Developments in Ceramic Materials Research

98
José M. Rojo, José L. Mesa and Teófilo Rojo
giromagnetic tensor that depends of the nature of the metal and of its S-number of spin.
The magnetic study indicates the existence of antiferromagnetic interactions in the
majority of the compounds, being detected for several metaphosphates a weak
ferromagnetic spin canting phenomenon at low temperatures. In the case of the Ti(III) a
ferromagnetic behavior has been observed. The values of the J-exchanger parameter have
been calculated using the Rushbrooke and Wood equations for 3D-antiferromagnetic
lattices with different values of the spin. In some case, the D-zero field splitting
parameter has been calculated from the simultaneously combination of the ESR and
susceptibility measurements. The neutron powder diffraction study of the M(PO3)3 (M III =
Cr, Mo and Fe) metaphosphates shows antiferromagnetic structures.
1. INTRODUCTION
The study of phosphates of transition elements with low oxidation states has received
great interest in recent years. These compounds exhibit interesting properties such as
magnetic, heterogeneous catalysis, ionic exchange, etc., with potential applications [1,2]. In
this way, the M(PO3)3 materials are interesting owing to the activity of Cr-containing
aluminophosphates (CrAPO-5 materials) which are well known for their catalytic
performance [3, 4].
The M(PO3)3 (M= Al, Sc, Ti, V, Mo, Rh, Ru,…) metaphosphates are the most condensed
phosphate systems in which the metal ions present unusual oxidation states [5-10]. We are
centering this work on the M III = Ti, V, Cr, Mo and Fe, metallic cations, which exhibit some
of them an unusual +III reduced oxidation state in their metaphosphate compounds. In the
molybdenum and vanadium metaphosphates only one crystalline phase, the monoclinic Ctype
[M(PO3)3] structure, has been isolated and the structures have recently been reported
[9,10]. However, six different polymorph phases denoted A, B, C, D, E and F have been
described in chromium and iron metaphosphates [11,12] but, to our knowledge, the crystal
structures of only two phases (B and C) are known [13-15]. One of them adopts the C-type
[Cr(PO3)3] structure and is characterized by one-dimensional infinite metaphosphate (PO3)∞
chains and isolated (CrO6) octahedra. The superstructure found in single crystals X-ray
investigations for this Cr(PO3)3 structure type, in which the monoclinic y axis is tripled, could
be the responsible of the three crystallographic different (CrO6) octahedral units of almost
ideal geometry [13]. The reason for this subcell effect is not immediately apparent in this
complex structure and could be necessary to perform an analysis of the symmetry in order to
elucidate any translational pseudosymmetry that may occur via concerted polyhedral rotation
[7]. The crystal structure for the other chromium phase, B-type [Cr(PO3)3], namely the
hexametaphosphate Cr2(P6O18), is formed by (CrO6) octahedra linked through (PO4)
tetrahedra belonging to (P6O18) 6- cycles.
Taking into account the structural features of these compounds and the nature of the
metallic cations, their magnetic properties are expected to be very interesting. In this way, the
magnetic study of these compounds reveals different types of magnetic exchange interactions,
which range from antiferromagnetic to ferromagnetic for the chromium, molybdenum or iron
phases and the titanium compound, respectively. In the vanadium metaphosphate, the
magnetic behavior is determined by both the antiferromagnetic interactions and the zero-field
splitting parameter characteristic of this cation. To understand the magnetic structures in the