Thermal Transformation of (NH4)[Fe(AsO4)F] Into the New Textural Porous Orthorhombic Fe(AsO4) Phase. Crystal Structures, Thermal Behavior, and Spectroscopic and Magnetic Properties

(NH4)[Fe(AsO4)F] 1 has been synthesized under mild hydrothermal conditions. Single crystals of this phase heated under air atmosphere at 525 °C in a tubular furnace gave single crystals of the new polymorphous Fe(AsO4) 2. The crystal structure of both compounds has been determined from X-ray single-crystal diffraction data. Compound 1 crystallizes in the orthorhombic Pna21 space group with unit-cell parameters of a = 13.270(2) (angstrom), b = 6.629(1) (angstrom), c = 10.866(1) (angstrom), V = 955.8(2) (angstrom)3, and Z = 8. Compound 2 belongs to the orthorhombic space group Imam, the unit cell parameters are a = 13.468(2) (angstrom), b = 6.525(1) (angstrom), c = 10.768(2) (angstrom), V = 946.3(3) (angstrom)3, and Z = 12. The crystal structure of compound 1 consists of a threedimensional framework with chains formed by alternating Fe(2)O4F2 or Fe(1)O4F2 octahedra and As(2)O4 or As(1)O4 tetrahedra, respectively, linked by a common oxygen vertex. These chains run along the a and b axes of the crystal structure. The crystal structure of 2 is formed by a three-dimensional skeleton, constructed from sheets stacked along the [001] direction and interconnected by chains of alternating Fe(2)O6 octahedra, sharing vertex with the As(2)O4 tetrahedra. Transmission electronic microscopy of 2 indicates the existence of unconnected external and internal cavities with a BET surface area of 1.57(2) m2 g-1. The diffuse reflectance spectra in the visible region show the forbidden electronic transitions characteristic of the d5 high spin cations in slightly distorted octahedral geometry. The Mossbauer spectra of both phases at 300 K are characteristic of iron(III) cations. For each phase two sextets with similar isomer shifts and internal hyperfine fields of approximately 50 T were observed at 4.2 K. The ESR spectra from room temperature to 4.2 K remain isotropic with variation in temperature; the g-value being 1.99(1). The magnetic measurements at low temperatures indicate the predominance of antiferromagnetic interactions, with the Neel temperature being 68.0 and 54.5 K for compounds 1 and 2, respectively. For both phases the existence of weak ferromagnetic components, giving rise to small hysteresis loops, was also detected at very low temperatures.