authors.initials: L
authors.lastname: Wen
authors.forename: Wei
authors.initials: W
authors.lastname: Sun
authors.forename: Ya-Xi
authors.initials: YX
authors.lastname: Huang